BK channels are ion channels with a large conductance for potassium ions. BK channels are mostly involved in controlling neuronal excitability and smooth muscle contraction. BK is not a classic cannabinoid receptor but its activity is modulated by cannabinoids. Since cannabinoids have a physiological effect on BK it is effectively a cannabinoid receptor.
BK channels are neuronal and are mostly found in neocortex (layer V/VI), pyriform cortex, septum (bed nucleus of the stria terminalis, lateral septum), thalamus (rhomboid nucleus).
Pertwee, R.G., Howlett, A.C., Abood, M.E., Alexander, S.P.H., Di Marzo, V., Elphick, M.R., Greasley, P.J., Hansen, H.S., Kunos, G., Mackie, K., et al. (2010). International Union of Basic and Clinical Pharmacology. LXXIX. cannabinoid receptors and their ligands: beyond CB₁ and CB₂. Pharmacol. Rev. 62, 588–631.