The physiology of locust hindgut muscle with reference to the possible effects of azadirachtin
The physiology of Locusta hindgut has been investigated using techniques developed for measuring both contractility of whole hindguts and the electrophysiological properties of hindgut superior longitudinal (SL) muscle, in order to examine the possible interaction of the tetranortriterpenoid, azadirachtin. The pentapeptide proctolin, a neurotransmitter/modulator candidate at the hindgut neuromuscular junction, produced highly reproducible dose dependent effects on the isolated hindgut preparation. The effect of other putative neurotransmitters/modulators, including acetylcholine, 5-HT, octopamine, GABA and FMRFamide, and the effects of high potassium Ringer, calcium free (EGTA) Ringer, Co2+ and verapamil on the proctolin response, were also assessed. Neurally evoked contractions could be produced by stimulating the proctodeal nerve, using a standard set of stimulation parameters. Under these conditions, neurally evoked responses comprised both a phasic and tonic component. The effect of verapamil, Co2+ and Ca2+ free Ringer was examined on these responses. Proctolin, at low doses, did not potentiate neurally evoked responses, although a potentiation could be seen if the stimulation parameters were significantly altered. In contrast, GABA dose dependently inhibited neurally evoked responses. Both cyclic adenosine monophosphate and inositol trisphosphate pathways are implicated in the neurally evoked response. Azadirachtin, under the conditions of the experiment, did not antagonize proctolin induced contractile responses, which is in contrast to a previous study (Mordue(Luntz) and Evans 1987) and had no consistent effect on neurally evoked responses. In addition, no major differences in the contractile behaviour were found between insects injected with azadirachtin in vivo and control insects, although hindguts from the azadirachtin treated group appeared more relaxed and flaccid than those of the control group, and produced larger responses on contraction.