Expression of neuronal nicotinic acetylcholine receptor subunit mRNAs within midbrain dopamine neurons

Many behavioral effects of nicotine result from activation of nigrostriatal and mesolimbic dopaminergic systems. Nicotine regulates dopamine release not only by stimulation of nicotinic acetylcholine receptors (nAChRs) on dopamine cell bodies within the substantia nigra and ventral tegmental area (SN/VTA), but also on presynaptic nAChRs located on striatal terminals. The nAChR subtype(s) present on both cell bodies and terminals is still a matter of controversy. The purpose of this study was to use double-labeling in situ hybridization to identify nAChR subunit mRNAs expressed within dopamine neurons of the SN/VTA, by using a digoxigenin-labeled riboprobe for tyrosine hydroxylase as the dopamine cell marker and (35)S-labeled riboprobes for nAChR subunits. The results reveal a heterogeneous population of nAChR subunit mRNAs within midbrain dopamine neurons. Within the SN, almost all dopamine neurons express alpha2, alpha4, alpha5, alpha6, beta2, and beta3 nAChR mRNAs, with more than half also expressing alpha3 and alpha7 mRNAs. In contrast, less than 10% express beta4 mRNA. Within the VTA, a similar pattern of nAChR subunit mRNA expression is observed except that most subunits are expressed in a slightly lower percentage of dopamine neurons than in the SN. Within the SN, alpha4, beta2, alpha7, and beta4 mRNAs are also expressed in a significant number of nondopaminergic neurons, whereas within the VTA this only occurs for beta4. The heterogeneity in the expression of nAChR subunits within the SN/VTA may indicate the formation of a variety of different nAChR subtypes on cell bodies and terminals of the nigrostriatal and mesolimbic pathways.