Morphology, ultrastructure and synapse distribution of putative serotonergic salivary neurons in the locust

Neurons innervating the locust salivary glands have been investigated using the electron microscope. We have examined the projections in the suboesophageal ganglion of two identified salivary neurons by differential double labelling with cobalt ions, followed by silver intensification. Numerous synaptic inputs occur on the arborizations of the salivary neurons, particularly on fine branches and on small spines arising from larger branches. Although a few instances occur of common input to both salivary neurons from a single presynaptic element, many of the appositions between branches seen in the light microscope do not represent functional connections. A few structures resembling presynaptic dense bodies have been observed in salivary neuron profiles, but with few synaptic vesicles. Large dense granules are present in some labelled profiles, but not in the vicinity of synapse-like membrane specializations. We have also examined unidentified neuron profiles within the acini of the salivary glands, which contain large dense granules and small vesicles. There is good evidence that these unidentified terminals correspond to the suboesophageal salivary neurons. The central arborizations of the salivary neurons appear to serve largely as sites of synaptic input, whereas the peripheral terminals are likely sites of transmitter release. The results are considered in the context of the known immunoreactivity of the salivary neurons with antibodies to 5-hydroxytryptamine, and compared with analogous systems in other insect groups.