DISTRIBUTION AND SUBCELLULAR-LOCALIZATION OF MOTILIN BINDING-SITES INTHE RABBIT BRAIN

We previously reported the existence of motilin receptors in the cereb ellum of the rabbit. We now explored the existence of motilin receptor s in other brain regions and determined their association with neurons by subcellular fractionation studies. Autoradiographic studies with [ I-125]nle(13)-porcine motilin on rabbit coronal brain sections reveale d specific binding sites in the hippocampus, thalamus, hypothalamus an d amygdaloid body. Receptor binding studies allowed the identification of two binding sites. In all regions the density of the high-affinity binding site was lower than in the cerebellum, but its affinity was t he same, except for the hypothalamus. No differences were found for af finity or density of the low-affinity receptor site. Homogenates of ra bbit cerebellum were subjected to differential centrifugation. The hig hest motilin binding (10-times more than in the postnuclear supernatan t) was found in the fraction which also showed maximal enrichment of [ 11-H-3]saxitoxin binding (selective marker for voltage sensitive Na+ c hannels), 6.9-fold, and cytochrome c oxidase activity (mitochondrial m arker), 2.4-fold. In discontinuous sucrose density gradient centrifuga tion the motilin and saxitoxin binding both peaked in the 0.85-1 M lay er, while cytochrome c oxidase was maximal in the 1.2 M layer. In conc lusion, motilin receptors exist in several regions of the rabbit brain and are probably associated with synaptosomes. These findings further support a neurotransmitter role for motilin in the brain. (C) 1997 El sevier Science B.V.