Distribution and density of GABA cells in intralaminar and adjacent nuclei of monkey thalamus

Sleep spindles in primates: Modeling the effects of distinct laminar thalamocortical connectivity in core, matrix, and reticular thalamic circuits

Sleep spindles are associated with the beginning of deep sleep and memory consolidation and are disrupted in schizophrenia and autism. In primates, distinct core and matrix thalamocortical (TC) circuits regulate sleep spindle activity through communications that are filtered by the inhibitory thalamic reticular nucleus (TRN); however, little is known about typical TC network interactions and the mechanisms that are disrupted in brain disorders. We developed a primate-specific, circuit-based TC computational model with distinct core and matrix loops that can simulate sleep spindles. We implemented novel multilevel cortical and thalamic mixing, and included local thalamic inhibitory interneurons, and direct layer 5 projections of variable density to TRN and thalamus to investigate the functional consequences of different ratios of core and matrix node connectivity contribution to spindle dynamics. Our simulations showed that spindle power in primates can be modulated based on the level of cortical feedback, thalamic inhibition, and engagement of model core versus matrix, with the latter having a greater role in spindle dynamics. The study of the distinct spatial and temporal dynamics of core-, matrix-, and mix-generated sleep spindles establishes a framework to study disruption of TC circuit balance underlying deficits in sleep and attentional gating seen in autism and schizophrenia.

Thalamocortical interactions in cognition and disease: the mediodorsal and anterior thalamic nuclei

The mediodorsal thalamus (MD) and anterior thalamic nuclei (ATN) are two adjacent brain nodes that support our ability to make decisions, learn, update information, form and retrieve memories, and find our way around. The MD and PFC work in partnerships to support cognitive processes linked to successful learning and decision-making, while the ATN and extended hippocampal system together coordinate the encoding and retrieval of memories and successful spatial navigation. Yet, while these distinctions may appear to be segregated, both the MD and ATN together support our higher cognitive functions as they regulate and are influenced by interconnected fronto-temporal neural networks and subcortical inputs. Our review focuses on recent studies in animal models and in humans. This evidence is re-shaping our understanding of the importance of MD and ATN cortico-thalamocortical pathways in influencing complex cognitive functions. Given the evidence from clinical settings and neuroscience research labs, the MD and ATN should be considered targets for effective treatments in neuropsychiatric diseases and disorders and neurodegeneration.

A causal role for the pulvinar in coordinating task-independent cortico-cortical interactions

The pulvinar is the largest nucleus in the primate thalamus and has topographically organized connections with multiple cortical areas, thereby forming extensive cortico-pulvino-cortical input-output loops. Neurophysiological studies have suggested a role for these transthalamic pathways in regulating information transmission between cortical areas. However, evidence for a causal role of the pulvinar in regulating cortico-cortical interactions is sparse and it is not known whether pulvinar's influences on cortical networks are task-dependent or, alternatively, reflect more basic large-scale network properties that maintain functional connectivity across networks regardless of active task demands. In the current study, under passive viewing conditions, we conducted simultaneous electrophysiological recordings from ventral (area V4) and dorsal (lateral intraparietal area [LIP]) nodes of macaque visual system, while reversibly inactivating the dorsal part of the lateral pulvinar (dPL), which shares common anatomical connectivity with V4 and LIP, to probe a causal role of the pulvinar. Our results show a significant reduction in local field potential phase coherence between LIP and V4 in low frequencies (4-15 Hz) following muscimol injection into dPL. At the local level, no significant changes in firing rates or LFP power were observed in LIP or in V4 following dPL inactivation. Synchronization between pulvinar spikes and cortical LFP phase decreased in low frequencies (4-15 Hz) both in LIP and V4, while the low frequency synchronization between LIP spikes and pulvinar phase increased. These results indicate a causal role for pulvinar in synchronizing neural activity between interconnected cortical nodes of a large-scale network, even in the absence of an active task state.

A Neuronal Network Model of the Primate Visual System: Color Mechanisms in the Retina, LGN and V1

Color plays a key role in human vision but the neural machinery that underlies the transformation from stimulus to perception is not well understood. Here, we implemented a two-dimensional network model of the first stages in the primate parvocellular pathway (retina, lateral geniculate nucleus and layer 4C[Formula: see text] in V1) consisting of conductance-based point neurons. Model parameters were tuned based on physiological and anatomical data from the primate foveal and parafoveal vision, the most relevant visual field areas for color vision. We exhaustively benchmarked the model against well-established chromatic and achromatic visual stimuli, showing spatial and temporal responses of the model to disk- and ring-shaped light flashes, spatially uniform squares and sine-wave gratings of varying spatial frequency. The spatiotemporal patterns of parvocellular cells and cortical cells are consistent with their classification into chromatically single-opponent and double-opponent groups, and nonopponent cells selective for luminance stimuli. The model was implemented in the widely used neural simulation tool NEST and released as open source software. The aim of our modeling is to provide a biologically realistic framework within which a broad range of neuronal interactions can be examined at several different levels, with a focus on understanding how color information is processed.

Thalamocortical Projection Neuron and Interneuron Numbers in the Visual Thalamic Nuclei of the Adult C57BL/6 Mouse

A key parameter to constrain predictive, bottom-up circuit models of a given brain domain is the number and position of the neuronal populations involved. These include not only the neurons whose bodies reside within the domain, but also the neurons in distant regions that innervate the domain. The mouse visual cortex receives its main subcortical input from the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior (LP) complex of the thalamus. The latter consists of three different nuclei: lateral posterior lateral (LPL), lateral posterior medial rostral (LPMR), and lateral posterior medial caudal (LPMC), each exhibiting specific patterns of connections with the various visual cortical areas. Here, we have determined the number of thalamocortical projection neurons and interneurons in the LP complex and dLGN of the adult C57BL/6 male mouse. We combined Nissl staining and histochemical and immunolabeling methods for consistently delineating nuclei borders, and applied unbiased stereological cell counting methods. Thalamic interneurons were identified using GABA immunolabeling. The C57BL/6 dLGN contains ∼21,200 neurons, while LP complex contains ∼31,000 total neurons. The dLGN and LP are the only nuclei of the mouse dorsal thalamus containing substantial numbers GABA-immunoreactive interneurons. These interneurons, however, are scarcer than previously estimated; they are 5.6% of dLGN neurons and just 1.9% of the LP neurons. It can be thus inferred that the dLGN contains ∼20,000 and the LP complex ∼30,400 thalamocortical projection neurons (∼12,000 in LPL, 15,200 in LPMR, and 4,200 in LPMC). The present dataset is relevant for constraining models of mouse visual thalamocortical circuits, as well as for quantitative comparisons between genetically modified mouse strains, or across species.

Tectal-derived interneurons contribute to phasic and tonic inhibition in the visual thalamus

The release of GABA from local interneurons in the dorsal lateral geniculate nucleus (dLGN-INs) provides inhibitory control during visual processing within the thalamus. It is commonly assumed that this important class of interneurons originates from within the thalamic complex, but we now show that during early postnatal development Sox14/Otx2-expressing precursor cells migrate from the dorsal midbrain to generate dLGN-INs. The unexpected extra-diencephalic origin of dLGN-INs sets them apart from GABAergic neurons of the reticular thalamic nucleus. Using optogenetics we show that at increased firing rates tectal-derived dLGN-INs generate a powerful form of tonic inhibition that regulates the gain of thalamic relay neurons through recruitment of extrasynaptic high-affinity GABA_A receptors. Therefore, by revising the conventional view of thalamic interneuron ontogeny we demonstrate how a previously unappreciated mesencephalic population controls thalamic relay neuron excitability.

Anatomical localization of Cav3.1 calcium channels and electrophysiological effects of T-type calcium channel blockade in the motor thalamus of MPTP-treated monkeys

Conventional anti-Parkinsonian dopamine replacement therapy is often complicated by side effects that limit the use of these medications. There is a continuing need to develop nondopaminergic approaches to treat Parkinsonism. One such approach is to use medications that normalize dopamine depletion-related firing abnormalities in the basal ganglia-thalamocortical circuitry. In this study, we assessed the potential of a specific T-type calcium channel blocker (ML218) to eliminate pathologic burst patterns of firing in the basal ganglia-receiving territory of the motor thalamus in Parkinsonian monkeys. We also carried out an anatomical study, demonstrating that the immunoreactivity for T-type calcium channels is strongly expressed in the motor thalamus in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. At the electron microscopic level, dendrites accounted for >90% of all tissue elements that were immunoreactive for voltage-gated calcium channel, type 3.2-containing T-type calcium channels in normal and Parkinsonian monkeys. Subsequent in vivo electrophysiologic studies in awake MPTP-treated Parkinsonian monkeys demonstrated that intrathalamic microinjections of ML218 (0.5 μl of a 2.5-mM solution, injected at 0.1-0.2 μl/min) partially normalized the thalamic activity by reducing the proportion of rebound bursts and increasing the proportion of spikes in non-rebound bursts. The drug also attenuated oscillatory activity in the 3-13-Hz frequency range and increased gamma frequency oscillations. However, ML218 did not normalize Parkinsonism-related changes in firing rates and oscillatory activity in the beta frequency range. Whereas the described changes are promising, a more complete assessment of the cellular and behavioral effects of ML218 (or similar drugs) is needed for a full appraisal of their anti-Parkinsonian potential.

A biologically constrained model of the whole basal ganglia addressing the paradoxes of connections and selection

The basal ganglia nuclei form a complex network of nuclei often assumed to perform selection, yet their individual roles and how they influence each other is still largely unclear. In particular, the ties between the external and internal parts of the globus pallidus are paradoxical, as anatomical data suggest a potent inhibitory projection between them while electrophysiological recordings indicate that they have similar activities. Here we introduce a theoretical study that reconciles both views on the intra-pallidal projection, by providing a plausible characterization of the relationship between the external and internal globus pallidus. Specifically, we developed a mean-field model of the whole basal ganglia, whose parameterization is optimized to respect best a collection of numerous anatomical and electrophysiological data. We first obtained models respecting all our constraints, hence anatomical and electrophysiological data on the intrapallidal projection are globally consistent. This model furthermore predicts that both aforementioned views about the intra-pallidal projection may be reconciled when this projection is weakly inhibitory, thus making it possible to support similar neural activity in both nuclei and for the entire basal ganglia to select between actions. Second, we predicts that afferent projections are substantially unbalanced towards the external segment, as it receives the strongest excitation from STN and the weakest inhibition from the striatum. Finally, our study strongly suggests that the intrapallidal connection pattern is not focused but diffuse, as this latter pattern is more efficient for the overall selection performed in the basal ganglia.

Comparison of numbers of interneurons in three thalamic nuclei of normal and epileptic rats

The inhibitory sources in the thalamic nuclei are local interneurons and neurons of the thalamic reticular nucleus. Studies of models of absence epilepsy have shown that the seizures are associated with an excess of inhibitory neurotransmission in the thalamus. In the present study, we used light-microscopic gamma-aminobutyric acid (GABA) immunocytochemistry to quantify the interneurons in the lateral geniculate (LGN), ventral posteromedial (VPM), and ventral posterolateral (VPL) thalamic nuclei, and compared the values from normal Wistar rats and genetic absence epilepsy rats from Strasbourg (GAERS). We found that in both Wistar rats and GAERS, the proportion of interneurons was significantly higher in the LGN than in the VPM and VPL. In the LGN of Wistar rats, 16.4% of the neurons were interneurons and in the GAERS, the value was 15.1%. In the VPM, the proportion of interneurons was 4.2% in Wistar and 14.9% in GAERS; in the VPL the values were 3.7% for Wistar and 11.1% for the GAERS. There was no significant difference between Wistar rats and the GAERS regarding the counts of interneurons in the LGN, whereas the VPM and VPL showed significantly higher counts in GAERS. Comparison of the mean areas of both relay cells and interneuronal profiles showed no significant differences between Wistar rats and GAERS. These findings show that in the VPL and the VPM there are relatively more GABAergic interneurons in GAERS than in Wistar rats. This may represent a compensatory response of the thalamocortical circuitry to the absence seizures or may be related to the production of absence seizures.

Comparing GABAergic cell populations in the thalamic reticular nucleus of normal and genetic absence epilepsy rats from Strasbourg (GAERS)

The GABAergic neurons of the thalamic reticular nucleus (TRN) play a critical role in the generation and control of spike-and-wave discharges (SWDs) in absence epilepsy. We have used the disector method to count the GABA+ve and GABA-ve neurons in the intermediate TRN sector of genetic absence epilepsy rats from Strasbourg (GAERS) and of Wistar rats during postnatal (P) development at P10, P20, P30, and P60 days. The same part of TRN was removed from each animal, the GABAergic neurons were labelled using light-microscopical GABA immunohistochemistry and the data were statistically analysed. Both the GAERS and Wistar animals showed an increase in the density of GABA+ve and GABA-ve cells from P10 to P20. From P20 to P60, Wistar animals showed no significant differences for either cell type, but in the GAERS a progressive decrease from P20 to P60 was observed in both GABA+ve and GABA-ve cells. The decrease of the GABA-ve cells was more pronounced than that of the GABA+ve cells. There were no significant differences between cell sizes for GAERS and Wistar rats at any developmental age. The lower density GABA+ve and GABA-ve neurons at P30 and P60 of GAERS compared to Wistar animals may contribute to the generation of SWDs in absence epilepsy.

Dopamine innervation in the thalamus: monkey versus rat

We recently identified the thalamic dopaminergic system in the human and macaque monkey brains, and, based on earlier reports on the paucity of dopamine in the rat thalamus, hypothesized that this dopaminergic system was particularly developed in primates. Here we test this hypothesis using immunohistochemistry against the dopamine transporter (DAT) in adult macaque and rat brains. The extent and density of DAT-immunoreactive (-ir) axons were remarkably greater in the macaque dorsal thalamus, where the mediodorsal association nucleus and the ventral motor nuclei held the densest immunolabeling. In contrast, sparse DAT immunolabeling was present in the rat dorsal thalamus; it was mainly located in the mediodorsal, paraventricular, ventral medial, and ventral lateral nuclei. The reticular nucleus, zona incerta, and lateral habenular nucleus held numerous DAT-ir axons in both species. Ultrastructural analysis in the macaque mediodorsal nucleus revealed that thalamic interneurons are a main postsynaptic target of DAT-ir axons; this suggests that the marked expansion of the dopamine innervation in the primate in comparison to the rodent thalamus may be related to the presence of a sizable interneuron population in primates. We remark that it is important to be aware of brain species differences when using animal models of human brain disease.

Long-range interneurons within the medial pulvinar nucleus of macaque monkeys

Like other thalamic nuclei, the primate pulvinar is considered not to have long-range intrinsic connections, either excitatory or inhibitory. Injections of biotinylated dextran amine (BDA) in the medial pulvinar, however, reveal retrogradely filled neurons up to 2.0 mm from the injection edge. Serial section reconstruction (n = 18) confirmed that retrogradely filled neurons projected to the injection site and showed that they had additional long-range collaterals within the posterior pulvinar. Arrays of small, beaded terminations occurred in multiple foci along the collaterals. Terminal arrays were up to 1.0 mm in length; foci were separated by about 0.7 mm. Somata were large (average area = 220 microm2), and dendritic arbors were radiate and also large (about 1.0 mm in diameter), but without either the appendages of classical interneurons or the hairlike spines characteristic of radiate pulvinocortical projection neurons. Double labeling for BDA and parvalbumin (PV) or BDA and gamma-aminobutyric acid (GABA) indicated that these large neurons were positive for both PV and GABA. Double labeling for PV and GABA, or PV and glutamic acid decarboxylase 67 (GAD67) revealed a small number of similarly large neurons in the posterior pulvinar that were positive for both substances. Thus, we propose that these neurons are a novel class of inhibitory interneuron, longer range than the classic thalamic local circuit interneurons. Future questions include how these neurons relate to other inhibitory systems and specific postsynaptic populations and whether they are located preferentially within the posterior pulvinar, possibly related to the multimodal character of this thalamic region.

Stereological analysis of the mediodorsal thalamic nucleus in schizophrenia: volume, neuron number, and cell types

The mediodorsal thalamic nucleus (MD) is the principal relay nucleus for the prefrontal cortex, a brain region thought to be dysfunctional in schizophrenia. Several, but not all, postmortem studies of the MD in schizophrenia have reported decreased volume and total neuronal number. However, it is not clear whether the findings are specific for schizophrenia nor is it known which subtypes of thalamic neurons are affected. We studied the left MD in 11 subjects with schizophrenia, 9 control subjects, and 12 subjects with mood disorders. Based on morphological criteria, we divided the neurons into two subclasses, presumably corresponding to projection neurons and local circuit neurons. We estimated MD volume and the neuron number of each subclass using methods based on modern unbiased stereological principles. We also estimated the somal volumes of each subclass using a robust, but biased, approach. In addition, we investigated the left MD in four cynomolgus monkeys chronically exposed to haloperidol and in four control monkeys in order to assess the possible effects of antipsychotic medications. The three human subject groups did not differ in any of the measures. In addition, no differences were observed between the two groups of monkeys. Thus, these findings do not support the hypothesis that the MD is a locus of pathology in schizophrenia, although they cannot rule out important functional or structural changes in parameters not measured. Like other studies, this investigation is subject to the limitations involved in sampling from a heterogeneous population emphasizing the need to continue to improve the application of robust, unbiased techniques to quantitative studies of this complex brain disorder.

Chemical anatomy of the human paraventricular thalamic nucleus

The paraventricular thalamic nucleus (Pa) lies in the most medial aspect of the thalamus and is considered one of the midline thalamic nuclei. In the present study, we carried out histochemical and immunohistochemical procedures in the Pa of normal individuals to visualize the pattern of distribution of acetylcholinesterase (AChE), calbindin D-28k (CB), parvalbumin (PV), calretinin (CR), limbic system-associated membrane protein (LAMP), substance P (SP), and enkephalin (ENK). Other cytoarchitectural and myeloarchitectural techniques, such as Nissl and Gallyas, were also employed to delineate the boundaries of the Pa. The main findings of this study are: 1) AChE staining in the Pa was heterogeneously distributed along its anteroposterior and mediolateral axes; 2) the Pa harbored numerous CB- and CR-immunoreactive (ir) cells and neuropil, but this nucleus was largely devoid of PV; 3) the Pa was highly enriched in LAMP and this protein appeared uniformly distributed through its whole extent; and, 4) the SP and ENK immunoreactivities in the Pa revealed numerous highly varicose fibers scattered throughout this nucleus, but no stained cells. This morphological study demonstrates that the Pa is a heterogeneous chemical structure in humans. The functional significance of these results is discussed in the light of similar data gathered in several mammalian species.

Development of the human motor-related thalamic nuclei during the first half of gestation, with special emphasis on GABAergic circuits

This study analyzed the expression of differentiation markers (Calbindin D28K: CaBP; parvalbumin: PARV; calretinin: CalR), gamma-aminobutyric acid (GABA) markers (GABA, glutamic acid decarboxylases: GAD65, GAD67; and GABA transporters: GAT1, GAT3), and other markers (neurotensin: NT, and neurofilament-specific protein: SMI32) in the human thalamus at 8-23 gestation weeks (g.w.), focusing on the motor-related nuclei. From 8-13 g.w. mainly CaBP was expressed in the cells while fiber bundles traversing the thalamus in addition to CaBP expressed all GABA markers except GAD67. CaBP and PARV expression patterns in different nuclei changed over the time course studied, whereas NT was expressed consistently along the anterior-lateral curvature of the thalamus. CalR and SMI were detectable at 23 g.w. in the ventral parts of the dorsal thalamus. Most remarkably, punctate GAD65 immunoreactivity in the neuropil was confined to the nigro- and pallidothalamic afferent receiving nuclei from 16 to about 21 g.w., overlapping with that of CaBP in some of these nuclei (subdivisions of the ventral anterior and mediodorsal nuclei) and with PARV in others (centromedian nucleus). During this period, GAD65 immunoreactivity can be considered a marker of the basal ganglia afferent receiving territory in the motor thalamus. GAD67-positive local circuit neurons were first detected at 12-13 g.w. in the thalamic nuclei outside the basal ganglia afferent receiving territory. In the ventral anterior and centromedian nuclei, GAD-containing local circuit neurons were not conspicuous even at 22-23 g.w. The cells of the reticular nucleus expressed GAD67 and PARV from 12 g.w. on starting in the lateral-posterior regions. By 23 g.w., both markers were expressed in about two-thirds of the nucleus except for its most medial-anterior part. The results imply spatially and temporally differential expression of GABA and differentiation markers in the developing human thalamus.

Two epochs in the development of gamma-aminobutyric acidergic neurons in the ferret thalamus

These studies chart the development of gamma-aminobutyric acid (GABA)-ergic neurons in the three divisions of the thalamus (ventral thalamus, dorsal thalamus, and epithalamus). GABAergic neurons were identified by in situ hybridization to localize mRNA for 67-kDa glutamic acid decarboxylase (GAD(67)) and related to the morphological maturation of the thalamus in fetal and postnatal brains and to expression of transcription factors Gbx-2 and Tbr-1. Origins of GABAergic neurons were sought in in vitro slice preparations incubated in bromodeoxyuridine or injected with a carbocyanine dye. GABA neurons of ventral thalamus (reticular nucleus, ventral lateral geniculate nucleus, zona incerta, and nucleus of the fields of Forel) and of epithalamus appear at least 14 days before those intrinsic to dorsal thalamus. Ventral thalamus GABA cells are derived from a region connecting the ventricular zone of the third ventricle to the caudal ganglionic eminence. This region is delimited ventrally by the Tbr-1-expressing prethalamic eminence and dorsally by the Gbx-2-expressing part of the dorsal thalamus. GABA neurons of epithalamus are derived from the embryonic pretectum. Neurons continue to be added to the ventral thalamus, perireticular nucleus, entopeduncular nucleus, and substantia nigra from the ganglionic eminence as development proceeds. GAD(67)-expressing cells of dorsal thalamus become detectable only at birth and populate the thalamus from posterior to anterior over the first week of life. Although a very small number reaches the dorsal lateral geniculate nucleus from the caudal ganglionic eminence, there is no obvious new source of proliferating neurons at this stage. Intrinsic GABA cells of dorsal thalamus may, therefore, derive from an early generated population of cells that turns on a GABAergic phenotype only late in development.

Efferent connections of the dorsomedial thalamic nuclei of the domestic chick (Gallus domesticus)

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in the thalamic anterior dorsomedial nucleus (DMA) of domestic chicks. The projections of the DMA covered the rostrobasal forebrain, ventral paleostriatum, nucleus accumbens, septal nuclei, Wulst, hyperstriatum ventrale, neostriatal areas, archistriatal subdivisions, dorsolateral corticoid area, numerous hypothalamic nuclei, and dorsal thalamic nuclei. The rostral DMA projects preferentially on the hypothalamus, whereas the caudal part is connected mainly to the dorsal thalamus. The DMA is also connected to the periaqueductal gray, deep tectum opticum, intercollicular nucleus, ventral tegmental area, substantia nigra, locus coeruleus, dorsal lateral mesencephalic nucleus, lateral reticular formation, nucleus papillioformis, and vestibular and cranial nerve nuclei. This pattern of connectivity is likely to reflect an important role of the avian DMA in the regulation of attention and arousal, memory formation, fear responses, affective components of pain, and hormonally mediated behaviors.

Thalamic organization and function after Cajal

Cajal's many contributions to understanding the thalamus have been hidden by his body of work on the cerebral cortex. He delineated many thalamic nuclei in rodents, defined afferent fibers, thalamocortical relay neurons and interneurons, was first to demonstrate thalamocortical fibers and their terminations in the cortex, and recognized the feed-back provided by corticothalamic fibers. This presentation outlines modern methods for identifying classes of thalamic neurons, their chemical characteristics, synaptology and differential connections, and describes the intrinsic circuitry of the thalamus, showing how interactions between GABAergic cells of the reticular nucleus and glutamatergic relay cells underlie rhythmic activities of neurons in the thalamo-cortico-thalamic network, activities associated with changes in the conscious state, and which are generated and maintained by the corticothalamic projection. Corticothalamic fibers interact with reticular nucleus cells and relay cells through NMDA, AMPA and metabotropic receptors while interactions between reticular nucleus cells and relay cells are mediated by GABAA and GABAB receptors. Differing strengths of synaptic input to the two cell types, from which oscillatory behavior commences, depend upon differential expression at individual synapses of specific AMPA receptor subunits which modulate excitatory postsynaptic conductances. Two classes of relay cells can be distinguished by differential staining for calbindin and parvalbumin. The first forms a matrix in the thalamus, unconstrained by nuclear borders; the second is concentrated in certain nuclei in which it forms the topographically organized core. In projecting diffusely to the cortex, calbindin cells provide a substrate for binding together activities of multiple cortical areas that receive focused input from single thalamic nuclei. This, and the presence of specific and diffuse corticothalamic projections may serve to promote coherent activity of large populations of cortical and thalamic neurons in perception, attention and conscious awareness.

Quantitative analysis of glutamic acid decarboxylase-immunoreactive neurons in the anterior thalamus of the human brain

Local circuit neurons in the human anterior thalamus (AT) were identified on the basis of glutamic acid decarboxylase immunoreactivity (GAD-IR). GAD-IR neurons of the AT displayed small diameter somas with thin, sparsely-branching dendrites, consistent with the morphological characteristics of local circuit neurons found in the thalami of other mammals. Sampling techniques revealed an average of 42% of all neurons within the AT were GAD-IR, one of the highest reported percentages of local circuit neurons in the mammalian thalamus. The presence of high proportion of local circuit neurons in the AT may indicate the extent to which the Papez circuit has evolved within the human brain in comparison to other mammals.

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.

Nucleus- and cell-specific expression of NMDA and non-NMDA receptor subunits in monkey thalamus

Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate receptor subtypes. Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) receptor subunits (GluR1-4; GluR5-7; NR1, NR2A-D) on a nucleus- and cell type-specific basis was examined by quantitative in situ hybridization histochemistry and by immunocytochemical staining for receptor subunits and colocalized gamma-aminobutyric acid (GABA) or calcium binding proteins. Levels of NMDA subunit expression, except NR2C, are higher than for the most highly expressed AMPA (GluR1,3,4) and kainate (GluR6) receptor subunits. Expression of NR2C, GluR2, GluR5, and GluR7 is extremely low. Major differences distinguish the reticular nucleus and the dorsal thalamus and, within the dorsal thalamus, the intralaminar and other nuclei. In the reticular nucleus, GluR4 is by far the most prominent, and NMDA receptors are at comparatively low levels. In the dorsal thalamus, NMDA receptors predominate. Anterior intralaminar nuclei are more enriched in GluR4 and GluR6 subunits than other nuclei, whereas posterior intralaminar nuclei are enriched in GluR1 and differ among themselves in relative NMDA receptor subunit expression. GABAergic intrinsic neurons of the dorsal thalamus express much higher levels of GluR1 and GluR6 receptor subunits than do parvalbumin- or calbindin-immunoreactive relay cells and low or absent NMDA receptors. Relay cells are dominated by NMDA receptors, along with GluR3 and GluR6 subunits not expressed by GABA cells. High levels of NR2B are found in astrocytes. Differences in NMDA and non-NMDA receptor profiles will affect functional properties of the thalamic GABAergic and relay cells.

Expression of 10 GABA(A) receptor subunit messenger RNAs in the motor-related thalamic nuclei and basal ganglia of Macaca mulatta studied with in situ hybridization histochemistry

In situ hybridization histochemistry technique with [35S]UTP-labelled riboprobes was used to study the expression pattern of 10 GABA(A) receptor subunit messenger RNAs in the basal ganglia and motor thalamic nuclei of rhesus monkey. Human transcripts were used for the synthesis of alpha2, alpha4, beta2, beta3, gamma1 and delta subunit messenger RNA probes. Rat complementary DNAs were used for generating alpha1, alpha3, beta1 and gamma2 subunit messenger RNA probes. Nigral, pallidal and cerebellar afferent territories in the ventral tier thalamic nuclei all expressed alpha1, alpha2, alpha3, alpha4, beta1, beta2, beta3, delta and gamma2 subunit messenger RNAs but at different levels. Each intralaminar nucleus displayed its own unique expression pattern. In the thalamus, gamma1 subunit messenger RNA was detected only in the parafascicular nucleus. Comparison of the expression patterns with the known organization of GABA(A) connections in thalamic nuclei suggests that (i) the composition of the receptor associated with reticulothalamic synapses, except for those in the intralaminar nuclei, may be alpha1alpha4beta2delta, (ii) receptors of various other subunit compositions may operate in the local GABAergic circuits, and (iii) the composition of receptors at nigro- and pallidothalamic synapses may differ, with those at nigrothalamic probably containing beta1 and gamma2 subunits. In the medial and lateral parts of the globus pallidus, the subthalamic nucleus and the substantia nigra pars reticularis, the alpha1, beta2 and gamma2 messenger RNAs were co-expressed at a high level suggesting that this subunit composition was associated with all GABAergic synapses in the direct and indirect striatal output pathways. Various other subunit messenger RNAs were also expressed but at a lower level. In the substantia nigra pars compacta the most highly expressed messenger RNAs were alpha3, alpha4 and beta3; all other subunit messenger RNAs studied, except for gamma1, alpha1 and alpha2, were expressed at a moderate to high level. In the striatum, the following subunit messenger RNAs were expressed (listed in order of decreasing signal intensity): alpha4, beta3, alpha2, alpha3, beta2, delta, gamma2, alpha1. The expression patterns found in the monkey were similar to those described in comparable nuclei in the rat by Wisden et al. [J. Neurosci. (1992), 12, p. 1040]; however, the monkey nuclei displayed a much greater variety of GABA(A) receptor subunit messenger RNAs.

A population of cholinergic neurons is present in the macaque monkey thalamus

Three new cholinergic markers were employed to study the cholinergic innervation in the thalamus of adult macaque monkeys. They were: two antibodies against choline acetyltransferase (ChAT), one polyclonal and one monoclonal; and a polyclonal antibody against the vesicular transporter of acetylcholine (VAChT), a powerful new marker that colocalizes with ChAT. This approach led to an unexpected finding: the three antibodies positively immunostained a population of neurons in the paracentral nucleus. The immunostained cells are confined to the dorsal region of this nucleus along its rostrocaudal extent. Measurement of the somatic areas of the immunostained neurons indicated that they correspond to a population of large neurons thought to be projection neurons. Because dorsal paracentral neurons are known to project to the dorsal striatum and specific cortical areas involved in visual and visuomotor mechanisms, these structures might be modulated by cholinergic thalamic neurons.

GABA(B) receptor gene expression in monkey thalamus

Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.

Short-term plasticity during intrathalamic augmenting responses in decorticated cats

The intrathalamic mechanisms of frequency-dependent augmenting responses were investigated in decorticated cats by means of intracellular recordings from thalamocortical (TC) neurons in ventrolateral (VL) nucleus, including simultaneous impalements from two TC neurons. Pulse trains (10 Hz) applied to VL nucleus elicited two types of augmenting responses: (1) in 68% of cells, the incremental responses occurred on a progressive depolarization associated with the decrease in IPSPs produced by preceding stimuli in the train; (2) in the remaining cells, progressively growing low-threshold (LT) responses resulted from the enhancement of Cl--dependent IPSPs, giving rise to postinhibitory rebound bursts, followed by a self-sustained sequence of spindle waves. Although in some TC cells the augmenting responses developed from LT responses once the latter reached a given level of depolarization, other neurons displayed augmenting responses immediately after the early antidromic spike that depolarized the neuron to the required level, without an intermediate step of LT rebound. Repeated pulse trains led to a progressive and persistent increase in slow depolarizing responses of TC cells, as well as to a persistent and prolonged decrease in the amplitudes of the IPSPs. On the basis of parallel experiments, we propose that the two types of augmentation in TC cells are a result of contrasting responses of thalamic reticular neurons evoked by repetitive thalamic stimuli: decremental responses, which may account for disinhibition leading to depolarizing responses in TC cells, and incremental responses, explaining the progressive hyperpolarization of TC cells. These data demonstrate that frequency-dependent changes in neuronal excitability are present in the thalamus of a decorticated hemisphere and suggest that short-term plasticity processes in the gateway to the cerebral cortex may decisively influence cortical excitability during repetitive responses.

GABA-like immunoreactivity in the thalamic nucleus submedius of the cat

The distribution of GABAergic elements and their synaptic contacts in the nucleus submedius, a specific nociceptive relay in the medial thalamus of the cat, was studied using light and electron-microscopic postembedding immunohistochemical methods. About one-fourth of the neurons in nucleus submedius were GABA immunoreactive. These neurons were generally smaller than the unlabeled neurons and are probably local circuit neurons. Electron microscopy showed GABA immunoreactivity in two types of vesicle-containing profiles, F-terminals and presynaptic dendrites. F-terminals formed simple synapses with the dendrites of presumed thalamocortical relay cells. Presynaptic dendrites were involved in more complex synaptic arrangements that included ascending trigeminothalamic and spinothalamic tract terminals and thalamocortical relay cell dendrites. Analysis of single sections showed that about 40% of the trigeminothalamic and spinothalamic tract terminals, identified by anterograde transport of horseradish peroxidase, were presynaptic to GABAergic presynaptic dendrites. These results show that GABAergic neurons are frequent in nucleus submedius and that the GABAergic elements make synaptic connections similar to those described for other sensory relay nuclei, including the somatosensory ventroposterior nucleus. This suggests that GABAergic mechanisms play an important role in the processing of nociceptive and thermoreceptive information.

GABAergic neurons in mammalian thalamus: a marker of thalamic complexity?

The present study evaluated the occurrence, distribution, and number of GABAergic neurons in the thalamus of different mammalian species (bat, mouse, rat, guinea pig, rabbit, cat, monkey, humans), by means of light microscopical immunoenzymatic localization of GABA or of its biosynthetic enzyme glutamic acid decarboxylase and by ultrastructural immunogold detection of GABA. Our data demonstrated that: 1) GABAergic local circuit neurons were detected in the thalamic visual domain in all the species analyzed, whereas in other thalamic nuclei their presence and number varied among species; 2) the number of GABAergic local circuit neurons progressively increased in the dorsal thalamus of species with more complex behavior; 3) the presence of local circuit neurons conferred a similar intrinsic organization to the dorsal thalamic nuclei, characterized by complex synaptic arrangements; 4) in the reticular thalamic nucleus, whose neurons were GABA-immunoreactive in all the examined species, the cellular density decreased from the bat to humans. These findings strongly suggest that thalamic GABAergic local circuit neurons are not directly related to the ability to perform specific sensorimotor tasks, but they are likely to reflect an increasing complexity of the local information processing that occurs at thalamic level.

Nucleus-specific expression of GABA(A) receptor subunit mRNAs in monkey thalamus

Expression of 10 GABAA receptor subunit genes was examined in monkey thalamus by in situ hybridization using cRNA probes specific for alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, beta 1, beta 2, beta 3, gamma 1, and gamma 2 subunit mRNAs. These displayed unique hybridization on patterns with significant differences from rodents. Alpha 1, beta 2, and gamma 2 transcripts were expressed at high levels in all dorsal thalamic nuclei, but expression was significantly higher in sensory relay nuclei-especially the dorsal lateral geniculate nucleus. Other transcripts showed nucleus-specific differences in levels of expression and in the range expressed. Alpha 5 and alpha 4 subunit transcripts were expressed in all nuclei except the intralaminar nuclei. Levels of alpha 2, alpha 3, beta 1, beta 3, and gamma 1 expression were very low, except in intralaminar nuclei. In the reticular nucleus, most subunit transcripts were not expressed, and only gamma 2 transcripts were consistently detected at modest levels. Thalamic GABAA receptors may be assembled from nucleus-specific groupings of subunit polypeptides.

Neuronal and synaptic organization of the centromedian nucleus of the monkey thalamus: a quantitative ultrastructural study, with tract tracing and immunohistochemical observations

The ultrastructure of the centromedian nucleus of the monkey thalamus was analysed qualitatively and quantitatively and projection neurons, local circuit neurons, and synaptic bouton populations identified. Projection neurons were mostly medium-sized, with oval-fusiform or polygonal perikarya, few primary dendrites, and frequent somatic spines; local circuit neurons were smaller. Four basic types of synaptic boutons were distinguished: (1) Small- to medium-sized boutons containing round vesicles (SR) and forming asymmetric contacts, identified as corticothalamic terminals. (2) Heterogeneous medium-sized boutons with asymmetric contacts and round vesicles, similar to the so-called large round (LR) boutons, which were in part of cortical origin. (3) Heterogeneous GAD-positive small- to medium-sized boutons, containing pleomorphic vesicles and forming symmetric contacts (F1 type), which included pallidothalamic terminals. (4) Presynaptic profiles represented by GAD-positive vesicle-containing dendrites of local circuit neurons. Complex synaptic arrangements, serial synapses and triads with LR and SR boutons engaging all parts of projection neuron dendrites and somata, were seen consistently, whereas classical glomeruli were infrequent. LR and SR boutons also established synapses on dendrites of local circuit neurons. F1 boutons established synapses on projection neuron somata, dendrites and initial axon segments. Compared to other previously studied motor-related thalamic nuclei, differences in synaptic coverage between proximal and distal projection neuron dendrites were less pronounced, and the density of synapses formed by local circuit dendrites on projection neuron dendrites was lower. Thus, compared to other thalamic nuclei, the overlap of different inputs was higher on monkey centromedian cells, and centromedian inhibitory circuits displayed a different organization.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

Thalamostriatal projection neurons in birds utilize LANT6 and neurotensin: a light and electron microscopic double-labeling study

Based on its location, connectivity and neurotransmitter content, the dorsal thalamic zone in birds appears to be homologous to the intralaminar, midline, and mediodorsal nuclear complex in the thalamus of mammals. We investigated the neuroactive substances used by thalamostriatal projection neurons of the dorsal thalamic zone in the pigeon. Single-labeling experiments showed that many neurons in the dorsal thalamic zone are immunoreactive for neurotensin and the neurotensin-related hexapeptide, (Lys8,Asn9)NT(8-13) (LANT6). Double-labeling experiments, using the retrograde fluorescent tracer, FluoroGold, combined with fluorescence immunocytochemistry for either LANT6 or neurotensin, showed that neurotensin- and LANT6-containing neurons in the dorsal thalamic zone project to the striatum of the basal ganglia. Immunofluorescence double-labeling experiments showed that neurotensin and LANT6 are often (possibly always) co-expressed in neurons in the dorsal thalamic zone. Electron microscopic immunohistochemical double-labeling showed that LANT6 terminals in the striatum make asymmetric contacts with heads of spines labeled for substance P and heads of spines not labeled for substance P, suggesting that these terminals synapse with both substance P-containing and non-substance P-containing medium spiny striatal projection neurons. These findings indicate that LANT6 and neurotensin may be utilized as neurotransmitters in thalamostriatal projections in birds and raise the possibility that this may also be the case in other amniotes.

Acetylcholinesterase histochemistry in the macaque thalamus reveals territories selectively connected to frontal, parietal and temporal association cortices

The patterns of histochemical staining for acetylcholinesterase (AChE) activity in the macaque thalamus were analyzed and compared with the distribution of cells and terminals labeled from injections of axonal tracers in the dorsolateral and orbital prefrontal cortex, in area 7a of the posterior parietal cortex and in the polysensory cortex of the superior temporal sulcus. AChE histochemistry is very useful in delineating the thalamic nuclei connected with the association cortex and in uncovering thalamic subdivisions that are barely evident on cytoarchitectonic grounds. Moreover, AChE activity reveals previously unrecognized heterogeneities within several thalamic nuclei, like the ventral anterior (VA), where a new ventromedial subdivision (VAvm) is described, the medial pulvinar (PulM) or the mediodorsal nucleus (MD). In this nucleus three distinct chemical domains are present: the medial, ventral and lateral sectors characterized by low, moderate and high AChE activities, respectively. The staining pattern of the lateral sector is markedly heterogeneous with patches of intense AChE activity surrounded by a moderately stained matrix. The MD medial sector is connected with the orbitofrontal cortex, whereas the AChE-rich patches in the lateral sector are selectively connected with the dorsolateral prefrontal, parietal and temporal association cortices. In the PulM, a dorsomedial AChE-rich patch is selectively connected with the orbitofrontal cortex, whereas the surrounding territory, which shows moderate AChE activity, is preferentially connected with the parietal and temporal cortices. Chemically specific domains in the anterior, ventral anterior, midline, and intralaminar thalamic nuclei are also connected with the examined association cortices. These findings indicate that the topographic patterns of the thalamo-cortical connections of primate association areas conform to the chemical architecture of the thalamus. This implies that because each cortical area is connected to a particular set of thalamic regions, the influence of the thalamus on cortical function is exclusive for each area, highly diverse among the various association areas, and subject to a wide range of modulation at the thalamic level.

Synaptic distribution of afferents from reticular nucleus in ventroposterior nucleus of cat thalamus

This study was aimed at determining the synaptic circuitry that contributes to the alterations in thalamic function that accompany changes in behavioral states. The somatosensory sector of the thalamic reticular nucleus (RTN) was identified by microelectrode recording in cats and injected with Phaseolus vulgaris-leucoagglutinin (PHA-L). The axons of labeled RTN cells gave rise to collaterals within the RTN and continued into the dorsal thalamus where they terminated predominately in the ventral posterior lateral nucleus (VPL). After small injections in the upper limb representation of RTN, most labeled terminations in VPL were confined to its medial part, suggesting the presence of a topographic organization in the projection. Terminations were concentrated in localized, focal aggregations of boutons. Combined electron microscopic immunocytochemistry, using immunogold labeling for gamma-aminobutyric acid (GABA), showed that the PHA-L labeled boutons were GABA-positive terminals that ended in symmetrical synapses. Eighty-two percent of these synapses were on dendrites of relay neurons, 8.5% on dendrites of interneurons, and 9.3% on somata. The terminals of RTN axons form the majority of axon terminals ending in symmetrical synapses in VPL. Their concentration on relay neurons probably underlies the capacity of the RTN projection to reduce background activity of VPL relay neurons in the awake state and to maintain oscillatory behavior of these neurons in drowsiness and early phases of sleep.

Distribution of four types of synapse on physiologically identified relay neurons in the ventral posterior thalamic nucleus of the cat

This study was aimed at providing quantitative data on the thalamic circuitry that underlies the central processing of somatosensory information. Four physiologically identified thalamocortical relay neurons in the ventral posterior lateral nucleus (VPL) of the cat thalamus were injected with horseradish peroxidase and subjected to quantitative electron microscopy after pre- or postembedding immunostaining for gamma-aminobutyric acid to reveal synaptic terminals of thalamic inhibitory neurons. The four cells all had rapidly adapting responses to light mechanical stimuli applied to their receptive fields, which were situated on hairy or glabrous skin or related to a joint. Their dendritic architecture was typical of cells previously described as type I relay cells in VPL, and they lacked dendritic appendages. Terminals ending in synapses on the injected cells were categorized as RL (ascending afferent), F (inhibitory), PSD (presynaptic dendrite), and RS (mainly corticothalamic) types and were quantified in reconstructions of serial thin sections. RL and F terminals formed the majority of the synapses on proximal dendrites (approximately 50% each). The number of synapses formed by RL terminals declined on intermediate dendrites, but those formed by F terminals remained relatively high, declining to moderate levels (20-30%) on distal dendrites. RS terminals formed moderate numbers of the synapses on intermediate dendrites and the majority (> 60%) of the synapses on distal dendrites. Synapses formed by PSDs were concentrated on intermediate dendrites and were few in number (approximately 6%). They formed synaptic triads with F terminals and rarely with RL terminals. On somata, only a few synapses were found, all made by F terminals. The total number of synapses per cell was calculated to be 5,584-8,797, with a density of 0.6-0.9 per micrometer of dendritic length. Of the total, RL terminals constituted approximately 15%, F terminals approximately 35%, PSD terminals approximately 5%, and RS terminals approximately 50%. These results provide the first quantitative assessment of the synaptic architecture of thalamic somatic sensory relay neurons and show the basic organizational pattern exhibited by representatives of the physiological type of relay neurons most commonly encountered in the VPL nucleus.

Nucleus reticularis thalami connections with the mediodorsal thalamic nucleus: a light and electron microscopic study in the monkey

Wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) were used as tracers to study nucleus reticularis (NRT) connections with the mediodorsal nucleus (MD). Injections of WGA-HRP in the MD resulted in retrograde labeling of cells in the anteromedial segment of the NRT, the so-called rostral NRT pole. Injections of WGA-HRP and BDA in this NRT region resulted in dense anterograde labeling in the MD. Labeled NRT fibers gave off several collaterals to different MD regions ending with terminal plexuses of thin varicose fibers. In the neuropil, the varicosities were distributed at random, and no tendency to form pericellular baskets was noted. Postembedding immunocytochemistry for GABA was performed on the tissue containing anterograde WGA-HRP label for identification of NRT boutons under electron microscope. The double-labeled boutons were of small to medium size, contained a large number of pleomorphic vesicles, few mitochondria, and formed multiple symmetric synaptic contacts. The number of contacts established by one bouton ranged from 1 to 4 with an average of 1.8 per bouton. About 60% of these boutons made synapses on distal dendrites of GABAergic local circuit neurons; 33% of synaptic contacts were on distal dendrites of thalamocortical neurons, and the rest on their proximal dendrites and soma. NRT boutons were also found in serial synapses and triads. The results demonstrate that the NRT input to the MD is organized so that a single fiber innervates; different MD regions and its terminals form numerous synaptic contacts mostly on the distal dendrites of a large number of local circuit neurons and projection neurons.

GABAergic interneurons in the somatosensory thalamus of the guinea-pig: a light and ultrastructural immunocytochemical investigation

This work was performed to confirm previous data reporting the presence of GABAergic interneurons in the ventrobasal complex of guinea-pig, and to investigate the intrinsic organization of this nucleus compared to that of thalamic nuclei lacking interneurons. Immunocytochemical experiments were performed on the thalamus of adult guinea-pigs perfused with mixed aldehydes using an anti-GABA serum. At light microscopy, the immunoreaction on floating Vibratome sections showed that GABAergic neurons are present only in the reticular and lateral geniculate nuclei and in the ventrobasal complex. Quantitative evaluation of their number indicated that they are 20 and 15% of the total neuronal population in lateral geniculate nucleus and ventrobasal complex, respectively, while they are less than 1% in ventrolateral nucleus. At the ultrastructural level, the postembedding immunogold procedure showed the presence, in the ventrobasal complex, of GABA-labeled profiles involved in complex synaptic arrangements similar to those found in carnivores and primates. Conversely, GABA-labeled terminals in thalamic nuclei devoid of interneurons formed exclusively axo-dendritic or axo-somatic contacts, like in rats and mice. The present data suggest that GABAergic neurons in the ventrobasal complex of guinea-pigs give rise to functionally important rearrangements of its intrinsic synaptic organization and that they represent the morphological basis for an intrinsic modulatory mechanism that is absent in other thalamic nuclei lacking inhibitory interneurons. The phylogenetic implications of these findings are also discussed in comparison to other animal species.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Chemical compartmentation and relationships between calcium-binding protein immunoreactivity and layer-specific cortical caudate-projecting cells in the anterior intralaminar nuclei of the cat

Neurons projecting to the parietal cortex or striatum and neurons showing immunoreactivity for the calcium-binding proteins parvalbumin and 28KD-calbindin were examined in the anterior intralaminar nuclei (IL) of the cat. Retrograde tracing from deep or superficial parietal cortical layers or from the caudate nucleus was coupled with immunohistochemistry to determine which of these proteins were expressed in the projection neurons. It was found that IL neurons project to deep as well as to superficial layers of the parietal cortex, that IL-cortical neurons could be differentiated into two populations according to their cortical projection pattern and their soma size, and that IL neurons projecting to the parietal cortex or to the striatum express 28KD calbindin immunoreactivity but not parvalbumin immunoreactivity. The distribution of immunoreactivity to 28KD calbindin and parvalbumin in the neuropil showed a consistent complementary distribution pattern in the IL. The compartments based on differential parvalbumin and 28KD calbindin expression may indicate the presence of functionally segregated units in IL.

The distribution of GABA-containing perikarya, fibers, and terminals in the forebrain and midbrain of pigeons, with particular reference to the basal ganglia and its projection targets

Immunohistochemical techniques were used to study the distributions of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in pigeon forebrain and midbrain to determine the organization of GABAergic systems in these brain areas in birds. In the basal ganglia, numerous medium-sized neurons throughout the striatum were labeled for GABA, while pallidal neurons, as well as a small population of large, aspiny striatal neurons, labeled for GAD and GABA. GAD+ and GABA+ fibers and terminals were abundant throughout the basal ganglia, and GABAergic fibers were found in all extratelencephalic targets of the basal ganglia. Most of these targets also contained numerous GABAergic neurons. In pallial regions, approximately 10-12% of the neurons were GABAergic. The outer rind of the pallium was more intensely labeled for GABAergic fibers than the core. The olfactory tubercle region, the ventral pallidum, and the hypothalamus were extremely densely labeled for GABAergic fibers, while GABAergic neurons were unevenly distributed in the hypothalamus. GABAergic neurons and fibers were abundant in the dorsalmost part of thalamus and the dorsal geniculate region, while GABAergic neurons and fibers were sparse (or lightly labeled) in the thalamic nuclei rotundus, triangularis, and ovoidalis. Further, GABAergic neurons were abundant in the superficial tectal layers, the magnocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the reticular formation. GABAergic fibers were particularly abundant in the superficial tectal layers, the parvocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the interpeduncular nucleus. These results suggest that GABA plays a role as a neurotransmitter in nearly all fore- and midbrain regions of birds, and in many instances the observed distributions of GABAergic neurons and fibers closely resemble the patterns seen in mammals, as well as in other vertebrates.

Immunoreactivity for GAD and three peptides in somatosensory cortex and thalamus of the raccoon

Immunocytochemical methods were used to determine the distributions of glutamic acid decarboxylase (GAD), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), and somatostatin (SOM) in the primary somatosensory cortex and somatosensory thalamus of adult raccoons. The cortex showed extensive immunoreactivity for GAD, revealing a large population of GABAergic neurons. GAD-labeled cells were numerous in all cortical layers, but were most concentrated in laminae II-IV. The cells were nonpyramidal and of varying morphology, typically with somata of small or medium size. GAD-immunoreactive puncta, presumably synaptic terminals, were widespread and often appeared to end on both GAD-negative and GAD-positive neurons. Immunoreactivity for the peptides was much less extensive than that for GAD, with the number of labeled neurons for VIP > CCK > SOM. Peptidergic cells were preferentially located in the upper and middle cortical layers, especially laminae II and III. The cells were nonpyramidal, often bitufted or bipolar in morphology, and small to medium in size. Their processes formed diffuse plexuses of fibers with terminal-like varicosities that occasionally surrounded nonpeptidergic neurons. The thalamus showed a clearly differentiated pattern of immunoreactivity for GAD, but little or no labeling for the three peptides. Nuclei adjoining the ventral posterior lateral (VPL)/ventral posterior medial (VPM) complex--including the reticular nucleus--contained many GAD-positive neurons and fibers. In contrast, the VPL and VPM nuclei displayed considerably less GAD immunoreactivity, somewhat surprising given the raccoon's highly developed somatosensory system. However, the ventral posterior inferior (VPI) nucleus revealed rather dense GAD labeling, perhaps related to a specialized role in sensory information processing. Thus, the primary somatosensory cortex of the raccoon showed patterns of immunoreactivity for GAD and peptides that were similar to those of other species; the somatosensory thalamus revealed a distinctive profile of GAD immunoreactivity, with labeling that was light to moderate in the VPL/VPM complex and relatively extensive in VPL.

Simultaneous Demonstration of Serotonin-immunoreactive Terminals and GABAergic Neurons in the VPL Nucleus of the Cat Thalamus

Pre-embedding immunoperoxidase (for serotonin) and postembedding immunogold (for gamma-aminobutyric acid; GABA) labelling were combined at light and electron microscopic levels to demonstrate the neuronal targets of serotonin (5-HT) afferents in the ventral posterior lateral nucleus (VPL) of the cat thalamus. 5-HT-immunoreactive fibres and terminal varicosities were found in close proximity to GABA-immunoreactive interneurons and non-GABAergic relay neurons. Ultrastructurally, the vast majority of 5-HT terminals made close membrane contacts without overt membrane specializations with GABAergic axon terminals, GABAergic presynaptic dendrites and GABAergic somata. A very small number of 5-HT terminals formed typical asymmetrical synapses with GABAergic presynaptic dendrites and with dendritic shafts of relay cells. Some 5-HT terminals participated with the presynaptic dendrites in triadic synaptic arrangements. These findings suggest a dual innervation pattern by 5-HT afferents in VPL and the release of 5-HT in large part at sites not associated with morphologically detectable synapses.

Chronic deafferentation in monkeys differentially affects nociceptive and nonnociceptive pathways distinguished by specific calcium-binding proteins and down-regulates gamma-aminobutyric acid type A receptors at thalamic levels

Chronic deafferentation of skin and peripheral tissues is associated with plasticity of representational maps in cerebral cortex and with perturbations of sensory experience that include severe "central" pain. This study shows that in normal monkeys the nonnociceptive, lemniscal component of the somatosensory pathways at spinal, brainstem, and thalamic levels is distinguished by cells and fibers immunoreactive for the calcium-binding protein parvalbumin, whereas cells of the nociceptive component at these levels are distinguished by immunoreactivity for 28-kDa calbindin. Long-term dorsal rhizotomies in monkeys lead to transneuronal degeneration of parvalbumin cells at brainstem and thalamic sites accompanied in the thalamus by a down-regulation of gamma-aminobutyric acid type A receptors and an apparent increase in activity of calbindin cells preferentially innervated by central pain pathways. Release from inhibition and imbalance in patterns of somatosensory inputs from thalamus to cerebral cortex may constitute subcortical mechanisms for inducing changes in representational maps and perturbations of sensory perception, including central pain.

Contrasting patterns in the localization of glutamic acid decarboxylase and Ca2+/calmodulin protein kinase gene expression in the rat central nervous system

The expression of the genes encoding the alpha subunit of type II calcium calmodulin-dependent protein kinase (CaM II kinase alpha) and the 67,000 mol. wt form of glutamic acid decarboxylase was examined throughout the rat central nervous system. In situ hybridization histochemistry, using cRNA probes, revealed a dense population of CaM II kinase alpha-expressing cells throughout the telencephalon and diencephalon. CaM II kinase alpha mRNA was also expressed in the midbrain, cerebellum and medulla oblongata, but at greatly reduced levels. No CaM II kinase alpha gene expression was detected in nuclei producing monoamines or acetylcholine. By contrast, the glutamic acid decarboxylase gene was moderately to highly expressed throughout the central nervous system. In several regions there was a complementarity in the distributions of cells expressing the glutamic acid decarboxylase or CaM II kinase alpha genes. Cells in certain nuclei such as the thalamic reticular nucleus or globus pallidus showed glutamic acid decarboxylase gene expression only; others such as the majority of the dorsal thalamic nuclei showed CaM II kinase alpha gene expression only. Several regions contained both glutamic acid decarboxylase and CaM II kinase alpha expressing cells. However, simultaneous immunostaining for both proteins revealed only two regions where CaM II kinase alpha and glutamic acid decarboxylase immunoreactivity were colocalized: the cerebellar Purkinje cells and the commissural nucleus of the stria terminalis. The results imply that CaM II kinase alpha is primarily expressed in non-GABAergic neurons. In several regions CaM II kinase alpha mRNA is concentrated in nuclei known to contain populations of neurons that use excitatory amino acid transmitters.