The locus coeruleus projects to the mesencephalic trigeminal nucleus in rats.

The ganglion-cells in the mesencephalic trigeminal nucleus (Me5) process proprioceptive signals from the masticatory muscles and the periodontal ligaments, and are considered to regulate the rhythm of biting and bite strength. The locus coeruleus (LC) is the major source of noradrenergic projections in the brain and plays an important role in stressful situations and aggressive behavior. The two nuclei are adjacently located to each other in the lateral part of the periaqueductal gray matter of the fourth ventricle. In the present study, a small number of neurons were labeled in the LC with a neuronal tracer biotinylated dextran amine. The labeled single axons were traced from the labeled LC neuronal somata to the ipsilateral Me5 region where they produced terminal-like swellings. Some of the swellings appeared to make contact with the ganglion-cells of the Me5. These results suggest that the LC regulates the bite strength by modifying the ganglion-cell activity in the Me5. Additionally, these findings shed light on the enigma of why the main part of the Me5 at the level of pons is located at the lateral end of the gray matter ventral to the fourth ventricle, instead of at the trigeminal ganglion.