Distinct temporal activity patterns in the rat M1 and red nucleus during skilled versus unskilled limb movement

The mammalian motor system contains multiple interconnected supraspinal networks, but little is known about their relative roles in producing different movements and behaviors, particularly given their apparently fused activity in primates. We tested whether the task context, as well as using a phylogenetically older mammal, rats, could distinguish the separate contributions of these networks. We obtained simultaneous multi-single neuron recordings from the forelimb motor cortex and magnocellular red nucleus as rats performed two contextually different, but kinematically similar, forelimb reach-like tasks: highly learned, skilled reaching for food through a narrow slot, a task requiring extensive training, versus the swing phases of treadmill locomotion. In both the M1 and the mRN, large subpopulations of neurons peaked in their spike firing rates near the onset and the end of the swing phase during treadmill locomotion. In contrast, neural subgroups in the two areas displayed different temporal sequences of activity during the skilled reaching task. In the mRN, the majority of task-modulated neurons peaked in their firing rate in the middle of the reach when the rat was preparing to project the arm through the slot, whereas large subgroups of M1 neurons displayed elevated firing rates during the initial and terminal phases of the reach. These results suggest that motor-behavioral context can alter the degree of overlapping activity in different supraspinal sensorimotor networks. Moreover, results for the skilled reaching task in rats may have highlighted a distinct processing role of the rubral complex: adapting natural muscle synergies across joints and limbs to novel task demands, in concert with cortically based learning.

Neural prosthetic devices for quadriplegia

Research into the use of neural prosthetic devices to treat paralysing disorders is expanding rapidly. Functional electrical stimulation of muscles is now used to activate electrically paralysed muscles to restore movements such as the hand-grasp. Sensory feedback signals recorded from peripheral nerves are used to improve the control of functional electrical stimulation systems. Functional electrical stimulation may also be used to stimulate neural circuits in the spinal cord. Electrical recordings from the brain will soon improve patients' control of neural prostheses. Non-invasive electroencephalographic recordings can currently be used to move a cursor across a computer screen. The restoration of near-to-normal limb movement control will, however, require recordings from large numbers of neurons within the brain. Preliminary feasibility has been demonstrated in experimental animals.

Real-time prediction of hand trajectory by ensembles of cortical neurons in primates

Signals derived from the rat motor cortex can be used for controlling one-dimensional movements of a robot arm. It remains unknown, however, whether real-time processing of cortical signals can be employed to reproduce, in a robotic device, the kind of complex arm movements used by primates to reach objects in space. Here we recorded the simultaneous activity of large populations of neurons, distributed in the premotor, primary motor and posterior parietal cortical areas, as non-human primates performed two distinct motor tasks. Accurate real-time predictions of one- and three-dimensional arm movement trajectories were obtained by applying both linear and nonlinear algorithms to cortical neuronal ensemble activity recorded from each animal. In addition, cortically derived signals were successfully used for real-time control of robotic devices, both locally and through the Internet. These results suggest that long-term control of complex prosthetic robot arm movements can be achieved by simple real-time transformations of neuronal population signals derived from multiple cortical areas in primates.

Principal component analysis of neuronal ensemble activity reveals multidimensional somatosensory representations

Principal components analysis (PCA) was used to define the linearly dependent factors underlying sensory information processing in the vibrissal sensory area of the ventral posterior medial (VPM) thalamus in eight awake rats. Ensembles of up to 23 single neurons were simultaneously recorded in this area, either during long periods of spontaneous behavior (including exploratory whisking) or controlled deflection of single whiskers. PCA rotated the matrices of correlation between these n neurons into a series of n uncorrelated principal components (PCs), each successive PC oriented to explain a maximum of the remaining variance. The fact that this transformation is mathematically equivalent to the general Hebb algorithm in linear neural networks provided a major rationale for performing it here on data from real neuronal ensembles. Typically, most information correlated across neurons in the ensemble was concentrated within the first 3-8 PCs. Each of these was found to encode distinct, and highly significant informational factors. These factor encodings were assessed in two ways, each making use of fact that each PC consisted of a matrix of weightings, one for each neuron. First, the neurons were rank ordered according to the locations of the central whiskers in their receptive fields, allowing their weightings within different PCs to be viewed as a function of their position within the whisker representation in the VPM. Each PC was found to define a distinctly different topographic mapping of the cutaneous surface. Next, the PCs were used to weight-sum the neurons' simultaneous activities to create population vectors (PVs). Each PV consisted of a single continuous time series which represented the expression of each PC's 'magnitude' in response to stimulation of different whiskers, or during behavioral events such as active tactile whisking. These showed that each PC functioned as a feature detector capable of selectively predicting significant sensory or behavioral events with far greater statistical reliability than could any single neuron. The encoding characteristics of the first few PCs were remarkably consistent across all animals and experimental conditions, including both spontaneous exploration and direct sensory stimulation: PC1 positively weighted all neurons, mainly according to their covariance. Thus it encoded global magnitude of ensemble activity, caused either by combined sensory inputs or intrinsic network activity, such as spontaneous oscillations. PC2 encoded spatial position contrast, generally in the rostrocaudal dimension, across the whole cutaneous surface represented by the ensemble. PC3 more selectively encoded contrast in an orthogonal (usually dorsoventral) dimension. A variable number of higher numbered PCs encoded local position contrast within one or more smaller regions of the cutaneous surface. The remaining PCs typically explained residual 'noise', i.e. the uncorrelated variance that constituted a major part of each neuron's activity. Differences in behavioral or sensory experience produced relatively little in the PC weighting patterns but often changed the variance they explained (eigenvalues) enough to alter their ordering. These results argue that PCA provides a powerful set of tools for selectively measuring neural ensemble activity within multiple functionally significant 'dimensions' of information processing. As such, it redefines the 'neuron' as an entity which contributes portions of its variance to processing not one, but several tasks.

Real-time control of a robot arm using simultaneously recorded neurons in the motor cortex

To determine whether simultaneously recorded motor cortex neurons can be used for real-time device control, rats were trained to position a robot arm to obtain water by pressing a lever. Mathematical transformations, including neural networks, converted multineuron signals into 'neuronal population functions' that accurately predicted lever trajectory. Next, these functions were electronically converted into real-time signals for robot arm control. After switching to this 'neurorobotic' mode, 4 of 6 animals (those with > 25 task-related neurons) routinely used these brain-derived signals to position the robot arm and obtain water. With continued training in neurorobotic mode, the animals' lever movement diminished or stopped. These results suggest a possible means for movement restoration in paralysis patients.

Simultaneous encoding of tactile information by three primate cortical areas

We used simultaneous multi-site neural ensemble recordings to investigate the representation of tactile information in three areas of the primate somatosensory cortex (areas 3b, SII and 2). Small neural ensembles (30-40 neurons) of broadly tuned somatosensory neurons were able to identify correctly the location of a single tactile stimulus on a single trial, almost simultaneously. Furthermore, each of these cortical areas could use different combinations of encoding strategies, such as mean firing rate (areas 3b and 2) or temporal patterns of ensemble firing (area SII), to represent the location of a tactile stimulus. Based on these results, we propose that ensembles of broadly tuned neurons, located in three distinct areas of the primate somatosensory cortex, obtain information about the location of a tactile stimulus almost concurrently.

Topographic and laminar organizations of the incertocortical pathway in rats

The topographic and laminar organizations of the projection system from the zona incerta to the neocortex were studied by using both retrograde and anterograde methods in the rat. Injections of retrograde fluorescent tracers into different cortical areas revealed that the incertocortical projection neurons have a rough topographic organization with respect to their cortical targets. Furthermore, the incertocortical projecting neurons were found mainly in the dorsal and rostral subdivisions of the zona incerta, and none were found in the ventral subdivision. In cases which included three different fluorescent tracers injected into the frontal, the parietal and the occipital cortices, retrogradely single-labelled cells were found intermingled within the dorsal zona incerta. Very few double-labelled cells were noted, and triple-labelled cells were absent. Injections of anterograde tracers into the dorsal zona incerta demonstrate that labelled fibres traverse the striatum and terminate most densely in the outer half of layer I of the neocortex. The density of incertocortical terminals was greatest in the somatosensory cortex, while the innervation of visual cortical areas was sparse. Very fine and sparse bouton-like swellings of labelled incertocortical fibres were found running parallel along the pial surface. Since it has recently been shown that the incertocortical projections derive from GABAergic neurons, the present results suggest that the diffuse and roughly topographic projection from the zona incerta to the cerebral cortex may play an inhibitory role in widespread areas of cerebral cortex. This inhibitory action may preferentially target the distal dendrites of cortical neurons, since the majority of incertocortical terminals were found in the outer part of layer I of the neocortex.

Neonatal whisker removal reduces the discrimination of tactile stimuli by thalamic ensembles in adult rats

Simultaneous recordings of up to 48 single neurons per animal were used to characterize the long-term functional effects of sensory plastic modifications in the ventral posterior medial nucleus (VPM) of the thalamus following unilateral removal of facial whiskers in newborn rats. One year after this neonatal whisker deprivation, neurons in the contralateral VPM responded to cutaneous stimulation of the face at much longer minimal latencies (15.2 +/- 8.2 ms, mean +/- SD) than did normal cells (8.8 +/- 5.3 ms) in the same subregion of the VPM. In 69% of these neurons, the initial sensory responses to stimulus offset were followed for up to 700 ms by reverberant trains of bursting discharge, alternating in 100-ms cycles with inhibition. Receptive fields in the deafferented VPM were also atypical in that they extended over the entire face, shoulder, forepaw, hindpaw, and even ipsilateral whiskers. Discriminant analysis (DA) was then used to statistically evaluate how this abnormal receptive field organization might affect the ability of thalamocortical neuronal populations to "discriminate" somatosensory stimulus location. To standardize this analysis, three stimulus targets ("groups") were chosen in all animals such that they triangulated the central region of the "receptive field" of the recorded multineuronal ensemble. In the normal animals these stimulus targets were whiskers or perioral hairs; in the deprived animals the targets typically included hairy skin of the body as well as face. The measured variables consisted of each neuron's spiking response to each stimulus differentiated into three poststimulus response epochs (0-15, 15-30, and 30-45 ms). DA quantified the statistical contribution of each of these variables to its overall discrimination between the three stimulus sites. In the normal animals, the stimulus locations were correctly classified in 88.2 +/- 3.7% of trials on the basis of the spatiotemporal patterns of ensemble activity derived from up to 18 single neurons. In the deprived animals, the stimulus locations were much less consistently discriminated (reduced to 73.5 +/- 12.6%; difference from controls significant at P < 0.01) despite the fact that much more widely spaced stimulus targets were used and even when up to 20 neurons were included in the ensemble. Overall, these results suggest that neonatal damage to peripheral sense organs may produce marked changes in the physiology of individual neurons in the somatosensory thalamus. Moreover, the present demonstration that these changes can profoundly alter sensory discrimination at the level of neural populations in the thalamus provides important evidence that the well-known perceptual effects of chronic peripheral deprivation may be partially attributable to plastic reorganization at subcortical levels.

Low-dose amphetamine elevates movement-related firing of rat striatal neurons

To study the striatal role in amphetamine's stimulant effects on motor behavior, single neurons were recorded in the dorsolateral striatum of unrestrained rats before and after amphetamine injection (0.5 or 1.0 mg/kg, i.p.). Comparisons of firing were made between similar motor behaviors before and after injection. Mean locomotor firing rates increased 5% to 276% within 30 min after injection and reversed within 2 h. Firing related to specific head- or forelimb-movements, which were similar in all measured parameters before and after injection, was elevated several hundred percent after injection and then reversed, the time course paralleling that of the stimulant effect on these movements. Elevation of movement-related striatal firing rates by low doses of the psychomotor stimulant is in line with established increases in firing rate normally observed for striatal neurons related to motor behavior.

The estrous cycle and the olivo-cerebellar circuit. I. Contrast enhancement of sensorimotor-correlated cerebellar discharge

Neuromodulation of Purkinje (Pnj) cell responses by monoamines and estrous hormones is well characterized in the cerebellum at the cellular level, but not at the level of neuronal circuits in the awake behaving animal. In the present study, simultaneous recordings of up to 16 single neurons from within the olivo-cerebellar circuit were obtained through chronically implanted microwire electrode bundles: arrays of Pnj cell like neurons (Pnj cln) in the paravermal cerebellum and neurons within the afferent source of its climbing fiber input, the rostral dorsal accessory olive (rDAO), were recorded simultaneously across 3-20 consecutive estrous cycles during constant or variable speed treadmill locomotion performance tasks. Over 90% of Pnj cln recorded during treadmill locomotion exhibited significant increases (80%) or decreases (10%) in activity correlated with the stance phase of locomotion. In contrast, cells from the rDAO increased activity during speed changes or when the rat failed to maintain the treadmill speed (position slip). On the night of behavioral estrus, which is triggered by elevations in circulating levels of 17 beta-estradiol and progesterone, the magnitude of both increases and decreases in stance-correlated Pnj cln activity increased by 85-115%. These results are consistent with our previous findings that 17 beta-estradiol and progesterone enhance excitatory and inhibitory responses of single Pnj cells to locally applied glutamate and GABA, respectively. This dual enhancement of both excitatory and inhibitory effects, apparently paradoxical at the cellular level, produced a marked heightening of the contrast of the neural population "signal" at the neuronal ensemble level. Furthermore, the stance-correlated discharge of Pnj cln during estrus preceded that during diestrus by approximately 120 ms. Frame-by-frame video analysis also suggested that the swing phase of the step cycle was shortened on estrus compared with diestrus (low hormone state). In addition, rDAO discharge correlated with speed change or position slip was also significantly increased (P < 0.05) on the night of behavioral estrus versus diestrus. Thus, estrus was associated with changes in both the amplitude and the timing of Pnj cln and rDAO discharge correlated with specific behavioral events. These estrous-associated changes in Pnj cell activity were well correlated (r = 0.84) with faster responses to random changes in treadmill speed, a motor performance task. Together, these findings suggest that the increases in the contrast of stance-correlated Phj cln discharge observed following peak circulating levels of sex steroid hormones are associated with improved motor performance on a randomly moving treadmill.

The estrous cycle and the olivo-cerebellar circuit. II. Enhanced selective sensory gating of responses from the rostral dorsal accessory olive

This study demonstrates that gating of responses of the rostral dorsal accessory olive (rDAO) to somatosensory stimulation varies across the estrous hormone cycle of the rat. The rDAO has been suggested as an "error" or event signal generator for the cerebellar cortex. Selective sensory gating of input to this structure may underlie this error signalling function. In the present study, as many as 23 single neurons were recorded simultaneously from either the forepaw or the snout areas of the rDAO. Responses of these neurons to electrical stimulation of peripheral afferents were determined during active movement or non-movement conditions. These results were then compared across the estrous cycle or after administration of the estrous hormones 17 beta-estradiol (E2) and/or progesterone (P) to rats on diestrus or following E2 priming. Elevations in circulating estrous hormones produced greater excitatory responses of rDAO neurons to stimulation during non-movement, and, conversely, enhanced inhibition of rDAO activity during active movement of the stimulated peripheral area compared with control diestrous conditions, suggesting that selective gating processes to the rDAO are enhanced by estrous hormones. The results of this study suggest that the night of behavioral estrus is associated with enhanced selective sensory gating processes associated with improved detection and processing of error signals.

Active tactile exploration influences the functional maturation of the somatosensory system

1. The hypothesis that active exploration of objects is required for the functional maturation of neuronal circuits subserving tactile perception was tested by subjecting 8- to 11-day old rats to a complete unilateral section of the facial nerve. This procedure selectively abolished whisker protraction movements without affecting the sensory innervation of the facial vibrissae, the tactile organs used by rats to discriminate object texture and shape. 2. Six to 14 mo after the facial nerve section, simultaneous recordings of neuronal ensembles located in the ventral posterior medial nucleus (VPM) of the thalamus revealed a marked reduction in receptive field (RF) size (in terms of number of whiskers), and the formation of abnormal RF surrounds, spanning the face and contiguous body regions. In addition, the directional organization of VPM RFs, represented by caudal to rostral shifts in RF centers over 30 ms following whisker stimulation, was greatly reduced in these animals. 3. These results suggest that neonatal active tactile exploration is required to establish normal spatiotemporal patterning of neuronal RFs within the somatosensory system, and consequently, to develop normal tactile perception.

Calbindin-containing non-specific thalamocortical projecting neurons in the rat

Immunoreactivity for calcium binding proteins was used to demonstrate the neurochemical profiles of non-specific thalamocortical neurons located in the ventromedial nucleus, the centrolateral nucleus, and the nucleus reuniens that project to the somatosensory cortex in the adult rat. Cortical injections of fluorescent tracers combined with immunohistochemistry for calcium binding proteins revealed that retrogradely labeled neurons in these three thalamic nuclei are immunoreactive for calbindin. The present results suggest the presence of a chemically distinct non-specific thalamocortical system which terminates in the neocortex.

Effects of systemic and local ethanol on responses of rat cerebellar Purkinje neurons to iontophoretically applied gamma-aminobutyric acid

The goals of this study were: (1) to determine the effects of acute systemic or local application of ethanol (ETOH) on the response of cerebellar Purkinje cells (P-cells) to iontophoretically applied gamma-aminobutyric acid (GABA) and (2) to characterize the effects of Ro15-4513, a putative antagonist of ETOH-GABA interactions, on ETOH-induced changes in GABA responsiveness. Male Sprague-Dawley rats (230-370 g) were anesthetized with halothane and implanted with intraperitoneal catheters for administration of ETOH (1.0-2.0 g/kg), before the recording session. Extracellular activity of single P-cells was recorded with the central barrel of a five-barrel micropipette, the other barrels of which were used for microiontophoresis of GABA and electro-osmosis of ETOH at the recording site. Spontaneous discharge and response of P-cells to GABA were monitored during a pre-ETOH control and for 1-1.5 h after systemic or electro-osmotic administration of ETOH. Transient suppression of spontaneous P-cell discharge was usually observed within 4-8 min of systemic ETOH injection. This effect lasted 2-4 min in 10 out of 19 rats tested. GABA-mediated inhibitory responses of cerebellar P-cells were increased by 45-50% relative to pre-ETOH values at 10 and 90 min post-ETOH injection. Prior administration of the imidazobenzodiazepine Ro15-4513 (4-6 mg/kg) failed to antagonize either the ETOH-induced enhancement of GABA-mediated inhibition or the transient inhibition of spontaneous P-cell activity rat cerebellar P-cell produced by ETOH. In these studies, electro-osmotically applied ETOH produced a potent suppression of spontaneous P-cell activity which precluded further augmentation of unit responses to GABA. These results show that doses of systemically administered ETOH which are mildly intoxicating in the awake, behaving animal, enhance the inhibitory action of GABA on cerebellar P-cell discharge.

Effects of systemic and local ethanol on responses of rat cerebellar Purkinje neurons to iontophoretically applied norepinephrine and gamma-aminobutyric acid

The goal of the present study was to determine the effect of acute ethanol (ETOH), administered intraperitoneally or electro-osmotically, on norepinephrine (NE) induced increases in gamma-aminobutyric acid (GABA) mediated inhibition of single cerebellar Purkinje neurons (P-cells). Male Sprague-Dawley rats (230-370g) were anesthetized with halothane and implanted with an intraperitoneal catheter for systemic administration of ETOH (1.0-1.5 g/kg) prior to the recording session. Extracellular activity of single P-cells was recorded before and after iontophoresis of GABA and NE using five-barrel glass micropipettes. GABA was administered at the recording site by microiontophoretic pulses before, during and after continuous iontophoretic application of NE. Spontaneous discharge, GABA responses and NE-GABA interactions in P-cells were monitored for each experiment before and 1-1.5 h following systemic administration of ETOH. As in our previous reports administration of NE, at low ejection currents (10-60 nA), augmented GABA mediated suppression of P-cell spontaneous discharge. Between 10 and 60 min after injection of ETOH, this NE induced augmentation of GABA inhibition was further potentiated. This potentiation involved increases in both the magnitude and the duration of the GABA inhibition observed after NE alone. NE-induced augmentation of GABA inhibition persisted for 2-13 min longer after ETOH administration than in the pre-ETOH control period. Local electro-osmotic application of ETOH, which resulted in strong depression of spontaneous activity and caused small increases in GABA-mediated inhibition, did not directly potentiate NE-induced augmentation of GABA action. These results indicate that NE-mediated augmentation of GABA inhibition of P-cell activity is potentiated following systemic, but not local, ETOH administration.

Sensorimotor encoding by synchronous neural ensemble activity at multiple levels of the somatosensory system

Neural ensemble processing of sensorimotor information during behavior was investigated by simultaneously recording up to 48 single neurons at multiple relays of the rat trigeminal somatosensory system. Cortical, thalamic, and brainstem neurons exhibited widespread 7- to 12-hertz synchronous oscillations, which began during attentive immobility and reliably predicted the imminent onset of rhythmic whisker twitching. Each oscillatory cycle began as a traveling wave of neural activity in the cortex that then spread to the thalamus. Just before the onset of rhythmic whisker twitching, the oscillations spread to the spinal trigeminal brainstem complex. Thereafter, the oscillations at all levels were synchronous with whisker protraction. Neural structures manifesting these rhythms also exhibited distributed spatiotemporal patterns of neuronal ensemble activity in response to tactile stimulation. Thus, multilevel synchronous activity in this system may encode not only sensory information but also the onset and temporal domain of tactile exploratory movements.

Development of direct GABAergic projections from the zona incerta to the somatosensory cortex of the rat

The postnatal development of direct thalamocortical projections from the zona incerta of the ventral thalamus to the whisker representation area of the rat primary somatosensory cortex was investigated. Cytoarchitectonic analysis based on Nissl staining, cytochrome oxidase histochemistry and immunohistochemistry for glutamic acid decarboxylase, GABA, parvalbumin and calbindin D28K revealed that the zona incerta can be clearly distinguished from surrounding diencephalic structures from the day of birth. Moreover, four distinct anatomical subdivisions of this nucleus were identified: the rostral, dorsal, ventral and caudal. Of these, the ventral subdivision is by far the most conspicuous, containing the highest density of neurons, and the highest levels of cytochrome oxidase, glutamate decarboxylase, GABA, parvalbumin and calbindin D28K. In contrast, the dorsal, rostral and caudal subdivisions contain fewer cells, lower levels of glutamic acid decarboxylase and GABA and very few parvalbumin-positive and calbindin-positive neurons. Small injections of rhodamine coated microspheres or Fluoro-gold in the primary somatosensory cortex of animals at different stages of development revealed the existence of retrogradely labeled neurons in the rostral and dorsal subdivisions of the zona incerta from postnatal day 1. At this age, retrogradely labeled cells were also found in the ventral lateral, ventral posterior medial, posterior medial, centrolateral, ventral medial and magnocellular subdivision of the medial geniculate nuclei of the dorsal thalamus. The density of the incertocortical projection reaches its maximum between the first and second postnatal weeks, decreasing subsequently, until an adult pattern of labeling is achieved. Tracer injections combined with immunohistochemistry revealed that the majority of the incertocortical projection derives from GABAergic neurons, implying a potentially inhibitory role for the incertocortical projection. These results demonstrate that the rat trigeminal system contains parallel thalamocortical pathways of opposite polarity, emerging from both the dorsal (glutamatergic, excitatory) and ventral (GABAergic, inhibitory) thalamus since the day of birth. As such, these findings suggest that, contrary to the classical notion, not only the dorsal but also the ventral thalamus may play a special role in both cortical maturation and function.

Behavioral associations of neuronal activity in the ventral tegmental area of the rat

The ventral tegmental area (VTA) is a central element in a system that mediates the reinforcing properties of natural stimuli (such as food), brain stimulation, and drugs of abuse. Although considerable effort has been applied to understanding how drugs of abuse influence this system, relatively little work has examined its function during conditioned reinforcement tasks in awake, behaving animals. In the present studies, bundles of four to eight microwire electrodes were chronically implanted in the VTA or prefrontal cortex (PFC) of male Wistar rats. Following recovery from surgery, simultaneous recordings from multiple single neurons and unit clusters were obtained in rats pressing a lever for a sucrose solution on a fixed-ratio schedule of reinforcement. Consistent with the hypothesis that these neurons encode information related to motivation, most of the neurons in both VTA and PFC showed significant modulation of firing rate associated with one or more events occurring within the response/reinforcement cycle. These events included lever presses, onset and end of a tone signaling sucrose delivery, and onset and end of sucrose consumption. Significant decreases in firing rate were observed, coincident with onset of the tone and sucrose delivery, or with consumption. These decreases were sustained through the end of sucrose consumption. A number of neurons also discharged bursts of activity associated with individual lever presses. These findings provide a clear demonstration that VTA neuronal activity is modulated during motivated behavior. Similar information about events within the ongoing response/reinforcement cycle appears to be distributed through many neurons within the VTA, and may be mirrored in target structures such as PFC.

Spatiotemporal structure of somatosensory responses of many-neuron ensembles in the rat ventral posterior medial nucleus of the thalamus

Classically, the rat ventral posterior medial (VPM) nucleus of the thalamus has been considered as a simple passive relay for single-whisker information to the primary somatosensory cortex (SI). However, recent reports have suggested that the VPM could contain a much more coarsely coded and spatiotemporally complex representation of the rat whisker pad. To address this possibility properly, we have carried out chronic simultaneous recordings of large numbers (up to 23) of single neurons, distributed across the entire VPM, in both awake and lightly anesthetized adult rats. Quantitative, computer-based reconstruction of receptive fields (RFs) revealed that single VPM neurons exhibit significant responses to discrete stimulation of as many as 20 single whiskers (mean +/- SD RF size, 13.7 +/- 4.8 whiskers). By defining multiple response magnitude (RM) thresholds it was possible to subdivide these large VPM RFs quantitatively into a prominent center (mean +/- SD, 1.41 +/- 0.70 whiskers, RM > 95%) and an excitatory surround (up to 18 whiskers, RM < 95%). VPM neurons exhibited both short-latency responses (SLRs, from 4 to 10 msec poststimulus) and/or long-latency responses (LLRs, 15-25 msec), each followed by inhibitory responses. Though LLRs were weaker (mean +/- SD, 47.19 +/- 33.34 Hz) than SLRs (119.63 +/- 50.12 Hz), they often defined RFs that differed considerably from those defined by the SLRs of the same cell. In particular, VPM cells with short-latency RFs centered in caudal whiskers (e.g., C1, D1, E1) showed a poststimulus time-dependent shift of these RF centers toward the rostral whiskers (e.g., C4, D4, E4). These caudal-to-rostral (C-->RF shifts occurred in neurons with the largest RFs of our sample (17.2 +/- 2.4 whiskers). On the other hand, VPM cells with short-latency RFs centered in rostral whiskers had the smallest RFs (13.1 +/- 4.1 whiskers) and usually did not exhibit time-dependent RF center shifts. Multivariate analysis revealed that these two groups of VPM neurons, C-->R shifting and rostral position (RP) cells, could be statistically distinguished according to a combination of three RF attributes (short-latency RF center location, RF size, and magnitude of RF center shift). Quantitative, computer-based reconstruction of "population response maps" demonstrated that the "place" coding for each single whisker in the VPM involved a distinct weighted contribution from a large proportion of the simultaneously recorded neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential phasic modulation of short and long latency afferent sensory transmission to single neurons in the primary somatosensory cortex in behaving rats

Single neurons were recorded in the forepaw area of the primary somatosensory (SI) cortex of awake rats during rest and running behavior. Locomotor step cycle dependent changes of the transmission of the short (4.5 +/- 0.1-10.5 +/- 0.1 ms, SEURs) and the long (10.6 +/- 0.6-28.5 +/- 2.3 ms, LEURs) latency evoked unit responses were tested by generating post-stimulus time histograms of these neurons' responses to stimulation through electrodes chronically implanted under the skin of the forepaw. Times of footfall were determined by way of frame-by-frame analyses of video recordings, and peri-footfall histograms were generated to differentiate a total of 55 SI cortical neurons into two types: footfall responsive (n = 37) and footfall unresponsive (n = 18) neurons. Peri-footfall gating patterns were determined for both types of cells. The SEURs and the LEURs showed significantly different phasic sensory modulation patterns. A major difference in sensory modulations between footfall responsive and footfall unresponsive cells was noted during the swing phase of the locomotor step cycle. In footfall responsive cells, the SEURs were suppressed most strongly just after footfall, while the LEURs were phasically suppressed following both footfall and footoff. The SEURs were disinhibited during the swing phase. In footfall unresponsive cells, the SEURs were tonically suppressed during the whole locomotor step cycle phases. However, the LEURs were phasically facilitated during the early swing phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential phasic modulation of short and long latency afferent sensory transmission to single neurons in the ventroposterolateral thalamus in behaving rats

Single neurons were recorded in the forepaw area of the ventroposterolateral (VPL) thalamus of awake rats during rest and running behaviors. Locomotor step cycle dependent changes of the transmission of the short (4.0 +/- 0.1-10.1 +/- 0.3 ms, SEURs) and the long (10.2 +/- 0.2-26.0 +/- 2.1 ms, LEURs) latency somatic sensory responses were tested by generating post-stimulus time histograms of these neurons' responses to stimulation through electrodes chronically implanted under the skin of the forepaw. The magnitudes of firing during these responses were measured and normalized as percent increases over background firing. Times of footfall were determined through frame-by-frame analyses of video recordings and peri-footfall histograms were generated to differentiate a total of 40 VPL thalamic neurons into two types, footfall responsive (n = 21) and unresponsive (n = 19) neurons. Perifootfall gating patterns were determined for both types of cells. The SEURs and the LEURs showed significantly different phasic sensory modulation patterns across the locomotor step cycle. Major difference of the sensory modulations between footfall responsive and footfall unresponsive cells was noted during swing phase of the locomotor step cycle. In footfall responsive cells, the SEURs were suppressed most strongly just after footfall, while the LEURs were tonically suppressed during late stance and swing phases. The SEURs were disinhibited during the swing phase, while the LEURs were disinhibited during the middle stance phase. In footfall unresponsive cells, the LEURs were suppressed more strongly around footfall event than the SEURs were.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic and distributed properties of many-neuron ensembles in the ventral posterior medial thalamus of awake rats

The traditional view that the map of the face in the ventral posterior medial thalamus (VPM) is static and highly discrete was derived largely from qualitative studies that reported only small, robust, and nonoverlapping receptive fields (RFs). Here, by using more quantitative techniques, we have provided evidence for an alternative hypothesis: the RFs in the VPM are large and overlapping and tend to shift as a function of post-stimulus time. These results were obtained through simultaneous recordings of up to 23 single neurons across the whisker representation in the VPM of rats. Under both awake and anesthetized conditions, these neurons responded robustly at short (4-6 ms) and/or long (15-25 ms) latencies to discrete vibromechanical stimulation of single facial whiskers. Computer graphics were used to construct three-dimensional plots depicting the magnitudes of neuronal responses to stimulation of each of several whiskers as a function of post-stimulus time. These "spatiotemporal RFs" demonstrated that (i) the RFs of VPM neurons are quite large, covering up to 20 whiskers and (ii) the spatial locations of these RFs may shift dramatically over the first 35 ms of post-stimulus time, especially from the caudal-most to the rostral-most whiskers on the face. These results suggest that the VPM contains a dynamic and distributed representation of the face, in which stimulus information is coded in both spatial and temporal domains.

Induction of immediate spatiotemporal changes in thalamic networks by peripheral block of ascending cutaneous information

Peripheral sensory deprivation induces reorganization within the somatosensory cortex of adult animals. Although most studies have focused on the somatosensory cortex, changes at subcortical levels (for example the thalamus) could also play a fundamental role in sensory plasticity. To investigate this, we made chronic simultaneous recordings of large numbers of single neurons across the ventral posterior medial thalamus (VPM) in adult rats. This allowed a continuous and quantitative evaluation of the receptive fields of the same sample of single VPM neurons per animal, before and after sensory deprivation. Local anaesthesia in the face induced an immediate and reversible reorganization of a large portion of the VPM map. This differentially affected the short latency (4-6 ms) responses (SLRs) and long latency (15-25 ms) responses (LLRs) of single VPM neurons. The SLRs and LLRs normally define spatiotemporally complex receptive fields in the VPM. Here we report that 73% of single neurons whose original receptive fields included the anaesthetized zone showed immediate unmasking of SLRs in response to stimulation of adjacent cutaneous regions, and/or loss of SLRs with preservation or enhancement of LLRs in response to stimulation of regions just surrounding the anaesthetized zone. This thalamic reorganization demonstrates that peripheral sensory deprivation may induce immediate plastic changes at multiple levels of the somatosensory system. Further, its spatiotemporally complex character suggests a disruption of the normal dynamic equilibrium between multiple ascending and descending influences on the VPM.

Somatotopic maps within the zona incerta relay parallel GABAergic somatosensory pathways to the neocortex, superior colliculus, and brainstem

Neurons located in the zona incerta (ZI) of the ventral thalamus project to several regions of the central nervous system, including the neocortex, superior colliculus, and brainstem. However, whether these projections are functionally segregated remains unknown. This issue was addressed here by combining neuroanatomical tracers with immunohistochemical staining for gamma-aminobutyric acid (GABA) and/or parvalbumin, coupled with neurophysiological mapping. GABAergic projection neurons were found in four distinct subregions of the ZI including: (1) the rostral pole of the ZI, from which neurons project to the supragranular layers of the neocortex (especially layer I); (2) the dorsal subregion of the ZI, where both ascending projections to the neocortex and descending projections to the pretectal area were observed; (3) the ventral subregion of the ZI, whose neurons project to the superior colliculus; and 3) the caudal pole of the ZI, from which descending projections to the lower brainstem and spinal cord were observed. Somatotopic representations of the contralateral cutaneous periphery were also identified in the dorsal and ventral subregions of ZI, both of which were found to receive dense direct afferent projections from the trigeminal complex, and dorsal column nuclei. These results suggest that the rat ZI is a major somatosensory relay in the ventral thalamus, carrying feed-forward inhibitory signals to neocortical and subcortical targets, in parallel with the excitatory somatosensory pathways.

Neonatal whisker removal in rats stabilizes a transient projection from the auditory thalamus to the primary somatosensory cortex

A normally transient cross-modal thalamocortical projection from the magnocellular subdivision of the medial geniculate nucleus (MGm) to the primary somatosensory (SI) cortex of rats was found to remain unchanged throughout adulthood following unilateral removal of whiskers in newborn animals. The normal MGm projection to the auditory cortex is not lost in these neonatally whisker-deprived adults rats but some of the MGm neurons send collaterals to both primary auditory and SI cortices. Parallel electrophysiological experiments demonstrated the multimodal character of some MGm neurons, since they responded to both auditory and cutaneous stimulation. These results suggest that the areal distribution in the cortex of thalamocortical projections arising from a multimodal thalamic nucleus, such as the MGm, may be determined during early postnatal development by the normal flow of sensory information from the periphery to the thalamus and that an early postnatal somatosensory deprivation may prevent the normal withdrawal of a cross-modal projection from the MGm to the SI.

Thalamic plasticity induced by early whisker removal in rats

Neurophysiological mapping was used to study the effects of early postnatal removal of mystacial whiskers on the organization of cutaneous receptive fields (RFs) within the ventral posterior thalamus (VP) of rats. This sensory deprivation induced an extensive reorganization of the thalamus, as reflected in larger facial or continuous overlapping face-body RFs and a higher proportion of slowly-adapting responses. Mapping of the VP of young rats (2-3 weeks old) demonstrated that the functional organization of the immature VP thalamus resembles that of the sensory-deprived VP, suggesting that an early postnatal sensory deprivation may interfere with the normal process of thalamic development.

Ontogeny of corticocortical projections of the rat somatosensory cortex

Rhodamine-coated microspheres (RCMs) were injected into the primary somatosensory cortex (SI) of rats ranging in age from postnatal (PN) day 1 to adulthood. Ipsilateral corticocortical and callosal projections within the SI were identified as early as PN day 1. At the end of the first PN week, ipsilaterally projecting neurons located in sublayer VIb were the first to assume an adult-like pattern of connectivity. Injections at subsequent postnatal ages revealed that an adult pattern of lamination of ipsilateral corticocortical projections within the SI is established between PN weeks 2 and 3, comprising projection neurons from layers II/III, layer V, and sublayer VIb. Therefore, local interactions in the rat SI are mediated not only by pyramidal neurons of layers III and V, derived from the cortical plate, but also by a subpopulation of ontogenetically older neurons located in the sublayer VIb, which may correspond to the subplate neurons of other species. Overall, these results suggest the existence of three independent short-range corticocortical systems of projections within the rat SI, which differ in terms of the laminar distribution and ontogenetic origin of their cells.

A region in the dorsolateral striatum of the rat exhibiting single-unit correlations with specific locomotor limb movements

1. To examine the activity of single units in the lateral striatum of the awake rat with respect to sensorimotor function, 788 units were recorded during locomotion and passive testing. The focus of this report is on 138 units (18%) that fired in relation to sensorimotor activity of a single limb. The remaining units were related to other body parts (16%), to general body movement (38%), or were unresponsive (28%). 2. Firing rates of limb-related units were near zero during resting behavior but increased markedly during treadmill locomotion. Each of the 138 units exhibited a rhythmic pattern of discharge in phase with the locomotor step cycle. Passive testing revealed that 86/97 units tested (89%) responded to passive manipulation of a single limb, exhibiting increased firing rates. Of these, 77 (90%) were related to contralateral and 9 (10%) to ipsilateral limbs. Sixty-one units (71%) were related to a forelimb and 25 (29%) to a hindlimb. Of the 86 units responding to passive manipulation. 34/48 units tested (71%) also responded to cutaneous stimulation of the same limb but no other part of the body. 3. To study in greater detail the rhythmic unit discharges in phase with the locomotor step cycle, computer-synchronized videotape recordings were used to generate perimovement time histograms constructed around discrete locomotor movements of each limb (n = 17 units). Activity of each unit was shown to be restricted to a specific portion of a particular limb's step cycle. The majority of units discharged throughout (8 units) or during a portion of (3 units) the swing phase, whereas other units fired during a portion of stance (3 units), footfall (2 units), or foot off (1 unit). 1. The specificity of unit firing was further demonstrated by the finding that rhythmic discharges, related to discrete locomotor limb movements in the forward direction, were completely absent during spontaneous deviations such as backward or disrupted locomotion. 5. Units related to limb movement were located in the far lateral, especially the dorsolateral, subregion of the striatum. This subregion extend rostrocaudally from A-P +1.6 to -1.0 mm relative to bregma. No clear somatotopic organization was observed, but this issue requires further study. 6. These results show that functional representations of individual limbs can be demonstrated in the lateral striatum of the rat, within a subregion containing terminals of projections from somatic sensorimotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Movement induced modulation of afferent transmission to single neurons in the ventroposterior thalamus and somatosensory cortex in rat

Single neurons were simultaneously recorded in the forepaw areas of the primary somatosensory (SI) cortex and ventroposterolateral (VPL) thalamus of awake rats during rest and running behaviors. Movement dependent changes in somatic sensory transmission were tested by generating post-stimulus histograms of these neurons' responses to stimulation through electrodes chronically implanted under the skin of the forepaw, while the animal ran on a timed treadmill. As viewed in post-paw-stimulus histograms, the evoked unit responses (EURs) could be differentiated into short (4.5 +/- 0.1-10.9 +/- 0.2 ms) and longer (12.9 +/- 0.4-31.3 +/- 0.9 ms) latency components ("SEURs" and "LEURs", respectively). The magnitudes of firing during these responses were measured and normalized as percent increases over background firing. By comparison with resting behavior, treadmill movement suppressed both SEURs and LEURs in the thalamus, as well as the cortex. The SEURs, however, were much more strongly suppressed in the SI cortex (-48.3 +/- 2.7%) than in the VPL thalamus (-28.1 +/- 6.7%). By contrast, similar magnitudes of suppression of LEURs were found in the SI (-25.8 +/- 8.6%) and VPL (-26.5 +/- 11.1%). These results suggest that the suppression of LEURs observed in the SI cortex may result from modulatory actions on subcortical circuits. Major suppression of SEURs, on the other hand, may occur intracortically, with a minor component occurring subcortically. Thus, VPL thalamus and SI cortex in the rat appear to be differentially subject to movement related modulation of sensory transmission.

A major direct GABAergic pathway from zona incerta to neocortex

Retrograde fluorescent tracers were used to demonstrate a previously unknown but sizable direct gamma-aminobutyric acid (GABA)-containing neuronal pathway from the zona incerta to the neocortex in rats. This incertocortical pathway was found to project bilaterally to the entire neocortex and exhibited a rough corticotopic organization. Many of the zona incerta neurons projecting to the parietal and occipital cortices could also be immunohistochemically stained with antibodies to glutamic acid decarboxylase and GABA. Few of these neurons were immunoreactive to tyrosine hydroxylase antibodies, which identify dopamine-containing neurons. Injections in the frontal and entorhinal cortices labeled many neurons near or within the dopaminergic A13 subdivision of the zona incerta. In addition, the incertocortical system was found to be significantly larger during early postnatal (2 to 3 weeks) development. The projection pattern of this newly discovered pathway resembles that of the monoaminergic and cholinergic systems, arising from the brainstem and forebrain, suggesting possible similarities of function.

Ketamine effects on somatosensory cortical single neurons and on behavior in rats

The neurophysiological effects of ketamine were studied at the single-neuron level in the somatosensory cortex of unanesthetized rats behaving in a treadmill movement paradigm. Chronically implanted 25-microns microwire electrodes were used to record spontaneous discharge, sensory responses, and sensorimotor-correlated activity of single neurons before and after ketamine administration. Extracellular action potentials of up to six single neurons were simultaneously recorded for several days, allowing ketamine effects to be tested repeatedly on the same neurons. Videotaped recordings obtained during each experiment were used to measure both the sensorimotor properties of the neurons and the changes in these measures caused by different doses of ketamine. Behaviorally, ketamine produced restless-hyperactive behavior at subanesthetic doses from 5 to 20 mg/kg (intramuscularly). At higher doses (30-50 mg/kg) the rats became cataleptic and immobile after the initial hyperactive period. Whereas the spontaneous rates of most neurons were reduced or unchanged after subanesthetic doses, a subgroup (27% of the total) exhibited markedly increased firing rates. This excitation was of a tonic nature, persisting for a dose-dependent duration in a manner that was not correlated with any of the behavioral effects of the drug. In further analyses, ketamine suppressed the sensory responses of virtually all of the recorded neurons. In particular, low doses of ketamine suppressed "sensorimotor" firing (mainly proprioceptive responses) of neurons in relation to active limb movement. It also suppressed virtually all neuronal sensory responses to the sudden onset of treadmill movement, although the time-course of this effect varied from neuron to neuron. These results reveal two separable effects of ketamine: (a) a strong inhibition of all somatosensory responsiveness in this area and (b) a tonic excitatory influence expressed heterogeneously on a subgroup of neurons. This coexistence of cortical neuronal excitation and sensory suppression in the same cortical region may explain in part the mechanism of dissociative anesthesia and hallucinatory side effects observed in humans during emergence from ketamine anesthesia.

Mapping the effects of motor cortex stimulation on somatosensory relay neurons in the rat thalamus: direct responses and afferent modulation

Single unit recordings were used to map the spatial distribution of motor (MI) cortical influences on thalamic somatosensory relay nuclei in the rat. A total of 215 microelectrode penetrations were made to record single neurons in tracks through the medial and lateral ventroposterior (VPM and VPL), ventrolateral (VL), reticular (nRt), and posterior (Po) thalamic nuclei. Single units were classified according to their: 1) location within the nuclei, 2) receptive fields, and 3) response to standardized microstimulation in deep layers of the forepaw-forelimb areas of MI cortex. For mapping purposes, only short latency (1-7 msec) excitatory neuronal responses to the MI cortex stimulation were considered. Percentages of recorded thalamic neurons responsive to the MI stimulation varied considerably across nuclei: VL: 42.6%, nRt: 23.0%, VPL: 15.7%, VPM: 9.3%, and Po: 3.9%. Within the VPL, most responsive neurons were found in "border" regions, i.e., areas adjacent to the VL, and (to a lesser extent) the nRt and Po thalamic nuclei. The same parameters of MI cortical stimulation were used in studies of corticofugal modulation of afferent transmission through the VPL thalamus. A condition-test (C-T) paradigm was implemented in which the cortical stimulation (C) was delivered at a range of time intervals before test (T) mechanical vibratory stimulation was applied to digit No. 4 of the contralateral forepaw. The time course of MI cortical effects was analyzed by measuring the averaged evoked unit responses of the thalamic neurons to the T stimuli, and plotting them as a function of C-T intervals from 5-50 msec. Of the 30 VPL neurons tested during MI stimulation, the average response to T stimulation was decreased a mean 43%, with the suppression peaking at about 30 msec after the C stimulus. This suppression was more pronounced in the VPL border areas (-52% in areas adjacent to VL and nRt) than in the VPL center (-25%).

Modulation of afferent transmission to single neurons in the ventroposterior thalamus during movement in rats

Single units (n = 135) were recorded in the ventroposterolateral nucleus of the thalamus in awake rats. The responsiveness of neurons to sensory activation during rest and treadmill locomotion was tested by stimulation through electrodes implanted under the skin of the forepaw. The averaged evoked unit response was suppressed by a mean 31% during movement as compared with rest. This is to be compared with the mean 71% sensory suppression observed previously in the somatosensory cortex. These findings are consistent with the hypothesis that sensory information ascending to, and within the SI cortex is successively modulated at several levels during movement.

Mapping the effects of SI cortex stimulation on somatosensory relay neurons in the rat thalamus: direct responses and afferent modulation

We have used single-unit recording techniques to map the spatial distribution of the primary somatosensory (SI) cortical influences on thalamic somatosensory relay nuclei in the rat. A total of 193 microelectrode penetrations were made to record single neurons in tracks through the medial and lateral ventroposterior (VPL and VPM), ventrolateral (VL), posterior (Po), and reticular (nRt) thalamic nuclei. Single units were classified according to their (1) location within the nuclei, (2) receptive fields, and (3) response to standardized microstimulation in deep layers of the SI cortical forepaw areas. The SI stimulation produced short-latency (1- to 7-msec) excitatory responses in different percentages of neurons recorded in the following thalamic nuclei: VPL, 42.0%; Po, 25.0%; nRt, 16.4%; VL, 13.6%; and VPM, 9.9%. Within the VPL, the highest proportion of responsive neurons was found in the anterior region. Although most of the VL region was unresponsive, the caudal subregion bordering the rostral VPL showed some responsiveness (13.6% of neurons). In general, the spatial pattern of corticothalamic influences appeared to reciprocate the known thalamocortical connection patterns, but with a heterogeneity that was unpredicted. The same parameters of SI cortical stimulation were used in studies of corticofugal modulation of afferent transmission through the VPL thalamus. A condition-test (C-T) paradigm was implemented in which the cortical stimulation (C) was delivered at a range of time intervals before test (T) mechanical vibratory stimulation was applied to digit 4 of the contralateral forepaw. The time course of cortical effects was analyzed by measuring the averaged evoked unit responses of thalamic neurons to the T stimuli, and plotting them as a function of C-T intervals from 5 to 50 msec. Of the 20 VPL neurons tested during SI stimulation, the average response to T stimulation was decreased a mean of 36%, with the suppression peaking (at 49% inhibition of the afferent response) about 15 msec after the C stimulus. Considerable rostrocaudal variation was observed, however. Whereas neurons in the rostral VPL (near VL) were strongly inhibited (-69%), neurons in the middle and caudal VPL exhibited facilitations at long and short C-T intervals, respectively. This study establishes a specific projection system from the forepaw region of SI cortex to different subregions of the VPL thalamus, producing specific temporal patterns of sensory modulation.

Mapping the effects of motor cortex stimulation on single neurons in the dorsal column nuclei in the rat: direct responses and afferent modulation

The major aim of this study was to define the topography of descending motor cortical influences on the dorsal column nuclei of the rat. A total of 1442 single neurons were recorded throughout the rat cuneate (Cu) or external cuneate (ECu) nuclei and classified according to their: 1) location within the nuclei, 2) receptive fields, and 3) response to standardized deep layer microstimulation in the forelimb area of the motor (MI) cortex. Excitatory neuronal responses to this MI cortical stimulation ranged in latency from 2-28 msec, with 80% of responses in the 4-12 msec range. Overall, the rostral extremity of the Cu (0.5-1.0 mm rostral to the obex), and the ECu contained the highest percentage of recorded neurons responding to the MI cortical stimulation (61%). By contrast, only 11% responded in the middle subregion of the Cu (+0.4 to -0.9 mm relative to the obex), and 28% responded in the caudal-most subregion (1.0-2.0 mm caudal to obex). A similar paradigm was used to investigate the topography of MI corticofugal modulation of afferent transmission through the Cu and ECu. The MI cortical stimulation was used as a conditioning (C) stimulus in a condition-test (C-T) paradigm in which the test (T) stimulus was standardized mechanical vibration of digit No. 4 of the contralateral forepaw. A total of 30 cells from different subregions were analyzed by measuring the averaged evoked unit responses to the T-stimuli, and plotting them as a function of C-T intervals from 0-50 msec.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol withdrawal increases sensory responsiveness of single somatosensory cortical neurons in the awake, behaving rat

This study investigated the effects of ethanol withdrawal on sensory responses of single neurons recorded in the somatosensory (SI) cortex of awake, behaving, ethanol-dependent rats. Eleven rats were fed an ethanol-containing liquid diet for 90-120 days, while 11 weight-matched controls were pair-fed an equivalent sucrose containing diet to equalize caloric intake. Single SI cortical neurons in the chronically treated rats were recorded continuously over several hours after withdrawal from ethanol, and after reintoxication induced by intraperitoneal injection of 10% (v/v) ethanol solution. Intoxication and withdrawal related changes in sensory responsiveness of these neurons were quantitatively measured by stimulating through electrodes chronically implanted under the skin of the forepaw. Sensory response histograms constructed from these stimuli showed a characteristic pattern, typically consisting of two early excitatory peaks (E1a and E1b), followed by an inhibition (I1), and in some neurons, a late excitatory response (E2). As withdrawal advanced, the sensory response histograms exhibited marked increases in the magnitudes of the E1a, I1, and E2 responses, coupled with a reduced spontaneous discharge rate. These changes are similar to, but quantitatively greater than, those which have previously been observed in normal and control rats during "immobile arousal" behaviors, which can be evoked when an experimenter holds the animals still, producing an immobile "aversive arousal." In withdrawing animals this same "holding" manipulation tended to markedly exacerbate and accelerate behavioral and neurophysiological signs of withdrawal. By contrast, the same manipulations had little effect when carried out during light intoxication or early stages of withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex steroids modulate motor-correlated increases in cerebellar discharge

Adult female rats implanted with a microelectrode drive unit were trained to walk on a computer-controlled treadmill apparatus (10 s on every 20 s for 2 h) during recording of single Purkinje neurons in the paravermal area of the anterior cerebellum. Vigorous increases in the firing rate of individual units were found to be correlated with movement of specific limbs in particular stages of the step cycle during treadmill locomotion. Both spontaneous and motor-evoked discharge of individual Purkinje neurons were monitored before and after s.c. injection of either 17 beta estradiol (E2, 100 ng/kg) or progesterone (P, 50 micrograms). The percent increase in firing rate during locomotion versus rest was determined as a measure of the evoked:spontaneous discharge ratio. Drug-induced changes in this ratio indicate differential effects on the individual parameters, rather than simple excitatory or inhibitory effects. For all neurons tested, E2 augmented the movement-evoked discharge over pre-E2 control levels. The onset for this effect occurred at 15 min post-steroid, with a peak response noted at 30-35 min post-steroid. By 60-90 min, a partial recovery of the evoked:spontaneous ratio was noted, although absolute increases in both parameters were still observed, indicating long-term effects on neuronal activity. These effects were independent of the stage of the estrous cycle. In contrast, P decreased absolute firing rates of Purkinje cells during stationary and locomotor phases. However, the evoked:spontaneous ratio was decreased to an even greater degree. The latency for this effect was 9-12 min, with recovery to control levels of response seen at 30 min post-steroid. This response was typical of cells tested on estrus and diestrus 1. Cells tested on proestrus or diestrus 2, when E2 levels are increasing, were not modulated by P using the above paradigm.

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. II. An in vitro study of the effects of iontophoretically applied norepinephrine on LH neuronal responses to gamma-aminobutyric acid (GABA)

The preceding studies demonstrated that norepinephrine (NE) can consistently augment synaptically mediated (70%) and gamma-aminobutyric acid (GABA)-induced (69%) inhibitory responses of lateral hypothalamic (LH) neurons in vivo. The present experiments further characterized the interactions of NE with LH neuronal responses to GABA in terms of alpha- and beta-receptor mechanisms and demonstrated the utility of the in vitro LH tissue slice preparation as a model for future extra- and intracellular studies of NE modulatory phenomena. Extracellular activity of LH cells was recorded from diencephalic slices (450 microns) incubated in artificial cerebrospinal fluid at 33 degrees C. Interactions between iontophoretically applied NE, isoproterenol (ISO) or phenylephrine (PE) and responses of LH neurons (n = 64) to GABA microiontophoresis were quantitated and characterized using computer-generated ratemeter and histogram records. This analysis revealed two distinct actions of NE on GABA-induced responses of LH neurons. In 8 of 32 cells tested (25%), locally applied NE markedly enhanced inhibitory responses to GABA iontophoresis in a manner identical to that observed in vivo. However, in 20 cells (62.5%), iontophoretic application of NE produced a clear antagonism of GABA responses. NE also exerted dual effects on the background firing rate of LH neurons, causing both inhibition and excitation. Overall, in those cells where NE administration increased spontaneous discharge, it either antagonized or had no effect on GABA-mediated inhibition. In contrast, spontaneous firing rate was never elevated above control levels in those cases where NE potentiated GABA responses. Additional experiments demonstrated that the GABA potentiating actions of the benzodiazepine, flurazepam, were preserved in LH tissue slice preparations. In addition, iontophoretic application of the beta-agonist, ISO, routinely suppressed the spontaneous activity of LH neurons and mimicked the facilitating action of NE on GABA. Likewise, microiontophoretic application of 8-bromo cyclic adenosine monophosphate (AMP) enhanced GABA-induced inhibition of LH firing rate in each of 11 cells tested. On the other hand, local administration of the alpha agonist, PE, routinely produced NE-like antagonism of GABA inhibition along with increases in spontaneous firing rate. Taken together these findings indicate that the commonly observed in vivo phenomena of NE augmentation of GABA and suppression of LH neuron spontaneous firing can be demonstrated in vitro, and most likely result from activation of beta adrenoceptors and subsequent elevation of cyclic AMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)

A paradigm for determination of direct drug-induced modulatory alterations in Purkinje cell activity during treadmill locomotion

This paper describes a paradigm employing chronic single unit recording techniques and videotape analysis of treadmill locomotion in order to determine drug-induced modulation of sensorimotor neuronal activity. Animals implanted with a chronic headstage microdrive unit and an indwelling jugular cannula are trained to walk on a treadmill (10 s on, 10 s off). Characteristically, cerebellar Purkinje cells recorded 1-1.5 mm from midline, exhibit increased rates of discharge in phase with movement of specific limbs during a particular stage of the step-swing cycle, as assessed by video analysis of locomotor patterns. Drug-induced alterations in this movement-correlated discharge relative to changes in the spontaneous firing rate can then be determined to assess drug-induced neuromodulatory effects beyond general non-specific excitatory or inhibitory actions.

Corticocortical connections within the primary somatosensory cortex of the rat

Corticocortical connections within the primary somatosensory (SI) cortex of rat were investigated by using discrete injections of retro- and orthogradely transported neuroanatomical tracers (including HRP, WGA, PHA-L, and 3H-leucine). Tangential and vertical connections were defined with respect to the cytoarchitectonic divisions within the rat SI, specifically: (1) the "granular zones" (GZs), characterized by their dense layer IV granular aggregates, which receive the majority of direct ventroposterior (VP) thalamocortical terminations, (2) the "perigranular zones" (PGZs), the less-granular cortical matrix just surrounding the GZs, and (3) the "dysgranular zones" (DZs), the larger dysgranular regions lying centrally within and just lateral to the SI. Receptive fields recorded in the granular zones are small and discrete, whereas in the perigranular zones and especially in dysgranular zones they exhibit complex sensory convergence. A major aim of this study was to determine whether the pattern of intracortical connectivity within the SI is compatible with these observed physiological differences. In general, the perigranular and dysgranular zones contained more profuse systems of corticocortical connections than did the granular zones. For example, discrete tracer injections in the perigranular zones produced "walls" of labelling throughout the adjacent perigranular zones, while adjacent granular zones were relatively empty. Nevertheless, the granular zones were filled with dendritic branches of neurons in adjacent perigranular zones. Since these dendrites could presumably receive direct VP thalamocortical contacts, they represent one path through which this thalamic sensory information might be transmitted to the perigranular zones. Further transmission to the dysgranular zones might be subserved by a topographically organized system of reciprocal interconnections that was found between the perigranular zones and dysgranular zones. In coronal sections, labelling produced by relatively distant injections of either retro or orthograde tracers generally appeared in a columnar distribution, and was localized in perigranular zones and dysgranular zones. Within these zones, orthograde labelling consisted of vertically oriented axons emitting collateral sprays of terminals in all layers. Retrograde neuronal labelling (composed almost exclusively of pyramidal cells) was greatest in supragranular layers. Proximal to the injection site, labelling tended to spread out from these columns into supra- and infragranular layers in adjacent granular zones.(ABSTRACT TRUNCATED AT 400 WORDS)

Progesterone alters GABA and glutamate responsiveness: a possible mechanism for its anxiolytic action

In this study, the neuromodulatory effects of progesterone were tested in an intact neuronal circuit of a model extrahypothalamic CNS area. Spontaneous discharge and responses of single cerebellar Purkinje neurons to microiontophoretically applied gamma-aminobutyric acid (GABA) and glutamate were monitored before, during and after either systemic injection, at physiologic doses, or local application of the steroid. By both means of administration, progesterone significantly enhanced inhibitory responses of Purkinje cells to GABA and suppressed glutamate excitation within 3-10 min post-steroid. These results are consistent with the anxiolytic actions of the steroid.

Acute ethanol effects on sensory responses of single units in the somatosensory cortex of rats during different behavioral states

We have investigated the dose-dependence and time-course of ethanol effects on the activity of single neurons in the somatosensory (SI) cortex of behaving, unanesthetized rats. Sensory responses of neurons recorded in the forepaw area of the SI cortex were quantitatively measured by constructing post-stimulus histograms to repetitive stimulation through electrodes chronically implanted in the forepaw. Single units were isolated and held throughout a protocol involving: (1) a control period, (2) intoxication produced by a single dose of ethanol administered IP or IV and (3) recovery for 60 minutes or more. Post-stimulus histograms were generated during three standard behaviors: (1) REST, (2) IMMOBILE AROUSAL (produced by holding the animal), and (3) MOVEMENT (running on a treadmill). In pre-ethanol controls, the immobile arousal condition slightly increased both excitatory and inhibitory components of the sensory response, while the movement condition strongly inhibited them. Ethanol reduced both of these types of behavioral modulation of sensory responses by abolishing the facilitation normally seen during immobile arousal, as well as the inhibitory gating normally seen during movement. Different latency response epochs of post-stimulus histograms were also used to compare the effect of ethanol on fast vs. slow conducting pathways to the SI cortex. Ethanol at low doses (0.3 g/kg bw, IP) was found to selectively reduce the longer latency excitatory response peaks, while sparing the shortest latency response peak. At moderate doses (1.0 g/kg), however, the shortest latency response peak was also reduced. This contrasted with the effects of halothane which, at anesthetic doses, exerted a much more selective reduction of the longer latency responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Laminar differences in sizes, shapes, and response profiles of cutaneous receptive fields in the rat SI cortex

Quantitative techniques were used to demonstrate cortical layer differences in cutaneous receptive fields (RF's) in the rat SI cortex. Two- and three-dimensional (2-D and 3-D) RF maps were constructed showing the responsiveness of single neurons to standardized punctate stimulation of each of a matrix of points on the skin or the mystacial vibrissa pad. These allowed a visualization not only of the overall sizes of such RF's, but also their shape and "response profile". Initial experiments showed that the sizes and response profiles of such RF's were similar whether they were mapped by sinusoidal mechanical vibration of skin, punctate touch, or direct intracutaneous electrical stimulation. This method was used to quantitatively determine distoproximal lengths of RF's of single units recorded at different depths in the forepaw area of the SI cortex. Plots of these RF lengths as a function of cortical depth showed that the smallest RF's were found in the granular layers (IV and deep III). RF's up to double that size were found in supragranular layers, and up to triple that size in infragranular layers. 3-D maps of RF's in the granular layers showed sharp central response peaks surrounded by very steep dropoffs to the RF boundaries. In the whisker areas, granular layer RF's were typically circular in shape and contained from 1-4 whiskers. By contrast, in supragranular layers they were often elongated in shape, and were oriented along rows or columns of whiskers. RF's in layer V resembled large, high plateaus, often supporting clearly separated peaks. RF's mapped in the fore- and hindpaw areas were similar, but, even in the granular layers, were often slightly elongated along the limb axis. In all regions of the SI, both the locations and shapes of the granular layer RF's appeared to be conserved as subsets of other more topographically heterogeneous RF's encountered elsewhere in the column. These findings may correlate with patterns of axonal connectivity in the rat SI.

Distribution of somatic sensory and active-movement neuronal discharge properties in the MI-SI cortical border area in the rat

The rat somatosensory (SI) cortex contains a precise map of the cutaneous periphery, yet its rostromedial edge, which includes part of the fore- and hind paw representation, has been reported to functionally overlap with the electrically excitable primary motor (MI) cortex. Thus, the MI cortex in the rat contains two subregions: (i) rostrally, the "MI-agranular" cortex (i.e., "typical" MI cortex), and (ii) caudally, the "MI-SI-granular" cortex (i.e., the MI-SI overlap). The aim of this study was to assess the degree of overlap in the physiologic properties of single neurons recorded across the MI-SI boundary zone in awake, freely moving rats. Computer techniques were used to characterize both somatosensory receptive fields (cutaneous or "passive joint-manipulation") and discharge correlates of active limb movement in these MI-agranular and MI-SI-granular subregions of the MI. "Active-movement" units were defined as those which discharged strongly in correlation with specific limb movements, but in a manner which could not be predicted by their observable somatosensory properties. Of 92 completely analyzed cells in the MI-SI-granular forepaw region, 86 exhibited specific cutaneous receptive fields on the palmar surface of the forepaw. By contrast with the similarity of these neurons' responses to passive stimulation, they varied markedly in their discharge during active limb movements. For example, many did not respond when their forepaw receptive fields touched the ground during stepping. Furthermore, 31 (of 92) neurons in this region were identified as active-movement, firing in correlation with reaching movements of the forelimb. Seven of these were completely unresponsive to any sensory stimuli, but 24 exhibited an apparent convergence of cutaneous and active-movement properties. Of 86 units recorded in the MI-agranular subregion, 46 responded strongly to passive joint manipulation, but only three responded exclusively to cutaneous stimulation. Twenty-eight (of 86) cells were defined as active-movement, discharging mainly in correlation with forelimb reaching movements. Thus, the active-movement properties of neurons in these two subregions were quite similar, whereas the somatosensory properties were markedly different.

Mapping the body representation in the SI cortex of anesthetized and awake rats

We have used single unit recording techniques to map the representation of cutaneous and joint somatosensory modalities in the primary somatosensory (SI) cortex of both anesthetized and awake rats. The cytoarchitectonic zones within the rat SI were divided into the following main categories: (1) granular zones (GZs)--areas exhibiting koniocortical cytoarchitecture (i.e., containing dense aggregates of layer IV granule cells), (2) perigranular zones (PGZs)--narrow strips of less granular cortex surrounding the GZs, and (3) dysgranular zones (DZs)--large areas of dysgranular cortex enclosed within the SI. The narrow strip between the SI and the rostrally adjacent frontal agranular cortex was termed the "transitional zone" (TZ). Initial computer-based studies of the properties of cutaneous receptive fields (RFs) in SI showed that, although there were differences in response threshold, adaptability, frequency response, and overall RF size and shape of adjacent neurons, the size and location of the "centers" of the RFs were quite constant and were similar to those seen in multiple unit recordings. The same was true of RFs of single neurons recorded through different anesthetic states. The body representation in SI was first mapped by determining single unit and unit cluster RFs within a total of 2,170 microelectrode penetrations in barbiturate-anesthetized rats. Cutaneous RFs in the GZs were quite discrete. Thus, a single, finely detailed, continuous map of the cutaneous periphery was definable within the GZs themselves. Only the forepaw had a double representation. RFs in the PGZs were larger and more diffuse, but since they covered roughly the same skin areas as the RFs in the most closely adjacent GZs, they fit into the same body map. Neurons in the DZs were unresponsive to any sensory stimuli in the anesthetized animal. In chronically implanted, freely moving, awake animals cutaneous RFs were larger and more volatile than in the anesthetized, but the accuracy of the map was clearly preserved by the fact that the locations of the RF centers (which often must be defined quantitatively) were unchanged. The PGZs and DZs in the awake animals exhibited a multimodal convergence of cutaneous and joint movement RFs within single vertical penetrations, or even on single neurons. Directionally specific and bilateral cutaneous RFs were also observed in the DZs. It was concluded the DZs are more associational or integrative areas within the SI, but they could not be shown to comprise a distinct and separate body representation.(ABSTRACT TRUNCATED AT 400 WORDS)