Knock-down of synapsin alters cell excitability and action potential waveform by potentiating BK and voltage-gated Ca2+ currents in Helix serotonergic neurons

Synapsins are an evolutionarily-conserved family of presynaptic proteins crucial for the fine-tuning of synaptic function. A large amount of experimental evidences has shown that the synapsins are involved in the development of epileptic phenotypes and several mutations in synapsin genes have been associated with epilepsy in humans and animal models. Synapsin mutations induce alterations in circuitry and neurotransmitter release, differentially affecting excitatory and inhibitory synapses, thus causing an excitation/inhibition imbalance in network excitability toward hyperexcitability that may be determinant with regard to the development of epilepsy. Another approach to investigate epileptogenic mechanisms is to understand how silencing synapsin affects cellular behavior of single neurons and is associated with the hyperexcitable phenotypes observed in epilepsy. Here, we examined the functional effects of antisense-RNA inhibition of synapsin expression in individually identified and isolated serotonergic cells of the Helix land snail. We found that Helix synapsin silencing increases cell excitability characterized by a slightly depolarized resting membrane potential, decreases the rheobase, reduces the threshold for action potential (AP) firing and increases the mean and instantaneous firing rates, with respect to control cells. The observed increase of Ca2+ and BK currents in synapsin-silenced cells seems to be related to changes in the shape of the AP waveform. These currents sustain the faster spiking in synapsin-deficient cells by increasing the after hyperpolarization and limiting the Na+ and Ca2+ channel inactivation during repetitive firing. This in turn speeds up the depolarization phase by reaching the AP threshold faster. Our results provide evidence that synapsin silencing increases intrinsic cell excitability associated with increased Ca2+ and Ca2+-dependent BK currents in the absence of excitatory or inhibitory inputs.