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PubMed1996

Possible release of nitric oxide from cholinergic axons in the thalamus by stimulation of the rat laterodorsal tegmental nucleus as measured with voltammetry.

By means of the differential direct current voltammetry technique with carbon fiber electrodes in urethane-anesthetized rats, we monitored nitric oxide (NO) concentrations in the thalamus in the basal condition and following electrical stimulation of the laterodorsal tegmental nucleus (LDT), whose neurons have the strongest activity of NADPH-diaphorase, or NO synthase, together with acetylcholine. NO levels, measured as the height of the peak at +970-1000 mV in the voltammetry (NO was soon oxidized in vivo to be detected at the voltage of this peak, so that NO levels in this report are, in the strict sense, levels of the oxidized metabolites reflecting very possibly those of NO in physiological conditions; see Section 2, Methods), increased just after repetitive stimulation of the LDT. Stimulation of the surrounding areas or the cerebellum produced virtually no change in NO levels. An intravenous injection of L-nitroarginine methyl ester reduced the basal level of NO, but stimulation of the LDT still increased NO levels, which may be due to very strong activity of NO synthase in the LDT neurons. These results are consistent with the notion that NO can be released from axons of the LDT neurons by their excitation.

Animals↗
PubMed2005

Effects of ibotenic acid lesions of the mediodorsal thalamus on memory: relationship with emotional processes in mice.

The effects of ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) on memory and fear reactivity in mice were studied. In the first experiment, MD subjects were submitted to a behavioral design allowing to study the relationship between memory and anxiety [Krazem A, Borde N, Beracochea D. Effects of diazepam and beta-CCM on working memory in mice: relationship with emotional reactivity. Pharmacol Biochem Behav 2001;68:235-44; Beracochea D, Krazem A, Jaffard R. Methyl beta carboline-3-carboxylate reverses the working memory deficits induced either by chronic alcohol consumption or mammillary body lesions in mice. Psychobiology 1995;23:52-8]. In a second experiment, MD-lesioned subjects were submitted to a GO/NOGO temporal alternation task involving two intertrial intervals (ITIs: 0 and 30 s). Lesioned subjects exhibited large bilateral mediodorsal thalamic lesions with small damage into the centromedial thalamic nucleus. In the first experiment, MD-lesioned animals performed normally a sequential alternation task involving fixed ITIs over seven successive trials (5 or 30 s); in contrast, MD-lesioned subjects exhibited deficits in the sequential task involving the same but mixed ITIs (30-5 s versus 5-30 s) the deficit being observed for the last trials of the series, regardless the ITIs used. MD lesions increased fear reactivity in an elevated-plus maze, and scores of anxiety were negatively correlated with performance in the mixed alternation schedule. The second experiment involving non spatial information extended results of the first experiment in showing that the deficit of MD-lesioned animals was not dependent on the ITIs separating trials. Overall, our data show that MD-lesioned subjects exhibit a cognitive impairment characterized by a difficulty to maintain an alternation rule in situations involving procedural variance, and this deficit could stem primarily from an increase of fear reactivity.

Animals↗
PubMed2005

Orexin (hypocretin) innervation of the paraventricular nucleus of the thalamus.

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus with projections to limbic forebrain areas such as the nucleus accumbens and amygdala. The orexin (hypocretin) peptides are synthesized in hypothalamic neurons that project throughout the CNS. The present experiments were done to describe the extent of orexin fiber innervation of the PVT in comparison to other midline and intralaminar thalamic nuclei and to establish the location and proportion of orexin neurons innervating the PVT. All aspects of the anteroposterior PVT were found to be densely innervated by orexin fibers with numerous enlargements that also stained for synaptophysin, a marker for synaptic vesicle protein associated with pre-synaptic sites. Small discrete injections of cholera toxin B into the PVT of rats resulted in the retrograde labeling of a relatively small number of orexin neurons in the medial and lateral hypothalamus. The results also showed a lack of topographical organization among orexin neurons projecting to the PVT. Previous studies indicate that orexin neurons and neurons in the PVT appear to be most active during periods of arousal. Therefore, orexin neurons and their projections to the PVT may be part of a limbic forebrain arousal system.

Animals↗
PubMed2006

Short-term facilitation in the anterior cingulate cortex following stimulation of the medial thalamus in the rat.

The present study examined the distribution and localization of synaptic activities (field potentials, multiunit activities and sink source currents) evoked in the anterior cingulate cortex (ACC) by electrical paired pulse stimulation of the ipsilateral medial thalamus (MT). Male Sprague-Dawley rats were anesthetized with halothane (1.0-1.5%), and electrical paired pulses stimuli (100-300 microA, inter-pulse interval, 100 ms) were delivered to the MT. Tungsten microelectrodes and a multichannel Michigan probe were used to record the evoked field potentials and multiunit activities in the ACC. Paired pulse stimulation facilitated field potentials and multiunit activities elicited from several MT nuclei. The second component of the negative field potential (com2) was augmented to about 2.5 times that of the first component (com1), and the integrated multiunit activities were facilitated by about 1.6-fold. Paired stimulation produced an expansion of the maximal negative potential from layer II/III into the deeper layers of the cingulate cortex area 1 (Cg1). Furthermore, the potentiated activity spread into adjacent secondary motor cortex (M2) and prelimbic cortex (PrL). Meanwhile, the area covered by the maximal integrated multiunit activities expanded from layer V (com1) to layers II-V (com2) in M2, Cg1 and PrL. The current source density (CSD) analysis revealed that the short latency sinks were located in layer II/III and layer V/VI. The sink currents were potentiated and expanded to more superficial and to deeper layers when a second pulse was delivered with a 100-ms time delay. Sink currents and the paired pulse facilitation (PPF) were reduced by morphine treatment (5 mg/kg, i.v.), and this effect could be blocked by naloxone. Electrical stimulation at 10 Hz in the MT induced more pronounced c-fos immunolabeling of neurons in the medial prefrontal cortex than did 1-Hz stimulation. The short-term facilitation occurred in the middle layers and expanded to the deeper layers of the ACC. These changes may mediate the effective signal transference in the specific frequency associated with painful responses.

Animals↗
PubMed2006

Depth electrode recorded cerebral responses with deep brain stimulation of the anterior thalamus for epilepsy.

OBJECTIVE: We investigated the relation between anterior thalamic stimulation and the morphology of the evoked cerebral responses (CRs) using intracerebral depth electrodes in patients with intractable epilepsy undergoing deep brain stimulation (DBS) of the thalamus. METHODS: Monopolar cathodic and bipolar stimuli were delivered at a rate of 2 or 1 Hz to the anterior nucleus (AN) and the dorsomedian nucleus (DM) of two patients using the programmable stimulation device (Medtronic ITREL II) or a GRASS stimulation device (S12). CRs were recorded from depth or DBS electrodes, situated bilaterally in mesial temporal (hippocampus, both patients), lateral temporal (one patient), orbitofrontal (Brodmann area 11, one patient) and anterior thalamic sites (one patient). RESULTS: The distribution and morphology of the CRs depended primarily on the site of stimulation within the anterior thalamic region. Overall, monopolar cathodic and bipolar stimulation of the AN elicited CRs mainly in ipsilateral mesial temporal cortical areas, whereas stimulation of the DM evoked high-amplitude CRs predominantly in ipsilateral orbitofrontal areas. The amplitude of the CR was positively related to the strength of the stimulus and generally higher with monopolar than with bipolar stimulation. The differences between CRs elicited during wakefulness or slow wave sleep were minimal. CONCLUSIONS: The distribution of the CRs corresponded with the intracerebral pathways of the involved structures and the findings are in good accordance with those of our previous study investigating the sources of CRs using statistical non-parametric mapping of low resolution electromagnetic tomography (LORETA) values. SIGNIFICANCE: Our findings indicate a certain degree of point-to-point specificity within the thalamocortical circuitry, which may make optimal localization of DBS electrodes important in patients with epilepsy.

Adult↗
PubMed2005

Glutamate and GABA(B) transmissions in lateral amygdala are involved in startle-like electromyographic (EMG) potentiation caused by activation of auditory thalamus.

The lateral amygdala nucleus (LA) receives auditory inputs from both the auditory thalamus (medial geniculate nucleus, MGN) and auditory association cortex (AAC). These auditory inputs are closely linked with glutamate and GABA(B) receptors in the LA. The LA has intra-amygdaloid connections with the central amygdala nucleus, which mediates auditory fear potentiation of startle (AFPS) via pathways to the startle circuits. The purpose of the present study was to establish an electromyographic (EMG) model for studying AFPS-related neural transmissions in the LA. Hind-limb startle-like EMG responses to single-pulse electrical stimulation of the trigeminal nucleus (TN) were recorded in anesthetized rats. These EMG responses were enhanced by single-pulse sub-threshold electrical stimulation of the MGN when the MGN stimulus led the TN stimulus at short inter-stimulus intervals (ISI). However, the EMG responses were not affected by single-pulse sub-threshold electrical stimulation of the AAC. Bilateral injection of the glutamate antagonist, kynurenic acid, into the LA decreased both the EMG enhancement caused by MGN stimulation at short ISIs and EMG responses to combined TN and AAC stimulation across various ISIs. Moreover, bilateral injection of the GABA(B) antagonist, phaclofen, into the LA increased both EMG responses to combined TN and MGN stimulation across various ISIs, and EMG responses to combined TN and AAC stimulation at short ISIs. These results suggest that the auditory inputs to the LA from the MGN and those from the AAC are affected differently by glutamate and GABA(B) receptors in the LA, and play differential roles in modulating startle responses.

Amygdala↗
PubMed2006

Reliable identification of the auditory thalamus using multi-modal structural analyses.

The medial geniculate body (MGB) of the thalamus is a key component of the auditory system. It is involved in relaying and transforming auditory information to the cortex and in top-down modulation of processing in the midbrain, brainstem, and ear. Functional imaging investigations of this region in humans, however, have been limited by the difficulty of distinguishing MGB from other thalamic nuclei. Here, we introduce two methods for reliably delineating MGB anatomically in individuals based on conventional and diffusion MRI data. The first uses high-resolution proton density weighted scanning optimized for subcortical grey-white contrast. The second uses diffusion-weighted imaging and probabilistic tractography to automatically segment the medial and lateral geniculate nuclei from surrounding structures based on their distinctive patterns of connectivity to the rest of the brain. Both methods produce highly replicable results that are consistent with published atlases. Importantly, both methods rely on commonly available imaging sequences and standard hardware, a significant advantage over previously described approaches. In addition to providing useful approaches for identifying the MGB and LGN in vivo, our study offers further validation of diffusion tractography for the parcellation of grey matter regions on the basis of their connectivity patterns.

Adult↗
PubMed2004

Subcortical afferents to the lateral mediodorsal thalamus in cynomolgus monkeys.

The mediodorsal (MD) nucleus of the thalamus has long been known to provide the principal source of subcortical input to the primate prefrontal cortex, as well as to other areas of the frontal lobe that are thought to contribute to higher-order cognitive functions. In this study, we used injections of retrograde tracers in the lateral portion of the monkey MD to assess the locations of labeled cells in subcortical structures. Three main patterns were identified in the distribution of subcortical connections. We found that the claustrum, superior colliculus and ventral midbrain regions were heavily labeled in the cases with injections in caudoventral MD. In these cases, labeled cells were also found in either the periaqueductal gray or zona incerta, depending on the specific case. In one case with an injection in anterodorsal MD, labeled cells were most numerous in the structures of the ventral midbrain, especially the ventral tegmental area. Finally, the claustrum and superior colliculus contained the largest percentage of labeled subcortical cells in cases with injections in ventrolateral MD. These three patterns of subcortical label corresponded to three equally distinctive trends in the distribution of MD connections with the cortex in these same cases [J Comp Neurol 473 (2004) 107]. Very few labeled cells were found in other areas such as the amygdala, globus pallidus and deep cerebellar nuclei, suggesting that pathways leading from these structures to dorsolateral and dorsomedial frontal cortices are not likely to include the lateral divisions of MD. In concert, these findings show that particular locales within lateral MD receive distinct profiles of subcortical afferents, and project into specific neocortical domains, suggesting that these different sites within lateral MD may participate in functionally distinct circuits of information processing.

Afferent Pathways↗
PubMed2006

Dopamine innervation of the monkey mediodorsal thalamus: Location of projection neurons and ultrastructural characteristics of axon terminals.

Dopamine (DA) axons and receptors have recently been identified in the primate thalamus, including the mediodorsal thalamic nucleus (MD). In order to determine whether the DA innervation of the primate MD shares the anatomical features of the mesocortical or nigrostriatal DA projections, we performed tract-tracing and immunocytochemistry studies in macaque monkeys (Macaca fascicularis) to identify the location of the DA neurons that project to MD and immuno-electron microscopy to determine the distribution of the dopamine transporter (DAT) in axons within the MD. Similar to the mesocortical projection, retrogradely-labeled, tyrosine hydroxylase-containing neurons were present in dorsal tier ventral mesencephalic nuclei, such as the ventral tegmental area and the dorsal portion of the substantia nigra pars compacta. In contrast, no dual-labeled neurons were present in the ventral tier nuclei, the primary origin of the nigrostriatal DA pathway. In addition, like the DA projection to the prefrontal cortex, DAT immunoreactivity was predominantly localized to the pre-terminal portion of axons in the MD, and was infrequently found in association with synaptic vesicles, in contrast to nigrostriatal DA axons. These findings indicate that the DA projection to the MD shares anatomical features with the mesocortical DA system, suggesting that the functional properties of DA neurotransmission in the MD might be more similar to those in the cortex than in the striatum.

Animals↗
PubMed2003

Chronic stress alters behavior in the conditioned defensive burying test: role of the posterior paraventricular thalamus.

In the present studies, we examined the effects of chronic restraint on behavior in the conditioned defensive burying paradigm, a well-validated test of anxiety. This test is based on the findings that rodents tend to cover or bury the source of a noxious or aversive stimulus. However, little is known about whether prior chronic stress exposure can alter this anxiety-related behavior. In the present study, we examined whether chronic restraint affects indices of behavior in the conditioned defensive burying paradigm. Furthermore, since the posterior division of the paraventricular thalamus (pPVTh) regulates neuroendocrine activity specifically in chronically stressed but not control rats, we hypothesized that the pPVTh may also regulate any chronic stress-induced changes in behavior observed in the defensive burying test. Chronically stressed rats (30-min restraint per day for seven consecutive days) exhibited decreased latency to bury compared to control rats regardless of the presence of lesions suggesting increased reactivity to the shock in these animals. Importantly, pPVTh-lesioned chronically stressed rats exhibited increased duration and height of burying compared to control rats with pPVTh lesions, whereas no differences existed between sham-lesioned control and chronically stressed rats. Since both burying height and duration of burying are considered indices of anxiety in the defensive burying test, the present results suggest that the intact pPVTh may be important in dampening behaviors related to anxiety in chronically stressed rats.

Animals↗
PubMed2002

Physiological properties of periodontal mechanosensitive neurones in the posteromedial ventral nucleus of rat thalamus.

Unitary discharges of periodontal mechanosensitive (PM) neurones responding to mechanical tooth stimulation were recorded from the posteromedial ventral nucleus (VPM) of rat thalamus. PM neurones are distributed in the ventromedial area in the rostral two-thirds of the VPM nucleus. Maxillary and mandibular tooth-sensitive neurones are arranged in dorsoventral sequence. Of the PM neurones, 36% were slowly adapting to pressure applied to the tooth and 67% were rapidly adapting. The majority of PM units were sensitive to the contralateral incisor tooth. Response magnitudes of the slowly adapting neurones varied with stimulus direction and were directionally selective to mechanical tooth stimulation. The optimal stimulus direction was labiolingual or linguolabial. Rapidly adapting neurones were directionally non-selective to tooth stimulation. The threshold for mechanical stimulation was <0.05 N. Mean response latencies evoked by electrical stimulation of the peripheral receptive fields were 4.6 ms in the slowly adapting neurones and 5.8 ms in the rapidly adapting neurones.

Adaptation, Physiological↗
PubMed2002

Startle gating in rats is disrupted by chemical inactivation but not D2 stimulation of the dorsomedial thalamus.

The neural regulation of sensorimotor gating, as measured by prepulse inhibition (PPI) of the startle reflex, has been a focus of interest based on the consistent deficits in PPI reported in schizophrenia patients. While dorsomedial thalamus (MD) dysfunction has been implicated in the clinical 'gating' deficits of schizophrenia patients, relatively little is known regarding the regulation of PPI by the MD. We previously reported that PPI in rats is reduced after intra-MD infusion of the GABA agonist muscimol, or after excitotoxic lesions of the MD. In the present study, we tested the regulation of PPI by D2 receptors in the MD. PPI was measured after intra-MD infusion of the D2 agonist quinpirole (0, 1 or 10 microg/side) in a within-subject design. Infusion placement was confirmed functionally in later tests by reversible inactivation of the MD via intra-MD infusion of tetrodotoxin (TTX; 10 ng/side), and subsequently by direct histological examination. Intra-MD infusion of quinpirole had no significant effect on PPI, using doses that significantly disrupt PPI after infusion into the ventral forebrain (nucleus accumbens). TTX infusion into the MD caused a significant loss of PPI; this effect was not reversed by pretreatment with the atypical antipsychotic quetiapine (7.5 mg/kg). The MD regulation of PPI in rats is not mediated via D2 receptors, but is clearly manifested via PPI deficits after reversible MD inactivation via TTX.

Animals↗
PubMed1986

Adult rat motor cortex connections to thalamus following neonatal and juvenile frontal cortical lesions: WGA-HRP and amino acid studies.

Frontal cortex was removed in 1- and 30-day-old rats. When both groups reached 90 days of age, the forelimb motor/sensory cortex in the unlesioned hemisphere was injected with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or tritiated leucine. Thalamic neurons were retrogradely labeled only ipsilateral to the WGA-HRP injection site in both neonatally and juvenile-lesioned subjects. Ventrolateral (VL), ventromedial (VM), centromedial (CM), centrolateral (CL), parafascicular (PF), posteromedial (POm), and posterior (PO) thalamic nuclei were labeled. This and the demonstration of only ipsilateral thalamocortical connections at birth helped explain the marked thalamic atrophy which developed ipsilateral to neonatal frontal cortex lesions. Death of thalamic neurons after neonatal removal of their normal cortical target could be due to their failure to sprout into the opposite cortex because that cortex was already innervated by the opposite thalamus at birth. Leucine motor/sensory cortex injections in both neonatally and juvenile-lesioned subjects labeled the ipsilateral VL, VM, CM, CL, PF, POm, and PO thalamic nuclei; contralateral CM, CL, and PF thalamic nuclei; ipsilateral medial, ventral, and lateral pontine nuclei; and parts of the contralateral pontine nuclei. The ipsilateral connections were always more robust than the contralateral connections. The contralateral corticothalamic and corticopontine projections, however, were much more numerous and widespread in neonatally compared to juvenile-lesioned subjects. The greater sparing of some motor functions said to occur in neonatal compared to adult motor cortex-lesioned subjects could be due to the plasticity of corticothalamic, corticopontine, and other corticofugal pathways, but not to the plasticity of thalamocortical pathways.

Age Factors↗
PubMed1999

Cholinergic projections to the visual thalamus and superior colliculus.

The parabrachial region of the brainstem reticular formation projects to the dorsal lateral geniculate nucleus of the thalamus and to the intermediate gray layer of the superior colliculus. We used the retrograde axonal transport of two fluorescent labels to demonstrate that individual parabrachial cells project to both structures. The results suggest that cholinergic cells of the parabrachial region may coordinate the relay of visuosensory information to the cortex with the onset of orienting movements.

Acetylcholine↗
PubMed1996

Specific binding of [3H]resiniferatoxin by human and rat preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus membrane preparations.

Specific [3H]resiniferatoxin (RTX) binding detects the vanilloid (capsaicin) receptors and provides a biochemical means for exploring their pharmacology. In the present study we demonstrate specific vanilloid (RTX) binding sites in various brain areas not known to be innervated by primary afferent neurons. Specific high-affinity binding of [3H]RTX could be detected in membrane preparations of the posterior ("hypothalamic") and anterior ("septal") parts of the preoptic area, locus ceruleus, medial hypothalamus, brainstem reticular formation and ventral thalamic nuclei from naive rats. The determined levels of binding at 4 nM [3H]RTX were 23.0 +/- 4.5, 7.1 +/- 1.6, 29.9 +/- 2.3, 23.5 +/- 2.4, 9.9 +/- 2.2 and 8.1 +/- 1.9 fmol/mg, respectively; unfortunately, the high levels of non-specific binding (higher than 80%) in the present experiments made it impossible for us to characterize fully the binding properties of the receptors. However, no detectable specific [3H]RTX binding was present in membranes of brain nuclei from rats pretreated with 300 mg/kg capsaicin, a treatment which causes loss of response to capsaicin. Significant specific [3H]RTX binding was also absent in membrane preparations of the midbrain central gray matter, somatosensory cortex and cerebellum either from naive or capsaicin treated rats. In human brain specific [3H]RTX binding measured at 4 nM [3H]RTX showed a pattern of distribution similar to that in the rat brain. The corresponding levels of specific [3H]RTX binding in the preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus were 44.9 +/- 2.4, 50.6 +/- 3.0, 36.1 +/- 2.9, 9.4 +/- 2.8 and 8.4 +/- 2.4 fmol/mg, respectively. Our findings corroborate previous biological evidence that vanilloid receptors are present in brain as well as in sensory afferent neurons.

Animals↗
PubMed2003

Negative feedback functions in chronically stressed rats: role of the posterior paraventricular thalamus.

A gradual decrement in hypothalamic-pituitary-adrenal (HPA) activity is observed following repeated exposure to the same stressor, such as repeated restraint. This decrement, termed habituation, may be partly due to alterations in corticosterone-mediated negative feedback inhibition of the HPA axis. We have previously found that the posterior division of the paraventricular thalamus (pPVTh) regulates habituated HPA activity without altering HPA responses to acute stress. Therefore, in the present study, we examined the role of the pPVTh in delayed feedback inhibition of plasma corticosterone responses to repeated restraint. Dexamethasone was administered subcutaneously 2 h prior to 30 min restraint to induce delayed negative feedback inhibition of the HPA axis. In the first experiment, we determined that a 0.05-mg/kg dose of dexamethasone produced submaximal suppression of corticosterone responses to acute restraint and used this dose in the remainder of the experiments. In Experiment 2, we examined dexamethasone-induced feedback inhibition to corticosterone responses to a single or eighth restraint exposure since negative feedback functions in chronically stressed rats are not well studied. We found that corticosterone levels following dexamethasone treatment were similar in repeatedly restrained compared to acutely restrained rats. In Experiment 3, we lesioned the pPVTh and examined dexamethasone-induced feedback inhibition of corticosterone responses to a single or eighth exposure to restraint. pPVTh lesions attenuated dexamethasone-induced inhibition of corticosterone at 30 min in chronically stressed rats but had no effect in acutely stressed rats. These data suggest that negative feedback functions are maintained in rats exposed to repeated restraint and implicate the pPVTh as a site that contributes to these negative feedback functions specifically under chronic stress conditions.

Analysis of Variance↗
PubMed1981

Morphology and histochemistry of the thalamus in the course of intoxication by ethyl-mercury-p-toluenesulphanilide (EMTS).

Organic Mercury compounds are known for their neurotoxicity. From our previous studies it appeared that a variable degree of damage is brought about in the individual structures of the CNS by ingestion of organic mercurials. A detailed study was undertaken on the histotopography of pathological changes occurring in the thalamus - the major integrator of various cerebral functions - following intragastric administration of EMTS, a fungicide widely applied in agriculture. Rats served as the experimental animals. They were treated intragastrically with a daily dose of 15 mg of EMTS over a period of 10 days ( the per os DL50 for rats is 100 mg/kg of body weight). The results of our study have revealed that chronic ingestion of large doses of EMTS brings about a considerable inhibition of the activities of AChE, BuTJ, acP and ATPase in many thalamic nuclei with a concomitant elevation of TPPase and NsE activities. There was considerable variation as to the degree of histoenzymatic changes in the individual thalamic nuclei. The accompanying glial reaction was evaluated aw well. An interpretation of the findings made in this study has been undertaken.

Acetylcholinesterase↗
PubMed2002

The intralaminar and midline nuclei of the thalamus. Anatomical and functional evidence for participation in processes of arousal and awareness.

The thalamic midline and intralaminar nuclei, long thought to be a non-specific arousing system in the brain, have been shown to be involved in separate and specific brain functions, such as specific cognitive, sensory and motor functions. Fundamental to the participation of the midline and intralaminar nuclei in such diverse functions seems to be a role in awareness. It is unknown whether the midline and intralaminar nuclei, together often referred to as the 'non-specific' nuclei of the thalamus, act together or whether each nucleus is involved idiosyncratically in separate circuits underlying cortical processes. Detailed knowledge of the connectivity of each of these nuclei is needed to judge the nature of their contribution to cortical functioning. The present account provides an overview of the results of neuroanatomical tracing studies on the connections of the individual intralaminar and midline thalamic nuclei in the rat, that have been performed over the past decade in our laboratory. The results are discussed together with those reported by other laboratories, and with those obtained in other species. On the basis of the patterns of the afferent and efferent projections, we conclude that the midline and intralaminar thalamic nuclei can be clustered into four groups. Each of the groups can be shown to have its own set of target and input structures, both cortically and subcortically. These anatomical relationships, in combination with functional studies in animals and in humans, lead us to propose that the midline and intralaminar nuclei as a whole play a role in awareness, with each of the groups subserving a role in a different aspect of awareness. The following groups can be discerned: (1) a dorsal group, consisting of the paraventricular, parataenial and intermediodorsal nuclei, involved in viscero-limbic functions; (2) a lateral group, comprising the central lateral and paracentral nuclei and the anterior part of the central medial nucleus, involved in cognitive functions; (3) a ventral group, made up of the reuniens and rhomboid nucleus and the posterior part of the central medial nucleus, involved in multimodal sensory processing; (4) a posterior group, consisting of the centre médian and parafascicular nuclei, involved in limbic motor functions.

Animals↗