STRUCTURAL AND FUNCTIONAL CHARACTERIZATION OF AN OXIME-BASED ACETYLCHOLINESTERASE REACTIVATOR WITH UNEXPECTED GABA AMINOTRANSFERASE INHIBITORY ACTIVITY AT GABAERGIC SYNAPSES

Previous research on exposure to organophosphorus compounds demonstrated that RM048, an oxime-based acetylcholinesterase reactivator, possesses anticonvulsant properties. Then, patch-clamp recordings revealed RM048's hyperpolarizing properties. We hypothesized that RM048 could modulate GABAergic signalling. We addressed this issue using primary hippocampal neuron cultures, which allow for the dissection of the underlying cellular and molecular mechanisms. We found that RM048 rapidly and reversibly decreases neuronal activity in hippocampal cultures. Pharmacological approaches using inhibitors of both glutamatergic and GABAergic receptors show that the decrease in [Ca2+]i is mainly due to the extrasynaptic GABA_A receptors that are responsible for the GABAergic tonic currents. Then, using in silico modelling approaches, we identified GABA aminotransferase (GABA-AT) as a high-affinity molecular target of RM048, suggesting that inhibition of GABA catabolism may increase GABA levels both in neurons and astrocytes, thereby promoting tonic inhibition through the activation of extrasynaptic GABA_ARs. Immunocytochemistry further revealed alterations in GABA_A receptor organisation, with a reduction in extrasynaptic α5 subunit receptors and an increase in synaptic α1 and α5 subunits. These results suggest that RM048 is a novel modulator of GABAergic signalling and support its potential application in treating disorders associated with neuronal hyperexcitability, such as epilepsy, neuropathies, and psychiatric disorders.