Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

Cholinergic modulation of spontaneous GABAergic currents (mIPSC) was studied using whole-cell patch methods in mouse accessory olfactory bulb slices. Carbachol (above 100 µM) administration produced an increase in the mIPSC frequency in mitral cells, but did not affect the responses of mitral cells to GABA. The carbachol effect persisted in the presence of combined ionotropic and metabotropic glutamatergic receptor antagonists. The carbachol effect was reduced by the muscarinic receptor type-1 and -4 (M1 and M4) antagonist pirenzepine (10 µM), but not by the M2 and M4 antagonist himbacine (10 µM). The KCNQ/Kv7 potassium channel openers retigabine (80 µM) and diclofenac (300 µM) blocked the carbachol action, while the KCNQ potassium channel blocker XE-911 (20 µM) increased the mIPSC frequency. XE-911's action persisted in the presence of glutamate receptor blockers. In the presence of carbachol, mIPSCs were abolished by Ni (200 µM), while being insensitive to the calcium channel blocker nimodipine (30 µM), suggesting a role for R-type calcium channels in the GABA release. These results suggest that carbachol closed KCNQ channels by stimulating M1 receptors on granule cell dendrites, and the resulting depolarized and unstable membrane promoted calcium influx, thus increasing the GABA release. The possible role of acetylcholine in facilitating formation of a pheromone memory in mice is also discussed.