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

Nobuya Harayama, Izumi Shibuya, Keiko Tanaka, Narutoshi Kabashima, Yoichi Ueta and Hiroshi Yamashita

1. Voltage-dependent Ca²⁺ 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.

2. The selective GABA_B agonist baclofen reversibly inhibited voltage-dependent Ca²⁺ currents elicited by voltage steps from a holding potential of -80 mV to depolarized potentials in a dose-dependent manner. The ED₅₀ of baclofen for inhibiting Ca²⁺ currents was 1·4 × 10^-6 M. Baclofen did not inhibit low threshold Ca²⁺ currents elicited by voltage steps from -120 to -40 mV.

3. Inhibition of high threshold Ca²⁺ 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.

4. The inhibition of Ca²⁺ 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 GDPS was included in the patch pipette. When GTPS was included in the patch pipette, baclofen produced irreversible inhibition of Ca²⁺ currents and this inhibition was again reversed by the prepulse procedure.

5. The inhibition of N-, P/Q-, L- and R-type Ca²⁺ channels by baclofen (10^-5 M) was 24·1, 10·5, 3·1 and 3·6 %, respectively, of the total Ca²⁺ currents. Only the inhibition of N- and P/Q-types was significant.

6. 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 Ca²⁺ 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.

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