Activation of protein kinase C antagonizes the opioid inhibition of calcium current in rat spinal dorsal horn neurons

Spinal dorsal horn (SDH) is one of important regions in both nociceptive transmission and antinociception. Opioid peptides produce analgesia via regulation of neurotransmitter release through modulation of voltage-dependent Ca²⁺ channel (VDCC) in neuronal tissues. The modulatory effect of μ-opioid receptor (MOR) activation on VDCC was investigated in acutely isolated rat SDH neurons under the conventional whole-cell patch-clamp recording mode. The Ba²⁺ current passing through VDCC was reversibly inhibited by a MOR agonist, [D-Ala²,N-MePhe⁴,Gly ⁵-ol]-enkephalin (DAMGO, 1 μM). Among 108 SDH neurons tested, VDCC of 39 neurons (36%) were inhibited by MOR activation, while other 69 neurons (64%) were not affected. The L-, N-, P/Q-, and R-type VDCC components shared 58.4±18.9%, 29.3±12.1%, 8.7±7.2%, and 3.4±4.8% of the total VDCC, respectively. Among VDCC subtypes inhibited by MOR activation, L- and N-types were 61.4±12.8% and 30.7±14.4%, respectively, while both P/Q- and R-types were 7.9±11.8%. A depolarizing pre-pulse increased the amplitude of VDCC and suppressed most of the inhibitory effect of MOR activation. Application of 1 μM phorbol-12-myristate-13-acetate completely antagonized the inhibitory effect of MOR activation without any alteration of basal VDCC amplitude. In contrast, the response of MOR activation was not altered by application of 4-α-phorbol (1 μM), 2-[3-Dimethylaminopropyl] indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 μM), forskolin (1 μM), N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89, 1 μM). These results indicate that activation of MOR coupled to G-proteins inhibits VDCC, and that this G-protein-mediated inhibition is antagonized by PKC-dependent phosphorylation.