Implication of protein kinase C in the orexin-induced elevation of extracellular dopamine levels and its rewarding effect.

In the present study, we investigated the role of orexinergic systems in the activation of midbrain dopamine neurons. In an in vitro study, exposure to either orexin A or orexin B under superfusion conditions produced a transient increase in the intracellular Ca(2+) concentration through the phospholipase C (PLC)/protein kinase C (PKC) pathway via G(q11)alpha or Gbetagamma subunits in midbrain cultured neurons, which were shown to be tyrosine hydroxylase (TH)-positive cells, but not in purified midbrain astrocytes. Here we show that in vivo injection with a selective PKC inhibitor chelerythrine chloride or 2-{8-[(dimethylamino)methyl]-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl}-3-1-methyl-1H-indol-3-ylmaleimide HCl (Ro-32-0432) into the ventral tegmental area (VTA) significantly suppressed the place preference and increased levels of dopamine in the nucleus accumbens (NAcc) induced by intra-VTA injection of orexins. These results strongly support the idea that activation of the orexin-containing neuron in the VTA leads to the direct activation of mesolimbic dopamine neurons through the activation of the PLC/PKC pathway via G(q11)alpha or Gbetagamma-subunit activation, which could be associated with the development of its rewarding effect.