Dopamine-immunoreactive neurons in the blowfly visual system: light and electron microscopic immunocytochemistry

Dopamine-immunoreactive (DA-IR) neurons were mapped in detail in the visual system of the blowfly, Calliphora erythrocephala. Three types of DA-IR neurons could be identified in the optic lobes. One type constitutes a population of several thousand columnar small field amacrine neurons in the second neuropil region, the medulla. The other two types are large field projection neurons innervating the next, more central, synaptic region comprising the lobula and the lobula plate, as well as centres of the midbrain. Their cell bodies are located latero-ventrally in the brain. No DA-IR neurons were seen in the most peripheral visual synaptic neuropil, the lamina. The two types of projection neurons form overlapping wide field arborizations in the lobula and lobula plate and cannot be distinguished from each other in this region. Their central connections are different, however. One type of projection neuron, BOD1, consists of two neurons that bilaterally connect the optic lobes and neuropil on each side of the oesophageal foramen in the posterior protocerebrum. The other type, BOD2, also consists of two bilateral neurons similar to BOD1, but with their central processes posteriorly in the lateral protocerebrum. The amacrine DA-IR neurons form lateral processes in three layers of the medulla synaptic neuropil. These neurons were also investigated by means of electron microscopical immunocytochemistry. They contain predominantly clear vesicles, but a few dense core vesicles could be resolved. The synaptic connections of the DA-IR amacrines suggest that they form centrifugal feedback circuits between the inner and the outer portion of the medulla. The present results indicate that dopamine may be a neurotransmitter in functionally different classes of neurons of the blowfly visual system: amacrines and projection neurons.