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Regulation of slow potential shifts in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex
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Electroencephalography and Clinical Neurophysiology, 40 (1976) 288--296 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
R E G U L A T I O N OF SLOW POTENTIAL SHIFTS IN NUCLEUS RETICULARIS THALAMI BY THE MESENCEPHALIC R E T I C U L A R FORMATION AND THE F R O N T A L G R A N U L A R CORTEX * JAMES E. SKINNER and CHARLES D. YINGLING
Physiology Department, Baylor College of Medicine; Biology Department, Rice University; and Neurophysiology Service, The Methodist Hospital, Houston, Texas 77025 (U.S.A.) (Accepted for publication: September 23, 1975)
We have previously suggested that nucleus reticularis thalami (R) has an inhibitory role in the generation of recruiting responses and EEG synchronization and we have demonstrated that unit activity in R is regulated jointly by the mesencephalic reticular formation and the mediothalamic-frontocortical system (Yingling and Skinner 1975). The present paper provides direct evidence that R has an inhibitory action on the thalamus, and examines the role of R in the mechanism mediating cortical slow potential (SP) shifts, which, like EEG synchronization and desynchronization, have been associated with changes in arousal and attention (Walter et al. 1964; Tecce 1972;McCallum and Knott 1973; Skinner and Lindsley 1973). Arduini et al. (1957) first showed that a negative SP shift could be produced on the surface of the frontal cortex in a cat by stimulation of sensory nerves or by stimulation of the mesencephalic reticular formation, These SPs persisted for many seconds following the stimulus, as did the desynchronization of the EEG. The effect of drowsiness or sleep, a condition which is related to a decrease in the mesencephalic reticular activation level
* This work was supported by Grants HL 05435 and HL 13837 from the National Heart and Lung Institute, NIH, U.S. Public Health Service. C.D. Yingling was the recipient of a predoctoral fellowship from
the National Science Foundation.
(see Jouvet 1967) has been shown to result in a tonic frontocortical SP that is of opposite polarity (i.e., surface-positive) to that elicited by mesencephalic reticular stimulation (Caspers 1963, 1965). Thus, it appears that conditions associated with an increased tendency for EEG synchronization result in SPs on the frontal lobe that are of positive polarity, whereas a condition that reduces EEG synchronization results in a negative frontocortical SP. Surface slow potentials can also be produced in the frontal cortex by a warning signal that is associated with a meaningful expected event. Walter et al. (1964) and Walter (1969) first demonstrated that a tonic SP of negative polarity, which they called the "contingent negative variation", was produced in the frontal region in man during conditioned expectancy. The SPs produced by this means have been implicated in a higher cerebral process that appears to be distinct from that of general arousal usually associated with a mesencephalic reticular mechanism. A previous study has implicated the mediothalamic-frontocortical system in the generation of the cortical SPs elicited by a warning signal, for blockade in the pathway interconnecting the medial thalamus and frontal cortex abolishes these conditioned SPs (Skinner 1971). Whether or n o t s u c h b l o c k a d e a f f e c t s SPs o f m e s e n c e p h a l i c
reticular origin is one of the problems that is investigated in the present study.
NUCLEUS RETICULARIS THALAMI In a recent study (Yingling and Skinner !975) it was found that analysis of unit activity in R clearly differentiated the roles of the mesencephalic reticular and mediothalamic-frontocortical aystems in the regulation of cortical synchronous activity. Since these two systems also have been shown to affect cortical SPs, we decided to look for SP shifts in the thalamus, especially in R. It was thought that analysis of such thalamic events might elucidate the roles of the two systems in the regulation of cortical SPs as it had in the regulation of cortical synchronization,
289 tained for chronic studies. All of the leads were placed in connector strips and secured to the skull with stainless-steel screws and dental cement in these chronic preparations. In cases where the signal-to-noise ratio was small, 15 responses were averaged on a Technical Measurements Corporation CAT 400. Toneshock conditioning trials were controlled by a BRS solid-state programming apparatus which delivered 0.5 sec tones followed in 4 sec by a mild shock to the hind limbs (100 c/sec, 0.5 msec, 30--50 msec train). The reinforcement shocks were delivered through stainless-steel wires sutured into the skin.
Methods Results A total of 8 chronic and 22 acute adult cat preparations were used in this study. The methods were identical to those described previously (Yingling and Skinner 1975}, with the exceptions noted below. DC-coupled, chopper-stabilized SP recordings were amplified by a Beckman Type R polygraph and recorded on a Precision Instruments FM tape recorder. The active electrodes were referenced to an electrode embedded in Gelfoam, moistened with saline, and placed in the most anterior part of the frontal sinus septal bone. Recordings from the back of the neck to the reference electrode showed negligible potentials of brain origin in response to various stimuli or from the eyes as a consequence of their spontaneous movement, Care was taken to avoid pain and abnormal • ventilation, and to maintain normal blood gas levels and normal body temperature in the immobilized preparation, for the slow potential shifts are qui~e sensitive to these factors, At the conclusion of the acute phase of the experiments, it was generally possible to allow the gallamine to be metabolized and within 1--2 h after the last injection, the animal would be sitting up and breathing regularly and responding appropriately to stimuli. The rapid recovery served as evidence that the state of the previously immobilized animal was good and permitted selected animals to be main-
The nucleus reticularis thalami, R, is a thin, shell-like structure that surrounds most of the thalamus. Capital letter subscripts of R are used to indicate the part of the thalamus lying adjacent to the particular segment of R studied. In the figures below that show evoked responses, the site of the recording will be abbreviated by capital letters, and the site of the evoking electric stimulus will be abbreviated by lower case subscripts. Fig. 1 shows that brief (50 msec) high-frequency (250 c/sec) stimulation of the mesencephalic reticular formation (MRF) evoked a large positive slow potential (SP) shift in RVA that had a similar time course to the surfacenegative shift in the frontal cortex (AS). Recordings from RLG showed a similar shift which was also initially positive after the MRF stimulus, though it was sometimes followed by a later negative component. The positive SPs in RVA were very large for extracellular potentials and ranged in amplitude from 1 to 20 mV. SPs almost identical to those produced by brief MRF stimulation were found to accompany orienting responses to novel stimuli, such as a sudden loud noise (Fig. 1, NOVEL STIM). The giant SPs in the thalamus were highly localized to R. No consistent SP responses were seen in recordings from medial thalamic nuclei (nucleus cen-
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orienting responses or MRF activation were localized to the frontal cortex in aIl of the cats; no responses were seen in recordings from posterior sigmoid, coronal, suprasylvian, visual, or auditory cortices. Further localization of the large subcortical SPs to R is shown in the results from an acute preparation (Fig. 2), in which movement of the recording electrode by as little as 0.5 mm outside the nucleus resulted in a loss of the SP elicited by a brief stimulus to the MRF.
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Fig. 1. Slow potential responses in frontal cortex (anterior sigmoid, AS) and nucleus reticularis thalami (R) accompanying mesencephalic reticular formation stimulation (STIM MRF) and orienting responses (NOVEL STIM). Transcortical recordings are shown
from AS and monopolar recordings referenced to frontal sinus septal bone are shown from areas of R adjacent to nucleus ventralis anterior (RvA) and the lateral geniculate body (RLG). On the left are shown the responses to 250 msec, 250 c/sec stimulation of the MRF (arrow). On the right, similar responses are evoked by the novel stimulus of striking the door of the recording chamber. Calibrations: 1000 #V; 10 sec.
tralis medialis, centre median, or ventralis anterior), lateral thalamic nuclei (ventralis lateralis, ventralis posterolateralis, lateral geniculate), or association nuclei (lateralis posterior), The negative cortical SPs accompanying either
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tical SPs were seen to accompany orienting responses or MRF stimulation (Fig. 1) suggests that the SPs evoked by the novel stimuli may be mediated by the MRF. In support of this suggestion, Fig. 3 shows that large negative SPs in t h e M R F a c c o m p a n y t h e t o n i c p o s i t i v e SPs in RVA whether they are evoked by a novel stimulus (first mark) or occur during a spontaneous scanning behavior a few seconds later (second mark). The EEG remained de-
synchronized throughout the period of experimentation, a result which suggests that SP shifts may be more sensitive indicators of changes in attentive states than synchronization and desynchronization of the EEG. SPs in the frontal cortex and RVA identical in polarity to those produced by either MRF
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t j; HL I m rf Ft. 12. O Fig. 2. Localization of the slow potential response in nucleus reticularis thalami (RvA) following stimulation of the mesencephalic reticular formation (MRF). Points 1 and 3, in nucleus ventralis anterior (VA) and the internal capsule (CI), respectively, show little or no response. Point 2, in RVA, shows a transient, slow positive shift of 12 mV that does not. return to baseline until after 20 sec. The three points are each separated by a distance of 1 ram. The diagram is taken from the atlas of Jasper and Ajmone Marsan (1954). Calibrations: 5000 pV; 10 sec.
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Fig. 3. Slow potentials in meseneephalic reticular formation (MRF), and nucleus reticularis thalami (RvA) evoked by novel stimuli. This segment from a continuous recording of a quiescent animal shows a negative potential in the MRF accompanied by a positive potential in RVA following a sudden loud noise (first mark). Several seconds later, similar potentials are seen arising spontaneously; these were accompanied by behavioral signs of attentiveness, i.e., the animal raised its head (second mark) and once again scanned the room for several seconds before resuming a quiet pose at the time the slow potential returned to baseline. Calibrations: 2000 pV; 20 sec.
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stimulation or novel stimuli were elicited in a tone--shock conditioning paradigm. However, the duration of the conditioned SPs depended u p o n the interstimulus interval in contrast to those prolonged SPs elicited by MRF or novel stimuli. Fig. 4, A shows such conditioned SPs which were elicited by a neutral t one that was reinforced by a mild electric shock delivered 4 sec later. After complete acquisition (500 trials), the 4 sec interstimulus interval was shifted temporarily to 6 sec, but the SPs continued to peak at the previously expected time of r e i n f o r c e m e n t (Fig. 4, B). Additional trials resulted in a change of the peak SP to 6 sec. After 50 presentations of an unreinforced tone of a slightly higher frequency (randomly interspersed among the reinforced
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Fig. 4. Slow potential responses recorded from the frontal cortex (anterior sigmoid, AS) and nucleus reticularis thalami (RvA) during a tone ( T ) l h o c k (S) conditioning paradigm. A : Responses after 500 trials. B: Responses immediately after changing interstimulus interval from 4 to 6 sec (INT CH). C: Reduction in slow potential amplitude to a nonreinforced tone of higher frequency (thicker trace), randomly interspersed between the reinforced tones (thinner trace) in a discrimination paradigm (DISCR). D: Responses after changing intertrial intervals from 3.0 + 1.5 min to 30 -+ 15 see (MASS TR). E: Responses after bilateral cryogenic blockade (5°C) of the inferior thalamic peduncle (COOL ITP). Each trace is the average of 15 responses. Calibrations: 2ooo pv; 1 sec.
stimuli) the SPs evoked by the unreinforced tones were only 50% of the amplitudes of those SPs evoked by the reinforced tones (Fig. 4, C). The SPs were reduced during massed trials when the mean intertrial interval was shifted from 3 min (randomly presented, 2--4 rain) to 30 sec (randomly presented, 15--45 sec), as illustrated in Fig. 4, D. Lengthening the intertrial interval restored the SPs to the levels shown in Fig. 4, A. Bilateral cryogenic blockade in the inferior thalamic peduncle was found to abolish the conditioned SPs in both the frontal cort ex and RVA (Fig. 4, E). The pre- and post-cooling controls were recorded before and 3 min after the cessation of cooling and were identical to those in Fig. 4, A. During cooling, a constant negative DC offset occurred in the frontal cortex and a constant positive one in RVA. Fig. 5, A shows that DC-coupled recordings of monophasic positive recruiting responses in RVA were superimposed upon a negative shift of the baseline; in the cortex, a positive baseline shift accompanied the usual monophasic negative recruiting responses. Unlike the tonic SP shifts that follow desynchronizing stimuli to the MRF, these shifts did n o t outlast the stimulus train for more than 100--200 msec. Cryogenic blockade of the inferior thalamic peduncle abolished these phasic SP shifts, as well as the accompanying recruiting responses, in RVA and the cortex, and in their places produced the sustained DC potential shifts that were n o t e d to be opposite in polarity to the electrically evoked SPs (Fig. 5, A, COOL ITP). The baseline shifts produced by the cryogenic blockade were sustained througho u t the period of cooling. However, the blockade did not affect the tonic positive shifts in RVA that were produced by MFR activation (Fig. 5, B) or by a strong cutaneous shock. SPs were no longer evoked, however, by mild cutaneous shocks (Fig. 4, E). The SPs produced by novel stimuli were still present in RVA during cooling but t hey were usually reduced in amplitude by 20--30%. Thus, stimulation of the mesencephalic reticular formation or its activation by strong or novel stim-
292
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J+ Fig. 5. Slow potential (SP) shifts produced by a stimulation of the medial thalamus and mesencephalic reticular formation and by cryogenic blockade (5°C) in the inferior thalamic peduncle. A : Stimulation (8 c/sec, dark line) of nucleus centralis medialis (ncm) produced monophasic negative recruiting responses (RRs) superimposed upon a positive SP shift, recorded from the anterior sigmoid cortex (AS); monophasic positive RRs superimposed upon a negative SP shift were simultaneously recorded from nucleus reticularis thalami (RvA). Cryogenic blockade of the inferior thalamic peduncle (COOL ITP) abolished both the RRs and SPs. B: Brief stimulation (250 c/sec, 250 msec) of the mesencephalic reticular formation (mrf) produced a surface-negative SP on the AS and a positive SP in RVA ; this latter response was not affected by ITP blockade. All responses are from the same chronic cat preparation. Calibrations: A, 500 pV, 1 sec;B, 1000 pV, 10 sec; negative polarity upward.
uli, produces long-lasting SP responses in RVA that are independent of the integrity of the inferior thalamic peduncle, in contrast to those phasic SPs produced by activation of the medial thalamus or t o n e - s h o c k conditioning, Direct activation of RVA, by stimulation through the same electrode that had previously recorded large SP shifts in response to
Fig. 6. Reduction of visual evoked potentials by stimulation in the anterior neuropil region of nucleus reticularis thalami (RvA). Stimulation of the optic tract (ot) produced evoked responses in the primary visual cortex (VC) that were totally abolished by a conditioning stimulus (10 c/sec, 500 msec) in RvA ending 150 msec preceding the ot stimulus (STIM RVA )' Chronic cat preparation. Calibrations: 400 ~V; 10 msec.
shock-reinforced tones, produced a dramatic inhibitory effect on primary visual evoked potentials, as illustrated in Fig. 6. Conditioning stimulation (8--10 c/sec) of the RVA electrode in this chronic preparation resulted in total suppression of the visual cortex evoked potential to optic tract stimuli delivered a n y time within 150 msec after the end of the conditioning t r a i n . A l l o f t h e components of the evoked potential were abolished, a result which suggests complete blockade of transmission a t the thalamic level. This result, however, was obtained in only one of four chronic preparations with electrodes in approximately the same region of RVA, a finding which suggests t h a t a v e r y critical locus within RVA exists for modulation of visual sensory input. Discussion In a previous paper, we demonstrated that the mediothalamic-frontocortical system has an excitatory effect and the M R F has an inhibitory effect on the unit activity in RVA (Yingling and Skinner 1975). The present results show that these two systems also regulate negative and positive SP shifts in RVA. Table I summarizes the characteristics of these and similar SPs elicited by natural stimuli that evoke states of arousal and attention.
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TABLE 1 Characteristics o f slow p o t e n t i a l shifts in the frontal granular c o r t e x and nucleus reticularis thalami.
Novel stimuli
Strong cutaneous shocks
M R F stimuli (250 c/sec, 50 msec trains)
Medial thalamic stimuli (8 c/sec trains)
Tone--shock conditioning
Frontal cortex Polarity Duration E f f e c t of ITP cooling *
Negative 20--30 sec Abolished
Negative 20--30 sec Abolished
Negative 20--30 sec Abolished
Positive Stim. train ** Abolished
Negative I n t e r s t i m . int. * * * Abolished
N. reticularis Polarity Duration E f f e c t o f ITP cooling *
Positive 20--30 sec Reduced t
Positive 20--30 sec Unaffected
Positive 20--30 sec Unaffected
Negative Stim. train ** Abolished
Positive I n t e r s t i m . int. * * * Abolished
• Cooling o f the inferior thalamic p e d u n c l e (ITP) p r o d u c e s a c o n s t a n t negative o f f s e t p o t e n t i a l in t h e frontal c o r t e x a n d a c o n s t a n t positive o n e in N. reticularis thalami. • * The slow p o t e n t i a l perists for 1 0 0 - - 2 0 0 m s e c after t h e last impulse in the stimulus train. • ** The d u r a t i o n is a f u n c t i o n o f t h e t i m e b e t w e e n t h e warning signal ( t o n e ) and t h e r e i n f o r c e m e n t (shock), i.e., the i n t e r s t i m u l u s interval. t The slow p o t e n t i a l a m p l i t u d e is usually r e d u c e d b y 20--30%.
The temporal congruity of the changes in unit activity in RVA with the SP shifts suggests that the negative SPs reflect cellular excitation, whereas the positive SPs reflect inhibition. For example, the 20--30 sec positive SPs produced in RVA by a brief stimulus to the M R F show a parallel time course to the period of unit inhibition in RVA evoked by the same stimulus (Yingling and Skinner 1975). Conversely, the negative SP produced by 8 c/sec medial thalamic stimulation is similar in duration to the concurrent period of increased unit activity; that is, both the SP shift and unit activity increase last only during and up to 100--200 msec after the last impulse in the stimulus train. A similar correlation was found by Rebert (1973) for neurons in the lateral geniculate, whose firing rate increased during negative SPs. During an electrically elicited 8--12 c/sec EEG spindle, intracellular records from RVA cells show a constant depolarizing membrane shift together with increased spike discharges (Waszak 1973, 1974). This result, together
with our observations, supports the concept that the polarity and duration of the SPs in RVA are related to changes in the level of membrane polarization of the neurons. The mechanism connecting the SPs to the excitability of the neurons is unknown. Slow potentials have been shown, however, to be related both to changes in metabolic activities in brain tissue (Rosenthal and Somjen 1973} and to processes in glial cells that appear to be regulating the levels of extracellular potassium (Castellucci and Goldring 1970; Grossman and Rosman 1971; Ransom and Goldring 1973a, b, c). The positive SPs in RVA that accompany the negative SPs in the frontal cortex during the orienting reactions to novel stimuli appear to be mediated by activation of the MRF. Stimulation of the M R F evokes SPs in both the frontal cortex and RVA that are identical to those produced by novel stimuli; also negative SPs of identical time course to those in RVA are produced in the M R F during behavioral responses to novel stimuli. In contrast to
294 the conditioned SPs, those evoked in RVA by novel events or MRF stimulation are independent of the integrity of the mediothalamicfrontocortical system, and thus they appear to result from a direct inhibitory projection from the MRF to RVA. The Scheibels (Scheibel and Scheibel 1966, 1967) have demonstrated a probable anatomical substrate for this electrophysiological phenomenon, the ventral leaf of the bifurcating projection ascending from the M R F t h a t traverses subthalamic and hypothalamic regions and terminates in RVA. The origin of the conditioned SPs is less clear. Their abolition in RVA following blockade in the inferior thalamic peduncle indicates that they do not originate from the MRF, because under these conditions, the MRFevoked SPs could still occur. Several alternarives exist which could explain the results, The blockade in the inferior thalamic peduncle could interrupt connections between: (1) the medial thalamus and frontal cortex (Scheibel and Scheibel 1967}; ( 2 ) t h e frontal cortex and RVA (Scheibeland Scheibel 1966); (3) the frontal cortex and the M R F (Nauta 1964); or (4) connections between other systems which could indirectly influence the generation of SPs in RVA. The present data do not permit a clear choice to be made between these alternative mechanisms. The only conclusion that can be made at this time is that the conditioned SPs in RVA require the functional integrity of the mediothalamicfrontocortical system, whereas those produced by novel or noxious stimuli, presumably mediated by activation of the MRF, do not require such connections, Since the negative SPs in the frontal cortex produced by either novel stimuli or conditioning are abolished during ITP blockade, it is necessary to rely on the subcortical response in RVA in order to differentiate the processes which mediate SPs in different behavioral contexts. Our results indicate that the SP responses in RVA that follow novel or noxious stimuli are mediated by the MRF. In contrast, the SPs in RVA elicited by a conditioned stim-
J.E. SKINNER, C.D. YINGLING ulus appear to be regulated by the mediothalamic-frontocortical system. These latter SPs may be related to the contingent negative variation (CNV) recorded in human subjects (Walter et al. 1964) since they both appear to reflect a preparatory process associated with the expectancy of r e c e i p t o f a s e c o n d m e a n i n g ful stimulus. If this analogy is valid, the present results would suggest that the CNV is not related to a general arousal process mediated by the MRF, but rather to a higher cerebral process mediated by an influence on RVA that descends froth the frontal cortex. In this and a previous paper (Yingling and Skinner 1975) we have argued that the frontal cortex and the MRF both regulate activity in RVA. The activation of RVA , in turn, appears to produce inhibition in the thalamic relay nuclei, at least in the lateral geniculate body, as illustrated by Fig. 6. The sensory specificity of the activation in R (i.e., which thalamic sensory nuclei are affected by a given stimulus in R) is presently unknown and requires further study, b u t it would appear that topographical organization between sense modalities may be present in the anterior neuropil region of RVA, since not all electrodes in the same general area of RVA inhibit visual evoked responses. The inferior thalamic peduncle, which projects into RVA (Scheibel and Scheibel 1966, 1967), does itself appear to contain a modality-specific topographical organization in its effect on sensory evoked potentials (Skinner and Lindsley 1971). The fact that direct electrical activation of R cells produces thalamic inhibition confirms previous studies that have shown a correlation between unit activity in RVA and thalamic inhibition (discussed in Yingling and Skinner 1975) and constitutes evidence that the relationship may be causal.
Summary Novel stimuli or electric stimulation of the mesencephalic reticular formation (MRF) produced large positive slow potentials (SPs) in
NUCLEUS RETICULARIS THALAMI rostral nucleus reticularis thalami (RvA) that accompanied the negative SPs known to occur in frontal cortex. SP durations {20- 30 sec) were similar to the periods of unit inhibition that occur in RVA following MRF stimulation, Trains of 8 c/sec medial thalamic stimuli produced phasic negative SPs in RVA similar in duration to the intervals of unit excitation that follow each stimulus pulse. These results suggest that the polarity and duration of the SPs in RVA reflect changes in excitation of the underlying neurons. Direct activation of a specific region of RVA produced complete inhibition of visual cortex responses evoked by optic tract stimuli, a finding which suggests that RVA has an inhibitory action on the thalamus. A tone reinforced by electric shock also elicited SPs in frontal cortex (negative) and RVA (positive). In contrast to the long duration of the MRF- or novelty-elicited SPs, the durations of the conditioned SPs were phasic and were regulated by the tone--shock interval. Bilateral cryogenic blockade of the interconnections between the frontal cortex and medial thalamus abolished SPs of all origins in the frontal cortex. The blockade also abolished conditioned SPs in RVA, but did not affect the MRF-elicited ones. Thus, the subcortical SPs that accompany orienting to novel stimuli are distinct from those which occur during the higher cognitive process of conditioned expectancy and require the integrity of the mediothalamic-frontocortical system,
R@sumd
Rdgulation des ddflections lentes de potentiels dans le noyau rdticulaire thalamique par la formation rdticulaire mdsencdphalique et le cortex granulaire frontal Des stimuli nouveaux ou une stimulation 61ectrique de la formation r6ticulaire m6senc~phalique (MRF) provoquent de grands potentiels lents positifs (SPs) au niveau du noyau r6ticulaire rostral du thalamus (RVA), qui ac-
295 compagnent le SPs n~gatif classique recueilli au niveau du cortex frontal. Les dur~es de SP {20 ~ 30 sec) sont du m~me ordre que les p~riodes d'inhibition unitaire qui surviennent dans les RVA apr6s stimulation de la MRF. Des trains de stimuli de 8 c/sec appliques au thalamus m~dian provoquent des SPs phasiques n~gatifs dans le RVA de dur~e similaire aux intervalles d'excitation unitaire cons~cutifs ~ chaque impulsion de stimulation. Ces r~sultats sugg~rent que la polarit~ et la dur~e des SP dans le RVA refl~tent des changements d'excitation des neurones sous-jacents. L'activation directe d'une r~gion sp~cifique du RVA provoque une inhibition complete des r~ponses du cortex visuel ~voqu@es par stimulation du tractus optique, donn~e qui sugg~re que le RVA a une action inhibitrice sur le thalamus. Un son renforc~ par un choc ~lectrique fait apparaftre ~galement des SPs dans le cortex frontal (n~gatifs) et les RVA (positifs). Par opposition ~ la longue dur@e des SPs mis en ~vidence par stimulation de la MRF ou par stimuli nouveaux, les dur~es des SPs conditionn~es sont phasiques et sont r~gul~es par l'intervalle s o n - c h o c . Le blocage cryog~nique bilateral des inter-connexions entre le cortex frontal. Ce blocage abolit ~galement les SPs conditionn~s dans le RVA mais n'affecte pas ceux qui sont dfis ~ la stimulation de la MRF. Ainsi les SPs souscorticaux qui accompagnent l'orientation vers des stimuli nouveaux sont distincts de ceux qui surviennent au cours des processus cognitifs plus ~lev~s de l'attente conditionn~e, et qui n~cessitent l'int~grit~ du syst6me m~diothalamique-fronto-cortical. The authors wish to thank Gregory L. King for his technical assistance. References Arduini, A., Mancia, M. and Mechelse, K. Slow potential changes in the cerebral cortex by sensory and reticular stimulation. Arch. ital. Biol., 1957, 95: 127--138.
Caspers,H. Relations of steady potential shifts in the cortex to the wakefulness--sleep spectrum. In
296 M.A.B. Brazier (Ed.), Brain function, Vol. I: Cortical excitability and steady potentials. University of California Press, Berkeley, 1963: pp. 117--123. Caspers, H. Shifts of the cortical steady potential during various stages of sleep. In M. Jouvet {Ed.), Aspects anatomo-fonctionnels de la physiologie du sommeil, Colloques Internationaux du Centre National de la Recherche Scientifique, No. 127, Paris, 1965, pp. 213--229. Castellucci, V.F. and Goldring, S. Contribution to steady potential shifts of slow depolarization in cells presumed to be glia. Electroenceph. clin. Neurophysiol., 1970, 28: 1 0 9 - 1 1 8 . Grossman, R.G. and Rosman, L.J. Intracellular potentials of inexcitable cells in epileptogenic cortex undergoing fibrillary gliosis after a local injury, Brain Res., 1971, 28: 181--201. Jasper, H.H. and Ajmone Marsan, C. A stereotaxic atlas of the diencephalon of the cat. National Research Council of Canada, Ottawa, 1954, 71 p. Jouvet, M. Neurophysiology of the states of sleep. In G.C. Quarton et al. (Eds.), The neurosciences: A study program. Rockefeller University Press, New York, 1967 : 529--576. McCaltum, W.C. and Knott, J.R. (Eds.). Event-related slow potentials of the brain: their relation to behavior. Electroenceph. clin. Neurophysiol., 1973, Suppl. 33: 1--390. Nauta, W.J.H. Some efferent connections of the prefrontal cortex in the monkey. In J.M. Warren and K. Akert (Eds.), The frontal granular cortex and behavior. McGraw-Hill, New York, 1964: 397-409. Ransom, B.R. and Goldring, S. Ionic determinants of membrane potential of cells presumed to be glia in cerebral cortex of cat. J. Neurophysiol., 1973a, 36: 855--868. Ransom, B.R. and Goldring, S. Slow depolarization in cells presumed to be glia in cerebral cortex of cat. J. Neurophysiol., 1973b, 36: 869--878. Ransom, B.R. and Goldring, S. Slow hyperpolarization in cells presumed to be glia in cerebral cortex of cat. J. Neurophysiol., 1973c, 36: 879--892. Rebert, C.S. Slow potential correlates of neuronal population responses in the cat's lateral geniculate nucleus. Electroenceph. clin. Neurophysiol., 1973, 35: 511--515. Rosenthal, M. and Somjen, G. Sustained evoked potentials, spreading depression and metabolic ac-
J.E. SKINNER, C.D. YINGLING tivity of the cerebral cortex. J. Neurophysiol., 1973, 36: 730--749. Scheibel, M.E. and Scheibel, A.B. The organization of the nucleus reticularis thalami: a Golgi study. Brain Res., 1966, 1: 43--62. Scheibel, M.E. and Scheibel, A.B. Structural organization of nonspecific thalamic nuclei and their proj e c t i o n toward cortex. Brain Res., 1967, 6: 60--94. Skinner, J.E. Abolition of a conditioned, surfacenegative, cortical potential during cryogenic blockade of the nonspecific thalamo-cortical system. Electroenceph. clin. Neurophysiol., 1971, 31: 197--201. Skinner, J.E. and Lindsley, D.B. Enhancement of visual and auditory evoked potentials during blockade of the nonspecific thalamo-cortical systern. Electroenceph. clin. Neurophysiol., 1971, 31: 1--6. Skinner, J.E. and Lindsley, D.B. The nonspecific mediothalamic frontocortical system: Its influence on electrocortical activity and behavior. In K. Pribram and A. Luria (Eds.), Psychophysiology of the frontal lobes. Academic Press, New York, 1973. Tecce, J.J. Contingent negative variation (CNV) and psychological processes in man. Psychol. Bull., 1972, 77: 73--108. Walter, W.G. Can " A t t e n t i o n " be defined in physiological terms? In C.R. Evans and T.B. Mulholland (Eds.), Attention in neurophysiology. Appleton-Century--Crofts, New York, 1969: 27--39. Walter, W.G., Cooper, R., Aldridge, V.J., McCallum, W.C. and Winter, A.L. Contingent negative variation: An electric sign of sensory-motor association and expectancy in the human brain. Nature (Lond.), 1964, 203: 380--384. Waszak, M. Effect of polarizing currents on potentials evoked in neurons of the ventral lead of nucleus reticularis thalami by intralaminar stimulation. Exp. Neurol., 1973, 40: 82--89. Waszak, M. Effect of barbiturate anesthesia on discharge pattern in nucleus reticularis thalami. Pharmacol. Biochem. Behav., 1974, 2: 339--345. Yingling, C.D. and Skinner, J.E. Regulation of unit activity in nucleusreticularis thalami by the mesencephalic reticular formation and the frontal granular cortex. Electroenceph. clin. Neurophysiol., 1975, 39: 635--642.
Electroencephalography and Clinical Neurophysiology, 40 (1976) 288--296 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
R E G U L A T I O N OF SLOW POTENTIAL SHIFTS IN NUCLEUS RETICULARIS THALAMI BY THE MESENCEPHALIC R E T I C U L A R FORMATION AND THE F R O N T A L G R A N U L A R CORTEX * JAMES E. SKINNER and CHARLES D. YINGLING
Physiology Department, Baylor College of Medicine; Biology Department, Rice University; and Neurophysiology Service, The Methodist Hospital, Houston, Texas 77025 (U.S.A.) (Accepted for publication: September 23, 1975)
We have previously suggested that nucleus reticularis thalami (R) has an inhibitory role in the generation of recruiting responses and EEG synchronization and we have demonstrated that unit activity in R is regulated jointly by the mesencephalic reticular formation and the mediothalamic-frontocortical system (Yingling and Skinner 1975). The present paper provides direct evidence that R has an inhibitory action on the thalamus, and examines the role of R in the mechanism mediating cortical slow potential (SP) shifts, which, like EEG synchronization and desynchronization, have been associated with changes in arousal and attention (Walter et al. 1964; Tecce 1972;McCallum and Knott 1973; Skinner and Lindsley 1973). Arduini et al. (1957) first showed that a negative SP shift could be produced on the surface of the frontal cortex in a cat by stimulation of sensory nerves or by stimulation of the mesencephalic reticular formation, These SPs persisted for many seconds following the stimulus, as did the desynchronization of the EEG. The effect of drowsiness or sleep, a condition which is related to a decrease in the mesencephalic reticular activation level
* This work was supported by Grants HL 05435 and HL 13837 from the National Heart and Lung Institute, NIH, U.S. Public Health Service. C.D. Yingling was the recipient of a predoctoral fellowship from
the National Science Foundation.
(see Jouvet 1967) has been shown to result in a tonic frontocortical SP that is of opposite polarity (i.e., surface-positive) to that elicited by mesencephalic reticular stimulation (Caspers 1963, 1965). Thus, it appears that conditions associated with an increased tendency for EEG synchronization result in SPs on the frontal lobe that are of positive polarity, whereas a condition that reduces EEG synchronization results in a negative frontocortical SP. Surface slow potentials can also be produced in the frontal cortex by a warning signal that is associated with a meaningful expected event. Walter et al. (1964) and Walter (1969) first demonstrated that a tonic SP of negative polarity, which they called the "contingent negative variation", was produced in the frontal region in man during conditioned expectancy. The SPs produced by this means have been implicated in a higher cerebral process that appears to be distinct from that of general arousal usually associated with a mesencephalic reticular mechanism. A previous study has implicated the mediothalamic-frontocortical system in the generation of the cortical SPs elicited by a warning signal, for blockade in the pathway interconnecting the medial thalamus and frontal cortex abolishes these conditioned SPs (Skinner 1971). Whether or n o t s u c h b l o c k a d e a f f e c t s SPs o f m e s e n c e p h a l i c
reticular origin is one of the problems that is investigated in the present study.
NUCLEUS RETICULARIS THALAMI In a recent study (Yingling and Skinner !975) it was found that analysis of unit activity in R clearly differentiated the roles of the mesencephalic reticular and mediothalamic-frontocortical aystems in the regulation of cortical synchronous activity. Since these two systems also have been shown to affect cortical SPs, we decided to look for SP shifts in the thalamus, especially in R. It was thought that analysis of such thalamic events might elucidate the roles of the two systems in the regulation of cortical SPs as it had in the regulation of cortical synchronization,
289 tained for chronic studies. All of the leads were placed in connector strips and secured to the skull with stainless-steel screws and dental cement in these chronic preparations. In cases where the signal-to-noise ratio was small, 15 responses were averaged on a Technical Measurements Corporation CAT 400. Toneshock conditioning trials were controlled by a BRS solid-state programming apparatus which delivered 0.5 sec tones followed in 4 sec by a mild shock to the hind limbs (100 c/sec, 0.5 msec, 30--50 msec train). The reinforcement shocks were delivered through stainless-steel wires sutured into the skin.
Methods Results A total of 8 chronic and 22 acute adult cat preparations were used in this study. The methods were identical to those described previously (Yingling and Skinner 1975}, with the exceptions noted below. DC-coupled, chopper-stabilized SP recordings were amplified by a Beckman Type R polygraph and recorded on a Precision Instruments FM tape recorder. The active electrodes were referenced to an electrode embedded in Gelfoam, moistened with saline, and placed in the most anterior part of the frontal sinus septal bone. Recordings from the back of the neck to the reference electrode showed negligible potentials of brain origin in response to various stimuli or from the eyes as a consequence of their spontaneous movement, Care was taken to avoid pain and abnormal • ventilation, and to maintain normal blood gas levels and normal body temperature in the immobilized preparation, for the slow potential shifts are qui~e sensitive to these factors, At the conclusion of the acute phase of the experiments, it was generally possible to allow the gallamine to be metabolized and within 1--2 h after the last injection, the animal would be sitting up and breathing regularly and responding appropriately to stimuli. The rapid recovery served as evidence that the state of the previously immobilized animal was good and permitted selected animals to be main-
The nucleus reticularis thalami, R, is a thin, shell-like structure that surrounds most of the thalamus. Capital letter subscripts of R are used to indicate the part of the thalamus lying adjacent to the particular segment of R studied. In the figures below that show evoked responses, the site of the recording will be abbreviated by capital letters, and the site of the evoking electric stimulus will be abbreviated by lower case subscripts. Fig. 1 shows that brief (50 msec) high-frequency (250 c/sec) stimulation of the mesencephalic reticular formation (MRF) evoked a large positive slow potential (SP) shift in RVA that had a similar time course to the surfacenegative shift in the frontal cortex (AS). Recordings from RLG showed a similar shift which was also initially positive after the MRF stimulus, though it was sometimes followed by a later negative component. The positive SPs in RVA were very large for extracellular potentials and ranged in amplitude from 1 to 20 mV. SPs almost identical to those produced by brief MRF stimulation were found to accompany orienting responses to novel stimuli, such as a sudden loud noise (Fig. 1, NOVEL STIM). The giant SPs in the thalamus were highly localized to R. No consistent SP responses were seen in recordings from medial thalamic nuclei (nucleus cen-
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orienting responses or MRF activation were localized to the frontal cortex in aIl of the cats; no responses were seen in recordings from posterior sigmoid, coronal, suprasylvian, visual, or auditory cortices. Further localization of the large subcortical SPs to R is shown in the results from an acute preparation (Fig. 2), in which movement of the recording electrode by as little as 0.5 mm outside the nucleus resulted in a loss of the SP elicited by a brief stimulus to the MRF.
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from AS and monopolar recordings referenced to frontal sinus septal bone are shown from areas of R adjacent to nucleus ventralis anterior (RvA) and the lateral geniculate body (RLG). On the left are shown the responses to 250 msec, 250 c/sec stimulation of the MRF (arrow). On the right, similar responses are evoked by the novel stimulus of striking the door of the recording chamber. Calibrations: 1000 #V; 10 sec.
tralis medialis, centre median, or ventralis anterior), lateral thalamic nuclei (ventralis lateralis, ventralis posterolateralis, lateral geniculate), or association nuclei (lateralis posterior), The negative cortical SPs accompanying either
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tical SPs were seen to accompany orienting responses or MRF stimulation (Fig. 1) suggests that the SPs evoked by the novel stimuli may be mediated by the MRF. In support of this suggestion, Fig. 3 shows that large negative SPs in t h e M R F a c c o m p a n y t h e t o n i c p o s i t i v e SPs in RVA whether they are evoked by a novel stimulus (first mark) or occur during a spontaneous scanning behavior a few seconds later (second mark). The EEG remained de-
synchronized throughout the period of experimentation, a result which suggests that SP shifts may be more sensitive indicators of changes in attentive states than synchronization and desynchronization of the EEG. SPs in the frontal cortex and RVA identical in polarity to those produced by either MRF
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t j; HL I m rf Ft. 12. O Fig. 2. Localization of the slow potential response in nucleus reticularis thalami (RvA) following stimulation of the mesencephalic reticular formation (MRF). Points 1 and 3, in nucleus ventralis anterior (VA) and the internal capsule (CI), respectively, show little or no response. Point 2, in RVA, shows a transient, slow positive shift of 12 mV that does not. return to baseline until after 20 sec. The three points are each separated by a distance of 1 ram. The diagram is taken from the atlas of Jasper and Ajmone Marsan (1954). Calibrations: 5000 pV; 10 sec.
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Fig. 3. Slow potentials in meseneephalic reticular formation (MRF), and nucleus reticularis thalami (RvA) evoked by novel stimuli. This segment from a continuous recording of a quiescent animal shows a negative potential in the MRF accompanied by a positive potential in RVA following a sudden loud noise (first mark). Several seconds later, similar potentials are seen arising spontaneously; these were accompanied by behavioral signs of attentiveness, i.e., the animal raised its head (second mark) and once again scanned the room for several seconds before resuming a quiet pose at the time the slow potential returned to baseline. Calibrations: 2000 pV; 20 sec.
NUCLEUS RETICULARIS THALAMI
291
stimulation or novel stimuli were elicited in a tone--shock conditioning paradigm. However, the duration of the conditioned SPs depended u p o n the interstimulus interval in contrast to those prolonged SPs elicited by MRF or novel stimuli. Fig. 4, A shows such conditioned SPs which were elicited by a neutral t one that was reinforced by a mild electric shock delivered 4 sec later. After complete acquisition (500 trials), the 4 sec interstimulus interval was shifted temporarily to 6 sec, but the SPs continued to peak at the previously expected time of r e i n f o r c e m e n t (Fig. 4, B). Additional trials resulted in a change of the peak SP to 6 sec. After 50 presentations of an unreinforced tone of a slightly higher frequency (randomly interspersed among the reinforced
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Fig. 4. Slow potential responses recorded from the frontal cortex (anterior sigmoid, AS) and nucleus reticularis thalami (RvA) during a tone ( T ) l h o c k (S) conditioning paradigm. A : Responses after 500 trials. B: Responses immediately after changing interstimulus interval from 4 to 6 sec (INT CH). C: Reduction in slow potential amplitude to a nonreinforced tone of higher frequency (thicker trace), randomly interspersed between the reinforced tones (thinner trace) in a discrimination paradigm (DISCR). D: Responses after changing intertrial intervals from 3.0 + 1.5 min to 30 -+ 15 see (MASS TR). E: Responses after bilateral cryogenic blockade (5°C) of the inferior thalamic peduncle (COOL ITP). Each trace is the average of 15 responses. Calibrations: 2ooo pv; 1 sec.
stimuli) the SPs evoked by the unreinforced tones were only 50% of the amplitudes of those SPs evoked by the reinforced tones (Fig. 4, C). The SPs were reduced during massed trials when the mean intertrial interval was shifted from 3 min (randomly presented, 2--4 rain) to 30 sec (randomly presented, 15--45 sec), as illustrated in Fig. 4, D. Lengthening the intertrial interval restored the SPs to the levels shown in Fig. 4, A. Bilateral cryogenic blockade in the inferior thalamic peduncle was found to abolish the conditioned SPs in both the frontal cort ex and RVA (Fig. 4, E). The pre- and post-cooling controls were recorded before and 3 min after the cessation of cooling and were identical to those in Fig. 4, A. During cooling, a constant negative DC offset occurred in the frontal cortex and a constant positive one in RVA. Fig. 5, A shows that DC-coupled recordings of monophasic positive recruiting responses in RVA were superimposed upon a negative shift of the baseline; in the cortex, a positive baseline shift accompanied the usual monophasic negative recruiting responses. Unlike the tonic SP shifts that follow desynchronizing stimuli to the MRF, these shifts did n o t outlast the stimulus train for more than 100--200 msec. Cryogenic blockade of the inferior thalamic peduncle abolished these phasic SP shifts, as well as the accompanying recruiting responses, in RVA and the cortex, and in their places produced the sustained DC potential shifts that were n o t e d to be opposite in polarity to the electrically evoked SPs (Fig. 5, A, COOL ITP). The baseline shifts produced by the cryogenic blockade were sustained througho u t the period of cooling. However, the blockade did not affect the tonic positive shifts in RVA that were produced by MFR activation (Fig. 5, B) or by a strong cutaneous shock. SPs were no longer evoked, however, by mild cutaneous shocks (Fig. 4, E). The SPs produced by novel stimuli were still present in RVA during cooling but t hey were usually reduced in amplitude by 20--30%. Thus, stimulation of the mesencephalic reticular formation or its activation by strong or novel stim-
292
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J+ Fig. 5. Slow potential (SP) shifts produced by a stimulation of the medial thalamus and mesencephalic reticular formation and by cryogenic blockade (5°C) in the inferior thalamic peduncle. A : Stimulation (8 c/sec, dark line) of nucleus centralis medialis (ncm) produced monophasic negative recruiting responses (RRs) superimposed upon a positive SP shift, recorded from the anterior sigmoid cortex (AS); monophasic positive RRs superimposed upon a negative SP shift were simultaneously recorded from nucleus reticularis thalami (RvA). Cryogenic blockade of the inferior thalamic peduncle (COOL ITP) abolished both the RRs and SPs. B: Brief stimulation (250 c/sec, 250 msec) of the mesencephalic reticular formation (mrf) produced a surface-negative SP on the AS and a positive SP in RVA ; this latter response was not affected by ITP blockade. All responses are from the same chronic cat preparation. Calibrations: A, 500 pV, 1 sec;B, 1000 pV, 10 sec; negative polarity upward.
uli, produces long-lasting SP responses in RVA that are independent of the integrity of the inferior thalamic peduncle, in contrast to those phasic SPs produced by activation of the medial thalamus or t o n e - s h o c k conditioning, Direct activation of RVA, by stimulation through the same electrode that had previously recorded large SP shifts in response to
Fig. 6. Reduction of visual evoked potentials by stimulation in the anterior neuropil region of nucleus reticularis thalami (RvA). Stimulation of the optic tract (ot) produced evoked responses in the primary visual cortex (VC) that were totally abolished by a conditioning stimulus (10 c/sec, 500 msec) in RvA ending 150 msec preceding the ot stimulus (STIM RVA )' Chronic cat preparation. Calibrations: 400 ~V; 10 msec.
shock-reinforced tones, produced a dramatic inhibitory effect on primary visual evoked potentials, as illustrated in Fig. 6. Conditioning stimulation (8--10 c/sec) of the RVA electrode in this chronic preparation resulted in total suppression of the visual cortex evoked potential to optic tract stimuli delivered a n y time within 150 msec after the end of the conditioning t r a i n . A l l o f t h e components of the evoked potential were abolished, a result which suggests complete blockade of transmission a t the thalamic level. This result, however, was obtained in only one of four chronic preparations with electrodes in approximately the same region of RVA, a finding which suggests t h a t a v e r y critical locus within RVA exists for modulation of visual sensory input. Discussion In a previous paper, we demonstrated that the mediothalamic-frontocortical system has an excitatory effect and the M R F has an inhibitory effect on the unit activity in RVA (Yingling and Skinner 1975). The present results show that these two systems also regulate negative and positive SP shifts in RVA. Table I summarizes the characteristics of these and similar SPs elicited by natural stimuli that evoke states of arousal and attention.
NUCLEUS RETICULARIS THALAMI
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TABLE 1 Characteristics o f slow p o t e n t i a l shifts in the frontal granular c o r t e x and nucleus reticularis thalami.
Novel stimuli
Strong cutaneous shocks
M R F stimuli (250 c/sec, 50 msec trains)
Medial thalamic stimuli (8 c/sec trains)
Tone--shock conditioning
Frontal cortex Polarity Duration E f f e c t of ITP cooling *
Negative 20--30 sec Abolished
Negative 20--30 sec Abolished
Negative 20--30 sec Abolished
Positive Stim. train ** Abolished
Negative I n t e r s t i m . int. * * * Abolished
N. reticularis Polarity Duration E f f e c t o f ITP cooling *
Positive 20--30 sec Reduced t
Positive 20--30 sec Unaffected
Positive 20--30 sec Unaffected
Negative Stim. train ** Abolished
Positive I n t e r s t i m . int. * * * Abolished
• Cooling o f the inferior thalamic p e d u n c l e (ITP) p r o d u c e s a c o n s t a n t negative o f f s e t p o t e n t i a l in t h e frontal c o r t e x a n d a c o n s t a n t positive o n e in N. reticularis thalami. • * The slow p o t e n t i a l perists for 1 0 0 - - 2 0 0 m s e c after t h e last impulse in the stimulus train. • ** The d u r a t i o n is a f u n c t i o n o f t h e t i m e b e t w e e n t h e warning signal ( t o n e ) and t h e r e i n f o r c e m e n t (shock), i.e., the i n t e r s t i m u l u s interval. t The slow p o t e n t i a l a m p l i t u d e is usually r e d u c e d b y 20--30%.
The temporal congruity of the changes in unit activity in RVA with the SP shifts suggests that the negative SPs reflect cellular excitation, whereas the positive SPs reflect inhibition. For example, the 20--30 sec positive SPs produced in RVA by a brief stimulus to the M R F show a parallel time course to the period of unit inhibition in RVA evoked by the same stimulus (Yingling and Skinner 1975). Conversely, the negative SP produced by 8 c/sec medial thalamic stimulation is similar in duration to the concurrent period of increased unit activity; that is, both the SP shift and unit activity increase last only during and up to 100--200 msec after the last impulse in the stimulus train. A similar correlation was found by Rebert (1973) for neurons in the lateral geniculate, whose firing rate increased during negative SPs. During an electrically elicited 8--12 c/sec EEG spindle, intracellular records from RVA cells show a constant depolarizing membrane shift together with increased spike discharges (Waszak 1973, 1974). This result, together
with our observations, supports the concept that the polarity and duration of the SPs in RVA are related to changes in the level of membrane polarization of the neurons. The mechanism connecting the SPs to the excitability of the neurons is unknown. Slow potentials have been shown, however, to be related both to changes in metabolic activities in brain tissue (Rosenthal and Somjen 1973} and to processes in glial cells that appear to be regulating the levels of extracellular potassium (Castellucci and Goldring 1970; Grossman and Rosman 1971; Ransom and Goldring 1973a, b, c). The positive SPs in RVA that accompany the negative SPs in the frontal cortex during the orienting reactions to novel stimuli appear to be mediated by activation of the MRF. Stimulation of the M R F evokes SPs in both the frontal cortex and RVA that are identical to those produced by novel stimuli; also negative SPs of identical time course to those in RVA are produced in the M R F during behavioral responses to novel stimuli. In contrast to
294 the conditioned SPs, those evoked in RVA by novel events or MRF stimulation are independent of the integrity of the mediothalamicfrontocortical system, and thus they appear to result from a direct inhibitory projection from the MRF to RVA. The Scheibels (Scheibel and Scheibel 1966, 1967) have demonstrated a probable anatomical substrate for this electrophysiological phenomenon, the ventral leaf of the bifurcating projection ascending from the M R F t h a t traverses subthalamic and hypothalamic regions and terminates in RVA. The origin of the conditioned SPs is less clear. Their abolition in RVA following blockade in the inferior thalamic peduncle indicates that they do not originate from the MRF, because under these conditions, the MRFevoked SPs could still occur. Several alternarives exist which could explain the results, The blockade in the inferior thalamic peduncle could interrupt connections between: (1) the medial thalamus and frontal cortex (Scheibel and Scheibel 1967}; ( 2 ) t h e frontal cortex and RVA (Scheibeland Scheibel 1966); (3) the frontal cortex and the M R F (Nauta 1964); or (4) connections between other systems which could indirectly influence the generation of SPs in RVA. The present data do not permit a clear choice to be made between these alternative mechanisms. The only conclusion that can be made at this time is that the conditioned SPs in RVA require the functional integrity of the mediothalamicfrontocortical system, whereas those produced by novel or noxious stimuli, presumably mediated by activation of the MRF, do not require such connections, Since the negative SPs in the frontal cortex produced by either novel stimuli or conditioning are abolished during ITP blockade, it is necessary to rely on the subcortical response in RVA in order to differentiate the processes which mediate SPs in different behavioral contexts. Our results indicate that the SP responses in RVA that follow novel or noxious stimuli are mediated by the MRF. In contrast, the SPs in RVA elicited by a conditioned stim-
J.E. SKINNER, C.D. YINGLING ulus appear to be regulated by the mediothalamic-frontocortical system. These latter SPs may be related to the contingent negative variation (CNV) recorded in human subjects (Walter et al. 1964) since they both appear to reflect a preparatory process associated with the expectancy of r e c e i p t o f a s e c o n d m e a n i n g ful stimulus. If this analogy is valid, the present results would suggest that the CNV is not related to a general arousal process mediated by the MRF, but rather to a higher cerebral process mediated by an influence on RVA that descends froth the frontal cortex. In this and a previous paper (Yingling and Skinner 1975) we have argued that the frontal cortex and the MRF both regulate activity in RVA. The activation of RVA , in turn, appears to produce inhibition in the thalamic relay nuclei, at least in the lateral geniculate body, as illustrated by Fig. 6. The sensory specificity of the activation in R (i.e., which thalamic sensory nuclei are affected by a given stimulus in R) is presently unknown and requires further study, b u t it would appear that topographical organization between sense modalities may be present in the anterior neuropil region of RVA, since not all electrodes in the same general area of RVA inhibit visual evoked responses. The inferior thalamic peduncle, which projects into RVA (Scheibel and Scheibel 1966, 1967), does itself appear to contain a modality-specific topographical organization in its effect on sensory evoked potentials (Skinner and Lindsley 1971). The fact that direct electrical activation of R cells produces thalamic inhibition confirms previous studies that have shown a correlation between unit activity in RVA and thalamic inhibition (discussed in Yingling and Skinner 1975) and constitutes evidence that the relationship may be causal.
Summary Novel stimuli or electric stimulation of the mesencephalic reticular formation (MRF) produced large positive slow potentials (SPs) in
NUCLEUS RETICULARIS THALAMI rostral nucleus reticularis thalami (RvA) that accompanied the negative SPs known to occur in frontal cortex. SP durations {20- 30 sec) were similar to the periods of unit inhibition that occur in RVA following MRF stimulation, Trains of 8 c/sec medial thalamic stimuli produced phasic negative SPs in RVA similar in duration to the intervals of unit excitation that follow each stimulus pulse. These results suggest that the polarity and duration of the SPs in RVA reflect changes in excitation of the underlying neurons. Direct activation of a specific region of RVA produced complete inhibition of visual cortex responses evoked by optic tract stimuli, a finding which suggests that RVA has an inhibitory action on the thalamus. A tone reinforced by electric shock also elicited SPs in frontal cortex (negative) and RVA (positive). In contrast to the long duration of the MRF- or novelty-elicited SPs, the durations of the conditioned SPs were phasic and were regulated by the tone--shock interval. Bilateral cryogenic blockade of the interconnections between the frontal cortex and medial thalamus abolished SPs of all origins in the frontal cortex. The blockade also abolished conditioned SPs in RVA, but did not affect the MRF-elicited ones. Thus, the subcortical SPs that accompany orienting to novel stimuli are distinct from those which occur during the higher cognitive process of conditioned expectancy and require the integrity of the mediothalamic-frontocortical system,
R@sumd
Rdgulation des ddflections lentes de potentiels dans le noyau rdticulaire thalamique par la formation rdticulaire mdsencdphalique et le cortex granulaire frontal Des stimuli nouveaux ou une stimulation 61ectrique de la formation r6ticulaire m6senc~phalique (MRF) provoquent de grands potentiels lents positifs (SPs) au niveau du noyau r6ticulaire rostral du thalamus (RVA), qui ac-
295 compagnent le SPs n~gatif classique recueilli au niveau du cortex frontal. Les dur~es de SP {20 ~ 30 sec) sont du m~me ordre que les p~riodes d'inhibition unitaire qui surviennent dans les RVA apr6s stimulation de la MRF. Des trains de stimuli de 8 c/sec appliques au thalamus m~dian provoquent des SPs phasiques n~gatifs dans le RVA de dur~e similaire aux intervalles d'excitation unitaire cons~cutifs ~ chaque impulsion de stimulation. Ces r~sultats sugg~rent que la polarit~ et la dur~e des SP dans le RVA refl~tent des changements d'excitation des neurones sous-jacents. L'activation directe d'une r~gion sp~cifique du RVA provoque une inhibition complete des r~ponses du cortex visuel ~voqu@es par stimulation du tractus optique, donn~e qui sugg~re que le RVA a une action inhibitrice sur le thalamus. Un son renforc~ par un choc ~lectrique fait apparaftre ~galement des SPs dans le cortex frontal (n~gatifs) et les RVA (positifs). Par opposition ~ la longue dur@e des SPs mis en ~vidence par stimulation de la MRF ou par stimuli nouveaux, les dur~es des SPs conditionn~es sont phasiques et sont r~gul~es par l'intervalle s o n - c h o c . Le blocage cryog~nique bilateral des inter-connexions entre le cortex frontal. Ce blocage abolit ~galement les SPs conditionn~s dans le RVA mais n'affecte pas ceux qui sont dfis ~ la stimulation de la MRF. Ainsi les SPs souscorticaux qui accompagnent l'orientation vers des stimuli nouveaux sont distincts de ceux qui surviennent au cours des processus cognitifs plus ~lev~s de l'attente conditionn~e, et qui n~cessitent l'int~grit~ du syst6me m~diothalamique-fronto-cortical. The authors wish to thank Gregory L. King for his technical assistance. References Arduini, A., Mancia, M. and Mechelse, K. Slow potential changes in the cerebral cortex by sensory and reticular stimulation. Arch. ital. Biol., 1957, 95: 127--138.
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