Evolution of morphological and histochemical changes in the adult cat cuneate nucleus following forelimb denervation.

Morphological and histochemical changes were studied in the ipsilateral cuneate nucleus between one and 52 weeks after forelimb denervation in adult cats. The deafferented nucleus and neighboring fasciculus were noticeably reduced in size within four weeks and decreased further by 13 weeks. The intensity of acetylcholinesterase staining decreased within one week and was further reduced one month after nerve transections. This reduction in acetylcholinesterase staining was transient, approaching control levels within one year. Parvalbumin immunostaining was also altered by the nerve transections; on the deafferented side, the neuropil staining in the cuneate nucleus and fasciculus decreased, but the number of parvalbumin-positive cells was consistently greater than in the contralateral side. These cell counts returned to normal levels within one year. One month after the injury, cytochrome oxidase activity was reduced. This reduction persisted and was even more apparent after one year. In parallel, the cell clusters of the nucleus became progressively less distinct. These observations in an adult mammal indicate that peripheral nerve injury imposes molecular and morphological changes on second-order sensory neurons which evolve differentially with time. Although some changes developed rapidly after deafferentation, the onset of others was slower; and whereas some seemed irreversible, others eventually regressed. Taken together with the functional studies of others, these findings suggest that early molecular changes observed in cuneate neurons reflect adaptive reactions to lesion-induced alterations in afferent activity. Permanent deprivation of the normal input, however, would eventually lead to chronic, and perhaps irreversible, degenerative changes.

Quantitative analysis of anatomical changes in the cuneate nucleus following forelimb denervation: a stereological morphometric study in adult cats.

The consequences on the cuneate nucleus of the transection of the major nerves of the forelimb in adult cats were studied quantitatively with stereological procedures on celloidin-embedded material. The cell cluster region of the normal cuneate was 2.93 +/- 0.41 (mean +/- SD) mm3. This volume decreased significantly 4.5 weeks after the injury. The decrease amounted to 11-23%, and persisted until the longest survival studied (36 weeks). Despite this reduction in nuclear volume, there was no significant loss of neurons. The normal cell cluster region contained 48.8 +/- 7.3 (mean +/- SD) x 1000 neurons. Neuronal density showed a significant 16.8% mean increase between 4 weeks and 36 weeks of deafferentation. Perikaryal volume decreased by an average of 15.2%, between 1 and 36 weeks, but since cell bodies make only a small fraction of the total volume, much of the overall volume reduction observed must be attributed to a concomitant reduction of the neuropil. The distribution of cell size suggested that there are two populations of neurons, presumably corresponding to interneurons and projection neurons. This bimodal distribution was maintained after deafferentation, but after 4 weeks it shifted to the left, showing an increase in small cells and a decrease of large cells. These findings demonstrate that peripheral deafferentation causes a substantial and persistent decrease of cytoplasmic mass in the cuneate nucleus, involving both neuropil and neuronal cell bodies, but does not lead to neuron loss, at least up to 36 weeks after injury. These effects suggest that the altered synaptic input and trophic support subsequent to deafferentation leave the cuneate nucleus in a permanently compromised, albeit seemingly stable, state.

The organization of two cutaneous submodalities in the forearm region of area 3b of cat somatosensory cortex.

Electrophysiological data were collected from the forelimb region of somatosensory cortex in barbiturate-anesthetized cats using low-impedance microelectrodes in both perpendicular and nearly horizontal penetrations. The data within cytoarchitectonic area 3b were classified according to receptive field locus and submodality. The forearm cortex was shown to consist of segregated regions of slowly adapting and rapidly adapting neurons arranged in a pattern unique to each animal. The general organization of each submodality consisted of interdigitating bands of submodality-specific neurons. Horizontal penetrations confirmed data obtained from vertical penetrations. The somatotopic representation within the forearm region was arranged in a way that was complementary to the submodality segregation. Each part of the forearm appeared to be represented by both the slowly adapting and rapidly adapting neurons so that area 3b contained two complete maps of the forearm. Yet, the slowly and rapidly adapting maps were organized so that the same body part was found in only one part of the cortex; the slowly and rapidly adapting regions for each body part tended to be adjacent to one another. Area 1 was incompletely sampled; however, there appeared to be a separate representation of the cutaneous surface located there.

Magnification functions and receptive field sequences for submodality-specific bands in SI cortex of cats.

Electrophysiological data were collected from the forelimb region of somatosensory cortex in barbiturate-anesthetized cats using low-impedance microelectrodes in long slanting trajectories. Subsequently, the brains were fixed and stained with thionin to locate the electrode trajectories and to correlate cytoarchitecture with neural activity. Confirming earlier experiments, regions of cortex, preferentially responsive to one submodality of afferent input, were observed to stretch in mediolateral bands across SI. Separate magnification functions were calculated for each band. In the forelimb region the magnification functions for the deep and cutaneous RA bands could be approximated by linear functions while the data for the cutaneous SA band was best described by a second-order equation. At least in the forelimb region this magnification function applied only along the anteroposterior dimension of the band because the body representation is anisotropic; along the anteroposterior dimension of the band, receptive field loci may change only millimeters on the skin when the electrode moves through 1 mm of cortex, whereas when an electrode moves in a mediolateral direction they change centimeters on the forearm for an equivalent cortical distance. Each representation is separated from the others by a transition zone having a minimal width less than 200 micron. Within the transition zones, neurons often have receptive field loci which are a compromise between loci found in the adjacent representations. In these zones responses are often difficult to elicit. Electrode penetrations encountering these transition zones contain data which fulfill the receptive field and modality criteria for the boundary of a cortical column. However, within the mediolateral length of each submodality band we have found no unit with a dimension greater than 200 micron which has a boundary or transition zone. Evidence for units of smaller size cannot be obtained because of limitations in the data collection techniques. Thus, within each submodality-specific band, large portions of the somatotopic map appear to be a mediolateral continuum while relatively abrupt changes occur in receptive field locus and in submodality when the electrode passes through a transition zone between adjacent bands.

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)

Cytoarchitecture and responsiveness of the medial ansate region of the cat primary somatosensory cortex.

To understand its relationship to somatosensory areas in other species, we studied the rostral bank of the medial ansate sulcus in adult cats. Neurons in the shoulder and upper part of the sulcal wall responded to low-threshold cutaneous stimuli much like neurons on the crown of the gyrus, whereas neurons in some deeper portions of the sulcus required more intense but innocuous somatic stimuli. Because we found much of the body surface re-represented in this area, we suggest that, besides the representation in area 3b, there is another cutaneous representation of the hindlimb and trunk located on the gyral crown near the medial end of the medial ansate sulcus and of the forelimb and trunk within the medial ansate sulcus. Posterior to this second cutaneous representation, many parts of the body were also represented in regions activated by more intense stimuli and having a different cytoarchitecture, suggesting that they were part of another body representation. Area 3b and the shoulder of the gyrus were distinguished by relatively intense acetylcholinesterase staining of layers III and IV. In the wall of the sulcus, all layers except layer I were uniformly stained to a point where electrophysiological recordings showed the cortex to be unresponsive, whereupon the outer two-thirds of layer I became very pale. Neurons activated by afferents from knee joints were found only in a small area; we did not find a mediolateral band serving joint afferents as is reported in primates. These data suggest that cat somatosensory cortex differs in some ways from primates but that it contains multiple representations of the body, as do most other mammals.

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.

Functional regions within the map of a single digit in raccoon primary somatosensory cortex.

Electrophysiological recordings were made at a large number of sites in the primary somatosensory cortex of six anesthetized raccoons. A high density of penetrations (110-229 per animal), within or near the representation of the fourth digit, allowed identification of three cortical regions with different physiological properties: a glabrous zone, containing a highly detailed, somatotopically ordered representation of the glabrous surface of the digit; rostral to this a claw-dominant zone, in which the neurons at most penetrations respond to stimulation of the claw of the fourth digit, but may also receive input from the hairy skin or surrounding glabrous skin; and a more rostral multidigit zone, in which the neurons respond to stimulation of two to five digits, with the dominant digit usually being the one represented caudally (i.e., the fourth digit at most of the sites sampled here). Claw-dominant zones with receptive fields restricted to digit three or five are also found rostral to the representations of the glabrous skin of the corresponding digit. The glabrous and claw-dominant zones constitute a complete map of the fourth digit. The multidigit region presumably is a separate map, since its neurons have different spatial convergence, higher thresholds, and a lower incidence of slowly adapting inputs than those in the claw-dominant and glabrous zones. A comparison between animals with lesions of the basal forebrain and intact animals found no differences in the organization of these zones or in the responses to peripheral input, suggesting that cholinergic inputs to the cortex are not essential to these properties. The detailed description of these regions and the proposed terminology should resolve some inconsistencies in the use of the term "heterogeneous zone" in this species.

Differential control of cortical activity by the basal forebrain in rats: a role for both cholinergic and inhibitory influences.

Using microdialysis and high-performance liquid chromatography, we measured acetylcholine (ACh) release simultaneously from two cortical sites in anesthetized rats. One site was always in the somatosensory cortex, and the other was in either the visual or the motor cortex. After baseline measurements were obtained, selected sites in the basal forebrain (BF) were stimulated to increase ACh release. Some BF sites provoked more release in one microdialysis probe than in the other, suggesting some degree of corticotropic organization of the cholinergic projections from the BF. BF sites optimal for release from the visual cortex were separated from optimal sites for release from the somatosensory cortex by greater distances than were the best sites for release from the somatosensory and the motor cortex. Stimulation of a single BF site often provoked similar release from the latter two cortical areas. Electrical stimulation of the BF also modified cortical neuronal activity. Activation of some BF sites provoked an intense discharge of many neurons in the vicinity of the cortical recording electrode, and the same stimulus site in the BF provoked release of large amounts of ACh in the cortex. Stimulation of other BF sites produced strong inhibition of ongoing cortical activity and no increase in cortical ACh release. When other sites were stimulated, they had no effect or they generated stereotyped bursting patterns in the cortex without any observable effect on ACh release. BF sites that generated inhibition of cortical neural activity were generally located near the sites that activated the cortex and provoked release of ACh. These data suggest an elaborate control of the sensory cortex by a mechanism involving both gamma-aminobutyric acid-containing and cholinergic neurons of the BF.

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.

The somatotopic organization of the ventroposterior thalamus of the squirrel monkey, Saimiri sciureus.

Multiunit microelectrode mapping techniques were used to investigate the organization of the somatosensory thalamus in squirrel monkeys. Receptive fields and response characteristics were determined for closely spaced recording sites along arrays of electrode penetrations that passed through the ventral thalamus dorsoventrally, rostrocaudally, or lateromedially. The results were related to thalamic architecture and led to the following conclusions: (1) A large, single, systematic representation of the body surface occupied most or all of the ventroposterior nucleus, VP. The nucleus was further defined by a distinct cytoarchitectonic appearance, produced by densely packed, deeply stained neurons. (2) Recording sequences in VP were characterized by (a) abrupt shifts in receptive field locations over short recording distances indicating that the electrode had crossed discontinuities or folds in the representation, (b) long sequences of overlapping receptive fields indicating regions of continuous representation and the maintenance of adjacency in the map, and (c) similar receptive field locations for sites along the trajectory of a penetration indicating regions of isorepresentation. Major somatotopic discontinuities were associated with crossing narrow cell-poor laminae that partially divided VP into subnuclei related to the hand, foot, trunk, and tail in lateral VP and the face in medial VP. Somatotopic discontinuities occurred for electrode penetrations in all three planes, but discontinuities were greater and more frequent for lateromedial electrode penetrations. Lines of isorepresentation and gradual change were most extensive in the rostrocaudal and dorsoventral planes. We hypothesize that the disruptions, regions of isorepresentation, and regions of gradual change result from the thickening, splitting, and folding of a two-dimensional representation of the skin surface to occupy a three-dimensional volume. (3) The magnifications of various skin surfaces in VP were variable so that some skin surfaces, especially the tips of the digits, occupied relatively large portions of the nucleus, while other skin surfaces such as the trunk activated little tissue. It appeared that regions of isorepresentation varied in extent according to magnification factor and position in the map. (4) Within VP, neurons could be classified as slowly adapting or rapidly adapting to maintained skin indentation. Each type of neuron formed small groups or clusters in the nucleus so that several successive recording sites typically encountered one type before a sequence of the other type was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Oral use of interferon-alpha stimulates ISG-15 transcription and production by human buccal epithelial cells.

ISG-15 is a 15-kDa protein encoded by an interferon (IFN)-stimulated gene (ISG), which is transcriptionally regulated by IFN-alpha and IFN-beta. Considered as part of the cytokine network, ISG-15 has the potential to amplify the immunomodulatory effects of these IFNs by enhancing IFN-gamma production, natural killer cell proliferation, and lymphokine-alphactivated killer cell cytotoxicity. To understand better the mechanism(s) of action of orally administered IFN-alpha, we have studied the effect of IFN-alpha on ISG-15 gene expression by human buccal epithelial cells (BEC). For in vitro studies, ISG-15 mRNA and protein levels were measured in BEC incubated for 0.5, 2, and 9 h with 100 or 1,000 IU/ml of human lymphoblastoid IFN-alpha. For in vivo studies, ISG-15 mRNA was measured in BEC samples collected at baseline, and 0.5, 2, and 9 h after 5-20 min of oral rinsing with 10 ml of IFN-alpha (1,000 IU/ml). ISG-15 mRNA was measured by reverse transcriptase polymerase chain reaction (RT-PCR), and ISG-15 protein production by Western Blot analysis. IFN-alpha augmented BEC ISG-15 gene expression in a concentration dependent manner both in vivo and in vitro. We conclude that orally administered IFN-alpha exerts its immunomodulatory effects in humans in part by upregulating the production of ISG-15 by BEC, thereby enhancing the immune reactivity of mucosa-associated lymphocytes.

Interferon-alpha upregulates gene expression of aquaporin-5 in human parotid glands.

Aquaporins are a family of homologous membrane proteins that function as highly selective water channels. Aquaporin-5 (AQP5) is uniquely present in lacrimal and salivary glands, where it accounts for normal tear and saliva production. We tested the hypothesis that orally administered human interferon-alpha (HuIFN-alpha) benefits persons with xerostomia by augmenting the production of AQP5 protein by parotid gland epithelium. Cells from three human parotid glands were cultured with and without human lymphoblastoid IFN-alpha, and assayed for AQP5 mRNA levels by reverse transcriptase polymerase chain reaction (RT-PCR), and AQP5 protein levels by Western blot. Intracellular localization of AQP5 protein was done using confocal microscopy. The functional integrity of the glandular tissue was confirmed by RT-PCR analysis of alpha-amylase 1 and basic proline-rich protein transcripts. AQP5 was constitutively expressed in human parotid gland tissue, with AQP5 protein restricted to the plasma membranes and cytoplasmic vesicles of acinar cells. IFN-alpha augmented AQP5 transcription and protein production in a concentration-dependent manner, and increased the size of intensity of staining of AQP5-containing cytoplasmic vesicles in acinar cells. We conclude that IFN-alpha upregulates AQP5 gene expression in human parotid acinar cells in vitro. To our knowledge, this is the first demonstration that IFN-alpha regulates the gene expression of an aquaporin.

Changes in cortical acetylcholine release in the rat during day and night: differences between motor and sensory areas.

By sampling simultaneously from two microdialysis probes placed in the left and right hindlimb somatosensory cortex, or in the somatosensory and visual or in the somatosensory and motor cortices, we compared the release of acetylcholine in functionally different regions. Samples were taken hourly from freely moving, adult male Sprague-Dawley rats for periods of 10-24h. A generalized increase in acetylcholine release occurred in all cortical regions with the transition to the night-time period of wakefulness and activity; however, the change was significantly greater in the two sensory regions (56%) than in the motor cortex (20%). Decrements in release during the active period seldom decreased the amount released below the values observed during sleep. During the active period, the amount of acetylcholine released in the somatosensory cortex was strongly correlated with the amount released in the contralateral somatosensory region and was only slightly less well correlated with the amount released in either the visual or motor cortex. The correlation between release in the somatosensory and motor cortex was not present during the day, when rats habitually sleep. These data confirm that a global change in the level of acetylcholine release occurs with a transition in behavioural state; however, because the change is not equal in all areas and, because the correlation between motor and sensory cortex can be uncoupled, it seems likely that there are additional mechanisms available for independent control of acetylcholine release within specific cortical regions.

The effects of strychnine on neurons in cat somatosensory cortex and its interaction with the inhibitory amino acids, glycine, taurine and beta-alanine.

In area 3b of primary somatosensory cortex, neurons may be classified as either rapidly adapting or slowly adapting to sustained stimuli and may be differentiated further by the presence or absence of a receptive field and by their threshold of activation. It is also possible to use the rate of adaptation of the background activity to a sustained stimulus to divide the cortex into slowly adapting regions or rapidly adapting regions. By blocking GABA-mediated inhibition with iontophoretically administered bicuculline methiodide, others have observed an increase in receptive field size in rapidly adapting regions but not in slowly adapting regions. The present study was designed to look for a different inhibitory transmitter which might control receptive field size in slowly adapting regions. Iontophoretically delivered strychnine was employed as an antagonist because it interferes with glycine-like inhibitory transmitters such as glycine, taurine and beta-alanine. Pharmacological tests were performed on 157 neurons in two series of experiments. In the first series three effects were documented. (i) In rapidly adapting regions, the size of the receptive field increased in 11 out of 25 cases whereas none of the 20 receptive fields tested in slowly adapting regions enlarged. (ii) In 13 of 24 cases a receptive field was revealed for previously unresponsive neurons in rapidly adapting regions whereas only 5 of 22 unresponsive cells tested in slowly adapting regions developed a receptive field. (iii) In 15 of 25 cells with receptive fields tested in rapidly adapting zones, strychnine reduced the threshold for somatic stimuli but only 8 of 20 cells isolated in slowly adapting zones showed this effect. In a second series of experiments, the effect of beta-alanine, glycine and taurine was examined on neurons of the rapidly adapting regions. beta-Alanine and taurine reduced the excitability of all neurons tested. Glycine inhibited most neurons. However, strychnine only antagonized the inhibitory effects of beta-alanine on responses to peripheral stimuli (9 of 11 cases). When neurons could not be driven by peripheral stimuli, the inhibition of spontaneous or glutamate-induced activity could not be blocked by strychnine (0 of 18 cases). We suggest that glycine-like amino acids contribute to the control of receptive field size and the control of neuronal excitability in rapidly adapting regions but not in slowly adapting regions. Our data suggest that strychnine-sensitive synapses are limited only to a subset of cortical neurons driven by somatic inputs.

Acetylcholine permits long-term enhancement of neuronal responsiveness in cat primary somatosensory cortex.

Acetylcholine (ACh) was administered iontophoretically to single neurons in cat somatosensory cortex. Using extracellular recording techniques, neuronal responsiveness was determined at regular intervals from the number of action potentials produced either by iontophoretically applied glutamate or by tactile stimulation of the cutaneous receptive field. The responses were altered in only 21% (13/61) of the neurons following the application of ACh alone. In contrast, 75% (66/88) of the neurons displayed altered responses during administration of ACh simultaneously with either iontophoretically administered glutamate or with tactile stimulation of the receptive field. Forty-seven percent (29/62) of the responses potentiated in the presence of ACh remained enhanced for periods lasting from 8 min to over 1 h. The responsiveness of cortical neurons to afferent inputs changes during the reorganization of somatotopic maps that occurs after deafferentation, and perhaps during some forms of learning. As ACh has been implicated in some of these processes, it may be that the changes in responsiveness observed here following iontophoretically applied ACh are similar to those which facilitate the acquisition of neuronal responses to altered or novel afferent inputs.

Evidence for homeostatic adjustments of rat somatosensory cortical neurons to changes in extracellular acetylcholine concentrations produced by iontophoretic administration of acetylcholine and by systemic diisopropylfluorophosphate treatment.

We describe the responses of single units in the awake (24 cells) or urethane-anesthetized (37 cells) rat somatosensory cortex during repeated iontophoretic pulses (1.0 s, 85 nA) of acetylcholine, both before and after systemic treatment with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (i.p., 0.3-0.5 LD50). The time-course of the response to acetylcholine pulses differed among cortical neurons but was characteristic for a given cell. Different time-courses included monophasic excitatory or inhibitory responses, biphasic (excitatory-inhibitory, inhibitory-excitatory, excitatory-excitatory, and inhibitory-inhibitory), and triphasic (excitatory-excitatory-inhibitory, inhibitory-inhibitory-excitatory, and inhibitory-excitatory-inhibitory) responses. Although the sign and time-course of the individual responses remained consistent, their magnitude fluctuated across time; most cells exhibited either an initial increase or decrease in response magnitude followed by oscillations in magnitude that diminished with time, gradually approaching the original size. The time-course of the characteristic response to an acetylcholine pulse appeared to determine direction and rate of change in response magnitude with successive pulses of acetylcholine. Diisopropylfluorophosphate treatment, given 1 h after beginning repeated acetylcholine pulses, often resulted in a gradual increase in spontaneous activity to a slightly higher but stable level. Superimposed on this change in background activity, the oscillations in the response amplitude reappeared and then subsided in a pattern similar to the decay seen prior to diisopropylfluorophosphate treatment. Our results suggest that dynamic, homeostatic mechanisms control neuronal excitability by adjusting the balance between excitatory and inhibitory influences within the cortical circuitry and that these mechanisms are engaged by prolonged increases in extracellular acetylcholine levels caused by repeated pulses of acetylcholine and by acetylcholinesterase inhibition. However, this ability of neurons in the cortical neuronal network to rapidly adjust to changes in extracellular levels of acetylcholine questions the potential efficacy of therapeutic treatments designed to increase ambient levels of acetylcholine as a treatment for Alzheimer's disease or to enhance mechanisms of learning and memory.

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.

The basal forebrain cholinergic system in the raccoon.

The distribution of neurons displaying choline acetyltransferase (ChAT) immunoreactivity was examined in the raccoon basal forebrain using a rabbit antiserum and a monoclonal antibody. Alternating sections were used for Nissl staining. ChAT-positive neurons were arranged in a continuous mass extending from the medial septum to the caudal pole of the pallidum. Based upon spatial relations to fibre tracts, the clustering of neuronal groups, and cytological criteria, the basal forebrain magnocellular complex can be subdivided into several distinct regions. Although clear nuclear boundaries were often absent, the ChAT-positive neurons were divided into: the nucleus tractus diagonalis (comprising pars septi medialis, pars verticalis and pars horizontalis); nucleus praeopticus magnocellularis; substantia innominata; and the nucleus basalis of Meynert. Comparison with Nissl-stained sections indicated the presence of varying proportions of non-cholinergic neurons clustered or arranged loosely within these basal forebrain subdivisions. These data provide a structural basis for studies concerned with the topographical and physiological aspects of the raccoon basal forebrain cholinergic projections and its comparison with the basal forebrains of other species.