Direct projections from the central amygdaloid nucleus to the mesencephalic trigeminal nucleus in rats

The amygdala is activated by fear and plays an important role in the emotional response to life-threatening situations. When rats feel threatened, they respond by biting fiercely. Bite strength is regulated by the trigeminal motor nucleus and the mesencephalic trigeminal nucleus (Me5). The Me5 relays proprioceptive signals from the masticatory muscles and the periodontal ligaments to the trigeminal motor and premotor nuclei. The amygdala projects to the trigeminal motor nucleus and the premotor reticular formation. However, it is unknown whether the amygdala projects directly to the Me5. In the present study, neurons of the central amygdaloid nucleus (ACe) were labeled following injection of a retrograde tracer, Fast Blue, into the caudal Me5, and fibers and terminal buttons from the ACe to the Me5 were examined after injections of an anterograde neuronal tracer, biotinylated dextran amine into the ACe. Furthermore, wheat germ agglutinin-conjugated to horseradish peroxidase was injected into the ACe, and labeled fibers and terminal buttons in the Me5 were examined by electron microscopy. Labeled terminal buttons on Me5 somata were more abundant in the caudal than the rostral Me5. Electron microscopic observation revealed that a part of these terminal buttons formed axo-somatic synapses. These results indicate that the ACe sends direct projections to the Me5, and suggest that the amygdala regulates bite strength by modifying neuronal activity in the Me5.