Selective inhibition by nicotine of shock-induced fighting in the rat.

The frequency of shock-induced fighting, posturing and "no reaction" (running/jumping or freezing), after acute SC injections of 0, 0.1, 0.2 and 0.4 mg/kg nicotine, was measured in 38 pairs of male and 12 pairs of female rats. Sensitivity to footshock was also measured, at the same nicotine doses, in males. Nicotine inhibited shock-induced fighting in 32 pairs of high-frequency fighting males in a dose-dependent fashion, with fighting being gradually replaced by posturing at the 0.1 and 0.2 mg/kg doses. There was a significant increase in the "no reaction" category (especially freezing behavior) only at 0.4 mg/kg, indicating that reduced activity may have been partly responsible for the reduction in fighting seen at that dose. None of the doses had a significant effect on sensitivity to footshock. Nicotine had no effect on shock-induced fighting in the six low-frequency fighting male pairs, and affected the female pairs only at the 0.4 mg/kg level, where fighting was also decreased due to an increase in the "no reaction" category. It could be concluded that small doses of nicotine, under favorable conditions, were capable of inhibiting shock-induced fighting in rats without altering shock sensitivity or depressing activity.