Receptor autoradiographic analysis of insulin-like growth factor-I (IGF-I) binding sites in rat forebrain and pituitary gland.

1. Specific 125I-labeled insulin-like growth factor-I [( 125I] IGF-I) binding sites in the rat forebrain and pituitary gland were investigated using quantitative receptor autoradiography. 2. High densities of [125I]IGF-I binding sites were present in the olfactory nerve layer, olfactory glomerular layer, choroid plexus, CA3 and CA4 of the hippocampus, basolateral amygdaloid nucleus, and endopiriform nucleus. Moderate to high binding densities were found in the cerebral cortex (II, VI), bed nucleus stria terminalis, accumbens nucleus, lateral septum, median preoptic nucleus, supraoptic nucleus, paraventricular hypothalamic nucleus, and ventroposterior thalamic nucleus. In the circumventricular organs, subfornical organ, vascular organ of the lamina terminalis, and median eminence, the binding sites were numerous. High densities of [125I]IGF-I binding sites were also observed in the anterior pituitary gland. 3. In kinetic experiments, [125I]IGF-I binding sites in the olfactory glomerular layer, choroid plexus, median eminence, and anterior pituitary gland were found to be single and of a high affinity. 4. Noteworthy was the difference in the potency of insulin in inhibiting the binding among the areas examined, a finding which suggests heterogeneity of IGF-I receptors. 5. The possibility that IGF-I plays the role of a neurotransmitter and/or neuromodulator in the central nervous system warrants further investigation.