Sensory signal processing in the central complex of the cockroach Rhyparobia maderae

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Insects like many vertebrates use the position of the sun as a guiding cue during spatial orientation. In addition, many species can also navigate using the polarization pattern of the blue sky. Neurobiological studies showed that the central complex in the center of the insect brain plays a key role in these tasks. Studies from my laboratory on the desert locust showed that a compass-like representation of directions underlies the columnar organization of the central complex, because neurons of neighboring columns differ in a systematic way with respect to their preferred orientation of the plane of polarized light and their preferred horizontal direction of a bright light spot. The orientation of the animal relative to sky signals is thus represented in a compass-like manner in the central complex. Individual neurons of this system have properties similar to those of head-direction cells in the brain of rats and other mammals. To obtain a more complete understanding of the functional role of the central complex, we plan to study the responses to sensory stimuli in a nocturnal insect, the cockroach Rhyparobia maderae. Extracellular recordings from the cockroach showed that neurons of the central complex encode the orientation of the animal relative to visual background and that neural activity can predict the direction of subsequent locomotion. Moreover, many units responded to mechanical stimulation of the antennae, which are used by cockroaches for orientation in the dark. Because of the extracellular recording technique, however, none of the recorded neurons has so far been identified morphologically. Preliminary work for our laboratory showed that the neuronal organization of the cockroach central complex differs substantially from that of the well-studied locust. We, therefore, plan in the current proposal to analyze the organization and sensory processing in the cockroach central complex at the level of single identified neurons. Dye injections and immunocytochemical stainings will analyze the contributing neuronal systems as well as the layering and columnar organization of the cockroach central complex. Intracellular recordings combined with subsequent dye injection into the recorded neuron will test whether neurons are sensitive to the plane of polarized light and/or encode the direction of a sun stimulus. We will analyze whether, as in the locust, these signals are represented in a compass-like manner in the central complex. Finally, intracellular recordings will show whether neurons of the central complex encode the position and movements of the antennae, and thereby employ a second sensory modality for the encoding of space. The project will substantially enhance our understanding of the functional role of the central complex in the insect brain and will provide important impulses for subsequent work also in well studied species like locust and fruit fly.