Effects of halothane and sevoflurane on inhibitory neurotransmission to medullary expiratory neurons in a decerebrate dog model

Background: In canine expiratory bulbospinal neurons, I minimum alveolar concentration (MAC) halothane and sevoflurane reduced the glutamatergic excitatory drive at a presynaptic site and enhanced the overall gamma-aminobutyric acid (GABA)-mediated inhibitory input. The authors investigated if this inhibitory enhancement was mainly caused by postsynaptic effects. Methods: Two separate anesthetic studies were performed in two sets of decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of I MAC halothane or sevoflurane on extracellularly recorded neuronal activity was measured during localized picoejection of the GABA(A) receptor agonist muscimol and the GABA(A) receptor antagonist bicuculline. Complete blockade of GABA(A)-mediated inhibition with bicuculline was used to assess the prevailing overall inhibitory input to the neuron. The neuronal response to muscimol was used to estimate the anesthetic effect on postsynaptic GABA(A) receptor function. Results: Halothane at I MAC depressed the spontaneous activity of 12 expiratory neurons 22.2 +/- 14.8% (mean +/- SD) and overall glutamatergic excitation 14.5 +/- 17.9%. Overall GABA-mediated Inhibition was enhanced 14.1 +/- 17.9% and postsynaptic GABA(A) receptor function 74.2 +/- 69.2%. Sevoflurane at I MAC depressed the spontaneous activity of 23 neurons 20.6 +/- 19.3% and overall excitation 10.6 +/- 21.7%. Overall inhibition was enhanced 15.4 +/- 34.0% and postsynaptic GABA(A) receptor function 65.0 +/- 70.9%. The effects of halothane and sevoflurane were not statistically different. Conclusion: Halothane and sevoflurane at I MAC produced a small increase in overall inhibition of expiratory premotor neuronal activity. The increase in inhibition results from a marked enhancement of postsynaptic GABA(A) receptor function that is partially offset by a reduction in presynaptic inhibitory input by the anesthetics.