Pituitary adenylate cyclase-activating polypeptide (PACAP) can act as determinant of the tyrosine hydroxylase phenotype of dopaminergic cells during retina development

In the chick retina, dopaminergic cells are generated between embryonic days 3 and 7 (E3/E7). However, the expression of tyrosine hydroxylase (TH), the first enzyme in the catecholamine synthetic pathway, is only detected after E11/E12. During the interval comprising E7 to E12, signals conveyed by cAMP are important to determine the TH phenotype. The present study shows that pituitary adenylyl cyclase-activating polypeptide (PACAP), via cAMP, is a major endogenous component in defining the TH phenotype of retina dopaminergic cells during development. PACAP type 1 receptor and its mRNA were detected in retinas since E6. PACAP was also immunodetected in cells localized in the inner nuclear layer of retinas since E8. This peptide promoted greater than 10-fold increase in cAMP accumulation of retinas obtained from embryos since E8, an effect that was blocked by PACAP6-38 (PAC1 receptor antagonist). In cultured retina cells from E8 and E9, maintained for 6 days in vitro with 10 nM PACAP (for 5 days), the number of dopaminergic cells expressing tyrosine hydroxylase increased 2.4-fold. The cAMP analog, 8-Br-cAMP and 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) also increased the number of tyrosine hydroxylase-positive cells by 4- to 6-fold. IBMX plus PACAP treatment resulted in 17-fold increase in the number of cells positive for tyrosine hydroxylase. Under this condition the amount of tyrosine hydroxylase expression, as detected by western blot analysis, was also increased. The protein kinase-A inhibitor, rp-cAMPS, significantly reduced the effect of PACAP. Our data show that this peptide is an important factor influencing the definition of the tyrosine hydroxylase phenotype of retina dopaminergic cells within a narrow window of development.