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Spontaneous Synaptic Activity Is Primarily GABAergic in Vestibular Nucleus Neurons of the Chick Embryo

¹ Department of Anatomy and Cell Biology and Neuroscience Program, George Washington University Medical Center, Washington DC 20037; ² Institut National de la Santé et de la Recherche Médicale U114, College de France, 75005 Paris, France

Submitted 27 January 2003; accepted in final form 18 April 2003

The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibular reflexes. In 16-day embryos, the application of glutamate receptor antagonists abolished the postsynaptic responses generated on vestibular-nerve stimulation, but spontaneous synaptic activity was largely unaffected. Here, spontaneous synaptic activity was characterized in principal cells from brain slices at E16 using whole cell voltage-clamp recordings. With KCl electrodes, the frequency of spontaneous inward currents was 3.1 Hz at –60 mV, and the reversal potential was +4 mV. Cs-gluconate pipette solution allowed the discrimination of glycine/GABA_A versus glutamate receptor-mediated events according to their different reversal potentials. The ratio for spontaneous excitatory to inhibitory events was about 1:4. Seventy-four percent of the outward events were GABA_A, whereas 26% were glycine receptor-mediated events. Both pre- and postsynaptic GABA_B receptor effects were shown, with presynaptic GABA_B receptors inhibiting 40% of spontaneous excitatory postsynaptic currents (sEPSCs) and 53% of spontaneous inhibitory postsynaptic currents (sIPSCs). With TTX, the frequency decreased 50% for EPSCs and 23% for IPSCs. These data indicate that the spontaneous synaptic activity recorded in the principal cells at E16 is primarily inhibitory, action potential-independent, and based on the activation of GABA_A receptors that can be modulated by presynaptic GABA_B receptors.

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