Variable properties in a single class of excitatory spinal synapse.

Although synaptic properties are specific to the type of synapse examined, there is evidence to suggest that properties can vary in individual synaptic populations. Here, a large sample of monosynaptic connections made by excitatory interneurons (EINs) onto motor neurons in the lamprey spinal cord locomotor network has been used to examine the properties of a single class of spinal synapse in detail. The properties and activity-dependent plasticity of EIN-evoked EPSPs varied considerably. This variability occurred at convergent inputs made by several EINs onto single motor neurons. This suggests that it was an intrinsic network property and not simply related to differences between animals or experiments. The activity-dependent plasticity of EIN-evoked EPSPs could be negatively or positively related to the initial EPSP amplitude (P1 and P2 connections, respectively). This reflected the development of facilitation and depression from either small or large initial EPSPs. To identify differences in presynaptic properties that could contribute to the synaptic variability, the quantal amplitude, release probability, number of release sites, and size of the available vesicle pool were examined. This analysis suggested that the variable amplitude and plasticity of EPSPs at P1 and P2 connections reflected an interaction between the release probability and the size of the available transmitter store. There is thus significant functional variability in EIN synaptic properties. Synapses ranged from strong (evoked postsynaptic spikes) to weak (small depressing EPSPs). The selection of interneurons with different synaptic properties could provide an intrinsic mechanism for modifying excitatory network interactions and the locomotor network output.