EVIDENCE FOR THE CHOLINERGIC NATURE OF C-TERMINALS ASSOCIATED WITH SUBSURFACE CISTERNS IN ALPHA-MOTONEURONS OF RAT

C-terminals can be distinguished at the ultrastructural level from oth er types of nerve endings on motoneurons by their prominent and regula rly occurring postsynaptic specializations termed subsurface cisterns (SSC). We have previously shown (Yamamoto et al., 1991) that an antibo dy directed against a sequence within the gap junction protein connexi n32 immunolabels these motoneuronal SSCs and can therefore serve as a immunohistochemical tool to visualize indirectly the location of C-ter minals on motoneurons at the light microscope level. Here we have used this anti-SSC antibody in combination with antibodies against choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) to determine whether C-terminals on motoneurons contain these cholinergic enzyme m arkers. In sections at all major spinal cord levels and in several cra nial motor nuclei examined, motoneuronal cell bodies and their proxima l dendrites were studded with large ChAT-immunoreactive (ChAT-IR) bout ons. Boutons having a similar distribution and appearance on motoneuro ns were also immunolabelled for AChE. In addition, motoneurons were su rrounded by a dense plexus of AChE-immunoreactive (AChE-IR) varicose f ibers and fine preterminal axons. In double-labeled sections, AChE-IR boutons corresponded to those immunolabeled for ChAT. In sections proc essed for simultaneous immunofluorescence detection of ChAT and SSCs, ChAT-IR boutons were very often found in apposition to immunolabeled S SCs. In sections processed for simultaneous labeling of AChE and SSCs, AChE-IR boutons were again frequently seen abutting labeled SSCs. The se results provide the first strong evidence at the LM level that a la rge proportion, if not the entirety, of C-terminals are cholinergic an d show that these terminals consist in part of relatively large varico sities along highly varicose axons that form en passant type contacts on motoneurons. At the same time, our results substantially narrow pos sibilities regarding the as yet undetermined source of C-terminals, wh ich can now be considered to originate from cholinergic neurons, such as those located in the brainstem and/or the spinal cord. (C) 1993 Wil ey-Liss, Inc.