Orexin-a activates phospholipase C- and protein kinase C-mediated Ca2+ signaling in dopamine neurons of the ventral tegmental area

The orexin-orexin receptor system has been implicated in the regulation of wakefulness/sleep states. Behavioral and psychostimulant effects of orexins have also been shown. Mesolimbic dopamine neurons in the ventral tegmental area (VTA) are implicated in the regulation of reward and wakefulness/sleep, In the present study, we examined the effect of orexin-A on cytosolic [Ca 2+ ] i concentration ([Ca 2+ ]) in the isolated rat VTA dopamine neurons. Orexin-A (10 -12 -10 -8 M) concentration dependently increased [Ca 2+ ] i in dopamine-containing neurons. The [Ca 2+ ] i responses to orexin-A were inhibited under Ca 2+ -free conditions and by blockers of voltage-gated Land N-type [Ca 2+ ] i channels, nitrendipine and (1)-conotoxin, respectively. The [Ca 2+ ] i responses were also abolished by a phosphatidylcholine-specific phospholipase C inhibitor, D609, and a protein kinase C (PKC) inhibitor, calphostin C. A PKC activator, TPA, mimicked orexin-A in increasing [Ca 2+ ] i . These results indicate that orexin-A increases [Ca 2+ ] i in VTA dopamine neurons via phosphatidylcholine-specific PLC- and PKC-mediated activation of L- and N-type Ca 2+ channels. This effect may serve as the mechanism by which orexin regulates wakefulness/sleep states and exerts its behavioral and psychostimulant effects.