Abstract

Recent studies have implicated the involvement of Ca²⁺-dependent mechanisms, in particular, calcium/calmodulin-protein kinase II in nicotine-induced antinociception using the tail-flick test. The spinal cord was suggested as a possible site of this involvement. The present study was undertaken to investigate the hypothesis that the β₂ nicotinic receptor subunit plays a central role in nicotine-induced spinal antinociception via calcium/calmodulin-dependent calmodulin protein kinase II activation. The antinociceptive effects of i.t. nicotine in the tail-flick test did not significantly differ in wild-type and α₇ knockout (KO) animals but were lost in β₂ knockout mice. When calcium/calmodulin-dependent calmodulin protein kinase II activity in the lumbar spinal cord after acute i.t. administration of nicotine was investigated in wild-type and β₂ and α₇ knockout mice, the increase in calcium/calmodulin-dependent calmodulin protein kinase II activity was not significant reduced in α₇ KO mice but was eliminated in the β₂ KO mice. In addition, L-type calcium channel blockers nimodipine and verapamil but not the N-methyl-d-aspartate antagonist MK-801 (dizocilpine maleate) blocked the increase in the kinase activity induced by nicotine. Taken together, these results are consistent with the hypothesis that increases in intracellular calcium result in activation of calcium-mediated second messengers in the spinal cord that play an important role in nicotine-induced antinociception as measured in the tail-flick test. Furthermore, our findings indicate that nicotinic stimulation of β₂-containing acetylcholine nicotinic receptors in the spinal cord can activate calcium/calmodulin-dependent calmodulin protein kinase II and produce nicotinic analgesia, which may require L-type calcium voltage and gated channels but not the intervention of glutamatergic transmission.