Facilitation of intracranial self-stimulation behavior in rats by environmental stimuli associated with nicotine.

Environmental stimuli associated with nicotine act as a trigger for nicotine-seeking behavior and make it difficult to quit smoking. The trigger action might be related to the activity of the mesolimbic dopamine "reward" system. Thus, in the present study, we investigated the effects of nicotine-associated stimuli on reward seeking, assessed by current intensity thresholds of intracranial self-stimulation (ICSS) in rats. Rats were unilaterally implanted with an electrode into the lateral hypothalamus and trained to press a lever to obtain electrical brain stimulation. After stable responses had been established, the rats underwent six conditioning sessions with subcutaneous nicotine at 0.4 mg/kg in particular visual and tactile contexts. The threshold for electrical stimulation was then tested in either the nicotine-conditioned environment or novel environment under saline or nicotine treatment. Nicotine enhanced the responding for ICSS and significantly lowered the threshold in both environments. Under saline treatment, the responding for ICSS was facilitated and the threshold was significantly lower in the nicotine-conditioned environment than in the novel environment. The present results demonstrate that nicotine-conditioned contextual stimuli may affect the mesolimbic dopamine system through ICSS threshold lowering effect.