Electron microscopy of serotonin-immunoreactive neuron branches and terminals in the locust central nervous system

Using a pre-embedding peroxidase technique, we have investigated the ultrastructure of elements that react with antibodies to 5-hydroxytryptamine in the central nervous system of the locust. Reactive neuron profiles are widespread, and contain a variety of vesicle types: small lucent vesicles, some of which are stained only on their outer membranes while others are stained internally, and large dense granules that again differ in the staining properties of their cores. Some of the reactive profiles contain synaptic specializations, while others receive synaptic inputs from unlabelled elements. The heterogeneity of reactive profiles makes it impossible to define ultrastructural characteristics of putative serotonergic terminals that might be generally applicable. The differential reactivity both of small vesicles and of large dense granules may indicate functional differences within these categories.