Role of α6 nAChRs in Ethanol Modulation of VTA Neurons

• Main Author: Shin, Samuel Injae
• Format: Others
• Published: BYU ScholarsArchive 2014
• Subjects: ethanol; nicotine; VTA; GABA; α6 nAChRs; Cell and Developmental Biology; Physiology
• Online Access: https://scholarsarchive.byu.edu/etd/3980; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=4979&context=etd

The prevailing view is that enhancement of dopamine (DA) transmission in the mesolimbic system leads to the rewarding properties of alcohol and nicotine (NIC). The mesolimbic DA system consists of DA neurons in the midbrain ventral tegmental area (VTA) that innervate the nucleus accumbens (NAc). DA neurotransmission is regulated by inhibitory VTA GABA neurons, whose excitability is a net effect of glutamate (GLU) and GABA neurotransmission that are modulated by NIC cholinergic receptors (nAChRs) on afferent terminals. We have previously demonstrated that VTA GABA neurons are excited by low-dose ethanol but are inhibited by moderate to high-dose ethanol, and they adapt to chronic ethanol, evincing marked hyperexcitability during withdrawal. The aim of this study was to evaluate the role of α6 nAChRs in ethanol effects on VTA GABA and DA neurons. In order to more conclusively demonstrate the role of α6 nAChRs in alcohol modulation in the VTA, we profiled the pharmacological interactions between ethanol and α6 nAChRs using recombinant α6 nAChRs in human epithelia (SH-EP1) cells and evaluated the effects of α6 nAChR antagonists on ethanol inhibition of GABA-mediated synaptic responses in dissociated GABA neurons of the VTA by recording mIPSCs; and assessed the effects of α6 nAChR antagonists on ethanol inhibition of VTA neurons, via eIPSCs on GABA neurons, sIPSCs on GABA neurons, and firing rate of DA neurons. We found that ethanol enhanced NIC currents in SH-EP1 cells via α6 nAChRs. Electrophysiology studies showed that superfusion of ethanol (5-30 mM) enhanced the frequency and amplitude of mIPSCs recorded in acutely dissociated VTA GABA neurons from GAD-GFP mice. Furthermore, the α6 nAChR antagonist α-conotoxin P1A (10 nM) prevented the ethanol-induced changes in mIPSC. In support, eIPSC experiments demonstrated that low doses of ethanol (1-5 mM) enhanced eIPSC peaks and decreased paired-pulse ratio, suggesting a presynaptic effect with ethanol. Alpha-conotoxin MII (α-CTX MII) blocked ethanol's effects on eIPSCs. This effect on VTA GABA neurons was also seen in sIPSCs, as ethanol decreased GABA firing rate. Similarly, the inhibition caused by ethanol was prevented by α-conotoxin P1A (10 nM). Additionally, CPP studies showed that α6 KO mice and WT mice treated with MEC, a non-competitive, non-α7 antagonist, did not show a preference for EtOH compartments that was found in WT mice. Taken together, these studies indicate that ethanol is acting through α6 nAChRs on GABA terminals to enhance GABA release, suggesting a possible mechanism of action of alcohol and nicotine co-dependence. Through these studies conducted to understand the role of α6 nAChRs in ethanol modulation, we hope to further outline how alcohol alters brain activity so that we can ultimately facilitate the development of therapies/medications for the treatment of alcoholism.