GASTRIC-INHIBITORY POLYPEPTIDE (GIP) BINDING-SITES IN RAT-BRAIN

Synthetic porcine gastric inhibitory polypeptide (GIP) was iodinated and purified by reverse-phase HPLC and used to localize saturable [I-125]GIP binding sites by radioligand binding to frozen sections of rat brain followed by autoradiography. Saturable [I-125]GIP binding sites were expressed in several brain regions including cerebral cortex, anterior olfactory nucleus, lateral septal nucleus, subiculum, inferior colliculus, and inferior olive. Saturable [I-125]GIP binding was time dependent, reversible, high affinity, and specific for GIP. Scatchard analysis of equilibrium binding resulted in an estimated dissociation constant (K(d)) of 16-62 pM for the rat brain [I-125]GIP binding sites. Peptides with amino acid sequences similar to GIP such as secretin, vasoactive intestinal polypeptide (VIP), glucagon, and peptide histidine isoleucine (PHI) only partially inhibited saturable [I-125]GIP binding at concentrations approximately 10,000-100,000-fold higher than GIP. Saturable [I-125]GIP binding was not observed in other rat organs surveyed such as spinal cord, pituitary, stomach, small intestine, colon, pancreas, liver, heart, or skeletal muscle. We conclude that a saturable [I-125]GIP binding site with the pharmacological properties of an authentic GIP receptor is expressed in certain regions of the rat brain.