Oculomotor control in calliphorid flies: GABAergic organization in heterolateral inhibitory pathways.

In calliphorid Diptera, motor neurons mediating visually evoked head movements can be excited or inhibited by visual stimuli, depending on the directionality of the stimulus and whether it is in the ipsi- or contralateral visual field. The level at which inhibition occurs is of special interest because binocular activation of homolateral tangential neurons in the lobula plate demonstrates that excitatory interaction must occur between the left and right optic lobes. Recordings and dye fillings demonstrate a variety of motion-sensitive heterolateral pathways between the lobula plates, or between them and contralateral deutocerebral neuropil, which provides descending pathways to neck motor centers. The profiles of heterolateral tangential cells correspond to neurons stained by an antibody against gamma-aminobutyric acid (GABA). Other GABA-immunoreactive interneurons linking each side of the brain correspond to uniquely identified motion-sensitive neurons linking the deutocerebral. Additional inhibitory pathways include heterolateral GABAergic descending and ascending neurons, as well as heterolateral GABAergic neurons in the thoracic ganglia. The functional significance of heterolateral GABAergic pathways was tested surgically by making selective microlesions and monitoring the oculomotor output. The results demonstrate an important new attribute of the insect visual system. Although lesions can initially abolish an excitatory or inhibitory response, this response is reestablished through alternative pathways that provide inhibitory and excitatory information to the same motor neurons.