Cellular effects of isoflurane on bulbar respiratory neurons in decerebrate cats

Effects of isoflurane on the membrane potential trajectory and synaptic activity in bulbar respiratory neurons were investigated in decerebrate, vagotomized and artificially ventilated cats. A 2-min inhalation of 1.6% end-tidal concentration of isoflurane produced depolarization of the membrane in 10 out of 18 inspiratory, 8 out of 15 post-inspiratory and 5 out of 12 expiratory neurons and hyperpolarization in the rest of the population recorded in the ventral respiratory group. In both depolarized and hyperpolarized cells, periodically occurring excitatory and inhibitory synaptic waves were decreased, and input resistance was increased. Concomitantly, isoflurane reduced the excitatory and inhibitory postsynaptic potentials evoked by electrical stimulation of the vagus nerve, superior laryngeal nerve and cervical spinal cord. The effects of isoflurane on membrane potential and input resistance became negligible when excitatory and inhibitory synaptic potentials were suppressed by iontophoretically applied tetrodotoxin. The present results suggest that the respiratory neuronal responses induced by isoflurane are attributed mainly to the decrease of excitatory and inhibitory synaptic interactions in the bulbar respiratory network of neurons.