Small Conductance Ca2+-Activated K+ Channels as Targets of CNS Drug Development

In most central neurons, small conductance Ca2+-activated K+ channels (SK channels) contribute to afterhyperpolarizations (AHPs), which control neuronal excitability. The medium AHP has pharmacological properties similar to recombinant SK channels, consistent with the hypothesis that SK channels generate this afterhyperpolarization component. It is still unclear how recombinant SK channels are functionally related to the slow AHP component. Cloned SK channels are heteromeric complexes of SK channel subunits and calmodulin. The channels are activated by Ca2+ binding to calmodulin that induces conformational changes resulting in channel opening. Channel deactivation is the reverse process brought about by dissociation of Ca2+ from calmodulin. In the mammalian brain, the three SK channel subunits (SK1-3) display partially overlapping distributions. Most of the higher brain regions such as the neocortex and hippocampus show expression of both genes encoding SK1 and SK2 channels, whereas phylogenetically older brain regions such as the thalamus, basal ganglia, cerebellum, and brainstem show high levels of SK3 gene expression. At present, it is still unclear whether native SK channels are generated as heteromeric or homomeric channels. Peptide toxins such as apamin and scyllatoxin, as well as organic compounds such as quaternary salts of bicuculline, dequalinium, UCL 1684 and UCL 1848 serve as non-specific SK channel blockers. The only known exceptions so far are the scorpion toxin tamapin and the peptide inhibitor Lei-Dab(7), which bind preferentially to SK2. Electrophysiological and behavioral studies indicate that blockade of SK channels by apamin increases excitability, lowers the threshold for the induction of synaptic plasticity, and facilitates hippocampusdependent memory. The potential value of pharmacological SK channel modulation in various pathological states such as increased epileptiform activity, cognitive impairment, pain, mood disorders and schizophrenia will be discussed.