Membrane-delimited coupling between sigma receptors and K+ channels in ratneurohypophysial terminals requires neither G-protein nor ATP

1. Receptor-mediated modulation of ion channels generally involves G-protei ns, phosphorylation, or both in combination. The sigma receptor, which modu lates voltage-gated K+ channels, is a novel protein with no homology to oth er receptors known to modulate ion channels. In the present study patch cla mp and photolabelling techniques were used to investigate the mechanism by which sigma receptors modulate K+ channels in peptidergic nerve terminals. 2. The sigma receptor photoprobe iodoazidococaine labelled a protein with t he same molecular mass (26 kDa) as the sigma receptor protein identified by cloning. 3. The sigma receptor ligands pentazocine and SKF10047 modulated K+ channel s, despite intra-terminal perfusion with GTP-free solutions, a Gi-protein i nhibitor (GDP beta S), a G-protein activator (GTP gamma S) or a non-hydroly sable ATP analogue (AMPPcP). 4. Channels in excised outside-out patches were modulated by ligand, indica ting that soluble cytoplasmic factors are not required. In contrast, channe ls within cell-attached patches were not modulated by ligand outside a patc h, indicating that receptors and channels must be in close proximity for fu nctional interactions. Channels expressed in oocytes without receptors were unresponsive to sigma receptor agonists, ruling out inhibition through a d irect drug interaction with channels. 5. These experiments indicate that sigma receptor-mediated signal transduct ion is membrane delimited, and requires neither G-protein activation nor pr otein phosphorylation. This novel transduction mechanism is mediated by mem brane proteins in close proximity, possibly through direct interactions bet ween the receptor and channel. This would allow for more rapid signal trans duction than other ion channel modulation mechanisms, which in the present case of neurohypophysial nerve terminals would lead to the enhancement of n europeptide release.