Radioautographic analysis of somatostatin receptor sub-type in rat hypothalamus

Hypothalamic somatostatin (SRIF) receptors were examined in a qualitative and quantitative radioautographic study using [125I-Tyr0,D-Trp8]SRIF14 and the stable octapeptide analog [125I-Tyr3]SMS 201-995 as radioligands. The latter has been shown to bind selectively to the high-affinity SS1 receptor subtype. Both radioligands labeled specifically and with high resolution various hypothalamic nuclei. In addition, the labeling patterns obtained with the two probes were identical; in both cases specific binding density was highest in the preoptic area and lowest in the ventromedial hypothalamic nucleus. Inhibition of the specific binding of each radioligand by either SRIF14 or the SS1-selective (SMS 201-995) unlabeled competitor was assessed on serial sections throughout the hypothalamus. The proportions of both non-selective and SS1-selective binding, remaining in the presence of either SRIF14 or SMS 201-995 (micromolar concentrations) were identical. These results indicate the existence of a homogeneous class of SRIF binding sites of the SS1 type in the hypothalamus.