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

Activation of large conductance Ca²⁺-activated potassium (BK) channels hastens action potential repolarisation and generates the fast afterhyperpolarisation in hippocampal pyramidal neurons. A rapid coupling of Ca²⁺ entry with BK channel activation is necessary for this to occur, which might 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-immunoprecipitation, we found that N-type channels were more abundantly associated with BK channels than L-type channels (Ca_V1.2) in rat brain. Expression of only the pore-forming -subunits of the N-type (Ca_V2.2) and BK (Slo₂₇) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of Ca_V2.2/Ca_V3 subunits with Slo₂₇ channels revealed rapid functional coupling. By contrast, extremely rare examples of rapid functional coupling were observed with co-expression of Ca_V1.2/Ca_V3 and Slo₂₇ channels. Action potential repolarisation in hippocampal pyramidal neurons was slowed by the N-type channel blocker -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.