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The efferent projections of the medial geniculate nucleus (MG) and its adjacent nuclei to the basal ganglia were studied in the rat by the antero- and retrograde tracing methods. Injections of wheat germ agglutinin conjugated to horseradish peroxidase into the caudal parts of the striatum and globus pallidus produced retrograde neuronal labeling in the medial division of the MG (MGm) and its adjacent structures including the suprageniculate, posterior intralaminar and peripeduncular nuclei, and substantia nigra pars lateralis. Injections of [3H]leucine into the MG and its surroundings resulted in anterograde labeling not only in the striatum but also in the globus pallidus. The resulting labeling was distributed exclusively in the caudal parts of these two nuclei. The electron microscopic autoradiography showed preferential radiolabeling of terminals and myelinated axons in both the globus pallidus and striatum. Labeled terminals in the pallidum mostly made symmetrical synapses on somata and major dendrites, while labeled terminals in the striatum established asymmetrical synapses on dendritic spines. These morphological differences in the synapses of the efferent systems originating from the MGm and its surrounding region suggest functional/chemical differentiations at their target sites in the basal ganglia.

The mechanisms controlling central and systemic oxytocin (OT) release were examined using in vivo microdialysis of the paraventricular (PVN) region. Dialysate and plasma samples were collected from conscious male rats and stimuli were administered via the dialysate fluid. Characterization studies showed that microdialysis was a viable technique for the study of peptide secretion in the conscious animal. OT was consistently detected in the PVN dialysate and a partially purified extract crossreacted in parallel fashion with the synthetic peptide. In vitro studies showed that peptide recovery was positively correlated with the pore size of the dialysis membrane and that there was an inverse relationship between flow rate and recovery. Hypertonic saline administered centrally caused an increase in dialysate and plasma oxytocin while the intravenous injection affected only plasma oxytocin. The excitatory amino acid, glutamate (0.05-0.5 M), caused an increase in plasma, but not dialysate oxytocin, while depolarization with potassium chloride (0.05-0.15 M) had no significant effects. Histological examination showed that the dialysis probe was located in the rostral, lateral PVN. Our results show that in vivo microdialysis provides a method for the delivery of drugs into specific brain regions as well as a useful technique for the evaluation of in vivo neuropeptide release.

A retrograde fluorescent double-labeling technique was employed to examine whether the cortico-striate fibers are collaterals of the corticofugal fibers directed to the inferior colliculus in the rat. Following injections of two different fluorescent tracers into the striatum and the inferior colliculus, double-labeled cells were found in layer V pyramidal cells of the temporal cortex. These double-labeled cells were located mostly in the area corresponding to the rat primary auditory cortex, and constituted 6.4% of the pyramidal cells projecting to the inferior colliculus. This study has revealed the existence of a common innervation of the basal ganglia and the auditory system by the pyramidal cell of the auditory cortex.

The ascending projections of pars compacta (SNc) neurons displaced within the pars reticulata (SNr) of the substantia nigra in the rat were examined using a fluorescent retrograde tracing. Following unilateral injections of a tracer into the striatum, SNc cells within the SNr were retrogradely labeled predominantly in the caudal parts, and to a lesser extent in the rostral portions of the nucleus. These nigrostriatal projections arising from the SNc cells within the SNr were only ipsilateral. Injections of a tracer into the nucleus accumbens (Acc) did not produce any labeling of these displaced SNc cells in contrast with cells in the SNc proper. Given that the SNc neurons within the SNr project only to the striatum but not to the Acc, and that they contain dopamine but not cholecystokinin, they might be involved in the motor but not in the limbic function.

Neuronal cell bodies in the medial terminal nucleus of the accessory optic system (MTN) were labeled with WGA-HRP which was injected ipsilaterally into the occipital cortex in the rat. We suggest that the label was first transported anterogradely to the pretectal nucleus of the optic tract, then moved transneuronally from axon terminals of occipital cortical neurons to axon terminals of MTN neurons, and finally transported retrogradely to reach cell bodies of MTN neurons.

The organization of the claustrocortical projections was examined in the rat using a fluorescent retrograde labeling technique. From injections of three different fluorescent tracers into the occipital, frontal and cingulate cortices, claustocortical projections were revealed to be topographically organized in both the rostrocaudal and dorsoventral direction. The dorsoventral topography is similar to that of the primates, but is different from that of the cat. On the basis of a quantitative analysis of the double and triple labeling of single claustral neurons, significant collateralization of the claustrocortical projections is referred.