Neuronal activity in normal and deafferented forelimb somatosensory cortex of the awake cat

Contributions of Nociresponsive Area 3a to Normal and Abnormal Somatosensory Perception

Traditionally, cytoarchitectonic area 3a of primary somatosensory cortex (SI) has been regarded as a proprioceptive relay to motor cortex. However, neuronal spike-train recordings and optical intrinsic signal imaging, obtained from nonhuman sensorimotor cortex, show that neuronal activity in some of the cortical columns in area 3a can be readily triggered by a C-nociceptor afferent drive. These findings indicate that area 3a is a critical link in cerebral cortical encoding of secondary/slow pain. Also, area 3a contributes to abnormal pain processing in the presence of activity-dependent reversal of gamma-aminobutyric acid A receptor-mediated inhibition. Accordingly, abnormal processing within area 3a may contribute mechanistically to generation of clinical pain conditions. PERSPECTIVE: Optical imaging and neurophysiological mapping of area 3a of SI has revealed substantial driving from unmyelinated cutaneous nociceptors, complementing input to areas 3b and 1 of SI from myelinated nociceptors and non-nociceptors. These and related findings force a reconsideration of mechanisms for SI processing of pain.

Rapid functional plasticity in the primary somatomotor cortex and perceptual changes after nerve block

The mature human primary somatosensory cortex displays a striking plastic capacity to reorganize itself in response to changes in sensory input. Following the elimination of afferent return, produced by either amputation, deafferentation by dorsal rhizotomy, or nerve block, there is a well-known but little-understood 'invasion' of the deafferented region of the brain by the cortical representation zones of still-intact portions of the brain adjacent to it. We report here that within an hour of abolishing sensation from the radial and medial three-quarters of the hand by pharmacological blockade of the radial and median nerves, magnetic source imaging showed that the cortical representation of the little finger and the skin beneath the lower lip, whose intact cortical representation zones are adjacent to the deafferented region, had moved closer together, presumably because of their expansion across the deafferented area. A paired-pulse transcranial magnetic stimulation procedure revealed a motor cortex disinhibition for two muscles supplied by the unaffected ulnar nerve. In addition, two notable perceptual changes were observed: increased two-point discrimination ability near the lip and mislocalization of touch of the intact ulnar portion of the fourth finger to the neighbouring third finger whose nerve supply was blocked. We suggest that disinhibition within the somatosensory system as a functional correlate for the known enlargement of cortical representation zones might account for not only the 'invasion' phenomenon, but also for the observed behavioural correlates of the nerve block.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Influence of the postlesion environment and chronic piracetam treatment on the organization of the somatotopic map in the rat primary somatosensory cortex after focal cortical injury

The influence of housing in an enriched or impoverished environment and anti-ischemic treatment (piracetam) on the organization of the intact regions of the somatosensory cortical maps adjacent to a focal cortical injury were investigated in adult rats. Response properties of small clusters of neurons were recorded in the area of the primary somatosensory cortex (SI) devoted to the contralateral forepaw representation. Electrophysiological maps were elaborated on the basis of the sensory "submodality" (cutaneous or noncutaneous) and the location of the receptive fields (RFs) of layer IV neurons. Recordings were made before, and 3 weeks after induction of a focal neurovascular lesion to the SI cortex. The main results were: 1) the focal ischemic injury induced a cellular loss which was less severe in the piracetam treated rats, regardless of the housing conditions; 2) the lesion resulted in a compression of the remaining forepaw map, a fragmentation of the representational zones serving the cutaneous surfaces (low-threshold inputs) and an enlargement of noncutaneous zones (high-threshold inputs) in the spared cortical sectors surrounding the lesion. These changes were found in all placebo rats, with the most detrimental effects in the animals exposed to an impoverished environment, and in the piracetam-plus-impoverished rats. In contrast, a limited compression of the forepaw map and a preservation of most representational sectors were observed in the piracetam-plus-enriched animals, 3) the size of the cutaneous RFs of the neurons within the intact cortical zones remained unchanged, regardless of environment or treatment; 4) consistent with the map changes, the skin surfaces lacking low-threshold cutaneous RFs increased after the lesion in all animal groups but the piracetam-plus-enriched rats; 5) cortical responsiveness as assessed with mechanical threshold evaluation was diminished in the placebo rats, whatever the environment, and in the piracetam-impoverished rats, but was not significantly affected in the piracetam-enriched animals. Our findings, based on the first double electrophysiological mapping in the rat SI cortex, highlight the protective effects of an environmental therapy associated with an anti-ischemic treatment on the neurophysiological properties of cortical neurons following a focal neurovascular injury to the neocortex.

Neuroprotective effects on somatotopic maps resulting from piracetam treatment and environmental enrichment after focal cortical injury

Acute and chronic postlesion reorganization of the cortical maps was examined in adult rats using electrophysiological mapping of the forepaw area in the primary somatosensory cortex. Recordings were made before and after (first 12 hr and 3 wk) induction of a focal thermal-ischemic lesion to a restricted part of the forepaw area. The influence of an anti-ischemic substance (piracetam) and housing in an enriched environment (EE) or impoverished environment (IE) on the organization of the spared regions of the cortical maps adjacent to the lesion was investigated. The results revealed (1) a gradual expansion of the injured zone and a cellular loss that were smaller in the piracetam-treated (PT) rats than in the placebo (PL) rats; (2) a better preservation of the somatotopic organization and the neuronal responsiveness in the maps of the PT rats during the first 12 hr after the lesion; (3) a gradual compression and fragmentation of the remaining forepaw map over the first 3 postlesion wk. These changes were found in all PL rats, with the most detrimental effects in the animals exposed to an IE. In the PT-EE animals, a contrasting substantial preservation of the map was observed. (4) Cortical responsiveness was diminished in the PL rats, whatever the environment, and in the PT-IE rats; but it was not significantly affected in the PT-EE animals. The findings demonstrate the protective function of acute piracetam treatment on electrophysiological properties of cortical neurons within the peri-infarct tissue and highlight the neuroprotective effects of an environmental therapy combined with the piracetam treatment during the weeks after ischemic damage.

The neural basis of Charles Bonnet hallucinations: a hypothesis

OBJECTIVES

To describe the hallucinations occurring as a result of a macular hole in each eye and to investigate the neural basis.

METHODS

Psychophysical observations including sketches of the hallucinations calibrated for size.

RESULTS

All the hallucinations were of the geometric (patterned) type and lasted for only a few days.

CONCLUSIONS

The observations can be explained on the basis of a "deafferentation" model, which is described in some detail. It is proposed that the hallucinations result from activation of the "blobs" of area V1 and the "stripes" of area V2 in the visual cortex. A theory is proposed to account for the disappearance of the hallucinations by a "filling in" mechanism.

Long-term changes in behavior and regional cerebral blood flow associated with painful peripheral mononeuropathy in the rat

We identified long-term (up to 12 weeks), bilateral changes in spontaneous and evoked pain behavior and baseline forebrain activity following a chronic constriction injury (CCI) of the sciatic nerve. The long-term changes in basal forebrain activation following CCI were region-specific and can be divided into forebrain structures that showed either: (1) no change, (2) an increase, or (3) a decrease in activity with regard to the short-term (2 weeks) changes we previously reported. All the rats showed spontaneous pain behaviors that persisted throughout the 12-week observation period, resembling the pattern of change found in four limbic system structures: the anterior dorsal thalamus, habenular complex, and the cingulate and retrosplenial cortices. In contrast, heat hyperalgesia was delayed in onset until 4 weeks following CCI, but then persisted, showing a nearly constant level of increased responsiveness. The forebrain activation that resembles this behavioral pattern of change is found in somatosensory cortex, and in the hypothalamic paraventricular nucleus and the basolateral amygdala. Finally, mechanical allodynia, which was maximal during the first 2 weeks following nerve injury and gradually recovered by the seventh post-operative week uniquely matches the time course of changes in ventrolateral and ventroposterolateral thalamic activity. Our results indicate that peripheral nerve damage results in persistent changes in behavior and resting forebrain systems that modulate pain perception. The persistent abnormalities in the somatosensory cortex and thalamus suggest that the sensory thalamocortical axis is functionally deranged in certain chronic pain states.

Intrinsic dynamics in neuronal networks. I. Theory

Many networks in the mammalian nervous system remain active in the absence of stimuli. This activity falls into two main patterns: steady firing at low rates and rhythmic bursting. How are these firing patterns generated? Specifically, how do dynamic interactions between excitatory and inhibitory neurons produce these firing patterns, and how do networks switch from one firing pattern to the other? We investigated these questions theoretically by examining the intrinsic dynamics of large networks of neurons. Using both a semianalytic model based on mean firing rate dynamics and simulations with large neuronal networks, we found that the dynamics, and thus the firing patterns, are controlled largely by one parameter, the fraction of endogenously active cells. When no endogenously active cells are present, networks are either silent or fire at a high rate; as the number of endogenously active cells increases, there is a transition to bursting; and, with a further increase, there is a second transition to steady firing at a low rate. A secondary role is played by network connectivity, which determines whether activity occurs at a constant mean firing rate or oscillates around that mean. These conclusions require only conventional assumptions: excitatory input to a neuron increases its firing rate, inhibitory input decreases it, and neurons exhibit spike-frequency adaptation. These conclusions also lead to two experimentally testable predictions: 1) isolated networks that fire at low rates must contain endogenously active cells and 2) a reduction in the fraction of endogenously active cells in such networks must lead to bursting.

Acute reorganization of the forepaw representation in the rat SI cortex after focal cortical injury: neuroprotective effects of piracetam treatment

Immediate postlesion reorganization of the somatosensory cortical representation was examined in adult rats. Response properties of small clusters of neurons were recorded in the area of the primary somatosensory cortex (SI) devoted to the contralateral forepaw representation. Electrophysiological maps were elaborated on the basis of the sensory 'submodality' (cutaneous or noncutaneous) and the location of the peripheral receptive fields (RFs) of layer IV neurons. Recordings were made prior to, and from 1 to 12 h after, induction of a focal neurovascular lesion to the SI cortex that initially destroyed a part (8.5%) of the cutaneous representation. Moreover, the influence of an anti-ischaemic substance (piracetam) on lesion-induced changes was analysed. The main observations were: (i) a gradual outward expansion of the area of the functional lesion, which was smaller in the piracetam-treated (PT) rats than in the control, placebo-treated (PL) rats; (ii) a substantial remodelling of the spared representational zones, both in cortical sectors adjoining the site of injury and those remote from the site; (iii) a significant postlesion increase in the size of cutaneous RFs in the PT rats, but not in the PL rats; (iv) a better preservation of RF submodality and topographic organization in the PT maps than in the PL maps; and (v) a decrease in neuronal responsiveness to cutaneous stimulation which was less pronounced in the PT than in the PL rats. Our results can be ascribed to a rapid change in the balance of excitatory and inhibitory connections which leads to unmasking of subthreshold inputs converging onto cortical neurons. Our findings also indicate that acute piracetam treatment exerts a protective function on the physiological response properties of cortical neurons after focal injury.

Short-term reorganization of the rat somatosensory cortex following hypodynamia-hypokinesia

This study was performed to determine if hypodynamia-hypokinesia (HH) could induce a reorganization of the rat somatosensory cortex. The cortical hindpaw representation was determined by stimulating the limb and recording multi-unit cortical activity. The size of the cutaneous receptive fields was also measured. After 14 days of HH, the size of the cortical hindpaw representation was decreased. The proportion of small cutaneous receptive fields decreased while the large ones increased. After 7 days of HH, no change in the two studied parameters was noticed in five animals. In the other rats, a number of sites unresponsive to cutaneous stimulation or with high thresholds was observed. This study provides evidence of a plasticity of the somatosensory cortex induced by a situation that reduces both sensory and motor functions. The cortical reorganization occurs in two stages.

Temporally structured impulse activity in spontaneously discharging somatosensory cortical neurons in the awake cat: recognition and quantitative description of four different patterns of bursts, post-recording GFAP immunohistology and computer reconstruction of the studied cortical surface

We elaborated two methods used in two previous publications [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129-140 [16]; H.H. Webster, I. Salimi, A.A. Myasnikov, R.W. Dykes. The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats, Brain Res. 750 (1997) 109-121 [21]]: (A) a procedure for detecting and classifying brief epochs of high-frequency extracellular impulse activity (bursts) recorded chronically in the somatosensory cortex of the awake cat, and (B) a modification of an immunohistochemical technique [L.A. Bevento, L.B. McCleary. An immunochemical method for marking microelectrode tracks following single-unit recordings in long surviving, awake monkeys, J. Neurosci. Meth. 41 (1992) 199-204 [5]] for visualization of electrode tracks and electrolytic lesions around the tip of tungsten-in-glass microelectrodes [D.M.D. Landis, The early reactions of non-neuronal cells to brain injury, Annu. Rev. Neurosci. 17 (1994) 133-151 [15]] weeks after lesions were made in cortex. The burst recognition and classification method uses an interval threshold to determine the beginning and end of one epoch [M. Armstrong-James, K. Fox, Effects of ionophoresed noradrenaline on spontaneous activity of neurons in rat primary somatosensory cortex, J. Physiol. (London), 335 (1983) 427-447 [3]] in the original sequence of interspike intervals (ISIs) to segregate and analyze separately a burst. The threshold is based on the duration of the shortest modal ISI found in the autocorrelogram [J. Martinson, H.H. Webster, A.A. Myasnikov, R.W. Dykes, Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats, Brain Res. 750 (1997) 129-140 [16]]. The technique allowed recognition of bursts with several distinctive patterns: (i) an initial, longer ISI followed by progressively shorter ones; (ii) an initially shorter ISI followed by progressively longer ones; (iii) patterns where the intermediate ISI could be either longer or shorter than surrounding ones; and (iv) consecutive ISIs of relatively equal duration. Among the cells discharging in bursts with equal ISIs, the technique allows recognition of cells generating only short (up to three to five intervals) bursts, and others generating mixtures of a short and long (up to six or more intervals) bursts. Finally, frequency distributions of the probability of encountering bursts having intervals of a stated length is described. The visualization of tracks from chronic recording experiments is important for relating neuronal function to a specific cytoarchitectural region and a specific cortical layer. Several modifications of the procedure of immunostaining for GFAP allows identification of recording sites in clearer relationship to the cytoarchitectonic details of cat somatosensory cortex.

Effects of D-AP5 and NMDA microiontophoresis on associative learning in the barrel cortex of awake rats

Experiments involving single-unit recordings and microiontophoresis were carried out in the barrel cortex of awake, adult rats subjected to whisker pairing, an associative learning paradigm where deflections of the recorded neuron's principle vibrissa (S2) are repeatedly paired with those of a non-adjacent one (S1). Whisker pairing with a 300 ms interstimulus interval was applied to 61 cells. In 23 cases, there was no other manipulation whereas in the remaining 38, pairing occurred in the presence of one of three pharmacological agents previously shown to modulate learning, receptive field plasticity and long-term potentiation: N-methyl-D-aspartic acid (NMDA) (n=8), the NMDA receptor antagonist AP5 (n=17) or the nitric oxide synthase inhibitor L-nitro-arginine-N-methyl-ester (L-NAME) (n=13). Non-associative (unpaired) experiments (n=14) and delivery of pharmacological agents without pairing (n=14) served as controls. Changes in neuronal responsiveness to S1 following one of these procedures were calculated and adjusted relative to changes in the responses to S2. On average, whisker pairing alone yielded a 7% increase in the responses to S1. This enhancement differed significantly from the 17% decrease obtained in the non-associative control condition and could not be attributed to variations in the state of the animals because analysis of the cervical and facial muscle electromyograms revealed that periods of increased muscular activity, reflecting heightened arousal, were infrequent (less than 4% of a complete experiment on average) and occurred randomly. The enhancement of the responses to S1 was further increased when whisker pairing was performed in the presence of L-NAME (27%) or NMDA (35%) whereas AP5 reduced it to 1%. During the delivery period, NMDA enhanced both neuronal excitability and responsiveness to S1 whereas AP5 depressed them. However, the effects of both substances disappeared immediately after administration had ended. L-NAME did not affect the level of ongoing activity and responses to S1 significantly. From these data, we concluded that, since the changes in the responses to S1 lasted longer than the periods of both whisker pairing and drug delivery, they were not residual excitatory or inhibitory drug effects on neuronal excitability. Thus, our results indicate that, relative to the unpaired controls, whisker pairing led to a 24% increase in the responsiveness of barrel cortex neurons to peripheral stimulation and that these changes were modulated by the local application of pharmacological agents that act upon NMDA receptors and pathways involving nitric oxide. We can infer that somatosensory cerebral cortex is one site where plasticity emerges following whisker pairing.

The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats

Single neurons (n = 356) were studied in the forelimb representation of awake, quietly resting cats. Thirty-five spontaneously bursting neurons in a sample of 206 cells recorded before forelimb deafferentation were compared to 39 spontaneously bursting neurons in a sample of 127 neurons studied 1-3 weeks after deafferentation. The probability of encountering bursting neurons increased significantly following deafferentation from 17% to 31% of the sample (P < 0.005). The same 5 classes of bursting cells were observed after deafferentation but there were significant changes in the duration of interspike intervals in some classes, in the probability of observing certain classes, and in the proportion of spikes found in bursts. The probability of encountering class III cells, a class thought to consist primarily of non-inactivating pyramidal burst neurons, nearly doubled and the average interspike interval length within the burst increased from 1.9 to 3.0 ms. The burst structure in the other classes did not change but they were found less frequently. These other classes may include inhibitory interneurons which receive less excitatory drive after deafferentation and therefore provide less inhibition to class III cells. The differential behavior of the different classes of bursting cells may be one reason why the overall level of spontaneous activity does not change after deafferentation and it suggests that there are homeostatic mechanisms in primary somatosensory cortex that maintain a certain level of neural activity.

Recognition of temporally structured activity in spontaneously discharging neurons in the somatosensory cortex in waking cats

We describe a method to automate the detection and analysis of structured neuronal activity obtained in relatively non-restrictive experiments in awake animals. Several different, regularly occurring, discharge patterns consisting of groups of spikes were identified in extracellular recordings from the somatosensory cortex of awake cats. The introduction of an interspike interval threshold made it possible to segregate these bursts from single spikes. The threshold interval was obtained from the modal interval in high-resolution autocorrelograms (up to 0.1 ms/bin) of the spontaneous neural activity. Single spikes were those separated by intervals greater than the threshold, while those within the group were of less than threshold value. When intervals were arranged and averaged according to their order of occurrence within the burst, four distinctive burst patterns were observed. These four patterns occurred in both normal and deafferented cortex and we believe them to be characteristic of particular cell types, a feature that will be useful for studying such cells in intact cellular networks.

Plasticity of adult sensorimotor representations

Plasticity of sensory and motor cortical and subcortical representations in the adult brain appears to be a general phenomenon in animals that has now been extended to humans. There is a growing understanding of the mechanisms and rules that regulate the form and extent of reorganization; these appear to include activity-dependent control of synaptic efficacy, details of circuit arrangements, and growth of new axonal arbors. Of particular relevance to plasticity of cerebral cortical sensorimotor representations is recent evidence for the participation of intracortical horizontal pathways. These fibers provide a substrate for reorganization and contain mechanisms for increases or decreases in synaptic efficacy that depend on particular spatiotemporal activation patterns.