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Using quantitative in vitro receptor autoradiography, minute distributions of 125 I-Tyr 11 -somatostatin (SS)-14 binding sites were investigated in the rat forebrain and diencephalon. In the cerebral cortex, there was a high density of receptors observed in layers V–VI and a low density in layers I–IV. The entorhinal cortex displayed the highest receptor density of the cerebral cortices. The olfactory system had a high SS receptor density. The anterior olfactory nucleus, nucleus of the lateral olfactory tract, medial habenular nucleus and the basolateral amygdaloid nucleus showed moderate densities. In the limbic system, the CA1 and subiculum regions had high receptor densities. More detailed observations revealed high receptor densities in the oriens, radiatum and lacunosum layers and a much lower density in the pyramidal cell layer. The caudate putamen and substantia nigra showed low receptor densities, while the claustrum displayed the highest density of receptors in the rat brain. These data were not consistent with those of previous studies using 125 I-SS-28 and 125 I-201-995, which had shown that the high receptor density area in the basolateral amygdaloid group was identified as the lateral amygdaloid nucleus, and that the pyramidal cell layer in the hippocampus showed high receptor densities.

GAD-IR neurons were roughly divided into those with medium sized perikarya and large perikarya. The medium-sized GAD-IR neurons accounted for about 85% of the GAD-IR neurons. The medium-sized perikarya were further divided into two, those with a smooth nuclear membrane and those with an indented nucleus. The former were very similar to medium-sized spiny neurons and the latter corresponded to medium-sized aspiny neurons. The GAD-IR large cells that were identified by light microscopy, had nuclear indentations and were divided into two classes based on their ultrastructural features, type 1 large cells received few synaptic inputs and type 2 large cells received many synaptic contacts from non-immunoreactive or immunoreactive boutons. The former resembles Type I large cells and the latter Type II large cells identified recently by Chang and Kitai; the latter are also similar to the second type of projecting neurons identified by Bolam et al.

Calcitonin gene-related peptide (CGRP)-like immunoreactive (CGRP-IR) structures have been studied in the rat caudate putamen using avidin-biotin peroxidase immunohistochemistry. Immunoreactivity was found in the axons of this nucleus but not in the perikarya. CGRP-IR fibers were most densely concentrated along the ventral border and in the caudal portion of the rat caudate putamen. CGRP-IR fibers were sparsely distributed throughout the rest of this nucleus. Almost all immunoreactive boutons which contained small clear vesicles had formed asymmetrical synapses. Postsynaptic targets included dendritic spines and shafts. Asymmetrical synapses in the caudate putamen are supposed to be extrinsic in origin. These observations, together with the results of other investigations, suggest that CGRP-IR boutons form synapses with spiny striatal neurons, which, most likely, are medium-sized spiny projecting neurons. Moreover, evidence indicates that these boutons are of extrinsic origin.

Enkephalin-like immunoreacitve (ENK-IR) neurons and fibers in the rat ventromedial hypothalamic nucleus (VMH) were examined by light and electron microscopy using the peroxidase-antiperoxidase immunocytochemistry. There were groups of ENK-IR neurons present in the ventrolateral part of the VMH, and such neurons were scattered elsewhere. These neurons had perikarya 10–25 μm in diameter with moderately developed cell organelles and enfolded nuclei that were often distributed eccentrically placed in the cell. The perikarya and dendrites contained diffuse, large-cored vesicles (LCV) (60–230 nm with a predominance in the 60–80 nm). ENK-IR neurons received synaptic inputs on the soma and dendrites from unlabeled axonal boutons containing many small, clear vesicles and occasional LCV. The ultrastructural features of the ENK-IR cells in the VMH seemed to correspond to the “common cells” described by Milhouse. Dense ENK-IR fibers were distributed in this nucleus throughout the rostrocaudal and ventrodorsal areas. Axonal boutons containing numerous small, clear vesicles, and dispersed LCV generally made synaptic contacts with the cell bodies and dendrites of unlabeled neurons. The findings suggest that opioid peptides directly influence VMH neurons through synaptic contacts.