5-HT3 receptors partially mediate halothane depression of spinal dorsal horn sensory neurons.

We recently reported that gamma-aminobutyric acid type A- and strychnine-sensitive glycine receptor systems partially mediate halothane depression of spinal dorsal horn low-threshold neurons. Serotonin subtype 3 (5-HT(3)) receptors belong to the same ligand-activated ion-channel family as gamma-aminobutyric acid type A- and strychnine-sensitive glycine receptors, so we examined the possible involvement of 5-HT receptor systems in halothane depression of spinal sensory neurons. Extracellular recordings of spinal low-threshold neurons were obtained in decerebrate, spinally transected rats. Receptive field size and brush-induced activity were recorded in the presence or absence of 5-HT antagonists and in the presence or absence of 1.1% (1 minimum alveolar anesthetic concentration) halothane. In the absence of halothane, antagonists had no effect on receptive field size or brush-induced activity. In the presence of halothane, methysergide, a nonselective 5-HT antagonist, and tropisetron, a selective 5-HT(3) antagonist, significantly reversed the halothane-induced reduction in receptive field size but did not alter halothane depression of brush-induced activity. Methiothepin, a 5-HT(1) antagonist, and ketanserin, a 5-HT(2) antagonist, did not reverse halothane depression. These results support the hypothesis that 5-HT(3) receptors partially mediate some inhibitory effects of halothane on spinal dorsal horn neurons.

Conclusions: The results of this study support the hypothesis that halothane depression of spinal sensory neuronal responses to low-intensity stimuli is mediated, to a minor extent, by serotonin subtype 3 neurotransmitter systems.