Effects of systemic nicotine on serotonin release in rat brain

We used in vivo microdialysis to examine the acute effects of systemically administered nicotine (0.8-8.0 mg/kg, s.c.) on extracellular levels of serotonin (5-HT) in the frontal cortex of awake rats and animals anesthetized with chloralose/urethane. In anesthetized animals, 5-HT efflux was elevated during the initial 15 min after nicotine administration (2-8 mg/kg), but then returned to baseline values. All of the effective nicotine doses also lowered and then raised blood pressure in these animals. However, other drugs which raised (methoxamine, 0.07 mg/kg, i.v.) or lowered (mecamylamine, 5 mg/kg, i.p.) blood pressure without directly activating nicotinic receptors failed to alter 5-HT release. Moreover, pretreatment with a centrally active dose of mecamylamine, a known nicotinic antagonist, blocked the effects of nicotine (4 mg/kg) on 5-HT release. For studies on awake rats the perfusion fluid also contained fluoxetine, since basal 5-HT levels were barely detectable without this uptake blocker. In such animals, 1.6 mg/kg of nicotine significantly increased 5-HT release, an effect apparent in the initial 20 min after treatment and persisting for at least 2 h. These observations demonstrate that systemically administered nicotine increases frontocortical 5-HT release, that this effect is independent of the cardiovascular responses to the drug, and that it probably results from the activation of previously described nicotinic receptors on raphe neurons. The present findings are consistent with the hypothesis that the appetitive and mood disturbances associated with nicotine withdrawal may be mediated by diminished serotoninergic transmission.