Presynaptic hyperexcitability reversed by positive allosteric modulation of a GABABR epilepsy variant.

GABABRs are key membrane proteins that continually adapt the excitability of the nervous system. These G-protein coupled receptors are activated by the brain's premier inhibitory neurotransmitter GABA. They are obligate heterodimers composed of GABA-binding GABABR1 and G-protein-coupling GABABR2 subunits. Recently, three variants (G693W, S695I, I705N) have been identified in the gene (GABBR2) encoding for GABABR2. Individuals that harbour any of these variants exhibit severe developmental epileptic encephalopathy and intellectual disability, but the underlying pathogenesis that is triggered in neurons, remains unresolved. Using a range of confocal imaging, flow cytometry, structural modelling, biochemistry, live cell Ca2+ imaging of presynaptic terminals, whole-cell electrophysiology of HEK-293T cells and neurons, and two-electrode voltage clamping of Xenopus oocytes we have probed the biophysical and molecular trafficking and functional profiles of G693W, S695I and I705N variants. We report that all three point mutations impair neuronal cell surface expression of GABABRs, reducing signalling efficacy. However, a negative effect evident for one variant perturbed neurotransmission by elevating presynaptic Ca2+ signalling. This is reversed by enhancing GABABR signalling via positive allosteric modulation. Our results highlight the importance of studying neuronal receptors expressed in nervous system tissue and provide new mechanistic insights into how GABABR variants can initiate neurodevelopmental disease whilst highlighting the translational suitability and therapeutic potential of allosteric modulation for correcting these deficits.

GABAergic disinhibition from the BNST to PNOCARC neurons promotes HFD-induced hyperphagia.

Activation of prepronociceptin (PNOC)-expressing neurons in the arcuate nucleus (ARC) promotes high-fat-diet (HFD)-induced hyperphagia. In turn, PNOCARC neurons can inhibit the anorexic response of proopiomelanocortin (POMC) neurons. Here, we validate the necessity of PNOCARC activity for HFD-induced inhibition of POMC neurons in mice and find that PNOCARC-neuron-dependent inhibition of POMC neurons is mediated by gamma-aminobutyric acid (GABA) release. When monitoring individual PNOCARC neuron activity via Ca2+ imaging, we find a subpopulation of PNOCARC neurons that is inhibited upon gastrointestinal calorie sensing and disinhibited upon HFD feeding. Combining retrograde rabies tracing and circuit mapping, we find that PNOC neurons from the bed nucleus of the stria terminalis (PNOCBNST) provide inhibitory input to PNOCARC neurons, and this inhibitory input is blunted upon HFD feeding. This work sheds light on how an increase in caloric content of the diet can rewire a neuronal circuit, paving the way to overconsumption and obesity development.