Origin and fine structure of substance P-containing nerve terminals in the facial nucleus of the rat: an immunohistochemical study.

The distribution, origin and fine structure of substance P-like immunoreactive (SPI) nerve terminals in the facial nucleus of the rat were investigated by means of immunocytochemistry. SPI-terminals were concentrated in the intermediate and dorsal subnuclei of the facial nucleus. Hemi-transection of the brainstem just rostral to the facial nucleus or at the most caudal level of the medulla oblongata did not cause any change of SPI-terminals in the facial nucleus. Electrical destruction of the various parts of the medulla oblongata clearly demonstrated that SPI-terminals in the intermediate subnucleus were supplied contralaterally from the SPI-neurons in the dorsomedial part of the medullary reticular formation. Most of the SPI-terminals (85%) in the intermediate subnucleus of the facial nucleus were observed to make asymmetric synaptic contacts with large dendrites (mean diameter; 1.26 micron). It was supposed that the contact sites are located on proximal parts of the dendrite. A few SPI-terminals (6%) formed axo-somatic contacts with large perikarya filled with numerous cytoplasmic organelles.