Analysis of G-protein-activated inward rectifying K+ (GIRK) channel currents upon GABAB receptor activation in rat supraoptic neurons

•Baclofen enhances GIRK currents via GABAB receptors in rat SON neurons.•Gi/o proteins are involved in the activation of GIRK currents by baclofen.•mRNAs for GIRK1-4 and GABABR1&2 are detected in the rat SON.•EC50 for GIRK enhancement is about 100 times higher than that for VDC inhibition.•Rat SON neurons may receive little influence from GIRK channels.

While magnocellular neurons in the supraoptic nucleus (SON) possess rich Gi/o-mediated mechanisms, molecular and cellular properties of G-protein-activated inwardly rectifying K+ (GIRK) channels have been controversial. Here, properties of GIRK channels are examined by RT-PCR and whole-cell patch-clamp techniques in rat SON neurons. Patch clamp experiments showed that the selective GABAB agonist, baclofen, enhanced currents in a high K+ condition. The baclofen-enhanced currents exhibited evident inward rectification and were blocked by the selective GABAB antagonist, CGP55845A, the IRK channel blocker, Ba2+, and the selective GIRK channel blocker, tertiapin, indicating that baclofen activates GIRK channels via GABAB receptors. The GIRK currents were abolished by N-ethylmaleimide pretreatment, and prolonged by GTPγS inclusion in the patch pipette, suggesting that Gi/o proteins are involved. RT-PCR analysis revealed mRNAs for all four GIRK 1-4 channels and for both GABABR1 and GABABR2 receptors in rat SON. However, the concentration-dependency of the baclofen-induced activation of GIRK currents had an EC50 of 110 µM, which is about 100 times higher than that of baclofen-induced inhibition of voltage-dependent Ca2+ channels. Moreover, baclofen caused no significant changes in the membrane potential and the firing rate. These results suggest that although GIRK channels can be activated by GABAB receptors via the Gi/o pathway, this occurs at high agonist concentrations, and thus may not be a physiological mechanism regulating the function of SON neurons. This property that the membrane potential receives little influence from GIRK currents seems to be uncommon for CNS neurons possessing rich Gi/o-coupled receptors, and could be a special feature of rat SON neurons.