Although thalamic projections to the dorsal striatum are well described in primates and other species, little is known about thalamic projections to the ventral or "limbic" striatum in the primate. This study explores the organization of the thalamic projections to the ventral striatum in the primate brain by means of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and Lucifer yellow (LY) retrograde tracer techniques. In addition, because functional and connective differences have been described for the core and shell components of the nucleus accumbens in the rat and are thought to be similar in the primate, this study also explores whether these regions of the nucleus accumbens can be distinguished by their thalamic input. Tracer injections are placed in different portions of the ventral striatum, including the medial and lateral regions of the ventral striatum; the central region of the ventral striatum, including the dorsal part of the core of the nucleus accumbens; and the shell region of the nucleus accumbens. Retrogradely labeled neurons are located mainly in the midline nuclear group (anterior and posterior paraventricular, paratenial, rhomboid, and reuniens thalamic nuclei) and in the parafascicular thalamic nucleus. Additional labeled cells are found in other portions of the intralaminar nuclear group as well as in other thalamic nuclei in the ventral, anterior, medial, lateral, and posterior thalamic nuclear groups. The distribution of labeled cells varies depending on the area of the ventral striatum injected. All regions of the ventral striatum receive strong projections from the midline thalamic nuclei and from the parafascicular nucleus. In addition, the medial region of the ventral striatum receives numerous projections from the central superior lateral nucleus, the magnocellular subdivision of the ventral anterior nucleus, and parts of the mediodorsal nucleus. After injection into the lateral region of the ventral striatum, few labeled neurons are seen scattered in nuclei of the intralaminar and ventral thalamic groups and occasional labeled cells in the mediodorsal nucleus. The central region of the ventral striatum, including the dorsal part of the core of the nucleus accumbens, receives a limited projection from the midline thalamic, predominantly from the rhomboid nucleus. It receives much smaller projections from the central medial nucleus and the ventral, anterior, and medial thalamic groups. The shell of the nucleus accumbens receives the most limited projection from the thalamus and is innervated almost exclusively by the midline thalamic nuclei and the central medial and parafascicular nuclei. The shell is distinguished from the rest of the ventral striatum in that it receives the fewest projections from the ventral, anterior, medial, and lateral thalamic nuclei.

The striatal return through the thalamus is largely neglected in current studies dealing with basal ganglia function, and its role within this circuitry remains obscure. In this contribution the thalamus is regarded as an important place of interaction between the input and the output organization of the basal ganglia. In support of this idea, a brief overview is provided of some of the most recent findings concerning the thalamus in relation to the basal ganglia circuitry. In particular, we have focused on the thalamostriatal projections themselves, on the output of the basal ganglia to the thalamus and also on the overlapping territories between the thalamic projection of the output nuclei and the thalamostriatal neurons. These data support the existence of several thalamic feedback circuits within the basal ganglia neural system. Finally, some considerations are provided upon the functional significance of these thalamic feedback circuits in the overall organization of the basal ganglia.

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Fluorescent tracers were injected into different regions of the caudate nucleus and HRP-WGA in the substantia nigra of the cat in order to analyse the thalamic distribution of retrogradely labelled thalamostriatal neurones and anterogradely labelled nigrothalamic terminals within the thalamus. Overlapping thalamic territories between the thalamostriatal neurones projecting to areas of the caudate nucleus and the nigrothalamic connections were observed in the rostral nuclei of the central thalamic group (ventral anterior nucleus, ventral anterior-ventral lateral complex and ventral medial nucleus) and, more restricted, in the rostral (rhomboid, paracentral, ventral lateral, dorsal mediodorsal nuclei) and caudal intralaminar nuclei (centromedian-parafascicular complex). This study provides evidence of the existence of thalamic areas in which the input and output of the basal ganglia converge.

The distribution of thalamostriatal neurons projecting to the cat caudate nucleus was examined by retrograde fluorescent tracers. Thus, Fast Blue and Diamidino Yellow were concomitantly injected in different rostrocaudal, dorsoventral, or mediolateral sectors of the caudate nucleus. The main findings of this study are as follows: (1) few double-labeled cells were found after two injections in different sectors of the caudate nucleus; (2) double-labeled neurons were more abundant after adjacent injections and they were mainly located in the caudal intralaminar nuclei, in the rhomboid nucleus and in the dorsal mediodorsal nucleus; and (3) there were variations in the spatial organization of the thalamostriatal neurons projecting to various sectors of the caudate nucleus in the different thalamic nuclei known to project to this part of the striatum.

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