Nicotine and ethanol cooperate to enhance ventral tegmental area AMPA receptor function via α6-containing nicotinic receptors

Abstract

Nicotine+ethanol co-exposure results in additive and/or synergistic effects in the ventral tegmental area (VTA) to nucleus accumbens (NAc) dopamine (DA) pathway, but the mechanisms supporting this are unclear. We tested the hypothesis that nAChRs containing α6 subunits (α6* nAChRs) are involved in the response to nicotine+ethanol co-exposure. Exposing VTA slices from C57BL/6 WT animals to drinking-relevant concentrations of ethanol causes a marked enhancement of α-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA) receptor (AMPAR) function in VTA neurons. This effect was sensitive to α-conotoxin MII (an α6β2* nAChR antagonist), suggesting that α6* nAChR function is required. In mice expressing hypersensitive α6* nAChRs (α6L9S mice), we found that lower concentrations (relative to C57BL/6 WT) of ethanol were sufficient to enhance AMPAR function in VTA neurons. Exposure of live C57BL/6 WT mice to ethanol also produced AMPAR functional enhancement in VTA neurons, and studies in α6L9S mice strongly suggest a role for α6* nAChRs in this response. We then asked whether nicotine and ethanol cooperate to enhance VTA AMPAR function. We identified low concentrations of nicotine and ethanol that were capable of strongly enhancing VTA AMPAR function when co-applied to slices, but that did not enhance AMPAR function when applied alone. This effect was sensitive to both varenicline (an α4β2* and α6β2* nAChR partial agonist) and α-conotoxin MII. Finally, nicotine+ethanol co-exposure also enhanced AMPAR function in VTA neurons from α6L9S mice. Together, these data identify α6* nAChRs as important players in the response to nicotine+ethanol co-exposure in VTA neurons.

α6-Containing nicotinic acetylcholine receptors in midbrain dopamine neurons are poised to govern dopamine-mediated behaviors and synaptic plasticity

“…Previously, we demonstrated that incubating brain slices from naïve animals with low concentrations of nicotine causes α6* nAChR activation and enhanced AMPAR function in VTA DA neurons (Engle et al, 2013, Engle et al, 2015). In this study, we sought to determine whether in vivo activation of α6* nAChRs is sufficient to support changes in VTA DA neuron excitability.…”

“…7A). AMPAR function was measured using local application of AMPA to the recorded cell using a puff-pipette as previously described and validated (Engle et al, 2012, Engle et al, 2013, Engle et al, 2015). Based on the dose-range found to be sufficient to stimulate locomotor activity in α6L9S mice (0.02 to 0.05 mg/kg; Fig.…”

“…These data are consistent with our previous results. When naïve brain slices containing VTA were treated with nicotine, AMPAR function (Engle et al, 2013) and AMPA/NMDA ratios (Engle et al, 2015) were enhanced via activation of α6* nAChRs. These prior studies also demonstrated, in naïve slices, that α4 subunits and NMDAR activity are also required for α6*-mediated increases in AMPAR activity (Engle et al, 2013).…”

“…Cells were held at −60 mV and AMPA receptor (AMPAR) function was analyzed as previously described (Engle et al, 2012, Engle et al, 2013, Engle et al, 2015). A micropipette filled with AMPA (100 μM) was loaded onto a single-dimension piezoelectric translator attached to a micromanipulator.…”

“…We also demonstrated that α4 subunits are required for most of these effects, highlighting the importance of α4α6* nAChRs (Drenan et al, 2010). More recently, we used these mice to show that selective activation of α6* nAChRs in DA neurons is sufficient to enhance glutamatergic synaptic plasticity, which is a key molecular change occurring during the nicotine dependence/addiction process (Engle et al, 2013, Engle et al, 2015). Despite these advances, important questions remain to be answered to further validate the α6L9S model for future drug discovery studies on mesolimbic DA system α6* nAChRs.…”