Quantitative autoradiography of somatostatin receptors in the rat limbic system

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Abstract
The distribution of somatostatin receptors (SRIF‐R) was analyzed in the limbic system of the adult rat by in vitro autoradiography with [125I‐Tyr0, DTrp8]S14 as a radioligand. Precise quantification of the density of binding sites, at 0.2 mm intervals throughout the different areas revealed a marked heterogeneity of labeling in most structures. In particular, SRIF‐R were concentrated in the basal (104.4 ± 3.3 fmol/mg proteins) and basolateral amygdaloid nuclei (94.8 ± 4.3 fmol/mg proteins), and in the nucleus of the lateral olfactory tract (121.6 ± 2.4 fmol/mg proteins), whereas moderate densities were detected in the amygdalo‐hippocampal nucleus (76.4 ± 2.8 fmol/mg proteins). The medial (41.3 ± 1.9 fmol/mg proteins) and the central (24.0 ± 1.4 fmol/mg proteins) amygdaloid nuclei contained lower SRIF‐R concentrations. It appears from these observations, in the light of the anatomical pathways of the amygdala, that intra‐amygdalian SRIF‐containing neurons project to the amygdalo‐hippocampal nucleus, and that SRIF‐R in the basolateral complex are the target of afferents from limbic cortical areas. SRIF‐R were detected at different levels of the hippocampal formation but their distribution was more restricted than that of SRIF‐containing fibers. The maximal density of sites was detected in the ventral and dorsal parts of the subiculum (115.0 ± 3.4 and 87.0 ± 2.8 fmol/mg proteins, respectively) and in the parasubiculum (100.1 ± 5.4 fmol/mg proteins). In Ammon's horn, the stratum oriens and stratum radiatum of the CA1 field were the only sites enriched in SRIF‐R (74.1 ± 2.0 and 74.6 ± 1.9 fmol/mg proteins, respectively). The apparent lack of receptors in the pyramidal cell laver indicated that, in Ammon's horn, SRIF is involved in intra‐hippocampal communication. Low levels of receptors were found in the hippocampal CA2 and CA3 fields. SRIF‐R in the dentate gyrus were mainly concentrated in the molecular layer (57.3 ± 1.2 fmol/mg proteins). A very high density of sites was also observed in the entorhinal cortex (up to 123.1 ± 1.5 fmol/mg proteins). A clear mismatch between SRIF and SRIF‐R was detected in the septum and the habenula. In the profound layers of the cingulum and retrosplenial cortex, a heterogeneous distribution of SRIF‐R was observed. High concentrations of sites were detected in the rostral zone of the cingulate cortex (93.4 ± 2.0 fmol/mg proteins) while the posterior cingulate only exhibited moderate concentrations of sites (66.5 ± 0.7 fmol/mg proteins). The density of SRIF‐R increased toward the caudal part of the retrosplenial cortex (94.7 ± 1.4 fmol/mg proteins). The results suggest that SRIF is likely to exert specific activity depending on field specialization in the deep layers of these cortical areas. The present findings show that in the rat the distribution of the receptor sites is more restricted than that of SRIF‐immunoreactive fibers in discrete areas of most subdivisions of the limbic system. Thus these data, together with the known localization of somatostatinergic perikarya and fibers, provide detailed information concerning the sites of action of SRIF in the various regions of the limbic system.