Computer simulations of high-pass filtering in zebrafish larval muscle fibres

Abstract

SUMMARY Larval somatic muscle of the zebrafish, Danio rerio, like that of some other organisms, responds to a sustained depolarization with one, and only one, action potential. Here, we report computer simulations, using the NEURON simulation programme, of sodium and potassium currents of somatic muscle of larval Danio rerio to investigate their possible contribution to once-only firing. Our computer model incorporated simulated sodium and potassium ion channels based on steady-state and kinetic parameters derived from a recent electrophysiological study. The model responded to sustained depolarizations with a single action potential at all levels of depolarization above threshold. By varying several parameters of the sodium and potassium currents systematically, the minimum changes necessary to produce repetitive firing were found to be a positive shift in the half-inactivation and a negative shift in the half-activation potentials for the sodium current, accompanied by a slowing of the rate of inactivation to half of the experimentally observed values. This suggests that once-only spiking can be attributed to the steady-state values of activation and inactivation of the sodium current, along with a slower rate of inactivation. Mapping of the resultant firing properties against steady-state and kinetic ion channel parameters revealed a high safety factor for once-only firing and showed that the time constant of inactivation of the sodium current was the key determinant of once-only or repetitive firing. The rapidly inactivating potassium current does not influence once-only firing or the maximum rate of firing in response to periodic excitation in these simulations. Although a contribution of other currents to produce once-only firing has not been excluded, this model suggests that the properties of the sodium current are sufficient to account for once-only firing.

The effects of amyloid peptides on A-type K+ currents of Drosophila larval cholinergic neurons: modeled actions on firing properties

“…1a(ii); V 0.5 activation simulated = 11.5 mV, slope = 9.2; V 0.5 activation measured = 4.7 mV, slope = 12; from Kidd et al 2006). The depolarizing shift in the voltage dependence of activation and the reduced slope of the Boltzmann equation had also been observed in a similar study (Buckingham and Ali 2005). We found that improvements could be made in the voltage dependence of activation by speeding activation and slowing inactivation.…”

“…1a(iii, iv); V 0.5 inactivation simulated = -56.1 mV, slope = 7.3). Following Buckingham and Ali (2005), this suggests that the simulated channels faithfully reproduce voltage-clamp experimental data. Figure 1b(i) shows the simulated delayed rectifier K + currents (Kdr) obtained in response to 50 ms depolarizing steps from -80 to 60 mV in 10 mV increments.…”

Nicotinic acetylcholine receptors (nAChRs) at zebrafish red and white muscle show different properties during development

“…Aside from their anatomical differences, these muscle types are physiologically distinct, with different electrophysiological phenotypes (Buss and Drapeau, ; Buckingham and Ali, ; Coutts et al, ) and functional roles (Buss and Drapeau, ), and arise from different precursor cells (Devoto et al, ). Red fibers do not express voltage gated Na + channels and do not support action potentials (Buckingham and Ali, ; Buckingham and Ali, ), thus they possess some of the hallmarks of tonic fibers. In addition, the red and white fibers are differentially innervated by motor neurons.…”

Protein kinase A modulates A‐type potassium currents of larval zebrafish (Danio rerio) white muscle fibres

“…In neurones the A‐type current also plays a role in affecting the frequency of action potentials (Connor & Stevens 1971, Rudy 1988, Tierney & Harris‐Warrick 1992). However, the fact that zebrafish muscle fibres only fire once, even in the presence of sustained excitatory input (Buckingham & Ali 2004, 2005, Coutts et al. 2006), suggests that modulation of the A‐type current in this preparation may have little effect on the frequency of firing.…”