Glutamate escape from a tortuous synaptic cleft of the hippocampal mossy fibre synapse

The time course of neurotransmitter in the synaptic cleft contributes substantially to the fast kinetics of synaptic signalling. Hippocampal mossy fibres (MFs), a well-characterised excitatory pathway from dentate granule cells to the hippocampus proper, form large glutamatergic synapses at branched spiny structures in CA3 pyramidal cell dendrites. To what extent transmission at these synapses is affected by retarded glutamate clearance from the large tortuous synaptic cleft is not known. Here, we propose a simple geometrical approximation representing the 'typical' geometry of thorny excrescences that form the tortuous cleft interface at a MF synapse. We then employ Monte Carlo simulations to monitor movements of 3000 individual glutamate molecules released within the cleft. The results predict that, in the absence of neuronal glutamate transporters, it should take approximately 10 ms for 50% and 60-70 ms for 90% of glutamate molecules to escape the MF synapse.