Ultrastructural features of synapse from dorsal parvocellular reticular formation neurons to hypoglossal motoneurons of the rat

The dorsal parvocellular reticular formation (PCRt) receives projection of the trigeminal mesencephalic nucleus neurons. It contains the dorsal group of interneurons that integrate and coordinate activity of the oral motor nuclei. Ultrastructural features of synaptic connection from the dorsal PCRt neurons to the motoneurons of the hypoglossal nucleus (XII) were examined at both the light and electron microscopic levels in rats. Biotinylated dextran amine (BDA) was initially iontophoresed into the dorsal part of PCRt unilaterally. Seven days later horseradish peroxidase (HRP) was injected into the body of the tongue. After histochemical reaction for visualization of HRP and BDA, the BDA-labeled fibers and terminals were seen distributing bilaterally in XII with ipsilateral predominance. BDA-labeled terminals were closely apposed upon HRP retrogradely labeled somata and dendrites of the XII motoneurons. A total of 1408 BDA-labeled boutons were examined ultrastructurally, which had mean size of 1.22+/-0.37 microm in diameter. Five hundred-ninety three of these boutons in both the ipsilateral (n=401) and contralateral (n=192) XII were seen to synapse on both the dendrites and somata of HRP-labeled motoneurons. The vast majorities of synapses were axodendritic (98%, 580/593), while 2% of them were axosomatic. Of the 1408 BDA-labeled boutons, 69.6% of them were S-type boutons containing small clear and spherical synaptic vesicles and 30.4% of them were PF-type boutons containing pleomorphic and flattened synaptic vesicles. Approximately 64% of synapses between BDA-labeled boutons and HRP-labeled motoneurons were asymmetric, and 33% of synapses were symmetric. No axoaxodendritic or axoaxosomatic synaptic triad was observed. The present study illustrated the anatomical pathway and synaptological characteristics of neuronal connection between the dorsal PCRt premotor neurons and the XII motoneurons. Its functional significance in coordinating activity of XII motoneurons during oral motor behaviors has been discussed.