Neurosteroids reverse tonic inhibitory deficits in Fragile X syndrome mouse model
Authors: Davies Paul A , Modgil Amit A , Parakala Manasa , Ackley Mike , Doherty James , Moss Stephen J 
Published: June 2021

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. In addition, there is increased incidence of anxiety, sleep irregularities, and seizure activity. The underlying cause of FXS is a loss of the fragile X mental retardation protein (FMRP) which has been shown to participate in the biosynthesis of δ subunits. Studies from both FXS patients and animal models have revealed reduced expression levels of GABAAR α4 and δ subunits with a reduced efficacy of tonic inhibition. Neurosteroids (NS) are known allosteric modulators of GABAAR channel function but recent studies from our laboratory have revealed that NS also exert persistent effects on the efficacy of tonic inhibition by increasing the PKC‐mediated phosphorylation of the α4 subunit which increases the membrane expression and boosts tonic inhibition. We have used a combination of biochemical and electrophysiological methods to assess alterations in GABAergic signaling in the hippocampus of FMRP knock‐out mouse (Fmr1 KO), a widely validated model of the human syndrome.

Our preliminary studies demonstrate that Fmr1 KO mice at p21 have a decrease in phosphorylation of S443 in the α4 subunit compared to WT and an increase in the phosphorylation of β3 subunits at the S408/409 site compared to WT. We have previously showed that phosphorylation of these residues changes the trafficking of the subunits so the changes observed in Fmr1 KO mice would predictably have consequences for trafficking of these essential inhibitory subunits. However, at p48–72 Fmr1 KO mice did not exhibit any deficits in GABAAR expression levels or phosphorylation demonstrating a critical developmental deficit in GABAAR expression. We noted that there was a significant decrease in tonic inhibition in dentate gyrus granule cells in p21 Fmr1 KO mice compared to WT. A 10 min exposure to 1μM THDOC followed by 330 min wash induced a >3 fold increase in tonic current in Fmr1 KO animals which was prevented with PKC inhibition. Using a perforated multi‐electrode array we have observed in horizontal cortical‐hippocampal slices seizure like events (SLE) propagating through the dentate gyrus and into the CA3 and CA1 regions of the hippocampus of Fmr1 KO mice but in WT mice SLEs did not propagate through the dentate gyrus. We predict the increase neuronal excitability seen in Fmr1 KO mice is due to the deficits in tonic inhibition.

We have observed that THDOC treatment effectively reversed tonic current deficits in Fmr1 KO mice. Future studies will focus on ameliorating behavioral deficits in Fmr1 KO mice with synthetic neuroactive steroids that display improved pharmacokinetic properties.

Support or Funding Information

This work was supported by grants from the National Institutes of Health (NIH)‐National Institute of Alcoholism and Alcohol Abuse grant AA017938 (P.A.D), NIH‐National Institute of Mental Health grant, MH097446, and DOD, AR140209 (PAD & SJM), NIH‐National Institute of Neurological Disorders and Stroke grant NS051195, NS056359, NS081735, NS080064, NS087662 (SJM), the Simons Foundation #206026 (S.J.M.).