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The Journal of Neurophysiology Vol. 88 No. 3 September 2002, pp. 1374-1386
Copyright ©2002 by the American Physiological Society
Department of Neuroscience, The Ohio State University, Columbus, Ohio 43210
Dziema, Heather and
Karl Obrietan.
PACAP Potentiates L-Type Calcium Channel Conductance in
Suprachiasmatic Nucleus Neurons by Activating the MAPK Pathway. J. Neurophysiol. 88: 1374-1386, 2002. 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.
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