Date

2-2014

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Behavioral Neuroscience

Institution

Oregon Health & Science University

Abstract

Alcohol use disorders (AUDs) are a group of heterogenous disorders of which no single animal model can capture the entirety of the disease. This dissertation examined the effect of a neuroactive steroid (NAS) analog ganaxolone (GAN) on measures of ethanol seeking and intake across multiple procedures in male C57BL/6J mice. NAS can act at both synaptic and extrasynaptic -aminobutyric acidA (GABAA) receptors, and it is unclear if the effects of GAN on ethanol intake are due to actions at synaptic GABAA receptors, extrasynaptic GABAA receptors, or both. In order to examine how activation of extrasynaptic GABAA receptors might be contributing to alterations in ethanol intake across the different procedures, I also examined the effect of gaboxadol (THIP), a GABAA receptor agonist with preference for extrasynaptic receptors, on ethanol seeking and consumption. My hypothesis was that both GAN and THIP would alter components of ethanol seeking and consumption, indicating a role for NAS and extrasynaptic GABAA receptor activation in altering ethanol intake, and that the nucleus accumbens (NAc) shell would be a brain region contributing to this effect. In Chapter 2, a continuous-access procedure was used in order to examine the time-related effects of GAN and THIP across 24-hours of ethanol access. In this experiment, GAN promoted the onset of ethanol consumption while decreasing overall ethanol intake. THIP produced decreases in intake that were present for the first 5 hours of access, and it also increased the latency to complete the first ethanol bout. These experiments revealed interesting divergent and comparable effects between GAN and THIP, particularly during the first hours of access, suggesting that GAN was likely exerting its effects on ethanol intake via an action at both synaptic and extrasynaptic GABAA receptors. xiv In Chapter 3, the first set of studies utilized a 2-hour limited-access procedure to take advantage of the high ethanol intakes and maximal drug effects during this shorter time period. This experiment revealed a dose-dependent decrease in ethanol intake with GAN, due to a trend for a decrease in bout frequency. Consistent with the continuous-access study, THIP dose-dependently decreased ethanol intake, and this was primarily due to a decrease in bout frequency and a delay in the onset of the first ethanol bout. Additionally, Chapter 3 utilized an operant procedure, in which lever pressing and subsequent drinking were temporally separated, in order to dissect the effects of GAN and THIP on appetitive versus consummatory behavior. GAN caused a slight, non-significant increase in both the appetitive and consummatory phases of self-administration, consistent with a possible role in promoting ethanol seeking and initiation of intake. To examine the specificity of this effect, I also tested the effects of GAN on sucrose self-administration and found that GAN promoted sucrose seeking but did not systematically alter sucrose intake, indicating that GAN may promote general seeking behavior. THIP on the other hand, decreased both the appetitive and consummatory phases of ethanol and sucrose self-administration, indicating a lack of specificity for ethanol seeking and consumption in this procedure. Importantly, this procedure again revealed divergent roles of GAN and THIP, primarily on ethanol seeking, indicating that GAN’s actions at synaptic GABAA receptors may be particularly important for promoting ethanol seeking. Chapter 4 examined whether activation of GABAA receptors in the NAc shell by GAN or THIP could account for the effects that were observed following the systemic injections of these drugs which were reported in Chapter 3. GAN infused into the NAc xv shell led to a decrease in ethanol intake that was primarily due to a decrease in bout frequency. Dorsal controls showed that effects of GAN were localized to the NAc shell. Infusions of THIP into the NAc shell also decreased ethanol intake, but the use of multiple cohorts revealed important order effects of the drug. Locomotor activity studies confirmed that there were no sedative effects following intra-NAc infusion with the highest dose of either drug that could have contributed to the decreases in ethanol intake observed. In total, these results suggest that the activation of GABAA receptors in the NAc shell by GAN or THIP is sufficient to account for the effects of systemic GAN and THIP on ethanol intake that were observed in the limited-access procedure. Finally, in Chapter 5, ethanol seeking was measured with the reinstatement procedure. Following extinction of operant ethanol self-administration, lever pressing was measured in the absence of gaining access to the ethanol reinforcer. GAN reinstated extinguished ethanol-reinforced responding, measured by an increase in pressing on the previously active lever. However, THIP did not alter reinstatement of ethanol seeking. Consistent with Chapter 3, these findings suggest that activation of extrasynaptic receptors at the GABA binding site is not sufficient to promote reinstatement of ethanol seeking. Collectively, these studies showed that GAN altered ethanol consumption by increasing appetitive ethanol seeking and initial ethanol intake, while suppressing overall ethanol intake. These studies also showed that THIP decreased both ethanol seeking and consumption across multiple procedures. Together these studies suggest that activation of extrasynaptic receptors alone, at the GABA binding site, is not sufficient to promote ethanol seeking in these procedures. The results demonstrate that alteration of NAS levels, as well as activation of extrasynaptic GABAA receptors, influence the xvi consumption of ethanol in mice. Importantly, the results also document the sufficiency of the NAc shell in mediating the effects on ethanol intake observed following systemic injection of GAN and THIP. These studies contribute to literature examining the mechanisms underlying ethanol intake and will hopefully aid in the ability to understand and treat AUDs. Data with GAN and THIP may be especially useful in light of their increased use in clinical trials, particularly for diseases (i.e. post-traumatic stress disorder, depression, nicotine dependence) that may have a high comorbidity with AUDs.

Identifier

doi:10.6083/M48K77CN

School

School of Medicine

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