Role of 5-Hydroxytryptamine_2CReceptors in Ca²⁺-Dependent Ethanol Potentiation of GABA Release onto Ventral Tegmental Area Dopamine Neurons

Abstract

Activation of ventral tegmental area (VTA)-dopaminergic (DA) neurons by ethanol has been implicated in the rewarding and reinforcing actions of ethanol. GABAergic transmission is thought to play an important role in regulating the activity of DA neurons. We have reported previously that ethanol enhances GABA release onto VTA-DA neurons in a brain slice preparation. Because intraterminal Ca 2ϩ levels regulate neurotransmitter release, we investigated the roles of Ca 2ϩ -dependent mechanisms in ethanol-induced enhancement of GABA release. Acute ethanol enhanced miniature inhibitory postsynaptic current (mIPSC) frequency in the presence of the nonspecific voltage-gated Ca 2ϩ channel inhibitor, cadmium chloride, even though basal mIPSC frequency was reduced by cadmium. Conversely, the inositol-1,4,5-triphosphate receptor inhibitor, 2-aminoethoxydiphenylborane, and the sarco/endoplasmic reticulum Ca 2ϩ ATPase pump inhibitor, cyclopiazonic acid, eliminated the ethanol enhancement of mIPSC frequency. Recent studies suggest that the G protein-coupled receptor, 5-hydroxytryptamine (5-HT) 2C , may modulate GABA release in the VTA. Thus, we also investigated the role of 5-HT 2C receptors in ethanol enhancement of GABAergic transmission. Application of 5-HT and the 5-HT 2C receptor agonist, Ro-60-0175 [(␣S)-6-chloro-5-fluoro-␣-methyl-1H-indole-1-ethanamine fumarate], alone enhanced mIPSC frequency of which the latter was abolished by the 5-HT 2C receptor antagonist, SB200646 [N-(1-methyl-5-indoyl)-N-(3-pyridyl)urea hydrochloride], and substantially diminished by cyclopiazonic acid. Furthermore, SB200646 abolished the ethanol-induced increase in mIPSC frequency and had no effect on basal mIPSC frequency. These observations suggest that an increase in Ca 2ϩ release from intracellular stores via 5-HT 2C receptor activation is involved in the ethanol-induced enhancement of GABA release onto VTA-DA neurons.

GABAergic Actions Mediate Opposite Ethanol Effects on Dopaminergic Neurons in the Anterior and Posterior Ventral Tegmental Area

“…The effects were reversible, concentration-dependent, and reproducible in the same neuron. This inhibition by EtOH is consistent with our previous observations (Xiao and Ye, 2008); but facilitation was reported in some other studies (Theile et al, 2008(Theile et al, , 2009Xiao and Ye, 2008). The VTA DA neurons in our previous study probably were located in the p-VTA, because we selected neurons stringently by such characteristics as disinhibition by MOR agonist, inhibition by 100 nM quinpirole, broad action potentials (half-width Ͼ1.2 ms), and expression of I h (Xiao et al, 2007;Xiao and Ye, 2008).…”

“…4). A similar effect was reported by Theile et al (2008Theile et al ( , 2009). It is note- jpet.aspetjournals.org worthy that those investigators reported EtOH-induced excitation of DA neurons with increased GABA release, suggesting that the EtOH-induced excitation of VTA DA neurons could also be mediated by other cellular mechanisms, including direct excitation (Brodie et al, 1999;Appel et al, 2003;Okamoto et al, 2006).…”

“…4B1). These results are consistent with previous studies (Theile et al, 2009). A K-S test of individual cells showed that EtOH systematically increased the amplitude of mIPSCs (Fig.…”

“…We did not study the mechanism by which EtOH facilitated GABA release in the current study. However, a previous study has suggested that this may involve activation of 5-hydroxytryptamine type 2C receptors (Theile et al, 2009). …”

“…These results support the notion that EtOH stimulates VTA DA neurons at least partly via MOR-mediated disinhibition. Under some conditions, EtOH was also reported to enhance spontaneous GABA release in VTA: 1) when endogenous opioidergic activity was negligible (Theile et al, 2008); 2) when neuronal firing was blocked (Theile et al, 2008(Theile et al, , 2009, and 3) when GABAergic neurons were silenced by a MOR agonist (Xiao and Ye, 2008). These findings further indicate a pivotal role of opioids in the disinhibition of VTA DA neurons by EtOH.…”