Gaboxadol Normalizes Behavioral Abnormalities in a Mouse Model of Fragile X Syndrome

Abstract

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and autism. FXS is also accompanied by attention problems, hyperactivity, anxiety, aggression, poor sleep, repetitive behaviors, and self-injury. Recent work supports the role of γ-aminobutyric-acid (GABA), the primary inhibitory neurotransmitter in the brain, in mediating symptoms of FXS. Deficits in GABA machinery have been observed in a mouse model of FXS, including a loss of tonic inhibition in the amygdala, which is mediated by extrasynaptic GABA_A receptors. Humans with FXS also show reduced GABA_A receptor availability. Here, we sought to evaluate the potential of gaboxadol (also called OV101 and THIP), a selective and potent agonist for delta-subunit-containing extrasynaptic GABA_A receptors (dSEGA), as a therapeutic agent for FXS by assessing its ability to normalize aberrant behaviors in a relatively uncharacterized mouse model of FXS (Fmr1 KO2 mice). Four behavioral domains (hyperactivity, anxiety, aggression, and repetitive behaviors) were probed using a battery of behavioral assays. The results showed that Fmr1 KO2 mice were hyperactive, had abnormal anxiety-like behavior, were more irritable and aggressive, and had an increased frequency of repetitive behaviors compared to wild-type (WT) littermates, which are all behavioral deficits reminiscent of individuals with FXS. Treatment with gaboxadol normalized all of the aberrant behaviors observed in Fmr1 KO2 mice back to WT levels, providing evidence of its potential benefit for treating FXS. We show that the potentiation of extrasynaptic GABA receptors alone, by gaboxadol, is sufficient to normalize numerous behavioral deficits in the FXS model using endpoints that are directly translatable to the clinical presentation of FXS. Taken together, these data support the future evaluation of gaboxadol in individuals with FXS, particularly with regard to symptoms of hyperactivity, anxiety, irritability, aggression, and repetitive behaviors.

Keywords: FMRP; GABA; OV101; THIP; aggression; anxiety; hyperactivity; stereotypy.

Figure 1

Gaboxadol normalizes hyperactivity in Fmr1 KO2 mice. Total distance traveled for 30 min for wild-type-littermates (WT) mice treated with vehicle (white bar), Fmr1 KO2 mice treated with vehicle (knockout, KO dark gray bar) and Fmr1 KO2 mice treated with gaboxadol (KO-0.5–5.0 mg/kg, light gray bars). Bars are means ± SEM, dots are raw data from individual mice. *p < 0.05, ****p < 0.0001 vs. WT-vehicle group; ns, not significant vs. WT-vehicle group; ⁺⁺⁺⁺p < 0.0001 vs. Fmr1 KO2-vehicle group. N = 10 per group.

Figure 2

Gaboxadol normalizes anxiety-related behaviors in Fmr1 KO2 mice. WT mice treated with vehicle (white bar), Fmr1 KO2 mice treated with vehicle (KO, dark gray bar), or Fmr1 KO2 mice treated with gaboxadol (KO-0.5–5.0 mg/kg, light gray bars) were subjected to the open field test (OFT), light/dark exploration test (LDT), and successive alleys test (SAT). (A) Total distance traveled in the center of the OFT. (B) The number of transitions between light and dark compartments in the LDT. (C) The number of entries into Alley 1 during the SAT. (D) The number of entries into Alley 2 during the SAT. Bars are means ± SEM, dots are raw data from individual mice. ****p < 0.0001, ***p < 0.001, **p < 0.01, *p < 0.05 vs. WT-vehicle group; ns, not significant vs. WT-vehicle group; ⁺⁺⁺⁺p < 0.0001, ⁺⁺⁺p < 0.001, ⁺⁺p < 0.01 vs. Fmr1 KO2 vehicle group. N = 10 per group.

Figure 3

Gaboxadol normalizes irritability and aggression behaviors in Fmr1 KO2 mice. Tail rattling (A), biting behavior (B), mounting behavior (C) and latency to attack a novel cage-mate (D) were measured in WT mice treated with vehicle (white bar), Fmr1 KO2 mice treated with vehicle (KO, dark gray bar), or Fmr1 KO2 mice treated with gaboxadol (KO-0.5–5.0 mg/kg, light gray bars). Bars are means ± SEM, dots are raw data from individual mice. ****p < 0.0001,***p < 0.001,**p < 0.01, *p < 0.05 vs. WT-vehicle group; ns, not significant vs. WT-vehicle group; ⁺⁺⁺⁺p < 0.0001, ⁺⁺⁺p < 0.001, ⁺⁺p < 0.01, ⁺p < 0.05 vs. Fmr1 KO2 vehicle group. N = 10 per group.

Figure 4

Gaboxadol normalizes repetitive behaviors in Fmr1 KO2 mice. WT mice treated with vehicle (white bar), Fmr1 KO2 mice treated with vehicle (KO, dark gray bar), or Fmr1 KO2 mice treated with gaboxadol (KO-0.5–5.0 mg/kg, light gray bars) were subjected to four tests that measure repetitive behaviors. (A) Counter-clockwise (CCW) revolutions were measured by infrared beam breaks during a 5-min test in the Open Field. (B) After a 5-min habituation to the test cage, time spent grooming for 3 min is shown. (C) Stereotypy counts (head bobbing events) during a 3-min testing period are shown. Bars are means ± SEM, dots are raw data from individual mice. ****p < 0.0001 vs. WT-vehicle group; ns, not significant vs. WT-vehicle group; ⁺⁺⁺⁺p < 0.0001 vs. Fmr1 KO2 vehicle group. N = 10 per group.

Figure 5

Overall behavioral effects of gaboxadol in Fmr1 KO2 mice. Summary of the results of behavioral assessment of Fmr1 KO2 mice treated with vehicle or gaboxadol (0.5–5.0 mg/kg) compared to WT littermate controls. The Fmr1 KO2 mice showed significant phenotypes in locomotor activity, anxiety-related behaviors, irritability and aggression, and repetitive behaviors. All of these phenotypes were consistently normalized to WT levels by treatment with gaboxadol at the 0.5 mg/kg dose. “Normal” behavior (green squares) is consistent with WT vehicle-treated mice and is statistically different from Fmr1 KO2 (KO) vehicle-treated mice. “Abnormal” behavior (red squares) is consistent with Fmr1 KO2 (KO) vehicle-treated mice and is statistically different from WT vehicle-treated mice. Yellow squares indicate a difference that is either not significantly different from WT mice or statistically different from Fmr1 KO2 (KO) vehicle-treated mice.