The cannabinoid 1 receptor antagonist AM251 produces nocifensive behavior via activation of ERK signaling pathway

Intrathecal (i.t.) injection of AM251, a cannabinoid 1 (CB₁) receptor antagonist, into the spinal lumbar space of mice elicited a behavioral response consisting of biting and licking with a few scratchings. In this study, we investigated to determine whether i.t. AM251 could influence the activity of extracellular signal-regulated kinase-1 and -2 (ERK1/2), a mitogen-activated protein kinase (MAPK) in neuronal nitric oxide synthase (nNOS) and inducible NOS (iNOS) activation. The CB₁ receptor agonist ACEA, neurokinin 1 (NK₁) receptor antagonists and NMDA receptor antagonists, inhibited i.t. AM251-induced behavioral response in a dose-dependent manner. The CB₂ receptor agonist, JWH-133 gave no effect on response elicited by i.t. AM251. Both non-selective NOS inhibitors, l-NAME and 7-NI, and N^ω-propyl-l-arginine, a selective inhibitor of nNOS resulted in a dose-dependent inhibition of i.t. AM251-induced behavioral response. The selective iNOS inhibitor, 1400W, in relatively large doses, inhibited in a non dose-dependent manner. The i.t. injection of AM251 produced a definite activation of ERK1/2 in the lumbar dorsal spinal cord. Behavioral experiments showed that U0126, a MAPK/ERK kinase (MEK) inhibitor, dose-dependently attenuated the behavioral response to i.t. AM251. Spinal activation of ERK1/2 following i.t. AM251 was reduced clearly by N^ω-propyl-l-arginine and U0126, while 1400W gave a significant effect on only ERK1 activation. These findings suggest that the nNOS-ERK pathway in spinal cord neurones plays an important role in AM251-induced nocifensive behavior and its inhibition may provide significant anti-nociception.