Maintaining Ca2+ homeostasis is extremely important for the nervous system. Fluctuations in extracellular Ca2+ concentration ([Ca2+]e) are very frequent, both under physiological and pathological conditions. Among extracellular calcium sensors, the Calcium-Sensing Receptor (CaSR), is a G-protein coupled receptor activated by [Ca2+]e and is ubiquitously expressed in the body. The CaSR is expressed in brain regions involved in memory and cognition, however the mechanisms by which CaSR regulates neuronal function remain unexplored.We therefore studied how CaSR regulates neuronal excitability in the CA1 hippocampal region. Lowering external calcium from 3.0 or 1.3 mM to 0.6 mM [Ca2+]e, increased neuronal excitability and reduced first-spike delay. We first tested if the reduction of the spike delay was mediated by the downregulation of Kv1 channels. We then tested the hypothesis of the gain of function of voltage-gated sodium (Nav) channels as a consequence of the reduction of [Ca2+]e. To do this, we deleted the genes encoding Nav1.2 or Nav1.6 in CA1 pyramidal neurons of hippocampal organotypic cultures using CRISPR/Cas9 editing. Our data indicate that the CaSR inactivation induced by the decrease in [Ca2+]e from 3.0 to 0.6 mM [Ca2+]e , induced an increase in the excitability of the CRISPR/Nav1.6 transfected neurons but not in CRISPR/Nav1.2 transfected ones. Additionally the decrease in [Ca2+]e from 1.3 to 0.6 mM didn’t induce any increase in the excitability of the CRISPR/Nav1.6 nor in the CRISPR/Nav1.2 transfected neurons. We also used pharmacological agents to activate/inactivate CaSR and observe the consequences on excitability at different extracellular Ca2+ concentrations.