GABA-immunoreactive neurons and terminals in the lateral cervical nucleus of the cynomolgus monkey

An antiserum against the inhibitory transmitter substance gamma-aminobutyric acid (GABA) was used to investigate the distribution of GABAergic nerve terminals and cell bodies in the lateral cervical nucleus (LCN) of the cynomolgus monkey. Light microscopic immunohistochemistry demonstrated GABA-immunoreactive puncta, suggestive of nerve terminals, scattered throughout the LCN. The terminal-like profiles are often present along the somata of unlabeled neurons, but most are located in the neuropil. GABA-immunoreactive neurons are present in the LCN, but constitute a very small number of the LCN neurons. Electron microscopy showed that the GABA-positive neurons are small with a relatively large nucleus. They are contacted by few somatic boutons. Numerous GABA-immunoreactive terminals containing densely packed round to oval synaptic vesicles were also found. Most GABA-positive terminals make synaptic contact with dendrites, but synapses with cell bodies are also present. Synaptic contacts between labeled and unlabeled terminals were not observed. Some GABA-positive terminals make contact with GABA-positive neurons. The present findings suggest that GABA is a major inhibitory transmitter substance in the LCN of the monkey. However, in comparison with other somatosensory relay nuclei, there are few GABA-immunoreactive neurons in the LCN. This may imply that the GABA-positive neurons branch extensively in the LCN or that an extrinsic source of GABAergic input exists.