| Research Abstract |
1)To elicidate the role of GABA_B receptors in the regulation of the magnocellular neurosecretory cells in the supraoptic nucleus (SON) of the hypothalamus, we measured the membrane potential, the firing frequency, and spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) in rat SON slice preparations, and also the voltage-gated Ca^<2+> currents (VGC) of acutely dissociated rat SON neurons by the whole-cell patch-clamp technique. The selective GABAGABA_B agonist, baclofen suppressed the action potential discharge of SON neurons, without causing marked hyperpolarization. Baclofen educed the frequency of both the sEPSCs and sIPSCs without affecting the amplitude. The time constant of the decay phase of both the sEPSCs and sIPS Cs remained unchanged after baclofen application. The reduction of the frequency of the synaptic currents by baclofen was dose-dependent. Baclofen inhibited VGC also in a dose-dependent manner. Only the inhibition of N- and P/Q-types was significant. These results indicate that GABAGABA_B receptors are present both at the pre- and post-synaptic sites of SON neurons and mediate inhibition of EPSCs and IPSCs, and of N- and P/Q-type Ca^<2+> channels. These multiple inhibitory mechanisms mediated by pre- and postsynaptic GABAGABA_B receptors may play important roles in the regulation of SON neurons by the GABA neurons.
2)To investigate the signal transduction mechanism of GABAGABA_B receptor-mediated cellular responses, poly (A)^+ RNA derived from rat brain cortex was coexpressed with CFTR CI channls or with GIRK channels. We found that upon GABAGABA_B activation, the Gbetagamma released from PTX-sensitive G-proteins activates the adenylate cyclase type II, and this process requires Gs activation by Gs-coupled receptors. We also found that GABAGABA_B receptors activated the cloned GIRKs composed GIRK1 and GIRK2 as heteromultimers.
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