Somatosensory neurons in partially deafferented rat hindlimb granular cortex subsequent to transection of the sciatic nerve: effects of glutamate and acetylcholine

Lesions of the nucleus basalis magnocellularis (Meynert) induce enhanced somatosensory responses and tactile hypersensitivity in rats

We used the immunotoxin 192 immunoglobulin G-saporin to produce a selective cholinergic lesion in the nucleus basalis of Meynert (NBM) of rats and investigated whether the NBM lesion led to tactile hypersensitivity in the forepaw. The paw mechanical threshold test showed that the lesioned rats had a decreased threshold compared to the control. Surprisingly, there was a significant positive correlation between mechanical threshold and survival rate of NBM cholinergic neurons. Furthermore, using local field potential (LFP) recordings and voltage-sensitive dye (VSD) imaging, we found that the forepaw-evoked response in the primary somatosensory cortex (S1) was significantly enhanced in both amplitude and spatial extent in the NBM-lesioned rats. The neurophysiological measures of S1 response, such as LFP amplitude and maximal activated cortical area depicted by VSD, were also correlated with withdrawal behavior. Additional pharmacological experiments demonstrated that forepaw-evoked responses were increased in naive rats by blocking S1 cholinergic receptors with mecamylamine and scopolamine, while the response decreased in NBM-lesioned rats with the cholinergic agonist carbachol. In addition, NBM burst stimulation, which facilitates acetylcholine release in the S1, suppressed subsequent sensory responses to forepaw stimulation. Taken together, these results suggest that neuronal loss in the NBM diminishes acetylcholine actions in the S1, thereby enhancing the cortical representation of sensory stimuli, which may in turn lead to behavioral hypersensitivity.

Excitation and inhibition jointly regulate cortical reorganization in adult rats

The primary somatosensory cortex (SI) retains its capability for cortical reorganization after injury or differential use into adulthood. The plastic response of SI cells to peripheral stimulation is characterized by extension of cortical representations accompanied by changes of the receptive field size of neurons. We used intracortical microstimulation that is known to enforce local, intracortical synchronous activity, to induce cortical reorganization and applied immunohistochemical methods in the same individual animals to investigate how plasticity in the cortical topographic maps is linked to changes in the spatial layout of the inhibitory and excitatory neurotransmitter systems. The results reveal a differential spatiotemporal pattern of upregulation and downregulation of specific factors for an excitatory (glutamatergic) and an inhibitory (GABAergic) system, associated with changes of receptive field size and reorganization of the somatotopic map in the rat SI. Predominantly local mechanisms are the specific reduction of the calcium-binding protein parvalbumin in inhibitory neurons and the low expression of the activity marker c-Fos. Reorganization in the hindpaw representation and in the adjacent SI cortical areas (motor cortex and parietal cortex) is accompanied by a major increase of the excitatory transmitter glutamate and c-Fos. The spatial extent of the reorganization appears to be limited by an increase of glutamic acid decarboxylase and the inhibitory transmitter GABA. The local and medium-range net effects are excitatory and can facilitate receptive field enlargements and cortical map expansion. The longer-range increase of inhibition appears suited to limit these effects and to prevent neurons from pathological hyperexcitability.

What has intrinsic signal optical imaging taught us about NGF-induced rapid plasticity in adult cortex and its relationship to the cholinergic system?

Intrinsic signal optical imaging (ISI) is a high-resolution functional brain mapping technique that is being used to further our understanding of the neocortex and its interaction with drugs. Recent studies using combination ISI and in vivo pharmacology have advanced our insight into the actions of both acetylcholine and neurotrophins on inducing rapid and large-scale cortical plasticity. In particular, it appears that acetylcholine (ACh), nicotinic ACh receptors, nerve growth factor (NGF), and NGF receptors (TrkA and p75) are involved in an important feedback loop between the basal forebrain cholinergic system (BFCS) and the neocortex. Specifically, recent data suggest that NGF expressed in the cortex may act on multiple time scales on the BFCS: acutely to increase BFCS release of acetylcholine, intermediately to induce sprouting of BFCS axons, and long-term to change gene expression of BFCS neurons. In this article, advances in understanding the links in vivo between the BFCS, neocortex, nicotinic ACh receptors, and NGF are reviewed.

Local circuit properties underlying cortical reorganization

Peripheral denervation has been shown to cause reorganization of the deafferented somatotopic region in primary somatosensory cortex (S1). However, the basic mechanisms that underlie reorganization are not well understood. In the experiments described in this paper, a novel in vivo/in vitro preparation of adult rat S1 was used to determine changes in local circuit properties associated with the denervation-induced plasticity of the cortical representation in rat S1. In the present studies, deafferentation of rat S1 was induced by cutting the radial and median nerves in the forelimb of adult rats, resulting in a rapid shift of the location of the forepaw/lower jaw border; the amount of the shift increased over the times assayed, through 28 days after denervation. The locations of both borders (i.e., original and reorganized) were marked with vital dyes, and slices from the marked region were used for whole-cell recording. Responses were evoked using electrical stimulation of supragranular S1 and recorded in supragranular neurons close to either the original or reorganized border. For each neuron, postsynaptic potentials (PSPs) were evoked by stimulation of fibers that crossed the border site (CB stim) and by equivalent stimulation that did not cross (NCB stim). Monosynaptic inhibitory postsynaptic potentials (IPSPs) were also examined after blocking excitatory transmission with 15 microM CNQX plus 100 microM DL-APV. The amplitudes of PSPs and IPSPs were compared between CB and NCB stimulation to quantify effects of the border sites on excitation and inhibition. Previous results using this preparation in the normal (i.e., without induced plasticity) rat S1 demonstrated that at a normal border both PSPs and IPSPs were smaller when evoked with CB stimulation than with NCB stimulation. For most durations of denervation, a similar bias (i.e., smaller responses with CB stimulation) for PSPs and IPSPs was observed at the site of the novel reorganized border, while no such bias was observed at the suppressed original border site. Thus changes in local circuit properties (excitation and inhibition) can reflect larger-scale changes in cortical organization. However, specific dissociations between these local circuit properties and the presence of the novel border at certain durations of denervation were also observed, suggesting that there are several intracortical processes contributing to cortical reorganization over time and that excitation and inhibition may contribute differentially to them.

Changes in sensitivity of cholinoceptors and adrenoceptors during transhemispheric cortical reorganisation in rat SmI

The reorganisation of primary somatosensory cortex that occurs after lesioning the corresponding cortex of the contralateral hemisphere in rat has been termed, 'transhemispheric cortical reorganisation'. Cholinergic and noradrenergic innervations are hypothesized to be involved in cortical plasticity. The present study investigated the change in responses of somatosensory neurones in the hindpaw representation area to muscarinic cholinoceptor and beta-adrenoceptor receptor stimulation, by iontophoretic application of acetylcholine, noradrenaline, propranolol and atropine, during the process of transhemispheric cortical reorganization at 3-4 days and at 20-21 days after lesioning the corresponding area in the contralateral hemisphere. Most neurones in control rats showed excitatory atropine-sensitive responses to acetylcholine, and inhibitory propranolol-sensitive responses to noradrenaline. A marked reduction in neurones exhibiting muscarinic responses (from 69% to 22%) and beta-noradrenoceptor-mediated responses (from 62% to 24%) were seen in rats 3-4 days post lesion. The proportion of neurones responding had recovered by 3 weeks but the direction of the responses had changed with muscarinic response becoming predominantly inhibitory and beta-noradrenoceptor responses predominantly excitatory. It is concluded that transhemispheric cortical reorganization involves both receptor types and that the reciprocal changes at different stages after injury maintain cortical plasticity.

Acetylcholine innervation of sensory and motor neocortical areas in adult cat: a choline acetyltransferase immunohistochemical study

Light microscopic choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution of the acetylcholine innervation in primary motor (4 gamma) and sensory (3a, 3b, 41 and 17) cortical areas of adult cat. In every area, scattered immuno-reactive cell bodies were present and a relatively dense meshwork of ChAT immunoreactive axons pervaded the whole cortical thickness. These axons were generally thin and bore innumerable varicosities of different sizes. A few thicker and smoother fibers and occasional clusters of unusually large varicosities were also visible. Overall, area 17 was less densely innervated than the other areas. In each area, layer I showed the densest innervation. Innervation of underlying layers was rather uniform in area 17, but patterned in other areas. In areas 4 gamma and 3a, layers II, upper III and V showed preferential innervation. Innervation of layer IV was the strongest in areas 3b and 41. Area 3a was transitional between 4 gamma and 3b. Except in area 17, the laminar pattern of acetylcholinesterase staining was consistent with that of ChAT. In the light of current data on the distribution of this cortical innervation in different species, and of its presumed ultrastructural features, it appears likely that such regional and laminar features subtend widespread, modulatory roles of ACh.

Evolution of cortical responsiveness subsequent to multiple forelimb nerve transections: an electrophysiological study in adult cat somatosensory cortex

Multiunit recordings along mediolateral rows in the primary somatosensory cortex of the animals described by C. Avendaño, D. Umbriaco, R.W. Dykes, and L. Descarries (1995, J. Comp. Neurol. 354:321-332) provided information about the functional status of the regions in and near the deafferented cortex. Responses changed along this axis from normally organized receptive fields in the hindlimb representation through a transition zone of unusually small receptive fields into the clearly deafferented forelimb representation, where receptive fields were uncommon and often had unusual characteristics. The most abrupt change along this axis was the appearance of a repetitive, bursting discharge pattern in the multiunit activity near the border of the deprived cortex. The appearance of this pattern was used as a reference to describe differences between normal and deprived cortices. The nature of these differences evolved with time. Much of the deprived cortex lacked identifiable receptive fields for months after the nerve transections and, 1 year later, still only about half of the recording sites within the deprived region displayed organized receptive fields. Some sites within the deprived region lacking definable receptive fields could be excited at long latencies by somatic stimuli anywhere on the body. With time, regions of normal cortex near the border with the deprived zone became more involved in these processes. Spontaneous activity and thresholds also changed with time in both normal and deprived cortices.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease and long-term recovery of choline acetyltransferase immunoreactivity in adult cat somatosensory cortex after peripheral nerve transections

The functional reorganization of cerebral cortex following peripheral deafferentation is associated with changes in a number of neurotransmitters and related molecules. Acetylcholine (ACh) enhances neuronal responsiveness and could play a role in activity-dependent cortical plasticity. In this study, choline acetyltransferase (ChAT) immunohistochemistry was used to investigate ACh innervation of the primary somatosensory cortex in cats sustaining complete unilateral forearm and paw denervations. Survival times of 2-52 weeks were examined. The deafferented contralateral cortex was defined electrophysiologically, and quantitative estimates of ChAT-immunoreactive fiber density were obtained from the forelimb and hindlimb sectors of area 3b in both hemispheres. In the 3b forelimb sector contralateral to the deafferentation, a decrease in density of ChAT-positive fibers relative to the ipsilateral hemisphere was apparent at 2 weeks and most pronounced at 13 weeks, involving all cortical layers except layer I. There was no such decrease in the hindlimb sector, but the loss of ChAT immunoreactivity extended to sectors representing proximal forelimb and trunk. Changes in ChAT immunoreactivity were no longer found after 1 year of survival. This long-lasting but reversible lowering of ChAT immunoreactivity could result from a loss of afferent activity in basalis neurons and/or trophic influences retrogradely exerted by cortex on these cells. Reduced ACh transmission might then contribute to the loss of gamma aminobutyric acid (GABA) inhibition in the deafferented cortex by decreasing the activation of inhibitory interneurons. The long-term recovery of a normal ChAT immunoreactivity in cortex could be a consequence of its functional reorganization.

Large unresponsive zones appear in cat somatosensory cortex immediately after ulnar nerve cut

The organization of the primary somatosensory cortex innervated by the ulnar nerve was studied before and immediately after ulnar nerve transection in 11 cats electrophysiologically mapped under Nembutal or Ketamine anesthesia. The cortex was reexamined a second time beginning 42 hr after nerve transection in four cats anesthetized with Nembutal. One additional sham-operated control was also mapped. The region of cortex formerly served by the ulnar nerve remained largely unresponsive to somatic stimulation independent of the type of anesthetic used during recording. Nonetheless, animals anesthetized with Ketamine had more new responsive sites in deafferented cortex following nerve cut than cats anesthetized with Nembutal. New responses, when observed, were evoked by stimulation of a region of skin adjacent to the region served by the ulnar nerve. These findings suggest that the immediate response to deafferentation of somatosensory cortex is a limited acquisition of novel responses restricted to a region immediately adjacent to cortex containing normal afferent input.

Reevaluation of area 3b in the cat based on architectonic and electrophysiological studies: regional variability with functional and anatomical consistencies

Most anatomical and electrophysiological studies of the cat primary somatosensory cortex rely on Hassler and Muhs-Clement's (J. Hirnforsch. 6:377-420, 1964) cyto- and myeloarchitectonic description distinguishing area 3a from area 3b; however, discrepancies in the delineation of these areas in published studies suggest that many workers have found it difficult to apply those criteria systematically. We examined the cytoarchitecture of area 3b in Nissl stained sagittal sections from which electrophysiological data had been obtained prior to sacrifice. Rostrocaudal rows of electrode penetrations placed at different mediolateral positions in the gyrus located regions responsive to stimulation of either cutaneous or deep structures. Small electrolytic lesions allowed these data to be related to the cytoarchitecture. A systematic study throughout the trunk and limb representations found cutaneous responses in cortical regions characterized by a thick and cell-dense granular layer IV, however these same regions had a variable population of medium-sized and/or large pyramidal cells in layer V. Pyramidal cells were practically absent from the forelimb representation, but were present to varying degrees in the trunk and hindlimb representations. Moreover, the relative thickness and cell-density in layer IV were greater in the forelimb than in the hindlimb representations. Deep responses were found in cortex characterized by a thinner layer IV. Since the characteristics of layer V in area 3a were variable, it was less useful for identification of the border between areas 3a and 3b. Clear changes in the intensity and laminar distribution of acetylcholinesterase staining occurred between areas 3a and 3b, making this a useful adjunct to the Nissl stain.

Muscarinic cholinergic receptor binding in rat hindlimb somatosensory cortex following partial deafferentation by sciatic nerve transection

Peripheral nerve injury or amputation leads to extensive changes within the central representations of the mammalian body surface. The mechanisms responsible for post-traumatic reorganization of these maps in adults may also, at least partly, underlie a more general feature of the somatosensory system--the capacity for stimulus-dependent plasticity. Acetylcholine has been implicated in both of these processes. We studied the binding of the ligands [3H]QNB and [3H]pirenzepine in rat hindlimb somatosensory cortex from 1 to 14 days following sciatic nerve transection. Although the [3H]QNB binding was not different from normal levels in tissue homogenates of the affected somatosensory cortex, differences were demonstrated when binding was measured on a layer-by-layer basis. [3H]QNB binding was changed only in certain layers, at certain times. The predominant effects appeared to be a decrease in binding in the middle layers from 4 to 14 days after the transection. Combining the [3H]QNB data with data obtained from the more M1-selective ligand [3H]pirenzepine suggested that complex changes occur among several muscarinic receptors, including receptors with non-M1 subtype characteristics. Moreover, unilateral nerve transection affects the hindlimb somatosensory regions in both hemispheres.

Chronological observations of primary somatosensory cortical maps in kittens following low thoracic (T12) spinal cord transection at 2 weeks of age

The present study investigated the reorganization of the somatosensory cortex in kittens following T12 spinal cord transection at 2 weeks of age. Multiunit electrophysiological methods were used to map the somatosensory cortex of kittens at 3, 6, and 9 weeks after the transection. The entire reorganized cortical region was driven by substitute cutaneous inputs, primarily from the trunk, at 3 weeks after spinal cord transection. Although the level of cortical responsiveness remained the same throughout the 9 weeks studied, internal trunk representation changed, and there was an increase in shoulder girdle representation and emergence of forelimb representation. Poor somatotopic and topographic order was observed in the reorganized cortex, regardless of time posttransection. Finally, trunk receptive fields displayed a wide variety of shapes, sizes, and orientations not seen in the normal cortex.

Basal forebrain lesions with or without reserpine injection inhibit cortical reorganization in rat hindpaw primary somatosensory cortex following sciatic nerve section

To test the hypothesis that cortical reorganization depends on acetylcholine and one or more of the monoamines, the hindpaw cortex was mapped in eight different groups of mature rats: (1) untreated; (2) after sciatic nerve transection; (3) after intraperitoneal injections of reserpine, to reduce the level of cortical monoamines; (4) after ibotenic acid lesion of the nucleus basalis of Meynert (NBM), to destroy cholinergic cells projecting to the cortex; (5) after reserpine treatment and transection; (6) after ibotenic acid lesion and transection; (7) after reserpine treatment and ibotenic acid lesion; and (8) after reserpine treatment, ibotenic acid lesion, and transection. Four days after transection, the cortex had reorganized in the transected group. However, this process of reorganization was prevented in transected animals with NBM lesions. Treatment with reserpine alone did not inhibit the process of reorganization, nor did it enhance the effect of NBM lesion. Nonetheless, the animals treated with reserpine and transected had higher response thresholds in the reorganized cortex than did the animals that were treated but not transected. These data suggest that acetylcholine plays an important role in the early reorganization that follows deafferentation, and that one or more of the monoamines may have other influences on reorganization of the primary somatosensory cortex of adult rats.

Age-dependent changes in acetylcholinesterase activity in the primary somatosensory cortex of the cat

As cortical reorganization in cat somatosensory cortex has been shown to be age-dependent and acetylcholinesterase and acetylcholine have been implicated in the shaping of sensory responses during the developmental process, we decided to investigate the biochemical changes that occur in acetylcholinesterase during postnatal development of the primary somatosensory cortex in normal cat. Somatosensory cortices were removed from cats at various ages between 4 and 144 postnatal days. Three fractions (total, membrane-bound and soluble) were analyzed for activity (esterase assay and sedimentation analysis) and amount of acetylcholinesterase (electrophoresis). Results indicated that both esterase activity levels and amounts were characterized by 4 distinct phases which included a large step increase in all fractions between postnatal days 10 and 12: a gradual rise between days 12 and 28: a 'dip' during the 42- to 82-day interval, and a subsequent recovery. Results may be attributed to concomitant developmental events. Furthermore, we suggest that the observed changes may relate to age-dependent differences in somatosensory cortex reorganization that occur after spinal cord transection.

Cholinergic manipulation alters stimulus-evoked metabolic activity in cat somatosensory cortex

The role of acetylcholine (ACh) in cerebral cortical activity has recently been reevaluated. It now seems clear that this neurotransmitter increases the magnitude of cortical responses. Although substantial information has been gathered regarding the role of ACh in sensory information processing, little is known about the participation of ACh in the organization of maps in the cerebral cortex. To address this issue, we used 2 methods to manipulate the supply of ACh in the somatosensory cortex of cats: 1) unilateral neurotoxic lesions of the basal forebrain and 2) unilateral topical applications of the cholinergic antagonist, atropine. For each experimental condition, the animal received an injection of 2-deoxyglucose (2DG) while identical somatic stimuli were delivered to the right and left forepaws. In the somatosensory cortex, the 2DG uptake most often occurred in the form of patches that extended from layer II to IV. When the patches were reconstructed into 2-dimensional maps of activity throughout the somatosensory cortex, they formed strips that ran in the rostrocaudal direction. The reconstructed maps revealed that the 2DG patterns in ACh-depleted and the normal cortex were similar in their overall topographic distribution. Depletion or antagonism of ACh, however, caused the stimulus-evoked metabolic label to be reduced in dimension and density. Measurements of background activity levels were obtained by using 1) cytochrome oxidase histochemistry or 2) metabolic activity values in regions of somatosensory cortex that were not specifically stimulated. This analysis indicated that background values in the ACh-depleted hemispheres were not different from those in the normal hemispheres. The absence of ACh therefore appears to reduce the cortical response to stimulation, while background activity values do not change. These observations indicate that ACh plays a significant role in the processing of sensory information and the organization of somatosensory cortical maps.

Effects of acetylcholine on single cortical somatosensory neurons in the unanesthetized rat

Experiments have been performed on unanesthetized rats using a chronic restraint system. The animal's head was held in a stereotaxic apparatus by means of two metallic tubes fixed on the skull with dental cement. Electrodes consisted of a recording micropipette (filled with 1 M NaCl and 2% Pontamine Blue) attached to a multibarreled micropipette for iontophoresis. Electrode penetrations were reconstructed on camera lucida drawings of frontal brain sections. The overall percentage of spontaneously active somatosensory neurons was 77% with a mean spontaneous activity of 5.9 impulse/s (n = 405). Yet differences were observed in the proportions of active cells as well as in the mean spontaneous activity between cortical layers (both parameters being significantly higher in layers V and VI). Comparison with results obtained under urethane anesthesia [Dykes R. W. and Lamour Y. (1988) J. Neurophysiol. 60, 703-724] shows that the percentage of the spontaneously active neurons and the mean spontaneous activity were both significantly higher in unanesthetized rats (77 vs 36%; 5.9 vs 2.6 impulse/s). Nevertheless, the laminar distribution of the most active cells was similar under both conditions. In the present study, 52.3% of the neurons (n = 380) were excited by acetylcholine and 46% (n = 198) by carbachol. Significantly larger percentages of neurons excited by acetylcholine were found in layers Vb and VIb. These effects of cholinergic agonists--observed for the first time in unanesthetized rats--differed significantly from those previously obtained under anesthesia (33 and 34% of neurons excited by acetylcholine and carbachol, respectively) [Lamour Y. et al. (1982) Neuroscience 7, 1483-1494].(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in choline acetyltransferase activity and high-affinity choline uptake, but not in acetylcholinesterase activity and muscarinic cholinergic receptors, in rat somatosensory cortex after sciatic nerve injury

Selected cholinergic markers (choline acetyltransferase, acetylcholinesterase, muscarinic acetylcholine receptor, high-affinity choline uptake) were studied in the hindlimb representation areas of the rat somatosensory cortex and within the visual cortex 1 to 63 days after unilateral transection of the sciatic nerve. In the contralateral somatosensory cortex, peripheral deafferentation resulted in a significant reduction of choline acetyltransferase activity (by 15%) 3 days after sciatic nerve injury, and in a significant reduction of high-affinity choline uptake (by 30%) 1 day after nerve transection, in comparison to untreated control rats. Investigations in individual cortical layers revealed that the decrease of both choline acetyltransferase activity and high-affinity choline uptake sites was mainly due to reductions in cortical layer V. Acetylcholinesterase activity and [3H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors were not affected by unilateral transection of the sciatic nerve. In the ipsilateral somatosensory cortex, as well as in the visual cortex at both cortical hemispheres, no significant changes in the cholinergic parameters studied could be detected. The data indicate that peripheral deafferentation of the somatosensory cortex results in a transient change of presynaptic cholinergic parameters within the affected somatosensory area as early as 1 to 3 days after the lesion; thus, they emphasize the involvement of cholinergic mechanisms in cortical reorganizational events.

Stereotaxic preparation of circumscribed cortical areas from rat brain for biochemical studies

A method for the rapid dissection of circumscribed areas of rat cortex is described. The technique does not depend on skull-derived landmarks but uses for stereotaxic orientation the cross-point of the interhemispheric gap with the caudal margin of the cortex. An application of this dissection method to the biochemical analysis of cholinergic markers within the hindlimb representation of the primary somatosensory cortex revealed that both the activity of the enzyme choline acetyltransferase as well as the binding of [3H] quinuclidinyl benzilate to muscarinic cholinergic receptors do not seem to be affected drastically three days after unilateral transection of the sciatic nerve. The only significant effect detected was a slight decrease in the activity of the choline acetyltransferase within the hindlimb representation of the primary somatosensory cortex contralateral to the transected sciatic nerve. In the primary visual cortex, the cholinergic markers investigated did not show significant alterations after sciatic nerve injury.

Cholinergic modulation of responses to single tones produces tone-specific receptive field alterations in cat auditory cortex

Acetylcholine (ACh), acting via muscarinic receptors, is known to modulate neuronal responsiveness in primary sensory neocortex. The administration of ACh to cortical neurons facilitates or suppresses responses to sensory stimuli, and these effects can endure well beyond the period of ACh application. In the present study, we sought to determine whether ACh produces a general change in sensory information processing, or whether it can specifically alter the processing of sensory stimuli with which it was "paired". To answer this question, we restricted acoustic stimulation in the presence of ACh to a single frequency, and determined single neuron frequency receptive fields in primary auditory cortex before and after this pairing. During its administration, ACh produced mostly facilitatory effects on spontaneous activity and on responses to the single frequency tone. Examination of frequency receptive fields after ACh administration revealed receptive field modifications in 56% of the cells. In half of these cases, the receptive field alterations were highly specific to the frequency of the tone previously paired with ACh. Thus ACh can produce stimulus-specific modulation of auditory information processing. An additional and unexpected finding was that the type of modulation during ACh administration did not predict the type of receptive field modulation observed after ACh administration; this may be related to the physiological "context" of the same stimulus in two different conditions. The implications of these findings for learning-induced plasticity in the auditory cortex is discussed.

Demonstration of muscarinic acetylcholine receptor-like immunoreactivity in the rat forebrain and upper brainstem

The distribution of muscarinic acetylcholine receptor protein (mAChR) in the rat forebrain and upper brainstem was described by using a monoclonal antibody (M35) raised against mAChR purified from bovine forebrain homogenates. A method is investigated for light microscopic (LM) and electronmicroscopic (EM) immunocytochemical visualization of reactivity to mAChR-proteins. Putative cholinoceptive neurons including their dendrites were found immunoreactive in the cortical mantle, hippocampus, basal ganglia, amygdala, thalamus and several midbrain regions. In the neocortex, immunoprecipitate with M35 was mainly present in layer 5 pyramidal cells, some layer 3 pyramidal neurons and layer 2 stellate cells, all including their characteristic dendritic profiles of both basal and apical dendrites. In the hippocampus, a variety of pyramidal, granular and non-pyramidal celltypes were stained in various hippocampal cell layers, in the dentate hilus and in stratum oriens of cornu ammonis. Moreover, positively reacting cells occurred in central and lateral amygdala, all parts of the basal ganglia and ventral pallidum. The thalamus was very richly provided with labeled neurons in several nuclei but notably numerous in the ventrolateral, anteroventral and geniculate nuclei. In cortex and hippocampus also some staining of astrocytes occurred. Electron microscopic study of the intracellular distribution of M35 immunoreactivity in all cases showed dense precipitates in the soma cytoplasm in close association with the golgi apparatus, but conspicuous absence near the endoplasmic reticulum. Immunoprecipitate can be followed within the dendritic tree along the microtubular transport system, up to proximal and distal postsynaptic membrane positions, apposing non labeled presynaptic endings. Muscarinic receptor subtype recognition by M35 will be discussed by comparing M35 distribution with cholinergic innervation patterns, muscarinic receptor ligand binding studies and localization of muscarinic receptor subtype mRNAs.

Quantitative study of glutamic acid decarboxylase-immunoreactive neurons and cytochrome oxidase activity in normal and partially deafferented rat hindlimb somatosensory cortex

Somatosensory cortex reorganizes following restricted deafferentation so that deprived neurons acquire new receptive fields. Electrophysiological data suggest that a decrease in inhibition might be one of the mechanisms contributing to these changes. This hypothesis was tested by evaluating quantitatively glutamic acid decarboxylase (GAD) immunoreactivity and cytochrome oxidase (CO) activity in normal and partially deafferented rat hindlimb somatosensory cortex. In normal animals, there were laminar differences in the frequencies of GAD+ cells that correlated with the levels of CO activity. Two weeks after transection of the sciatic nerve, CO levels were reduced in all layers of the hindlimb somatosensory cortex contralateral to the nerve transection whereas the frequencies of GAD+ cells were unchanged except in layer IV where a 16% decrease was observed. This observation is consistent with the hypothesis that the expression of GAD in layer IV is partially controlled by the amount of afferent input. The ability of novel inputs to develop stable patterns of excitation in deafferented somatosensory cortex may depend upon the reduction of GABAergic inhibition which follows deafferentation.

An electrophysiological laminar analysis of single somatosensory neurons in partially deafferented rat hindlimb granular cortex subsequent to transection of the sciatic nerve

A sample of 302 neurons in rat hindlimb granular cortex was studied between 2 and 3 weeks after transection and ligation of the sciatic nerve. These neurons were compared to a control sample obtained from normal rats under similar experimental conditions. After sciatic nerve transection the proportion of neurons driven by somatic stimuli applied to the hindlimb (29.1%) was not significantly different from the proportion observed in the control sample (25.8%). The proportion of neurons with cutaneous receptive fields was also the same before and after nerve transection although the proportion of neurons responding to higher threshold taps was reduced. Spontaneously active neurons were encountered more frequently after sciatic nerve transection than in the control (45% vs 32%) and their mean discharge frequency was higher (8.6 vs 6.4 imp/s). Twice as many (10.2% vs 5.2%) spontaneously active neurons whose discharge was modulated by afferent stimuli were found after sciatic nerve transection. This and several other lines of evidence suggest that the cortical neurons were released from inhibition by the sciatic nerve transection. The number of spontaneously active neurons and the mean discharge rate were increased in each cortical lamina suggesting that the increased excitability seen after deafferentation occurred in all cortical layers. Neurons throughout the portion of the hindlimb representation studied could be driven from the remaining innervated region of the hindlimb, including one and sometimes two digits, part of the palm and most of the dorsum of the foot. More neurons were driven by receptive fields on the ankle than was the case for normal rats. Most neurons with cutaneous receptive fields were distributed in the same layers as those in the control group and had normal shapes and appearances. However, there were few of small size. Most were of moderate dimensions well within the normal range. Seven examples were found with unusually large proportions of their field extending from the foot onto the ankle and covering most of the posterior quadrant of the animal. In normal cortex the largest receptive fields were found in the middle layers. This distribution was not as clear after sciatic nerve section and a mixture of large and medium-sized receptive fields occurred at each depth. Further, some neurons with cutaneous receptive fields were found above 300 micron and below 1000 micron, depths where receptive fields were uncommon in the normal sample.

Somatosensory cortical neurons with an identifiable electrophysiological signature

In both cats and rats, neurons with a distinctively narrow action potential were recognized as a small subset of all neurons isolated in the somatosensory cortex. These cells were characterized by generally having a spontaneous activity, some evidence of an afferent input, a sensitivity to glutamate but a relative resistance to depolarization block induced by glutamate and a marked insensitivity to acetylcholine. Two were filled with horseradish peroxidase (HRP) and recovered. Although others have suggested that such neurons are interneurons, following reconstruction it was apparent that the two cells filled with HRP were pyramidal cells. These observations suggest that there may be more than one class of cortical neurons with thin spikes.