Abstract
The binding of radiolabeled vasoactive intestinal polypeptide (VIP) to rat brain membranes was investigated. Specific binding of 125I-labeled VIP was reversible and saturable (Bmax = 2.2 pmol/g of wet tissue). Brain membranes exhibited a high affinity for 125I-labeled VIP (KD = 1 nM) at a single class of noninteracting sites. Binding of 125I-labeled VIP paralleled its immunohistochemical localization, being enriched in cerebral cortex, hippocampus, striatum, and thalamus, with the notable exception of the hypothalamus, which had low levels of binding. The density of sites was greater in synaptosomal fractions relative to mitochondrial or nuclear fractions. Secretin and partial sequences of it and VIP inhibited binding to brain membranes with an order of potency similar to that found in other systems. The findings suggest the existence of a unique new class of brain receptors.