DEMONSTRATION AND CHARACTERIZATION OF MOTILIN-BINDING SITES IN THE RABBIT CEREBELLUM

This is the first report on central motilin receptors. Autoradiography on cerebellar slices revealed specific motilin-binding sites in the m olecular layer of the cortex. Scatchard analysis of cold saturation st udies showed the existence of a high- (PKd,hi = 9.07 +/- 0.09, where p K(d) is the negative logarithm of the dissociation constant) and a low -affnity binding site (pK(d,lo) = 6.56 +/- 0.09). Similar affinities w ere found with rabbit motilin and with the porcine (po) antagonist [Ph e(3),Leu(13)]po-motilin. Feline and canine motilin had a markedly lowe r affinity for the low-affinity site (pK(d,lo) = 5.29 and 4.58, respec tively); chicken motilin had a lower affinity for both sites (PKd,hi = 8.36, PKd,lo = 3.97). Erythromycin A and its derivative N-trimethyl e rythromycin A enol ether also bound to cerebellar motilin receptors (p K(d,hi) = 7.29 and 8.91, respectively). Structure-activity studies wit h motilin fragments and the potency ranking of agonists suggest that a novel subtype receptor of motilin may exist in the brain. Guanosine 5 '-O-(3-thiotriphosphate) (0.1 mM) reduced the number and the affinity for the high-affinity binding sites, which is evidence for G protein-c oupled receptors. Our findings open new perspectives for the study of the physiological role of motilin.