Immediate thalamic sensory plasticity depends on corticothalamic feedback

Instructive role of a peripheral pattern for the central patterning of the trigeminal projection at the brainstem and thalamus revealed by an artificially altered whisker pattern

The central patterning mechanism of neuronal circuits is an important issue in developmental neuroscience. We report here the role of a peripheral whisker pattern for the patterning of the trigeminal projection at the brainstem and thalamus in the mouse somatosensory system. The whisker pattern was manipulated by infecting the embryonic epidermis with adenovirus harboring Shh. The ectopic expression of Shh led to the induction of extra whiskers and displacement of whiskers, where these whiskers were histologically normal. The altered whisker pattern was isomorphically represented in the brainstem (barrelette: subnuclei principalis and subnuclei interpolaris), thalamus (barreloid) and cortex (barrel) as revealed by cytochrome oxidase staining. The barrelette-like pattern of the parvalbumin became discernible by immunostaining at P7 in subnuclei principalis and at P4 in subnuclei interpolaris in normal mice. These are the barrelette neurons projecting to the thalamus and the local circuit within the barrelette. The barrelette-like parvalbumin pattern also exhibits the altered whisker pattern induced by the adenovirus harboring Shh. These results highlight the role the peripheral whisker pattern for the central patterning of the brainstem, thalamus, and cortex in the mouse somatosensory system.

Different composition of glutamate receptors in corticothalamic and lemniscal synaptic responses and their roles in the firing responses of ventrobasal thalamic neurons in juvenile mice

Thalamic ventrobasal (VB) relay neurons receive information via two major types of glutamatergic synapses, that is, from the medial lemniscus (lemniscal synapses) and primary somatosensory cortex (corticothalamic synapses). These two synapses influence and coordinate firing responses of VB neurons, but their precise operational mechanisms are not yet well understood. In this study, we compared the composition of glutamate receptors and synaptic properties of corticothalamic and lemniscal synapses. We found that the relative contribution of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to non-NMDA receptor-mediated EPSCs was significantly greater in corticothalamic synapses than in lemniscal synapses. Furthermore, NMDA receptor 2B-containing NMDA receptor- and kainate receptor-mediated currents were observed only in corticothalamic synapses, but not in lemniscal synapses. EPSCs in corticothalamic synapses displayed the postsynaptic summation in a frequency-dependent manner, in which the summation of the NMDA receptor-mediated component was largely involved. The summation of kainate receptor-mediated currents also partially contributed to the postsynaptic summation in corticothalamic synapses. In contrast, the contribution of NMDA receptor-mediated currents to the postsynaptic summation of lemniscal EPSCs was relatively minor. Furthermore, our results indicated that the prominent NMDA receptor-mediated component in corticothalamic synapses was the key determinant for the late-persistent firing of VB neurons in response to corticothalamic stimuli. In lemniscal synapses, in contrast, the onset-transient firing in response to lemniscal stimuli was regulated mainly by AMPA receptors.

Reversible inactivations of rat medial prefrontal cortex impair the ability to wait for a stimulus

In simple reaction time tasks, lesions of rat dorsomedial prefrontal cortex impair the ability to wait for trigger stimuli and result in increased premature responding. This effect could be due to impairments in attending to trigger stimuli, estimating the timing of trigger stimuli, or inhibitory control of the motor response. Here, we examined these issues by reversibly inactivating dorsomedial prefrontal cortex during simple reaction time tasks with variable or fixed foreperiods. There were three consistent effects of dorsomedial prefrontal cortex inactivation: 1) increased premature responding, 2) increased variability in the timing of premature responses, and 3) speeded response latencies, especially on trials with short foreperiods in tasks with variable foreperiods. We observed these effects independent of differences in foreperiod duration, foreperiod variability, and stimulus probabilities. Therefore, dorsomedial prefrontal cortex appears not to be involved in attending to the trigger stimulus or in time estimation. Instead, we suggest that dorsomedial prefrontal cortex is critical for inhibiting responses before the maximum foreperiod duration, i.e. the "deadline" [Ollman RT, Billington MJ (1972) The deadline model for simple reaction times. Cognit Psychol 3:311-336], after which the rat should respond even if the trigger stimulus has not occurred.

Hippocampal inactivation enhances taste learning

Learning tasks are typically thought to be either hippocampal-dependent (impaired by hippocampal lesions) or hippocampal-independent (indifferent to hippocampal lesions). Here, we show that conditioned taste aversion (CTA) learning fits into neither of these categories. Rats were trained to avoid two taste stimuli, one novel and one familiar. Muscimol infused through surgically implanted intracranial cannulae temporarily inactivated the dorsal hippocampus during familiarization, subsequent CTA training, or both. As shown previously, hippocampal inactivation during familiarization enhanced the effect of that familiarization on learning (i.e., hippocampal inactivation enhanced latent inhibition of CTA); more novel and surprising, however, was the finding that hippocampal inactivation during training sessions strongly enhanced CTA learning itself. These phenomena were not caused by specific aspects of our infusion technique--muscimol infusions into the hippocampus during familiarization sessions did not cause CTAs, muscimol infusions into gustatory cortex caused the expected attenuation of CTA, and hippocampal inactivation caused the expected attenuation of spatial learning. Thus, we suggest that hippocampal memory processes interfere with the specific learning mechanisms underlying CTA, and more generally that multiple memory systems do not operate independently.

Balancing bilateral sensory activity: callosal processing modulates sensory transmission through the contralateral thalamus by altering the response threshold

Rats tactually explore a nearly spherical space field around their heads with their whiskers. The information sampled by the two sets of whiskers is integrated bilaterally at the cortical level in an activity dependent manner via the corpus callosum. We have recently shown that sensory activity in one barrel field cortex (BFC) modulates the processing of incoming sensory information to the other BFC. Whether interhemispheric integration is dynamically linked with corticothalamic modulation of incoming sensory activity is an important hypothesis to test, since subcortical relay neurons are directly modulated by cortical neurons through top-down processes. In the present study, we compared the direct sensory responses of single thalamic relay neurons under urethane anesthesia before and after inactivating the BFC contralateral to a thalamic neuron. The data show that silencing one BFC reduces response magnitude in contralateral thalamic relay neurons, significantly and reversibly, in response to test stimuli applied to the principal whisker at two times response threshold (2T) intensity for each unit. Neurons in the ventral posterior medial (VPM) nucleus and the medial division of the posterior nucleus (POm) react in a similar manner, although POm neurons are more profoundly depressed by inactivation of the contralateral BFC than VPM neurons. The results support the novel idea that the subcortical relay of sensory information to one hemisphere is strongly modulated by activity levels in the contralateral as well as in the ipsilateral SI cortex. The mechanism of the modulation appears to be based on shifting the stimulus-response curves of thalamic neurons, thereby rendering them more or less sensitive to sensory stimuli. We conclude that global sensory processing is created by combining activity in each cerebral hemisphere and continually balancing the flow of information to cortex by adjusting the responsiveness of ascending sensory pathways.

Relief of synaptic depression produces long-term enhancement in thalamocortical networks

Thalamocortical synapses may be able to undergo activity-dependent long-term changes in efficacy, such as long-term potentiation. Indeed, studies conducted in vivo have found that theta-burst stimulation (TBS) of the thalamus induces a long-term enhancement (LTE) of field potential responses evoked in the neocortex of adult rodents. Because the thalamus and neocortex form a complex interconnected network that is highly active in vivo, it is possible that a change in thalamic excitability would be reflected in the neocortex. To test this possibility, we recorded from barrel neocortex and applied TBS to the thalamic radiation while the somatosensory thalamus was inactivated with muscimol. Thalamocortical LTE was absent when the thalamus was inactivated, suggesting that changes in thalamic excitability are involved. Single-unit recordings from thalamocortical cells revealed that TBS causes a significant reduction in the spontaneous firing rate of thalamocortical cells. Reducing the spontaneous firing of thalamocortical cells directly enhances the efficacy of the thalamocortical pathway because it relieves the tonic depression of the thalamocortical connection caused by thalamocortical activity. Because these changes in thalamic excitability are triggered by corticothalamic activity, this may be a useful top-down mechanism to regulate afferent sensory input to the neocortex during behavior as a function of experience.

Cortical influences on sizes and rapid plasticity of tactile receptive fields in the dorsal column nuclei

The cerebral cortex influences subcortical processing. In the somatosensory system, descending cortical inputs contribute in specific ways to the sizes and plasticity of tactile receptive fields (RFs) in the thalamus, but less is known about cortical influences on these aspects of brainstem RFs. The present studies evaluated how loss of cortical inputs affects sizes and plasticity of RFs in the brainstem dorsal column nuclei (DCN) when peripheral inputs were normal and when peripheral inputs were acutely disrupted. Loss of cortical inputs was produced by acute lesion of somatosensory, motor, and adjacent cortex, whereas disruption of peripheral inputs was produced by cutaneous microinjection of lidocaine (LID). Modest or no changes in sizes of DCN RFs, comparable to changes during control periods of no treatment, were seen in response to cortical lesion. LID caused rapid enlargements in RFs when cortex was intact. LID also caused rapid RF enlargements after cortical lesion, and these enlargements were greater than post-LID enlargements when cortex was intact. These results indicate that normally sized RFs continue to be produced in the DCN after loss of cortical input. Cortex is also not required for RF enlargements after LID; however, cortical inputs have a constraining effect on these enlargements. Considered with findings from previous thalamic studies, these results suggest that cortical influences on RF size and plasticity in the DCN and thalamus differ in some respects.

Chronic suppression of activity in barrel field cortex downregulates sensory responses in contralateral barrel field cortex

Numerous lines of evidence indicate that neural information is exchanged between the cerebral hemispheres via the corpus callosum. Unilateral ablation lesions of barrel field cortex (BFC) in adult rats induce strong suppression of background and evoked activity in the contralateral barrel cortex and significantly delay the onset of experience-dependent plasticity. The present experiments were designed to clarify the basis for these interhemispheric effects. One possibility is that degenerative events, triggered by the lesion, degrade contralateral cortical function. Another hypothesis, alone or in combination with degeneration, is that the absence of interhemispheric activity after the lesion suppresses contralateral responsiveness. The latter hypothesis was tested by placing an Alzet minipump subcutaneously and connecting it via a delivery tube to a cannula implanted over BFC. The minipump released muscimol, a GABA(A) receptor agonist at a rate of 1 mul/h, onto one barrel field cortex for 7 days. Then with the pump still in place, single cells were recorded in the contralateral BFC under urethan anesthesia. The data show a approximately 50% reduction in principal whisker responses (D2) compared with controls, with similar reductions in responses to the D1 and D3 surround whiskers. Despite these reductions, spontaneous firing is unaffected. Fast spiking units are more sensitive to muscimol application than regular spiking units in both the response magnitude and the center/surround ratio. Effects of muscimol are also layer specific. Layer II/III and layer IV neurons decrease their responses significantly, unlike layer V neurons that fail to show significant deficits. The results indicate that reduced activity in one hemisphere alters cortical excitability in the other hemisphere in a complex manner. Surprisingly, a prominent response decrement occurs in the short-latency (3-10 ms) component of principal whisker responses, suggesting that suppression may spread to neurons dominated by thalamocortical inputs after interhemispheric connections are inactivated. Bilateral neurological impairments have been described after unilateral stroke lesions in the clinical literature.

Mechanisms underlying reorganization of fractured tactile cerebellar maps after deafferentation in developing and adult rats

Our previous studies showed that fractured tactile cerebellar maps in rats reorganize after deafferentation during development and in adulthood while maintaining a fractured somatotopy. Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was caused by mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps and found that the expansion of the upper incisor is not caused by a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not caused by unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.

Computing with thalamocortical ensembles during different behavioural states

A series of recent studies have indicated that ensembles of neurones, distributed within the neural structures that form the primary thalamocortical loop (TCL) of the trigeminal component of the rat somatosensory system, change the way they respond to similar tactile stimuli, according to both the behavioural strategy employed by animals to gather information and the animal's internal brain states. These findings suggest that top-down influences, which are more likely to play a role during active discrimination than during passive whisker stimulation, may alter the pattern of neuronal firing within both the distinct layers of the primary somatosensory cortex (S1) and the ventral posterior medial nucleus (VPM). We propose that through this physiological process, which involves concurrent dynamic modulations at both cellular and circuit levels in the TCL, rats can either optimize the detection of novel or hard to sense stimuli or they can analyse complex patterns of multi-whisker stimulation, during natural exploration of their surrounding environment.

Dynamics of sensory thalamocortical synaptic networks during information processing states

The thalamocortical network consists of the pathways that interconnect the thalamus and neocortex, including thalamic sensory afferents, corticothalamic and thalamocortical pathways. These pathways are essential to acquire, analyze, store and retrieve sensory information. However, sensory information processing mostly occurs during behavioral arousal, when activity in thalamus and neocortex consists of an electrographic sign of low amplitude fast activity, known as activation, which is caused by several neuromodulator systems that project to the thalamocortical network. Logically, in order to understand how the thalamocortical network processes sensory information it is essential to study its response properties during states of activation. This paper reviews the temporal and spatial response properties of synaptic pathways in the whisker thalamocortical network of rodents during activated states as compared to quiescent (non-activated) states. The evidence shows that these pathways are differentially regulated via the effects of neuromodulators as behavioral contingencies demand. Thus, during activated states, the temporal and spatial response properties of pathways in the thalamocortical network are transformed to allow the processing of sensory information.

Propofol suppresses synaptic responsiveness of somatosensory relay neurons to excitatory input by potentiating GABA(A) receptor chloride channels

Propofol is a widely used intravenous general anesthetic. Propofol-induced unconsciousness in humans is associated with inhibition of thalamic activity evoked by somatosensory stimuli. However, the cellular mechanisms underlying the effects of propofol in thalamic circuits are largely unknown. We investigated the influence of propofol on synaptic responsiveness of thalamocortical relay neurons in the ventrobasal complex (VB) to excitatory input in mouse brain slices, using both current- and voltage-clamp recording techniques. Excitatory responses including EPSP temporal summation and action potential firing were evoked in VB neurons by electrical stimulation of corticothalamic fibers or pharmacological activation of glutamate receptors. Propofol (0.6 - 3 microM) suppressed temporal summation and spike firing in a concentration-dependent manner. The thalamocortical suppression was accompanied by a marked decrease in both EPSP amplitude and input resistance, indicating that a shunting mechanism was involved. The propofol-mediated thalamocortical suppression could be blocked by a GABAA receptor antagonist or chloride channel blocker, suggesting that postsynaptic GABAA receptors in VB neurons were involved in the shunting inhibition. GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked in VB neurons by electrical stimulation of the reticular thalamic nucleus. Propofol markedly increased amplitude, decay time, and charge transfer of GABAA IPSCs. The results demonstrated that shunting inhibition of thalamic somatosensory relay neurons by propofol at clinically relevant concentrations is primarily mediated through the potentiation of the GABAA receptor chloride channel-mediated conductance, and such inhibition may contribute to the impaired thalamic responses to sensory stimuli seen during propofol-induced anesthesia.

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.

Recovery of saccadic dysmetria following localized lesions in monkey superior colliculus

Damage to the monkey superior colliculus (SC) produces deficits in the generation of saccadic eye movements. Recovery of the accuracy of saccades is rapid, but saccadic latency and peak velocity recover slowly or not at all. In the present experiments we revisited the issue of recovery of function following localized lesions of the SC using three methodological advances: implantation of wire recording electrodes into the SC for the duration of the experiment to ensure that we were recording from the same site on the SC map on successive days; quantification of changes in saccadic accuracy, latency, and velocity using a standard grid of target points in the visual field contralateral to the SC lesion; measurement of movement field size to quantitatively determine any changes following the lesion. We confirmed a decrease in saccadic accuracy following electrolytic lesions of the SC, and we found that this dysmetria recovered within about 4 days. Saccadic latency increased for saccades to the lesion area and this deficit persisted. Peak saccadic velocity decreased immediately after the lesion and decreased further during the 10 days to 2 weeks of the experiment. We found no indication of an expansion of the movement fields of neurons adjacent to the lesion area. This lack of reorganization suggests that movement field changes within the SC cannot mediate the recovery in accuracy of the saccade. The persistence of the latency and velocity deficits despite the recovery of amplitude deficits indicates that saccadic latency and peak velocity are dependent upon the SC whereas saccadic amplitude is not.

Corticothalamic modulation on formalin-induced change of VPM thalamic activities

Spontaneous activities of single cells were extracellularly recorded in ventral posterior medial (VPM) thalamus of anesthetized rats to characterize the corticothalamic modulation on formalin-induced changes of spontaneous thalamic firing. Formalin injected into the peripheral receptive field, dose-dependently induced the reversible facilitation of spontaneous activities of VPM. However, when the primary somatosensory (SI) cortex was inactivated by muscimol, the pattern of formalin-induced changes of VPM firing was altered. This altered responsiveness included both first and second phase of facilitated spontaneous activities. Bicuculline infused into SI cortex did not alter the pattern of formalin-induced thalamic changes. These results suggest that the pain reactivity of VPM thalamus may be modulated by cortex via corticothalamic pathway during the generation of inflammatory pain.

Changes in corticothalamic modulation of receptive fields during peripheral injury-induced reorganization

The influence of corticothalamic projections on the thalamus during different stages of reorganization was determined in anesthetized raccoons that had undergone previous removal of a single forepaw digit. Single-unit recordings were made from 522 sites in the somatosensory nucleus of the thalamus (ventroposterior lateral nucleus) before and after lesioning parts of primary somatosensory cortex. In those parts of ventroposterior lateral nucleus that had intact input from the periphery, the cortical lesion resulted in an immediate 85% increase in receptive field (RF) size. In animals studied 2-6 weeks after digit amputation, peripherally denervated thalamic neurons had unique RFs that were larger than normal, and these were not further enlarged by cortical lesion. However, at longer periods of reorganization (>4 mo), when the new RFs of denervated neurons had decreased in size, cortical lesion again produced expansion of RF size. These data demonstrate that corticothalamic fibers modulate the spatial extent of thalamic RFs in intact animals, probably by controlling intrathalamic inhibition. This corticothalamic modulation is ineffective during the early stages of injury-induced reorganization when new RFs are being formed, but is reinstated after the new RFs have become stabilized. The fact that neurons in the denervated thalamic region retained their unique RFs after cortical lesion indicates that their new inputs are not being relayed from a reorganized cortex and support the view that some plasticity occurs in or below the thalamus.

Functional imaging of the human trigeminal system: Opportunities for new insights into pain processing in health and disease

Peripheral inflammation or nerve damage result in changes in nervous system function, and may be a source of chronic pain. A number of animal studies have indicated that central neural plasticity, including sensitization of neurons within the spinal cord and brain, is part of the response to nervous system insult, and can result in the appearance of altered sensation, including pain. It cannot be assumed, however, that data obtained from animal models unambiguously reflects CNS changes that occur in humans. Currently, the only noninvasive approach to determining objective changes in neural processing and responsiveness within the CNS in humans is the use of functional imaging techniques. It is now possible to use functional magnetic resonance imaging (fMRI) to measure CNS activation in the trigeminal ganglion, spinal trigeminal nucleus, the thalamus, and the somatosensory cortex in healthy volunteers, in a surrogate model of hyperalgesia, and in patients with trigeminal pain. By offering a window into the temporal and functional changes that occur in the damaged nervous system in humans, fMRI can provide both insight into the mechanisms of normal and pathological pain and, potentially, an objective method for measuring altered sensation. These advances are likely to contribute greatly to the diagnosis and treatment of clinical pain conditions affecting the trigeminal system (e.g., neuropathic pain, migraine).

Effects of restricted cochlear lesions in adult cats on the frequency organization of the inferior colliculus

Restricted cochlear lesions in adult animals result in plastic changes in the representation of the lesioned cochlea, and thus in the frequency map, in the contralateral auditory cortex and thalamus. To examine the contribution of subthalamic changes to this reorganization, the effects of unilateral mechanical cochlear lesions on the frequency organization of the central nucleus of the inferior colliculus (ICC) were examined in adult cats. Lesions typically resulted in a broad high-frequency hearing loss extending from a frequency in the range 15-22 kHz. After recovery periods of 2.5-18 months, the frequency organization of ICC contralateral to the lesioned cochlea was determined separately for the onset and late components of multiunit responses to tone-burst stimuli. For the late response component in all but one penetration through the ICC, and for the onset response component in more than half of the penetrations, changes in frequency organization in the lesion projection zone were explicable as the residue of prelesion responses. In half of the penetrations exhibiting nonresidue type changes in onset-response frequency organization, the changes appeared to reflect the unmasking of normally inhibited inputs. In the other half it was unclear whether the changes reflected unmasking or a dynamic process of reorganization. Thus, most of the observed changes were explicable as passive consequences of the lesion, and there was limited evidence for plasticity in the ICC. The implications of the data with respect to the primary locus of the changes and to the manner in which they contribute to thalamocortical reorganization are considered.

Chronic, multisite, multielectrode recordings in macaque monkeys

A paradigm is described for recording the activity of single cortical neurons from awake, behaving macaque monkeys. Its unique features include high-density microwire arrays and multichannel instrumentation. Three adult rhesus monkeys received microwire array implants, totaling 96-704 microwires per subject, in up to five cortical areas, sometimes bilaterally. Recordings 3-4 weeks after implantation yielded 421 single neurons with a mean peak-to-peak voltage of 115 +/- 3 microV and a signal-to-noise ratio of better than 5:1. As many as 247 cortical neurons were recorded in one session, and at least 58 neurons were isolated from one subject 18 months after implantation. This method should benefit neurophysiological investigation of learning, perception, and sensorimotor integration in primates and the development of neuroprosthetic devices.

Corticothalamic feedback and sensory processing

Although nearly half of the synaptic input to neurons in the dorsal thalamus comes from the cerebral cortex, the role of corticothalamic projections in sensory processing remains elusive. Although sensory afferents certainly establish the basic receptive field properties of thalamic neurons, increasing evidence indicates that feedback from the cortex plays a crucial role in shaping thalamic responses. Here, we review recent work on the corticothalamic pathways associated with the visual, auditory, and somatosensory systems. Collectively, these studies demonstrate that sensory responses of thalamic neurons result from dynamic interactions between feedforward and feedback pathways.

Physiological changes in primate somatosensory thalamus induced by deafferentation are dependent on the spinal funiculi that are sectioned and time following injury

The importance of spike bursts in thalamo-cortical processing of sensory information has received an increasing amount of interest over the past several years. Previously it has been reported that short high-frequency spike trains (3-8 action potentials occurring at 67-167 Hz), or spike bursts, are increased in both human and non-human primate thalamus following deafferentation. Here we examine the effects of lesion of the ventral spinal quadrant alone versus combined lesion of the ventral and dorsal spinal quadrants on the evoked and spontaneous spike trains in thalamic neurons. A total of 1175 neurons were sampled from 13 animals, three intact, six with ventral quadrant lesions (three with prolonged survival and three with short-term survival after spinal lesion) and four with combined ventral and dorsal quadrant lesions. Detailed analysis was conducted on 256 of these neurons, which revealed that thalamic neurons of animals with ventral quadrant lesions had elevated burst and non-burst spike rates while neurons from animals with combined ventral-dorsal lesions showed two types of change. Neurons in the forelimb areas showed increased bursts without a change in non-burst activity, while neurons in lateral VPL without receptive fields showed very low non-burst activity, but high burst spike rates. The magnitude of the effects produced by ventral-lateral spinal lesions was more pronounced in the short-term survival animals than in the long-term survival animals. These results show that the effects of deafferentation on the physiological properties of thalamic neurons are dependent on the afferent tract or tracts that are lesioned and the time after lesion.

Kinaesthetic neurons in thalamus of humans with and without tremor

Increased afferent input may alter receptive field sizes, properties and somatotopographic representation in the cortex. Changes in the motor thalamus may also occur as a result of altered afferent input. Such plasticity has been implicated in both sensory and movement disorders. Using tremor as a model of augmented afferent input to kinaesthetic/deep neurons representing the shaking limbs, we studied the representation and properties of these neurons in human thalamus in patients with resting tremor (RestTr) from Parkinson's disease, patients with action- or posture-induced tremor (ActionTr), and patients without tremor (NoTr). Data were collected during stereotactic thalamotomy or insertion of deep brain stimulators for relief of pain or movement disorder. Using microelectrode recording, 58 kinaesthetic neurons responding to wrist and/or elbow movement were studied by mapping the receptive field, carefully isolating each joint during testing. There were no significant differences in the proportions of single and multijoint responsive neurons in the different patient groups (RestTr, ActionTr and NoTr). The borders between tactile-cutaneous, deep-kinaesthetic and voluntary cell representations in the thalamus were mapped in 74 patients and compared between the different tremor groups. A significant difference in kinaesthetic representation was found: both the RestTr and ActionTr groups had a significantly greater kinaesthetic representation than the NoTr patients. There was an expansion of kinaesthetic representation in patients with chronic increased afferent drive from tremor, without alteration in RF size. No decrease in tactile representation was found, suggesting that the increase in kinaesthetic representation does not occur at the expense of tactile representation. These data suggest that plasticity can occur at the thalamic level in humans and may contribute to the pathogenesis of tremor.

[Neurobiology of trigeminal pain]

The brainstem trigeminal complex integrates somatosensory inputs from orofacial areas and meninges. Recent studies have shown the existence of a double representation of pain within the brainstem, at the level of both caudalis and oralis subnuclei. Noxious messages are mainly conveyed by C-fibers that activate the subnucleus caudalis neurons. These neurons in turn activate the subnucleus oralis whose neurons share similar features with the deep spinal dorsal horn neurons. In contrast with the nearness of the laminar organization of the dorsal horn, the vertical organization of the trigeminal complex offers an easier access for the study of segmental mechanisms of nociceptive processing. This model allowed us to show the existence of subtle NMDA-related mechanisms of segmental nocious processing. The trigeminal complex conveys nociceptive messages to several brainstem and thalamic relays that activate a number of cortical areas responsible for pain sensations and reactions. Cortical processing is sustained by reciprocal interactions with thalamic areas and also by a direct modulation of their pre-thalamic relays. The dysfunction of these multiple modulatory mechanisms probably plays a key role in the pathophysiology of chronic trigeminal pain.

Induction of bilateral plasticity in sensory cortical maps by small unilateral cortical infarcts in rats

Behavioural impairments caused by brain lesions show a considerable, though often incomplete, recovery. It is hypothesized that cortical and subcortical plasticity of sensory representations contribute to this recovery. In the hindpaw representation of somatosensory cortex of adult rats we investigated the effects of focal unilateral cortical lesions on remote areas. Cortical lesions with a diameter of approximately 2 mm were induced in the parietal cortex by photothrombosis with the photosensitive dye Rose Bengal. Subsequently, animals were kept in standard cages for 7 days. On day seven, animals were anaesthetized and cutaneous receptive fields in the cortical hindpaw representations ipsi- and contralateral to the lesion were constructed from extracellular recordings of neurons in layer IV using glass microelectrodes. Receptive fields in the lesioned animals were compared to receptive fields measured in nonlesioned animals serving as controls. Quantitative analysis of receptive fields revealed a significant increase in size in the lesioned animals. This doubling in receptive field size was observed equally in the hemispheres ipsi- and contralateral to the lesion. The results indicate that the functional consequences of restricted cortical lesions are not limited to the area surrounding the lesion, but affect the cortical maps on the contralateral, nonlesioned hemisphere.

Dynamic shifting in thalamocortical processing during different behavioural states

Recent experiments in our laboratory have indicated that as rats shift the behavioural strategy employed to explore their surrounding environment, there is a parallel change in the physiological properties of the neuronal ensembles that define the main thalamocortical loop of the trigeminal somatosensory system. Based on experimental evidence from several laboratories, we propose that this concurrent shift in behavioural strategy and thalamocortical physiological properties provides rats with an efficient way to optimize either the detection or analysis of complex tactile stimuli.

Is there a thalamic component to experience-dependent cortical plasticity?

Sensory deprivation and injury to the peripheral nervous system both induce plasticity in the somatosensory system of adult animals, but in different places. While injury induces plasticity at several locations within the ascending somatosensory pathways, sensory deprivation appears only to affect the somatosensory cortex. Experiments have been performed to detect experience-dependent plasticity in thalamic receptive fields, thalamic domain sizes and convergence of thalamic receptive fields onto cortical cells. So far, plasticity has not been detected with sensory deprivation paradigms that cause substantial cortical plasticity. Part of the reason for the lack of thalamic plasticity may lie in the synaptic properties of afferent systems to the thalamus. A second factor may lie in the differences in the organization of cortical and thalamic circuits. Many deprivation paradigms induce plasticity by decreasing phasic lateral inhibition. Since lateral inhibition appears to be far weaker in the thalamus than the cortex, sensory deprivation may not cause large enough imbalances in thalamic activity to induce plasticity in the thalamus.

Reversible changes of presumable synaptic connections between primary somatosensory cortex and ventral posterior lateral thalamus of rats during temporary deafferentation

Many single neurons were simultaneously recorded from forepaw areas of both primary somatosensory cortex and the ventral posterior lateral thalamus of anesthetized rats to characterize the changes of presumable excitatory synaptic connections between two nuclei following temporary deafferentation (TD). Thalamic and cortical interactions were examined by analyzing spike-triggered cross-correlation histograms (STCCHs, n=426). Before TD, 46.48% of STCCHs exhibited thalamocortical (TC) excitation and 7.51% of STCCHs showed corticothalamic (CT) connectivity. After TD, these connections were less frequently observed (after 20 min of TD, TC: 13.38% of STCCHs, CT: 5.40% of STCCHs). Fifty-seven TC and nine CT connections were reversibly suppressed during TD. However, 23 CT connections were reversibly augmented following TD. These results imply that temporary blocking of afferent information may induce system-wide plasticity involving corticofugal modulation.

Short-term dynamics of thalamocortical and intracortical synapses onto layer 6 neurons in neocortex

Layer 6 is the main source of neocortical connections back to specific thalamic nuclei. Corticothalamic (CT) systems play an important role in shaping sensory input, but little is known about the functional circuitry that generates CT activity. We recorded from the two main types of neurons in layer 6, regular-spiking (RS; pyramidal neurons) and fast-spiking (FS; inhibitory interneurons) cells and compared the physiological properties of different excitatory inputs. Thalamic stimulation evoked two monosynaptic inputs with distinct properties: suspected thalamocortical (TC) synaptic events had short latencies, short-term synaptic depression, and paired-pulse responses that suggested subnormal axonal conduction. A second group of synaptic responses likely originated from intracortical collaterals of CT cells that were antidromically activated from the thalamus. These intracortical responses had longer latencies, short-term synaptic facilitation, and were transmitted by axons with supernormal conduction. Suspected TC inputs to FS cells had significantly larger amplitudes than those onto RS cells. Dual recordings from neighboring neurons in layer 6 revealed both facilitating and depressing synaptic connections; the depressing synapses were probably formed by layer 6 cells that do not project to the thalamus, and thus were not sampled by thalamic stimulation. We conclude that layer 6 neurons integrate a variety of inputs with distinct temporal dynamics that are determined by the presynaptic cell type.

Plasticity and corticofugal modulation for hearing in adult animals

The descending (corticofugal) auditory system adjusts and improves auditory signal processing in the subcortical auditory nuclei. The auditory cortex and corticofugal system evoke small, short-term changes of the subcortical auditory nuclei in response to a sound repetitively delivered to an animal. These changes are specific to the parameters characterizing the sound. When the sound becomes significant to the animal through conditioning (associative learning), the changes are augmented and the cortical changes become long-term. There are two types of reorganizations: expanded reorganization resulting from centripetal shifts in tuning curves of neurons toward the values of the parameters characterizing a sound and compressed reorganization resulting from centrifugal shifts in tuning curves of neurons away from these values. The two types of reorganizations are based on a single mechanism consisting of two components: facilitation and inhibition.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Thalamocortical [correction of Thalamcortical] optimization of tactile processing according to behavioral state

We propose a conceptual model that describes the operation of the main thalamocortical loop of the rat somatosensory system. According to this model, the asynchronous convergence of ascending and descending projections dynamically alters the physiological properties of thalamic neurons in the ventral posterior medial (VPM) nucleus as rats shift between three behavioral states. Two of these states are characterized by distinct modes of rhythmic whisker movements. We posit that these simultaneous shifts in exploratory behavioral strategy and in the physiological properties of VPM neurons allow rats to either (i) optimize the detection of stimuli that are novel or difficult to sense or (ii) process complex patterns of multi-whisker stimulation.

Centripetal and centrifugal reorganizations of frequency map of auditory cortex in gerbils

As repetitive acoustic stimulation and auditory conditioning do, electric stimulation of the primary auditory cortex (AI) evokes reorganization of the frequency map of AI, as well as of the subcortical auditory nuclei. The reorganization is caused by shifts in best frequencies (BFs) of neurons either toward (centripetal) or away from (centrifugal) the BF of stimulated cortical neurons. In AI of the Mongolian gerbil, we found that focal electrical stimulation evoked a centripetal BF shift in an elliptical area centered at the stimulated neurons and a centrifugal BF shift in a zone surrounding it. The 1.9-mm long major and 1.1-mm long minor axes of the elliptical area were parallel and orthogonal to the frequency axis, respectively. The width of the surrounding zone was 0.2-0.3 mm. Such "center-surround" reorganization has not yet been found in any sensory cortex except AI of the gerbil. The ellipse is similar to the arborization pattern of pyramidal neurons, the major source of excitatory horizontal connections in AI, whereas the surrounding zone is compatible to the arborization range of small basket cells (inhibitory neurons) in AI.

Cerebello-thalamic synapses and motor adaptation

The cerebellum's influence on voluntary movement is mediated, in large part, through the cerebello-thalamo-cortical (CTC) pathway. Of particular relevance here are those neurons in the cerebellar nuclei that project, via thalamus, to pyramidal tract neurons in primary motor cortex. Several lines of evidence implicate cerebello-thalamic (CT) synaptic plasticity as a neural substrate underlying movement adaptation in adult animals. CT synapses exhibit a number of structural characteristics suggestive of a capacity for both formation of new synapses, and alterations in efficacy of transmission across existing synapses. Long-term potentiation can be evoked across CT synapses in vitro by high frequency stimulation, albeit in young animals. Evidence regarding the contribution of CT synaptic plasticity to two different types of movement adaptation in adult animals is conflicting. Adaptation involving a strengthening and re-coordination of voluntary movement is associated with an increase in density of CT synaptic boutons and an increase in number of synaptic vesicles available for immediate neurotransmitter release within each bouton. On the other hand, adaptation involving associative conditioning of a reduced sensorimotor neural circuit is associated with plasticity at thalamo-cortical but not CT synapses. These conflicting findings may reflect differences in the extent of synaptic re-organization that occurs at thalamic versus cortical levels, differences in the neural circuitry mediating each behavior, and/or differences in the spatio-temporal convergence of activity in the thalamus during the adaptive processes. It is concluded that CT synaptic plasticity can underlie movement adaptation if the adaptation requires reorganization of the cerebellum's influence on cerebral cortex.

Corticofugal modulation of duration-tuned neurons in the midbrain auditory nucleus in bats

Animal sounds, as well as human speech sounds, are characterized by multiple parameters such as frequency, intensity, duration, etc. The central auditory system produces neurons tuned to particular durations and frequencies of sounds emitted by a species. In bats, "duration-tuned" neurons are mostly sensitive to short durations and high frequencies of sounds used for echolocation. They are scattered in the frequency maps of the inferior colliculus and auditory cortex. We found that electric stimulation of cortical duration-tuned neurons modulates collicular duration-tuned neurons in both duration and frequency tuning only when collicular and cortical neurons paired for studies are within +/-4 ms in best duration and within +/-6 kHz in best frequency. There are four types of modulations: sharpening or broadening of duration tuning, and lengthening or shortening of best duration. Sharpening is observed in "matched" collicular neurons whose best durations are the same as those of stimulated cortical neurons, and it is accompanied by augmentation of the auditory responses at their best durations. The other three types of modulations are observed in "unmatched" collicular neurons whose best durations are different from those of stimulated cortical neurons. Lengthening or shortening of best duration is linearly related to the amount of the difference in best duration between collicular and cortical neurons. Corticofugal modulation is specific and systematic according to relationships in both duration and frequency between stimulated cortical and recorded collicular neurons.

Techniques for long-term multisite neuronal ensemble recordings in behaving animals

Advances in our understanding of neural systems will go hand in hand with improvements in the experimental techniques used to study these systems. This article describes a series of methodological developments aimed at enhancing the power of the methods needed to record simultaneously from populations of neurons over broad regions of the brain in awake, behaving animals. First, our laboratory has made many advances in electrode design, including movable bundle and array electrodes and smaller electrode assemblies. Second, to perform longer and more complex multielectrode implantation surgeries in primates, we have modified our surgical procedures by employing comprehensive physiological monitoring akin to human neuroanesthesia. We have also developed surgical implantation techniques aimed at minimizing brain tissue damage and facilitating penetration of the cortical surface. Third, we have integrated new technologies into our neural ensemble, stimulus and behavioral recording experiments to provide more detailed measurements of experimental variables. Finally, new data analytical techniques are being used in the laboratory to analyze increasingly large quantities of data.

The neural basis of perceptual learning

Perceptual learning is a lifelong process. We begin by encoding information about the basic structure of the natural world and continue to assimilate information about specific patterns with which we become familiar. The specificity of the learning suggests that all areas of the cerebral cortex are plastic and can represent various aspects of learned information. The neural substrate of perceptual learning relates to the nature of the neural code itself, including changes in cortical maps, in the temporal characteristics of neuronal responses, and in modulation of contextual influences. Top-down control of these representations suggests that learning involves an interaction between multiple cortical areas.

Behavioral properties of the trigeminal somatosensory system in rats performing whisker-dependent tactile discriminations

To address several fundamental questions regarding how multiwhisker tactile stimuli are integrated and processed by the trigeminal somatosensory system, a novel behavioral task was developed that required rats to discriminate the width of either a wide or narrow aperture using only their large mystacial vibrissae. Rats quickly acquired this task and could accurately discriminate between apertures of very similar width. Accurate discriminations required a large number of intact facial whiskers. Systematic removal of individual whiskers caused a decrease in performance that was directly proportional to the number of whiskers removed, indicating that tactile information from multiple whiskers is integrated as rats gauge aperture width. In different groups of rats, different sets of whiskers were removed in patterns that preferentially left whisker rows or whisker arcs intact. These different whisker removals caused similar decreases in performance, indicating that individual whiskers within the vibrissal array are functionally equivalent during performance of this task. Lesions of the barrel cortex abolished the ability of rats to discriminate, demonstrating that this region is critically involved in this tactile behavior. Interestingly, sectioning the facial nerve, which abolished whisker movements, did not affect the ability to perform accurate discriminations, indicating that active whisker movements are not necessary for accurate performance of the task. Collectively, these results indicate that the trigeminal somatosensory system forms internal representations of external stimuli (in this case, aperture width) by integrating tactile input from many functionally equivalent facial whiskers and that the vibrissal array can function as a fine-grained distance detector without active whisker movements.

Somatotopic reorganization in the brainstem and thalamus following peripheral nerve injury in adult primates

Injury-induced reorganization of central somatotopic maps is a phenomenon that has proven to be useful for elucidating the mechanisms and time course of neural plasticity. To date, the overwhelming majority of this line of research has focused on such plastic events in cortical areas, at the expense of subcortical structures. In this study, we used multi-unit electrophysiological recording techniques to assess the somatotopic organization of brainstem and thalamic areas following chronic survival from paired median and ulnar nerve section in adult squirrel monkeys. We report that the extent of cutaneously-driven reorganization in both the cuneate nucleus of the brainstem and the ventroposterior lateral nucleus of the thalamus is comparable to that previously documented for area 3b of cortex. These observations are consistent with those previously reported in thalamus, and are unique for brainstem.

Behavioral properties of the trigeminal somatosensory system in rats performing whisker-dependent tactile discriminations

To address several fundamental questions regarding how multiwhisker tactile stimuli are integrated and processed by the trigeminal somatosensory system, a novel behavioral task was developed that required rats to discriminate the width of either a wide or narrow aperture using only their large mystacial vibrissae. Rats quickly acquired this task and could accurately discriminate between apertures of very similar width. Accurate discriminations required a large number of intact facial whiskers. Systematic removal of individual whiskers caused a decrease in performance that was directly proportional to the number of whiskers removed, indicating that tactile information from multiple whiskers is integrated as rats gauge aperture width. In different groups of rats, different sets of whiskers were removed in patterns that preferentially left whisker rows or whisker arcs intact. These different whisker removals caused similar decreases in performance, indicating that individual whiskers within the vibrissal array are functionally equivalent during performance of this task. Lesions of the barrel cortex abolished the ability of rats to discriminate, demonstrating that this region is critically involved in this tactile behavior. Interestingly, sectioning the facial nerve, which abolished whisker movements, did not affect the ability to perform accurate discriminations, indicating that active whisker movements are not necessary for accurate performance of the task. Collectively, these results indicate that the trigeminal somatosensory system forms internal representations of external stimuli (in this case, aperture width) by integrating tactile input from many functionally equivalent facial whiskers and that the vibrissal array can function as a fine-grained distance detector without active whisker movements.

Bilateral integration of whisker information in the primary somatosensory cortex of rats

The isomorphic representation of the contralateral whisker pad in the rodent cerebral cortex has served as a canonical example in primary somatosensory areas that the contralateral body surface is spatially represented as a topographic map. By characterizing responses evoked by multiwhisker stimuli, we provide direct evidence that the whisker region of the rat primary somatosensory cortex (SI) integrates information from both contralateral and ipsilateral whisker pads. The proportions of SI neurons responsive to ipsilateral whisker stimuli, as well as their response probabilities, increased with the number of ipsilateral whiskers stimulated. Under bilateral whisker stimulation, the responses of 95% of neurons recorded were affected by stimulation of ipsilateral whiskers. Contralateral tactile responses of SI neurons were profoundly influenced by preceding ipsilateral stimuli and vice versa. This effect depended on both the spatial location and the relative timing of bilateral whisker stimuli, leading to both spatial and temporal asymmetries of interaction. Bilateral whisker stimulation resulted in only modest changes in evoked response latency. Previous ipsilateral stimulation was also shown to affect tactile responses evoked by later ipsilateral stimuli. Inactivation of the opposite SI abolished ipsilaterally evoked responses as well as their influence on subsequently evoked contralateral responses in the intact SI. Based on these results, we conclude that the rat SI integrates information from both whisker pads and propose that such interactions may underlie the ability of rats to discriminate bilateral tactile stimuli.

Bilateral integration of whisker information in the primary somatosensory cortex of rats

The isomorphic representation of the contralateral whisker pad in the rodent cerebral cortex has served as a canonical example in primary somatosensory areas that the contralateral body surface is spatially represented as a topographic map. By characterizing responses evoked by multiwhisker stimuli, we provide direct evidence that the whisker region of the rat primary somatosensory cortex (SI) integrates information from both contralateral and ipsilateral whisker pads. The proportions of SI neurons responsive to ipsilateral whisker stimuli, as well as their response probabilities, increased with the number of ipsilateral whiskers stimulated. Under bilateral whisker stimulation, the responses of 95% of neurons recorded were affected by stimulation of ipsilateral whiskers. Contralateral tactile responses of SI neurons were profoundly influenced by preceding ipsilateral stimuli and vice versa. This effect depended on both the spatial location and the relative timing of bilateral whisker stimuli, leading to both spatial and temporal asymmetries of interaction. Bilateral whisker stimulation resulted in only modest changes in evoked response latency. Previous ipsilateral stimulation was also shown to affect tactile responses evoked by later ipsilateral stimuli. Inactivation of the opposite SI abolished ipsilaterally evoked responses as well as their influence on subsequently evoked contralateral responses in the intact SI. Based on these results, we conclude that the rat SI integrates information from both whisker pads and propose that such interactions may underlie the ability of rats to discriminate bilateral tactile stimuli.

Effects of acetylcholine and atropine on plasticity of central auditory neurons caused by conditioning in bats

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.

Neural assemblies: technical issues, analysis, and modeling

Neurons often work together to compute and process information, and neural assemblies arise from synaptic interactions and neural circuits. One way to study neural assemblies is to simultaneously record from several or many neurons and study the statistical relations among their spike trains. From this analysis researchers can try to understand the nature of the assemblies, which can also lead to attempts at modeling the underlying mechanisms. In this review we discuss three important parts of this process: (1) technical issues related to simultaneously recording more than one single unit, (2) ways of analyzing the data and (3) recent models offering hypothetical mechanisms of neural assemblies, especially models which incorporate feedback.

The role of cortical activity in experience-dependent potentiation and depression of sensory responses in rat barrel cortex

The role of cortical activity in experience-dependent cortical plasticity was studied in the rat barrel cortex. Plasticity was induced by depriving every other whisker in a chessboard pattern, which is known to cause depression of responses to deprived whisker stimulation and potentiation of responses to spared whisker stimulation. Postsynaptic activity was blocked by muscimol released from elvax slow-release polymer located under the dura and over the barrel field. Spared whisker responses potentiated 2.5-fold in layer II/III and 2.9-fold in layer IV of the near-neighbor barrel in animals implanted with saline-elvax. In contrast, in whisker-deprived animals implanted with muscimol-elvax, responses were indistinguishable from those in undeprived animals. Similarly, in the spared barrel itself, spared whisker responses potentiated 1.3-fold in layer IV in animals implanted with saline-elvax but not at all in muscimol-treated animals. Whiskers that were deprived and then allowed to regrow showed depressed responses in saline-elvax-treated animals, in which 40% of the cells in layer II/III and 26% in layer IV were unresponsive to their principal whisker. These values fell to 17 and 3% for layers II/III and IV, respectively, in muscimol-treated animals, and the response magnitude distributions were indistinguishable from undeprived cases. Cortical activity block had no acute effect on the ventroposteriomedial nucleus responses and had a transient facilitatory effect after 4 d of muscimol treatment, which returned to baseline as the muscimol treatment wore off. We conclude from these studies that cortical activity is required for potentiation and depression of sensory responses in barrel cortex.

The role of cortical activity in experience-dependent potentiation and depression of sensory responses in rat barrel cortex

The role of cortical activity in experience-dependent cortical plasticity was studied in the rat barrel cortex. Plasticity was induced by depriving every other whisker in a chessboard pattern, which is known to cause depression of responses to deprived whisker stimulation and potentiation of responses to spared whisker stimulation. Postsynaptic activity was blocked by muscimol released from elvax slow-release polymer located under the dura and over the barrel field. Spared whisker responses potentiated 2.5-fold in layer II/III and 2.9-fold in layer IV of the near-neighbor barrel in animals implanted with saline-elvax. In contrast, in whisker-deprived animals implanted with muscimol-elvax, responses were indistinguishable from those in undeprived animals. Similarly, in the spared barrel itself, spared whisker responses potentiated 1.3-fold in layer IV in animals implanted with saline-elvax but not at all in muscimol-treated animals. Whiskers that were deprived and then allowed to regrow showed depressed responses in saline-elvax-treated animals, in which 40% of the cells in layer II/III and 26% in layer IV were unresponsive to their principal whisker. These values fell to 17 and 3% for layers II/III and IV, respectively, in muscimol-treated animals, and the response magnitude distributions were indistinguishable from undeprived cases. Cortical activity block had no acute effect on the ventroposteriomedial nucleus responses and had a transient facilitatory effect after 4 d of muscimol treatment, which returned to baseline as the muscimol treatment wore off. We conclude from these studies that cortical activity is required for potentiation and depression of sensory responses in barrel cortex.

High-pass filtering of corticothalamic activity by neuromodulators released in the thalamus during arousal: in vitro and in vivo

The thalamus is the principal relay station of sensory information to the neocortex. In return, the neocortex sends a massive feedback projection back to the thalamus. The thalamus also receives neuromodulatory inputs from the brain stem reticular formation, which is vigorously activated during arousal. We investigated the effects of two neuromodulators, acetylcholine and norepinephrine, on corticothalamic responses in vitro and in vivo. Results from rodent slices in vitro showed that acetylcholine and norepinephrine depress the efficacy of corticothalamic synapses while enhancing their frequency-dependent facilitation. This produces a stronger depression of low-frequency responses than of high-frequency responses. The effects of acetylcholine and norepinephrine were mimicked by muscarinic and alpha(2)-adrenergic receptor agonists and blocked by muscarinic and alpha-adrenergic antagonists, respectively. Stimulation of the brain stem reticular formation in vivo also strongly depressed corticothalamic responses. The suppression was very strong for low-frequency responses, which do not produce synaptic facilitation, but absent for high-frequency corticothalamic responses. As in vitro, application of muscarinic and alpha-adrenergic antagonists into the thalamus in vivo abolished the suppression of corticothalamic responses induced by stimulating the reticular formation. In conclusion, cholinergic and noradrenergic activation during arousal high-pass filters corticothalamic activity. Thus, during arousal only high-frequency inputs from the neocortex are allowed to reach the thalamus. Neuromodulators acting on corticothalamic synapses gate the flow of cortical activity to the thalamus as dictated by behavioral state.

Differential synaptic distribution of AMPA receptor subunits in the ventral posterior and reticular thalamic nuclei of the rat

Although the mechanisms by which the cerebral cortex controls its ascending input are still poorly understood, it is known that cortical control at the thalamic level is via direct glutamatergic projections to relay nuclei and to the reticular nucleus. Here we confirm previous light microscopic reports of a high expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, GluR4, in reticular and ventral posterior thalamic nuclei of the rat, and moderate staining using an antibody recognizing both GluR2 and GluR3. In contrast only low levels of staining for GluR2, and barely detectable levels of GluR1 immunoreactivity were observed. After injections of biotinylated dextran, electron microscopy revealed that anterogradely-labeled cortical synapses in both thalamic nuclei were small with fewer mitochondria and more densely-packed vesicles than terminals likely to arise from intrinsic and ascending pathways. We performed post-embedding immunogold to provide quantitative data on the density of AMPA receptor subunits at morphologically-defined groups of synapses. We found that corticothalamic synapses in the reticular thalamic nucleus contain twice as much GluR2/3, and at least three times more GluR4 protein than do intrathalamic synapses. In the ventral posterior nucleus, corticothalamic synapses contain similar amounts of GluR2/3, but four times more GluR4 than do those from ascending afferents. Corticothalamic synapses in reticular nucleus contain slightly more GluR2/3, and three times more GluR4, than those in ventral posterior nucleus. We conclude that enrichment of GluR4 at morphologically-defined cortical synapses is a feature common to both thalamic nuclei, and those in the reticular nucleus express higher levels of AMPA receptors. The rapid kinetics of GluR4-rich AMPA receptors we suggest indicate that cortical descending control may be more temporally precise than previously recognized.

Up-regulation of metabotropic glutamate receptor 3 mRNA expression in the cerebral cortex of monoarthritic rats

Metabotropic glutamate receptors (mGluR) have been shown to play a role in the modulation of acute and inflammatory pain. Additionally, we have recently detected time-dependent changes in the mRNA expression of several mGluR subtypes in thalamic nuclei of monoarthritic (MA) rats. In the present study, mGluR1, -3, -4, and -7 subtype mRNA expression was analyzed by in situ hybridization with radioactively labelled oligonucleotide probes in cerebral cortical regions of normal and MA rats at 2, 4, and 14 days of the disease. The mGluR1, -4, and -7 mRNAs were at background level in normal rats and did not change in MA animals. In contrast, mGluR3 mRNA expression was abundant in normal rats and was significantly increased in cortical areas of MA rats at all time points. Higher changes were detected bilaterally at 4 days, predominantly in layers IV/V, in the motor, primary, and secondary somatosensory cortices (average increases of 50-75%), but maximum rises occurred in the contralateral cingulate cortex (+138%). No changes were detected in the auditory cortex. The present data show an up-regulation of mGluR3 mRNA expression in the motor, somatosensory, and limbic cortices of MA rats. This possibly reflects the occurrence of central mechanisms counteracting the increased transmission of nociceptive input arising from the inflamed paw and the impaired motor behavior of these rats. Changes in the cingulate cortex may be related to the motivational-affective component of nociception.