Acupuncture Research Today is a free monthly online journal that collates and summarizes the latest research about Acupuncture, including details on alternative medicine, uses, benefits, treatment, chinese medicine.

Involvement of serotonin 2A receptors in the analgesic effect of tramadol in mono-arthritic rats.

Xie H, Dong ZQ, Ma F, Bauer WR, Wang X, Wu GC

Department of Integrative Medicine, Institute of Acupuncture Research, Shanghai Medical College of Fudan University, Shanghai 200032, China; Department of Anesthesiology, Tianjin Medical University, Tianjin 300070, China.

The analgesic effects of tramadol are considered to be mediated by both the opioid system and the serotonergic system. This study investigated the involvement of a subtype of serotonin receptors, 5-hydroxytryptamine (5-HT)(2A) receptor, in the analgesic effect of tramadol. The intraperitoneal (i.p.) injection of tramadol reduced the paw withdrawal latency (PWL) to radiant heat testing in mono-arthritic rats. The antagonistic effect of i.p. ketanserin (a 5-HT(2A) receptor antagonist) on tramadol analgesia was observed. The expression of the 5-HT(2A) receptor mRNA in the nucleus of raphe magnus (NRM), ventrolateral periaqueductal gray (vlPAG) and spinal dorsal horn of mono-arthritic rats after a ten-day treatment with tramadol was measured with in situ hybridization. Either single injections or 10 days of tramadol treatment dose-dependently elevated PWL of arthritic rats while ketanserin could partially antagonize the tramadol analgesic effect. Expression of the 5-HT(2A) receptor mRNA in NRM, ipsilateral vlPAG, and the ipsilateral spinal dorsal horn of arthritic rats was significantly increased after tramadol treatment. These results suggest that 5-HT(2A) receptors are involved in the analgesic effect of tramadol. This study provides evidence for involvement of 5-HT(2A) receptors in the tramadol analgesia of inflammatory pain. The increase in this receptor mRNA in the chronic study may contribute to the sustaining effect of tramadol long-term treatments in clinical practice.

Published 12 May 2008 in Brain Res, 1210: 76-83.
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