Differential facilitation of N- and P/Q-type calcium channels during trains of action potential-like waveforms

Inhibition of presynaptic voltage-gated calcium channels by direct G-protein βγ subunit binding is a widespread mechanism that regulates neurotransmitter release. Voltage-dependent relief of this inhibition (facilitation), most likely to be due to dissociation of the G-protein from the channel, may occur during bursts of action potentials. In this paper we compare the facilitation of N- and P/Q-type Ca²⁺ channels during short trains of action potential-like waveforms (APW-s) using both native channels in adrenal chromaffin cells and heterologously expressed channels in tsA201 cells. While both N- and P/Q-type Ca²⁺ channels exhibit facilitation that is dependent on the frequency of the APW train, there are important quantitative differences. Approximately 20 % of the voltage-dependent inhibition of N-type I_Ca was reversed during a train while greater than 40 % of the inhibition of P/Q-type I_Ca was relieved. Changing the duration or amplitude of the APW dramatically affected the facilitation of N-type channels but had little effect on the facilitation of P/Q-type channels. Since the ratio of N-type to P/Q-type Ca²⁺ channels varies widely between synapses, differential facilitation may contribute to the fine tuning of synaptic transmission, thereby increasing the computational repertoire of neurons.