Light microscopic analysis of contacts between pairs of identified leech neurons with combined use of horseradish peroxidase and lucifer yellow

Intracellular injections of horseradish peroxidase (HRP) and Lucifer Yellow (LY) were used to determine the locations of possible synaptic contacts between pairs of identified leech neurons. The pairs studied were ipsilateral touch sensory neurons (T cells), T cells plus the large longitudinal motoneuron (L cell), and T cells plus the annulus erector motoneuron (AE cell). The joint use of epi-fluorescence and transmitted light permitted the simultaneous observation of the processes of the HRP- and LY-filled cells. Our results indicate that, within the resolution of the light microscope, (a) over 40% of the varicosities of a T cell are in contact with those of another ipsilateral T cell, (b) both ipsilateral and contralateral T cells make numerous contacts with the two motoneurons studied, and (c) T cell-motoneuron contacts are distributed widely within the region where the cells overlap in the neuropil and occur on both major and minor processes of the motoneurons.