High salt intake recruits tonic activation of NR2D subunit-containing extrasynaptic nmdars in vasopressin neurons

Abstract

In addition to producing a classical excitatory postsynaptic current via activation of synaptic NMDA receptors (NMDARs), glutamate in the brain also induces a tonic NMDAR current (INMDA) via activation of extrasynaptic NMDARs (eNMDARs). However, since Mg21 blocks NMDARs in nondepolarized neurons, the potential contribution of eNMDARs to the overall neuronal excitatory/inhibitory (E/I) balance remains unknown. Here, we demonstrate that chronic (7 d) salt loading (SL) recruited NR2D subunit-containing NMDARs to generate an Mg21-resistant tonic INMDA in nondepolarized [Vh (holding potential) 270mV] vasopressin (VP; but not oxytocin) supraoptic nucleus (SON) neurons in male rodents. Conversely, in euhydrated (EU) and 3 d SL mice, Mg21-resistant tonic INMDA was not observed. Pharmacological and genetic intervention of NR2D subunits blocked the Mg21resistant tonic INMDA in VP neurons under SL conditions, while an NR2B antagonist unveiled Mg21-sensitive tonic INMDA but not Mg21-resistant tonic INMDA. In the EU group VP neurons, an Mg21-resistant tonic INMDA was not generated by increased ambient glutamate or treatment with coagonists (e.g., D-serine and glycine). Chronic SL significantly increased NR2D expression but not NR2B expression in the SON relative to the EU group or after 3d under SL conditions. Finally, Mg21-resistant tonic INMDA selectively upregulated neuronal excitability in VP neurons under SL conditions, independent of ionotropic GABAergic input. Our results indicate that the activation of NR2D-containing NMDARs constitutes a novel mechanism that generates an Mg21-resistant tonic INMDA in nondepolarized VP neurons, thus causing an E/I balance shift in VP neurons to compensate for the hormonal demands imposed by a chronic osmotic challenge.