Localization of a leech inhibitory synapse by photo-ablation of individual dendrites.

An inhibitory motor neuron (cell 1) in the leech nervous system has a powerful inhibitory connection onto an excitatory motor neuron (cell 3) that is functionally important in behaviours such as swimming and local bending. The anatomical location of this connection was explored using focal ultraviolet irradiation of cell 3 dendrites filled with Lucifer yellow. Ablation of the main neurite of cell 3 in the middle of the ganglion eliminated 72% of the inhibitory postsynaptic potential (IPSP), showing that most of the synaptic contacts are in the dendritic field contralateral to the cell body. Ablation of a particular dendritic branch (d1), one of several that run anteriorly from the main neurite in the contralateral ganglion, eliminated 70% of the IPSP in some cases but only 4% in others. In these latter cases, subsequent ablation of a more distal dendrite (d2) eliminated from 41% to 83% of the IPSP. These findings suggest that the synapses onto cell 3 from cell 1 are primarily mediated by either one of these dendrites or the other, but not both. This synaptic specificity might be due to a developmental mechanism involving competition between dendrites for occupation of synaptic sites.