title: Tramadol and its metabolite m1 selectively suppress transient receptor potential ankyrin 1 activity, but not transient receptor potential vanilloid 1 activity.
abstract: BACKGROUND
  The transient receptor potential vanilloid 1 (TRPV1) and the transient receptor potential ankyrin 1 (TRPA1), which are expressed in sensory neurons, are polymodal nonselective cation channels that sense noxious stimuli. Recent reports showed that these channels play important roles in inflammatory, neuropathic, or cancer pain, suggesting that they may serve as attractive analgesic pharmacological targets. Tramadol is an effective analgesic that is widely used in clinical practice. Reportedly, tramadol and its metabolite (M1) bind to μ-opioid receptors and/or inhibit reuptake of monoamines in the central nervous system, resulting in the activation of the descending inhibitory system. However, the fundamental mechanisms of tramadol in pain control remain unclear. TRPV1 and TRPA1 may be targets of tramadol; however, they have not been studied extensively.
  
  
  METHODS
  We examined whether and how tramadol and M1 act on human embryonic kidney 293 (HEK293) cells expressing human TRPV1 (hTRPV1) or hTRPA1 by using a Ca imaging assay and whole-cell patch-clamp recording.
  
  
  RESULTS
  Tramadol and M1 (0.01-10 μM) alone did not increase in intracellular Ca concentration ([Ca]i) in HEK293 cells expressing hTRPV1 or hTRPA1 compared with capsaicin (a TRPV1 agonist) or the allyl isothiocyanate (AITC, a TRPA1 agonist), respectively. Furthermore, in HEK293 cells expressing hTRPV1, pretreatment with tramadol or M1 for 5 minutes did not change the increase in [Ca]i induced by capsaicin. Conversely, pretreatment with tramadol (0.1-10 μM) and M1 (1-10 μM) significantly suppressed the AITC-induced [Ca]i increases in HEK293 cells expressing hTRPA1. In addition, the patch-clamp study showed that pretreatment with tramadol and M1 (10 μM) decreased the inward currents induced by AITC.
  
  
  CONCLUSIONS
  These data indicate that tramadol and M1 selectively inhibit the function of hTRPA1, but not that of hTRPV1, and that hTRPA1 may play a role in the analgesic effects of these compounds.
authors: From the *Division of Cancer Pathophysiology, National Cancer Center Research Institute, Chuo-ku, Tokyo, Japan; †Department of Anesthesiology and Critical Care Medicine, Jichi Medical University, Shimotsuke, Tochigi, Japan; ‡Tsumura Research Labs, Tumura & Co., Inashiki-gun, Ibaraki, Japan; §Division of Biostatistics, Tohoku University Graduate School of Medicine, Clinical Research Data Center, Tohoku University Hospital, Sendai, Miyagi, Japan; ∥Department of Palliative Medicine, Seirei Sakura Citizen Hospital, Sakura-shi, Chiba, Japan; and ¶Department of Palliative Medicine, Aomori Prefectural Central Hospital, Aomori-city, Aomori, Japan.
authors: Kanako
authors: K
authors: Miyano
authors: Kouichiro
authors: K
authors: Minami
authors: Toru
authors: T
authors: Yokoyama
authors: Katsuya
authors: K
authors: Ohbuchi
authors: Takuhiro
authors: T
authors: Yamaguchi
authors: Satoshi
authors: S
authors: Murakami
authors: Seiji
authors: S
authors: Shiraishi
authors: Masahiro
authors: M
authors: Yamamoto
authors: Motohiro
authors: M
authors: Matoba
authors: Yasuhito
authors: Y
authors: Uezono
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D002118:Calcium / Q000737:chemistry; D015220:Calcium Channels; D002211:Capsaicin / Q000737:chemistry; D055724:Electrophysiological Phenomena; D057809:HEK293 Cells; D006801:Humans; D007249:Inflammation; D017879:Isothiocyanates / Q000737:chemistry; D008564:Membrane Potentials; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors*; D018408:Patch-Clamp Techniques; D017450:Receptors, Opioid, mu / Q000378:metabolism; D000074025:TRPA1 Cation Channel; D050916:TRPV Cation Channels / Q000037:antagonists & inhibitors*; D014147:Tramadol / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology*; D050051:Transient Receptor Potential Channels / Q000037:antagonists & inhibitors*
chemical_list: D000701:Analgesics, Opioid; D015220:Calcium Channels; D017879:Isothiocyanates; D009419:Nerve Tissue Proteins; D017450:Receptors, Opioid, mu; D000074025:TRPA1 Cation Channel; C473350:TRPA1 protein, human; D050916:TRPV Cation Channels; C494482:TRPV1 protein, human; D050051:Transient Receptor Potential Channels; C080580:O-demethyltramadol; D014147:Tramadol; C004471:allyl isothiocyanate; D002211:Capsaicin; D002118:Calcium