Mechanism of catecholamine synthesis inhibition by neuropeptide Y: role of Ca2+ channels and protein kinases.

We have previously demonstrated that neuropeptide Y (NPY) inhibits depolarization-stimulated catecholamine synthesis in rat pheochromocytoma (PC12) cells differentiated to a sympathetic neuronal phenotype with nerve growth factor (NGF). The present study uses multiple selective Ca2+ channel and protein kinase agonists and antagonists to elucidate the mechanisms by which NPY modulates catecholamine synthesis as determined by in situ measurement of DOPA production in the presence of the decarboxylase inhibitor m-hydroxybenzylhydrazine (NSD-1015). The L-type Ca2+ channel blocker nifedipine inhibited the depolarization-induced stimulation of DOPA production by approximately 90% and attenuated the inhibitory effect of NPY. In contrast, the N-type Ca2+ channel blocker omega-conotoxin GVIA inhibited neither the stimulation of DOPA production nor the effect of NPY. Antagonism of Ca2+/calmodulin-dependent protein kinase (CaM kinase) greatly inhibited the stimulation of DOPA production by depolarization and prevented the inhibitory effect of NPY, whereas alterations in the cyclic AMP-dependent protein kinase pathway modulated DOPA production but did not prevent the effect of NPY. Stimulation of Ca2+/phospholipid-dependent protein kinase (PKC) with phorbol 12-myristate 13-acetate (PMA) did not affect the basal rate of DOPA production in NGF-differentiated PC12 cells but did produce a concentration-dependent inhibition of depolarization-stimulated DOPA production. In addition, NPY did not produce further inhibition of DOPA production in the presence of PMA, and the inhibition by both PMA and NPY was attenuated by the specific PKC inhibitor chelerythrine. These results indicate that NPY inhibits Ca2+ influx through L-type voltage-gated Ca2+ channels, possibly through a PKC-mediated pathway, resulting in attenuation of the activation of CaM kinase and inhibition of depolarization-stimulated catecholamine synthesis.