Tolperisone is a centrally acting muscle relaxant drug, which inhibits segmental spinal reflexes via a blockade of voltage gated sodium and calcium conductances (Kocsis et al. 2005). The rat hemisected spinal cord model was used for characterizing spinal reflex inhibitory effects of drugs in vitro. Spinal cords were isolated from 6-day-old rats anesthetized with ether and cooled down by crushed ice until their respiration stopped. After removal, the spinal cord was hemisected along the midline, and transferred to a recording chamber. Tolperisone inhibited the monosynaptic component of the dorsal root stimulation-evoked ventral root reflex in this preparation with an IC₅₀ value of 52±8 μM. In the presence of the antitussive agent dextromethorphan (0.25 μM) tolperisone was significantly more potent (IC₅₀=24±2 μM; p<0.05) than alone. Further, we analyzed the possible mechanisms underlying the potentiation of the pharmacological actions of tolperisone by dextromethorphan. Dextromethorphan has multiple pharmacological actions; among others it inhibits serotonin transporters. In a binding study on membranes isolated from rat brain, dextromethorphan displaced [³H]sertraline with a K_i of 26 nM. In a recent study, we found that several selective serotonin reuptake inhibitors, such as paroxetine or fluoxetine, enhanced the sodium channel inhibitory action of a variety of blockers, among them tolperisone, via involvement of 5-HT2 receptors and PKC (Thán et al. 2007). To investigate if similar mechanisms play a role in the action of dextromethorphan, we investigated this interaction in the presence of the 5-HT2 receptor antagonist ketanserin (1 μM). The enhancement of the reflex inhibitory effect of tolperisone by dextromethorphan, as well as by paroxetine and fluoxetine, were prevented by ketanserin, suggesting similar mechanisms. In whole cell patch clamp recordings on dorsal root ganglion cells, tolperisone inhibited tetrodotoxin-sensitive voltage gated sodium currents concentration-dependently (IC₅₀=193 μM). Activation of 5-HT2 receptors with the receptor agonist DOI (10 μM) enhanced the effect of tolperisone (IC₅₀=80 μM). This positive interaction was prevented by blockade of protein kinase C (PKC) with staurosporine (1 μM; intracellular application), suggesting the involvement of 5-HT2 receptor mediated PKC dependent phosphorylation of sodium channels. In summary, an increased serotonin tone due to inhibition of the transporter, followed by activation of 5-HT2 receptors and PKC seem to underlie the modulation of tolperisone’s action by dextromethorphan.