MECHANISMS AND ROLE OF SUBTHRESHOLD DEPOLARIZING SPONTANEOUS FLUCTUATIONS IN CORTICAL NEURONS

Depolarizing Spontaneous Fluctuations (DSFs) are intrinsic membrane-potential instabilities that promote action potential generation and are well studied in peripheral sensory neurons. Despite recurrent descriptions of similar subthreshold events in neurons from the central nervous system, a systematic characterization of DSFs in these cells remains lacking. Here, we used whole-cell patch-clamp recordings to investigate DSFs in cultured mouse cortical neurons (n = 124 cells). We characterized DSF properties and development during maturation, identified their molecular substrate, and assessed their functional impact on action potential firing and spontaneous rhythmic network activity. Cortical neurons exhibit robust DSFs whose occurrence increases with depolarization. Pharmacological studies showed that these events depend on tetrodotoxin-sensitive Na⁺ channels, with Na_V1.2 identified as their main molecular substrate. Functionally, DSFs are closely associated with irregular action-potential firing, and Na_V1.2 inhibition selectively disrupts this type of firing. Moreover, DSFs are enhanced during excitatory synaptic events, indicating that synaptic drive promotes their emergence. Na_V1.2 inhibition abolishes the generation of spontaneous rhythmic network activity reminiscent of cortical up-states, suggesting a potential role for DSFs and irregular firing in their generation. Overall, our results indicate that Na_V1.2-mediated DSFs constitute a previously unrecognized intrinsic mechanism shaping cortical excitability and synaptic activity, and position Na_V1.2 as a candidate therapeutic target in disorders characterized by aberrant cortical hyperexcitability.