α4α6β2* Nicotinic Acetylcholine Receptor Activation on Ventral Tegmental Area Dopamine Neurons Is Sufficient to Stimulate a Depolarizing Conductance and Enhance Surface AMPA Receptor Function

Abstract

Tobacco addiction is a serious threat to public health in the United States and abroad, and development of new therapeutic approaches is a major priority. Nicotine activates and/or desensitizes nicotinic acetylcholine receptors (nAChRs) throughout the brain. nAChRs in ventral tegmental area (VTA) dopamine (DA) neurons are crucial for the rewarding and reinforcing properties of nicotine in rodents, suggesting that they may be key mediators of nicotine's action in humans. However, it is unknown which nAChR subtypes are sufficient to activate these neurons. To test the hypothesis that nAChRs containing a6 subunits are sufficient to activate VTA DA neurons, we studied mice expressing hypersensitive, gain-offunction a6 nAChRs (a6L99S mice). In voltage-clamp recordings in brain slices from adult mice, 100 nM nicotine was sufficient to elicit inward currents in VTA DA neurons via a6b2* nAChRs. In addition, we found that low concentrations of nicotine could act selectively through a6b2* nAChRs to enhance the function of 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) receptors on the surface of these cells. In contrast, a6b2* activation did not enhance N-methyl-D-aspartic acid receptor function. Finally, AMPA receptor (AMPAR) function was not similarly enhanced in brain slices from a6L99S mice lacking a4 nAChR subunits, suggesting that a4a6b2* nAChRs are important for enhancing AMPAR function in VTA DA neurons. Together, these data suggest that activation of a4a6b2* nAChRs in VTA DA neurons is sufficient to support the initiation of cellular changes that play a role in addiction to nicotine. a4a6b2* nAChRs may be a promising target for future smoking cessation pharmacotherapy.

Differential Expression and Function of Nicotinic Acetylcholine Receptors in Subdivisions of Medial Habenula

“…␣4 subunits are exclusively found in the lateral portion of the MHbV (referred to as MHbVL) (Fig. 3C), consistent with previous reports Fonck et al, 2009). In contrast to ␣4, ␣6 subunits are mainly found in the inferior portion of MHbV (MHbVI), with additional faint expression in the lateral portion of MHbD (Fig.…”

“…Thus, we reasoned that some IPN neurons likely express and use AMPAtype glutamate receptors (AMPARs). To study AMPARs, we puff-applied AMPA to IPR/IPDM neurons held in voltage-clamp mode similar to our previous study (Engle et al, 2013). As with action potential firing, we found two types of responses to AMPA that cosegregated with action potential firing in response to positive current injection.…”

“…Several rodent studies indicate a key role for ␣5-and/or ␤4-containing nAChRs in modulating nicotine intake. For example, ␣5 KO mice consume significantly more nicotine than their ␣5 WT counterparts, and reexpression of ␣5 subunits in MHb of ␣5 KO mice restores aversion to high doses of nicotine (Fowler et al, 2011). In a complementary study, Frahm et al (2011) found that overexpression of ␤4-containing nAChRs in MHb enhances aversion to, and consequently reduces consumption of, nicotine.…”

“…Functional AMPAR and nAChR responses were examined as previously described (Engle et al, , 2013. Briefly, a drugfilled glass pipette, identical to a typical recording pipette, was connected to a micropressure ejection system (PV-820 Pneumatic PicoPump; World Precision Instruments).…”

“…Robin Lester and colleagues identified ␣3␤4* (* ϭ subunits in addition to those listed may be found in the indicated pentameric receptor) nAChRs as the predominant subtype in MHbV, with a minor contribution from ␤2* nAChRs in some cells (Quick et al, 1999). Studies in mice expressing gain-offunction ␣4 nAChR subunits demonstrated expression and function of ␣4* nAChRs in lateral MHbV neurons Fonck et al, 2009).…”