THE MEMBRANE ORIGIN OF EPILEPTIC-RELATED PAROXYSMS: THE INTERPLAY BETWEEN HYPERPOLARIZATION-ACTIVATED INWARD CURRENT AND CA<SUP>2+</SUP> CURRENTS, A MATHEMATICAL APPROXIMATION AND EXPERIMENTAL CORRELATION

Paroxysmal depolarization shifts (PDS) are a hallmark of epileptic activity and have traditionally been interpreted as giant excitatory postsynaptic potentials arising from network synchronization that favors excitation over inhibition. However, studies in isolated single cells from invertebrate models challenge this view, showing that PDS can occur independently of synaptic inputs. In this context, the serotonergic C1 Helix neuron develops PDS during drug-induced epileptic-like activity, making it a useful model to explore intrinsic cellular mechanisms. We analyzed membrane potential dynamics using a mathematical model incorporating leakage currents (I_L), persistent and transient voltage-dependent Na⁺ currents (I_NaP and I_NaT), a voltage-dependent K⁺ current (I_K), and a hyperpolarization-activated inward current (I_h). The model revealed that modifications in I_h properties, such as conductance and voltage dependence, are sufficient to drive a transition from regular action potential firing to doublets and PDS. Previous evidence indicated that increased Ca²⁺ currents (I_Ca) enhance the propensity of C1 neurons to generate PDS. Accordingly, we used electrophysiology, calcium imaging, and antibodies to identify and characterize the C1 dominant Ca²⁺ channels, examined their distribution in neurites and varicosities, and evaluated their contribution to action potential firing. We further demonstrated that increased I_Ca correlates with enhanced I_h-related voltage sag and rebound depolarization, and intracellular Ca²⁺ levels increase during epileptic-like activity. Together, these findings indicate that epileptic-like activity in isolated neurons can arise from intrinsic membrane mechanisms, driven by I_h modulation under the influence of increased I_Ca. Modeling intrinsic PDS mechanisms enables cost-effective in silico screening of anticonvulsant molecules without animal use.