Regional distribution of somatostatin binding sites in the human hypothalamus: a quantitative autoradiographic study

Using in vitro quantitative autoradiography and [125I]Tyr0-D-Trp8SRIF 14 as radioligand, we characterized the detailed distribution of somatostatin binding sites in human hypothalamus of both infants and adults. Guanosine triphosphate pretreatment, before incubation, allowed us to detect higher [125I]Tyr0-D-Trp8SRIF 14 binding site densities in hypothalamic structures such as preoptic and anterior hypothalamic areas and ventromedial and dorsomedial nuclei. In contrast, guanosine triphosphate was without effect in the other hypothalamic regions. The regional effects of guanosine triphosphate pretreatment were not different in infant and adult hypothalamus. Scatchard analysis showed that in a guanosine triphosphate-sensitive region (preoptic area) and a guanosine triphosphate-insensitive area (infundibular nucleus), [125I]Tyr0-D-Trp8SRIF 14 bound to a single class of binding sites. Affinities were similar in both regions, not modified by guanosine triphosphate pretreatment and not different in the adult (1.5 +/- 1.2 nM vs 3.2 +/- 2.1 nM for preoptic area and infundibular nucleus, respectively) and infant (0.9 +/- 0.5 nM vs 2.4 +/- 1.7 nM for preoptic area and infundibular nucleus). [125I]Tyr0-D-Trp8SRIF 14 binding sites were widely distributed in the anterior, mediobasal and posterior hypothalamus. Somatostatin 28 was twice as potent as somatostatin 14 to displace [125I]Tyr0-D-Trp8SRIF 14 binding in the preoptic area and infundibular nucleus. However, IC50s were 30 times lower in the preoptic area as compared with the infundibular nucleus. In adult as well as in infant, high densities were found mainly in the diagonal band of Broca, preoptic area and infundibular nucleus. Intermediate densities were localized in the anterior hypothalamic area, ventromedial, dorsomedial and lateral mammillary nuclei. The dorsal hypothalamic area, the paraventricular and medial mammillary nuclei displayed low but measurable densities. The only marked difference in the distribution of [125I]Tyr0-D-Trp8SRIF 14 binding sites in adult vs infant was observed in the medial and tuberal nuclei where the concentrations were seven-fold higher in adult hypothalamus.