LOCALIZATION AND CHARACTERIZATION OF PACAP RECEPTORS IN THE RAT CEREBELLUM DURING DEVELOPMENT - EVIDENCE FOR A STIMULATORY EFFECT OF PACAP ON IMMATURE CEREBELLAR GRANULE CELLS

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its rec eptors are abundant in the brain and particularly in the cerebellum of adult rats. In contrast, the occurrence of PACAP binding sites has no t been investigated during ontogenesis. The aim of the present study w as to determine the distribution and biochemical characteristics of PA CAP binding sites in the rat cerebellum during postnatal development, and to examine the effect of PACAP on immature cerebellar granule cell s. Autoradiographic studies revealed that PACAP binding sites are tran siently expressed in a germinative matrix of the cerebellar cortex, th e external granule cell layer, and in the medulla, from postnatal days 8 to 25. A population of PACAP binding sites persisted in the interna l granule cell layer in the mature cerebellum. Emulsion-coated cytoaut oradiography, performed on cultured immature granule cells from eight- day-old rat cerebellum, demonstrated that transient PACAP binding site s are expressed by cerebellar immature granule cells. Biochemical char acterization of binding revealed the occurrence of two classes of PACA P recognition sites exhibiting, respectively, high (K(d) = 0.39 +/- 0. 08 nM) and low (K(d) = 21.2 +/- 9.4 nM) affinity for PACAP27. The two naturally occurring forms PACAP38 and PACAP27 were equipotent in compe ting for [I-125]PACAP27 binding. In contrast, the [Des-His1]PACAP38 an alog was eight times less efficient and vasoactive intestinal polypept ide only induced weak displacement of the binding. Exposure of culture d immature granule cells to PACAP27 resulted in a dose-dependent stimu lation of the production of cAMP, indicating that PACAP binding sites represent authentic receptors positively coupled to adenylate cyclase. These results show that PACAP receptors are actively expressed in the cerebellum of rats during postnatal development. The presence of func tional PACAP receptors in the external granule cell layer suggests tha t PACAP may play a role in the control of proliferation and/or differe ntiation of granule cells.