Functional Nicotinic Acetylcholine Receptors Containing α6 Subunits Are on GABAergic Neuronal Boutons Adherent to Ventral Tegmental Area Dopamine Neurons

Abstract

Diverse nicotinic acetylcholine receptor (nAChR) subtypes containing different subunit combinations can be placed on nerve terminals or soma/dendrites in the ventral tegmental area (VTA). nAChR ␣6 subunit message is abundant in the VTA, but ␣6*-nAChR cellular localization, function, pharmacology, and roles in cholinergic modulation of dopaminergic (DA) neurons within the VTA are not well understood. Here, we report evidence for ␣6␤2*-nAChR expression on GABA neuronal boutons terminating on VTA DA neurons. ␣-Conotoxin (␣-Ctx) MII labeling coupled with immunocytochemical staining localizes putative ␣6*-nAChRs to presynaptic GABAergic boutons on acutely dissociated, rat VTA DA neurons. Functionally, acetylcholine (ACh) induces increases in the frequency of bicuculline-, picrotoxin-, and 4-aminopyridine-sensitive miniature IPSCs (mIPSCs) mediated by GABA A receptors. These increases are abolished by ␣6*-nAChR-selective ␣-Ctx MII or ␣-Ctx PIA (1 nM) but not by ␣7 (10 nM methyllycaconitine) or ␣4* (1 M dihydro-␤-erythroidine)-nAChR-selective antagonists. ACh also fails to increase mIPSC frequency in VTA DA neurons prepared from nAChR ␤2 knock-out mice. Moreover, ACh induces an ␣-Ctx PIA-sensitive elevation in intraterminal Ca 2ϩ in synaptosomes prepared from the rat VTA. Subchronic exposure to 500 nM nicotine reduces ACh-induced GABA release onto the VTA DA neurons, as does 10 d of systemic nicotine exposure. Collectively, these results indicate that ␣6␤2*-nAChRs are located on presynaptic GABAergic boutons within the VTA and modulate GABA release onto DA neurons. These presynaptic ␣6␤2*-nAChRs likely play important roles in nicotinic modulation of DA neuronal activity.

Acute Ethanol Inhibits Dopamine Release in the Nucleus Accumbens via α6 Nicotinic Acetylcholine Receptors

“…These channels can exist in either homomeric or heteromeric form, with the a7 subunit composing the most common homomeric nAChR in the CNS, and pentameric mixtures of a (a2-a10) and b (b2-b4) subunits, with a4b2*-nAChRs as the most common heteromeric CNS receptor (Taylor et al, 2013a). Although nAChRs are of a wide variety in the VTA (Wooltorton et al, 2003), heteromeric a6*-nAChRs are highly expressed in the mesolimbic DA system (Champtiaux et al, 2003;Yang et al, 2009bYang et al, , 2011 and a6 is the primary a subunit that plays a prominent role in DA release (Quik et al, 2011), and they are predominantly expressed in catecholaminergic systems (Brunzell, 2012). a6*-nAChR subunits are functional in recombinant systems when paired with b subunits or hybrids of a subunits.…”

“…However, our results would suggest that there are interneurons regulating DA release in the NAc. It is likely that nAChRs are present on the terminals of these interneurons that then modulate the release of DA from DAergic neurons (Yang et al, 2011;Taylor et al, 2013b). Under this model, activation of the nAChRs on the interneurons would decrease the level of evoked DA release recorded by FSCV.…”

“…Comparing these physiologically relevant results with our ex vivo experimentation, we conclude that MEC is antagonizing the a6*-nAChRs in the NAc against the attenuation of ethanol. Given that a6*-nAChRs are located on GABA terminals and known to enhance GABA release (Yang et al, 2011), it is possible that ethanol inhibition of evoked DA release results from activation of a6*-nAChRs on GABA terminals that inhibit evoked DA release. However, we acknowledge that the mechanism of nAChRs in the NAc may be much more complex due to the compelling evidence of nAChRs present on DA terminals (Zoli et al, 2002;Exley et al, 2012).…”

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

“…Re-expression of each subunit in the VTA of the respective KO mouse rescues nicotine self-administration, indicating that activation of nicotinic receptors containing ␣4, ␣6, and/or ␤2 subunits, specifically in the VTA, are sufficient for nicotine reinforcement (Maskos et al, 2005;Pons et al, 2008). Although ␣4 and ␤2 nAChR subunits are expressed in both VTA DAergic and GABAergic neurons, expression of ␣6 subunits have predominantly been localized to DAergic neurons (Champtiaux et al, 2003;Grady et al, 2007;Drenan et al, 2008), although a recent report indicates they may also be expressed in GABAergic presynaptic boutons (Yang et al, 2011). More recently, Exley et al (2011) examined intracranial nicotine self-administration in WT, ␣4 KO, and ␣6 KO mice.…”

Long-Term Nicotine Exposure Depresses Dopamine Release in Nonhuman Primate Nucleus Accumbens

“…In fact, nicotine treatment upregulates mesolimbic ␣4␤2* nAChRs on GABAergic neurons in the ventral tegmental area, which leads to an increase in their firing and a decline in dopaminergic activity (Nashmi et al, 2007). In addition, long-term nicotine treatment may desensitize ␣6␤2* and/or ␣4␤2* nAChRs on GABAergic neurons in the ventral tegmental area to enhance mesolimbic glutamatergic transmission (Mansvelder et al, 2002;Nashmi et al, 2007;Yang et al, 2011). This altered GABAergic and glutamatergic activity in the ventral tegmental area may lead to downstream changes that depress dopaminergic activity in the nucleus (Thomas et al, 2000).…”