Abstract
The centrally acting muscle relaxatn cyclobenzaprine was thought to be an α 2-adrenoceptor agoinst that reduced muscle tone by decreasing the activity of descending noradrenergic neurons. In the present study, we examined the effects of cyclobenzaprine on descending neurons by measuring the monosynaptic reflex in rats. Cyclobenzaprine reduced the monosynaptic reflex amplitude dose dependently and this effect was not inhibited by the α 2-adrenoceptor antagonists idazoxan and yohimbine. Cyclobenzaprine-induced monosynaptic reflex depression was not attenuated by noradrenergic neuronal lesions produced by 6-hydroxydopamine. However, cyclobenzaprine inhibited monosynaptic reflex facilitation induced by (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane, a 5-HT 2 receptor agonist, in spinalized rats markedly, and 5-HT depletion by dl- p-chlorophenylalanine inhibited the depressive effect of cyclobenzaprine on the monosynaptic reflex. These results suggests that cyclobenzaprine is a 5-HT 2 receptor antagonist and that its muscle relaxant effect is due to inhibition of serotonergic, not noradrenergic, descending systems in the spinal cord.
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