GABA(A) receptor-mediated inhibition in the nucleus ambiguus motoneuron

GABA-sensitive ambiguus motoneurons were investigated by microiontophoretic application of GABAergic drugs including bicuculline and muscimol in alpha-chloralose- and urethane-anaesthetized rats. Ambiguus motoneurons were activated by recurrent laryngeal nerve stimulation through a small cuff electrode and identified as laryngeal motoneurons when they met the conventional criteria for antidromic activation. GABA(A) antagonist, bicuculline, and its agonist, muscimol, were iontophoretized on ambiguus motoneurons through a three-multibarrel electrode glued to the recording microelectrode. One-hundred and nineteen out of 155 neurons sampled from the loose formation and its vicinity were found to be respiratory neurons, most of which were inspiratory neurons. A small proportion (32 neurons) was classified as laryngeal motoneurons according to the criteria. A majority of laryngeal motoneurons was found to be GABA sensitive. Namely, application of GABA and its antagonist and agonist affected the antidromic spikes in a dose-dependent manner; GABA and muscimol usually decreased the amplitude and slowed the slope in the spike, whereas bicuculline, reversed these inhibitory effects. The dose-dependent relationships were limited exclusively to the measurements analysed in the negative-going phase but not in the positive-going phase in the antidromic spike. GABA and muscimol decreased but bicuculline increased the ratios to control in these measurements. The effects of distributions of the histograms shifting towards the opposite direction were statistically significant. The line of evidence suggests strongly that a majority of laryngeal motoneurons located in the nucleus ambiguus presumably possesses GABA(A) receptors on their postsynaptic membrane. These GABA-sensitive laryngeal motoneurons may receive inputs either from inhibitory interneurons subserving the reciprocal inhibition in the reflexive integration or from inhibitory respiratory interneurons which control the synchronized glottic movements during vocalization and respiration.