Receptors of the serotonin 1C subtype expressed from cloned DNA mediate the closing of K+ membrane channels encoded by brain mRNA.

The modulation of K+ channels by serotonin (5-HT) receptors was studied by coinjecting Xenopus oocytes with mRNA transcribed in vitro from a cloned 5-HT 1C subtype (5-HT1C) receptor gene, together with size-fractionated mRNA isolated from rat cerebral cortex that expresses K+ channels. After intracellular loading with EGTA to block Ca2(+)-dependent chloride currents, these oocytes responded to 5-HT with an inward current associated with a decrease in membrane conductance. Membrane current responses were small or absent in oocytes injected with either mRNA alone. We conclude that 5-HT1C receptors are able to cause the closing of a class of K+ channels expressed by cortex mRNA in a Ca2(+)-independent manner. The coupling between the receptors and channels appears to be mediated by the inositol phospholipid second messenger pathway, since activation of this pathway by application of serum evoked a similar closing current.