Differential development of insulin-like growth factor-I binding in the hypothalamus of hamster and rat

We investigated the binding of [125]insulin-like growth factor-I ([125I]IGF-I) within the hamster suprachiasmatic nucleus (SCN) and median eminence (ME) by quantitative autoradiography and compared the development of binding to the same regions in the rat. Binding in the hamster SCN was to a single class of sites (estimated Kd = 6 x 10(-10) M). Binding in the ME was approx. 1.5-fold that of the SCN. Full displacement of binding from the SCN and ME of adults and neonates was achieved by 10(-8) M IGF-I, 10(-8) M IGF-II or 10(-6) M insulin. Binding within the hamster SCN was evident by E15, peaked between P5 and P7 and decreased to adult levels by P20 while binding in the rat SCN peaked perinatally and declined to adult levels by P6. Binding in the hamster ME was evident at P4, peaked by P12 and decreased to adult levels by P20 while binding in rat ME was present by P2, peaked by P7 and declined to adult levels by P9. These results demonstrate a different developmental time course for [125I]IGF-I binding between the SCN and ME of hamster and rat. The peak binding in the SCN of each species correlates with previously reported time courses for onset of retinohypothalamic innervation of the SCN. Further study of IGFs in these regions may help elucidate the developmental role of brain IGFs.