Nicotinic effects on midbrain dopamine neurons : a dual mechanism of action

Abstract: The neurons of the mesocorticolimbic dopamine system are located in the ventral tegmental area (VTA) in the mesencephalon and send their projections to forebrain structures, such as the prefrontal cortex, the amygdala and the nucleus accumbens. The dopamine system is important in regulating mood and motivation as well as cognitive function and appear to be primarily involved in drug dependence, since most drugs abused by man stimulate this system. Nicotine stimulates dopamine neurons both by increasing their single spike activity and burst activity. Burst activity is characterized by action potentials fired rapidly in short episodes with longer intervals in between and is particularly effective in releasing dopamine in the terminal areas and increasing the postsynatic activation of the protoonco gene c-fos. Nicotine-induced dopamine release in the nucleus accumbens seems to be more or less exclusively dependent on the neuronal activation elicited via stimulation of nicotinic acetylcholine receptors (nAChRs) in the VTA. Burst firing of dopamine neurons is controlled by excitatory amino acids (EAAs) acting at NMDA receptors in the VTA. In many parts of brain, nicotine, instead of exciting neurons directly, as first thought, has been shown to act preferentially indirectly, i.e. by facilitating the release of EAAs. Recent evidence also suggests that nAChRs of the [alpha]7' subtype may be involved in presynaptic facilitation of EAA release. Therefore, the specific aim of the present project has been to analyze the role of EAAs in the stimulatory effects of nicotine on the mesocorticolimbic dopamine system and, in particular, the role of [alpha]7' nAChRs in this regard. Using microdialysis we found that the effect of nicotine (0.5 mg/kg s.c. free base) on dopamine release in the nucleus accumbens was attenuated by concomitant infusion in the VTA of the NMDA receptor antagonist AP-5, but not the AMPA/kainate receptor antagonist CNQX Nicotine also increased the levels of EAAs in the VTA an effect that was prevented by concomitant infusion of the 0 selective nAChR antagonist methyllycaconitine (MLA). Local infusion of MLA in the VTA also blocked nicotine-induced dopamine release in the nucleus accumbens. These data indicate that nicotine may act presynaptically at [alpha]7' nAChRs to stimulate the release of EAAs. Presynaptic [alpha]7' nAChRs may reside on [alpha]7' originating in the prefrontal cortex, since they provide an important source of glutamatergic [alpha]7' for dopamine neurons in the VTA. Therefore, excitotoxic lesions were made in the prefrontal cortex and autoradiography binding with nAChR ligands was performed on sections of the VTA. [alpha]-bungarotoxin binding, which represents binding to 0 nAChRs, was reduced by 30% in lesioned animals compared to controls, providing anatomical support for our notion. The postsynaptic consequences in the nucleus accumbens and medial prefrontal cortex of the corresponding pharmacological manipulations within the VTA was measured with Fos-immunohistochemistry. Nicotine increased Fos-expression in the nucleus accumbens and the medial prefrontal cortex. nAChR and NMDA receptor blockade in the VTA by mecamylamine or AP-5 prevented the effect of systemic nicotine on Fosexpression in the nucleus accumbens but not in the medial prefrontal cortex. MLA (6.0 mg/kg i.p.) antagonized the effect of nicotine on Fos expression both in the nucleus accumbens and the prefrontal cortex. MLA also inhibited nicotine-induced burst firing in dopamine neurons without affecting nicotine's effect on firing rate. In contrast the [alpha]4[beta]2 selective antagonist dihydro-[beta]-erythroidine [DH[beta]E, 1.0 mg/kg s.c.) prevented nicotine's effect on firing rate but did nor antagonize the nicotine-induced increase in burst firing. Accordingly, DH[beta]E did not affect the nicotine-induced Fos-expression in the nucleus accumbens and the prefrontal cortex. Also, we found that an 0 agonist (AR-R-17779) increased burst firing in dopamine neurons without affecting single spike activity, whereas an [alpha]4[beta]2 agonist (A-85380) increased single spike firing without affecting burst activity. These data suggest that [alpha]7' nAChRs are primarily involved in the burst firing enhancing effects of nicotine, whereas [alpha]4[beta]2 nAChRs appear to be primarily responsible for the nicotine-induced increase in single spike firing. Consequently, nicotine may activate dopamine neurons by two separate but convergent mechanisms. These mechanisms are both likely to contribute to different aspects of nicotine dependence. Selective ligands for nAChRs subtypes may be less dependence producing and might, therefore be used therapeutically since nicotine exerts several potentially beneficial effects on a number of CNS functions.