Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) and Its Receptors in the Zebrafish Ovary: Potential Evidence for Dual Roles of PACAP in Controlling Final Oocyte Maturation

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide originally purified from ovine hypothalamus for its potent activity to stimulate cAMP production. However, its presence and action have also been demonstrated in various peripheral tissues including the ovary. In the zebrafish, two forms of PACAP (PACAP₃₈-1, adcyap1a; and PACAP₃₈-2, adcyap1b) and three PACAP receptors (PAC1-R, adcyap1r1; VPAC1-R, vipr1; and VPAC2-R, vipr2) were all expressed in the ovary. Interestingly, while both follicle cells and oocytes express adcyap1b, the expression of adcyap1a was restricted to the oocytes only. Among the three receptors, adcyap1r1 and vipr2 were expressed in the oocytes whereas the expression of vipr1 was exclusively located in the follicle cells. Temporal expression analysis of PACAP ligands and receptors during folliculogenesis suggested that PACAP might play differential roles in regulating follicle growth and maturation through different receptors. The two receptors that are expressed in the oocyte (adcyap1r1 and vipr2) showed a significant increase in expression at the transition from the primary growth (PG) stage to pre-vitellogenic (PV) stage and their levels maintained high during follicle growth. However, when the follicle development approached full-grown (FG) stage, these two receptors both decreased significantly in expression. In contrast, vipr1, the receptor expressed in the follicle cells, showed little change in expression at the PG-PV transition and afterwards during follicle growth; however, its expression surged dramatically at FG stage prior to oocyte maturation. Based on these results, we hypothesized that PACAP might play dual roles in regulating follicle growth and maturation through different receptors located in different compartments. PACAP may stimulate oocyte growth but block its maturation in early follicles by acting directly on the oocyte via PAC1-R and VPAC2-R, whose expression is dominant in growth phase; however, PACAP may promote oocyte maturation in the maturation phase via VPAC1-R on the follicle cells, whose expression surges in FG follicles prior to maturation and is consistently high in the follicles undergoing final maturation. This hypothesis was further supported by the observation that PACAP promoted maturation of follicle-enclosed oocytes but suppressed spontaneous maturation of denuded oocytes in vitro. This study provides strong evidence for a PACAP-mediated signaling network in the zebrafish ovarian follicle, which may play roles in orchestrating follicle growth and maturation via different types of receptors located in different compartments of the follicle.