Co-assembly of N-type Ca²⁺ and BK channels underlies functional coupling in rat brain

Activation of large conductance calcium-activated potassium (BK) channels hastens action potential repolarization and generates the fast afterhyperpolarization in hippocampal pyramidal neurons. A rapid coupling of calcium (Ca2+) entry with BK channel activation is necessary for this to occur, which may result from an identified coupling of Ca2+ entry through N-type Ca2+ channels to BK channel activation. This selective coupling was extremely rapid and resistant to intracellular BAPTA, suggesting that the two channel types are close. Using reciprocal co-immunoprecipitations, we have found that N-type channels were more abundantly associated with BK channels than L-type channels (CaV1.2) in rat brain. Expression of only the pore-forming a-subunits of the N-type (CaV2.2) and BK (Slo27) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of CaV2.2/CaVb3 subunits with Slo27 channels revealed rapid functional coupling. In contrast, extremely rare examples of rapid functional coupling were observed with co-expression of CaV1.2/CaVb3 and Slo27 channels. Action potential repolarization in hippocampal pyramidal neurons was slowed by the N-type channel blocker w-conotoxin GVIA, but not by the L-type channel blocker isradipine. These data showed that selective functional coupling between N-type Ca2+ and BK channels provided rapid activation of BK channels in central neurons.