5-ht3 Receptors Partially Mediate Halothane Depression of Spinal Dorsal Horn Sensory Neurons

We recently reported that -aminobutyric acid type Aand strychnine-sensitive glycine receptor systems partially mediate halothane depression of spinal dorsal horn low-threshold neurons. Serotonin subtype 3 (5HT3) receptors belong to the same ligand-activated ionchannel family as -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-HT3 antagonist, significantly reversed the halothaneinduced reduction in receptive field size but did not alter halothane depression of brush-induced activity. Methiothepin, a 5-HT1 antagonist, and ketanserin, a 5-HT2 antagonist, did not reverse halothane depression. These results support the hypothesis that 5-HT3 receptors partially mediate some inhibitory effects of halothane on spinal dorsal horn neurons. (Anesth Analg 2003;96:1027–31)