Direct amygdaloid projections to the superior salivatory nucleus: a light and electron microscopic study in the cat

Amygdaloid projections to the superior salivatory nucleus (SSN) were investigated in the cat by using the anterograde and retrograde tracing techniques of horseradish peroxidase (HRP). After HRP injections were made into the lingual nerve, retrogradely labeled SSN neurons were located in the lateral tegmental field medial to the spinal trigeminal nucleus from the middle level of the superior olivary nucleus to the caudal level of the facial nucleus. These labeled neurons, triangular, oval or polygonal in shape, were small to medium-sized (12-29 microns) and formed loosely packed clusters. In further HRP studies, HRP injections were made into the amygdala and in the reticular formation containing the SSN neurons. The results suggested that the SSN receives direct afferents from the central nucleus of the amygdala with ipsilateral predominance. Final proof of such direct connections from amygdala to the SSN can be obtained only by electron microscopic study. Therefore, HRP injections were made into the lingual nerve and in the amygdala in the same animal and electron microscopic observations were carried out on the SSN. It appeared that anterogradely labeled amygdalo-tegmental fibers formed axosomatic and axodendritic synaptic contacts with retrogradely labeled SSN neurons.