High-conductance calcium-activated potassium channels in rat brain: Pharmacology, distribution, and subunit composition

In rat brain, high-conductance Ca2+-activated K+ (BK) channels are targeted to axons and nerve terminals [Knaus, H. G., et al. (1996) J. Neurosci. 16, 955-963], but absolute levels of their regional expression and subunit com position have not yet been fully established. To investigate these issues, an IbTX analogue ([I-125]IbTX-D19Y/Y36F) was employed that selectively bind s to neuronal BK channels with high affinity (K-d = 21 pM) Cross-linking ex periments with [I-125]IbTX-D 19Y/Y3GF in the presence of a bifunctional rea gent led to covalent incorporation of radioactivity into a protein with an apparent molecular mass of 25 kDa, Deglycosylation and immunoprecipitation studies with antibodies raised against alpha- and smooth muscle beta-subuni ts of the BK channel suggest that the beta-subunit that is associated with the neuronal BK channel is a novel protein. Quantitative receptor autoradio graphy reveals the highest levels of BK channel expression in the outer lay ers of the neocortex, hippocampal perforant path projections, and the inter peduncular nucleus. This distribution pattern has also been confirmed in im munocytochemical experiments with a BK channel-selective antibody. Taken to gether, these findings imply that neuronal BK channels exhibit a restricted distribution in brain and have a subunit composition different from those of their smooth muscle congeners.