Chronic Nicotine Selectively Enhances α4β2*Nicotinic Acetylcholine Receptors in the Nigrostriatal Dopamine Pathway

These electrophysiological experiments, in slices and intact animals, study the effects of in vivo chronic exposure to nicotine on functional alpha 4 beta 2* nAChRs in the nigrostriatal dopaminergic (DA) pathway. Recordings were made in wild-type and alpha 4 nicotinic acetylcholine receptor (nAChR) subunit knock-out mice. Chronic nicotine enhanced methyllycaconitine citrate hydrate-resistant, dihydro-beta-erythroidine hydrobromide-sensitive nicotinic currents elicited by 3-1000 mu M ACh in GABAergic neurons of the substantia nigra pars reticulata (SNr), but not in DA neurons of the substantia nigra pars compacta (SNc). This enhancement leads to higher firing rates of SNr GABAergic neurons and consequently to increased GABAergic inhibition of the SNc DA neurons. In the dorsal striatum, functional alpha 4* nAChRs were not found on the neuronal somata; however, nicotine acts via alpha 4 beta 2* nAChRs in the DA terminals to modulate glutamate release onto the medium spiny neurons. Chronic nicotine also increased the number and/or function of these alpha 4 beta 2* nAChRs. These data suggest that in nigrostriatal DA pathway, chronic nicotine enhancement of alpha 4 beta 2* nAChRs displays selectivity in cell type and in nAChR subtype as well as in cellular compartment. These selective events augment inhibition of SNc DA neurons by SNr GABAergic neurons and also temper the release of glutamate in the dorsal striatum. The effects may reduce the risk of excitotoxicity in SNc DA neurons and may also counteract the increased effectiveness of corticostriatal glutamatergic inputs during degeneration of the DA system. These processes may contribute to the inverse correlation between tobacco use and Parkinson's disease.