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

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 receptor current (INMDA) via activation of extrasynaptic NMDARs (eNMDARs). However, since Mg2+ blocks NMDARs in non-depolarized neurons, the potential contribution of eNMDARs to the overall neuronal excitation/inhibition balance remains unknown. Here, we demonstrate that chronic (7-day) salt loading (SL) recruited NR2D subunit-containing NMDARs to generate an Mg2+-resistant tonic INMDA in non-depolarized (Vh, -70 mV) vasopressin (VP) (but not oxytocin (OT)) supraoptic nucleus (SON) neurons in male rodents. Conversely, in euhydrated and 3-day SL mice, Mg2+-resistant tonic INMDA was not observed. Pharmacological and genetic intervention of NR2D subunits blocked the Mg2+-resistant tonic INMDA in VP neurons under SL conditions, while an NR2B antagonist unveiled Mg2+-sensitive tonic INMDA but not Mg2+-resistant tonic INMDA In the EU-group VP neurons, an Mg2+-resistant tonic INMDA was not generated by increased ambient glutamate or treatment with co-agonists (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 3 days under SL conditions. Finally, Mg2+-resistant tonic INMDA selectively upregulated neuronal excitability in VP neurons under SL conditions, independent of ionotropic GABAergic input. Our results indicate that activation of NR2D-containing NMDARs constitutes a novel mechanism that generates an Mg2+-resistant tonic INMDA in non-depolarized VP neurons, thus causing an excitatory E/I balance shift in VP neurons to compensate for the hormonal demands imposed by a chronic osmotic challenge.SIGNIFICANCE STATEMENT:The hypothalamic supraoptic nucleus (SON) consists of two different types of magnocellular neurosecretory cells (MNCs) that synthesize and release two peptide hormones: vasopressin (VP), necessary for regulation of fluid homeostasis, and oxytocin (OT), which plays a major role in lactation and parturition. NMDA receptors (NMDARs) play important roles in shaping neuronal firing patterns and hormone release from the SON MNCs in response to various physiological challenges. Our results show that prolonged (7-day) salt loading generated a Mg2+ resistant tonic NMDA current mediated by NR2D subunit-containing receptors, which efficiently activated non-depolarized VP (but not OT) neurons. Our findings support the hypothesis that NR2D subunit-containing NMDARs play an important adaptive role in adult brain in response to a sustained osmotic challenge.