Mapping and ultrastructure of serotonin-immunoreactive neurons in the optic lobes of three insect species.

With antibodies to serotonin (5-HT) we have mapped immunoreactive neurons in the optic lobes of three species, the blowfly Calliphora, the desert ant Cataglyphis, and the worker bee Apis. The main emphasis in this investigation is on a system of 5-HT-positive neurons connecting the most peripheral neuropil of the optic lobes, the lamina, to more central neuropil regions. To aid in electron microscopical identification of these neurons we used immunocytochemistry at the EM-level and Golgi-EM for Calliphora and horseradish peroxidase (HRP) labelling for the other two insects. The immunoreactive terminals in Calliphora and the HRP-labelled ones in the other insects contain large (c. 100 nm) granular vesicles and smaller (c.60 nm) clear vesicles. In Cataglyphis and Apis the profiles with granular vesicles are presynaptic to second order neurons of the lamina, whereas in Calliphora no synaptic contacts were found. In this animal the 5-HT-positive terminals are situated distal to the synaptic layer of the lamina, in a region of retinal photoreceptor axons and perikarya of the lamina monopolar neurons. In Catagylphis and Apis the interactions of the 5-HT-neurons with the laminar neurons might occur through chemical synapses, whereas in Calliphora neuroactive substance could be released non-synaptically from varicosities distal to the synaptic layer. The possible involvement of 5-HT in control of neuronal activity in the optic lobes is discussed.