Functional organization of the direct and indirect projection via the reticularis thalami nuclear complex from the motor cortex to the thalamic nucleus ventralis lateralis

The thalamic reticular nucleus-lateral habenula circuit regulates depressive-like behaviors in chronic stress and chronic pain

Chronic stress and chronic pain are two major predisposing factors to trigger depression. Enhanced excitatory input to the lateral habenula (LHb) has been implicated in the pathophysiology of depression. However, the contribution of inhibitory transmission remains unclear. Here, we dissect an inhibitory projection from the sensory thalamic reticular nucleus (sTRN) to the LHb, which is activated by acute aversive stimuli. However, chronic restraint stress (CRS) weakens sTRN-LHb synaptic strength, and this synaptic attenuation is indispensable for CRS-induced LHb neural hyperactivity and depression onset. Moreover, artificially inhibiting the sTRN-LHb circuit induces depressive-like behaviors in healthy mice, while enhancing this circuit relieves depression induced by both chronic stress and chronic pain. Intriguingly, neither neuropathic pain nor comorbid mechanical hypersensitivity in chronic stress is affected by this pathway. Altogether, our study demonstrates an sTRN-LHb circuit in establishing and modulating depression, thus shedding light on potential therapeutic targets for preventing or managing depression.

Homologous organization of cerebellar pathways to sensory, motor, and associative forebrain

Cerebellar outputs take polysynaptic routes to reach the rest of the brain, impeding conventional tracing. Here, we quantify pathways between the cerebellum and forebrain by using transsynaptic tracing viruses and a whole-brain analysis pipeline. With retrograde tracing, we find that most descending paths originate from the somatomotor cortex. Anterograde tracing of ascending paths encompasses most thalamic nuclei, especially ventral posteromedial, lateral posterior, mediodorsal, and reticular nuclei. In the neocortex, sensorimotor regions contain the most labeled neurons, but we find higher densities in associative areas, including orbital, anterior cingulate, prelimbic, and infralimbic cortex. Patterns of ascending expression correlate with c-Fos expression after optogenetic inhibition of Purkinje cells. Our results reveal homologous networks linking single areas of the cerebellar cortex to diverse forebrain targets. We conclude that shared areas of the cerebellum are positioned to provide sensory-motor information to regions implicated in both movement and nonmotor function.

Pisano et al. use transsynaptic tracing and whole-brain light-sheet microscopy to quantitatively map cerebellar paths to and from the forebrain, including relatively dense projections to the prefrontal neocortex. Divergence of paths from single injection sites suggests that a single cerebellar region can influence multiple thalamic and neocortical targets at once.

Rodent somatosensory thalamocortical circuitry: Neurons, synapses, and connectivity

As our understanding of the thalamocortical system deepens, the questions we face become more complex. Their investigation requires the adoption of novel experimental approaches complemented with increasingly sophisticated computational modeling. In this review, we take stock of current data and knowledge about the circuitry of the somatosensory thalamocortical loop in rodents, discussing common principles across modalities and species whenever appropriate. We review the different levels of organization, including the cells, synapses, neuroanatomy, and network connectivity. We provide a complete overview of this system that should be accessible for newcomers to this field while nevertheless being comprehensive enough to serve as a reference for seasoned neuroscientists and computational modelers studying the thalamocortical system. We further highlight key gaps in data and knowledge that constitute pressing targets for future experimental work. Filling these gaps would provide invaluable information for systematically unveiling how this system supports behavioral and cognitive processes.

Early lesion of the reticular thalamic nucleus disrupts the structure and function of the mediodorsal thalamus and prefrontal cortex

The thalamic reticular nucleus (TRN), part of the thalamus, is a thin GABAergic cell layer adjacent to the relay nuclei of the dorsal thalamus. It receives input from the cortex and other thalamic nuclei and provides major inhibitory input to each thalamic nucleus, particularly the mediodorsal nucleus (MD). As the MD is important for supporting optimal cortico-thalamo-cortical interactions during brain maturation, we hypothesized that that early damage to the TRN will cause major disturbances to the development and the functioning of the prefrontal cortex (PFC) and the MD. Rat pups at P4 were randomized in three groups: electrolytic lesion of TRN, TRN-sham-lesion group, and the classical control group. Seven weeks later, all rats were tested with several behavioral and cognitive paradigms, and then perfused for histological and immunohistochemical studies. Results showed that TRN lesion rats exhibited reduced spontaneous activity, high level of anxiety, learning and recognition memory impairments. Besides the behavioral effects observed after early TRN lesions, our study showed significant cytoarchitectural and functional changes in the cingulate cortex, the dorsolateral and prelimbic subdivisions of the PFC, as well as in the MD. The assessment of the basal levels of neuronal activity revealed a significant reduction of the basal expression of C-Fos levels in the PFC. These experiments, which are the first to highlight the effects of early TRN lesions, provided evidence that early damage of the anterior part of the TRN leads to alterations that may control the development of the thalamocortical-corticothalamic pathways.

Functional Diversity of Thalamic Reticular Subnetworks

The activity of the GABAergic neurons of the thalamic reticular nucleus (TRN) has long been known to play important roles in modulating the flow of information through the thalamus and in generating changes in thalamic activity during transitions from wakefulness to sleep. Recently, technological advances have considerably expanded our understanding of the functional organization of TRN. These have identified an impressive array of functionally distinct subnetworks in TRN that participate in sensory, motor, and/or cognitive processes through their different functional connections with thalamic projection neurons. Accordingly, "first order" projection neurons receive "driver" inputs from subcortical sources and are usually connected to a densely distributed TRN subnetwork composed of multiple elongated neural clusters that are topographically organized and incorporate spatially corresponding electrically connected neurons-first order projection neurons are also connected to TRN subnetworks exhibiting different state-dependent activity profiles. "Higher order" projection neurons receive driver inputs from cortical layer 5 and are mainly connected to a densely distributed TRN subnetwork composed of multiple broad neural clusters that are non-topographically organized and incorporate spatially corresponding electrically connected neurons. And projection neurons receiving "driver-like" inputs from the superior colliculus or basal ganglia are connected to TRN subnetworks composed of either elongated or broad neural clusters. Furthermore, TRN subnetworks that mediate interactions among neurons within groups of thalamic nuclei are connected to all three types of thalamic projection neurons. In addition, several TRN subnetworks mediate various bottom-up, top-down, and internuclear attentional processes: some bottom-up and top-down attentional mechanisms are specifically related to first order projection neurons whereas internuclear attentional mechanisms engage all three types of projection neurons. The TRN subnetworks formed by elongated and broad neural clusters may act as templates to guide the operations of the TRN subnetworks related to attentional processes. In this review article, the evidence revealing the functional TRN subnetworks will be evaluated and will be discussed in relation to the functions of the various sensory and motor thalamic nuclei with which these subnetworks are connected.

Spatiotemporal dynamics of cortical perfusion in response to thalamic deep brain stimulation

Deep brain stimulation (DBS) has revolutionized the treatment of movement disorders. The parameters of electrical stimulation are important to its therapeutic effect and remain a source of clinical controversy. DBS exerts its actions not only locally at the site of stimulation but also remotely through afferent and efferent connections, which are vital to its clinical effects. Yet, only a few studies have examined how cortical activity changes in response to various electrical parameters. Here, we investigated how the parameters of thalamic DBS alter cortical perfusion in rats using intrinsic optical imaging. We hypothesized that thalamic DBS will increase perfusion in primary motor cortex (M1), proportional to amplitude, pulse width, or frequency of the stimulation applied. We applied 45 different combinations of amplitude, pulse width and frequency in the ventro-lateral (VL) nucleus of the thalamus in anesthetized rats while measuring perfusion in M1. VL thalamic DBS reduced cortical reflectance, which corresponds to an increase in cortical perfusion. We computed the maximum change in reflectance (MCR) as well as the spatial spread of MCR in each trial. Both MCR and spatial spread increased linearly with increases in current amplitude or pulse width of stimulation; however, the effect of frequency was non-linear. Stimulation at 20 Hz was significantly different from that at higher frequencies while stimulation at higher frequencies did not differ significantly from each other. Moreover, the effect of pulse width on MCR was larger than the effect of amplitude. The proportional increase in M1 perfusion due to increase in amplitude or pulse width suggests that both activate more neural elements and increase the volume of tissue activated. These results should help clinicians set parameters of DBS. The use of optical imaging to monitor effects of DBS on M1 may not only help understand DBS mechanisms, but may also provide feedback for closed loop DBS devices.

Synaptic circuit organization of motor corticothalamic neurons

Corticothalamic (CT) neurons in layer 6 constitute a large but enigmatic class of cortical projection neurons. How they are integrated into intracortical and thalamo-cortico-thalamic circuits is incompletely understood, especially outside of sensory cortex. Here, we investigated CT circuits in mouse forelimb motor cortex (M1) using multiple circuit-analysis methods. Stimulating and recording from CT, intratelencephalic (IT), and pyramidal tract (PT) projection neurons, we found strong CT↔ CT and CT↔ IT connections; however, CT→IT connections were limited to IT neurons in layer 6, not 5B. There was strikingly little CT↔ PT excitatory connectivity. Disynaptic inhibition systematically accompanied excitation in these pathways, scaling with the amplitude of excitation according to both presynaptic (class-specific) and postsynaptic (cell-by-cell) factors. In particular, CT neurons evoked proportionally more inhibition relative to excitation (I/E ratio) than IT neurons. Furthermore, the amplitude of inhibition was tuned to match the amount of excitation at the level of individual neurons; in the extreme, neurons receiving no excitation received no inhibition either. Extending these studies to dissect the connectivity between cortex and thalamus, we found that M1-CT neurons and thalamocortical neurons in the ventrolateral (VL) nucleus were remarkably unconnected in either direction. Instead, VL axons in the cortex excited both IT and PT neurons, and CT axons in the thalamus excited other thalamic neurons, including those in the posterior nucleus, which additionally received PT excitation. These findings, which contrast in several ways with previous observations in sensory areas, illuminate the basic circuit organization of CT neurons within M1 and between M1 and thalamus.

Noradrenergic control of neuronal firing in cerebellar nuclei: modulation of GABA responses

The effects of noradrenaline (NA) on inhibitory responses to gamma aminobutyric acid (GABA) in neurones of the deep cerebellar nuclei were studied in vivo in rats, using extracellular single-unit recordings and microiontophoretic drug application. NA application altered GABA-evoked responses in 95 % of the neurones tested, but the effects differed between nuclei. Application of NA depressed GABA responses in the medial (MN) and posterior interpositus (PIN) nuclei, but enhanced GABA responses in the anterior interpositus nucleus (AIN). Comparable proportions of enhancing (57 %) and depressive (43 %) effects were found in the lateral nucleus (LN). The alpha2 noradrenergic receptor agonist clonidine mimicked the depressive effect of NA on GABA responses in MN and PIN and its enhancing effects in AIN and LN, while the alpha2 antagonist yohimbine partially blocked these effects. The beta-adrenergic agonist isoproterenol and antagonist timolol respectively induced and partially blocked enhancements of GABA responses in all nuclei except for LN, where isoproterenol had a weak depressive effect. It is concluded that NA modulates GABA responses by acting on both alpha2 and beta receptors. Activation of these receptors appears to be synergistic in the AIN and opposite in the remaining deep nuclei. These results support the hypothesis that the noradrenergic system participates in all the regulatory functions involving the cerebellum in a specific and differential manner, and suggest that any change in NA content, as commonly observed in ageing or stress, could influence cerebellar activity.

Circuits formultisensory integration and attentional modulation through the prefrontal cortex and the thalamic reticular nucleus in primates

Converging evidence from anatomic and physiological studies suggests that the interaction of high-order association cortices with the thalamus is necessary to focus attention on a task in a complex environment with multiple distractions. Interposed between the thalamus and cortex, the inhibitory thalamic reticular nucleus intercepts and regulates communication between the two structures. Recent findings demonstrate that a unique circuitry links the prefrontal cortex with the reticular nucleus and may underlie the process of selective attention to enhance salient stimuli and suppress irrelevant stimuli in behavior. Unlike other cortices, some prefrontal areas issue widespread projections to the reticular nucleus, extending beyond the frontal sector to the sensory sectors of the nucleus, and may influence the flow of sensory information from the thalamus to the cortex. Unlike other thalamic nuclei, the mediodorsal nucleus, which is the principal thalamic nucleus for the prefrontal cortex, has similarly widespread connections with the reticular nucleus. Unlike sensory association cortices, some terminations from prefrontal areas to the reticular nucleus are large, suggesting efficient transfer of information. We propose a model showing that the specialized features of prefrontal pathways in the reticular nucleus may allow selection of relevant information and override distractors, in processes that are deranged in schizophrenia.

Prefrontal projections to the thalamic reticular nucleus form a unique circuit for attentional mechanisms

The inhibitory thalamic reticular nucleus (TRN) intercepts and modulates all corticothalamic and thalamocortical communications. Previous studies showed that projections from sensory and motor cortices originate in layer VI and terminate as small boutons in central and caudal TRN. Here we show that prefrontal projections to TRN in rhesus monkeys have a different topographic organization and mode of termination. Prefrontal cortices projected mainly to the anterior TRN, at sites connected with the mediodorsal, ventral anterior, and anterior medial thalamic nuclei. However, projections from areas 46, 13, and 9 terminated widely in TRN and colocalized caudally with projections from temporal auditory, visual, and polymodal association cortices. Population analysis and serial EM reconstruction revealed two distinct classes of corticoreticular terminals synapsing with GABA/parvalbumin-positive dendritic shafts of TRN neurons. Most labeled boutons from prefrontal axons were small, but a second class of large boutons was also prominent. This is in contrast to the homogeneous small TRN terminations from sensory cortices noted previously and in the present study, which are thought to arise exclusively from layer VI. The two bouton types were often observed on the same axon, suggesting that both prefrontal layers V and VI could project to TRN. The dual mode of termination suggests a more complex role of prefrontal input in the functional regulation of TRN and gating of thalamic output back to the cortex. The targeting of sensory tiers of TRN by specific prefrontal areas may underlie attentional regulation for the selection of relevant sensory signals and suppression of distractors.

Bioenergetic abnormalities in discrete cerebral motor pathways presage spinal cord pathology in the G93A SOD1 mouse model of ALS

Multiple cell death pathways are implicated in the etiology of amyotrophic lateral sclerosis (ALS), but the cause of the characteristic motor neuron degeneration remains unknown. To determine whether CNS metabolic defects are critical for ALS pathogenesis, we examined the temporal evolution of energetic defects in the G93A SOD1 mouse model of familial ALS. [14C]-2-deoxyglucose in vivo autoradiography in G93A mice showed that glucose utilization is impaired in components of the corticospinal and bulbospinal motor tracts prior to either pathologic or bioenergetic changes in the spinal cord. This was accompanied by significant depletions in cortical ATP content in presymptomatic mice, which was partially ameliorated by creatine administration. Findings suggest that bioenergetic defects are involved in the initial stages of mSOD1-induced toxicity in G93A mice and imply that the selective dysfunction and degeneration of spinal cord motor neurons in this model may be secondary to dysfunction within cerebral motor pathways.

Structure and connections of the thalamic reticular nucleus: Advancing views over half a century

The advance of knowledge of the thalamic reticular nucleus and its connections has been reviewed and Max Cowan's contributions to this knowledge and to the methods used for studying the nucleus have been summarized. Whereas 50 years ago the nucleus was seen as a diffusely organized cell group closely related to the brain stem reticular formation, it can now be seen as a complex, tightly organized entity that has a significant inhibitory, modulatory action on the thalamic relay to cortex. The nucleus is under the control, on the one hand, of topographically organized afferents from the cerebral cortex and the thalamus, and on the other of more diffuse afferents from brain stem, basal forebrain, and other regions. Whereas the second group of afferents can be expected to have global actions on thalamocortical transmission, relevant for overall attentive state, the former group will have local actions, modulating transmission through the thalamus to cortex with highly specific local effects. Since it appears that all areas of cortex and all parts of the thalamus are linked directly to the reticular nucleus, it now becomes important to define how the several pathways that pass through the thalamus relate to each other in their reticular connections.

Effect of stimulation of beta-adrenergic receptors on neuronal activity in primary motor cortex of the rat

Modulatory influence of beta1- and beta2-subtypes of adrenergic receptors (ARs) on the background neuronal activity and cerebellocortical transmission was characterized in the rat primary motor cortex (M1). Microiontophoretic administration of non-selective beta-ARs agonist isoproterenol significantly decreased firing rate and responses to superior cerebellar peduncle stimulation in 82% of studied neurons in a dose-dependent manner. Similar changes were induced by ejection of selective beta1-ARs agonist dobutamine, while fenoterol (selective beta2-ARs agonist) increased or reduced firing rate in 32% and 19% of M1 neurons, respectively. Non-selective beta-ARs antagonist propranolol enhanced both the background and evoked activity in 84% of tested neurons. These data provided the functional evidence for beta-ARs-mediated inhibition (predominantly through beta1-subtype) of cerebellocortical input to M1. The possible mechanism of the positive therapeutic effect of propranolol in tremor-predominant Parkinson's disease is discussed.

Synaptic convergence of motor and somatosensory cortical afferents onto GABAergic interneurons in the rat striatum

Cortical afferents to the basal ganglia, and in particular the corticostriatal projections, are critical in the expression of basal ganglia function in health and disease. The corticostriatal projections are topographically organized but also partially overlap and interdigitate. To determine whether projections from distinct cortical areas converge at the level of single interneurons in the striatum, double anterograde labeling from the primary motor (M1) and primary somatosensory (S1) cortices in the rat, was combined with immunolabeling for parvalbumin (PV), to identify one population of striatal GABAergic interneurons. Cortical afferents from M1 and S1 gave rise to distinct, but partially overlapping, arbors of varicose axons in the striatum. PV-positive neurons were often apposed by cortical terminals and, in many instances, apposed by terminals from both cortical areas. Frequently, individual cortical axons formed multiple varicosities apposed to the same PV-positive neuron. Electron microscopy confirmed that the cortical terminals formed asymmetric synapses with the dendrites and perikarya of PV-positive neurons as well as unlabelled dendritic spines. Correlated light and electron microscopy revealed that individual PV-positive neurons received synaptic input from axon terminals derived from both motor and somatosensory cortices. These results demonstrate that, within areas of overlap of functionally distinct projections, there is synaptic convergence at the single cell level. Sensorimotor integration in the basal ganglia is thus likely to be mediated, at least in part, by striatal GABAergic interneurons. Furthermore, our findings suggest that the pattern of innervation of GABAergic interneurons by cortical afferents is different from the cortical innervation of spiny projection neurons.

The Somatotopic Organization Within the Rabbit's Thalamic Reticular Nucleus

The organization of the somatosensory representation within the rabbit's thalamic reticular nucleus (TRN) was studied. Focal injections of horseradish peroxidase (HRP), wheatgerm agglutinin conjugated to HRP, or [3H]proline were made into somatosensory cortical area 1 (S1). The resultant labelling in the thalamus was analysed. Single injections into S1 result in single zones of terminal labelling in TRN that are restricted to the centroventral part of the sheet-like nucleus. In reconstructions from horizontal sections these zones of labelling resemble 'slabs', which lie in the plane of the nucleus parallel to its borders, occupy only a fraction of the thickness of the reticular sheet, and are elongated in the dorsoventral and oblique rostrocaudal dimensions. Thus, the slabs of S1 terminals, which represent various loci of the body surface, and the main distribution of the reticular dendrites have a similar orientation. In comparisons of the zones of labelling following single or double injections at different cortical sites in S1, an inner (medial) to outer (lateral) shift in labelling in the ventrobasal complex (VB) is accompanied by an inner (medial) to outer (lateral) shift in labelling along the thickness of the reticular sheet; a rostral to caudal shift in labelling in VB is accompanied by a rostral to caudal shift in labelling along the plane of the reticular sheet. Thus, like VB, the reticular nucleus receives a topographically accurate projection from S1. Further, the somatotopic map conveyed from S1 to TRN lies perpendicular to the plane of the nucleus and repeats the spatial organization of the map in VB. These findings, together with those for the visual sector of the rabbit's TRN, indicate that the representation of the cortical sheet is broken up into significant parcels at the inner and outer borders of the reticular sheet.

Cortical projections from the suprasylvian gyrus to the reticular thalamic nucleus in the cat

The cat's suprasylvian gyrus was injected iontophoretically with either 4% wheat germ agglutinin-horseradish peroxidase, 4% dextran-fluororuby or 4% dextran-biotin. The locations of labelled fibres, presumed terminals and cell bodies were determined with the aid of a camera lucida attachment and computer aided stereometry. Cells from the crown of the suprasylvian gyrus project to the dorsal-most portion of the rostral half of the reticular nucleus. The region or 'sector' is distinct, albeit with some overlap, from the visual sector of the reticular nucleus defined by projections from adjacent extrastriate visual cortices. The projection from the suprasylvian gyrus to the reticular nucleus has a rough topography such that the caudal areas project to the more caudal aspects of the sector and rostral areas project to the more rostral areas of the reticular nucleus. There is a large degree of overlap of rostrocaudal projections from the suprasylvian gyrus within the sector, however, the projections originating from rostral sites are situated in a more ventral location compared to the projection originating from the caudal suprasylvian gyrus. Analysis of the distribution of biotin labelled presumptive terminals did not support the notion of 'slabs' or regional variation in terminal density across the mediolateral thickness of the reticular nucleus. In addition, a number of presumptive terminals were found within the internal capsule which coincided with the position of retrogradely labelled cells in the internal capsule following thalamic injections and appears to be part of the perireticular nucleus. The results suggest that the reticular nucleus may be segregated into sectors connected with modality specific cortical areas (e.g. striate and extrastriate visual areas) and nonspecific sectors connected with polymodal (e.g. area 7) cortical regions. The reticular nucleus and its connections with the suprasylvian gyrus may form an important link in binding eye movements to sensory integrative process through visuomotor and auditory thalamic connections.

Distribution of calbindin, parvalbumin, and calretinin immunoreactivity in the reticular thalamic nucleus of the marmoset: evidence for a medial leaflet of incertal neurons

The placement of the reticular thalamic nucleus (RTN) between the dorsal thalamus and the cortex and the inhibitory nature of reticulothalamic projections has led to suggestions that it "gates" the flow of sensory information to the cortex. The New World diurnal monkey, the marmoset, Callithrix jacchus is emerging as an important "model primate" for the study of sensory processing. We have examined the distribution of Nissl-stained somata and calbindin, parvalbumin, and calretinin immunoreactivity in the ventral thalamus for comparison with other species. Cells were labeled using standard immunohistochemistry, ExtraAvidin-HRP, and diaminobenzidine reaction products. The RTN is constituted by a largely homogeneous population of parvalbumin immunoreactive cells with respect to size and orientation. Calbindin and calretinin immunoreactive cells were only found along the medial edge of the RTN adjacent to the external medullary lamina of the dorsal thalamus and laterally near the ventral RTN. These cells were considered to be part of the zona incerta (ZI). The marmoset ZI could be subdivided into dorsal and ventral regions on the basis of its immunoreactivity to calcium binding proteins. Both the ZI and nucleus subthalamicus Luysi contained scattered calbindin and calretinin immunoreactive cells with well-defined dendritic processes. These cells were clearly different to cells in the dorsal thalamus. Parvalbumin immunoreactive cells in RTN, ZI, and subthalamic nucleus were on average larger than neurons positive for the other calcium binding proteins. Future studies reporting the afferent and efferent projections to the RTN must view their results in terms of the close apposition of RTN and ZI somata.

Thalamic reticular nucleus parcellation delineated by VIP and TRH gene expression in the rat

The distribution of the mRNAs encoding VIP (vasoactive intestinal peptide) and TRH (thyrotropin releasing hormone) was examined in the thalamic reticular nucleus of the adult rat using hybridization histochemistry with S35-labeled oligoprobes. Low levels of TRH expression were found in a medial tier. High levels of VIP expression were found in neurons located in a lateral shell of the same portion. High levels of TRH expression were found in a tier located dorsally and in a tier located ventrally to the first one. In these regions no VIP expression could be detected. These data suggest a parcellation of this nucleus according to the differential expression patterns of TRH and VIP.

Transplants of fetal frontal cortex grafted into the occipital cortex of newborn rats receive a substantial thalamic input from nuclei normally projecting to the frontal cortex

A number of molecular and hodological experiments have provided evidence that there is an early commitment of neocortical neurons to express features unique to a certain cortical area. However, the findings of several transplantation experiments have indicated that late embryonic cortical tissue heterotopically grafted into the neocortex of newborn rats receives a set of thalamic projections appropriate for the host cortical locus within which it develops. To provide further information on the extent to which neocortical neurons are predetermined to develop area-specific systems of connections, in this study we have compared the pattern of thalamic afferents to grafts of embryonic day 16 occipital or frontal neocortex transplanted into the occipital cortex of newborn rats. Two months after grafting, a retrograde neurotracer (cholera toxin, subunit b) was injected into the grafts to precisely assess the number of cells in the visual- and/or motor-related nuclei of the host thalamus projecting to each category of transplants (occipital-to-occipital or frontal-to-occipital). Transplants of embryonic occipital cortex received significant input from several visual-related thalamic nuclei, i.e. the lateral posterior and lateral dorsal nuclei, and no input from motor-related thalamic nuclei. However, only few labeled cells were found in the dorsal lateral geniculate nucleus, which was systematically affected by a severe atrophy, probably in response to the lesion of the occipital cortex performed prior to the transplantation. By comparison, transplants of frontal origin received a substantial input from the ventrolateral and ventromedial thalamic nuclei, which normally project to the frontal cortex, but received a weak input from the lateral posterior and lateral dorsal nuclei. Neocortical neurons grafted heterotopically into the neocortex of newborn hosts are not only able to contact cortical and subcortical targets appropriate for their embryonic site of origin, but are also susceptible to derive thalamic inputs closely related to their embryonic origin.

Analysis of spontaneous activity patterns of human thalamic ventrolateral neurons and their modifications due to functional brain changes

In the human thalamic ventralis lateralis nucleus the spontaneous activity of 235 single units during 38 stereotactic operations in locally anaesthetized parkinsonian patients was analysed. Two basic cell types (A and B) were shown to exist in this nucleus: (i) with unitary irregular (2-40/s) discharges characterized by a tendency to spike grouping in the range of 4-6 Hz and 10-30 Hz (A-type, 74%), (ii) with bursting discharges firing in short trains (5-30 ms) characterized by an unstable rhythmic 3-6 Hz pattern similar to a low-threshold Ca2+ intrinsic burst structure of discharges (B-type, 26%). The functional brain changes after a motor tests performance were accompanied by the appearance of two different transient modifications of activity of A-cells pattern into rhythmic burst discharges: (i) in the range of 3-6 Hz, similar to the bursts found for B-cells and recorded mainly in the anterior ventrolateral region in rigid patients, (ii) in the range of 5 +/- 1 Hz, characterized by other interspike interval and recorded in the posterior ventrolateral region in patients with tremor. Modifications during short-term anaesthesia resulted in 10-15 Hz burst discharges that were associated with gradual disappearance of A-cells activity. In contrast to what happens for A-cells, the activity of bursting B-units was characterized by an invariant intrinsic structure of discharges irrespective of the functional brain changes or the forms of parkinsonian pathology. The nature of A- and B-units as well as the mechanisms of transient modifications of their spontaneous activity patterns due to the functional brain changes are discussed.

Analysis of evoked activity patterns of human thalamic ventrolateral neurons during verbally ordered voluntary movements

In the human thalamic ventralis lateralis nucleus the responses of 184 single units to verbally ordered voluntary movements and some somatosensory stimulations were studied by microelectrode recording technique during 38 stereotactic operations on parkinsonian patients. The tests were carried out on the same previously examined population of neurons classified into two groups, named A- and B-types according to the functional criteria of their intrinsic structure of spontaneous activity patterns. The evaluation of the responses of these units during functionally different phases of a voluntary movement (preparation, initiation, execution, after-effect) by means of the principal component analysis and correlation techniques confirmed the functional differences between A- and B-types of neurons and their polyvalent convergent nature. Four main conclusions emerge from the studies. (1) The differences of the patterns of A- and B-unit responses during the triggering and the execution phases of a voluntary movement indicate the functionally different role of these two cell types in the mechanisms of motor signal transmission. (2) The universal non-specific form of anticipatory A- and B-unit responses during the movement preparation and initiation of various kinds of voluntary movements reflect the integrative "triggering" processes connected with the processing and programming of some generalized parameters of a motor signal and not with the performance of a certain forthcoming motor act. (3) The expressed intensity of these "triggered" non-specific processes in the anterior parts of the ventralis lateralis nucleus indicates their relation not only to the motor but to the cognitive attentional functions forming a verbally ordered voluntary movement. (4) The appearance of the transient cross-correlations between the activities of adjacent A- and B-cells and also the synchronization of their 5 +/- 1 Hz frequency during and/or after motor test performances point to the contribution of these two populations to central mechanisms of the voluntary movement and the parkinsonian tremor. The functional role of two A- and B-cell types is discussed with references to the central mechanisms of verbally ordered voluntary movements and the parkinsonian tremor.

A new intrathalamic pathway linking modality-related nuclei in the dorsal thalamus

Transmission of sensory information through the dorsal thalamus involves two types of modality-related nuclei, first order and higher order, between which there are thought to be no intrathalamic interactions. We now show that within the somatosensory thalamus, cells in one nucleus, the ventrobasal complex, can influence activity in another nucleus, the medial division of the posterior complex. Stimulation of ventrobasal complex cells evoked inhibitory postsynaptic currents in cells of the medial division of the posterior complex. These currents exhibited the reversal potential and pharmacology of a GABAA receptor-mediated chloride conductance, indicating that they result from the activation of a disynaptic pathway involving the GABAergic cells of the thalamic reticular nucleus. These findings provide the first direct evidence for intrathalamic interactions between dorsal thalamic nuclei.

Paying attention to the thalamic reticular nucleus

The thalamic reticular nucleus can be divided into a number of sectors, each concerned with a different function (sight, touch, hearing, movement or 'limbic' functions). Each sector is connected to more than one thalamic nucleus and to more than one cortical area, and each sector has topographically mapped connections with the thalamus and the cortex. We consider the known details of these connections and show: (1) that they are not the same for each sector; (2) that the reticular nucleus serves as a nexus, where several functionally related cortical areas and thalamic nuclei can interact, modifying thalamocortical transmission through the inhibitory connections that go from the reticular cells to thalamic relay cells; and (3) that we need much more detailed information about these highly organized connections before we can understand exactly how the thalamic reticular nucleus might be influencing thalamocortical pathways in attentional mechanisms or in other, as yet undefined, roles.

Topography of fibre organisation in the corticofugal pathways of rats

The organisation of the long descending corticofugal pathways is poorly understood. We have examined these pathways to determine the fibre relationships along the extent of their course through the internal capsule, cerebral peduncle, longitudinal pontine fasciculus, pyramid, pyramidal decussation, and dorsal column of the spinal cord. Different cytoarchitectonic regions (e.g., lateral agranular and granular) of the rat's neocortex were injected with the axonal tracer biotinylated dextran. In other experiments, each animal had different-coloured fluorescent tracers (Fluoro Ruby and dextran-fluorescein) injected into separate cortical areas. Our results show that in the anterior and posterior limbs of the internal capsule, axons arising from spatially separate sites in rat neocortex occupy distinct regions of the cross-sectional area of the pathway. More caudally, within the cerebral peduncle and the longitudinal pontine fasciculus, axons from more distant cortical areas remain largely separate, but those from adjacent cortical areas begin to overlap. By the medullary pyramid, the pyramidal decussation, and the dorsal column of the spinal cord, the representations of all the cortical regions injected overlap completely; in these structures, the axons arising from each cortical area are widely intermingled. Thus, along the rostral-to-caudal course of the corticofugal pathways, there is a change in the organisation of axons. At rostral levels, the order corresponds roughly to the spatial distribution of the cells of origin, but more caudally, this changes to an arrangement of axons that has no readily apparent order. A similar change has been observed along the course of the retinofugal pathway, where a decrease of spatial order in the fibre distribution has been associated with a reordering of axons according to their temporal sequence of outgrowth.

Cerebellar input to corticothalamic neurons in layers V and VI in the motor cortex

To investigate whether corticothalamic (CT) neurons in the motor cortex (Mx) receive cerebellar input via the ventroanterior-ventrolateral nucleus of the thalamus (VA-VL), we recorded intracellular potentials from neurons in the Mx of anesthetized cats and examined effects of stimulation of the VA-VL and the brachium conjunctivum on them. After this electrophysiological identification, horseradish peroxide (HRP) was injected iontophoretically into the recorded neurons for morphological analysis. We identified 34 neurons as CT neurons by their antidromic response to stimulation of the VA-VL, of which 13 were layer VI CT neurons and 21 were layer V CT neurons. A majority of the CT neurons of both layers VI and V received monosynaptic excitatory postsynaptic potentials (EPSPs) from the VA-VL and di- or polysynaptic EPSPs from the cerebellum. The laminar distribution and morphological characteristics of single CT neurons receiving cerebellar input were analyzed on 19 HRP-labeled CT neurons. Eight layer V and six layer VI CT neurons were reconstructed from serial sections. All the reconstructed layer VI CT neurons were modified pyramidal neurons whose apical dendrites ended in layer III or V, and all the stained layer V CT neurons were typical pyramidal neurons, although the laminar and tangential distribution of recurrent collaterals of these neurons varied from neuron to neuron.

c-fos induction in sensory pathways of rats exploring a novel complex environment: shifts of active thalamic reticular sectors by predominant sensory cues

In normal rats exploring a novel, complex environment, in comparison to control nonexploring rats, there is induction of the FOS protein, a marker of neuronal activity, in all layers of the striate visual cortex (particularly in the granular and supragranular layers), in the stratum griseum superficiale of the superior colliculus, and in the dorsal lateral geniculate nucleus, as well as in all layers of the whiskers barrel field in the somatosensory cortex. A surprising finding was a selective activation of the visual sector of the thalamic reticular nucleus, in dorsocaudal parts of the nucleus. To the contrary, in visually deprived rats exploring a novel environment which would depend critically on whiskers tactile clues for exploration there was instead a selective activation of the somatic sector in central parts of the thalamic reticular nucleus, in conjunction with activation of cortical whiskers barrel field. From these results it is concluded: (1) Different sensory sectors of the rat thalamic reticular nucleus are activated depending on prevalent sensory channels used in recognition of the environment, suggesting a role of thalamic reticular nucleus in optimizing thalamocortical transmission of essential external cues to guide adequate behaviour. (2) In the awake state, the granular and supragranular layers of the visual and somatosensory cortices are more active when attention is paid to sensory stimuli that are essential for recognition of the environment. (3) The selective induction of c-fos in the visual and somatosensory cortices, and in the stratum griseum superficiale of superior colliculus of rats exploring a novel, complex environment might be related to plastic changes that have been demonstrated in these centres in rats raised in complex environments. These plastic changes are likely to be the result of target late-response genes activated by c-fos.

Organization in the somatosensory sector of the cat's thalamic reticular nucleus

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the somatosensory part of the dorsal thalamus in the cat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the ventrobasal complex (VB) and the medial division of the posterior complex (POm) of the thalamus. The resultant retrograde labelling in TRN was analyzed. Large injections of a tracer in VB label many reticular cells that are restricted to a centroventral, or somatosensory, sector of TRN. Small injections of a tracer in VB produce narrow zones of labelled cells in this sector. In reconstructions these zones resemble thin "slabs," which lie parallel to the plane of TRN along its oblique rostrocaudal dimension and occupy only a fraction of its thickness. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different nuclei of VB, inner cells, intermediate cells, and outer cells across the thickness of TRN project to the ventral posteromedial, the medial division of the ventral posterolateral, and the lateral division of the ventral posterolateral nuclei, respectively. Furthermore, shifts in injected areas along the dorsoventral dimension of VB produce similar shifts in zones of labelled cells in TRN. Thus, reticular cells form an accurate map on the basis of their connections with VB. Large injections of a tracer in the ventral subdivision of POm label many reticular cells that are also restricted to the centroventral sector of TRN. Small injections of a tracer in ventral POm produce broad zones of labelled cells in this sector. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different regions of ventral POm, cells that project to these regions are scattered across the thickness of TRN and occupy overlapping territories. Large injections of a tracer in either VB or ventral POm also label cells in a restricted centroventral region of the perireticular nucleus. Double injections of different tracers in VB and ventral POm produce many cells in TRN that are labelled from both of these dorsal thalamic structures and fewer cells that are labelled from only one or the other of these structures. These results indicate that there is a dual organization in the projections of cells in the somatosensory sector of TRN to dorsal thalamus: Projections to VB are topographically organized whereas those to ventral POm lack a topographical organization. Furthermore, both of these mapped and nonmapped projections can arise from single reticular cells in the somatosensory sector.

Connections between the reticular nucleus of the thalamus and pulvinar-lateralis posterior complex: a WGA-HRP study

The present study utilises the capacity of wheat germ agglutinin-conjugated horseradish peroxidase to label both afferent and efferent projections from selected regions of the thalamic reticular nucleus (TRN) to the pulvinar lateralis-posterior complex (Pul-LP) of the cat. Fourteen injections into the TRN located between anterior-posterior levels 8.5 and 4.5 were analysed. The projection of the TRN to the Pul-LP complex is roughly organised in a topographic manner and is not widespread within the thalamus. Anterograde labelling in the Pul-LP extended rostrocaudally with a slight oblique dorsoventral orientation. Projections to the medial LP were predominantly but not exclusively from rostral areas of TRN, while projections to the lateral LP were largely from caudal areas of the TRN. Projections to other areas of the Pul-LP were sparse. The connections between TRN and Pul-LP were reciprocal, although the distribution of labelled cells and anterograde labelling was not completely overlapping. Reciprocal connections with the dorsal lateral geniculate nucleus were largely with the C-laminae and the medial interlaminar nucleus. The results are discussed with reference to the corticothalamic projections and the visuotopy of the Pul-LP.

Organization of connections between the thalamic reticular and the anterior thalamic nuclei in the rat

The thalamic reticular nucleus (TRN) receives topographically organized input from specific sensory nuclei such as the lateral geniculate nucleus. The present study shows this in the rat. However, the pattern of thalamic connections to the limbic reticular sector is unknown. Injecting biocytin into the ventral parts of anteroventral and anteromedial nuclei labeled neurons and axons in the rostral TRN. Filled axon collaterals and their terminals occupied a rectangular sheet in a plane close to the horizontal, and were confined to the inner zone (the medial portion) of the limbic TRN. Retrogradely filled cells were in the middle of the rostral pole in the same horizontal plane, receiving synapses from surrounding labeled boutons. In electron micrographs, thalamic terminals were found to contain round, densely packed synaptic vesicles and formed asymmetrical synapses onto reticular somata and dendritic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axis in the anterior nuclei produced corresponding shifts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not follow this pattern. Axons from the anterodorsal nucleus occupied the rostralmost tip of both inner and outer zones of the dorsal limbic sector. In accordance with earlier reports, the limbic sector was found to represent several dorsal thalamic nuclei parallel to each other medio-laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence circumscribed areas of the limbic thalamus.

The AMPA antagonist NBQX protects thalamic reticular neurons from degeneration following cardiac arrest in rats

Thalamic reticular (RT) neurons are selectively vulnerable to degeneration following global ischemia. The degenerative mechanism is thought to involve an excitotoxic component, mediated in part by sustained post-ischemic activation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) type excitatory amino acid (EAA) receptors. In order to test this hypothesis, the selective competitive AMPA type EAA antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F) quinoxalinedione) was administered at 30 mg/kg to rats 1, 3, and 6 h after resuscitation from 10 min cardiac arrest. NBQX treatment resulted in a 2-fold increase of spared RT neurons, from a mean density of 3.6 +/- 0.8 x 10(3) neurons/mm3 in cardiac arrest cases to 7.4 +/- 1.1 x 10(3) neurons/mm3 in the NBQX treated group, which represents sparing of 41.7% of the normal population of RT neurons, and protection of 26.9% of vulnerable RT neurons. Neurons within the central core of the RT manifest both a higher degree of vulnerability to ischemic degeneration, > 92% loss, and a higher sensitivity to sparing following NBQX administration, 460% increased sparing, than neuronal sub-populations in the medial or lateral 1/3 of the RT. Protection by post-arrest administration of NBQX suggests that sustained post-arrest stimulation of AMPA receptors is an important component in the process of ischemic degeneration of RT neurons.

The partial mu opiate agonist buprenorphine protects a sub-population of thalamic reticular neurons following cardiac arrest in rats

Extensive loss of neurons occurs from the middle region of the thalamic reticular nucleus (RT) in rats resuscitated after 10 min of cardiac arrest. Administration of the partial mu opiate agonist buprenorphine 45 min after resuscitation produces a small but significant increase in spared neurons along the medial and lateral margins of the middle RT, regions where mu receptors have been localized. Systemic administration of mu agonists may augment endogenous opiate mechanisms that contribute to the relative ischemic resistance of subpopulations of RT neurons.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: light and electron microscopy

The morphology and synaptic organization of neurons in the ventroanterior-ventrolateral nucleus of rats was examined using in vivo intracellular staining techniques. Neurons were characterized electrophysiologically based on intrinsic membrane properties and synaptic responses to stimulation of motor cortex and cerebellar nuclei, as described in the companion paper. Cerebellar-responsive neurons were stained intracellularly with either horseradish peroxidase or biocytin. All stained ventroanterior-ventrolateral nucleus neurons were identified as thalamocortical neurons on anatomical (and often electrophysiological) grounds, consistent with previous findings that rat ventroanterior-ventrolateral nucleus is interneuron-sparse. Ventroanterior-ventrolateral nucleus neurons had three to eight thick primary dendrites. Proximal dendrites often exhibited a tufted branching pattern, from which many thinner, higher order dendrites arose. Dendrites branched to form a funnel-like infiltration of the neuropil that resulted in a spherical, roughly homogeneous dendritic field. The axon originated from the cell body or a proximal dendrite and coursed laterally and dorsally to innervate motor cortex. One to five axon collaterals were emitted in the rostral dorsolateral sector of the thalamic reticular nucleus; collaterals were not observed in the ventroanterior-ventrolateral nucleus or other nuclei in dorsal thalamus. The synaptic organization of the ventroanterior-ventrolateral nucleus was examined with electron microscopy, including two intracellularly labeled ventroanterior-ventrolateral nucleus neurons that were shown electrophysiologically to receive monosynaptic inputs from the cerebellum. The neuropil of rat ventroanterior-ventrolateral nucleus lacked the complexity and diversity found in corresponding thalamic nuclei of felines and primates, due to the paucity of interneurons. Vesicle-containing dendrites, dendrodendritic synapses and glomeruli were not observed. Three broad classes of presynaptic terminals were identified. (1) Small round boutons: small boutons containing densely-packed, small round vesicles that formed asymmetric synapses predominantly with the distal dendrites of thalamocortical neurons. These were the most prevalent type of bouton in the ventroanterior-ventrolateral nucleus (78% of presynaptic elements) and likely arose from the cerebral cortex. (2) Large round boutons: large terminals with loosely packed small round vesicles that made multiple asymmetric synapses with proximal and intermediate dendrites. Large round boutons comprised 8% of the neuropil, and likely arose from the cerebellar nuclei. (3) Medium size boutons with pleomorphic vesicles: medium-sized profiles containing pleomorphic vesicles that formed symmetric synapses with proximal, intermediate and distal dendrites and, less frequently, with cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of cortical afferents to the rostral, limbic sector of the rat thalamic reticular nucleus

The organization of limbic cortical afferents to the thalamic reticular nucleus (TRN) is described. Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), biocytin, neurobiotin, or fluorescent dextrans was delivered into the rat cingulate, retrosplenial, and, for comparison, somatosensory cortices. In other species a slab-like arrangement of cortical terminals has been described for sensory TRN sectors. Here this is seen in the rat somatosensory sector. Terminals from limbic cortices did not cluster into slabs but were found to fill the entire thickness of distinct rostral TRN regions. The cingulate and retrosplenial recipient TRN regions overlap, as do the projections from these cortical areas to anterior thalamic nuclei. Retrosplenial fibres contacted the dorsal and rostral TRN, which is known to be connected to the retrosplenial-recipient anteroventral, anterodorsal, and laterodorsal thalamic nuclei. Cingulate terminals occupied more ventral regions of the rostral TRN. This area is connected to thalamic nuclei also innervated by the cingulate cortex: the mediodorsal and anteromedial nuclei. A loose, but clear, topography could be defined for the cingulate-reticular pathway: rostrocaudal and mediolateral directions in the cortex are represented by ventrodorsal and rostrocaudal directions in the TRN, respectively. This organization of limbic corticoreticular pathway corresponds to the arrangement of limbic corticothalamic connections. The ultrastructure of the limbic cortical axon terminals was similar to that of the cortical boutons (D-type) described previously. The labelled terminals formed asymmetrical synapses onto dendritic profiles of reticular neurons. These findings, together with data in the literature, show significant morphological and connectional differences within the TRN that imply functional heterogeneities.

New views of the thalamic reticular nucleus in the adult and the developing brain

The thalamic reticular nucleus plays a crucial role in modifying the patterns of activity that can reach the cerebral cortex from the thalamus. Although the nucleus is generally viewed as a cell group with widespread and nonspecific thalamic and cortical connections, recent evidence has begun to stress the extent to which at least some of the reticular pathways transmit well-defined maps with a clear local sign from the cortex and the thalamus. Further, evidence from the adult structure of the nucleus and ongoing developmental studies suggest that the reticular nucleus plays an important part in organizing the earliest connections between cortex and thalamus and that the developmental sequence may explain the complex connections formed in the adult.

Behavioural effects after cholinergic stimulation of the reticular thalamic nucleus in rats

This study investigated the functional relationship between the experimentally induced changes in the activity of the cholinergic, muscarinergic system of the rostral area of the nucleus reticularis thalami (TRN) and the motor behaviour. The effect of direct stimulation of the rostral TRN by the cholinergic agonist carbachol on the behaviour of freely moving rats was observed. Unilateral injection of carbachol (0.2-3.2 micrograms/0.5 microliters) into the rostral TRN caused catalepsy which appeared rapidly and was short-lasting. Furthermore, it induced impairment of the performance on the rota rod. Both effects were dose-dependent. The cholinergic antagonist scopolamine (6.66 micrograms) coadministered with the equimolar dose of carbachol (3.2 micrograms) antagonized the effects of carbachol on both behavioural tests. The described effects seem to be cholinergic- and site-specific within the rostral TRN. The present results suggest that activation of the cholinergic, muscarinergic receptors in the rostral TRN modulate the motor function of rats.

Depth distribution of frontal cortical field potential to acoustic stimulation in unrestrained non-anaesthetized rats

Intracortical depth profiles of acoustically evoked potentials (AEP) through the frontal cortex were studied with moveable electrodes in 5 freely behaving rats to identify the generators of the AEP components with current source-density analysis in pharmacologically uninfluenced relaxed wakefulness. The main negative AEP components (N21 and N38) which change their polarity across the cortical depth are generated by extracellular current sinks within the V cortical lamina. Since both components were polarity reversed in different cortical depths and have their current sources in different laminae, the participation of different generation modi are assumed for both AEP components.

Projection from the external pallidum to the reticular thalamic nucleus in the squirrel monkey

Small injections of biocytin in the external segment of the pallidum (GPe) of the squirrel monkey (Saimiri sciureus) led to anterograde labeling of fibers in the thalamic reticular nucleus (NRT). These fibers reached NRT by coursing along the ventral tip of the internal capsule or by directly piercing the internal capsule more dorsally. They arborized profusely within the entire rostrocaudal extent of the nucleus. Within NRT, biocytin-labeled fibers were long, slightly varicosed, and emitted numerous short collaterals whose terminal portions consisted of clusters of large varicosities. Some of these varicosities were closely apposed to cell bodies and proximal dendrites of NRT neurons. Small injections of wheat germ-agglutinated horseradish peroxidase in the rostral pole of NRT led to retrograde cell labeling within the entire rostrocaudal extent of GPe. These retrogradely-labeled cells did not display immunoreactivity for choline acetyltransferase. Hence, beside the well-established projection from the internal pallidum to the thalamus, our findings support the existence of another pallidothalamic projection whereby GPe neurons could exert a powerful influence upon the thalamocortical neurons via a relay in NRT.