Nicotine and morphine differentially activate brain dopamine in prefrontocortical and subcortical terminal fields: effects of acute and repeated injections.

Acute systemic injections of nicotine and morphine produce increased locomotion and repeating these injections produces an enhanced locomotor effect. Different lines of evidence suggest that both the acute and sensitized locomotion elicited by morphine are linked to activity in the mesolimbic dopamine (DA) system. The present experiments assessed the effect of acute and repeated injections of nicotine on DA utilization [3,4-dihydroxyphenylacetic acid (DOPAC)/DA] in subcortical and prefrontocortical DA terminal fields and compared these to the effects of morphine. Acute nicotine produced substantial increases in the DOPAC/DA ratio in the nucleus accumbens (N. Acc.) and the antero-medial striatum but not in the dorso-lateral striatum or the medial prefrontal cortex (mPFC). Repeated injections greatly reduced or abolished the drug's effects in the first two terminal fields and produced an increase in DA utilization in the mPFC. In contrast, acute morphine produced large increases in the DOPAC/DA ratio in both the N.Acc. and the mPFC. Consistent with previous findings, repeated morphine increased its effects further in the N. Acc. and produced a small decrease in the mPFC. Neither nicotine nor morphine produced changes in baseline DA utilization in any site. Finally, repeated exposure to nicotine did not enhance the locomotor response to a subsequent injection of morphine. These findings suggest that nicotine differs from morphine in the way it elicits locomotion following repeated injection and possibly in its relation to DA as a mediator of reward. They are also consistent with evidence for a functional interaction between the effects of DA in subcortical and prefrontocortical sites.