LOCALIZATION AND PHARMACOLOGICAL CHARACTERIZATION OF SOMATOSTATIN RECOGNITION SITES IN THE HUMAN CEREBELLUM

Radioligand binding studies were performed in membranes of human cereb ellum using [I-125][Tyr(3)]octreotide also known as [I-125]204-090, [I -125]LTT-SRIF-28 ([Leu(8), D-Trp(22), I-125-Tyr(25)]SRIF-28) and [I-12 5]CGP 25]c[Asu-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) to characteri ze the nature of cerebellar somatostatin receptors. Saturation experim ents performed with [I-125]204-090 suggest the presence of a single cl ass of binding sites with high affinity: B-max = 55.7 +/- 9.7 fmol/mg protein, pK(d) = 9.57 +/- 0.04. The pharmacological profile of [I-125] 204-090 and [I-125]LTT-SRIF-28 labelled sites in human cerebellar memb ranes was overlapping (correlation coefficient r = 0.998) and correlat ed very significantly with that of recombinant human sst(2) receptors (r = 0.987). By contrast, there was very little correlation with those of recombinant human sst(3) (r = 0.208) or human sst(5) receptors (r = 0.547). In contrast to [I-125]204-090 or [I-125]LTT-SRIF-28 binding, [I-125]CGP 23996 binding (in 5 mM MgCl2 buffer) in cerebellar membran es was heterogeneous as indicated by biphasic competition curves produ ced by sst(2) receptor selective ligands such as seglitide or octreoti de. The pharmacological profile of the major component was closely cor related with that of human sst(2) receptors (r = 0.989), whereas the m inor component correlated equally well with human sst(1) or sst(4) rec eptors (r = 0.902 and 0.941, respectively). In vitro autoradiographic studies performed in cerebellar slices using [I-125]204-090 and [I-125 ]LTT-SRIF-28 demonstrated the presence of binding sites predominantly in the molecular layer, whereas weaker labelling was detected in the g ranular layer. The distribution of sites labelled by both radioligands was very similar. Using [I-125]CGP 23996 (in 120 mM NaCl buffer), no clear difference between labeling of the molecular and granular layers was detectable; the dentate nucleus demonstrated binding sites for [I -125]CGP 23996; in contrast to the very low level of binding observed with both, [I-125]204-090 and [I-125]LTT-SRIF-28. Together, the presen t data demonstrate the presence of SRIF receptors in the adult human c erebellar cortex which are,for the major population, best characterize d as sst(2). The SRIF receptors in the minor population of the cerebel lar cortex and the dentate nucleus most probably represent sst(1) and/ or sst(4) sites. Copyright (C) 1996 Elsevier Science Ltd.