A light and electron microscopical localisation of the superior salivatory nucleus of the rat.

The present study localised the superior salivatory nucleus (SSNc) in the reticular formation in rats using the retrograde axonal transport of fluorogold (FG). An ultrastructural study using the retrograde transport of horseradish peroxidase (HRP) was also carried out. After FG or cholera toxin conjugated-HRP (CT-HRP) injections into the lingual nerve, the labelling of SSNc in both the experiments was comparable. The retrogradely labelled cells were located in the ipsilateral parvocellular reticular formation (PCRt), dorsolateral to the facial nucleus and medial to the nucleus of the spinal tract of the trigeminal nerve. The rostrocaudal extent of the nucleus was about 0.9-1.05 mm, between the levels of the ascending facial nerve fibres caudally and the root of the facial nerve rostrally. The number of cells labelled was between 300-360, most of them being small to medium sized (14 x 22 mm) and appeared fusiform or polygonal. In rats receiving intraglandular injections of the tracers, the localisation and the rostrocaudal extent of the SSNc were similar, but the number of its containing labelled cells was only about 1/2 of the above. The SSNc cells that supplied the intra-glandular submandibular ganglion were not topographically arranged. On electron microscopy, the SSNc cells showed the typical features of preganglionic autonomic neurons. The cells contained a prominent round or oval nucleus. Their cytoplasm was rich in organelles especially conspicuous was the aggregations of the cisternae of rough endoplasmic reticulum in distinct clumps. In the neuropil between the SSNc cells were present numerous dendrites, axons, axon terminals and neuroglial cells. Most of the axon terminals contained predominantly round agranular vesicles with a few dense-cored or flattened agranular vesicles. The majority of the synapses associated with the HRP-labelled SSNc cells were on the dendrites with occasional axosomatic or axoaxonal synaptic contacts. A remarkable feature was the occurrence of synaptic contacts between HRP-labelled axons and some labelled soma.