Control of grasshopper sound production by the central complex

The central complex in the protocerebrum coordinates the type, intensity and timing of sound signals used for mate attraction, courtship and rivalry of acoustically communicating grasshoppers.



Sound production depends on the balance of fast and slow excitation and inhibition in central complex neuropils and various transmitters, modulators and intracellular signalling pathways that promote (ACh, proctolin, dopamine) or suppress (GABA, NO) sound production have been identified by pharmacological stimulation and confirmed by anatomical studies. Two of these signaling pathways have been associated with particular behavioral situations. Hearing and recognizing conspecific song activates cholinergic projections to the central complex leading to both nicotinic excitation of yet unknown targets and muscarinic excitation of columnar neurons. The latter is mediated by phospholipase C and adenylyl cyclase-initiated intracellular signaling pathways. Expression of muscarinic ACh receptors in the central complex is limited to a subset of columnar neurons with their cell bodies located in the pars intercerebralis, which are thought to contact pre-motor elements in the lateral accessory lobes. In contrast, a different set of pars intercerebralis neurons with columnar projections in the upper division of the central body and tangential neurons with cell bodies in the ventro-median protocerebrum contain the enzyme nitric oxide synthase and accumulate citrulline in situations that are unfavorable for sound production. Since liberation of nitric oxide in the central body inhibits sound production via soluble guanylyl cyclase activation and cyclic GMP production in the central body lower division, these citrulline-accumulating central complex neurons may translate inappropriate behavioral situations into nitric oxide-mediated suppression of sound production. By applying multiple antibodies directed against components of signaling pathways that contribute to the control of grasshopper sound production to the central complex and conducting physiological studies on pre-identified central complex neurons in primary cell culture, we are attempting to identify the points of convergence of different signals in order to trace the flow of information within the central complex.