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 (I_NMDA) via activation of extrasynaptic NMDARs (eNMDARs). However, since Mg²⁺ 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 Mg²⁺-resistant tonic I_NMDA in nondepolarized [V_h (holding potential) –70 mV] vasopressin (VP; but not oxytocin) supraoptic nucleus (SON) neurons in male rodents. Conversely, in euhydrated (EU) and 3 d SL mice, Mg²⁺-resistant tonic I_NMDA was not observed. Pharmacological and genetic intervention of NR2D subunits blocked the Mg²⁺-resistant tonic I_NMDA in VP neurons under SL conditions, while an NR2B antagonist unveiled Mg²⁺-sensitive tonic I_NMDA but not Mg²⁺-resistant tonic I_NMDA. In the EU group VP neurons, an Mg²⁺-resistant tonic I_NMDA 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 3 d under SL conditions. Finally, Mg²⁺-resistant tonic I_NMDA 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 Mg²⁺-resistant tonic I_NMDA 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.

SIGNIFICANCE STATEMENT The hypothalamic supraoptic nucleus (SON) consists of two different types of magnocellular neurosecretory cells (MNCs) that synthesize and release the following two peptide hormones: vasopressin (VP), which is 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 d) salt loading generated a Mg²⁺-resistant tonic NMDA current mediated by NR2D subunit-containing receptors, which efficiently activated nondepolarized 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.

除了通过激活突触 NMDA 受体 (NMDARs) 产生经典的兴奋性突触后电流外，大脑中的谷氨酸还通过激活突触外 NMDARs (eNMDARs)诱导强直 NMDAR 电流 ( I _NMDA )。然而，由于 Mg ²⁺阻断非去极化神经元中的 NMDARs，eNMDARs 对整体神经元兴奋/抑制 (E/I) 平衡的潜在贡献仍然未知。在这里，我们表明，慢性（7 d）盐负荷（SL）招募NR2D含亚基- NMDA受体以生成镁²⁺抗性补药我_NMDA在nondepolarized [ V _ħ（保持电位）–70 mV] 雄性啮齿动物中的加压素（VP；但不是催产素）视上核 (SON) 神经元。相反，在正常水合 (EU) 和 3 d SL 小鼠中，未观察到Mg ²⁺抗性强直I _NMDA。NR2D 亚基的药理学和遗传干预阻断了SL 条件下 VP 神经元中的 Mg ²⁺抗性补品I _NMDA，而 NR2B 拮抗剂揭示了 Mg ²⁺敏感补品I _NMDA但不是 Mg ²⁺抗性补品I _NMDA。在 EU 组 VP 神经元中，抗 Mg ²⁺滋补剂I _NMDA 不是由环境谷氨酸增加或用促效剂处理产生的（例如， d-丝氨酸和甘氨酸）。相对于 EU 组或在 SL 条件下 3 天后，慢性 SL 显着增加了 SON 中的 NR2D 表达但不增加 NR2B 表达。最后，在 SL 条件下，Mg ²⁺抗性补品I _NMDA选择性上调 VP 神经元中的神经元兴奋性，独立于离子型 GABA 能输入。我们的结果表明，含有 NR2D 的 NMDAR 的激活构成了一种新机制，该机制在非去极化的 VP 神经元中产生 Mg ²⁺抗性强直I _NMDA，从而导致 VP 神经元中的 E/I 平衡转变，以补偿由慢性渗透挑战。

重要性声明下丘脑视上核 (SON) 由两种不同类型的大细胞神经分泌细胞 (MNC) 组成，它们合成和释放以下两种肽类激素：加压素 (VP)，它是调节体液稳态所必需的；催产素 (OT)，它在泌乳和分娩中起主要作用。NMDA 受体 (NMDAR) 在塑造神经元放电模式和 SON MNC 的激素释放方面发挥重要作用，以应对各种生理挑战。我们的结果表明，延长（7 天）的盐负荷产生了 Mg ²⁺由含有 NR2D 亚基的受体介导的抗性强直 NMDA 电流，可有效激活非去极化的 VP（但不是 OT）神经元。我们的研究结果支持这样的假设，即含有 NR2D 亚基的 NMDAR 在成人大脑中发挥重要的适应性作用，以响应持续的渗透挑战。