Methods for grouping shapes of synaptic currents recorded from sets of synapses

Synaptic currents due to transmitter release at sets of synapses, such as those formed by a single nerve terminal on adjacent smooth muscle cells or a nerve terminal on a dendrite, possess different amplitudes and time courses, that is different shapes. It is possible that different shape groupings exist that may be each associated with a particular synapse in the set recorded from. Two methods, involving the distance between two synaptic currents, namely multidimensional scaling and hierarchical clustering, were used to suggest possible groupings. A test of differences between a pair of synaptic currents was developed by comparing the distances from pairs of synaptic currents with the distances from pairs of failures from the same experiment. This allowed observation of significance levels between and within the suggested groups. These methods were applied to loose-patch recordings of excitatory junctional currents (EJCs) in mouse vas deferens made with extracellular electrodes, some of which were visualised using DiOC2(5) staining to identify varicosities. Groups of EJC shapes could be distinguished supporting a hypothesis of several categories of shapes. However, each method showed that the number of shape-groupings was much larger than the number of visualised varicosities. Possible reasons for this discrepancy are analysed, including a model in which different shape groupings can be generated by a single synaptic site.