kappa- and mu-opioid inhibition of N-type calcium currents is attenuated by 4 beta-phorbol 12-myristate 13-acetate and protein kinase C in rat dorsalganglion neurons

In rat dorsal root ganglion neurons, activation of kappa- and mu-opioid rec eptors decreases N-type calcium current, whereas a constitutively active fo rm of protein kinase C (LKC; i.e., PKM, a PKC catalytic subunit fragment) i ncreases N-type calcium current. PKC also attenuates inhibition of calcium current by several G protein-linked neurotransmitter systems. We examined t he effects of activation of endogenous PKC by 4 beta-phorbol 12-myristate 1 3-acetate (PMA) and dialysis of cells with PKM and a pseudosubstrate inhibi tor PKC(19-31) (PKC-I) on kappa- and mu-opioid-mediated inhibition of calci um current, calcium current amplitude, and rundown. PMA modestly increased peak calcium current and substantially reduced calcium current "rundown," e ffects blocked by PKC-I. In contrast, PKC-I decreased calcium current and i ncreased current rundown. PMA attenuated morphine-, dynorphin A-, and U50,4 88- but not pentobarbitol-related inhibition of calcium current. Similar ef fects were seen with intracellular dialysis of PKM. Intracellular PKC-I did not block opioid inhibition of calcium current but did reverse PMA and PKM effects on opioid receptor coupling to calcium channels. Because neither P MA nor PKM changed the proportion of omega-CgTX-inhibited current, their ef fects were not due to a decrease in the proportion of N-type current. After omega-CgTX treatment, there were no differences in the dynorphin A effects on control and PMA- or PKM-treated neurons, suggesting that PKC primarily affected coupling to N-type calcium channels. These data suggest that in ac utely dissociated rat dorsal root ganglion neurons, endogenous PKC is requi red for maintenance of calcium current, may play a role in regulation of ne uronal calcium channels, and could be involved in tolerance and/or cross-ta lk inhibition of opioid responsiveness.