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For an organism to survive, it must maintain its homeostasis despite living in an ever-changingenvironment. In vertebrates, the hypothalamus integrates hormonal and metabolic signals to generate appropriate behavioural and physiological responses. Although hypothalamic nuclei are functionally specialized, their output depends on finely tuned synaptic regulation at both pre- and postsynaptic levels. Understanding how hormonal and metabolic states shape synaptic transmission within hypothalamiccircuits is therefore central to understanding hypothalamic function.

This thesis investigates how physiological state shapes synaptic transmission in two key hypothalamic nucleiusing rodent models. In the medial preoptic nucleus (MPN), a region critically involved in the regulation ofcomplex social behaviours, including reproduction, neuroactive steroids are known to modulate inhibitory transmission through chloride-permeable ion channels. However, whether steroid-dependent alterations in presynaptic chloride dynamics also regulate excitatory neurotransmission has remained unresolved. Using electrophysiological recordings combined with targeted pharmacological manipulation in acutely dissociated hypothalamic neurons from rats, we examined the chloride-dependent control of glutamate release in the MPN, complemented by immunogold electron microscopy to determine the ultrastructural localization ofrelevant membrane chloride transporters.

In parallel, we examined how metabolic state regulates synaptic function in the paraventricular nucleus (PVN) in mice, a critical node in feeding regulation. Specifically, we focused on αCaMKII-positive neurons, which sense nutrient availability through O-GlcNAc transferase (OGT). Combining patch-clamp recordings and immunohistochemistry in wild-type and OGT-deficient neurons from mice under glucose fluctuations ,we assessed how OGT links metabolic signals to postsynaptic receptor regulation.

Our results demonstrate that activation of presynaptic chloride-permeable GABA A receptors predominantly enhances glutamate release in the MPN, whereas presynaptic chloride-permeable glycine receptors exert heterogeneous effects, likely reflecting differences in chloride extrusion capacity across presynaptic terminals. These findings support a chloride-dependent presynaptic mechanism through which neuroactive steroids can modulate excitatory transmission within hypothalamic circuits. In the PVN, OGT deletion increases neuronal excitability, destabilizes synaptic activity during acute glucose shifts, and alters GABA A -receptor subunit composition without affecting baseline intrinsic membrane properties.

Together, these findings identify novel mechanisms of synaptic regulation in hypothalamic neurons, revealing both ion-dependent presynaptic control of neurotransmitter release and metabolically driven postsynaptic molecular adaptations. By linking hormonal and nutritional states to specific synaptic modifications, this work provides an integrated framework for understanding how hypothalamic circuitry maintains homeostasis.