title: Halothane enhances gamma-aminobutyric acid receptor type A function but does not change overall inhibition in inspiratory premotor neurons in a decerebrate dog model.
abstract: BACKGROUND
  Inspiratory premotor neurons in the caudal ventral medulla relay excitatory drive to phrenic and inspiratory intercostal motoneurons in the spinal cord. These neurons are subject to tonic gamma-aminobutyric acid type A (GABA(A))-mediated (GABA(A)ergic) inhibition. In a previous study, 1 minimum alveolar concentration (MAC) halothane depressed overall glutamatergic excitatory drive but did not change overall inhibitory drive to the neurons. This study investigated in further detail the effects of halothane on GABA(A)ergic inhibition by examining postsynaptic GABA(A) receptor activity in these neurons.
  
  
  METHODS
  Studies were performed in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 MAC halothane on extracellularly recorded neuronal activity was measured during localized picoejection of the GABA(A) receptor antagonist bicuculline and the GABA(A) agonist muscimol. Complete blockade of GABAergic inhibition by bicuculline allowed estimation of the prevailing overall inhibition of the neuron. The neuronal response to muscimol was used to assess the anesthetic effect on the postsynaptic GABA(A) receptor function.
  
  
  RESULTS
  One minimum alveolar concentration halothane depressed the spontaneous activity of 19 inspiratory premotor neurons by 22.9 +/- 29.1% (mean +/- SD; P < 0.01). Overall excitatory drive was depressed 23.6 +/- 16.9% (P < 0.001). Overall GABAergic inhibition was not changed (+8.7 +/- 27.5%; P = 0.295), but the postsynaptic GABA(A) receptor function was increased by 110.3 +/- 97.5% (P < 0.001).
  
  
  CONCLUSIONS
  One minimum alveolar concentration halothane greatly enhanced GABA(A) receptor function on inspiratory premotor neurons but did not change overall synaptic inhibition, indicating that the presynaptic inhibitory input was reduced. Therefore, the anesthetic depression of spontaneous inspiratory premotor neuronal activity in the intact brainstem respiratory network is mainly due to a decrease in overall glutamatergic excitation.
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, USA.
authors.forename: Astrid G
authors.initials: AG
authors.lastname: Stucke
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Zuperku
authors.forename: Viseslav
authors.initials: V
authors.lastname: Tonkovic-Capin
authors.forename: Mirko
authors.initials: M
authors.lastname: Krolo
authors.forename: Francis A
authors.initials: FA
authors.lastname: Hopp
authors.forename: John P
authors.initials: JP
authors.lastname: Kampine
authors.forename: Eckehard A E
authors.initials: EA
authors.lastname: Stuth
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D003655:Decerebrate State; D004285:Dogs; D006221:Halothane / Q000494:pharmacology*; D008526:Medulla Oblongata / Q000187:drug effects* / Q000502:physiology; D009046:Motor Neurons / Q000187:drug effects*; D015221:Potassium Channels / Q000502:physiology; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000502:physiology; D012125:Respiratory Center / Q000187:drug effects* / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects
chemical_list: D018685:Anesthetics, Inhalation; D015221:Potassium Channels; D011963:Receptors, GABA-A; D006221:Halothane