An evaluation of synapse independence

If, as is widely believed, information is stored in the brain as distribute d modifications of synaptic efficacy, it can be argued that the storage cap acity of the brain will be maximized if the number of synapses that operate independently is as large as possible. The majority of synapses in the bra in are glutamatergic; their independence will be compromised if glutamate r eleased at one synapse can significantly activate receptors at neighboring synapses. There is currently no agreement on whether "spillover" after the liberation of a vesicle will significantly activate receptors at neighborin g synapses. To evaluate the independence of central synapses, it is necessa ry to compare synaptic responses with those generated at neighboring synaps es by glutamate spillover. Here, synaptic activation and spillover response s are simulated in a model, based on data for hippocampal synapses, that in cludes an approximate representation of the extrasynaptic space. Recently-p ublished data on glutamate transporter distribution and properties are inco rporated. Factors likely to influence synaptic or spillover responses are i nvestigated. For release of one vesicle, it is estimated that the mean resp onse at the nearest neighboring synapse will be <5% of the synaptic respons e. It is concluded that synapses can operate independently.