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 visual system are parts of motion-sensitive pathways that detect objects approaching on a collision course. The dendritic processes of the LGMD1 and -2 in the lobula are localised to discrete regions, allowing the dendrites of each neuron to be distinguished uniquely. As was described previously for the LGMD1, the afferent processes onto the LGMD2 synapse directly with each other, and these synapses are immediately adjacent to their outputs onto the LGMD2. Here we present immunocytochemical evidence, using antibodies against choline-protein conjugates and a polyclonal antiserum against choline acetyltransferase (ChAT; Chemicon Ab 143), that the LGMD1 and -2 and the retinotopic units presynaptic to them contain acetylcholine (ACh). It is proposed that these retinotopic units excite the LGMD1 or -2 but inhibit each other. It is well established that ACh has both excitatory and inhibitory effects and may provide the substrate for a critical race in the LGMD1 or -2, between 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 chiasm between the lamina and medulla. In the brain, the antennal lobes contained neurons that reacted positively for ACh. Silver- or haematoxylin and eosin-stained sections through the optic lobe confirmed the identities of the positively immunostained neurons.