LOCALIZATION AND PHARMACOLOGICAL CHARACTERIZATION OF SOMATOSTATIN SST(2) SITES IN THE RAT CEREBELLUM

Radioligand binding studies were performed in membranes of rat cerebel lum using [I-125]-[Tyr(3)]octreotide ([I-125]204-090) to characterize the nature of cerebellar somatostatin receptors. Saturation experiment s suggest the presence of a single class of binding sites with high af finity, pK(d) = 9.53 +/- 0.11, but low receptor density, B-max = 12.7 +/- 1.0 fmol/mg protein. The pharmacological profile of [I-125]204-090 sites in cerebellar membranes was established using a range of ligand s known to interact with SSTR-2 (now called sst(2)) and other somatost atin (SRIF) receptors. SRIF analogues such as octreotide (SMS 201-995) , seglitide (MK 678) and somatuline (BIM 23014) displayed very high af finity for cerebellar [I-125]204-090 binding sites. The data were comp ared to results obtained using the same ligand in rat cerebral cortex membranes known to represent sst(2) binding. The pharmacological chara cteristics of the cerebellar sites were in close correlation with thos e of the cerebral cortex (r = 0.976, n = 19, p < 0.001) and CHO-cells expressing human recombinant sst(2) receptor (r = 0.977, n = 19, p < 0 .001). By contrast, there was very little correlation between cerebell ar binding and published affinities for rat sst(5) receptors (r = 0.46 5), for which octreotide has also high affinity. In vitro autoradiogra phic studies performed in cerebellar slices using [I-125]204-090 demon strated the presence of binding sites in the molecular layer of the ra t cerebellum. In situ hybridization studies using sst(2) receptor mRNA selective oligoprobes confirmed the presence of sst(2) receptor mRNA in the rat cerebellum. Together, the present data demonstrate the pres ence of a low density of SRIF receptors in the molecular layer of the adult rat cerebellum which are best characterized as sst(2). This is t he first pharmacological characterization and localization of sst(2) r eceptors in the adult rat cerebellum.