Pallidal afferent territory of the Macaca mulatta thalamus: neuronal and synaptic organization of the VAdc

Structural Plasticity of GABAergic Pallidothalamic Terminals in MPTP-Treated Parkinsonian Monkeys: A 3D Electron Microscopic Analysis

The internal globus pallidus (GPi) is a major source of tonic GABAergic inhibition to the motor thalamus. In parkinsonism, the firing rate of GPi neurons is increased, and their pattern switches from a tonic to a burst mode, two pathophysiological changes associated with increased GABAergic pallidothalamic activity. In this study, we used high-resolution 3D electron microscopy to demonstrate that GPi terminals in the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM), the two main GPi-recipient motor thalamic nuclei in monkeys, undergo significant morphometric changes in parkinsonian monkeys including (1) increased terminal volume in both nuclei; (2) increased surface area of synapses in both nuclei; (3) increased number of synapses/GPi terminals in the CM, but not VApc; and (4) increased total volume, but not number, of mitochondria/terminals in both nuclei. In contrast to GPi terminals, the ultrastructure of putative GABAergic nonpallidal terminals was not affected. Our results also revealed striking morphological differences in terminal volume, number/area of synapses, and volume/number of mitochondria between GPi terminals in VApc and CM of control monkeys. In conclusion, GABAergic pallidothalamic terminals are endowed with a high level of structural plasticity that may contribute to the development and maintenance of the abnormal increase in pallidal GABAergic outflow to the thalamus in the parkinsonian state. Furthermore, the evidence for ultrastructural differences between GPi terminals in VApc and CM suggests that morphologically distinct pallidothalamic terminals from single pallidal neurons may underlie specific physiological properties of pallidal inputs to VApc and CM in normal and diseased states.

Structural plasticity of GABAergic and glutamatergic networks in the motor thalamus of parkinsonian monkeys

In the primate thalamus, the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM) receive GABAergic projections from the internal globus pallidus (GPi) and glutamatergic inputs from motor cortices. In this study, we used electron microscopy to assess potential structural changes in GABAergic and glutamatergic microcircuits in the VApc and CM of MPTP-treated parkinsonian monkeys. The intensity of immunostaining for GABAergic markers in VApc and CM did not differ between control and parkinsonian monkeys. In the electron microscope, three major types of terminals were identified in both nuclei: (a) vesicular glutamate transporter 1 (vGluT1)-positive terminals forming asymmetric synapses (type As), which originate from the cerebral cortex, (b) GABAergic terminals forming single symmetric synapses (type S1), which likely arise from the reticular nucleus and GABAergic interneurons, and (c) GABAergic terminals forming multiple symmetric synapses (type S2), which originate from GPi. The density of As terminals outnumbered that of S1 and S2 terminals in VApc and CM of control and parkinsonian animals. No significant change was found in the abundance and synaptic connectivity of S1 and S2 terminals in VApc or CM of MPTP-treated monkeys, while the prevalence of "As" terminals in VApc of parkinsonian monkeys was 51.4% lower than in controls. The cross-sectional area of vGluT1-positive boutons in both VApc and CM of parkinsonian monkeys was significantly larger than in controls, but their pattern of innervation of thalamic cells was not altered. Our findings suggest that the corticothalamic system undergoes significant synaptic remodeling in the parkinsonian state.

Drivers of the primate thalamus

The activity of thalamocortical neurons is primarily determined by giant excitatory terminals, called drivers. These afferents may arise from neocortex or from subcortical centers; however, their exact distribution, segregation, or putative absence in given thalamic nuclei are unknown. To unravel the nucleus-specific composition of drivers, we mapped the entire macaque thalamus using vesicular glutamate transporters 1 and 2 to label cortical and subcortical afferents, respectively. Large thalamic territories were innervated exclusively by either giant vGLUT2- or vGLUT1-positive boutons. Codistribution of drivers with different origin was not abundant. In several thalamic regions, no giant terminals of any type could be detected at light microscopic level. Electron microscopic observation of these territories revealed either the complete absence of large multisynaptic excitatory terminals (basal ganglia-recipient nuclei) or the presence of both vGLUT1- and vGLUT2-positive terminals, which were significantly smaller than their giant counterparts (intralaminar nuclei, medial pulvinar). In the basal ganglia-recipient thalamus, giant inhibitory terminals replaced the excitatory driver inputs. The pulvinar and the mediodorsal nucleus displayed subnuclear heterogeneity in their driver assemblies. These results show that distinct thalamic territories can be under pure subcortical or cortical control; however, there is significant variability in the composition of major excitatory inputs in several thalamic regions. Because thalamic information transfer depends on the origin and complexity of the excitatory inputs, this suggests that the computations performed by individual thalamic regions display considerable variability. Finally, the map of driver distribution may help to resolve the morphological basis of human diseases involving different parts of the thalamus.

Glutamic acid decarboxylase isoform 65 immunoreactivity in the motor thalamus of humans and monkeys: γ-aminobutyric acidergic connections and nuclear delineations

The neurotransmitter γ-aminobutyric acid (GABA) plays an important role in the motor thalamic nuclei. This report analyzes the distribution of the GABA-producing enzyme glutamic acid decarboxylase isoform 65 (GAD65), stained with monoclonal antibody, in human and rhesus monkey thalami and compares it with staining patterns of some widely used cytoskeletal and calcium binding protein markers. GAD65 immunoreactivity distinctly labeled two systems: fibers and terminals of basal ganglia thalamic afferents and local circuit neurons, revealing fine features of GABAergic circuitry in the human thalamus. Gross distribution patterns of GAD65 were identical in human and rhesus monkey thalami. The area displaying specific staining of large-caliber beaded fibers coincided with nigro- and pallidothalamic afferent territories previously identified in monkeys with anterograde tracers. Accordingly, a similarly stained region in the human thalamus was considered basal ganglia territory. Except for cytoarchitecture, no specific markers differentiating between the nigro- and pallidothalamic projection zones within this territory were found. GAD65 staining in the cerebellar afferent territory reflected organization of its local circuit neuron network, distinguishing it from adjacent nuclei. Specific GAD65 staining pattern and negative calcium binding protein immunoreactivity identify the cerebellar afferent territory in humans. It is subdivided further into ventral and dorsal regions based on the cytoskeletal protein SMI31 staining pattern. The nuclear outlines revised according to the results are compared with those of Hassler (Schaltenbrand G and Bailey P [1959] Einfuhrung in die stereotaktishen Operationen mit einem Atlas des menschlichen Gehirns, vol 3. Stuttgart: Thieme) and discussed in light of the ongoing controversy regarding delineations of the motor thalamic nuclei in humans.

A direct projection from the subthalamic nucleus to the ventral thalamus in monkeys

The current basal ganglia model considers the internal division of the globus pallidus and the substantia nigra pars reticulata as the sole sources of basal ganglia output to the thalamus. However, following the delivery of retrograde tracers into the ventral anterior/ventral lateral thalamic nuclei, a moderate number of labeled neurons were found within the subthalamic nucleus (STN) in control cases, MPTP-treated monkeys and animals with levodopa-induced dyskinesias. Furthermore, dual tracing experiments showed that subthalamo-thalamic and subthalamo-pallidal projections arise from different subpopulations of STN efferent neurons. Moreover, upregulated expression of the mRNA coding the vesicular glutamate transporter 2 (vGlut2) was found in retrogradely-labeled STN neurons in MPTP-treated monkeys. By contrast, there is a reduction in vGlut2 mRNA expression in subthalamo-thalamic neurons in dyskinetic monkeys. In conclusion, our findings support the presence of a direct projection from the STN to the ventral thalamus that appears to be functionally modulated by dopaminergic activity.

Parafascicular thalamic nucleus activity in a rat model of Parkinson's disease

Parkinson's disease is associated with increased oscillatory firing patterns in basal ganglia output, which are thought to disrupt thalamocortical activity. However, it is unclear how specific thalamic nuclei are affected by these changes in basal ganglia activity. The thalamic parafascicular nucleus (PFN) receives input from basal ganglia output nuclei and directly projects to the subthalamic nucleus (STN), striatum and cortex; thus basal ganglia-mediated changes on PFN activity may further impact basal ganglia and cortical functions. To investigate the impact of increased oscillatory activity in basal ganglia output on PFN activity after dopamine cell lesion, PFN single-unit and local field potential activities were recorded in neurologically intact (control) rats and in both non-lesioned and dopamine lesioned hemispheres of unilateral 6-hydroxydopamine lesioned rats anesthetized with urethane. Firing rates were unchanged 1-2 weeks after lesion; however, significantly fewer spontaneously active PFN neurons were evident. Firing pattern assessments after lesion showed that a larger proportion of PFN spike trains had 0.3-2.5 Hz oscillatory activity and significantly fewer spike trains exhibited low threshold calcium spike (LTS) bursts. In paired recordings, more PFN-STN spike oscillations were significantly correlated, but as these oscillations were in-phase, results are inconsistent with feedforward control of PFN activity by inhibitory oscillatory basal ganglia output. Furthermore, the decreased incidence of LTS bursts is incompatible with inhibitory basal ganglia output inducing rebound bursting in PFN after dopamine lesion. Together, results show that robust oscillatory activity observed in basal ganglia output nuclei after dopamine cell lesion does not directly drive changes in PFN oscillatory activity.

The presence and absence of prosencephalic cell groups relaying striatal information to the medial and lateral thalamus in tenrec

Although there are remarkable differences regarding the output organization of basal ganglia between mammals and non-mammals, mammalian species with poorly differentiated brain have scarcely been investigated in this respect. The aim of the present study was to identify the pallidal neurons giving rise to thalamic projections in the Madagascar lesser hedgehog tenrec (Afrotheria). Following tracer injections into the thalamus, retrogradely labelled neurons were found in the depth of the olfactory tubercle (particularly the hilus of the Callejal islands and the insula magna), in subdivisions of the diagonal band complex, the peripeduncular region and the thalamic reticular nucleus. No labelled cells were seen in the globus pallidus. Pallidal neurons were tentatively identified on the basis of their striatal afferents revealed hodologically using anterograde axonal tracer substances and immunohistochemically with antibodies against enkephalin and substance P. The data showed that the tenrec's medial thalamus received prominent projections from ventral pallidal cells as well as from a few neurons within and ventral to the cerebral peduncle. The only regions projecting to the lateral thalamus appeared to be the thalamic reticular nucleus (RTh) and the dorsal peripeduncular nucleus (PpD). On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. There was no evidence of entopeduncular (internal pallidal) neurons being present within the RTh/PpD complex, neuropils of which did not stain for enkephalin and substance P. The ventrolateral portion of RTh, the only region eventually receiving a striatal input, projected to the caudolateral rather than the rostrolateral thalamus. Thus, the striatopallidal output organization in the tenrec appeared similar, in many respects, to the output organization in non-mammals. This paper considers the failure to identify entopeduncular neurons projecting to the rostrolateral thalamus in a mammal with a little differentiated cerebral cortex, and also stresses the discrepancy between this absence and the presence of a distinct external pallidal segment (globus pallidus).

Role of individual basal ganglia nuclei in force amplitude generation

The basal ganglia-thalamo-cortical loop is an important neural circuit that regulates motor control. A key parameter that the nervous system regulates is the level of force to exert against an object during tasks such as grasping. Previous studies indicate that the basal ganglia do not exhibit increased activity with increasing amplitude of force, although these conclusions are based mainly on the putamen. The present study used functional magnetic resonance imaging to investigate which regions in the basal ganglia, thalamus, and motor cortex display increased activity when producing pinch-grip contractions of increasing force amplitude. We found that the internal portion of the globus pallidus (GPi) and subthalamic nucleus (STN) had a positive increase in percent signal change with increasing force, whereas the external portion of the globus pallidus, anterior putamen, posterior putamen, and caudate did not. In the thalamus we found that the ventral thalamic regions increase in percent signal change and activation volume with increasing force amplitude. The contralateral and ipsilateral primary motor/somatosensory (M1/S1) cortices had a positive increase in percent signal change and activation volume with increasing force amplitude, and the contralateral M1/S1 had a greater increase in percent signal change and activation volume than the ipsilateral side. We also found that deactivation did not change across force in the motor cortex and basal ganglia, but that the ipsilateral M1/S1 had greater deactivation than the contralateral M1/S1. Our findings provide direct evidence that GPi and STN regulate the amplitude of force output. These findings emphasize the heterogeneous role of individual nuclei of the basal ganglia in regulating specific parameters of motor output.

Differential wiring of local excitatory and inhibitory synaptic inputs to islet cells in rat spinal lamina II demonstrated by laser scanning photostimulation

The substantia gelatinosa (lamina II) of the spinal dorsal horn contains inhibitory and excitatory interneurons that are thought to play a critical role in the modulation of nociception. However, the organization of the intrinsic circuitry within lamina II remains poorly understood. We used glutamate uncaging by laser scanning photostimulation to map the location of neurons that give rise to local synaptic inputs to islet cells, a major class of inhibitory interneuron in lamina II. We also mapped the distribution of sites on the islet cells that exhibited direct (non-synaptic) responses to uncaging of excitatory and inhibitory transmitters. Local synaptic inputs to islet cells arose almost entirely from within lamina II, and these local inputs included both excitatory and inhibitory components. Furthermore, there was a striking segregation in the location of sites that evoked excitatory versus inhibitory synaptic inputs, such that inhibitory presynaptic neurons were distributed more proximal to the islet cell soma. This was paralleled in part by a differential distribution of transmitter receptor sites on the islet cell, in that inhibitory sites were confined to the peri-somatic region while excitatory sites were more widespread. This differential organization of excitatory and inhibitory inputs suggests a principle for the wiring of local circuitry within the substantia gelatinosa.

The Motor Thalamus in Neurosurgery

THE MOTOR THALAMUS is an important target for the treatment of tremor. It receives afferents from the cerebellum, globus pallidus internus, and substantia nigra and projects mainly to the motor cortex, premotor cortex, and supplementary motor area. Various nomenclatures have been proposed to subdivide the motor thalamus, none of which are universally accepted. Both thalamic lesions and high-frequency stimulation ameliorate tremor in diverse pathological conditions. Modern neurophysiological techniques have allowed the recording of the activity of thalamic neurons in patients with different clinical conditions. This has provided a better understanding of the functions of the motor thalamus in humans. The aim of the present article is to briefly review the major anatomic and physiological aspects of the motor thalamus as well as the electrophysiological findings described in humans undergoing surgical procedures.

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.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Reevaluation of the primary motor cortex connections with the thalamus in primates

Six injections (approximately 1 mm in diameter) of biotinylated dextran amine (BDA) were placed in different locations of the primary motor cortex of the rhesus monkey. Anterograde and retrograde labeling patterns in the thalamus were charted and individual labeled axons traced in continuous serial sections. Both anterograde and retrograde labeling in the thalamus was extensive, spanning several millimeters mediolaterally and including ventral lateral, ventral anterior, centromedian, and centrolateral nuclei. Paracentral, mediodorsal, lateral posterior, and medial pulvinar nuclei were also labeled. Two basic types of corticothalamic axons were identified: small to medium-width, type 1 axons that formed large terminal fields with small boutons, and thick, type 2 axons that formed small terminal fields with large boutons. Within each group, subtypes were identified based on specific features of the axons and terminals: two subtypes of type 1 axons and four subtypes of type 2 axons. The results revealed multiple modes of corticothalamic connectivity: sparsely distributed type 1 axons, dense plexuses of type 1 axons, type 2 axon terminal fields either singly or in clusters, and mixed plexuses of type 1 and type 2 axons. Only some cells in the plexuses were retrogradely labeled; some plexuses did not contain any labeled neurons, and many retrogradely labeled neurons were in the regions devoid of anterograde labeling. These connectivity patterns differed between thalamic nuclei. The results revealed much more complex relationships between M1 and thalamus than were previously thought to exist. It is suggested that this connectivity is neither of exclusively a feedback nature nor perfectly reciprocal but is subserved by a multitude of channels, most likely originating from different populations of cortical neurons, and feeding into a variety of functionally different neuronal networks, with each processing specific information.

Motor thalamic circuits in primates with emphasis on the area targeted in treatment of movement disorders

The ventral region of the motor thalamus that receives cerebellar afferents has been and still is the target of stereotactic interventions for movement disorders. According to Hassler, this area includes ventro-oralis posterior (Vop) and ventral intermedius (Vim) nuclei, although some investigators believe that Vop is associated with the pallidothalamic pathway. We sought to correlate our experimental data on distribution of nigral, pallidal, and cerebellar afferents to the monkey thalamus with Hassler's motor thalamic parcelations. We concluded that Hassler's parcelations retained their value, although some adjustments were needed to relate them to the current neuroanatomic data; particularly, the cerebellothalamic zone that represents the monkey ventral lateral nucleus (VL) corresponds topographically to Hassler's Vop, Vim, and most of Voi. Electron microscopic tracing studies have shown very complex circuitry in this region of the monkey thalamus, as the cerebellar and cortical afferents innervating it are engaged in complex synapses with thalamocortical projection neurons, and this interaction is strongly modulated by local circuit neurons and the input from the reticular thalamic nucleus, which are both inhibitory and gamma-aminobutyric acid (GABA)ergic. Spinothalamic afferents also reach the VL, but this input is less studied in the monkey. The circuitry subserving the activity of thalamocortical projection neurons in the VL should be considered while interpreting the functional data obtained in stereotactic surgery.

The place of the thalamus in frontal cortical-basal ganglia circuits

The thalamus has long been thought to convey subcortical information to the cortex. Indeed, models of basal ganglia function attribute the primary role for the thalamus to a simple relay of information processed in the basal ganglia to the cortex. The thalamic nuclear groups that are associated primarily with this function are the ventral anterior and ventral lateral nuclei and the mediodorsal thalamic nucleus. However, recent studies have shown that the corticothalamic projection is important for the dynamics of the thalamocortical processing. Furthermore, the relay nuclei that carry basal ganglia output to the cortex have recently been shown to project back to the basal ganglia directly. These two recent developments indicate a more dynamic role for the thalamus in basal ganglia information processing than a passive relay.

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.