Differential development of insulin-like growth factor I binding in the suprachiasmatic nucleus and median eminence of the rat hypothalamus.

We investigated the in vitro binding of [125I]insulin-like growth factor I ([125I]IGF-I) within the suprachiasmatic nucleus (SCN) and median eminence (ME) of the rat by quantitative autoradiography. Binding of [125I]IGF-I within the adult SCN was saturable, reversible and to a single class of sites with an estimated Kd of 3.01 x 10(-10) M and Bmax of 131 fmol/mg protein. Competition studies revealed [125I]IGF-I binding within the SCN and ME to have the pharmacological specificity characteristic of the brain IGF-I receptor. The most potent competitors were IGF-I and IGF-II, while insulin displaced binding with a much lower potency and nerve growth factor 7S had no effect. Binding within the SCN was evident by embryonic day 18, peaked perinatally, declined to adult levels by day 6 and remained constant through middle age. The potency of IGF-I to displace [125I]IGF-I binding from SCN was similar among all ages. Binding in the ME was not evident until postnatal day 2, peaked near the end of the 1st week, declined to adult levels by day 9 and remained constant thereafter. There was no difference in binding in SCN or ME between day and early night, in animals with acute or long-term eye enucleation, or in animals fed a high sucrose diet. These results demonstrate differential development of [125I]IGF-I binding within the SCN and ME and support a role for IGFs in both these nuclei during development and maturity.