Pituitary Adenylate Cyclase-Activating Polypeptide Regulates Brain-Derived Neurotrophic Factor Exon IV Expression through the VPAC1 Receptor in the Amphibian Melanotrope Cell

In mammals, pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors PAC1-R, VPAC1-R, and VPAC2-R play a role in various physiological processes, including proopiomelanocortin (POMC) and brain-derived neurotrophic factor (BDNF) gene expression. We have previously found that PACAP stimulates POMC gene expression, POMC biosynthesis, and alpha-MSH secretion in the melanotrope cell of the amphibian Xenopus laevis. This cell hormonally controls the process of skin color adaptation to background illumination. Here, we have tested the hypothesis that PACAP is involved in the regulation of Xenopus melanotrope cell activity during background adaptation and that part of this regulation is through the control of the expression of autocrine acting BDNF. Using quantitative RT-PCR, we have identified the Xenopus PACAP receptor, VPAC1-R, and show that this receptor in the melanotrope cell is under strong control of the background light condition, whereas expression of PAC1-R was absent from these cells. Moreover, we reveal by quantitative immunocytochemistry that the neural pituitary lobe of white-background adapted frogs possesses a much higher PACAP content than the neural lobe of black-background adapted frogs, providing evidence that PACAP produced in the hypothalamic magnocellular nucleus plays an important role in regulating the activity of Xenopus melanotrope cells during background adaptation. Finally, an in vitro study demonstrates that PACAP stimulates the expression of BDNF transcript IV.