Effects of early‐life exposure to THIP on brainstem neuronal excitability in the Mecp2‐null mouse model of Rett syndrome before and after drug withdrawal

Rett syndrome (RTT) is mostly caused by mutations of the X-linked MECP2 gene. Although the causal neuronal mechanisms are still unclear, accumulating experimental evidence obtained from Mecp2-/Y mice suggests that imbalanced excitation/inhibition in central neurons plays a major role. Several approaches may help to rebalance the excitation/inhibition, including agonists of GABAA receptors (GABAAR). Indeed, our previous studies have shown that early-life exposure of Mecp2-null mice to the extrasynaptic GABAAR agonist THIP alleviates several RTT-like symptoms including breathing disorders, motor dysfunction, social behaviors, and lifespan. However, how the chronic THIP affects the Mecp2-/Y mice at the cellular level remains elusive. Here, we show that the THIP exposure in early lives markedly alleviated hyperexcitability of two types of brainstem neurons in Mecp2-/Y mice. In neurons of the locus coeruleus (LC), known to be involved in breathing regulation, the hyperexcitability showed clear age-dependence, which was associated with age-dependent deterioration of the RTT-like breathing irregularities. Both the neuronal hyperexcitability and the breathing disorders were relieved with early THIP treatment. In neurons of the mesencephalic trigeminal nucleus (Me5), both the neuronal hyperexcitability and the changes in intrinsic membrane properties were alleviated with the THIP treatment in Mecp2-null mice. The effects of THIP on both LC and Me5 neuronal excitability remained 1 week after withdrawal. Persistent alleviation of breathing abnormalities in Mecp2-/Y mice was also observed a week after THIP withdrawal. These results suggest that early-life exposure to THIP, a potential therapeutic medicine, appears capable of controlling neuronal hyperexcitability in Mecp2-/Y mice, which occurs in the absence of THIP in the recording solution, lasts at least 1 week after withdrawal, and may contribute to the RTT-like symptom mitigation.