Nigral modulation of cerebello-thalamo-cortical transmission in the ventral medial thalamic nucleus

Summary In the rat, the highly active GABAergic neurons of the substantia nigra pars reticulata (SNR) are known to exert a tonic inhibitory influence on cells in the ventral medial thalamic nucleus (VM). Considering that this nucleus is involved in the transfer of cerebellar signals towards motor cortex, we investigated the role played by SNR in that transmission. For this purpose we examined how changes in nigral background activity are reflected in the reactivity of VM cells to their cerebellar input. We report here that a GABA induced nigral pause increases the efficacy of cerebellar afferent volleys in VM, whereas an increase of nigral background by bicuculline, interrupts cerebello-thalamo-cortical transmission. It is concluded that nigrothalamic neurons subserve a permanent gating of cerebellothalamo-cortical transmission in VM.     

Projections to the rostral reticular thalamic nucleus in the rat

Summary Afferent pathways to the rostral reticular thalamic nucleus (Rt) in the rat were studied using anterograde and retrograde lectin tracing techniques, with sensitive immunocytochemical methods. The analysis was carried out to further investigate previously described subregions of the reticular thalamic nucleus, which are related to subdivisions of the dorsal thalamus, in the paraventricular and midline nuclei and three segments of the mediodorsal thalamic nucleus. Cortical inputs to the rostral reticular nucleus were found from lamina VI of cingulate, orbital and infralimbic cortex. These projected with a clear topography to lateral, intermediate and medial reticular nucleus respectively. Thalamic inputs were found from lateral and central segments of the mediodorsal nucleus to the lateral and intermediate rostral reticular nucleus respectively and heavy paraventricular thalamic inputs were found to the medial reticular nucleus. In the basal forebrain, afferents were found from the vertical and horizontal limbs of the diagonal band, substantia innominata, ventral pallidum and medial globus pallidus. Brainstem projections were identified from ventrolateral periaqueductal grey and adjacent sites in the mesencephalic reticular formation, laterodorsal tegmental nucleus, pedunculopontine nucleus, medial pretectum and ventral tegmental area. The results suggest a general similarity in the organisation of some brainstem Rt afferents in rat and cat, but also show previously unsuspected inputs. Furthermore, there appear to be at least two functional subdivisions of rostral Rt which is reflected by their connections with cortex and thalamus. The studies also extend recent findings that the ventral striatum, via inputs from the paraventricular thalamic nucleus, is included in the circuitry of the rostral Rt, providing further evidence that basal ganglia may function in concert with Rt. Evidence is also outlined with regard to the possibility that rostral Rt plays a significant role in visuomotor functions.Abbreviations ac anterior commissure - aca anterior commissure, anterior - Acb accumbens nucleus - AI agranular insular cortex - AM anteromedial thalamic nucleus - AV anteroventral thalamic nucleus - BST bed nucleus of stria terminalis - Cg cingulate cortex - CG central gray - CL centrolateral thalamic nucleus - CM central medial thalamic nucleus - CPu caudate putamen - DR dorsal raphe nucleus - DTg dorsal tegmental nucleus - EP entopeduncular nucleus - f fornix - Fr2 Frontal cortex, area 2 - G gelatinosus thalamic nucleus - GP globus pallidus - Hb habenula - HDB horizontal limb of diagonal band - IAM interanterodorsal thalamic nucleus - ic internal capsule - INC interstitial nucleus of Cajal - IF interfascicular nucleus - IL infralimbic cortex - IP interpeduncular nucleus - LC locus coeruleus - LDTg laterodorsal tegmental nucleus - LH lateral hypothalamus - LHb lateral habenular nucleus - ll lateral lemniscus - LO lateral orbital cortex - LPB lateral parabrachial nucleus - MD mediodorsal thalamic nucleus - MDL mediodorsal thalamic nucleus, lateral segment - Me5 mesencephalic trigeminal nucleus - MHb medial habenular nucleus - mlf medial longitudinal fasciculus - MnR median raphe nucleus - MO medial orbital cortex - mt mammillothalamic tract - OPT olivary pretectal nucleus - pc posterior commissure - PC paracentral thalamic nucleus - PF parafascicular thalamic nucleus - PPTg pedunculopontine tegmental nucleus - PrC precommissural nucleus - PT paratenial thalamic nucleus - PV paraventricular thalamic nucleus - PVA paraventricular thalamic nucleus, anterior - R red nucleus - Re reuniens thalamic nucleus - RRF retrorubral field - Rt reticular thalamic nucleus - Scp superior cerebellar peduncle - SI substantia innominata - sm stria medullaris - SNR substantia nigra, reticular - st stria terminalis - TT tenia tecta - VL ventrolateral thalamic nucleus - VO ventral orbital cortex - VP ventral pallidum - VPL ventral posterolateral thalamic nucleus - VTA ventral tegmental area - 3 oculomotor nucleus - 3V 3rd ventricle - 4 trochlear nucleus     

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

Summary Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4–6 ms that may last for 10–15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4–22 ms. These action potentials were considered to be antidromic because 1) they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8–2.0 ms. The antidromic responses failed to follow low frequency stimulation (< 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses 1) occur at the same latency, 2) respond in the same manner to twin pulse and repetitive stimulation and 3) are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.     

Inhibitory nigral influence on tectospinal neurons,a possible implication of basal ganglia in orienting behavior

Summary We have established in previous electrophysiological studies that the substantia nigra pars reticulata (SNr) exerts a potent inhibitory influence on cells located in the intermediate and deep tectal strata. The present study demonstrates that, in the rat, the tectospinal neurons constitute one of the cellular populations of the tectum on which the SN exerts its influence.Tectospinal neurons were identified using the antidromic activation method. Following SNr stimulation 17/37 (45%) of these cells showed a shortlatency (1.5–2 ms) short-duration (7–15 ms) inhibition. This effect was revealed by a blockade of spontaneous and peripherally evoked discharges. Moreover, in some cases the nigral inhibition delayed the antidromic invasion of the somato-dendritic portion of the neuron. Tectospinal neurons have been considered as one of the neuronal substrate by which the superior colliculus (SC) can promote head orienting movements. The evidence that the SNr influences the responsiveness of these cells to their peripheral sensory inputs suggests that the basal ganglia and in particular the SNr are involved in the integrative sensorimotor processes underlying head orienting movements.This work was supported by DGRST Grant 83 C 0336     

Comparison of caudate nucleus and substantia nigra control of medial thalamic cell activities in the rat

Systematic mapping of the intralaminar thalamic cells was performed in rats. We studied the characteristics of extracellular spontaneous activity and how the stimulation of caudate nucleus and substania nigra changes firing rate.The majority of cells exhibited a decrease in their activity after the caudate nucleus and substania nigra stimulation; the duration of the decrease was significantly different for each structures stimulated. We propose the participation of a relay structure mediating the decrease of thalamic activity. The responses to caudate nucleus and substantia nigra stimulation could be due to a cortical disfacilitation on the thalemic cells.     

Neural pathway mediating somatic evoked responses in the caudate nucleus of cats

The course of somatic afferents to caudate nucleus (CN) was investigated. In curaized cats discrete lesions in intralaminar nuclei, n. centralis medialis (NCM) affect evoked response (ER) in CN to radial nerve stimulation. NCM lesions totally abolish these somatic responses, while after subthalamic lesions ER appear only when a high stimulus intensity is used. Lesions in the specific relay nuclei, ventralis posterior lateralis (VPL) have no effect on somatic ER in CN. Barbiturates (10 mg/kg) abolish somatic ER in CN. Stimulation of the ponto-mesencephalic reticular formation (RF) in a region where visual and somatic responses are recorded shows lowest threshold values for eliciting evoked responses in NCM and CN. The potentials in CN to RF stimulation are restricted to this nucleus and disappear in the internal capsule.     

Direct projections of the hypothalamic nuclei to the thalamic mediodorsal nucleus in the cat

Direct projections of the hypothalamic nuclei to the thalamic mediodorsal nucleus (MD) were studied using retrograde and anterograde transport of horseradish peroxidase (HRP) and wheat germ agglutinin (WGA)-HRP. HRP and WGA-HRP were injected into the MD, thalamic paraventricular, lateral habenular and hypothalamic nuclei. The results indicate that the MD, particularly its medial part, receives a moderate amount of hypothalamic afferents, and that most of these afferents originate in the medial part of the lateral hypothalamic nucleus at anterior levels, while a limited number are derived from the dorsal, dorsomedial, ventromedial and anterior hypothalamic and lateral preoptic nuclei.     

Behavioural effects mediated by unilateral nigral dopamine receptor stimulation in the rat

Summary Unilateral intranigral injections of dopamine in conscious rats pretreated with nialamide resulted in either ipsiversive or contraversive rotation depending upon the site of injection. Injection of dopamine (50 g) into the zona compacta of the substantia nigra induced weak ipsiversive or mixed ipsiversive and contraversive rotation. Injection of dopamine (12.5–50.0 g) into zona reticulata of substantia nigra induced only contraversive circling. Destruction of the ipsilateral medial forebrain bundle (MFB) using 6-hydroxydopamine (6-OHDA) abolished ipsiversive circling but enhanced contraversive circling produced by dopamine or apomorphine. The combination of a unilateral 6-OHDA lesion of MFB with a kainic acid or electrolesion of the ipsilateral strio-nigral and pallido-nigral pathways reduced contraversive circling to intranigral apomorphine (10 g). Ipsiversive circling produced following intranigral injection of dopamine is dependent upon the integrity of ascending dopamine neurones. Contraversive rotation is independent of ascending dopamine pathways but is reliant upon afferent input to the substantia nigra from the striatum and/or globus pallidus.     

Dopamine function in the prefrontal cortex of the rat is sensitive to a reduction of tonic GABA-mediated inhibition in the thalamic mediodorsal nucleus

Summary Dopamine (DA) utilisation has been determined in the medial bank of the prefrontal cortex (FCx) and the agranular insular cortex (AgCx) of the rat in response to a unilateral reduction of -aminobutyric acid (GABA)-mediated inhibition in the thalamic mediodorsal nucleus (MD). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC) DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA): DA were used as indices of DA utilisation. A bilateral increase in both ratios was found in FCx and AgCx following unilateral infusion of GABA antagonists (1 mM) into MDc. When this concentration was infused into one MD_L no change was detected in DA utilisation of FCx, although a bilateral increase was observed in AgCx. However, a correspondence with the known anatomical connections was attained following infusion of lower concentrations (0.5 mM) into MD_L in that a significant bilateral elevation of DA utilisation was shown in FCx. The changes induced in these ratios by the above treatments were, in general, due to increases in the concentration of metabolite and slight decreases in that of DA. However, unilateral lesions to the presumed GABA-containing neurones of the rostrodorsal thalamic reticular nucleus (TRNd), which topographically innervate MD_L, produced increases in both metabolite and DA concentrations in FCx of both hemispheres, whilst those in AgCx were unaffected. Despite the slightly different results obtained using these two experimental approaches, it is argued that a reduction of tonic GABA-mediated inhibition in MD may tend to activate the DA system in cortical target regions.     

Muscarinic action of acetylcholine in the rat ventromedial thalamic nucleus

Summary Several lines of evidence suggest a role for ACh in the mediation of cerebello-thalamic transmission. The physiological, pharmacological and biochemical experiments described were designed to test this hypothesis for the rat cerebello-thalamic pathway. Unilateral electrolytic lesions of the superior cerebellar peduncle resulted in modest falls of CAT from both ventromedial thalamic nuclei (contralateral 35%, ipsilateral 15%). Iontophoretic application of ACh to relay cells evokes three types of response (i) excitation (ii) inhibition (iii) polyphasic combinations of (i) and (ii). The type of response evoked was directly related to the firing pattern of the cell. Thus, for example, excitatory responses were never recorded during high-frequency bursting but were easily evoked following a switch to tonic, single-spike activity. All responses to ACh and synaptic responses to cerebellar stimulation were sensitive to muscarinic but not to nicotinic cholinergic antagonists. The nicotinic antagonist mecamylamine was a potent blocker of excitant amino acid responses but had no effect on cerebellarevoked synaptic responses. Cholinergic and anticholinergic agents had a profound action on relay cell firing pattern. ACh promoted single-spike activity whereas atropine promoted high-frequency bursting. The actions of ACh are discussed with reference to recently discovered voltage-sensitive ionic conductances. Because of the modulatory action of ACh on relay cell firing pattern and excitability no firm conclusion can be reached concerning the hypothesis under test here. We tentatively suggest a dual role for ACh as both neurotransmitter and neuromodulator.     

Regulation of cerebello-cortical transmission in the rat ventromedial thalamic nucleus

Summary On the basis of antidromic stimulation we have identified two distinct neuronal populations in the rat ventromedial thalamic nucleus. The largest population (96%) are thalamo-cortical relay cells which project via the internal capsule to the cerebral cortex. The smaller population of cells (4%) project caudally to the reticular formation and superior colliculus. These two cell types could be distinguished further on the basis of their patterns of spontaneous discharge. Relay cells fluctuate between two activity patterns (i) a rhythmic pattern characterized by periods of high-frequency bursting, and (ii) a more tonic discharge pattern of single spikes. The caudally projecting cells had a characteristic fast, regular type of spontaneous firing. Brachium conjunctivum stimulation evokes two distinct responses in thalamic relay cells, (i) a short-latency single spike, (ii) a longer latency, rhythmic response of 2–3 spikes. Both excitatory responses are followed by a period of cell quiescence. The type of response is dependent upon the cell's firing pattern. The short-latency response occurs during tonic, single-spike activity whilst the longer latency response occurs during highfrequency bursting activity. The short-latency response can be altered to the long latency response by increasing the level of anaesthesia or by applying a conditioning shock to known inhibitory pathways. Conversely the long latency response can be altered to the short-latency response by decreasing anaesthesia or by stimulation of the reticular formation. It is argued that both response types are evoked monosynaptically by activation of the same cerebello-thalamic fibres but that different ionic conductances which are active at different levels of membrane polarization are responsible for the two response patterns. Efficient time-locked cerebellothalamo-cortical transmission occurs only during tonic single-spike activity, when cerebellar stimulation evokes a short-latency response. Such transmission is allowed or disallowed by the fine balance between converging excitatory and inhibitory afferents. In addition to a monosynaptic excitatory input from the cerebellar nuclei, relay cells received converging synaptic inputs from the substantia nigra, cerebral cortex, reticular formation and superior colliculus. Due to the anatomical arrangement in the rat it proved impossible to assess the role of the pallidum. The population of caudally projecting cells also received several converging synaptic inputs, but unlike those influencing relay cells, these inputs were all excitatory. We have obtained no clear physiological evidence for the occurrence of local interneurones within the ventromedial nucleus. However, a powerful recurrent inhibitory circuit is activated following antidromic activation of relay cells. The interneurones responsible for this inhibition appear to lie in the thalamic nucleus reticularis.Abbreviations ACh Acetylcholine - ACG Autocorrelollogram - BC Brachium Conjunctivum - EEG Electroencephalogram - GABA Gamma aminobutyric acid - GP Globus Pallidus - IPSP Inhibitory post-synaptic potential - IC Internal capsule - ISIH Interspike interval histogram - MRF Mesencephalic reticular formation - PSTH Post stimulus time histogram - St Striatum - SN Substantia Nigra - SC Superior Colliculus - VM Ventromedial thalamic nucleus     

Auditory neurons in the rat thalamic reticular nucleus

Summary In the thalamic reticular nucleus (TR) of the rat a cluster of neurons has been located which receives auditory inputs and acts as a source of inhibition for relay neurons of the medial geniculate nucleus (MG). These TR neurons (auditory thalamic reticular neurons; A-TR neurons) showed a repetitive burst of grouped discharge upon electrical stimulation of the inferior colliculus (IC) or of the auditory cortex. Many of them responded to tonal stimuli such as clicks or pips.Adjacent to the cluster of A-TR neurons there were the cluster of TR neurons receiving visual inputs (V-TR neurons) and that receiving somatosensory inputs (S-TR neurons). The cluster of A-TR neurons was situated ventrally to the cluster of V-TR neurons, both extending caudally from the level of the rostral tip of the dorsal lateral geniculate nucleus. The S-TR neurons distributed rostrally to the clusters of A- and V-TR neurons. Some of the sensory TR neurons, usually found around the boundaries between the clusters of different sensory modalities, were activated from stimulation of different central sensory pathways.Single electric shocks directly applied to the cluster of A-TR neurons suppressed discharges of relay neurons of the MG, either spontaneous or evoked by click stimuli or by electric shocks to the IC. The postexcitatory suppression of MG relay neurons was similar in time course to the suppression following electrical stimulation of A-TR neurons.Response latencies of the A-TR neurons to IC shocks were found to be 1.0–1.5 ms longer than those of the MG relay cells with respect to the modal and shortest values. It is suggested that A-TR neurons are intercalated in the axon collateral circuit of the thalamocortical projection arising from relay neurons of the MG.Dedicated to Dr. Kitsuya Iwama, Emeritus Professor of Osaka University Medical School, on the occasion of his retirement     

The pathways connecting the hippocampal formation, the thalamic reuniens nucleus and the thalamic reticular nucleus in the rat

Most dorsal thalamic nuclei send axons to specific areas of the neocortex and to specific sectors of the thalamic reticular nucleus; the neocortex then sends reciprocal connections back to the same thalamic nucleus, directly as well indirectly through a relay in the thalamic reticular nucleus. This can be regarded as a 'canonical' circuit of the sensory thalamus. For the pathways that link the thalamus and the hippocampal formation, only a few comparable connections have been described. The reuniens nucleus of the thalamus sends some of its major cortical efferents to the hippocampal formation. The present study shows that cells of the hippocampal formation as well as cells in the reuniens nucleus are retrogradely labelled following injections of horseradish peroxidase or fluoro-gold into the rostral part of the thalamic reticular nucleus in the rat. Within the hippocampal formation, labelled neurons were localized in the subiculum, predominantly on the ipsilateral side, with fewer neurons labelled contralaterally. Labelled neurons were seen in the hippocampal formation and nucleus reuniens only after injections made in the rostral thalamic reticular nucleus (1.6-1.8 mm caudal to bregma). In addition, the present study confirmed the presence of afferent connections to the rostral thalamic reticular nucleus from cortical (cingulate, orbital and infralimbic, retrosplenial and frontal), midline thalamic (paraventricular, anteromedial, centromedial and mediodorsal thalamic nuclei) and brainstem structures (substantia nigra pars reticularis, ventral tegmental area, periaqueductal grey, superior vestibular and pontine reticular nuclei). These results demonstrate a potential for the thalamo-hippocampal circuitry to influence the functional roles of the thalamic reticular nucleus, and show that thalamo-hippocampal connections resemble the circuitry that links the sensory thalamus and neocortex.     

丘脑室旁核形态和功能研究进展

丘脑室旁核（PVT）为丘脑中线核团的重要组成部分,是多种行为的中继传导核团及整合中心,参与动物觉醒、摄食、成瘾、奖赏、恐惧记忆等多种行为的调节。PVT内主要分布着表达囊泡谷氨酸转运体-2（VGluT2）的谷氨酸能兴奋性神经元,却无γ-氨基丁酸（GABA）能抑制性神经元。基于PVT的复杂功能与其内神经元相对单一的兴奋性属性,有必要对PVT内兴奋性神经元进行分类。在本综述中,我们主要对PVT的形态及电生理特点、传入和传出联系、前后两段的形态和功能差异进行总结,并以纤维联系和神经化学性质作为分类标准对PVT的兴奋性神经元进行分类,以便为阐明PVT的复杂功能提供帮助。     

Electron microscopic demonstration of terminations of posterior thalamic axons on identified rubrospinal neurons in the rat

In the rat, the major output of the posterior thalamic nucleus (PT) ends in the ventrolateral sector of the rostral two-thirds of the red nucleus. The aim of this study is to identify the rubral cells contacted by these thalamic efferents. In a first set of experiments, an anterograde neurotracer (PHA-L) was injected into the rostral part of the red nucleus. The only structure consistently and densely labeled was the contralateral spinal cord. Therefore, in a second set of experiments, massive HRP injections were made at different cervical levels in the spinal cord in combination with either electrolytic lesion or PHA-L injection in the contralateral PT. Both anterograde and retrograde labelings obtained in the RN were examined by correlated light and electron microscopy. Our findings indicate that anterogradely degenerated or labeled axons arising from the PT form synaptic contacts on HRP-filled dendritic processes of rubrospinal neurons. The thalamo-rubral articulation is direct and seems to be mainly axo-dendritic. These results support the possible participation of the PT in the modulatory control of spinal interneurons through the rubrospinal tract.     

Cholinergic stimulation of the nucleus basalis of Meynert and reticular thalamic nucleus affects spike-and-wave discharges in WAG/Rij rats

The role of the cholinergic innervated nucleus basalis of Meynert (NB) and reticular thalamic nucleus (RT) in the generation or modulation of spontaneously occurring spike-and-wave discharges (SWDs) was investigated in the WAG/Rij rat model of absence epilepsy. The cholinergic agonist carbachol and the muscarinic antagonist scopolamine were injected in the NB and RT in the doses of 0.55 and 5.5 nmol while the EEG was recorded. Carbachol injected in the NB decreased the number and the mean duration of SWDs. Scopolamine alone had no influence on SWDs, but could antagonize the effects of carbachol if administered simultaneously in NB. Injections of carbachol in the RT inhibited the occurrence of SWDs, but did not affect the mean duration. Scopolamine administered in the RT had no influence on seizure activity. It is concluded that cholinergic stimulation of the NB or the RT inhibits the cortical synchronous activity characterizing SWDs.     

Ascending projections to the lateral thalamic nuclei from the substantia grisea centralis in the rat: a retrograde WGA-HRP study

Ascending projections to the lateral thalamic nuclear group from the substantia grisea centralis (SGC) were studied by injections of wheat germ agglutinin-conjugated with HRP (WGA-HRP) into the laterodorsal (LD) and lateroposterior (LP) thalamic nuclei. The present study demonstrated that the pars ventralis of the SGC at the levels of the intercollicular region and of the locus ceruleus sent fibers to both the LD and LP on both sides with homolateral predominance. Distribution pattern and morphological characteristics of the LD-projection neurons in the SGC were similar to those of SGC-LP projection cells. WGA-HRP injections into the LD or LP labeled also a considerable number of neurons in the dorsal raphe nucleus and the dorsal tegmental nucleus bilaterally with homolateral predominance, but the nucleus of Darkschewitsch contained labeled neurons only after the LD injection.     

The synaptic activation of neurones of the feline ventrolateral thalamic nucleus: Possible cholinergic mechanisms

Summary The responses of individual neurones of nucleus ventrolateralis thalami (VL) have been recorded extracellularly following stimulation of the brachium conjunctivum (BC), nucleus entopeduncularis (EN) and precruciate cortex. In anaesthetized cats stimulation of these structures produced either short latency single spike responses or brief bursts of action potentials with somewhat longer latency: the latter responses could be converted to single spikes by the electrophoretic application of acetylcholine or an excitatory amino acid to the neurone. Atropine attenuated the effect of BC stimulation but did not alter excitations from the cortex or EN. Acetylcholine was found to depress the excitation of VL neurones from EN. Collateral fibres of the EN neurones were shown to innervate neurones in the lateral parts of the centrum medianum — parafascicular complex and in VL.It was concluded that VL neurones receive monosynaptic inputs from cortex, EN and the cerebellar nuclei, but that only-the last may have a significant cholinergic component.     

Calcium-binding proteins in the laterodorsal thalamic nucleus during development of the guinea pig

The laterodorsal thalamic nucleus (LD) is often treated as a part of the anterior thalamic nuclei (ATN) because of its location and similar connectivity. Our previous studies have shown that distribution of three calcium-binding proteins, i.e. calbindin D_28k (CB), calretinin (CR) and parvalbumin (PV), changes within the ATN during development of the guinea pig. The aim of this study is to examine the immunoreactivity pattern of these proteins in the LD in the guinea pig ontogeny. Brains from animals ranging from 40th embryonic day to 80th postnatal day were used in the study. Two methods were applied: a single-labelling immunoenzymatic method and double-labelling immunofluorescence. No changes of the distribution pattern of the substances were observed throughout the examined developmental stages. CB and CR were the most abundantly expressed proteins in perikarya of the LD. Numerous CB- and CR-immunoreactive cell bodies were found throughout the whole extent of the nucleus. In most of these cell bodies both proteins colocalized vastly. The highest immunoreactivity of the perikarya containing CB and CR was observed in the mediodorsal part of the LD and in its rostral portion. In regard to PV, single cell bodies were observed mostly in the dorsal part of the nucleus. PV did not colocalize with the other proteins. In summary, all the studied calcium-binding proteins were already present in the LD at prenatal developmental stages and the pattern of distribution remained virtually constant until adulthood. Thus, the LD differs considerably from the ATN in an aspect of neurochemical cell differentiation.     

Distribution of γ-aminobutyrate in the rat thalamus: Specific decreases in thalamic γ-aminobutyrate following lesion or electrical stimulation of the substantia nigra

Various subnuclei of the rat's thalamus were punched out or finely microdissected from frozen coronal sections of brain and assayed for γ-aminobutyrate by microdansylation. Highest levels were recorded in the ventromedial and parafascicular regions, corresponding to the sites of termination of nigrothalamic neurones. One week after placing a unilateral electrolytic lesion in the zona reticulata of the substantia nigra there were significant decreases in γ-aminobutyrate levels in the ipsilateral ventromedial (?14%) and parafascicular (?11%) nuclei, but not in other nuclei tested. Falls in the level of γ-aminobutyrate of 19–26% were found to be confined to the ventromedial region in microdissections of brain slices containing this nucleus, two weeks after injecting 1 μg kainic acid into the substantia nigra. The concentrations of glutamate, aspartate, glutamine, glycine and alanine were not altered in the deafferented ventromedial thalamus. Pathological effects of kainate were registered as a complete loss of neuronal perikarya at the injection site coupled with intense gliosis. Mild gliosis, but no postsynaptic neuronal damage, was visible in the corresponding ventromedial and parafascicular nuclei in the thalamus, although degenerating electron-dense terminal boutons were occasionally seen in electron micrographs of this tissue.In rats in which thalamic glutamate decar?ylase activity was inhibited by systemic injection of 3-mercaptopropionic acid, trains of biphasic electrical pulses delivered to the nigra (100 Hz, 0.1 mA, width 0.6 ms for 15 min) evoked a stimulus-dependent decrease in ventromedial (?17%) and parafascicular (?10%) γ-aminobutyrate content, but not in that of other thalamic nuclei. Similar post-stimulus falls in the level of γ-aminobutyrate in the ventromedial nucleus were noted after direct stereotaxic injection of this nucleus with the enzyme inhibitor.These findings are consistent with the existence of γ-aminobutyrate-containing nigrothalamic fibres whose cell bodies lie in the medial part of the substantia nigra zona reticulata and which terminate in the ventromedial and parafascicular nuclear regions of the thalamus.