Broadened Action Potentials in the Presynaptic Cell Increase Synaptic Delay in Crayfish Neuromuscular Junctions

Abstract

Broadened action potentials are predicted to reduce the rate of Ca²⁺ influx into presynaptic nerve terminals, decrease the single channel Ca²⁺ current, and thus reduce the rate of Ca²⁺ diffusion from microdomains to binding sites. Therefore we proposed that broadened action potentials would increase synaptic delay. Our results showed that the presence of 3,4- DAP increased synaptic delay, and doubling its concentration further increased delay. However, while initial concentrations of 3,4-DAP and TEA together increased synaptic delay, doubling their concentrations decreased the delay. Washing the cells, however, increased average synaptic delay again. Also, initial concentrations of TEA increased synaptic delay, but doubled concentrations decreased the delay. Thus our hypothesis is supported, except in the presence of higher concentrations of TEA. We propose that higher concentrations of TEA must affect some event in synaptic transmission other than K⁺ channels, causing the subsequent decrease in delay. Amplitude of the EPSP was also found to decrease using the higher concentration of TEA (a result not demonstrated by 3,4-DAP), so perhaps TEA blocks ion channels in the post-synaptic membrane, and thus reduces the rate of Na⁺ influx, reducing depolarization of the postsynaptic membrane and amplitude of the EPSP.