PACAP potentiates L-type calcium channel conductance in suprachiasmatic nucleus neurons by activating the MAPK pathway

The endogenous pacemaker activity of the suprachiasmatic nuclei (SCN; the master clock in mammals) is regulated by photic information relayed from the retina to the SCN via the retinohypothalamic tract (RHT). Recent work has revealed that glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP) are stored in RHT nerve terminals and function in a coordinated manner to regulate clock timing. To address this interaction on a cellular level, Fura-2 Ca2+ digital imaging was employed and the effects of PACAP on glutamate evoked Ca2+ transients in SCN neurons were examined. Pretreatment of SCN neurons with PACAP markedly potentiated Ca2+ transients elicited by both exogenous glutamate application and synaptically released glutamate. Many neurons became responsive to glutamate only after PACAP administration, suggesting that PACAP sets the lower concentration threshold required for glutamate to initiate a robust rise in postsynaptic cytosolic Ca2+. Facilitation of glutamate-induced Ca2+ transients was inhibited by nimodipine, indicating that PACAP potentiates L-type Ca2+ channel activity. The modulatory actions of PACAP were inhibited by antagonizing signaling via the p42/44 mitogen-activated protein kinase (MAPK) signal transduction cascade. Immunocytochemistry and Western analysis confirmed that PACAP stimulates MAPK activity at doses and time points shown to potentiate Ca2+ influx. Down-regulation of protein kinase C (PKC) with the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) or PKC inhibition with bisindolylmaleimide attenuated the actions of PACAP, indicating that PKC also couples PACAP to potentiation of depolarization-induced Ca2+ transients. The data presented here identify potentially important mechanisms by which PACAP regulates SCN physiology.