In rat pheochromocytoma (PC-12) cells the dopamine D 2 receptor agonists apomorphine (APO) and n-propylnorapomorphine (NPA) produced a concentration dependent inhibition of K + -evoked neuropeptide Y release (NPY-ir). The effect of APO was blocked by the dopamine D 2 -receptor antagonist, eticlopride, but not the D 1 /D 3 or the D 4 /D 2 antagonists, SCH23390 or clozapine, respectively. The D 1 /D 5 receptor agonist, SKF38393 or the D 3 agonists PD128907 and 7-OH DPAT had no effect. Selective N and L-type voltage gated Ca 2+ channel blockers, ω-Conotoxin GVIa (CtxGVIa) and nifedipine, respectively, produced a concentration dependent inhibition of NPY-ir release but were not additive with APO. The Ca 2+ /calmodulin-dependent protein kinase (CaM kinase) II inhibitor KN-62 produced a concentration-dependent inhibition of NPY-ir release but the combination of KN-62 and APO produced no further inhibition. PMA-mediated protein kinase C stimulation significantly increased both basal and K + -evoked release of NPY-ir, and in the presence of PMA APO had no inhibitory effect. The PKC antagonist, chelerythrine, inhibited K + -evoked NPYir release but was not additive with APO. Neither forskolin-mediated adenylate cyclase activation and the active cAMP analog Sp-cAMPS, nor the adenylate cyclase inhibitor SQ 22536, and the competitive inhibitor of cAMP-dependent protein kinases Rp-cAMPS, had any significant effect on K + -evoked NPY-ir release. This suggests the inhibitory effect of APO on K + -evoked release of NPYir from PC 12 cells is most likely mediated through activation of dopamine D 2 receptors leading to direct inhibition of N and L-type voltage gated Ca 2+ channels, or indirect inhibition of PKC, both of which would reduce [Ca 2+ ] i and inactivate CaM kinase.

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