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Cholinergic modulation of sensory responses in rat primary somatic sensory cortex
Brain Research, 408 (1987) 367-371 Elsevier
367
BRE 22175
Cholinergic modulation of sensory responses in rat primary somatic sensory cortex John P. Donoghue and Kristen L. Carroll Center for Neural Science, Brown University, Providence, R102912 (U.S.A.)
(Accepted 16 December 1986) Key words: Acetylcholine; Rat; Somatic sensory cortex; Neural modulation
Responses evoked in single neurons of the primary somatic sensory cortex following tactile stimulation were examined before, during and after local iontophoretic application of acetylcholine (ACh) in urethane-anesthetized rats. The most common effect of ACh was an enhancement of the discharge evoked by sensory stimuli. Some cells responded to sensory input only in the presence of ACh. Response enhancement was observed in both supra- and infragranular layers, whereas response suppression was the most common effect in layer IV. These studies suggest that cholinergic systems modify sensory processing in cerebral neocortex by modulating the effectiveness of afferent inputs to cortical neurons in all layers.
The primary somatic sensory cortex (SI) receives a widespread cholinergic input from the basal forebrain and may have an additional intrinsic source of cholinergic innervation 5"7'9'13. Previous studies have shown that iontophoretic application of acetylcholine (ACh) in SI and in other cortical areas most commonly increases the ongoing (or background) discharge of neurons; these effects are primarly found in layer V and, to a lesser extent, in layer VI 6'11'12'14'22-24. Cholinergic modulation of background discharge has been observed less frequently in the superficial cortical layers (II and III), but both decreases and increases following A C h application have been reported lt,13,18,19'21. The results of these studies predict that the amount of discharge evoked by a sensory input to SI would be enhanced by A C h and that this enhancement would be largely restricted to the deep layers. The consequence of this action would be an increased sensory activation of cortical neurons that project directly to brainstem and spinal targets, since these cells reside within layer V. In contrast, these results predict that neurons in the more superficial layers, which project intracortically, would show little or no enhancement effect, To test these hypotheses the effect of A C h applica-
tion on the sensory-evoked discharge of single cortical neurons was examined in the SI whisker representation in 13 adult albino rats. Rats were anesthetized with urethane (Sigma, 50% in saline, 1.8 g/kg, i.p.), then the region of the whisker representation was exposed and covered with an agar-saline solution. Glass multi-barrel electrodes were used to record single unit activity and to apply drugs. Each electrode consisted of a single-recording barrel filled with 3 M NaCI (2-5 Mff2 impedance at 1 kHz) that was cemented to an array of 5 drug-containing pipettes. This array was pulled into a confluent tip and broken so that the tip of each drug barrel had a diameter of about 1.5/~m. Drug barrels were filled with ACh chloride (Sigma, 0.2 M, pH 4), atropine sulfate (Sigma, 0.1 M, pH 5), v-aminobutyric acid ( G A B A ) (Kodak, 0.1 M, pH 4) or sodium glutamate (Sigma, 0.1 M, pH 8) and 165 mM NaCI (for current balance). Receptive fields of single SI neurons were first identified by hand, then air puffs were delivered at 1.2 s intervals to the whisker that yielded a maximal response. Controlled air puffs of either 200, 500 or 1000 ms duration were delivered through a 28-gauge cannula that was aimed at that whisker. Stimulus duration and amplitude were controlled with a high-
Correspondence: J.P. Donoghue, Center for Neural Science, Box 1953, Brown University, Providence, RI 02912, U.S.A.
0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division )
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Fig. t. Response enhancement in SI neurons during ACh application. A - C : histograms and raster displays showing the response of a layer III neuron in SI to air puffs delivered to the C 3 whisker in (A) control condition; (B) during application of 20 nA of ACh; and (C) during application of atropine (20 hA) and ACh (20 nA). Arrow and vertical line on each raster mark the onset of whisker stimulation. Note the increase in the peak discharge and the small increase in background activity. Atropine reduced this effect. D - F : gating of a sensory response by ACh in a layer VI cell. This cell showed no clear response to whisker stimulation in the control condition (D). However, in the presence of ACh (E) a response was clearly present. This response was diminished when atropine was also applied (F). Rasters/histogram displays each span 2 s, each horizontal row in the raster in a single trial and each vertical mark indicates the occurrence of an action potential. Histogram bin width is 5 ms; vertical scale is averaged frequency in impulses/s.
369 speed solenoid valve and pressure regulator. Each isolated cell was tested during whisker stimulation under 3 basic conditions (a) with no applied drug (b) during local application of ACh (mean ejection current 34 _+ 22 nA, range 5-100nA) and (c) after a 3and/or 5-rain recovery period. In some cases atropine was then co-applied with ACh to test for the specificity of ACh action. The occurrence of unit discharge and stimulus characteristics were recorded on a PDP 11-23 computer for later analysis of unit activity. The laminar distribution of cells was identified by correlating electrode depth and unit discharge characteristics with histological reconstructions of electrode tracts in thionin-stained sections through SI cortex. The present analysis is based on 68 SI neurons that were modulated by ACh. Among the group of modulated cells, ACh application most frequently increased the discharge elicited by whisker stimulation (Fig. 1A-C). This response enhancement was manifest in peristimulus time histograms as an increase in discharge amplitude and duration. Inspection of raster displays showed that these changes were a result of an increase in the number of action potentials evoked or an increase in the probability of discharge following a peripheral stimulus. Of the SI cells tested, 76% (41 of 54) increased their peak firing frequency, measured from post-stimulus time histograms, when the sensory stimulus was delivered concurrently with ACh iontophoresis. Most commonly the background discharge rate also increased during ACh application, but peak response was increased without changing background activity in 8 cells. Eleven out of 54 cells tested (20.4%) were activated by whisker stimulation only during ACh application. These cells, which generally had low rates of spontaneous activity, did not respond to air puffs or mechanical stimulation of any of the whiskers in the absence of ACh. However, they responded consistently to air puffs delivered to the whiskers when presented in combination with ACh iontophoresis (Fig. 1D-F). The ACh-dependent receptive field of these cells was centered about the same whisker as nearby neurons that were driven by tactile stimulation in the absence of ACh. Most often the response of a cell returned to near control levels after the termination of ACh application. However, in 11 cells some evidence of response enhancement was still present after 5 min. In 5 cells
that were followed for longer periods of time, response enhancement remained at 20 min, although the amplitude of this effect was less than observed during ACh application. All forms of response enhancement were blocked if atropine was co-applied with the ACh, suggesting that these effects are mediated by muscarinic receptors. The enhancement of sensory-evoked discharge by ACh was observed both in supragranular and infragranular layers. In layers II and III, 83% (10 of 12) of the cells tested increased their mean peak response (measured over 20 trials) following ACh application, while the sensory evoked discharge increased for 94% (29 of 32) of layer V and V1 cells during ACh application. The remaining 2 superficial and 3 deep cells decreased sensory response during ACh application. The amount of response enhancement in superficial and deep layers was calculated as the (PeakAch-Peakco.tro0/PeakcontroI × 100% where peak ACh is the peak discharge amplitude (averaged over 20 trials)evoked by whisker stimulation during ACh application; PeakcontroI is the peak discharge before ACh application. Peak discharge had a mean increase of 76 _ 64 % (S.D.) in layers V and VI; changes as high as 245% were observed. A similar average increase in peak response of 74 _+ 48% was found for cells in the superficial layers. Cells for which receptive fields could be found only after ACh application were also present in layers II-III and V-VI, but were not included in the analysis of peak enhancement. In contrast to the response enhancement elicited in supra- and infragranular layers, the peak discharge evoked by whisker stimulation most commonly decreased in recordings that corresponded most closely with layer IV. Eiectrophysiologically, cells located in the region of layer IV, about 600-800/~m below the cortical surface, exhibited a distinctive set of electrophysiological features: they are often more difficult to isolate than cells at other depths, they are extremely sensitive to low amplitude deflections of a single whisker and their receptive fields include one or, at most, a few whiskers. These features are characteristic of neurons in layer I V 3'19'22 a layer distinguished histologically in SI cortex by the presence of densely packed granular cells. Reconstructions of
370 penetrations from histological sections confirmed that 10 well-isolated single neurons with these elcctrophysiological features were recorded m the layer 1V of SI cortex. The response of 8 of these to air puff stimuli was decreased an average of 24.6 + 2tVi during ACh application, while discharge increased under the same conditions in two layer IV cells. The mean amplitude of iontophoretic current used in layer 1V was not significantly different from that used in other layers (Mann-Whitney U-test). Tests of ACh effects during multi-unit recordings confirmed a general lack of ACh-induced response increase in layer IV. The present results show that ACh can increase the amount of somatic sensory evoked discharge in neurons located in superficial (excluding layer I) as well as deep layers of cerebral neocortex, presumably by acting on muscarinic receptors. Earlier studies of SI cortex indicated that the excitatory effects of ACh were largely restricted to the deeper cortical layers, based on measurements of change in background discharge rate 14'15. In contrast, recent studies have identified ACh-induced excitation in superficial layers of some cortical areas. McCormack and Prince 16 demonstrated muscarinic cholinergic increases in background discharge in more than 90% of the identified pyramidal cells studied in vitro in a population of 253 layer I I - I I I neurons intracellularly recorded in rat cingulate cortex slices. Similarly, iontophoretic application of ACh enhanced visual responses of a large percentage of neurons in both superficial and deep layers of cat primary visual cortex 21. The reason for the discrepancy between more recent and older studies is not clear, it may reflect a sampling bias towards cells with ongoing activity, anesthetic agent employed, or the use of different techniques to test for ACh effects. However, there is now strong evidence for a very broadly distributed action for cortical cholinergic systems, Most of the response-enhanced cells increased both background and evoked discharge in the presence of ACh. This background modulation may in turn increase the excitability of cells upon which that neuron projects. A burst of activity against this background could then produce a stronger activation of the target of the ACh modulated neuron. The effect of ACh was not always an overall increase in activity, for evoked response increased in the absence of any
detectable change in background activity in 20% of the cells examined. This action might be explained bv a voltage dependency of ACh action in these neurons ie4s. ('ells with relatively little ongoing discharge might be hyperpolarized beyond the range at which voltage-dependent ACh effects could be expressed. This explanation would clarify the previously described positive correlation between spontaneous discharge rate and degree of ACh effect I". The activation of an afferent input by peripheral stimulation could bring a cell into a range where ACh is effective in increasing excitability, thus accounting for an increase in evoked discharge without background modulation. The low ongoing activity of these cells is consistent with this explanation. The restriction of ACh modulation to the sensory-related discharge improves the signal to noise ratio of afferent input to these cortical neurons, suggesting that one action of ACh is selective gain control of sensory inputs. A further subset of cells responded to sensory stimuli only during ACh application. Thus, ACh may also act as a gate that alters the set of cells that participate in intracortical processing. Each of these actions of ACh are consistent with the hypothesis that ACh has a modulatory role in the cortex. The ratio of response-enhanced to response-supressed cells was reversed for layer IV compared to deeper or more superficial layers. Thus, the predominant effect of ACh in layer IV appears to be a suppression of the evoked sensory response. Sillito and Kemp 21 also found that cortical cells inhibited by ACh were most common in the midcorlical depths in cat visual cortex, although these cells were generally localized to the deep part of layer Ili. The functional significance of response suppression among the cells that are most likely to receive direct thalamic input is unclear but this selective action could serve as a filtering mechanism that eliminates weak inputs and preserve strong ones at the earliest of intracortical processing. We would like to thank Dr. J. Michael Walker for his generous assistance and guidance in the initial phases of this project, Dr. Ford Ebner for his cornments of an earlier version of this manuscript, and Ms. Mary Ellen Flinn-Butera for her outstanding secretarial assistance. Supported by NIMH Grant MH 40972-01 and NIH Grant NS 22517-0l.
371 1 Bernado, L.S. and Prince, D.A., Ionic mechanisms of cholinergic excitation in mammalian hippocampal pyramidal cells, Brain Research, 249 (1982) 333-344. 2 Brown, D.A., Slow cholinergic excitation- a mechanism for increasing neuronal excitability, Trends Neurosci. 6 (1983) 302-307. 3 Chapin. J.K. and Lin, C.S., Mapping the body representation in the SI cortex of anesthetized and awake rats, J. Comp. Neurol., 229(1984) 199-213. 4 Cole. A.E. and Nicoll, R.A., The pharmacology of cholinergic excitatory responses in hippocampal pyramidal cells, Brain Research, 305 (1984b)283-290. 5 Coyle, J.T., McKinney, M., Johnson, M.V. and Hedreen, J.C., Synaptic neurochemistry of the basal forebrain cholinergic projection, Psychopharmacol. Bull., 19 (1983) 441-447. 6 Crawford, J.M. and Curtis, D.R., Pharmacological studies on Betz cells, J. Physiol. (London), 186 (1966) 121-138. 7 Fibiger, H.C., The organization and some projections of cholinergic neurons of the mammalian forebrain, Brain Research, 257 (1982) 327-388. 8 Halliwell, J.V. and Adams, P.R., Voltage clamp analysis of muscarinic excitation in hippocampal neurons, Brain Research, 250 (1982) 71-92. 9 Johnston, M.V., McKinney, M. and Coyle, J.T., Neocortical cholinergic innervation: Description of extrinsic and intrinsic components in the rat, Exp. Brain Res., 43 (1981) 159-172. 10 Jones, R.S. and Olpe, H., On the role of baseline firing rate in determining responsiveness of Cingulate cortical neurons to iontophoretically applied SP and ACh J. Pharm. Pharmacol., 36 (1984)623-625. 11 Krnjevic, K. and Phillis, J.W., Pharmacological properties of acetylcholine sensitive cells in the cerebral cortex, J. Physiol. (London), 166 (1963) 328-350. 12 Krnjevic, K., Pumain, R. and Renaud, L., The mechanism of excitation by acetylcholine in the cerbral cortex, J. Phys-
iol. (London), 215 (1971) 247-268. 13 Lamour, Y., Dutar, P. and Jobert, A., Topographic organization of basal forebrain projections to the rat cerebral cortex, Neurosci. Lea., 34 (1982) 117-122. 14 Lamour, Y., Dutar, P. and Jobert, A., Excitatory effect of acetylcholine on different types of neurons in the first somatosensory neocortex of the rat: laminar distribution and pharmacological characteristics, Neuroscience, 7 (1982) 1483-1494. 15 Lamour, Y., Dutar, Y. and Jobert, A., A comparative study of two populations of acetylcholine sensitive neurons in rat somatosensory cortex, Brain Research, 289 (1983) 157-167. 16 McCormick, D.A. and Prince, D.A., Two types of muscarinic response to acetylcholine in mammalian cortical neurons, Proc, Natl. Acad. Sci. U.S.A., 82 (1985) 6344-6348. 17 Deleted. 18 Phillis, J.W. and York, D.H., Cholinergic inhibition in the cerebral cortex, Brain Research, 5 (1967) 517-520. 19 Pincince, T..I. and Donoghue, J.P., Receptive filed architecture in the whisker representation of the rat somatosensensory cortex, Soc. Neurosci. Abstr., 14 (1986) 1433. 20 Randic, M., Smirnoff, R. and Straughn, D.W., Acetylcholine depression of cortical neurons, Exp. Neurol., 9 (1964) 236-242. 21 Sillito, A. and Kemp, J.A., Cholinergic modulation of the functional organization of the cat visual cortex, Brain Research, 289 (1983) 143-155. 22 Simons, D.J., Response properties of vibrissa units in the rat SI somatosensory neocortex, J. Neurophysiol., 41 (1979) 798-820. 23 Stone, T.W., Cholinergic mechanisms in the rat somatic sensory motor cortex, J. Physiol. (London), 225 (1972) 485-499. 24 Woody, C.D., Swartz, B.E. and Gruen, E., Effects of acetylcholine and cyclic GMP on input resistance in awake cats, Brain Research, 158 (1978) 373-395.
367
BRE 22175
Cholinergic modulation of sensory responses in rat primary somatic sensory cortex John P. Donoghue and Kristen L. Carroll Center for Neural Science, Brown University, Providence, R102912 (U.S.A.)
(Accepted 16 December 1986) Key words: Acetylcholine; Rat; Somatic sensory cortex; Neural modulation
Responses evoked in single neurons of the primary somatic sensory cortex following tactile stimulation were examined before, during and after local iontophoretic application of acetylcholine (ACh) in urethane-anesthetized rats. The most common effect of ACh was an enhancement of the discharge evoked by sensory stimuli. Some cells responded to sensory input only in the presence of ACh. Response enhancement was observed in both supra- and infragranular layers, whereas response suppression was the most common effect in layer IV. These studies suggest that cholinergic systems modify sensory processing in cerebral neocortex by modulating the effectiveness of afferent inputs to cortical neurons in all layers.
The primary somatic sensory cortex (SI) receives a widespread cholinergic input from the basal forebrain and may have an additional intrinsic source of cholinergic innervation 5"7'9'13. Previous studies have shown that iontophoretic application of acetylcholine (ACh) in SI and in other cortical areas most commonly increases the ongoing (or background) discharge of neurons; these effects are primarly found in layer V and, to a lesser extent, in layer VI 6'11'12'14'22-24. Cholinergic modulation of background discharge has been observed less frequently in the superficial cortical layers (II and III), but both decreases and increases following A C h application have been reported lt,13,18,19'21. The results of these studies predict that the amount of discharge evoked by a sensory input to SI would be enhanced by A C h and that this enhancement would be largely restricted to the deep layers. The consequence of this action would be an increased sensory activation of cortical neurons that project directly to brainstem and spinal targets, since these cells reside within layer V. In contrast, these results predict that neurons in the more superficial layers, which project intracortically, would show little or no enhancement effect, To test these hypotheses the effect of A C h applica-
tion on the sensory-evoked discharge of single cortical neurons was examined in the SI whisker representation in 13 adult albino rats. Rats were anesthetized with urethane (Sigma, 50% in saline, 1.8 g/kg, i.p.), then the region of the whisker representation was exposed and covered with an agar-saline solution. Glass multi-barrel electrodes were used to record single unit activity and to apply drugs. Each electrode consisted of a single-recording barrel filled with 3 M NaCI (2-5 Mff2 impedance at 1 kHz) that was cemented to an array of 5 drug-containing pipettes. This array was pulled into a confluent tip and broken so that the tip of each drug barrel had a diameter of about 1.5/~m. Drug barrels were filled with ACh chloride (Sigma, 0.2 M, pH 4), atropine sulfate (Sigma, 0.1 M, pH 5), v-aminobutyric acid ( G A B A ) (Kodak, 0.1 M, pH 4) or sodium glutamate (Sigma, 0.1 M, pH 8) and 165 mM NaCI (for current balance). Receptive fields of single SI neurons were first identified by hand, then air puffs were delivered at 1.2 s intervals to the whisker that yielded a maximal response. Controlled air puffs of either 200, 500 or 1000 ms duration were delivered through a 28-gauge cannula that was aimed at that whisker. Stimulus duration and amplitude were controlled with a high-
Correspondence: J.P. Donoghue, Center for Neural Science, Box 1953, Brown University, Providence, RI 02912, U.S.A.
0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division )
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Fig. t. Response enhancement in SI neurons during ACh application. A - C : histograms and raster displays showing the response of a layer III neuron in SI to air puffs delivered to the C 3 whisker in (A) control condition; (B) during application of 20 nA of ACh; and (C) during application of atropine (20 hA) and ACh (20 nA). Arrow and vertical line on each raster mark the onset of whisker stimulation. Note the increase in the peak discharge and the small increase in background activity. Atropine reduced this effect. D - F : gating of a sensory response by ACh in a layer VI cell. This cell showed no clear response to whisker stimulation in the control condition (D). However, in the presence of ACh (E) a response was clearly present. This response was diminished when atropine was also applied (F). Rasters/histogram displays each span 2 s, each horizontal row in the raster in a single trial and each vertical mark indicates the occurrence of an action potential. Histogram bin width is 5 ms; vertical scale is averaged frequency in impulses/s.
369 speed solenoid valve and pressure regulator. Each isolated cell was tested during whisker stimulation under 3 basic conditions (a) with no applied drug (b) during local application of ACh (mean ejection current 34 _+ 22 nA, range 5-100nA) and (c) after a 3and/or 5-rain recovery period. In some cases atropine was then co-applied with ACh to test for the specificity of ACh action. The occurrence of unit discharge and stimulus characteristics were recorded on a PDP 11-23 computer for later analysis of unit activity. The laminar distribution of cells was identified by correlating electrode depth and unit discharge characteristics with histological reconstructions of electrode tracts in thionin-stained sections through SI cortex. The present analysis is based on 68 SI neurons that were modulated by ACh. Among the group of modulated cells, ACh application most frequently increased the discharge elicited by whisker stimulation (Fig. 1A-C). This response enhancement was manifest in peristimulus time histograms as an increase in discharge amplitude and duration. Inspection of raster displays showed that these changes were a result of an increase in the number of action potentials evoked or an increase in the probability of discharge following a peripheral stimulus. Of the SI cells tested, 76% (41 of 54) increased their peak firing frequency, measured from post-stimulus time histograms, when the sensory stimulus was delivered concurrently with ACh iontophoresis. Most commonly the background discharge rate also increased during ACh application, but peak response was increased without changing background activity in 8 cells. Eleven out of 54 cells tested (20.4%) were activated by whisker stimulation only during ACh application. These cells, which generally had low rates of spontaneous activity, did not respond to air puffs or mechanical stimulation of any of the whiskers in the absence of ACh. However, they responded consistently to air puffs delivered to the whiskers when presented in combination with ACh iontophoresis (Fig. 1D-F). The ACh-dependent receptive field of these cells was centered about the same whisker as nearby neurons that were driven by tactile stimulation in the absence of ACh. Most often the response of a cell returned to near control levels after the termination of ACh application. However, in 11 cells some evidence of response enhancement was still present after 5 min. In 5 cells
that were followed for longer periods of time, response enhancement remained at 20 min, although the amplitude of this effect was less than observed during ACh application. All forms of response enhancement were blocked if atropine was co-applied with the ACh, suggesting that these effects are mediated by muscarinic receptors. The enhancement of sensory-evoked discharge by ACh was observed both in supragranular and infragranular layers. In layers II and III, 83% (10 of 12) of the cells tested increased their mean peak response (measured over 20 trials) following ACh application, while the sensory evoked discharge increased for 94% (29 of 32) of layer V and V1 cells during ACh application. The remaining 2 superficial and 3 deep cells decreased sensory response during ACh application. The amount of response enhancement in superficial and deep layers was calculated as the (PeakAch-Peakco.tro0/PeakcontroI × 100% where peak ACh is the peak discharge amplitude (averaged over 20 trials)evoked by whisker stimulation during ACh application; PeakcontroI is the peak discharge before ACh application. Peak discharge had a mean increase of 76 _ 64 % (S.D.) in layers V and VI; changes as high as 245% were observed. A similar average increase in peak response of 74 _+ 48% was found for cells in the superficial layers. Cells for which receptive fields could be found only after ACh application were also present in layers II-III and V-VI, but were not included in the analysis of peak enhancement. In contrast to the response enhancement elicited in supra- and infragranular layers, the peak discharge evoked by whisker stimulation most commonly decreased in recordings that corresponded most closely with layer IV. Eiectrophysiologically, cells located in the region of layer IV, about 600-800/~m below the cortical surface, exhibited a distinctive set of electrophysiological features: they are often more difficult to isolate than cells at other depths, they are extremely sensitive to low amplitude deflections of a single whisker and their receptive fields include one or, at most, a few whiskers. These features are characteristic of neurons in layer I V 3'19'22 a layer distinguished histologically in SI cortex by the presence of densely packed granular cells. Reconstructions of
370 penetrations from histological sections confirmed that 10 well-isolated single neurons with these elcctrophysiological features were recorded m the layer 1V of SI cortex. The response of 8 of these to air puff stimuli was decreased an average of 24.6 + 2tVi during ACh application, while discharge increased under the same conditions in two layer IV cells. The mean amplitude of iontophoretic current used in layer 1V was not significantly different from that used in other layers (Mann-Whitney U-test). Tests of ACh effects during multi-unit recordings confirmed a general lack of ACh-induced response increase in layer IV. The present results show that ACh can increase the amount of somatic sensory evoked discharge in neurons located in superficial (excluding layer I) as well as deep layers of cerebral neocortex, presumably by acting on muscarinic receptors. Earlier studies of SI cortex indicated that the excitatory effects of ACh were largely restricted to the deeper cortical layers, based on measurements of change in background discharge rate 14'15. In contrast, recent studies have identified ACh-induced excitation in superficial layers of some cortical areas. McCormack and Prince 16 demonstrated muscarinic cholinergic increases in background discharge in more than 90% of the identified pyramidal cells studied in vitro in a population of 253 layer I I - I I I neurons intracellularly recorded in rat cingulate cortex slices. Similarly, iontophoretic application of ACh enhanced visual responses of a large percentage of neurons in both superficial and deep layers of cat primary visual cortex 21. The reason for the discrepancy between more recent and older studies is not clear, it may reflect a sampling bias towards cells with ongoing activity, anesthetic agent employed, or the use of different techniques to test for ACh effects. However, there is now strong evidence for a very broadly distributed action for cortical cholinergic systems, Most of the response-enhanced cells increased both background and evoked discharge in the presence of ACh. This background modulation may in turn increase the excitability of cells upon which that neuron projects. A burst of activity against this background could then produce a stronger activation of the target of the ACh modulated neuron. The effect of ACh was not always an overall increase in activity, for evoked response increased in the absence of any
detectable change in background activity in 20% of the cells examined. This action might be explained bv a voltage dependency of ACh action in these neurons ie4s. ('ells with relatively little ongoing discharge might be hyperpolarized beyond the range at which voltage-dependent ACh effects could be expressed. This explanation would clarify the previously described positive correlation between spontaneous discharge rate and degree of ACh effect I". The activation of an afferent input by peripheral stimulation could bring a cell into a range where ACh is effective in increasing excitability, thus accounting for an increase in evoked discharge without background modulation. The low ongoing activity of these cells is consistent with this explanation. The restriction of ACh modulation to the sensory-related discharge improves the signal to noise ratio of afferent input to these cortical neurons, suggesting that one action of ACh is selective gain control of sensory inputs. A further subset of cells responded to sensory stimuli only during ACh application. Thus, ACh may also act as a gate that alters the set of cells that participate in intracortical processing. Each of these actions of ACh are consistent with the hypothesis that ACh has a modulatory role in the cortex. The ratio of response-enhanced to response-supressed cells was reversed for layer IV compared to deeper or more superficial layers. Thus, the predominant effect of ACh in layer IV appears to be a suppression of the evoked sensory response. Sillito and Kemp 21 also found that cortical cells inhibited by ACh were most common in the midcorlical depths in cat visual cortex, although these cells were generally localized to the deep part of layer Ili. The functional significance of response suppression among the cells that are most likely to receive direct thalamic input is unclear but this selective action could serve as a filtering mechanism that eliminates weak inputs and preserve strong ones at the earliest of intracortical processing. We would like to thank Dr. J. Michael Walker for his generous assistance and guidance in the initial phases of this project, Dr. Ford Ebner for his cornments of an earlier version of this manuscript, and Ms. Mary Ellen Flinn-Butera for her outstanding secretarial assistance. Supported by NIMH Grant MH 40972-01 and NIH Grant NS 22517-0l.
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