C-terminals on motoneurons: electron microscope localization of cholinergic markers in adult rats and antibody-induced depletion in neonates.

C-terminals on motoneurons are defined as those accompanied by characteristic postsynaptic specializations termed subsurface cisterns. We have previously shown, by light microscope immunolabelling methods, that subsurface cisterns occur regularly beneath choline acetyltransferase- and acetylcholinesterase-containing boutons on motoneurons. In the present study, the cholinergic nature of C-terminals suggested by these results was further investigated by immunohistochemistry and electron microscopy in adult rats and in neonates treated with a murine monoclonal acetylcholinesterase antibody which was previously shown to cause immunological lesions of central cholinergic systems. In both the facial nucleus and lumbar segment of spinal cord of adult rats, C-terminals were seen intensely immunostained for the cholinergic markers choline acetyltransferase and acetylcholinesterase. Immunolabelled terminals made contact with either neuronal somata or large calibre dendrites, which were positive for the cholinergic markers, and exhibited club-shaped or thin elongated morphologies suggestive of terminal or en passant type synaptic interactions. The close relationship found between cholinergic markers and immunolabelled subsurface cisterns in adults was maintained on motoneurons of eight-day-old rats. While subcutaneous treatment of newborn rat with acetylcholinesterase antibody appeared to have no effect on the distribution of immunopositive subsurface cisterns in motoneurons when examined on postnatal day 8, the density of labelling for the two cholinergic markers around these neurons was reduced. Areas of neuropil immediately surrounding motoneurons in treated animals often showed signs of extensive swelling and deterioration indicative of a lesion event, and these motoneurons frequently displayed subsurface cisterns unapposed to C-terminals. These results support our earlier conclusion, based on light microscope investigation, that the majority if not all C-terminals are cholinergic in the areas investigated and demonstrate the potential utility of immunolesion methods in the study of C-terminal function.