Effects of opioid antagonism on operant ethanol self-administration in adolescence and characterization of extracellular GABA in the ventral tegmental area

The endogenous opioid peptide system is hypothesized to be involved in ethanol self-administration and relapse behaviors. Naltrexone, a nonselective opioid antagonist, is an approved medication for alcohol use disorder which has been shown to decrease drinking in adult animal models and select clinical populations, but little is known about the efficacy of naltrexone in animal models that begin drinking ethanol in adolescence. Therefore, we investigated the effects of systemic naltrexone administration in an adolescent rat model of operant ethanol self-administration. We found that naltrexone significantly reduced ethanol intake and motivation to obtain ethanol in adolescent and adult rats. Following a period of abstinence, naltrexone also significantly reduced “relapse” to alcohol seeking in both age groups. These results extend findings that naltrexone is effective at reducing ethanol intake to an adolescent animal model and support opioid antagonism as a treatment strategy for decreasing problem drinking in late adolescents and young adults. One potential mechanism underlying the effects of opioid receptor blockade on ethanol self-administration implicates γ-aminobutyric acid (GABA) neurons within the ventral tegmental area (VTA). Inhibitory signaling in the VTA is involved in the mechanism of action of many drugs of abuse yet there are few studies measuring extracellular GABA concentration in this region. Therefore, the remaining experiments focused on developing methods to quantify extracellular GABA in the VTA. We first describe a novel, sensitive fluorescence method to quantify GABA concentration using high performance liquid chromatography (HPLC) of an ophthalaldehyde/sulfite derivative, previously reported to produce low fluorescence not suitable for in vivo microdialysis applications. Next, we used quantitative microdialysis under transient conditions to characterize basal extracellular GABA concentration and the influence of uptake mechanisms in the VTA. Our results show that inhibition of GABA uptake significantly increased extracellular GABA concentration and reduced in vivo extraction fraction of the probe. Reduced in vivo extraction fraction caused significant underestimation of the increase in extracellular GABA by conventional microdialysis. Together, these results establish the foundation for future studies to investigate the regulation of extracellular GABA concentration and uptake mechanisms in the VTA in mediating the effects of ethanol, opioid antagonism and associated drug-related behaviors.