Somatostatin receptors in the rhesus monkey brain: localization and pharmacological characterization

To characterize the nature and distribution of somatostatin (SRIF) receptors, radioligand binding studies and in vitro receptor autoradiography were performed in Rhesus monkey brain using either [125I]LTT-SRIF-28 ([Leu8, D-Trp22, 125I-Tyr25]SRIF-28) alone or in the presence of 3 nM seglitide (to block sst2 sites), [125I]Tyr3-octreotide or [125I]CGP 23996 (c[Asu-Lys-Asn-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) in buffer containing either 120 mM Na+ or 5 mM Mg2+. [125I]Tyr3 -octreotide labelled an apparently homogeneous population of sites in cerebral and cerebellar cortex (Bmax = 27.3 +/- 2.8 fmol/mg protein and 52.6 +/- 8.6 fmol/mg protein, PKd = 9.46 +/- 0.03 and] 9.93 +/- 0.03, respectively). The pharmacological profile of these sites correlated highly significantly with that of human recombinant sst2 receptors (r = 0.996), but not or much less with that of human recombinant sst3 and sst5 receptors (r = 0.12 and 0.45, respectively). [125I]CGP 23996 (in Na(+)-buffer) also labelled an apparently homogeneous population of sites in Rhesus monkey cerebral cortex membranes (Bmax = 3.1 +/- 0.3 fmol/mg protein, pKd = 10.57 +/- 0.08), the pharmacological profile of which was highly significantly correlated with the profiles of human recombinant sst1 and sst4 receptors (r = 0.98 and 0.96, respectively). Using receptor autoradiography, high levels of [125I]LTT-SRIF-28 and [125I]Tyr3 -octreotide recognition sites were found in basal ganglia, molecular and granular layers of the cerebellum and layers III, V and VI of entorhinal cortex. In these regions, the addition of 3 nM seglitide produced a marked decrease of [125I]LTT-SRIF-28 binding. Low levels of [125I]LTT-SRIF-28 binding were observed in subiculum, pituitary and choroid plexus. By contrast, [125I]CGP 23996 labelling in the presence of Mg2+ as well as Na+ ions was highest in pituitary and choroid plexus. However, [125I]CGP 23996 binding was diversely affected by these ionic conditions in several regions of hippocampus and cerebral cortex. Displacement of [125I]CGP 23996 (in Mg(2+)-buffer) with seglitide in the molecular layer of the cerebellum, deep layers of the entorhinal cortex, layers I, II and V of the insular cortex and frontal pole yielded complex competition curves suggesting the presence of two populations of SRIF receptors. By contrast, [125I]CGP 23996 binding (in Mg(2+)-buffer) in the choroid plexus, hilus of the dentate gyrus and stratum oriens and radiatum of the CA3 field of hippocampus was not affected by seglitide up to 10 microM, suggesting only sst1 and/or sst4 sites which have a negligible affinity for seglitide to be present in these structures. Taken together, these results suggest that [125I]CGP 23996 (in the presence of Na+) labels exclusively SRIF-2 receptors (sst1 and/or sst4), whereas in the presence of Mg2+ ions, [125I]CGP 23996 labels both SRIF-2 and SRIF-1 receptors (sst2, sst3 and sst5). The present study also demonstrates the presence and differential distribution of sst2 and sst1/sst4 receptors in the Rhesus monkey brain.