Labile Calcium-Permeable AMPA Receptors Constitute New Glutamate Synapses Formed in Hypothalamic Neuroendocrine Cells during Salt Loading

ABSTRACT Magnocellular neuroendocrine cells of the hypothalamus play a critical role in the regulation of fluid and electrolyte homeostasis. They undergo a dramatic structural and functional plasticity under sustained hyperosmotic conditions, including an increase in afferent glutamatergic synaptic innervation. We tested for a postulated increase in glutamate AMPA receptor expression and signaling in magnocellular neurons of the male rat hypothalamic supraoptic nucleus (SON) induced by chronic salt loading. While without effect on GluA1-4 subunit mRNA, salt loading with 2% saline for 5-7 days resulted in a selective increase in AMPA receptor GluA1 protein expression in the SON, with no change in GluA2-4 protein expression, suggesting an increase in the ratio of GluA1 to GluA2 subunits. Salt loading induced a corresponding increase in excitatory postsynaptic currents in both oxytocin and vasopressin neurons, with properties characteristic of calcium-permeable AMPA receptor-mediated currents. Unexpectedly, the emergent AMPA synaptic currents were silenced by blocking protein synthesis and mTOR activity in the slices, suggesting that the new glutamate synapses induced by salt loading require continuous dendritic protein synthesis for maintenance. These findings indicate that chronic salt loading leads to the induction of highly labile glutamate synapses in oxytocin and vasopressin neurons that are comprised of calcium-permeable homomeric GluA1 AMPA receptors. The glutamate-induced calcium influx via calcium-permeable AMPA receptors would be expected to play a key role in the induction and/or maintenance of activity-dependent synaptic plasticity that occurs in the magnocellular neurons during chronic osmotic stimulation. SIGNIFICANCE STATEMENT Oxytocin- and vasopressin-secreting neurons of the hypothalamus undergo robust synaptic plasticity during chronic osmotic stimulation, including an increase in excitatory synapses. Here, we show that chronic salt loading results in the emergence of calcium-permeable AMPA receptor-mediated synaptic currents in oxytocin and vasopressin neurons by inducing an increase in GluA1 AMPA receptor subunits without a coordinate increase in GluA2 subunits. The salt loading-induced calcium-permeable AMPA currents were rapidly silenced by inhibiting protein synthesis and mTOR activity, suggesting that continuous dendritic protein synthesis is required to functionally maintain the new AMPA synapses. Calcium influx through calcium-permeable AMPA receptors should play a key role in the induction and/or maintenance of synaptic plasticity in oxytocin and vasopressin neurons by chronic osmotic stimulation.