Trigeminovestibular and trigeminospinal pathways in rats: Retrograde tracing compared with glutamic acid decarboxylase and glutamate immunohistochemistry

This study identified neurons in the sensory trigeminal complex with connections to the medial (MVN), inferior (IVN), lateral (LVN), and superior (SVN) vestibular nuclei or the spinal cord. Trigeminovestibular and trigeminospinal neurons were localized by injection of retrograde tracers. Immunohistochemical processing revealed gamma-aminobutyric acid (GABA)- and glutamate-containing neurons in these two populations. Trigeminovestibular neurons projecting to the MVN and the IVN were in the caudal principal nucleus (5P), pars oralis (5o), interpolaris (5i), and caudalis (5c) and scattered throughout the rostra] 5P. Projections were bilateral to the IVN, with an ipsilateral dominance to the MVN, except from the rostral 5P, which was contralateral. Neurons projecting to the LVN were numerous in the ventral caudal 5P and the 5o and less abundant in the rostral 5P, 5i, and 5c. Our results suggested that only 5P and 5o project to the dorsal LVN. Neurons projecting to the SVN were in the dorsal 5P, 5o, and 5i but not in 5c. Trigeminospinal neurons were mainly in the ventral 5o and 5i and in the lateral 5c, rarely or never in 5P. Among trigeminovestibular neurons, most of the somas were immunoreactive for glutamate, but some reacted for GABA. Among trigeminospinal neurons, the number of somas immunoreactive for each of the two amino acids was similar. Trigeminal terminals were observed in contact with vestibulospinal neurons in the IVN and LVN, giving evidence of a trigeminovestibulospinal pathway. Therefore, inhibitory and excitatory facial inputs may contribute through trigeminospinal or trigeminovestibulospinal pathways to the control of head/neck movements.