Membrane current-based mechanisms for excitability transitions in neurons of the rat mesencephalic trigeminal nuclei

Since Hodgkin's first description of three classes of excitability in crustacean nerve axons (1948), theoretical studies have used mathematical models to demonstrate that small changes in the parameters describing ionic currents could result in transitions between classes of membrane excitability. However, these transitions have rarely been investigated experimentally. Here, we show that states of excitability in rat mesencephalic V (Mes V) neurons can be classified into three groups, with manipulations of the 4-aminopyridine sensitive K+ current (I4-AP) or persistent Na+ current (INaP) leading to the corresponding transitions. However, alterations in the hyperpolarization-activated cation current (Ih), tetraethylammonium (TEA)-sensitive K+ current, or Cd2+-sensitive Ca2+ current were ineffective in causing these transitions. These results provide experimental evidence for the excitability transitions predicted by Hodgkin and characterize their ionic mechanisms in Mes V neurons.