Action potential mechanism of mammalian cortical neurons in cell culture

Rat cortical neurons in 4- to 7-week-old dissociated cell cultures have action potentials (APs) which are tetrodotoxin-sensitive and appear to be generated by conventional sodium mechanisms. Approximately 10% of neurons can also generate APs when sodium channels are blocked. These latter APs are smaller, slower and sensitive to cobalt. When voltage sensitive potassium channels are blocked by TEA or when Ba is substituted for calcium, all the neurons are capable of generating large, overshooting, slow and prolonged, calcium-dependent APs. The amplitudes of these APs are proportional to the log of the divalent cation concentration and the APs are blocked by either cobalt or verapamil. We conclude that under physiological conditions, calcium currents are not likely to play a major role in the AP morphology in 'mature' neurons, but are more likely to be involved with other aspects of cell excitability and neurotransmitter function. APs in younger neurons (3-4 weeks in vitro) have a large calcium component even under physiological conditions.