Neural processing of chemosensory information from the locust ovipositor

Abstract

Contact chemoreceptors (basiconic sensilla) located on the ovipositor and genital segments of the locust serve to control the chemical features of the substrate before and during oviposition. They occur dispersed and also crowded in fields between mechanosensory exteroceptors sensitive to touch or wind (trichoid and filiform sensilla). The central nervous projections of their four chemosensory and one mechanosensory neurons from single basiconic sensilla were stained selectively, focussing on receptors on the ovipositor valves, which usually contact the substrate during the pre oviposition probing movements. All axons and neurites from one contact chemoreceptor usually stay close together in most of their projections. Segregation occurs mainly when single axons terminate in one neuromere while the others proceed to a different neuromere or ganglion. For projections from one chemoreceptor, there is evidence neither for functional segregation of mechanosensory from chemosensory afferent terminals nor for specific segregation between different chemosensory afferents. The projections from sensilla of dorsal cuticle tend to project rather uniformly along the midline of the terminal ganglion. Comparative staining of touch and wind sensitive hair receptor neurons shows mostly central projections, similar to those of neighbouring contact chemoreceptors. From the typical intersegmental projections of most primary afferents and from the lack of segregation into glomerular structures, it is concluded that integration of chemosensory information from the genital segments is distributed in the terminal and the 7th abdominal ganglion. Signals from the ovipositor receptors could influence the various modes of behaviour of ovipositing locusts. The basiconic sensilla on the ventral ovipositor valve responses to different attractant or repellent chemicals. Responses were seen to aqueous solutions of salts (NaCl, 0.01 M to 3,0 M), sugar (glucose, 0.01 M to 3,0 M), acids (citric acid, 0.01M to 1,0 M), oviposition aggregation pheromones (veratrole and acetophenoneat 1,0% and 0,1%), neurotransmitters (GABA, 0,1%), neuromodulators (serotonin, and octopamine at 0,1%), alkaloides (quinine and tomatine at 0,1%) and phenolic compounds (salicin, 0,1%). With the classical tip recording and stimulation method (Hodgson et al 1955) usually three sensory neurons of these sensilla were excited, followed by rapid adaptation at lower concentrations. Local and ascending interneurones of the terminal abdominal ganglion process chemosensory information from the ventral ovipositor. This study focussed on interneurons extending in the anterolateral region of the eighth abdominal neuromere with some of their collaterals ascending to more anterior abdominal ganglia. The projecting interneurons of these respond only to one or two chemical substances (sugars or salts, or salts and acids together) with excitatory or inhibitory responses. The physiological and morphological differences between the interneurons suggest that there is no specific center for processing taste information in the locust terminal ganglion