Nicotine-Mediated Activation of Dopaminergic Neurons in Distinct Regions of the Ventral Tegmental Area

Abstract

Nicotine activation of nicotinic acetylcholine receptors (nAChRs) within the dopaminergic (DAergic) neuron-rich ventral tegmental area (VTA) is necessary and sufficient for nicotine reinforcement. Here we show that rewarding doses of nicotine activated VTA DAergic neurons in a region-selective manner, preferentially activating neurons in the posterior VTA (pVTA) but not the anterior VTA (aVTA) or tail VTA (tVTA). Nicotine (1µM) directly activated pVTA DAergic neurons in adult mouse midbrain slices but had little effect on DAergic neurons within the aVTA. Quantification of nAChR subunit gene expression revealed that pVTA DAergic neurons expressed higher levels of α4, α6, and β3 transcripts, compared to aVTA DAergic neurons. Activation of nAChRs containing the α4 subunit (α4* nAChRs) was necessary and sufficient for activation of pVTA DAergic neurons: Nicotine failed to activate pVTA DAergic neurons in α4 knock-out animals; in contrast pVTA α4* nAChRs were selectively activated by nicotine, in mutant mice expressing agonist hypersensitive α4* nAChRs (Leu9’Ala mice). In addition, whole-cell currents induced by nicotine in DAergic neurons were mediated by α4* nAChRs and were significantly larger in pVTA neurons compared to aVTA neurons. Infusion of an α6* nAChR antagonist into the VTA blocked activation of pVTA DAergic neurons in WT mice and Leu9’Ala mice at nicotine doses that only activate the mutant receptor indicating that α4 and α6 subunits coassemble to form functional receptors in these neurons. Thus, nicotine selectively activates DAergic neurons within the pVTA via α4α6* nAChRs. These receptors represent novel targets for smoking cessation therapies.

Nicotine Persistently Activates Ventral Tegmental Area Dopaminergic Neurons via Nicotinic Acetylcholine Receptors Containing α4 and α6 Subunits

“…1B), and expression of tyrosine hydroxylase (Fig. 1C), as described previously (Zhao-Shea et al, 2011). In addition, action potentials were recorded in the presence of a cocktail of inhibitors to block muscarinic receptors, GABA A receptors, and glutamatergic signaling (see Materials and Methods) in an effort to facilitate isolation of nicotine effects directly mediated by nAChRs.…”

“…In addition, DH␤E is not truly specific for ␣4␤2 nAChRs, because it can also block ␣6* nAChRs (Drenan et al, 2008), precluding identification of functional nAChRs containing only ␣4 and ␤2 subunits. Previously, we have shown that nAChRs containing ␣4 and ␣6 subunits are critical for activation of DAergic neurons in the VTA in response to rewarding nicotine doses (i.e., doses that condition a place preference) (Zhao-Shea et al, 2011). This is also supported by studies measuring nicotine-induced DA release from striatal synaptasomes, which indicate that nAChRs containing ␣4 and ␣6 subunits have the highest affinity for nicotine recorded to date, with an EC 50 of ϳ230 nM , well within the range of nicotine blood concentrations achieved by smoking (Russell et al, 1980), as opposed to nAChRs containing (non-␣4)␣6␤2 nAChRs, which have an EC 50 of 1.52 M nicotine.…”

“…Non-TH-expressing neurons were excluded from analysis. DAergic neurons from the posterior portion of the VTA were recorded because previous data indicate that these are most sensitive to activation by nicotine (Zhao-Shea et al, 2011). Pipette solution contained 121 mM KCl, 4 mM MgCl 2 ⅐6H 2 O, 11 mM EGTA, 1 mM CaCl 2 ⅐2H 2 O, 10 mM HEPES, 0.2 mM GTP, and 4 mM ATP.…”

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

“…These methods were adapted from Zhao-Shea et al (2011). VTA neurons were studied using K 1 gluconate-based internal solution (see recipe above) made with diethylpyrocarbonate-treated water.…”

Interaction of nicotinic receptors with bupropion: Structural, functional, and pre-clinical perspectives

“…In the ventral tegmental area (VTA), 7 (i.e., homomeric) as well as non-7 (i.e., heteromeric) AChRs are expressed on dopaminergic, GABAergic, and glutamatergic neurons [32][33][34][35] . In particular, BP blocks presynaptic 42-containing AChRs on GABAergic neurons which diminishes tonic inhibition of VTA neurons [36] .…”