Dynorphin A and cAMP-dependent protein kinase independently regulate neuronal calcium currents

The kappa-selective opioid peptide dynorphin A (DYN) inhibits neuronal adenylate cyclase activity and reduces neuronal voltage-dependent calcium currents. It is not yet known, however, whether the regulation of calcium channel activity is dependent on or independent of the adenylate cyclase/cAMP system. We used the whole-cell variation of the patch clamp technique to show that DYN reversibly reduced, in a naloxone-sensitive manner, calcium currents in acutely dissociated rat nodose ganglion neurons. DYN slowed the rate of current activation and had a greater effect on currents evoked from relatively negative holding potentials. These actions were mimicked by guanosine 5'-[gamma-thio]triphosphate, which activates GTP-binding proteins (G proteins), and were blocked by pretreatment with pertussis toxin, which inactivates Gi- and Go-type G proteins. In contrast, calcium currents recorded in the presence of the catalytic subunit of the cAMP-dependent protein kinase (AK-C), included in the recording pipette, increased in magnitude throughout the recording. DYN was applied to neurons before and after the effect of AK-C became apparent; the reduction of calcium currents by DYN was greater in the presence of AK-C than in its absence. We conclude that the acute reduction of neuronal calcium currents by DYN occurred by means of activation of pertussis toxin-sensitive Gi- or Go-type G proteins. The persistence of the action of DYN in the presence of AK-C indicates, however, that this effect was independent of a reduction of the activity of the adenylate cyclase/cAMP system and suggests in addition that phosphorylated channels may be preferentially inhibited by DYN.