Nucleotide-independent modulation of Ca(2+)-dependent K+ channel current by a mu-type opioid receptor

Physiological responses to opiates and opioid peptides are transduced via receptors coupled to G proteins. The effectors for these G proteins are often ion channels or second messenger systems that modulate channel activity. In cultured bovine adrenal medullary chromaffin cells (BAMCCs), the activity of a calcium-dependent, voltage-sensitive, potassium (BK) channel is robustly potentiated by a mu-type opioid receptor, an effect consistent with the inhibitory role of opioids versus neural excitability. Patch-clamp electrophysiology was used to investigate coupling between the mu receptor and BK channel, leading to rather surprising results. Potentiation of BK channel activity by the mu-selective agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10 nM) was unaffected by all attempts to disrupt or alter G protein function, including incubation of cells with pertussis toxin (PTX) and inclusion of guanosine 5'-O-(2-thio)diphosphate (GDP beta S) or guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) in intracellular recording solutions. However, dopamine D2 receptor potentiation of BK current in these same cells was affected by PTX, GDP beta S, and GTP gamma S in predictable fashion. Thus, PTX and GDP beta S inhibited dopamine potentiation of BK current, and GTP gamma S prolonged reversal of dopamine action. These results suggest that the BAMCC BK channel is not coupled to the mu receptor via a GTP-dependent mechanism, whereas in the same cells the dopamine D2 receptor modulates BK channel activity in a conventional GTP-dependent manner. In addition, replacement of both ATP and GTP with nonhydrolyzable analogs also failed to affect either potentiation or recovery of BK channel activity in response to [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin. These results indicate that in BAMCCs the mu-opioid receptor modulates BK channel activity independently of either G proteins or phosphorylation-dependent processes.