The regional distribution of receptors for vasoactive intestinal polypeptide (VIP) in the rat central nervous system

The regional distribution of receptors for vasoactive intestinal polypeptide (VIP) was studied in the rat central nervous system (CNS). The specific binding was highest in cerebral cortex, limbic forebrain and cerebellum, whereas moderate to low binding was found in hypothalamus, thalamus, brainstem and pituitary. The lowest binding was observed in pons and spinal cord. Scatchard analysis showed curvilinear plots with upward concavity, which was interpreted as two classes of binding sites. The Kd values were similar in all regions and calculated as 2.4 and 62 nmol/liter, respectively. The variations of specific [125I]VIP binding were due to differences in the total amount of receptors and were in the range of 1.7-8.6 pmol per mg protein. The regional distribution of VIP receptors was parallel with the occurrence of VIP-containing nerve terminals with exceptions of cerebellum, olfactory areas and nucleus caudatus, where a greater number of receptors than expected from the VIP content was found. In these regions, VIP may interact with receptors for a different, but homologous neuropeptide. In conclusion, the regional distribution of VIP receptors in CNS gives further evidence for the role of VIP as a central neurotransmitter.