Pharmacological characterization of somatostatin receptors in the rat cerebellum during development

Somatostatin (SRIF) receptors (SRIF-Rs) are transiently expressed in a germinative lamina of the rat cerebellum, the external granule cell layer. The appearance of SRIF-Rs coincides with the expression of SRIF-like immunoreactivity in the cerebellum. However, the cellular location of SRIF-Rs does not overlap with the distribution of SRIF-like immunoreactivity, with the latter being restricted to ascending fibers arising from the brainstem, to perikarya within the white matter, and to some Purkinje cells. The characterization of SRIF-Rs in the immature (13-day-old) rat cerebellum was conducted by means of binding experiments in membrane-enriched preparations and autoradiography, using two radioligands, [125I-Tyr0,D-Trp8]SRIF-14 [( 125I-Tyr0,D-Trp8]S14) and 125I-SMS 204-090. The pharmacological profile of cerebellar SRIF-Rs was compared with that of adult cortical SRIF-Rs. Saturation studies performed in 13-day-old rat cerebellum showed that the KD values for [125I-Tyr0,D-Trp8]S14 and 125I-SMS 204-090 binding were 0.35 +/- 0.04 and 0.39 +/- 0.01 nM, respectively. The corresponding Bmax values were 52.7 +/- 4.8 and 49.9 +/- 5.3 fmol/mg of protein, a result indicating that radioligands with high specific radioactivity (2,000 Ci/mmol) bind to a single class of high-affinity sites (SS1). Competition studies showed that different D-Trp-substituted analogs displaced [125I-Tyr0,D-Trp8]S14 binding with Hill coefficients less than 1, a finding indicating the existence of different subtypes of binding sites. When [Tyr0,D-Trp8]S14 was used as a competitor, two sites were resolved by Scatchard analysis in both 13-day-old cerebellum and adult cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)