Nitrous oxide activates GABAergic neurons in the spinal cord in Fischer rats

Abstract

Background: Findings to date indicate that nitrous oxide exerts its antinociceptive effect by activating descending noradrenergic neurons. The mechanism whereby descending inhibitory neurons, including noradrenergic neurons, produce antinociceptive effect remains unclear. Using c-Fos protein as a marker for neuronal activation, we examined whether spinal cord neurons activated by nitrous oxide are γ-aminobutyric acid - mediated (GABAergic) neurons. Methods: Adult male Fischer (a strain in which nitrous oxide shows strong antinociceptive properties) and Lewis (a strain in which nitrous oxide lacks antinociceptive properties) rats were exposed to either air (control) or nitrous oxide. Frozen sections of the spinal cord were either stained for c-Fos or double-stained for c-Fos and glutamic acid decarboxylase (a rate-limiting enzyme for GABA synthesis) and analyzed by standard or confocal microscopy. Results: In Fischer rats, 90 min of 75% N2O administration increased the number of c-Fos - positive cells in the spinal cord approximately threefold as compared with the control group. The c-Fos - positive cells induced by nitrous oxide were almost entirely colocalized with glutamic acid decarboxylase - positive cells. In contrast, exposure did not change the number of c-Fos - positive cells in the spinal cord in Lewis rats. Conclusions: Exposure to nitrous oxide activates GABAergic neurons in the spinal cord. The dose-dependence of GABAergic neuronal activation in the Fischer rats and its absence in the Lewis rat correlate with antinociceptive responses previously reported in these same circumstances. Together, we interpret these data to indicate that activation of GABAergic neurons in the spinal cord are involved in the antinociceptive action of nitrous oxide.