Date of Award

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Sciences

Track

Neuroscience

Research Advisor

Scott A. Heldt, Ph.D.

Committee

Joseph C. Callaway, Ph.D. Matthew Ennis, Ph.D. Robert C. Foehring, Ph.D. Kristin Hamre, Ph.D. Kristen O'Connell, Ph.D.

Keywords

Amygdala, Anxiety, GABA

Abstract

The investigation of the differential roles GABAA receptor (GABAAR) subtypes play in mediating various behaviors such as fear and anxiety was an intriguing research topic over the past decade. At present, most evidence suggests that benzodiazepine (BZ)-induced anxiolysis is primarily mediated by GABAARs containing the α2-subunit (α2-subtype). However, there is conflicting evidence as to whether α1- and α3-subtypes might also be involved in BZ-induced anxiolysis. In an attempt to further discern the role played by different α-subtype GABAARs in BZ-induced anxiolysis both systemically and within the basolateral amygdala (BLA), a brain region crucial for anxiety-like behaviors, we examined the anxiolytic-like effects, as measured by elevated-plus maze test (EPM), of several subtype selective and non-selective GABAAR positive allosteric modulators (PAMs) both in wild type mice and in mutant mice that express BZ-insensitive GABAARs of specific α-subtypes.

In our experiments, systemic injections of the α1-selective PAM zolpidem in WT mice produced slight anxiolytic-like effects with a narrow therapeutic window that overlapped with prominent motor-inhibiting effects. Systemic injection of the α3-selective PAM TP003 produced marked anxiolytic-like effects in WT mice that were accompanied by motor-stimulating effects. Systemic injection of the α2-, α3-, and α5-selective PAM L-838417 elicited significant anxiolytic-like effects in WT, and the effects were weakened in the α3(H126R) mice. Similarly, anxiolytic-like effects were observed when these selective PAMs were administered via microinjection into the BLA; however, these local injections did not significantly affect motor activity at the doses tested. In the experiment examining systemic injections of the non-selective BZ chlordiazepoxide (CDP), we found that CDP induced robust anxiolytic-like effects in both male and female WT mice. These effects were potentiated in female α1(H101R) mice, and were reduced in α2(H101R) mice of both sexes, as well as male α3(H126R) mice. Interestingly, intra-BLA microinjection of CDP produced few effects in WT, α1(H101R), or α2(H101R) mice, but showed some anxiolytic-like effects in α3(H126R) mice.

Taken together, our results suggests (i) all three (α1-, α2-, and α3-) GABAAR subtypes are involved in BZ-induced anxiolysis, but subtle differences do exist; (ii) augmentation of the α1-subtype GABAARs exerts anxiolytic-like effects; however, the therapeutic window is narrow; (iii) augmentation of the α2-, α3-, (and α5-) subtype GABAARs exerts anxiolytic-like effects and motor-stimulating effects, and these effects are weakened in α3(H126R) mice at doses tested, (iv) augmentation of the α3-subtype GABAARs exerts anxiolytic-like effects, accompanied by motor-stimulating effects; (v) BLA is an important brain region that is sufficient to mediate the anxiolytic-like effects, but not the motor-stimulating or inhibiting effects of subtype selective GABAAR PAMs; and (vi) intra-BLA microinjection of CDP yielded an inconclusive behavioral outcome, possibly due to the complex GABAergic intra-amygdaloidal microcircuitries which might antagonize each other when multiple subtypes of GABAARs are simultaneously modulated by BZs. Taken together, our results provide novel evidence that may benefit the current development of subtype selective drugs for treating clinical anxiety disorders.

ORCID

http://orcid.org/0000-0002-2371-4693

DOI

10.21007/etd.cghs.2016.0390

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