Abstracts

Rationale: Fragile-X Syndrome (FXS), caused by a mutation in the Fmr1 gene and subsequent dysfunction of its gene product FMRP, is the leading cause of inherited mental retardation. Patients with FXS display symptoms of commonly comorbid disorders involving network hyperexcitability such as autism, anxiety disorders, and ADHD. In addition, at some point in their childhood up to 25% of FXS patients will have epilepsy. Therefore, the study of FXS provides a platform to investigate common mechanisms of epilepsy comorbidities in a system in which these comorbidities innately occur. To this end we chose to focus our studies on the basolateral amygdala (BLA), a region of interest central to dysfunction in epilepsy and its comorbidities of anxiety disorders, autism, and ADHD. According to our recent studies in Fmr1 KO mice, this brain region shows decreased phasic GABAergic inhibition coupled with principal excitatory neuron hyperexcitability in vitro. This hyperexcitability can be rescued with gaboxadol, or THIP, a superagonist at extrasynaptic, ?-subunit-containing GABA receptors, in the acute BLA slice, indicating a possible role of tonic GABAergic inhibition in controlling BLA excitability. Additionally, hyperactivity of Fmr1 KO mice is reduced in vivo by gaboxadol. This study aims to characterize both the ?- and ?5-subunit mediated contributions to tonic GABAergic inhibition in the BLA, and their role in controlling BLA neuron excitability specifically in a mouse model of epilepsy and its comorbidities. Methods: We utilized single and paired whole-cell patch clamp recordings in acute coronal brain slices to investigate tonic GABAergic currents in principal excitatory neurons and interneurons in the BLA of wild-type and Fmr1 KO mice. Bath and local application of SR95531 (gabazine, 50 ?M), the ?-subunit agonist, THIP (10 ?M), and the ?5 inverse agonist, ?5IA (1.5 ?M), were used to examine total and relative subunit contributions to tonic currents in BLA neurons. Results: We report physiological deficits in both ?5- and ?-subunit mediated tonic inhibition of principal and inhibitory neurons in the Fmr1 KO BLA versus wild-type. Furthermore, we observed a major contribution of the ?5-mediated component to the total tonic current observed in principal neurons and some interneuron subclasses. ?5-subunit mediated tonic inhibition specifically also displayed marked outward rectification. Conclusions: Our results indicate decreased tonic GABAergic currents in BLA neurons of Fmr1 KO mice. Additionally, the rectification of ?5-containing GABA receptors implies that tonic currents mediated by these subunit-containing receptors may provide strong control over changes in membrane potential close to firing threshold compared to tonic inhibition mediated by ?-containing receptors, and therefore may be a relative target to control BLA hyperexcitability associated with epilepsy and its comorbidities.