SUN-472 Labile Ca-Permeable AMPA Receptors Comprise New Synapses Following Salt Loading-Induced Plasticity in Hypothalamic Magnocellular Neurons

Magnocellular neuroendocrine cells of the hypothalamus play an important role in the regulation of fluid and electrolyte homeostasis. They undergo a dramatic structural plasticity under sustained physiological activation, including an increase in glutamatergic synaptic innervation. We tested for plasticity in the glutamate AMPA receptor expression in magnocellular neurons of the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) induced by sustained activation by chronic salt loading-induced dehydration. We found that salt loading by with 2% saline drinking water for 5-7 days resulted in a significant selective increase in GluA1 protein expression in the rat SON, while no change in the mRNA expression or membrane localization of any of the AMPA receptor subunits was observed, suggesting a post-transcriptional up regulation of GluA1 at excitatory synapses on SON neurons. We performed whole-cell recordings of excitatory postsynaptic currents (EPSCs) to determine the contribution of Ca(2+) -permeable AMPA receptors (CP-AMPARs) to synaptic transmission by the EPSC rectification index and the sensitivity of EPSCs to the selective antagonist of CP-AMPA receptors, 1-naphthyl-acetyl-spermine (NAS). We found that neurons from the salt-loaded group showed a significantly greater inward rectification and an increased sensitivity to NAS compared to the control group. To further determine the role of rapid protein turnover in the AMPAR plasticity, we preincubated slices for over 60 min in the translational inhibitor cycloheximide to block local protein synthesis, which. Blocking protein synthesis reverted the Ca permeability and reduced the EPSC frequency and EPSC rectification in SON neurons from in the salt-loaded group to the levels of controls. These findings together suggest that salt loading induces highly labile glutamate synapses in magnocellular neurons that are comprised almost exclusively of CP-AMPARs, and indicate an essential role for rapid protein synthesis in the maintenance of the new synapses. This glutamate receptor plasticity should result in an increase in glutamate-induced calcium influx, which could play a key role in the activity-dependent neuronal-glial remodeling of the magnocellular neurons that occurs during chronic dehydration. Supported by NIH R01 and NSF IOS grants.