Pituitary Adenylate Cyclase Activating Polypeptide (PACAP1-38) Exerts Both Pro and Anti-Apoptotic Effects on Postnatal Retinal Development in Rat

PACAP1-38, a ubiquitous and multifunctional regulator has been in the focus of neurotoxicity research due to its impressive neuroprotective potential. Although the literature extensively demonstrated its repressive effect on the apoptotic machinery in neurodegenerative models, there is a striking absence of analysis on its role in normal development. We performed quantitative analyses on caspase activity in developing retina upon 100, 50, 25 or 1 pmol intravitreal PACAP1-38 injection from postnatal day 1 (P1) through P7 in Wistar rats. Retinas were harvested at 6, 12, 18, 24 or 48 h post-injection. Apoptotic activity was revealed using fluorescent caspase 3/7 enzyme assay, western blots and TUNEL assay. Unexpectedly, we found that 100 pmol PACAP1-38 increased the activity of caspase 3/7 at P1 and P5 whereas it had no effect at P7. At P3, as a biphasic effect, PACAP1-38 repressed active caspase 3/7 at 18 h post-injection while increased their activity in 24 h post-injection. Amounts, smaller than 100 pmol, could not inhibit apoptosis whereas 50, 25 or 1 pmol PACAP1-38 could evoke significant elevation in caspase 3/7 activity. TUNEL-positive cells appeared in the proximal part of inner nuclear as well as ganglion cell layers in response to PACAP1-38 treatment. The fundamental novelty of these results is that PACAP1-38 induces apoptosis during early postnatal retinogenesis. The dose as well as stage-dependent response suggests that PACAP1-38 has a Janus face in apoptosis regulation. It not only inhibits development-related apoptosis, but as a long-term effect, facilitates it.