SOMATOSTATIN RECEPTORS IN THE RHESUS-MONKEY BRAIN - LOCALIZATION AND PHARMACOLOGICAL CHARACTERIZATION

To characterize the nature and distribution of somatostatin (SRIF) rec eptors, radioligand binding studies and in vitro receptor autoradiogra phy were performed in Rhesus monkey brain using either [I-125]LTT-SRIF -28 ([Leu(8), D-Trp(22), I-125-Tyr(25)]SRIF-28) alone or in the presen ce of 3 nM seglitide (to block sst(2) sites), [I-125]Tyr(3)-octreotide or [I-125]CGP 23996 (c[Asu-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) in buffer containing either 120 mM Na+ or 5 mM Mg2+. [I-125]Tyr(3)-oct reotide labelled an apparently homogeneous population of sites in cere bral and cerebellar cortex (B-max = 27.3+/-2.8 fmol/mg protein and 52. 6+/-8.6 fmol/mg protein, pK(d) = 9.46+/-0.03 and 9.93+/-0.03, respecti vely). The pharmacological profile of these sites correlated highly si gnificantly with that of human recombinant sst(2) receptors (r = 0.996 ), but not or much less with that of human recombinant sst(3) and sst( 5) receptors (r = 0.12 and 0.45, respectively). [I-125]CGP 23996 (in N a+-buffer) also labelled an apparently homogeneous population of sites in Rhesus monkey cerebral cortex membranes (B-max = 3.1+/-0.3 fmol/mg protein, pK(d) = 10.57+/-0.08), the pharmacological profile of which was highly significantly correlated with the profiles of human recombi nant sst(1) and sst(4) receptors (r = 0.98 and 0.96, respectively). Us ing receptor autoradiography, high levels of [I-125]LTT-SRIF-28 and [I -125]Tyr(3)-octreotide recognition sites were found in basal ganglia, molecular and granular layers of the cerebellum and layers LII, V and VI of entorhinal cortex. In these regions, the addition of 3 nM seglit ide produced a marked decrease of [I-125]LTT-SRIF-28 binding. Low leve ls of [I-125]LTT-SRIF-28 binding were observed in subiculum, pituitary and choroid plexus. By contrast, [I-125]CGP 23996 labelling in the pr esence of Mg2+ as well as Na+ ions was highest in pituitary and choroi d plexus. However, [I-125]CGP 23996 binding was diversely affected by these ionic conditions in several regions of hippocampus and cerebral cortex. Displacement of [I-125]CGP 23996 (in Mg2+-buffer) with segliti de in the molecular layer of the cerebellum, deep layers of the entorh inal cortex, layers I, II and V of the insular cortex and frontal pole yielded complex competition curves suggesting the presence of two pop ulations of SRIF receptors. By contrast, [I-125]CGP 23996 binding (in Mg2+-buffer) in the choroid plexus, hilus of the dentate gyrus and str atum oriens and radiatum of the CA3field of hippocampus was not affect ed by seglitide up to 10 mu M, suggesting only sst(1) and/or sst(4) si tes which have a negligible affinity for seglitide to be present in th ese structures. Taken together, these results suggest that [I-125]CGP 23996 (in the presence of Na+) labels exclusively SRIF-2 receptors (ss t(1) and/or sst(4)), whereas in the presence of Mg2+ ions, [I-125]CGP 23996 labels both SRIF-2 and SRIF-1 receptors (sst(2), sst(3) and sst( 5)) The present study also demonstrates the presence and differential distribution of sst(2) and sst(1)/sst(4) receptors in the Rhesus monke y brain.