- Email: [email protected]
Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation
Brain Research 798 Ž1998. 7–17
Research report
Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation John J. Rutter b
a, )
, Michael H. Baumann b , Barry D. Waterhouse
a
a Department of Neurobiology and Anatomy, Allegheny UniÕersity of the Health Sciences, Philadelphia, PA 19102-1192, USA Clinical Psychopharmacology Section, National Institute of Drug Abuser National Institutes of Health, Intramural Research Program, Addiction Research Center, Baltimore, MD 21224, USA
Accepted 17 March 1998
Abstract Previous studies have shown that systemically administered cocaine can transiently alter responses of primary somatosensory cortical neurons to threshold level stimulation of peripheral receptive fields. The goal of the present investigation was 2-fold: Ž1. characterize the effects of systemic cocaine on stimulus-evoked responses of the ventral posterior medial ŽVPM. thalamic neurons which relay somatosensory information to the cortex and Ž2. determine the time course and magnitude of changes in monoamine levels within the somatosensory thalamus following systemic administration of cocaine. Extracellularly recorded responses of single VPM thalamic neurons to whisker stimulation were monitored before and after cocaine administration in halothane anaesthetized rats. Each cell was first characterized by assessing its response profile to a range of perithreshold level deflections of the optimal whisker on the contralateral face. Drug effects on stimulus-response curves, response magnitude and latency were determined from quantitative analysis of spike train data. The results indicate that cocaine elicits a predictable augmentation or attenuation of the sensory response magnitude, with the direction of the change inversely related to the initial magnitude of the stimulus-evoked discharge. In addition, cocaine consistently reduced the response time of somatosensory thalamic neurons to peripheral receptive field stimulation. At the same dose and over the same time period, cocaine also produced marked elevation of norepinephrine and serotonin levels within the ventrobasal thalamus, as determined by in vivo microdialysis. These results suggest that cocaine-induced increases in norepinephrine and serotonin are responsible for drug-related modulation of the transfer of sensory signals through primary thalamocortical relay circuits. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Somatosensory thalamus; Cocaine; Microdialysis; Sensory system; Norepinephrine; Serotonin
1. Introduction At the cellular level, cocaine exerts many of its effects on the CNS through its ability to block the reuptake of dopamine ŽDA., norepinephrine ŽNE., and serotonin Ž5-HT. w13,15,36x. For example, the acute locomotor stimulant effects and rewarding properties of cocaine depend upon an increase in dopamine Žsubsequent to uptake blockade. within the nucleus accumbens w47,48x. However, other physiological and behavioral consequences of cocaine administration are not mimicked by DA-specific uptake blockers, and might, therefore, be attributed to the ability )
Corresponding author. MS a408, Department of Neurobiology and Anatomy, Allegheny University of the Health Sciences, 3200 Henry Ave., Philadelphia, PA 19129. Fax: q1-215-843-9082; E-mail: [email protected] 0006-8993r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 3 2 0 - 5
of cocaine to alter 5-HT andror NE neurotransmission w8,24x. In order to address such a possibility, and in an effort to identify a potential substrate for the reported alterations in sensory perception which accompany cocaine self-administration w7,16x, we have been characterizing the ability of cocaine to alter signals transmitted through the rodent trigeminal somatosensory system. This noradrenergically and serotonergically innervated sensory pathway w26– 28,31x relays information from the mystacial vibrissae, and, for a number of reasons, is a good model for drug studies in intact animals. First, the anatomy of this ascending pathway is well established, with relays in the trigeminal nuclei and thalamus en route to the somatosensory cortex w6x. Second, electrophysiological responses of single neurons to vibrissae stimulation have been well characterized in these regions w20,39,41,43x. Finally, the vibrissae
8
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
can be reliably and precisely stimulated by a computerdriven piezoelectric device w42x. Previous work from our laboratory suggests that cocaine, via elevation of 5-HT and NE levels in central circuits, should be capable of modulating the feature extraction properties of sensory neurons w30,46x. For example, systemically administered cocaine selectively enhanced long-latency excitatory responses of somatosensory cortical neurons to suprathreshold whisker stimulation w2x. The goals of the present study were 2-fold: to characterize the effects of cocaine on receptive field properties Ži.e., amplitude dependence. by measuring somatosensory thalamic neuron responses to perithreshold stimulation of peripheral receptors, and to determine the magnitude and time course of changes in extracellular NE and 5-HT following systemic injection of cocaine. Extracellular responses of single units in the ventral posterior medial ŽVPM. thalamus to graded mechanical displacement of individual whiskers on the contralateral face were recorded from halothane-anaesthetized rats. The magnitude and latency of such stimulus-evoked discharges were determined from post-stimulus time histograms of cell activity before and after acute cocaine challenge. In addition, in vivo microdialysis experiments were performed in this region of the thalamus to correlate neurochemical alterations with electrophysiological effects of cocaine. The results indicate that the magnitude and latency of stimulus-evoked discharge can be altered independently following acute cocaine challenge, and that these changes are predictable when analyzed with respect to the basal cell response characteristics. In addition, cocaine administration increased extracellular 5-HT and NE with a time course that correlated well with the observed electrophysiological effects.
2% pontamine sky blue. Action potential discharges were monitored using conventional methods. Individual spikes were converted to uniform voltage pulses by a window discriminator. The discriminator output was led to a computer interface ŽCED 1401, Cambridge Electronic Design, Cambridge, England., which generated and displayed cell activity in the form of post-stimulus time histograms ŽPSTHs.. Synaptic inputs to VPM neurons were activated by mechanical displacement of individual vibrissae contralateral to the recording sites in the thalamus. The whisker in the center of the cell’s receptive field which produced an optimal response to movement Žthe ‘central whisker’. was initially determined by deflecting individual whiskers with a hand-held probe and detecting evoked spike discharges with an audio monitor. Once identified, uniform, repetitive Ž0.5 Hz. displacement of individual whiskers was accomplished by a computer-triggered piezoelectric stimulator w42x. The stimulus probe was positioned so that it was just touching the whisker at a distance approximately 10 mm from the face, while adjacent whiskers were trimmed so as to not interfere with deflection of the central whisker. With the aid of a stimulator ŽGrass S88, Grass Instruments, Quincy, MA., triggering pulses from the computer were converted into square wave pulses 100 ms in duration that varied in amplitude depending upon the voltage output sent to the piezoelectric stimulator. This allowed determi-
2. Materials and methods 2.1. Subjects and surgical procedures, electrophysiology experiments Male Long–Evans hooded rats, 250–400 g, were initially anaesthetized with halothane Ž2% in oxygen., incubated and allowed to breathe a mixture Ž0.75–1.0%. of halothane in oxygen for the duration of the experiment. Animals were fixed in a stereotaxic apparatus and body temperature was maintained at 378C with a heating pad. The skull and the dura over the vibrissae area of the VPM were removed, and the exposed brain covered with 2% agar in balanced salt solution. 2.2. Thalamic unit responses to whisker stimulation Extracellular activity of individual ventral posterior medial ŽVPM. neurons was recorded using glass microelectrodes Ž5–12 M V impedance. filled with 3 M NaCl with
Fig. 1. Characteristic stimulus-response profile of VPM thalamic units selected for this study. Plotted here is the evoked discharge rate as a function of graded mechanical displacement Ž45–105 m . of the principal vibrissae on the contralateral face. Following a gross determination of the cell’s sensitivity to vibrissae displacement, stimuli of different magnitude were presented, in random order, in blocks of 40 trials Žstimulus frequency s 0.5 Hz, stimulus durations100 ms.. A comparison of such control responses to those elicited following systemic cocaine administration provided a means for quantitatively assessing the drug’s modulatory effect on this receptive field property Žsee Fig. 4..
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
nation of the recorded neuron’s response to a range of motion of the central whisker, with ‘amplitude dependence’ curves constructed from graded whisker displacements ranging from approximately 40–200 m m ŽFig. 1.. In accordance with results from previous investigations, the responses of individual cells to stimulation varied greatly as a population; each cell, however, responded to a characteristic range of deflection amplitudes w20,43x. Once this ‘stimulus range’ was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison of cell activity across identical stimulus trials. 2.3. Subjects and surgical procedures, microdialysis experiments Male Long–Evans Hooded rats, 300–450 g, were anaesthetized with sodium pentobarbitol Ž50 mgrkg. and placed in a stereotaxic apparatus. The top of the skull was exposed, and a plastic intracerebral guide cannula ŽCMA 12, Bioanalytical Systems, West Lafayette, IN. was implanted level with the dorsal aspect of the ventral basal thalamus ŽAP: y3.0, ML: 3.0, D: 4.4 mm from bregma. according to the coordinate system of Paxinos and Watson w32x. The guide cannula was fixed to the skull using stainless steel anchor screws and dental acrylic. Microdialysis experiments were performed 4 to 6 days after surgery. 2.4. Microdialysis procedure On the morning of an experiment, rats were cannulated and intubated, and maintained under halothane anesthesia as described in Section 2.3; once the animal was secured in the stereotaxic apparatus, a microdialysis probe ŽCMA 12, 2 = 0.05 mm, Bioanalytical Systems. was slowly lowered into the guide cannula. Artificial Ringers’ solution Ž147.0 mM NaCl, 4.0 mM KCl and 2.0 mM CaCl 2 , unadjusted pH 6.5. was pumped through the probe at 1.0 m lrmin with a microinfusion pump Žmodel CMAr100, Bioanalytical Systems.. Beginning 2 to 3 h after probe insertion, 10 m l dialysate samples were collected into polyethylene tubes at 10-min intervals. The samples were immediately frozen in dry ice, and stored at y808 C until analysis. The first 3 samples collected before any treatment were considered baseline samples, and all subsequent 5-HT or NE measures were expressed as a percent of this baseline. At the conclusion of each experiment, microdialysis sites were confirmed histologically. Data included in this report are from rats with probes that were confirmed to be within or adjacent to the ventral posterior thalamus. 2.5. NE and 5-HT assay Aliquots of the dialysate Ž5 m l. were injected directly into a microbore HPLC column Ž3 m m, C18, 100 = 1 mm,
9
Sepstik, Bioanalytical Systems.. A glassy carbon working electrode was set at a potential of q650 mV relative to a AgrAgCl reference. The mobile phase, which consisted of 9.0 g of NaH 2 PO4 , 436 mg sodium dodecyl sulfate, 10 mg Na 2 EDTA, 100 m l triethylamine, 12% MeOH, and 12% CH 3 CNrl of distilled water Žfinal pH 5.6. was pumped Žmodel 260D, ISCO, Lincoln, NE. at a rate of 60 m lrmin to maintain a column pressure at 2000 to 2500 psi. Chromatographic data were acquired on-line and exported to a MAXIMA 820 software system ŽWaters Associates, Milford, MA. for peak amplification, integration and analysis. Single component standards of NE and 5-HT were run daily before dialysate samples and standard curves were linear over a wide range of concentrations Ž2–2000 pg.. A monoamine standard mix containing NE, 5-HT, 5-hydroxyindole acetic acid, and homovanillic acid was injected before, during and after the experiment to ensure validity of the constituent retention times. Peak heights of unknowns were compared with peak heights of standards and the minimal detectable amount of 5-HT and NE Ž3 times baseline noise. was routinely less than 0.2 pg Ž1.0 fmol. per 5 m l of injection. 2.6. Drug administration Solutions of cocaine HCl Ž0.75 mgrkg. in NaCl vehicle or vehicle were injected via an indwelling jugular catheter. For the electrophysiology experiments, individual perievent histograms of cell activity were collected for each deflection amplitude used both before and after drug injection. For the microdialysis experiments, four baseline samples were obtained, then either drug or vehicle were delivered via the jugular catheter. Microdialysis samples were collected every 10 min until 30 min after injection. 2.7. Data analysis Drug effects on spontaneous versus stimulus-evoked discharge of single VPM cells were quantitatively evaluated by comparing spike activity Žhistograms depict spikesr40 stimuli; values reported throughout the text and in figure legends are the average spikesrstimulusrepoch. during identical stimulation epochs Ž40 stimulirepoch. and periods of spontaneous activity in control and post-drug injection histograms. Spontaneous activity was subtracted from evoked responses in order to obtain an absolute measure of stimulus evoked discharge. The spontaneous activity was determined for the first 100 ms of each histogram, whereas excitatory response epochs were selected by visual inspection during the control condition and fixed for all remaining post-drug injection histograms. The period of a stimulus-evoked response was selected to begin at the particular bin where counts deviated from baseline and terminate where counts reapproached this baseline.
10
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
A subclass of cells showed a decrease in the latency to response with an increase in whisker deflection amplitude. For these cells, the average latency to response for the second stimulus epoch following cocaine delivery was compared, via Signed Rank test, to the like stimulus epoch during the control period. Results from selected experiments were subjected to a more rigorous analysis ŽKruskal–Wallis ANOVA on ranks. in which comparisons were made between the pre- and post-cocaine response magnitude andror latency on a trial by trial basis. Finally, the dialysis data were evaluated by one-way analysis of variance across time. When significant F values were obtained for this data, Newman–Keuls post-hoc tests were performed to compare group means. p - 0.05 was chosen as the minimum criterion for statistical significance.
3. Results 3.1. Characterization of recorded units The extracellular activity of 23 vibrissa-sensitive thalamic neurons was recorded Žone cell per animal. before and after i.v. injection of cocaine. The VPM cells selected for study reacted to stimulation of caudal vibrissae with what previous investigators have described as rapidly adapting w43x or transient w20x responses to maintained whisker deflection. These cells were characterized in terms of their dependence upon the amplitude of deflection for two reasons. First, amplitude dependence of the vibrissae-responsive cells has been described in both of the subcortical nuclei w20,39,43x and in the cortex w19,41x. Second, graded deflections of the individual whiskers can be reliably produced with a computer-driven piezoelectric device mounted onto a microdrive w42x. Previous investigations from other laboratories have shown that cells in the VPM w20x and cortex w19x exhibit a broad range of responsiveness Ži.e., firing rate changes. to whisker deflection. Since the stimulator’s primary plane of movement was dorsoventral, preference was given to those cells which responded optimally to vibrissa motion in these directions. In accordance with results from a previous investigation, the responses of individual cells to stimulation varied greatly as a population; however, each cell responded to a characteristic range of deflection amplitudes w20,43x. Once this stimulus sequence was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison between similar amplitude stimulation epochs. Since the latency and duration of the response to the initial deflection Žthe ‘on’ response. depended somewhat on the amplitude of deflection, average values were determined from cell responses to the largest Ž‘near-maximal’. deflection for a particular experiment. Accordingly, the mean latency Ž"S.E.M.. for the ‘on’ response Ž n s 23.
was 5.7 ms " 0.4, while the mean duration Ž"S.E.M.. was 8.7 ms " 0.1. Some cells Ž n s 12, 52%. also responded with a short latency discharge to the return of the whisker to the initial position Žthe ‘off’ response; see Fig. 6., with a mean latency and duration of 7.2 ms " 0.3 and 15.3 ms " 0.5, respectively. Recorded neurons were well isolated and exhibited stable responses to single vibrissa stimulation for at least 20 min prior to vehicle or drug injection. The response properties of cells described in this report match those reported previously for ‘barreloid’ neurons in the VPM thalamus w10,20,43x. 3.2. Effects of cocaine on the magnitude of stimulus-eÕoked discharge of barreloid thalamic neurons Previously, our laboratory reported that administration of cocaine Žat doses between 0.25 and 2.0 mgrkg i.v.. did not significantly alter the early excitatory component of ‘barrel field’ somatosensory cortical neuron response to suprathreshold vibrissa stimulation w2x. In the present study, however, our goal was to determine if cocaine could selectively alter response components of the thalamic input that drive this early cortical response w6,10,11x, provided graded, submaximal whisker deflections were used as stimuli. Once the appropriate range of whisker deflections was determined under control conditions Žsee Fig. 1., the same range of stimuli Žpresented in randomized order. was used to evoke responses following administration of cocaine Ž0.75 mgrkg i.v... This dose of cocaine is within the effective range for behavioral reinforcement in rodents; i.e., under conditions in which drug-administration is selfpaced, laboratory animals will bar-press for single infusions of cocaine Ž0.75–1.0 mgrkg i.v.. at a rate of approximately 5–6 times an hour w4x. In addition, our previous results in cerebral cortex revealed a reliable alteration in electrophysiological response to suprathreshold whisker deflection within 2 min of cocaine administration in this dose range w2x. Data from an experiment in which the VPM cell exhibited an ‘on’ response to whisker movements over a range of values from 45 to 105 m is depicted in Fig. 2. In this case, cocaine augmented the excitatory response to initial whisker deflection at all but the lowest stimulus amplitude. In contrast, other cells exhibited a decrease in response amplitude to stimulation following cocaine treatment ŽFig. 3.. The magnitude of response, as depicted by stimulus response curves ŽFig. 4., was consistently altered by more than 10% following acute systemic cocaine in the majority of cases Ž28 of 35, 80%.. Because of the variable nature of cocaine on amplitude-sensitive thalamic units, and because a previous study of psychostimulant-induced stereotyped movements suggested that the magnitude of pretreatment response was an important determinant of the post-drug effect w33x, we analyzed the data in the following way: Ž1. For each case, the evoked response to a suprathreshold Žnear maximal. displacement of the central whisker was determined, and
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
11
Fig. 2. Cocaine Ž0.75 mgrkg i.v..-induced increase in the magnitude of a whisker-evoked excitatory discharge. Perievent histograms depict short latency excitatory responses of an individual VPM thalamic somatosensory neuron to graded mechanical displacement Žindicated by arrowhead. of a single whisker on the contralateral face. The amplitude of deflection Žin microns. is displayed in the box above histograms. Numbers adjacent to the excitatory peak indicate the average frequency ŽF. of discharge per response. Each histogram represents cell activity during 40 presentations of the same stimulus. Note the overall increase in the magnitude of responses from control ŽA. to post-cocaine ŽB. conditions.
all subsequent evoked responses were expressed as a percentage of this maximal response Žthereby normalizing the data.. Ž2. Although 4 to 6 different amplitudes were used
to generate each input–output curve, only the normalized responses from the second through the sixth minute Žstimulus epochs 2–4. after cocaine administration were
Fig. 3. Cocaine Ž0.75 mgrkg i.v..-induced decrease in magnitude of a whisker-evoked excitatory discharge. Perievent histograms depict short latency excitatory responses of an individual VPM thalamic somatosensory neuron to graded mechanical displacement of a single whisker on the contralateral face. Labeling conventions are the same as those in Fig. 2. Note the overall decrease in the magnitude of responses from control ŽA. to post-cocaine ŽB. conditions.
12
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
Fig. 4. The graphs in A and B illustrate stimulus-response curves for two VPM thalamic neurons before Žfilled circles, solid line. and after Žopen triangles, dashed line. cocaine Ž0.75 mgrkg i.v.. administration. Frequency of evoked discharge Ži.e., cell response. is plotted as a function of amplitude of primary whisker deflection. Each point represents the average response of the cell to 40 stimulus presentations.
used for statistical comparison, since previous experiments from our laboratory w2x have shown that cocaine effects are most robust during this time frame. Ž3. A ‘low’ control animal was one in which the majority of the responses prior to cocaine were below 50% of the maximal response used for normalization; conversely, a ‘high’ control animal had a majority of pre-drug responses above 50%. Ž4. Once
a subject was categorized in this fashion, the pretreatment values were averaged and compared to the corresponding stimulus epochs following cocaine. Ž5. The data were then subjected to a two-way repeated measures ANOVA with pretreatment magnitude Ž‘low’ vs. ‘high’. and treatment Žpre- vs. post-drug. both as between-response variables. Further analysis used Newman–Keuls post-hoc tests to determine the source of the detected significances in the ANOVA Ž p - 0.05.. This analysis of the data supported the hypothesis that the low and high pretreatment groups responded differently to acute cocaine administration. Specifically, the low pretreatment group showed a significant increase in evoked response after cocaine, while the high pretreatment group showed a significant decrease Ž p - 0.05. Žsee Fig. 5.. 3.3. Effect of cocaine on the stimulus response latency of thalamic neurons
Fig. 5. The plots shown here illustrate the results of two way repeated measures ANOVA to determine significance between pretreatment stimulus-response ratios and cocaine-induced shifts in response magnitude. Cell responses to cocaine were placed into either a low or high pre-drug group depending on whether the control responses to stimulation averaged less or more than 50%, respectively. Pairwise post-hoc comparisons ŽNewman–Keuls. revealed a significant difference between the groups during the control period, as well as a significant response to cocaine administration for both groups Ž p- 0.05..
A portion Ž48%. of the neurons studied exhibited variable response latencies Žspecifically, deviations in time to response of at least 30%. that were inversely proportional to the amplitude of whisker deflection. In the example shown in Fig. 6, this latency shift under control conditions is evident for both the ‘on’ and the ‘off’ components of the cell’s response to whisker movement. Cocaine significantly decreased the response latency for each of these components, as well as reduced the trial to trial variability in response latency ŽFigs. 6 and 7.. The drug-induced shift in average response-latency was consistent for all of the cells studied. Fig. 8 illustrates a comparison of response latencies for equivalent stimuli before and after cocaine, for individual cells. This analysis revealed a significant
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
13
Fig. 6. Vertically oriented cumulative raster illustrates cocaine’s effect on the spike train activity of a single VPM thalamic neuron during ON Žsolid arrowhead accompanied by a stimulus artifact for all but the lowest amplitude stimulations. and OFF Žopen arrowhead. movement of the cell’s primary whisker. The ordinate depicts the first 0.3 s of each trial; each vertical line of dots represents the cell’s activity during one stimulus trial. The abscissa indicates the running time of the experiment and the amplitude of whisker deflection for a given time period. The solid arrowhead on the horizontal axis indicates the time at which cocaine Ž0.75 mgrkg i.v.. was injected. Note that the magnitude of the OFF response for all stimulus intensities was considerably larger following cocaine administration. Likewise, following cocaine a prominent OFF response to 45 m whisker deflection was revealed Žcompare period marked with solid bars Pre- vs. Post-cocaine..
decrease in the average response latency Ž n s 12, p 0.0001.. 3.4. Cocaine facilitation of subthreshold synaptic inputs Although stimulus amplitudes which produced a detectable response during pre-drug characterization were
generally selected for study, there were occasional instances when a stimulus level that was subthreshold for eliciting a response was tested. In two such cells, cocaine revealed excitatory responses to vibrissae stimulation which were not apparent during control conditions. For the case illustrated in Fig. 6 Ž‘off’ response., a whisker deflection
Fig. 7. Graphs illustrate cocaine-induced shift in average response latency for the VPM thalamic neuron depicted in Fig. 6. Latency to response is plotted as a function of amplitude of primary whisker deflection Ž A s ms from stimulus onset; B s ms from stimulus offset.. Each point represents the mean response" S.E.M. of the cell to 40 like-stimulus presentations. Filled circless before cocaine Žcontrol.; open triangless after cocaine injection. Trial by trial examination of the response to cocaine revealed a statistically significant effect of the drug on latency for amplitudes 62–150 m for the ‘on’ response, and 45-105 m for the ‘off’ response ŽNewman–Keuls p - 0.05, asterisks omitted for sake of clarity..
14
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
of 45 m evoked no discernable response during the control period. However, following cocaine administration an obvious excitatory response to the same stimulus emerged. This ‘gating effect’ on responses to otherwise subthreshold sensory stimuli has been described previously for NE in the visual cortex w45,46x and for cocaine in the somatosensory cortex w22x. 3.5. Effect of cocaine on extracellular NE and 5-HT leÕels in the VPM thalamus region In a separate group of animals Ž n s 7., in vivo microdialysis experiments were conducted as a means of correlating the time course of cocaine-induced changes in stimulus-evoked responses of thalamic neurons with altered monoamine levels in the VPM thalamus. To this end, probes targeted for the barreloid region of the VPM were implanted in halothane-anaesthetized rats, and following a suitable stabilization period Žapproximately 2–3 h., basal levels of NE and 5-HT were determined Žsee Section 2.. The baseline level Žmean pg 5 m lr10 min " S.E.M.. of NE was 2.02 " 0.12, while the baseline level of 5-HT was 2.31 " 0.85 Ž n s 7.. Low amounts of dopamine were sometimes detectable in this region, with a mean baseline level of 0.52 " 0.15. All values were expressed as the percent of baseline Žeach rat served as its own control. to account for interindividual variability in resting monoamine levels.
Fig. 9. Effect of cocaine on extracellular NE and 5-HT in the ventrobasal thalamus. The graph illustrates extracellular levels of NE and 5-HT in the ventrobasal thalamus for baseline and post-cocaine conditions. Values indicated are based upon means"S.E.M Ž ns 7–8.. Dialysis samples were taken every 10 min for 30-min periods before and after cocaine Ž0.75 mgrkg i.v.. administration. Cocaine injected at times 0. After cocaine, there were significant differences from baseline for both monoamines for at least 20 min Žasterisks denote significance, p- 0.05..
Acute i.v. cocaine Ž0.75 mgrkg. significantly increased both extracellular NE and 5-HT for 20–30 min following injection ŽFig. 9.. It is noteworthy that the maximum response for both NE and 5-HT Žboth to approximately 200% of pretreatment baseline. occurred in the first 10 min after cocaine administration, which corresponds well with the time course of the observed electrophysiologic changes produced by the drug. The cocaine-induced elevation in 5-HT was more transient than that observed for NE, but the extracellular levels for both monoamines was significantly different from baseline for at least 20 min after treatment. In contrast, drug-induced changes in extracellular DA were inconsistent and failed to reach statistical significance Ž p ) 0.05, data not shown.. Likewise, salinetreated controls showed no significant deviation from baseline for either 5-HT or NE Ž p ) 0.05, data not shown..
4. Discussion Fig. 8. Cocaine-induced decrease in response latency. All cells which exhibited an inverse correlation between amplitude of whisker movement and latency to response Ž ns12; see Fig. 6 for example. were analyzed to determine the effect of cocaine on latency. In all cases the average response latency to stimuli delivered 2–3 min after cocaine Žpost-cocaine. injection was computed and compared to the average response latency before cocaine Žpre-cocaine.. Each line represents the shift in average response latency for one cell from the pre- to post-cocaine conditions. The cocaine-induced decrease in response latency was highly significant Žpaired t-test, p- 0.0001..
The present report is based on data obtained from a population of rat somatosensory thalamic ŽVPM. neurons which responded with excitatory discharges of variable magnitude and latency to graded, submaximal peripheral tactile stimulation. While all VPM cells examined showed some degree of amplitude dependence, the cells chosen for analysis in the present study resemble those described as highly amplitude-dependent in previous work w20x. Since
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
VPM neurons are morphologically homogenous w18x, such variations in the control responses of cells studied here are presumably related to differential connectivity within the trigeminal somatosensory circuitry Žsee also discussion in Ref. w20x.. Systemic cocaine altered the stimulus-evoked discharges of these cells, effecting either a positive Žincrease. or negative Ždecrease. shift in the magnitude of these responses, as well as a decrease in the response latency of those cells which exhibited an inverse relationship between latency and the strength of the sensory stimulus employed. That such changes were related to cocaine-induced enhancement of extracellular 5-HT and NE is supported by microdialysis measurements in the region of the VPM, which revealed an increase in both monoamines with a time course that correlated well with the observed electrophysiological changes. Overall, these results complement previous studies in our laboratory which showed that systemic cocaine can differentially influence specific components of sensory cortical neuron responses to supra-threshold activation of the rat somatosensory system w2x. There are several compelling reasons to expect that cocaine can alter the transmission of sensory signals as a consequence of increased synaptic levels of 5-HT and NE. First, studies in both humans and animals indicate that cocaine can alter sensory perception. For example, individuals who self-administer cocaine report alterations in effect such as heightened arousal and a sense of well-being, along with enhanced sensory perception w12,16,17x. In rodents, cocaine also produces arousal and increased sensitivity to environmental changes including auditory, visual and tactile stimuli w38,47,48x, electrical shock w25x, and temperature changes w34x. Second, as a potent inhibitor of the uptake of 5-HT and NE w13,15,36x, cocaine influences monoaminergic transmission in two systems which have broad efferent projections to forebrain sensory areas including subcortical sensory relay nuclei and primary sensory cortical sites w21,27,29,31x. Finally, recent investigations from our laboratory and others indicate that iontophoretically applied NE and 5-HT can alter specific parameters of sensory cortical neuron responsiveness to afferent pathway stimulation w30,35,44,46x. The VPM cells selected for the present study reacted to stimulation of caudal vibrissae with what previous investigators had described as rapidly adapting w43x or transient w20x responses to maintained whisker deflection. These cells were characterized in terms of their dependence upon the amplitude of deflection for two reasons. First, amplitude dependence of the vibrissae-responsive cells has been described in both of the subcortical relays w20,39,43x and in the cortex w19,41x. Second, graded deflections of the whisker can be reliably produced with a computer-driven piezoelectric device mounted onto a microdrive w42x. In accordance with results from a previous investigation, the responses of individual cells to stimulation varied greatly as a population, however each cell had a characteristic set
15
of deflection amplitudes that evoked a response w20,43x. Once this ‘sensory signature’ was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison between similar amplitude stimulation epochs. The results indicate that a behaviorally relevant dose of cocaine Žsee Section 3. had an effect on transmission of the amplitude dependent signal, since the majority of the individual cell curves were shifted following administration of the drug. Further analysis of the data showed that cocaine’s effect on individual cells was predictable on the basis of the proximity of the control level response to the maximum evokable discharge of the cell. This analysis was prompted by the work of Pederson et al. w33x, who recently described a ‘rate dependent’ effect of psychostimulants on the discharge properties of striatal neurons. Our results show that cells whose averaged control response was approximately 25% of maximal Žsee Fig. 5 and Section 3. exhibited a increased response to stimuli following cocaine Ž‘low pre-drug group’., while the drug decreased the magnitude of discharge for those cells whose averaged control response was near maximal at the outset Ž‘high pre-drug group’.. While this analysis supports the notion that cocaine alters the amplitude sensitivity of vibrissae-driven cells in the VPM in a predictable fashion, the underlying mechanism for this effect is unclear at the present time. Previous observations that the VPM is comprised of a homogenous array of relay neurons whose activity is regulated by inhibitory influences from the neighboring reticular thalamic nucleus ŽNRT. w18,40x make it seem unlikely that the functional heterogeneity described here could be ascribed to unique morphological or electrophysiological characteristics of the recorded cell. In addition, the finding that individual cells in our study can exhibit both ‘on’ and ‘off’ responses which change in opposite directions following cocaine administration argues against a nonspecific mechanism such as a drug-induced shift in the plane of anesthesia. It is feasible that cocaine could elicit either a facilitation or suppression of the vibrissae response depending upon a variable elevation in the monoamines. Previous investigations have shown, for example, that increases in spontaneous activity in the somatosensory system w1x, or potentiation of discharges in the visual system w46x, occurs at lower iontophoretic applications of NE than those which produce a generalized suppression of spontaneous or stimulus-evoked activity. Again, such a mode of action would not explain those cases in which the responses to onset and offset of the applied stimulus within the same animal were altered in an opposing fashion following cocaine. Differences in the magnitude of response evoked by the ‘on’ and the ‘off’ phases of whisker deflection presumably reflect differential activation of this glutamatergically-mediated pathway w23,37x. It seems possible that the bimodal nature of the cocaine-induced changes was based upon the interaction between synaptically released monoamines and the
16
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
variable excitatory tone of the pathway. Further clarification of this issue may come from in vitro studies currently underway in our laboratory. The preliminary results of these experiments have shown that the modulatory effect of the same amount of iontophoretically-applied NE depends upon the initial level of the glutamate-evoked discharge w9x. Another predictable effect of cocaine was a decrease in the mean latency to response, in a group of cells which exhibited an inverse relationship between latency to response and deflection amplitude ŽFig. 6.. The latency of evoked spikes to vibrissae stimulation is a commonly measured receptive field property of this sensory pathway w20,39,43x, and has been positively correlated with response threshold w20,43x. Furthermore, the response latency appears to be mutable in some cases by altering the local monoaminergic tone. For example, similar decreases in the latency of an evoked response have been demonstrated in both the somatosensory w14x and visual systems w46x following iontophoretic application of NE. The neurochemical data presented here support the aforementioned anatomical and physiological observations that cocaine-induced changes in the processing of sensory signaling are mediated by NE and 5-HT. Further, there is a good correlation between the time course of the cocaineinduced NE and 5-HT elevations reported here Žsignificant elevation of both monoamines for 20 min post-cocaine. and the previously reported time course of cocaine actions on sensory cortical neurons w2x. In addition, a similar monoaminergic specificity for cocaine’s impact on sensory signal transmission was observed in a recent study in which selective uptake inhibitors for NE and 5-HT, but not DA, mimicked the changes observed after cocaine administration w3x. It is important to note that in several cases Ž3 of 8. reliable baseline levels of DA were present, and cocaine had the predictable effect of elevating the extracellular DA. The lack of a consistent effect of cocaine on DA in this study, therefore, presumably reflects the lack of a significant DA innervation in the ventral posterior thalamus w5x. Given the limited spatial resolution of the dialysis probe, it seems likely that in those cases where baseline DA was higher, the probe was within diffusion distance of a region with a greater DA innervation, such as the ventral medial thalamus w5,32x. While the observed increase in extracellular 5-HT and NE in the thalamus does not preclude the possibility that cocaine could affect sensory transmission at other sites, it does suggest the possibility that some of the drug-induced changes reported here could be mediated locally. Sitespecific, as opposed to circuit-level effects, are further supported by a previous investigation in our laboratory, in which it was observed that local iontophoretic application of cocaine could alter responses of cortical neurons to iontophoretically applied glutamate w22x. Future experiments, using iontophoresis in conjunction with monoamine agonists and antagonists, should help clarify the extent to
which the observed effects can be attributed to local NE andror 5-HT alterations in the VPM.
Acknowledgements The authors would like to thank Dr. E.M. Gould and M. Ayestas for technical support. This work was supported by grant RO1 DA05117 to BDW, and by the Intramural Research Program ŽMHB., from National Institute of Drug Abuse.
References w1x M. Armstrong-James, K. Fox, Effects of iontophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex, J. Physiol. 335 Ž1983. 427–447. w2x I. Bekavac, B.D. Waterhouse, Systemically administered cocaine selectively enhances long-latency responses of rat barrel field cortical neurons to vibrissae stimulation, J. Pharmacol. Exp. Ther. 272 Ž1995. 333–342. w3x I. Bekavac, E.M. Gould, B.D. Waterhouse, Monoamine substrates underlying cocaine-induced enhancement of somatosensory evoked discharges in rat barrel field cortical neurons, J. Pharmacol. Exp. Ther. 279 Ž1996. 582–592. w4x M.A. Bozarth, R.A. Wise, Toxicity associated with long term intravenous heroin and cocaine self-administration in the rat, JAMA 254 Ž1985. 81–83. w5x A. Bjorklund, O. Lindvall, Dopamine-containing systems in the CNS, in: A. Bjorklund, T. Hokfelt ŽEds.., Classical Transmitters in the CNS, Part 1: Handbook of Chemical Neuroanatomy, Vol. 2, Elsevier, Amsterdam, 1984, pp. 55–122. w6x J. Chapin, C.-S. Lin, The somatic sensory cortex of rat, in: B. Kolb, R.C. Tees ŽEds.., The Cerebral Cortex of the Rat, MIT Press, Cambridge, 1990, pp. 341–380. w7x S. Cohen, Cocaine: acute medical and psychiatric complications, Psychiatr. Ann. 14 Ž1984. 747–749. w8x K.A. Cunningham, J.M. Paris, N.E. Goeders, Serotonin neurotransmission in cocaine sensitization, in: P.W. Kalivas, H.H. Sampson ŽEds.., The Neurobiology of Drug and Alcohol Addiction, The New York Academy of Sciences, New York, 1992, pp. 117–127. w9x D.M. Devilbiss, B.D. Waterhouse, Effect of increasing tonic levels of norepinephrine on glutamate evoked discharge of single neurons in layers IIrIII and V of rat barrel field cortex, Soc. Neurosci. Abstr. 240 Ž1996. 10. w10x M.E. Diamond, M. Armstrong-James, F.F. Ebner, Somatic sensory responses in the rostral sector of the posterior group ŽPOm . and in the ventral posterior medial nucleus of the rat thalamus, J. Comp. Neurol. 318 Ž1992. 462–476. w11x M.E. Diamond, M. Armstrong-James, M.J. Budway, F.F. Ebner, Somatic sensory responses in the rostral sector of the posterior group ŽPOm . and in the ventral posterior medial nucleus ŽVPM. of the rat thalamus: dependence on the barrel field cortex, J. Comp. Neurol. 319 Ž1992. 66–84. w12x M.W. Fischman, C.R. Schuster, N.A. Krasnegor, Physiological and behavioral effects of intravenous cocaine in man, in: E.H. Ellinwood, M.M. Kilbey ŽEds.., Cocaine and Other Stimulants, Plenum, New York, 1977, pp. 647–664. w13x E. Friedman, S. Gershon, J. Rotrosen, Effects of acute cocaine treatment on the turnover of 5-hydroxytryptamine in the rat brain, Br. J. Pharmacol. 54 Ž1975. 61–64. w14x M.J. George, Modification of receptive fields of posteriomedial
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
w15x
w16x
w17x
w18x
w19x w20x w21x w22x
w23x
w24x
w25x w26x w27x
w28x
w29x
w30x
w31x
barrel subfield neocortical single units by known concentrations of iontophoresed noradrenaline in the rat, Int. J. Neurosci. 65 Ž1992. 69–81. J. Glowinski, J. Axelrod, Effects of drugs on the uptake, release and metabolism of 3 H-norepinephrine, J. Pharmacol. Exp. Ther. 149 Ž1965. 43–49. M.S. Gold, Cocaine Žand crack.: clinical aspects, in: J.H. Lowinsin, P. Ruiz, R.B. Millman, J.G. Langrod ŽEds.., Substance Abuse: A Comprehensive Textbook, Williams and Wilkins, Baltimore, 1992, pp. 205–221. M.S. Gold, N.S. Miller, J.M. Jonas, Cocaine Žand crack.: neurobiology, in: J.H. Lowinsin, P. Ruiz, R.B. Millman, J.G. Langrod ŽEds.., Substance Abuse: A Comprehensive Textbook, Williams and Wilkins, Baltimore, 1992, pp. 222–235. R.M. Harris, Morphology of physiologically identified thalamocortical relay neurons in the rat ventrobasal thalamus, J. Comp. Neurol. 251 Ž1986. 491–505. M. Ito, Some quantitative aspects of vibrissa-driven neuronal responses in rat neocortex, J. Neurophysiol. 46 Ž1981. 705–715. M. Ito, Response properties and topography of vibrissa-sensitive VPM neurons in the rat, J. Neurophysiol. 60 Ž1988. 1181–1197. B.L. Jacobs, E.C. Azmitia, Structure and function of the brain serotonin system, Physiol. Rev. 72 Ž1992. 165–229. C.A. Jimenez-Rivera, B.D. Waterhouse, Effects of systemically and locally applied cocaine on cerebrocortical neuron responsiveness to afferent synaptic inputs and glutamate, Brain Res. 546 Ž1991. 287– 296. T. Klockgether, Excitatory amino acid receptor-mediated transmission of somatosensory evoked potentials in the rat thalamus, J. Physiol. 394 Ž1987. 445–461. M.J. Kuhar, M.C. Ritz, D. Grigoriadis, R. Lew, A cocaine receptor associated with dopamine transport and drug self-administration, in: J.M. Lakowski, M.P. Galloway, F.J. White ŽEds.., Cocaine: Pharmacology, Physiology and Clinical Strategies, CRC Press, Boca Raton, 1992, pp. 191–202. H. Lal, R.D. Chessick, Lethal effects of aggregation and electric shock in mice treated with cocaine, Nature 208 Ž1965. 295–296. P. Levitt, R.Y. Moore, Noradrenaline neuron innervation of the neocortex in the rat, Brain Res. 139 Ž1978. 219–231. P. Levitt, R.Y. Moore, Origin and organization of brainstem catecholamine innervation in the rat, J. Comp. Neurol. 186 Ž1979. 505–528. H.G.W. Lidov, R. Grzanna, M.E. Molliver, The serotonin innervation of the cerebral cortex in the rat—an immunohistochemical analysis, Neuroscience 5 Ž1980. 207–227. O. Lindvall, A. Bjorklund, A. Nobin, U. Stenevi, The adrenergic innervation of the rat thalamus as revealed by the glyoxylic acid fluorescence method, J. Comp. Neurol. 154 Ž1974. 317–348. J. McLean, B.D. Waterhouse, Noradrenergic modulation of cat area 17 neuronal responses to moving visual stimuli, Brain Res. 667 Ž1994. 83–97. J.H. Morrison, S.L. Foote, Noradrenergic and serotonergic innerva-
w32x w33x
w34x w35x
w36x
w37x w38x
w39x
w40x
w41x w42x w43x
w44x
w45x
w46x
w47x
w48x
17
tion of cortical, thalamic and tectal visual structures in old and new world monkeys, J. Comp. Neurol. 243 Ž1986. 117–138. G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press, Sydney, 1986, 146 pp. C.L. Pederson, M. Wolske, L.L. Peoples, M.O. West, Firing rate dependent effect of cocaine on single neurons of the rat lateral striatum, Brain Res. 760 Ž1997. 261–265. D. Peterson, J. Hardinge, The effect of various environmental factors on cocaine and ephedrine toxicity, J. Pharmacol. 19 Ž1967. 810–814. M.A. Rogawski, G.K. Aghajanian, Norepinephrine and serotonin: opposite effects on the activity of lateral geniculate neurons evoked by optic pathway stimulation, Exp. Neurol. 69 Ž1980. 678–684. S.B. Ross, A.L. Renyi, Inhibition of the uptake of tritiated 5-hydroxytryptamine in brain tissue, Eur. J. Pharmacol. 7 Ž1969. 270– 277. T.E. Salt, Mediation of thalamic sensory input by both NMDA receptors and non-NMDA receptors, Nature 322 Ž1986. 263–265. J. Scheel-Kruger, C. Braestrup, M. Nielsen, R. Golembiowski, E. Mogilnicka, Cocaine: discussion of the role of dopamine in the biochemical mechanism of action, in: E. Ellinwood, M.M. Kilbey ŽEds.., Cocaine and Other Stimulants, Plenum, New York, 1977, pp. 373–407. M.T. Shipley, Response characteristics of single units in the rat’s trigeminal nucleus to vibrissae displacements, J. Neurophysiol. 37 Ž1974. 73–90. A. Shosaku, A comparison of receptive field properties of vibrissa neurons between the rat thalamic reticular and ventrobasal nuclei, Brain Res. 347 Ž1985. 36–40. D.J. Simons, Response properties of vibrissa units in rat SI somatosensory neocortex, J. Neurophysiol. 41 Ž1978. 798–820. D.J. Simons, Multi-whisker stimulation and its effects on vibrissa units in rat SmI barrel cortex, Brain Res. 276 Ž1983. 178–182. P.M.E. Waite, The responses of cells in the rat thalamus to mechanical movements of the whiskers, J. Physiol. ŽLondon. 228 Ž1973. 541–561. B.D. Waterhouse, H.C. Moises, D.J. Woodward, Interaction of serotonin with somatosensory cortical neuronal responses to afferent synaptic inputs and putative transmitter substances, Brain Res. Bull. 17 Ž1986. 507–518. B.D. Waterhouse, F.M. Sessler, J.-T. Cheng, D.J. Woodward, S.A. Azizi, H.C. Moises, New evidence for a gating action of norepinephrine in central neuronal circuits of mammalian brain, Brain Res. Bull. 21 Ž1988. 425–432. B.D. Waterhouse, S.A. Azizi, R.A. Burne, D.J. Woodward, Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis, Brain Res. 514 Ž1990. 276–292. M.C. Wilson, J.A. Bedford, J. Buelke, A.H. Kibbe, Acute pharmacological activity of intravenous cocaine in the rhesus monkey, Psychopharmacol. Commun. 2 Ž1976. 251–262. R.A. Wise, M.A. Bozarth, A psychomotor stimulant theory of addiction, Psychol. Rev. 94 Ž1987. 469–492.
Research report
Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation John J. Rutter b
a, )
, Michael H. Baumann b , Barry D. Waterhouse
a
a Department of Neurobiology and Anatomy, Allegheny UniÕersity of the Health Sciences, Philadelphia, PA 19102-1192, USA Clinical Psychopharmacology Section, National Institute of Drug Abuser National Institutes of Health, Intramural Research Program, Addiction Research Center, Baltimore, MD 21224, USA
Accepted 17 March 1998
Abstract Previous studies have shown that systemically administered cocaine can transiently alter responses of primary somatosensory cortical neurons to threshold level stimulation of peripheral receptive fields. The goal of the present investigation was 2-fold: Ž1. characterize the effects of systemic cocaine on stimulus-evoked responses of the ventral posterior medial ŽVPM. thalamic neurons which relay somatosensory information to the cortex and Ž2. determine the time course and magnitude of changes in monoamine levels within the somatosensory thalamus following systemic administration of cocaine. Extracellularly recorded responses of single VPM thalamic neurons to whisker stimulation were monitored before and after cocaine administration in halothane anaesthetized rats. Each cell was first characterized by assessing its response profile to a range of perithreshold level deflections of the optimal whisker on the contralateral face. Drug effects on stimulus-response curves, response magnitude and latency were determined from quantitative analysis of spike train data. The results indicate that cocaine elicits a predictable augmentation or attenuation of the sensory response magnitude, with the direction of the change inversely related to the initial magnitude of the stimulus-evoked discharge. In addition, cocaine consistently reduced the response time of somatosensory thalamic neurons to peripheral receptive field stimulation. At the same dose and over the same time period, cocaine also produced marked elevation of norepinephrine and serotonin levels within the ventrobasal thalamus, as determined by in vivo microdialysis. These results suggest that cocaine-induced increases in norepinephrine and serotonin are responsible for drug-related modulation of the transfer of sensory signals through primary thalamocortical relay circuits. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Somatosensory thalamus; Cocaine; Microdialysis; Sensory system; Norepinephrine; Serotonin
1. Introduction At the cellular level, cocaine exerts many of its effects on the CNS through its ability to block the reuptake of dopamine ŽDA., norepinephrine ŽNE., and serotonin Ž5-HT. w13,15,36x. For example, the acute locomotor stimulant effects and rewarding properties of cocaine depend upon an increase in dopamine Žsubsequent to uptake blockade. within the nucleus accumbens w47,48x. However, other physiological and behavioral consequences of cocaine administration are not mimicked by DA-specific uptake blockers, and might, therefore, be attributed to the ability )
Corresponding author. MS a408, Department of Neurobiology and Anatomy, Allegheny University of the Health Sciences, 3200 Henry Ave., Philadelphia, PA 19129. Fax: q1-215-843-9082; E-mail: [email protected] 0006-8993r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 3 2 0 - 5
of cocaine to alter 5-HT andror NE neurotransmission w8,24x. In order to address such a possibility, and in an effort to identify a potential substrate for the reported alterations in sensory perception which accompany cocaine self-administration w7,16x, we have been characterizing the ability of cocaine to alter signals transmitted through the rodent trigeminal somatosensory system. This noradrenergically and serotonergically innervated sensory pathway w26– 28,31x relays information from the mystacial vibrissae, and, for a number of reasons, is a good model for drug studies in intact animals. First, the anatomy of this ascending pathway is well established, with relays in the trigeminal nuclei and thalamus en route to the somatosensory cortex w6x. Second, electrophysiological responses of single neurons to vibrissae stimulation have been well characterized in these regions w20,39,41,43x. Finally, the vibrissae
8
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
can be reliably and precisely stimulated by a computerdriven piezoelectric device w42x. Previous work from our laboratory suggests that cocaine, via elevation of 5-HT and NE levels in central circuits, should be capable of modulating the feature extraction properties of sensory neurons w30,46x. For example, systemically administered cocaine selectively enhanced long-latency excitatory responses of somatosensory cortical neurons to suprathreshold whisker stimulation w2x. The goals of the present study were 2-fold: to characterize the effects of cocaine on receptive field properties Ži.e., amplitude dependence. by measuring somatosensory thalamic neuron responses to perithreshold stimulation of peripheral receptors, and to determine the magnitude and time course of changes in extracellular NE and 5-HT following systemic injection of cocaine. Extracellular responses of single units in the ventral posterior medial ŽVPM. thalamus to graded mechanical displacement of individual whiskers on the contralateral face were recorded from halothane-anaesthetized rats. The magnitude and latency of such stimulus-evoked discharges were determined from post-stimulus time histograms of cell activity before and after acute cocaine challenge. In addition, in vivo microdialysis experiments were performed in this region of the thalamus to correlate neurochemical alterations with electrophysiological effects of cocaine. The results indicate that the magnitude and latency of stimulus-evoked discharge can be altered independently following acute cocaine challenge, and that these changes are predictable when analyzed with respect to the basal cell response characteristics. In addition, cocaine administration increased extracellular 5-HT and NE with a time course that correlated well with the observed electrophysiological effects.
2% pontamine sky blue. Action potential discharges were monitored using conventional methods. Individual spikes were converted to uniform voltage pulses by a window discriminator. The discriminator output was led to a computer interface ŽCED 1401, Cambridge Electronic Design, Cambridge, England., which generated and displayed cell activity in the form of post-stimulus time histograms ŽPSTHs.. Synaptic inputs to VPM neurons were activated by mechanical displacement of individual vibrissae contralateral to the recording sites in the thalamus. The whisker in the center of the cell’s receptive field which produced an optimal response to movement Žthe ‘central whisker’. was initially determined by deflecting individual whiskers with a hand-held probe and detecting evoked spike discharges with an audio monitor. Once identified, uniform, repetitive Ž0.5 Hz. displacement of individual whiskers was accomplished by a computer-triggered piezoelectric stimulator w42x. The stimulus probe was positioned so that it was just touching the whisker at a distance approximately 10 mm from the face, while adjacent whiskers were trimmed so as to not interfere with deflection of the central whisker. With the aid of a stimulator ŽGrass S88, Grass Instruments, Quincy, MA., triggering pulses from the computer were converted into square wave pulses 100 ms in duration that varied in amplitude depending upon the voltage output sent to the piezoelectric stimulator. This allowed determi-
2. Materials and methods 2.1. Subjects and surgical procedures, electrophysiology experiments Male Long–Evans hooded rats, 250–400 g, were initially anaesthetized with halothane Ž2% in oxygen., incubated and allowed to breathe a mixture Ž0.75–1.0%. of halothane in oxygen for the duration of the experiment. Animals were fixed in a stereotaxic apparatus and body temperature was maintained at 378C with a heating pad. The skull and the dura over the vibrissae area of the VPM were removed, and the exposed brain covered with 2% agar in balanced salt solution. 2.2. Thalamic unit responses to whisker stimulation Extracellular activity of individual ventral posterior medial ŽVPM. neurons was recorded using glass microelectrodes Ž5–12 M V impedance. filled with 3 M NaCl with
Fig. 1. Characteristic stimulus-response profile of VPM thalamic units selected for this study. Plotted here is the evoked discharge rate as a function of graded mechanical displacement Ž45–105 m . of the principal vibrissae on the contralateral face. Following a gross determination of the cell’s sensitivity to vibrissae displacement, stimuli of different magnitude were presented, in random order, in blocks of 40 trials Žstimulus frequency s 0.5 Hz, stimulus durations100 ms.. A comparison of such control responses to those elicited following systemic cocaine administration provided a means for quantitatively assessing the drug’s modulatory effect on this receptive field property Žsee Fig. 4..
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
nation of the recorded neuron’s response to a range of motion of the central whisker, with ‘amplitude dependence’ curves constructed from graded whisker displacements ranging from approximately 40–200 m m ŽFig. 1.. In accordance with results from previous investigations, the responses of individual cells to stimulation varied greatly as a population; each cell, however, responded to a characteristic range of deflection amplitudes w20,43x. Once this ‘stimulus range’ was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison of cell activity across identical stimulus trials. 2.3. Subjects and surgical procedures, microdialysis experiments Male Long–Evans Hooded rats, 300–450 g, were anaesthetized with sodium pentobarbitol Ž50 mgrkg. and placed in a stereotaxic apparatus. The top of the skull was exposed, and a plastic intracerebral guide cannula ŽCMA 12, Bioanalytical Systems, West Lafayette, IN. was implanted level with the dorsal aspect of the ventral basal thalamus ŽAP: y3.0, ML: 3.0, D: 4.4 mm from bregma. according to the coordinate system of Paxinos and Watson w32x. The guide cannula was fixed to the skull using stainless steel anchor screws and dental acrylic. Microdialysis experiments were performed 4 to 6 days after surgery. 2.4. Microdialysis procedure On the morning of an experiment, rats were cannulated and intubated, and maintained under halothane anesthesia as described in Section 2.3; once the animal was secured in the stereotaxic apparatus, a microdialysis probe ŽCMA 12, 2 = 0.05 mm, Bioanalytical Systems. was slowly lowered into the guide cannula. Artificial Ringers’ solution Ž147.0 mM NaCl, 4.0 mM KCl and 2.0 mM CaCl 2 , unadjusted pH 6.5. was pumped through the probe at 1.0 m lrmin with a microinfusion pump Žmodel CMAr100, Bioanalytical Systems.. Beginning 2 to 3 h after probe insertion, 10 m l dialysate samples were collected into polyethylene tubes at 10-min intervals. The samples were immediately frozen in dry ice, and stored at y808 C until analysis. The first 3 samples collected before any treatment were considered baseline samples, and all subsequent 5-HT or NE measures were expressed as a percent of this baseline. At the conclusion of each experiment, microdialysis sites were confirmed histologically. Data included in this report are from rats with probes that were confirmed to be within or adjacent to the ventral posterior thalamus. 2.5. NE and 5-HT assay Aliquots of the dialysate Ž5 m l. were injected directly into a microbore HPLC column Ž3 m m, C18, 100 = 1 mm,
9
Sepstik, Bioanalytical Systems.. A glassy carbon working electrode was set at a potential of q650 mV relative to a AgrAgCl reference. The mobile phase, which consisted of 9.0 g of NaH 2 PO4 , 436 mg sodium dodecyl sulfate, 10 mg Na 2 EDTA, 100 m l triethylamine, 12% MeOH, and 12% CH 3 CNrl of distilled water Žfinal pH 5.6. was pumped Žmodel 260D, ISCO, Lincoln, NE. at a rate of 60 m lrmin to maintain a column pressure at 2000 to 2500 psi. Chromatographic data were acquired on-line and exported to a MAXIMA 820 software system ŽWaters Associates, Milford, MA. for peak amplification, integration and analysis. Single component standards of NE and 5-HT were run daily before dialysate samples and standard curves were linear over a wide range of concentrations Ž2–2000 pg.. A monoamine standard mix containing NE, 5-HT, 5-hydroxyindole acetic acid, and homovanillic acid was injected before, during and after the experiment to ensure validity of the constituent retention times. Peak heights of unknowns were compared with peak heights of standards and the minimal detectable amount of 5-HT and NE Ž3 times baseline noise. was routinely less than 0.2 pg Ž1.0 fmol. per 5 m l of injection. 2.6. Drug administration Solutions of cocaine HCl Ž0.75 mgrkg. in NaCl vehicle or vehicle were injected via an indwelling jugular catheter. For the electrophysiology experiments, individual perievent histograms of cell activity were collected for each deflection amplitude used both before and after drug injection. For the microdialysis experiments, four baseline samples were obtained, then either drug or vehicle were delivered via the jugular catheter. Microdialysis samples were collected every 10 min until 30 min after injection. 2.7. Data analysis Drug effects on spontaneous versus stimulus-evoked discharge of single VPM cells were quantitatively evaluated by comparing spike activity Žhistograms depict spikesr40 stimuli; values reported throughout the text and in figure legends are the average spikesrstimulusrepoch. during identical stimulation epochs Ž40 stimulirepoch. and periods of spontaneous activity in control and post-drug injection histograms. Spontaneous activity was subtracted from evoked responses in order to obtain an absolute measure of stimulus evoked discharge. The spontaneous activity was determined for the first 100 ms of each histogram, whereas excitatory response epochs were selected by visual inspection during the control condition and fixed for all remaining post-drug injection histograms. The period of a stimulus-evoked response was selected to begin at the particular bin where counts deviated from baseline and terminate where counts reapproached this baseline.
10
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
A subclass of cells showed a decrease in the latency to response with an increase in whisker deflection amplitude. For these cells, the average latency to response for the second stimulus epoch following cocaine delivery was compared, via Signed Rank test, to the like stimulus epoch during the control period. Results from selected experiments were subjected to a more rigorous analysis ŽKruskal–Wallis ANOVA on ranks. in which comparisons were made between the pre- and post-cocaine response magnitude andror latency on a trial by trial basis. Finally, the dialysis data were evaluated by one-way analysis of variance across time. When significant F values were obtained for this data, Newman–Keuls post-hoc tests were performed to compare group means. p - 0.05 was chosen as the minimum criterion for statistical significance.
3. Results 3.1. Characterization of recorded units The extracellular activity of 23 vibrissa-sensitive thalamic neurons was recorded Žone cell per animal. before and after i.v. injection of cocaine. The VPM cells selected for study reacted to stimulation of caudal vibrissae with what previous investigators have described as rapidly adapting w43x or transient w20x responses to maintained whisker deflection. These cells were characterized in terms of their dependence upon the amplitude of deflection for two reasons. First, amplitude dependence of the vibrissae-responsive cells has been described in both of the subcortical nuclei w20,39,43x and in the cortex w19,41x. Second, graded deflections of the individual whiskers can be reliably produced with a computer-driven piezoelectric device mounted onto a microdrive w42x. Previous investigations from other laboratories have shown that cells in the VPM w20x and cortex w19x exhibit a broad range of responsiveness Ži.e., firing rate changes. to whisker deflection. Since the stimulator’s primary plane of movement was dorsoventral, preference was given to those cells which responded optimally to vibrissa motion in these directions. In accordance with results from a previous investigation, the responses of individual cells to stimulation varied greatly as a population; however, each cell responded to a characteristic range of deflection amplitudes w20,43x. Once this stimulus sequence was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison between similar amplitude stimulation epochs. Since the latency and duration of the response to the initial deflection Žthe ‘on’ response. depended somewhat on the amplitude of deflection, average values were determined from cell responses to the largest Ž‘near-maximal’. deflection for a particular experiment. Accordingly, the mean latency Ž"S.E.M.. for the ‘on’ response Ž n s 23.
was 5.7 ms " 0.4, while the mean duration Ž"S.E.M.. was 8.7 ms " 0.1. Some cells Ž n s 12, 52%. also responded with a short latency discharge to the return of the whisker to the initial position Žthe ‘off’ response; see Fig. 6., with a mean latency and duration of 7.2 ms " 0.3 and 15.3 ms " 0.5, respectively. Recorded neurons were well isolated and exhibited stable responses to single vibrissa stimulation for at least 20 min prior to vehicle or drug injection. The response properties of cells described in this report match those reported previously for ‘barreloid’ neurons in the VPM thalamus w10,20,43x. 3.2. Effects of cocaine on the magnitude of stimulus-eÕoked discharge of barreloid thalamic neurons Previously, our laboratory reported that administration of cocaine Žat doses between 0.25 and 2.0 mgrkg i.v.. did not significantly alter the early excitatory component of ‘barrel field’ somatosensory cortical neuron response to suprathreshold vibrissa stimulation w2x. In the present study, however, our goal was to determine if cocaine could selectively alter response components of the thalamic input that drive this early cortical response w6,10,11x, provided graded, submaximal whisker deflections were used as stimuli. Once the appropriate range of whisker deflections was determined under control conditions Žsee Fig. 1., the same range of stimuli Žpresented in randomized order. was used to evoke responses following administration of cocaine Ž0.75 mgrkg i.v... This dose of cocaine is within the effective range for behavioral reinforcement in rodents; i.e., under conditions in which drug-administration is selfpaced, laboratory animals will bar-press for single infusions of cocaine Ž0.75–1.0 mgrkg i.v.. at a rate of approximately 5–6 times an hour w4x. In addition, our previous results in cerebral cortex revealed a reliable alteration in electrophysiological response to suprathreshold whisker deflection within 2 min of cocaine administration in this dose range w2x. Data from an experiment in which the VPM cell exhibited an ‘on’ response to whisker movements over a range of values from 45 to 105 m is depicted in Fig. 2. In this case, cocaine augmented the excitatory response to initial whisker deflection at all but the lowest stimulus amplitude. In contrast, other cells exhibited a decrease in response amplitude to stimulation following cocaine treatment ŽFig. 3.. The magnitude of response, as depicted by stimulus response curves ŽFig. 4., was consistently altered by more than 10% following acute systemic cocaine in the majority of cases Ž28 of 35, 80%.. Because of the variable nature of cocaine on amplitude-sensitive thalamic units, and because a previous study of psychostimulant-induced stereotyped movements suggested that the magnitude of pretreatment response was an important determinant of the post-drug effect w33x, we analyzed the data in the following way: Ž1. For each case, the evoked response to a suprathreshold Žnear maximal. displacement of the central whisker was determined, and
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
11
Fig. 2. Cocaine Ž0.75 mgrkg i.v..-induced increase in the magnitude of a whisker-evoked excitatory discharge. Perievent histograms depict short latency excitatory responses of an individual VPM thalamic somatosensory neuron to graded mechanical displacement Žindicated by arrowhead. of a single whisker on the contralateral face. The amplitude of deflection Žin microns. is displayed in the box above histograms. Numbers adjacent to the excitatory peak indicate the average frequency ŽF. of discharge per response. Each histogram represents cell activity during 40 presentations of the same stimulus. Note the overall increase in the magnitude of responses from control ŽA. to post-cocaine ŽB. conditions.
all subsequent evoked responses were expressed as a percentage of this maximal response Žthereby normalizing the data.. Ž2. Although 4 to 6 different amplitudes were used
to generate each input–output curve, only the normalized responses from the second through the sixth minute Žstimulus epochs 2–4. after cocaine administration were
Fig. 3. Cocaine Ž0.75 mgrkg i.v..-induced decrease in magnitude of a whisker-evoked excitatory discharge. Perievent histograms depict short latency excitatory responses of an individual VPM thalamic somatosensory neuron to graded mechanical displacement of a single whisker on the contralateral face. Labeling conventions are the same as those in Fig. 2. Note the overall decrease in the magnitude of responses from control ŽA. to post-cocaine ŽB. conditions.
12
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
Fig. 4. The graphs in A and B illustrate stimulus-response curves for two VPM thalamic neurons before Žfilled circles, solid line. and after Žopen triangles, dashed line. cocaine Ž0.75 mgrkg i.v.. administration. Frequency of evoked discharge Ži.e., cell response. is plotted as a function of amplitude of primary whisker deflection. Each point represents the average response of the cell to 40 stimulus presentations.
used for statistical comparison, since previous experiments from our laboratory w2x have shown that cocaine effects are most robust during this time frame. Ž3. A ‘low’ control animal was one in which the majority of the responses prior to cocaine were below 50% of the maximal response used for normalization; conversely, a ‘high’ control animal had a majority of pre-drug responses above 50%. Ž4. Once
a subject was categorized in this fashion, the pretreatment values were averaged and compared to the corresponding stimulus epochs following cocaine. Ž5. The data were then subjected to a two-way repeated measures ANOVA with pretreatment magnitude Ž‘low’ vs. ‘high’. and treatment Žpre- vs. post-drug. both as between-response variables. Further analysis used Newman–Keuls post-hoc tests to determine the source of the detected significances in the ANOVA Ž p - 0.05.. This analysis of the data supported the hypothesis that the low and high pretreatment groups responded differently to acute cocaine administration. Specifically, the low pretreatment group showed a significant increase in evoked response after cocaine, while the high pretreatment group showed a significant decrease Ž p - 0.05. Žsee Fig. 5.. 3.3. Effect of cocaine on the stimulus response latency of thalamic neurons
Fig. 5. The plots shown here illustrate the results of two way repeated measures ANOVA to determine significance between pretreatment stimulus-response ratios and cocaine-induced shifts in response magnitude. Cell responses to cocaine were placed into either a low or high pre-drug group depending on whether the control responses to stimulation averaged less or more than 50%, respectively. Pairwise post-hoc comparisons ŽNewman–Keuls. revealed a significant difference between the groups during the control period, as well as a significant response to cocaine administration for both groups Ž p- 0.05..
A portion Ž48%. of the neurons studied exhibited variable response latencies Žspecifically, deviations in time to response of at least 30%. that were inversely proportional to the amplitude of whisker deflection. In the example shown in Fig. 6, this latency shift under control conditions is evident for both the ‘on’ and the ‘off’ components of the cell’s response to whisker movement. Cocaine significantly decreased the response latency for each of these components, as well as reduced the trial to trial variability in response latency ŽFigs. 6 and 7.. The drug-induced shift in average response-latency was consistent for all of the cells studied. Fig. 8 illustrates a comparison of response latencies for equivalent stimuli before and after cocaine, for individual cells. This analysis revealed a significant
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
13
Fig. 6. Vertically oriented cumulative raster illustrates cocaine’s effect on the spike train activity of a single VPM thalamic neuron during ON Žsolid arrowhead accompanied by a stimulus artifact for all but the lowest amplitude stimulations. and OFF Žopen arrowhead. movement of the cell’s primary whisker. The ordinate depicts the first 0.3 s of each trial; each vertical line of dots represents the cell’s activity during one stimulus trial. The abscissa indicates the running time of the experiment and the amplitude of whisker deflection for a given time period. The solid arrowhead on the horizontal axis indicates the time at which cocaine Ž0.75 mgrkg i.v.. was injected. Note that the magnitude of the OFF response for all stimulus intensities was considerably larger following cocaine administration. Likewise, following cocaine a prominent OFF response to 45 m whisker deflection was revealed Žcompare period marked with solid bars Pre- vs. Post-cocaine..
decrease in the average response latency Ž n s 12, p 0.0001.. 3.4. Cocaine facilitation of subthreshold synaptic inputs Although stimulus amplitudes which produced a detectable response during pre-drug characterization were
generally selected for study, there were occasional instances when a stimulus level that was subthreshold for eliciting a response was tested. In two such cells, cocaine revealed excitatory responses to vibrissae stimulation which were not apparent during control conditions. For the case illustrated in Fig. 6 Ž‘off’ response., a whisker deflection
Fig. 7. Graphs illustrate cocaine-induced shift in average response latency for the VPM thalamic neuron depicted in Fig. 6. Latency to response is plotted as a function of amplitude of primary whisker deflection Ž A s ms from stimulus onset; B s ms from stimulus offset.. Each point represents the mean response" S.E.M. of the cell to 40 like-stimulus presentations. Filled circless before cocaine Žcontrol.; open triangless after cocaine injection. Trial by trial examination of the response to cocaine revealed a statistically significant effect of the drug on latency for amplitudes 62–150 m for the ‘on’ response, and 45-105 m for the ‘off’ response ŽNewman–Keuls p - 0.05, asterisks omitted for sake of clarity..
14
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
of 45 m evoked no discernable response during the control period. However, following cocaine administration an obvious excitatory response to the same stimulus emerged. This ‘gating effect’ on responses to otherwise subthreshold sensory stimuli has been described previously for NE in the visual cortex w45,46x and for cocaine in the somatosensory cortex w22x. 3.5. Effect of cocaine on extracellular NE and 5-HT leÕels in the VPM thalamus region In a separate group of animals Ž n s 7., in vivo microdialysis experiments were conducted as a means of correlating the time course of cocaine-induced changes in stimulus-evoked responses of thalamic neurons with altered monoamine levels in the VPM thalamus. To this end, probes targeted for the barreloid region of the VPM were implanted in halothane-anaesthetized rats, and following a suitable stabilization period Žapproximately 2–3 h., basal levels of NE and 5-HT were determined Žsee Section 2.. The baseline level Žmean pg 5 m lr10 min " S.E.M.. of NE was 2.02 " 0.12, while the baseline level of 5-HT was 2.31 " 0.85 Ž n s 7.. Low amounts of dopamine were sometimes detectable in this region, with a mean baseline level of 0.52 " 0.15. All values were expressed as the percent of baseline Žeach rat served as its own control. to account for interindividual variability in resting monoamine levels.
Fig. 9. Effect of cocaine on extracellular NE and 5-HT in the ventrobasal thalamus. The graph illustrates extracellular levels of NE and 5-HT in the ventrobasal thalamus for baseline and post-cocaine conditions. Values indicated are based upon means"S.E.M Ž ns 7–8.. Dialysis samples were taken every 10 min for 30-min periods before and after cocaine Ž0.75 mgrkg i.v.. administration. Cocaine injected at times 0. After cocaine, there were significant differences from baseline for both monoamines for at least 20 min Žasterisks denote significance, p- 0.05..
Acute i.v. cocaine Ž0.75 mgrkg. significantly increased both extracellular NE and 5-HT for 20–30 min following injection ŽFig. 9.. It is noteworthy that the maximum response for both NE and 5-HT Žboth to approximately 200% of pretreatment baseline. occurred in the first 10 min after cocaine administration, which corresponds well with the time course of the observed electrophysiologic changes produced by the drug. The cocaine-induced elevation in 5-HT was more transient than that observed for NE, but the extracellular levels for both monoamines was significantly different from baseline for at least 20 min after treatment. In contrast, drug-induced changes in extracellular DA were inconsistent and failed to reach statistical significance Ž p ) 0.05, data not shown.. Likewise, salinetreated controls showed no significant deviation from baseline for either 5-HT or NE Ž p ) 0.05, data not shown..
4. Discussion Fig. 8. Cocaine-induced decrease in response latency. All cells which exhibited an inverse correlation between amplitude of whisker movement and latency to response Ž ns12; see Fig. 6 for example. were analyzed to determine the effect of cocaine on latency. In all cases the average response latency to stimuli delivered 2–3 min after cocaine Žpost-cocaine. injection was computed and compared to the average response latency before cocaine Žpre-cocaine.. Each line represents the shift in average response latency for one cell from the pre- to post-cocaine conditions. The cocaine-induced decrease in response latency was highly significant Žpaired t-test, p- 0.0001..
The present report is based on data obtained from a population of rat somatosensory thalamic ŽVPM. neurons which responded with excitatory discharges of variable magnitude and latency to graded, submaximal peripheral tactile stimulation. While all VPM cells examined showed some degree of amplitude dependence, the cells chosen for analysis in the present study resemble those described as highly amplitude-dependent in previous work w20x. Since
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
VPM neurons are morphologically homogenous w18x, such variations in the control responses of cells studied here are presumably related to differential connectivity within the trigeminal somatosensory circuitry Žsee also discussion in Ref. w20x.. Systemic cocaine altered the stimulus-evoked discharges of these cells, effecting either a positive Žincrease. or negative Ždecrease. shift in the magnitude of these responses, as well as a decrease in the response latency of those cells which exhibited an inverse relationship between latency and the strength of the sensory stimulus employed. That such changes were related to cocaine-induced enhancement of extracellular 5-HT and NE is supported by microdialysis measurements in the region of the VPM, which revealed an increase in both monoamines with a time course that correlated well with the observed electrophysiological changes. Overall, these results complement previous studies in our laboratory which showed that systemic cocaine can differentially influence specific components of sensory cortical neuron responses to supra-threshold activation of the rat somatosensory system w2x. There are several compelling reasons to expect that cocaine can alter the transmission of sensory signals as a consequence of increased synaptic levels of 5-HT and NE. First, studies in both humans and animals indicate that cocaine can alter sensory perception. For example, individuals who self-administer cocaine report alterations in effect such as heightened arousal and a sense of well-being, along with enhanced sensory perception w12,16,17x. In rodents, cocaine also produces arousal and increased sensitivity to environmental changes including auditory, visual and tactile stimuli w38,47,48x, electrical shock w25x, and temperature changes w34x. Second, as a potent inhibitor of the uptake of 5-HT and NE w13,15,36x, cocaine influences monoaminergic transmission in two systems which have broad efferent projections to forebrain sensory areas including subcortical sensory relay nuclei and primary sensory cortical sites w21,27,29,31x. Finally, recent investigations from our laboratory and others indicate that iontophoretically applied NE and 5-HT can alter specific parameters of sensory cortical neuron responsiveness to afferent pathway stimulation w30,35,44,46x. The VPM cells selected for the present study reacted to stimulation of caudal vibrissae with what previous investigators had described as rapidly adapting w43x or transient w20x responses to maintained whisker deflection. These cells were characterized in terms of their dependence upon the amplitude of deflection for two reasons. First, amplitude dependence of the vibrissae-responsive cells has been described in both of the subcortical relays w20,39,43x and in the cortex w19,41x. Second, graded deflections of the whisker can be reliably produced with a computer-driven piezoelectric device mounted onto a microdrive w42x. In accordance with results from a previous investigation, the responses of individual cells to stimulation varied greatly as a population, however each cell had a characteristic set
15
of deflection amplitudes that evoked a response w20,43x. Once this ‘sensory signature’ was determined under control conditions, it could be accurately repeated following drug administration, thus allowing for statistical comparison between similar amplitude stimulation epochs. The results indicate that a behaviorally relevant dose of cocaine Žsee Section 3. had an effect on transmission of the amplitude dependent signal, since the majority of the individual cell curves were shifted following administration of the drug. Further analysis of the data showed that cocaine’s effect on individual cells was predictable on the basis of the proximity of the control level response to the maximum evokable discharge of the cell. This analysis was prompted by the work of Pederson et al. w33x, who recently described a ‘rate dependent’ effect of psychostimulants on the discharge properties of striatal neurons. Our results show that cells whose averaged control response was approximately 25% of maximal Žsee Fig. 5 and Section 3. exhibited a increased response to stimuli following cocaine Ž‘low pre-drug group’., while the drug decreased the magnitude of discharge for those cells whose averaged control response was near maximal at the outset Ž‘high pre-drug group’.. While this analysis supports the notion that cocaine alters the amplitude sensitivity of vibrissae-driven cells in the VPM in a predictable fashion, the underlying mechanism for this effect is unclear at the present time. Previous observations that the VPM is comprised of a homogenous array of relay neurons whose activity is regulated by inhibitory influences from the neighboring reticular thalamic nucleus ŽNRT. w18,40x make it seem unlikely that the functional heterogeneity described here could be ascribed to unique morphological or electrophysiological characteristics of the recorded cell. In addition, the finding that individual cells in our study can exhibit both ‘on’ and ‘off’ responses which change in opposite directions following cocaine administration argues against a nonspecific mechanism such as a drug-induced shift in the plane of anesthesia. It is feasible that cocaine could elicit either a facilitation or suppression of the vibrissae response depending upon a variable elevation in the monoamines. Previous investigations have shown, for example, that increases in spontaneous activity in the somatosensory system w1x, or potentiation of discharges in the visual system w46x, occurs at lower iontophoretic applications of NE than those which produce a generalized suppression of spontaneous or stimulus-evoked activity. Again, such a mode of action would not explain those cases in which the responses to onset and offset of the applied stimulus within the same animal were altered in an opposing fashion following cocaine. Differences in the magnitude of response evoked by the ‘on’ and the ‘off’ phases of whisker deflection presumably reflect differential activation of this glutamatergically-mediated pathway w23,37x. It seems possible that the bimodal nature of the cocaine-induced changes was based upon the interaction between synaptically released monoamines and the
16
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
variable excitatory tone of the pathway. Further clarification of this issue may come from in vitro studies currently underway in our laboratory. The preliminary results of these experiments have shown that the modulatory effect of the same amount of iontophoretically-applied NE depends upon the initial level of the glutamate-evoked discharge w9x. Another predictable effect of cocaine was a decrease in the mean latency to response, in a group of cells which exhibited an inverse relationship between latency to response and deflection amplitude ŽFig. 6.. The latency of evoked spikes to vibrissae stimulation is a commonly measured receptive field property of this sensory pathway w20,39,43x, and has been positively correlated with response threshold w20,43x. Furthermore, the response latency appears to be mutable in some cases by altering the local monoaminergic tone. For example, similar decreases in the latency of an evoked response have been demonstrated in both the somatosensory w14x and visual systems w46x following iontophoretic application of NE. The neurochemical data presented here support the aforementioned anatomical and physiological observations that cocaine-induced changes in the processing of sensory signaling are mediated by NE and 5-HT. Further, there is a good correlation between the time course of the cocaineinduced NE and 5-HT elevations reported here Žsignificant elevation of both monoamines for 20 min post-cocaine. and the previously reported time course of cocaine actions on sensory cortical neurons w2x. In addition, a similar monoaminergic specificity for cocaine’s impact on sensory signal transmission was observed in a recent study in which selective uptake inhibitors for NE and 5-HT, but not DA, mimicked the changes observed after cocaine administration w3x. It is important to note that in several cases Ž3 of 8. reliable baseline levels of DA were present, and cocaine had the predictable effect of elevating the extracellular DA. The lack of a consistent effect of cocaine on DA in this study, therefore, presumably reflects the lack of a significant DA innervation in the ventral posterior thalamus w5x. Given the limited spatial resolution of the dialysis probe, it seems likely that in those cases where baseline DA was higher, the probe was within diffusion distance of a region with a greater DA innervation, such as the ventral medial thalamus w5,32x. While the observed increase in extracellular 5-HT and NE in the thalamus does not preclude the possibility that cocaine could affect sensory transmission at other sites, it does suggest the possibility that some of the drug-induced changes reported here could be mediated locally. Sitespecific, as opposed to circuit-level effects, are further supported by a previous investigation in our laboratory, in which it was observed that local iontophoretic application of cocaine could alter responses of cortical neurons to iontophoretically applied glutamate w22x. Future experiments, using iontophoresis in conjunction with monoamine agonists and antagonists, should help clarify the extent to
which the observed effects can be attributed to local NE andror 5-HT alterations in the VPM.
Acknowledgements The authors would like to thank Dr. E.M. Gould and M. Ayestas for technical support. This work was supported by grant RO1 DA05117 to BDW, and by the Intramural Research Program ŽMHB., from National Institute of Drug Abuse.
References w1x M. Armstrong-James, K. Fox, Effects of iontophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex, J. Physiol. 335 Ž1983. 427–447. w2x I. Bekavac, B.D. Waterhouse, Systemically administered cocaine selectively enhances long-latency responses of rat barrel field cortical neurons to vibrissae stimulation, J. Pharmacol. Exp. Ther. 272 Ž1995. 333–342. w3x I. Bekavac, E.M. Gould, B.D. Waterhouse, Monoamine substrates underlying cocaine-induced enhancement of somatosensory evoked discharges in rat barrel field cortical neurons, J. Pharmacol. Exp. Ther. 279 Ž1996. 582–592. w4x M.A. Bozarth, R.A. Wise, Toxicity associated with long term intravenous heroin and cocaine self-administration in the rat, JAMA 254 Ž1985. 81–83. w5x A. Bjorklund, O. Lindvall, Dopamine-containing systems in the CNS, in: A. Bjorklund, T. Hokfelt ŽEds.., Classical Transmitters in the CNS, Part 1: Handbook of Chemical Neuroanatomy, Vol. 2, Elsevier, Amsterdam, 1984, pp. 55–122. w6x J. Chapin, C.-S. Lin, The somatic sensory cortex of rat, in: B. Kolb, R.C. Tees ŽEds.., The Cerebral Cortex of the Rat, MIT Press, Cambridge, 1990, pp. 341–380. w7x S. Cohen, Cocaine: acute medical and psychiatric complications, Psychiatr. Ann. 14 Ž1984. 747–749. w8x K.A. Cunningham, J.M. Paris, N.E. Goeders, Serotonin neurotransmission in cocaine sensitization, in: P.W. Kalivas, H.H. Sampson ŽEds.., The Neurobiology of Drug and Alcohol Addiction, The New York Academy of Sciences, New York, 1992, pp. 117–127. w9x D.M. Devilbiss, B.D. Waterhouse, Effect of increasing tonic levels of norepinephrine on glutamate evoked discharge of single neurons in layers IIrIII and V of rat barrel field cortex, Soc. Neurosci. Abstr. 240 Ž1996. 10. w10x M.E. Diamond, M. Armstrong-James, F.F. Ebner, Somatic sensory responses in the rostral sector of the posterior group ŽPOm . and in the ventral posterior medial nucleus of the rat thalamus, J. Comp. Neurol. 318 Ž1992. 462–476. w11x M.E. Diamond, M. Armstrong-James, M.J. Budway, F.F. Ebner, Somatic sensory responses in the rostral sector of the posterior group ŽPOm . and in the ventral posterior medial nucleus ŽVPM. of the rat thalamus: dependence on the barrel field cortex, J. Comp. Neurol. 319 Ž1992. 66–84. w12x M.W. Fischman, C.R. Schuster, N.A. Krasnegor, Physiological and behavioral effects of intravenous cocaine in man, in: E.H. Ellinwood, M.M. Kilbey ŽEds.., Cocaine and Other Stimulants, Plenum, New York, 1977, pp. 647–664. w13x E. Friedman, S. Gershon, J. Rotrosen, Effects of acute cocaine treatment on the turnover of 5-hydroxytryptamine in the rat brain, Br. J. Pharmacol. 54 Ž1975. 61–64. w14x M.J. George, Modification of receptive fields of posteriomedial
J.J. Rutter et al.r Brain Research 798 (1998) 7–17
w15x
w16x
w17x
w18x
w19x w20x w21x w22x
w23x
w24x
w25x w26x w27x
w28x
w29x
w30x
w31x
barrel subfield neocortical single units by known concentrations of iontophoresed noradrenaline in the rat, Int. J. Neurosci. 65 Ž1992. 69–81. J. Glowinski, J. Axelrod, Effects of drugs on the uptake, release and metabolism of 3 H-norepinephrine, J. Pharmacol. Exp. Ther. 149 Ž1965. 43–49. M.S. Gold, Cocaine Žand crack.: clinical aspects, in: J.H. Lowinsin, P. Ruiz, R.B. Millman, J.G. Langrod ŽEds.., Substance Abuse: A Comprehensive Textbook, Williams and Wilkins, Baltimore, 1992, pp. 205–221. M.S. Gold, N.S. Miller, J.M. Jonas, Cocaine Žand crack.: neurobiology, in: J.H. Lowinsin, P. Ruiz, R.B. Millman, J.G. Langrod ŽEds.., Substance Abuse: A Comprehensive Textbook, Williams and Wilkins, Baltimore, 1992, pp. 222–235. R.M. Harris, Morphology of physiologically identified thalamocortical relay neurons in the rat ventrobasal thalamus, J. Comp. Neurol. 251 Ž1986. 491–505. M. Ito, Some quantitative aspects of vibrissa-driven neuronal responses in rat neocortex, J. Neurophysiol. 46 Ž1981. 705–715. M. Ito, Response properties and topography of vibrissa-sensitive VPM neurons in the rat, J. Neurophysiol. 60 Ž1988. 1181–1197. B.L. Jacobs, E.C. Azmitia, Structure and function of the brain serotonin system, Physiol. Rev. 72 Ž1992. 165–229. C.A. Jimenez-Rivera, B.D. Waterhouse, Effects of systemically and locally applied cocaine on cerebrocortical neuron responsiveness to afferent synaptic inputs and glutamate, Brain Res. 546 Ž1991. 287– 296. T. Klockgether, Excitatory amino acid receptor-mediated transmission of somatosensory evoked potentials in the rat thalamus, J. Physiol. 394 Ž1987. 445–461. M.J. Kuhar, M.C. Ritz, D. Grigoriadis, R. Lew, A cocaine receptor associated with dopamine transport and drug self-administration, in: J.M. Lakowski, M.P. Galloway, F.J. White ŽEds.., Cocaine: Pharmacology, Physiology and Clinical Strategies, CRC Press, Boca Raton, 1992, pp. 191–202. H. Lal, R.D. Chessick, Lethal effects of aggregation and electric shock in mice treated with cocaine, Nature 208 Ž1965. 295–296. P. Levitt, R.Y. Moore, Noradrenaline neuron innervation of the neocortex in the rat, Brain Res. 139 Ž1978. 219–231. P. Levitt, R.Y. Moore, Origin and organization of brainstem catecholamine innervation in the rat, J. Comp. Neurol. 186 Ž1979. 505–528. H.G.W. Lidov, R. Grzanna, M.E. Molliver, The serotonin innervation of the cerebral cortex in the rat—an immunohistochemical analysis, Neuroscience 5 Ž1980. 207–227. O. Lindvall, A. Bjorklund, A. Nobin, U. Stenevi, The adrenergic innervation of the rat thalamus as revealed by the glyoxylic acid fluorescence method, J. Comp. Neurol. 154 Ž1974. 317–348. J. McLean, B.D. Waterhouse, Noradrenergic modulation of cat area 17 neuronal responses to moving visual stimuli, Brain Res. 667 Ž1994. 83–97. J.H. Morrison, S.L. Foote, Noradrenergic and serotonergic innerva-
w32x w33x
w34x w35x
w36x
w37x w38x
w39x
w40x
w41x w42x w43x
w44x
w45x
w46x
w47x
w48x
17
tion of cortical, thalamic and tectal visual structures in old and new world monkeys, J. Comp. Neurol. 243 Ž1986. 117–138. G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press, Sydney, 1986, 146 pp. C.L. Pederson, M. Wolske, L.L. Peoples, M.O. West, Firing rate dependent effect of cocaine on single neurons of the rat lateral striatum, Brain Res. 760 Ž1997. 261–265. D. Peterson, J. Hardinge, The effect of various environmental factors on cocaine and ephedrine toxicity, J. Pharmacol. 19 Ž1967. 810–814. M.A. Rogawski, G.K. Aghajanian, Norepinephrine and serotonin: opposite effects on the activity of lateral geniculate neurons evoked by optic pathway stimulation, Exp. Neurol. 69 Ž1980. 678–684. S.B. Ross, A.L. Renyi, Inhibition of the uptake of tritiated 5-hydroxytryptamine in brain tissue, Eur. J. Pharmacol. 7 Ž1969. 270– 277. T.E. Salt, Mediation of thalamic sensory input by both NMDA receptors and non-NMDA receptors, Nature 322 Ž1986. 263–265. J. Scheel-Kruger, C. Braestrup, M. Nielsen, R. Golembiowski, E. Mogilnicka, Cocaine: discussion of the role of dopamine in the biochemical mechanism of action, in: E. Ellinwood, M.M. Kilbey ŽEds.., Cocaine and Other Stimulants, Plenum, New York, 1977, pp. 373–407. M.T. Shipley, Response characteristics of single units in the rat’s trigeminal nucleus to vibrissae displacements, J. Neurophysiol. 37 Ž1974. 73–90. A. Shosaku, A comparison of receptive field properties of vibrissa neurons between the rat thalamic reticular and ventrobasal nuclei, Brain Res. 347 Ž1985. 36–40. D.J. Simons, Response properties of vibrissa units in rat SI somatosensory neocortex, J. Neurophysiol. 41 Ž1978. 798–820. D.J. Simons, Multi-whisker stimulation and its effects on vibrissa units in rat SmI barrel cortex, Brain Res. 276 Ž1983. 178–182. P.M.E. Waite, The responses of cells in the rat thalamus to mechanical movements of the whiskers, J. Physiol. ŽLondon. 228 Ž1973. 541–561. B.D. Waterhouse, H.C. Moises, D.J. Woodward, Interaction of serotonin with somatosensory cortical neuronal responses to afferent synaptic inputs and putative transmitter substances, Brain Res. Bull. 17 Ž1986. 507–518. B.D. Waterhouse, F.M. Sessler, J.-T. Cheng, D.J. Woodward, S.A. Azizi, H.C. Moises, New evidence for a gating action of norepinephrine in central neuronal circuits of mammalian brain, Brain Res. Bull. 21 Ž1988. 425–432. B.D. Waterhouse, S.A. Azizi, R.A. Burne, D.J. Woodward, Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis, Brain Res. 514 Ž1990. 276–292. M.C. Wilson, J.A. Bedford, J. Buelke, A.H. Kibbe, Acute pharmacological activity of intravenous cocaine in the rhesus monkey, Psychopharmacol. Commun. 2 Ž1976. 251–262. R.A. Wise, M.A. Bozarth, A psychomotor stimulant theory of addiction, Psychol. Rev. 94 Ž1987. 469–492.