Involvement of the opioid system in the effects induced by nicotine on anxiety-like behaviour in mice

Recent studies have revealed the participation of the endogenous opioid system in several behavioural responses induced by nicotine including antinociception, rewarding properties, and physical drug dependence.

The present study was designed to examine the possible involvement of the various opioid receptors in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice.

The acute administration of low (0.05) or high (0.8 mg/kg) doses of nicotine subcutaneously produced opposite effects in the elevated plus maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. Animals were only exposed once to nicotine. The effects of the pretreatment with the mu-opioid receptor antagonist, beta-funaltrexamine (5 mg/kg), the delta-opioid antagonist, naltrindole (2.5 mg/kg) and the kappa-opioid antagonist, nor-binaltorphimine (2.5 mg/kg) intraperitoneally were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine.

beta-funaltrexamine, but not nor-binaltorphimine or naltrindole, abolished nicotine-induced anxiolytic-like effects, suggesting an involvement of mu-opioid receptors in this behavioural response. On the other hand, naltrindole, but not nor-binaltorphimine or beta-funaltrexamine, increased the anxiogenic-like responses of nicotine, suggesting an involvement of delta-receptors in this behavioural effect.

These results demonstrate that the endogenous opioid system is involved in the effects induced by nicotine on anxiety-like behaviour and provide new findings to further clarify the interaction between these two neurochemical systems.