Papers of the Week

Dexmedetomidine, a highly selective alpha-2 adrenergic receptor agonist and novel sedative drug with minimal respiratory suppression, have shown anti-nociceptive activity in various pain models by poorly understood mechanisms. Because alpha-2 adrenergic receptor is co-localized with TRPV1 polymodal nociceptive receptor in dorsal root ganglion neurons and up-regulated in neuropathic pain animal models, the analgesic activity might be mediated through inhibition of TRPV1 in the peripheral nervous system. In an effort to elucidate whether modulatory effect of dexmedetomidine on TRPV1 activity could be the potential peripheral mechanism underlying the antinociceptive effect of dexmedetomidine, intracellular calcium concentration after capsaicin application was investigated in mice dorsal root ganglion (DRG) neurons, with and without pretreatment of dexmedetomidine. Dexmedetomidine (10 μM) reduced capsaicin-induced calcium responses by 29.7 ± 7.39% (n = 34, p < 0.0001), in dose-dependent manner. Higher level of inhibition was observed with increased dose of dexmedetomidine (50 μM, 45.1 ± 8.58%, n = 15, p = 0.0002), and lower inhibition by decreased dose (1 μM, 18.8 ± 1.48%, n = 148, p = 0.004). RT-PCR analysis revealed expression of TRPV1 and alpha-2A, alpha-2B and alpha-2C subtypes of adrenergic receptor in mice DRG neurons, and immunocytochemical analysis revealed co-expression of TRPV1 and alpha-2A receptors in primary cultured DRG neurons. In summary, these results suggested the inhibition of TRPV1 expressed in the primary sensory neurons as a potential mechanism that contributes to the anti-nociceptive action of dexmedetomidine.