Leucine-enkephalin-containing neuron system in the facial nucleus of the rat with special reference to its fine structure.

The light and electron microscopic localization of leucine-enkephalin-containing terminals in the facial nucleus of the rat were investigated by means of the peroxidase-antiperoxidase (PAP) immunocytochemical technique. By light microscopy, leucine-enkephalin-like immunoreactive (LEI) terminals were unevenly distributed in the facial nucleus. The greatest accumulation of the terminals was seen in the medial part of the nucleus. Electron microscopic examination of LEI-terminals in the medial part of the nucleus revealed that the predominant type of synaptic contacts of LEI-terminals in this area were axo-dendritic contacts (about 75%). These dendrites which made synapses with LEI-terminals were relatively large and rich in cytoplasmic organella, suggesting that they belonged to the proximal segment of the dendrite. A small number of LEI-terminals was found to make synaptic contact with neuronal perikarya (5%). These perikarya were very large and had nuclei with less chromatin particles. These findings suggest that LEI-terminals make contact with neurons which exist in the facial nucleus. The rest of the LEI-terminals (20%) were in apposition to the non-labelled axon terminals which contain small, clear and round vesicles.