Immunocytochemical evidence that collision sensing neurons in the locust visual system contain acetylcholine

The lobula giant movement detector (LGMD1 and -2) neurons in the locust vis ual system are parts of motion-sensitive pathways that detect objects appro aching on a collision course. The dendritic processes of the LGMD1 and -2 i n the lobula are localised to discrete regions, allowing the dendrites of e ach neuron to be distinguished uniquely. As was described previously for th e LGMD1, the afferent processes onto the LGMD2 synapse directly with each o ther, and these synapses are immediately adjacent to their outputs onto the LGMD2. Here we present immunocytochemical evidence, using antibodies again st choline-protein conjugates and a polyclonal antiserum against choline ac etyltransferase (ChAT; Chemicon Ab 143), that the LGMD1 and -2 and the reti notopic units presynaptic to them contain acetylcholine (ACh). It is propos ed that these retinotopic units excite the LGMD1 or -2 but inhibit each oth er. It is well established that ACh has both excitatory and inhibitory effe cts and may provide the substrate for a critical race in the LGMD1 or -2, b etween excitation caused by edges moving out over successive photoreceptors , and inhibition spreading laterally resulting in the selective response to objects approaching on a collision course. In the optic lobe, ACh was also found to be localised in discrete layers of the medulla and in the outer c hiasm between the lamina and medulla. In the brain, the antennal lobes cont ained neurons that reacted positively for ACh. Silver- or haematoxylin and eosin-stained sections through the optic lobe confirmed the identities of t he positively immunostained neurons. (C) 2000 Wiley-Liss, Inc.