Divalent cation depletion enhances intrinsic neuronal excitability through CaSR-dependent modulation of threshold ion channels.

External calcium ([Ca2+]e) and magnesium ([Mg2+]e) concentrations fluctuate across physiological and pathological brain states. For example, [Ca2+]e decreases during intense neuronal activity and epilepsy, whereas it rises during sleep. Similarly, [Mg2+]e varies with the sleep/wake cycle and is reduced in epilepsy. Lowering either [Ca2+]e or [Mg2+]e increases intrinsic excitability and hyperpolarizes the action potential (AP) threshold, yet the underlying mechanisms remain unclear. Here, we confirm that reducing [Ca2+]e or [Mg2+]e enhances intrinsic excitability and hyperpolarizes the AP threshold of CA1 pyramidal neurons. Physiological reductions in [Mg2+]e (0.8 → 0.4 mM) have minimal effect, whereas decreases from supraphysiological levels (2.0 → 0.4 mM) robustly increase excitability. Using pharmacology and CRISPR/Cas9 gene editing, we identify the calcium-sensing receptor (CaSR) as a mediator of these effects. The calcilytic NPS-2143 mimics and largely occludes both the intrinsic excitability increase and the AP threshold hyperpolarization, while genetic reduction of CaSR produces similar outcomes. We further show that AP threshold hyperpolarization induced by low-divalent cations involves both Kv1 and Nav1.2 channels. Together, these findings reveal that CaSR participate in the enhancement of intrinsic neuronal excitability induced by external divalent cation levels.