Reaction-diffusion modeling of neurotransmitter processing at a high frequency synapse

In the weakly electric fish Eigenmannia (glass knifefish), high frequency (200-550Hz) electric organ discharge (EOD) is driven by high frequency cholinergic synaptic input onto the electrocytes at their electroplaques. Assuming periodic release of ACh into the cylindrical synaptic gap, we solve numerically a one dimensional reaction-diffusion model at 200Hz and 500Hz. The model included the diffusion of ACh and its interactions with AChesterase (AChE) and AChRs. At 500Hz a higher AChE/ACh ratio is needed to remove ACh from the cleft between consecutive ACh releases. Only a small fraction of the ACh molecules reaches the AChRs, and there are residual amounts of ACh molecules from the preceding release. Previous computational studies showed that the persistently present ACh should not impede high frequency electrocyte firing, provided the ensuing cholinergic current is subthreshold for triggering firing. Our results suggest that the EOD’s upper bound frequency is attained when that persistent cholinergic current exceeds the firing threshold. The observed maximum frequency in Eigenmannia individuals is around 550Hz.