Inhibition of N- and P/Q-type calcium channels by postsynaptic GABAB receptor activation in rat supraoptic neurones

Voltage-dependent Ca 2+ currents of dissociated rat supraoptic nucleus (SON) neurones were measured using the whole-cell configuration of the patch-clamp technique to examine direct postsynaptic effects of GABA B receptor activation on SON magnocellular neurones. The selective GABA B agonist baclofen reversibly inhibited voltage-dependent Ca 2+ currents elicited by voltage steps from a holding potential of −80 mV to depolarized potentials in a dose-dependent manner. The ED 50 of baclofen for inhibiting Ca 2+ currents was 1.4 × 10 −6 M. Baclofen did not inhibit low threshold Ca 2+ currents elicited by voltage steps from −120 to −40 mV. Inhibition of high threshold Ca 2+ currents by baclofen was rapidly and completely reversed by the selective GABA B antagonists, CGP 35348 and CGP 55845A, when the antagonists were added at the molar ratio vs . baclofen of 10 : 1 and 0.01 : 1, respectively. It was also reversed by a prepulse to +150 mV lasting for 100 ms. The inhibition of Ca 2+ currents was abolished when the cells were pretreated with pertussis toxin for longer than 20 h or with N- ethylmaleimide for 2 min. It was also abolished when GDPβS was included in the patch pipette. When GTPγS was included in the patch pipette, baclofen produced irreversible inhibition of Ca 2+ currents and this inhibition was again reversed by the prepulse procedure. The inhibition of N-, P/Q-, L- and R-type Ca 2+ channels by baclofen (10 −5 M) was 24.1, 10.5, 3.1 and 3.6 %, respectively, of the total Ca 2+ currents. Only the inhibition of N- and P/Q-types was significant. These results suggest that GABA B receptors exist in the postsynaptic sites of the SON magnocellular neurones and mediate selective inhibitory actions on voltage-dependent Ca 2+ channels of N- and P/Q-types via pertussis toxin-sensitive G proteins, and that such inhibitory mechanisms may play a role in the regulation of SON neurones by the GABA neurones.