A description of the GABAergic neurons and axon terminals in the motor nuclei of the cat thalamus.

The GABA neurons and their processes in the cat motor thalamic nuclei were identified and studied with glutamic acid decarboxylase (GAD) immunocytochemistry at both the light and electron microscopic levels. The three nuclei that comprise the motor thalamus, ventral anterior (VA), ventral medial (VM), and ventral lateral (VL), each displayed a characteristic distribution pattern of GAD-positive structures that was consistent with their afferent and intrinsic neuronal organization. All three thalamic nuclei displayed a population of small, GAD-positive cells the dendrites of which contained synaptic vesicles and participated in complex synaptic arrays such as serial synapses, triads, and glomeruli. Based on their ultrastructural features, these GAD-containing cells were identified as local circuit neurons. In contrast, the larger, GAD-negative cells were presumed to be the thalamocortical projection neurons. The axons of GAD-positive local circuit neurons could not be identified in these preparations. The number of GAD-positive dendrites in the neuropil was different for the three thalamic nuclei. In the VA and VM, the GAD-positive dendrites were numerous and formed symmetric synapses with dendrites of GAD-negative cells, mainly in association with corticothalamic boutons. Within VL, the GAD-containing dendrites were more numerous than in VA and VM and formed synapses at influential locations on presumed thalamocortical projection neurons, such as bases of primary dendrites, and bifurcation sites of primary and secondary dendrites. The VA and anterolateral VM nuclei that receive inhibitory GABAergic afferents from the entopeduncular nucleus and substantia nigra contained the highest concentration of large GAD-positive axon terminals. These boutons contained pleomorphic vesicles and numerous mitochondria and formed symmetric synapses and multiple puncta adherentes with dendrites and somata of presumed thalamocortical projection neurons. The size, ultrastructural features, and distribution of these GAD-positive boutons were similar to those features described for basal ganglia terminals in the motor thalamus of the cat. In addition, similar large-size GAD-positive boutons were observed in the medial VM, which receives basal ganglia afferents exclusively from the substantia nigra. The concentration of these terminals in medial VM along the dendrites of thalamocortical projection neurons was much less than that in VA and anterolateral VM. The VL nucleus which lacks basal ganglia input did not contain any large GAD-positive boutons.(ABSTRACT TRUNCATED AT 400 WORDS

A cortical motor nucleus drives the basal ganglia-recipient thalamus in singing birds

The pallido-recipient thalamus transmits information from the basal ganglia to the cortex and is critical for motor initiation and learning. Thalamic activity is strongly inhibited by pallidal inputs from the basal ganglia, but the role of nonpallidal inputs, such as excitatory inputs from cortex, remains unclear. We simultaneously recorded from presynaptic pallidal axon terminals and postsynaptic thalamocortical neurons in a basal ganglia–recipient thalamic nucleus that is necessary for vocal variability and learning in zebra finches. We found that song-locked rate modulations in the thalamus could not be explained by pallidal inputs alone and persisted following pallidal lesion. Instead, thalamic activity was likely driven by inputs from a motor cortical nucleus that is also necessary for singing. These findings suggest a role for cortical inputs to the pallido-recipient thalamus in driving premotor signals that are important for exploratory behavior and learning.National Institutes of Health (U.S.) (Grant R01DC009183)National Institutes of Health (U.S.) (Grant K99NS067062)Damon Runyon Cancer Research Foundation (Postdoctoral Fellowship)Charles A. King Trust (Postdoctoral Fellowship

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 lo cal 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

Multiple cortical targets of one thalamic nucleus: the projections of the ventral medial nucleus in the cat studied with retrograde tracers

The organization of the cortical projections of the ventral medial thalamic nucleus (VM) was studied in the cat with retrograde tracers. The extent of the VM-cortical projections was first investigated with horseradish peroxidase injected in different cortical fields. The results obtained in the experiments indicated that the main target of VM efferents is represented by a large territory anterior to the cruciate sulcus involving area 6 and the gyrus proreus and extending into the anterior part of the medial cortical surface. The afferents to these precruciate fields arise from throughout the VM. In addition, the lateral third of VM projects upon the lateral precruciate cortex that is coextensive with the precruciate part of area 4, whereas VM efferents do not extend into the posterior sigmoid gyrus. A second major target of VM efferents is represented by the insular cortex in the anterior sylvian gyrus. VM projections also reach the prepyriform cortex and the cingulate gyrus. An anteroposterior decrease of density was found in the VM-cingulate projections. Sparse VM projections reach the temporal cortex, the adjacent posterior sylvian and ectosylvian fields, and the anterior ectosylvian gyrus. No VM projections were found either upon the visual areas 17 and 18 or upon the primary auditory cortex. The interrelations between some VM-cortical cell populations and their divergent collateralization were studied by using double retrograde labeling with fluorescent tracers. The results of these experiments demonstrated that a relatively high number (at least 20%) of VM cells projecting to the insula are also connected to the precruciate fields by means of axon collaterals. This finding indicates that VM is a highly collateralized structure of the cat's thalamus. Very few branched cells were found in the other combinations of cortical fields here examined (precruciate vs. posterior sylvian fields, lateral precruciate vs. proreal cortex, anterior vs. posterior cingulate fields). Altogether these data indicate that VM branched cells preferentially interconnect the two main cortical targets of the nucleus, i.e., precruciate and insular fields. The results of the present study are discussed in regard to the literature on the VM projections in the rat and the previously available data in the cat, to the afferent VM organization in the cat, to the relationships between VM and the nucleus submedius, and to the anatomical and functional role of VM in relation to the so-called "nonspecific" thalamocortical system