Morphology of visual sector thalamic reticular neurons in the macaque monkey suggests retinotopically specialized,parallel stream‐mixed input to the lateral geniculate nucleus

The thalamic reticular nucleus (TRN) is a unique brain structure at the interface between the thalamus and the cortex. Because the TRN receives bottom‐up sensory input and top‐down cortical input, it could serve as an integration hub for sensory and cognitive signals. Functional evidence supports broad roles for the TRN in arousal, attention, and sensory selection. How specific circuits connecting the TRN with sensory thalamic structures implement these functions is not known. The structural organization and function of the TRN is particularly interesting in the context of highly organized sensory systems, such as the primate visual system, where neurons in the retina and dorsal lateral geniculate nucleus of the thalamus (dLGN) are morphologically and physiologically distinct and also specialized for processing particular features of the visual environment. To gain insight into the functional relationship between the visual sector of the TRN and the dLGN, we reconstructed a large number of TRN neurons that were retrogradely labeled following injections of rabies virus expressing enhanced green fluorescent protein (EGFP) into the dLGN. An independent cluster analysis, based on 10 morphological metrics measured for each reconstructed neuron, revealed three clusters of TRN neurons that differed in cell body shape and size, dendritic arborization patterns, and medial‐lateral position within the TRN. TRN dendritic and axonal morphologies are inconsistent with visual stream‐specific projections to the dLGN. Instead, TRN neuronal organization could facilitate transmission of global arousal and/or cognitive signals to the dLGN with retinotopic precision that preserves specialized processing of foveal versus peripheral visual information. J. Comp. Neurol. 525:1273–1290, 2017. © 2016 Wiley Periodicals, Inc.     

Auditory thalamic reticular nucleus of the rat: anatomical nodes for modulation of auditory and cross-modal sensory processing in the loop connectivity between the cortex and thalamus

The auditory sector of the thalamic reticular nucleus (TRN) plays a pivotal role in gain and/or gate control of auditory input relayed from the thalamus to cortex. The TRN is also likely involved in cross-modal sensory processing for attentional gating function. In the present study, we anatomically examined how cortical and thalamic afferents intersect in the auditory TRN with regard to these two functional pathways. Iontophoretic injections of biocytin into subregions of the auditory TRN, which were made with the guidance of electrophysiological recording of auditory response, resulted in retrograde labeling of cortical and thalamic cells, indicating the sources of afferents to the TRN. Cortical afferents from area Te1 (temporal cortex, area 1), which contains the primary and anterior auditory fields, topographically intersected thalamic afferents from the ventral division of the medial geniculate nucleus at the subregions of the auditory TRN, suggesting tonotopically organized convergence of afferents, although they innervated a given small part of the TRN from large parts. In the caudodorsal and rostroventral parts of the auditory TRN, cortical afferents from nonprimary visual and somatosensory areas intersected thalamic afferents from auditory, visual, and somatosensory nuclei. Furthermore, afferents from the caudal insular cortex and the parvicellular part of the ventral posterior thalamic nucleus, which are associated with visceral processing, converged to the rostroventral end of the auditory TRN. The results suggest that the auditory TRN consists of anatomical nodes that mediate tonotopic and/or cross-modal modulation of auditory and other sensory processing in the loop connectivity between the cortex and thalamus.     

Postnatal development of cholinergic input to the thalamic reticular nucleus of the mouse

The thalamic reticular nucleus (TRN), a shell‐like structure comprised of GABAergic neurons, gates signal transmission between thalamus and cortex. While TRN is innervated by axon collaterals of thalamocortical and corticothalamic neurons, other ascending projections modulate activity during different behavioral states such as attention, arousal, and sleep‐wake cycles. One of the largest arise from cholinergic neurons of the basal forebrain and brainstem. Despite its integral role, little is known about how or when cholinergic innervation and synapse formation occurs. We utilized genetically modified mice, which selectively express fluorescent protein and/or channelrhodopsin‐2 in cholinergic neurons, to visualize and stimulate cholinergic afferents in the developing TRN. Cholinergic innervation of TRN follows a ventral‐to‐dorsal progression, with nonvisual sensory sectors receiving input during week 1, and the visual sector during week 2. By week 3, the density of cholinergic fibers increases throughout TRN and forms a reticular profile. Functional patterns of connectivity between cholinergic fibers and TRN neurons progress in a similar manner, with weak excitatory nicotinic responses appearing in nonvisual sectors near the end of week 1. By week 2, excitatory responses become more prevalent and arise in the visual sector. Between weeks 3–4, inhibitory muscarinic responses emerge, and responses become biphasic, exhibiting a fast excitatory, and a long‐lasting inhibitory component. Overall, the development of cholinergic projections in TRN follows a similar plan as the rest of sensory thalamus, with innervation of nonvisual structures preceding visual ones, and well after the establishment of circuits conveying sensory information from the periphery to the cortex.     

The thalamic reticular nucleus and schizophrenia

Background: The thalamic reticular nucleus (TRN) is a shell-shaped gamma amino butyric acid (GABA)ergic nucleus, which is uniquely placed between the thalamus and the cortex, because it receives excitatory afferents from both cortical and thalamic neurons and sends inhibitory projections to all nuclei of the dorsal thalamus. Method: A review of the evidence suggesting that the TRN is implicated in the neurobiology of schizophrenia. Results: TRN-thalamus circuits are implicated in bottom-up as well as top-down processing. TRN projections to nonspecific nuclei of the dorsal thalamus mediate top-down processes, including attentional modulation, which are initiated by cortical afferents to the TRN. TRN-thalamus circuits are also involved in bottom-up activities, including sensory gating and the transfer to the cortex of sleep spindles. Intriguingly, deficits in attention and sensory gating have been consistently found in schizophrenics, including first-break and chronic patients. Furthermore, high-density electroencephalographic studies have revealed a marked reduction in sleep spindles in schizophrenics. Conclusion: On the basis of our current knowledge on the molecular and anatomo-functional properties of the TRN, we suggest that this thalamic GABAergic nucleus may be involved in the neurobiology of schizophrenia.     

Fixational saccade‐related activity of pedunculopontine tegmental nucleus neurons in behaving monkeys

Fixational saccades are small, involuntary eye movements that occur during attempted visual fixation. Recent studies suggested that several cognitive processes affect the occurrence probability of fixational saccades. Thus, there might be an interaction between fixational saccade‐related motor signals and cognitive signals. The pedunculopontine tegmental nucleus (PPTN) in the brainstem has anatomical connections with numerous saccade‐related and limbic areas. Previously, we reported that a group of PPTN neurons showed transient phasic bursts or a pause in activity during large visually guided and spontaneous saccades, and also showed sustained tonic changes in activity with task context. We hypothesised that single PPTN neurons would relay both fixational saccade‐related and task context‐related signals, and might function as an interface between the motor and limbic systems. We recorded the activity of PPTN neurons in behaving monkeys during a reward‐biased task, and analysed neuronal activity for small fixational saccades during visual fixation, and compared it with the activity for large visually guided targeting saccades and large spontaneous saccades during intertrial intervals. A population of PPTN neurons exhibited a fixational saccade‐related phasic increase in activity, and the majority of them also showed activity modulation with large targeting saccades. In addition, a group of these neurons showed a task‐related tonic increase in activity during the fixation period, and half of them relayed the saccade signal only when the neuron exhibited higher tonic activity during the task execution period. Thus, fixational saccade‐related signals of PPTN neurons overlap with tonic task‐related signals, and might contribute to the cognitive modulation of fixational saccades.     

Central thalamic inactivation impairs the expression of action‐ and outcome‐related responses of medial prefrontal cortex neurons in the rat

The mediodorsal (MD) and adjacent intralaminar (IL) and midline nuclei provide the main thalamic input to the medial prefrontal cortex (mPFC) and are critical for associative learning and decision‐making. MD neurons exhibit activity related to actions and outcomes that mirror responses of mPFC neurons in rats during dynamic delayed non‐match to position (dDNMTP), a variation of DNMTP where start location is varied randomly within an open octagonal arena to avoid confounding behavioral events with spatial location. To test whether the thalamus affects the expression of these responses in mPFC, we inhibited the central thalamus unilaterally by microinjecting muscimol at doses and sites found to affect decision‐making when applied bilaterally. Unilateral inactivation reduced normalized task‐related responses in the ipsilateral mPFC without disrupting behavior needed to characterize event‐related neuronal activity. Our results extend earlier findings that focused on delay‐related activity by showing that central thalamic inactivation interferes with responses related to actions and outcomes that occur outside the period of memory delay. These findings are consistent with the broad effects of central thalamic lesions on behavioral measures of reinforcement‐guided responding. Most (7/8) of the prefrontal response types affected by thalamic inactivation have also been observed in MD during dDNMTP. These results support the hypothesis that MD and IL act as transthalamic gates: monitoring prefrontal activity through corticothalamic inputs; integrating this information with signals from motivational and sensorimotor systems that converge in thalamus; and acting through thalamocortical projections to enhance expression of neuronal responses in the PFC that support adaptive goal‐directed behavior.     

Diverse subthreshold cross‐modal sensory interactions in the thalamic reticular nucleus: implications for new pathways of cross‐modal attentional gating function

Our attention to a sensory cue of a given modality interferes with attention to a sensory cue of another modality. However, an object emitting various sensory cues attracts attention more effectively. The thalamic reticular nucleus (TRN) could play a pivotal role in such cross‐modal modulation of attention given that cross‐modal sensory interaction takes place in the TRN, because the TRN occupies a highly strategic position to function in the control of gain and/or gating of sensory processing in the thalamocortical loop. In the present study cross‐modal interactions between visual and auditory inputs were examined in single TRN cells of anesthetised rats using juxta‐cellular recording and labeling techniques. Visual or auditory responses were modulated by subthreshold sound or light stimuli, respectively, in the majority of recordings (46 of 54 visual and 60 of 73 auditory cells). However, few bimodal sensory cells were found. Cells showing modulation of the sensory response were distributed in the whole visual and auditory sectors of the TRN. Modulated cells sent axonal projections to first‐order or higher‐order thalamic nuclei. Suppression predominated in modulation that took place not only in primary responses but also in late responses repeatedly evoked after sensory stimulation. Combined sensory stimulation also evoked de‐novo responses, and modulated response latency and burst spiking. These results indicate that the TRN incorporates sensory inputs of different modalities into single cell activity to function in sensory processing in the lemniscal and non‐lemniscal systems. This raises the possibility that the TRN constitutes neural pathways involved in cross‐modal attentional gating.     

Synaptic targets of thalamic reticular nucleus terminals in the visual thalamus of the cat

A major inhibitory input to the dorsal thalamus arises from neurons in the thalamic reticular nucleus (TRN), which use gamma‐aminobutyric acid (GABA) as a neurotransmitter. We examined the synaptic targets of TRN terminals in the visual thalamus, including the A lamina of the dorsal lateral geniculate nucleus (LGN), the medial interlaminar nucleus (MIN), the lateral posterior nucleus (LP), and the pulvinar nucleus (PUL). To identify TRN terminals, we injected biocytin into the visual sector of the TRN to label terminals by anterograde transport. We then used postembedding immunocytochemical staining for GABA to distinguish TRN terminals as biocytin‐labeled GABA‐positive terminals and to distinguish the postsynaptic targets of TRN terminals as GABA‐negative thalamocortical cells or GABA‐positive interneurons. We found that, in all nuclei, the TRN provides GABAergic input primarily to thalamocortical relay cells (93–100%). Most of this input seems targeted to peripheral dendrites outside of glomeruli. The TRN does not appear to be a significant source of GABAergic input to interneurons in the visual thalamus. We also examined the synaptic targets of the overall population of GABAergic axon terminals (F1 profiles) within these same regions of the visual thalamus and found that the TRN contacts cannot account for all F1 profiles. In addition to F1 contacts on the dendrites of thalamocortical cells, which presumably include TRN terminals, another population of F1 profiles, most likely interneuron axons, provides input to GABAergic interneuron dendrites. Our results suggest that the TRN terminals are ideally situated to modulate thalamocortical transmission by controlling the response mode of thalamocortical cells. J. Comp. Neurol. 440:321–341, 2001. © 2001 Wiley‐Liss, Inc.     

Functional properties of the primary motor cortex and ventral premotor cortex in the monkey during a visually guided jaw-movement task with a delay period

This study investigated single neuronal activity in the face area of the primary motor cortex (MI) and ventral part of the premotor cortex (PMv) while a monkey performed a visually guided jaw-movement task with a delay period. When the monkey executed the jaw movements, 48 MI and 53 PMv neurons showed statistically significant activities time-locked to jaw movements and were defined as movement-related neurons. The activities of movement-related neurons could be classified into phasic, phasic-tonic and tonic patterns based on the changes in discharge rate. Most of the neurons exhibiting phasic and phasic-tonic activities probably contributed to the initiation of jaw movements, since they exhibited transient responses immediately after the onset of the go-cue indicating the jaw movement. In contrast, the sustained activity of the movement-related neurons exhibiting phasic-tonic and tonic activities may be involved in controlling and/or maintaining jaw position. Sustained activity was also detected during the delay period in 4 MI and 29 PMv neurons and these neurons were defined as set-related neurons. It is thought that these set-related neurons are involved in the preparation for the subsequent jaw movement, since the masticatory muscles showed no significant changes during the delay period. These findings suggest that the MI may be involved predominantly in the initiation and control of jaw movements, and that the PMv may be involved in motor preparation, and may play a role as a higher-order motor area related to the initiation and control of jaw movements.     

Are there connections between the thalamic reticular nucleus and the brainstem reticular formation?

Increasing awareness that the thalamic reticular nucleus (TRN) plays an important role in controlling the output of cortically projecting cells in nuclei of the dorsal thalamus has focused attention on the question of whether there exist ascending projections to the TRN from the mesencephalic or other parts of the brainstem reticular formation (BRF). We have examined this and the related question of whether the neurons of TRN project to the BRF, by anterograde and retrograde tracing experiments with horseradish peroxidase (HRP) and HRP conjugated to wheat germ agglutinin. Injections of tracer were placed stereotaxically in the BRF at various depths and rostrocaudal and mediolateral coordinates, and the TRN and adjacent nuclei were examined in serial coronal sections, using tetramethylbenzidine as the principal chromogen. Retrogradely labelled cell bodies were consistently seen in hypothalamus and zona incerta but never in TRN, suggesting that, in the rat, TRN neurons do not project caudal to the thalamus. After 54 out of 60 injections, no terminal label was detected in any part of the TRN although such label was present in other parts of the thalamus, including the intralaminar nuclei, in the same sections. We therefore conclude that direct projections from the BRF to the TRN must be extremely sparse, and that those effects of BRF stimulation upon thalamocortical transmission that are mediated by the TRN (rather than by direct projections to dorsal thalamic nuclei) probably depend chiefly on indirect polysynaptic pathways.     

Axonal projections of single auditory neurons in the thalamic reticular nucleus: implications for tonotopy-related gating function and cross-modal modulation

Tonotopically comparable subfields of the primary auditory area (AI) and nonprimary auditory areas (non-AI), i.e. posterodorsal area (PD) and ventral auditory area (VA), in the rat cortex have similar topographies in the projection to the ventral division of the medial geniculate nucleus (MGV), but reverse topographies in the projection to the thalamic reticular nucleus (TRN). In this study, we examined axonal projections of single auditory TRN cells, using juxtacellular recording and labeling techniques, to determine features of TRN projections and estimate how the TRN mediates corticofugal inhibition along with the reverse topographies of cortical projections to the TRN. Auditory TRN cells sent topographic projections to limited parts of the MGV in a manner that relays cortical inputs from tonotopically comparable subfields of the AI and non-AI (PD and VA) to different parts of the MGV. The results suggest that corticofugal excitations from the AI and non-AI modulate thalamic cell activity in the same part of the MGV, whereas corticofugal inhibitions via the TRN modulate cell activity in different parts of the MGV with regard to tonotopic organization. The AI and non-AI could serve distinctive gating functions for auditory attention through the differential topography of inhibitory modulation. In addition, we obtained an intriguing finding that a subset of auditory TRN cells projected to the somatosensory but not to the auditory thalamic nuclei. There was also a cell projecting to the MGV and somatosensory nuclei. These findings extend the previously suggested possibility that TRN has a cross-modal as well as an intramodal gating function in the thalamus.     

Functional analysis of the thalamocortical pathways in eye movements

Although the roles of the thalamocortical pathways in somatic movements are well documented, their roles in eye movements have only recently been examined. The oculomotor-related areas in the frontal cortex receive inputs from the basal ganglia and the cerebellum via the thalamus. Consistent with this, neurons in the paralaminar part of the ventrolateral (VL), ventroanterior (VA), and mediodorsal (MD) nuclei and those in the intralaminar nuclei exhibit a variety of eye movement-related responses. To date, the thalamocortical pathways are known to play at least 2 roles in eye movements. First, they are involved in the generation of volitional, but not reactive, saccades. Thalamic neurons discharge during anti-saccades, which are known to be impaired in several neurological and psychiatric disorders, such as Parkinson's disease, attention deficit/hyperactivity disorder, and schizophrenia. In addition, neurons in the thalamus also exhibit a gradual increase in firing rate that predicts the timing of self-initiated saccades. Recent inactivation experiments have established the causal roles of these thalamic signals in the generation of volitional saccades. Second, the thalamocortical pathways transmit the efference copy signals for eye movements. During inactivation of the MD thalamus, which relays signals from the superior colliculus to the frontal eye field (FEF), the accuracy of the saccade is reduced in tasks requiring efference copy signals. In addition, inactivation of the same pathways reduces the predictive visual response associated with saccades in neurons in the FEF. Moreover the VL thalamus has been reported to play a role in monitoring smooth pursuit. While the functional analysis of thalamocortical pathways in eye movements is just a beginning, the anatomical data suggest their important roles. Analysis of eye movement control may shed light on the functions of the thalamocortical pathways in general, and may reveal the neural mechanisms of cerebro-cerebellar, cerebro-basal ganglia, and cerebro-thalamic interactions.     

Spatiotemporal organization and thalamic modulation of seizures in the mouse medial thalamic-anterior cingulate slice

Purpose: Seizure‐like activities generated in anterior cingulate cortex (ACC) are usually classified as simple partial and are associated with changes in autonomic function, motivation, and thought. Previous studies have shown that thalamic inputs can modulate ACC seizure, but the exact mechanisms have not been studied thoroughly. Therefore, we investigated the role of thalamic inputs in modulating ACC seizure‐like activities. In addition, seizure onset and propagation are difficult to determine in vivo in ACC. We studied the spatiotemporal changes in epileptiform activity in this cortex in a thalamic–ACC slice to clearly determine seizure onset. Methods: We used multielectrode array (MEA) recording and calcium imaging to investigate the modulatory effect of thalamic inputs in a thalamic–ACC slice preparation. Key Findings: Seizure‐like activities induced with 4‐aminopyridine (4‐AP; 250 μm ) and bicuculline (5–50 μm ) in ACC were attenuated by glutamate receptor antagonists, and the degree of disinhibition varied with the dose of bicuculline. Seizure‐like activities were decreased with 1 Hz thalamic stimulation, whereas corpus callosum stimulation could increase ictal discharges. Amplitude and duration of cingulate seizure‐like activities were augmented after removing thalamic inputs, and this effect was not observed with those induced with elevated bicuculline (50 μm ). Seizure‐like activities were initiated in layers II/III and, after thalamic lesions, they occurred mainly in layers V/VI. Two‐dimensional current‐source density analyses revealed sink signals more frequently in layers V/VI after thalamic lesions, indicating that these layers produce larger excitatory synchronization. Calcium transients were synchronized after thalamic lesions suggesting that ACC seizure‐like activities are subjected to desynchronizing modulation by thalamic inputs. Therefore, ACC seizure‐like activities are subject to desynchronizing modulation from medial thalamic inputs to deep layer pyramidal neurons. Significance: Cingulate seizure‐like activities were modulated significantly by thalamic inputs. Repeated stimulation of the thalamus efficiently inhibited epileptiform activity, demonstrating that the desynchronization was pathway‐specific. The clinical implications of deep thalamic stimulation in the modulation of cingulate epileptic activity require further investigation.     

Functional connectivity between the thalamus and visual cortex under eyes closed and eyes open conditions: A resting‐state fMRI study

The thalamus and visual cortex are two key components associated with the alpha power of electroencephalography. However, their functional relationship remains to be elucidated. Here, we employ resting‐state functional MRI to investigate the temporal correlations of spontaneous fluctuations between the thalamus [the whole thalamus and its three largest nuclei (bilateral mediodorsal, ventrolateral and pulvinar nuclei)] and visual cortex under both eyes open and eyes closed conditions. The whole thalamus show negative correlations with the visual cortex and positive correlations with its contralateral counterpart in eyes closed condition, but which are significantly decreased in eyes open condition, consistent with previous findings of electroencephalography desynchronization during eyes open resting state. Furthermore, we find that bilateral thalamic mediodorsal nuclei and bilateral ventrolateral nuclei have remarkably similar connectivity maps, and resemble to those of the whole thalamus, suggesting their crucial contributions to the thalamus‐visual correlations. The bilateral pulvinar nuclei are found to show distinct functional connectivity patterns, compatible with previous findings of the asymmetry of anatomical and functional organization in the nuclei. Our data provides evidence for the associations of intrinsic spontaneous neuronal activity between the thalamus and visual cortex under different resting conditions, which might have implications on the understanding of the generation and modulation of the alpha rhythm. Hum Brain Mapp 2009. © 2009 Wiley‐Liss, Inc.     

Functional organization of surgically created visual circuits

Lesions of cerebral targets of the retina in newborn hamsters, when combined with transection of lemniscal pathways to the primary auditory or somatosensory thalamic nuclei or the secondary thalamic visual nucleus, can induce the formation of permanent retinal projections to the deafferented non-visual structures. These projections are retinotopically organized and form functional synapses. Consequently, neurons in the auditory or somatosensory cortices, which normally are not driven by visual stimuli, become visually responsive and have receptive field properties that ressemble, in several important ways, those of neurons in the visual cortex of normal animals. The surgically-induced retinothalamo-cortical pathways can mediate visually guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the thalamic dorsal lateral geniculate nucleus, is missing.     

Spatial tuning of reaching activity in the medial parieto-occipital cortex (area V6A) of macaque monkey

We recorded neural activity from the medial parieto-occipital area V6A while three monkeys performed an instructed-delay reaching task in the dark. Targets to be reached were in different spatial positions. Neural discharges were recorded during reaching movements directed outward from the body (towards visual objects), during the holding phase (when the hand was on the target) and during inward movements of the hand towards the home button (which was near the body and outside the field of view). Reach-related activity was observed in the majority of 207 V6A cells, during outward (78%) and inward (65%) movements as well as during the holding phase (62%). Most V6A reaching neurons (84%) were modulated in more than one phase of the task. The reach-related activity in V6A could depend on somatosensory inputs and/or on corollary discharges from the dorsal premotor cortex. Although visual and oculomotor inputs are known to have a strong influence on V6A activity, we excluded the possibility that the reach-related activity which we observed was due to visual stimulation and/or oculomotor activity. Reach-related activity for movements towards different locations was spatially modulated during outward (40%) and inward (47%) reaching movements. The position of the hand/arm in space modulated about 40% of V6A cells. Preferred reach directions and spatial locations were represented uniformly across the workspace. These data suggest that V6A reach-related neurons are able to code the direction of movement of the arm and the position of the hand/arm in space. We suggest that the V6A reach-related neurons are involved in the guidance of goal-directed arm movements, whether these actions are visually guided or not.     

Parvalbumin‐producing striatal interneurons receive excitatory inputs onto proximal dendrites from the motor thalamus in male mice

In rodents, the dorsolateral striatum regulates voluntary movement by integrating excitatory inputs from the motor‐related cerebral cortex and thalamus to produce contingent inhibitory output to other basal ganglia nuclei. Striatal parvalbumin (PV)‐producing interneurons receiving this excitatory input then inhibit medium spiny neurons (MSNs) and modify their outputs. To understand basal ganglia function in motor control, it is important to reveal the precise synaptic organization of motor‐related cortical and thalamic inputs to striatal PV interneurons. To examine which domains of the PV neurons receive these excitatory inputs, we used male bacterial artificial chromosome transgenic mice expressing somatodendritic membrane–targeted green fluorescent protein in PV neurons. An anterograde tracing study with the adeno‐associated virus vector combined with immunodetection of pre‐ and postsynaptic markers visualized the distribution of the excitatory appositions on PV dendrites. Statistical analysis revealed that the density of thalamostriatal appositions along the dendrites was significantly higher on the proximal than distal dendrites. In contrast, there was no positional preference in the density of appositions from axons of the dorsofrontal cortex. Population observations of thalamostriatal and corticostriatal appositions by immunohistochemistry for pathway‐specific vesicular glutamate transporters confirmed that thalamic inputs preferentially, and cortical ones less preferentially, made apposition on proximal dendrites of PV neurons. This axodendritic organization suggests that PV neurons produce fast and reliable inhibition of MSNs in response to thalamic inputs and process excitatory inputs from motor cortices locally and plastically, possibly together with other GABAergic and dopaminergic dendritic inputs, to modulate MSN inhibition.     

Bicuculline injections into the rostral and caudal motor thalamus of the monkey induce different types of dystonia

The pathophysiology of dystonia remains unclear in comparison with other movement disorders. Recent data suggest that there may exist in dystonia an increased thalamic drive to the mesial premotor cortex. To test this hypothesis, we induced overactivity of the motor thalamus by injecting a GABA-A (gamma-aminobutyric acid) antagonist (bicuculline) into the rostral (pallidal) and caudal (cerebellar) ventrolateral nuclei of the thalamus in both hemispheres of one monkey. Dystonic postures were observed in the contralateral limbs and axis. Electromyographic recordings revealed bursts of muscular activation with co-contractions during spontaneous dystonic movements and alterations in muscular patterns during sequential visually guided arm movements. The type of dystonia depended on the site of injections. Rostral thalamic injections induced more severe dystonic postures, whereas myoclonic jerks predominated following caudal injections. We conclude that these two distinct clinical patterns, which are frequently associated in humans, are probably due to a dysfunctioning of segregated thalamic projections to the supplementary motor area (from the rostral part) and to the primary motor cortex (from the caudal part).     

Neurochemical heterogeneity of the thalamic reticular and perireticular nuclei in developing rabbits: patterns of calbindin expression

The thalamic reticular nucleus (TRN) forms an essential part of the circuits that link the thalamus to the cortex, whereas the perireticular thalamic nucleus (PRN) consists of scattered neurons that are located in the internal capsule, in close relation to the TRN. A common feature of these nuclei in different species is the immunoreactivity for some calcium binding proteins with a developmental pattern of expression. In the present study, sections from rabbits at different ages were examined to determine the calbindin (CB) expression in the developing TRN and PRN at the first stages of development. These CB-expressing cells constitute an important subpopulation of neurons in the caudal half of the developing TRN. In the adult, there are still positive CB somata in the middle and caudal halves of the nucleus. In the PRN, where the developmental pattern of CB expression has not been described before, the number of CB perireticular cells decreases progressively. Our results, together with previous data in the rabbit suggest the existence of remarkable neurochemical heterogeneity in the TRN and PRN of the rabbit.     

Electrophysiologic study of globus pallidus projections to the thalamic reticular nucleus

This study was designed to explore the electrophysiological relationships between the globus pallidus (GP), the substantia nigra pars reticulata (SNr) and the thalamic reticular nucleus (TRN) in urethane-anesthetized rats. The neuronal activity of the rostral part of the TRN was recorded by microelectrodes. Single pulse electrical stimulation of the GP and SNr produced inhibition of the spontaneous activity of the majority of TRN neurons. Stimulation of the GP by microinjections of bicuculline (25 ng/300 nl) produced also inhibition of the spontaneous activity of the reticular neurons. This could lead to facilitation of the cerebral cortex, as the reticular nucleus is reciprocally connected to, and inhibits, the thalamic motor nuclei, that in turn excite the motor cortex.