Mechanisms Controlling Modulation by Haemolymph Amino Acids of Gustatory Responsiveness in The Locust

Abstract

Previous work has shown that fifth-instar nymphs of Locusta migratoria (L.) adjust their feeding behaviour to compensate for variation in dietary protein and carbohydrate levels. These changes in behaviour are accompanied by nutrient-specific changes in the responsiveness of taste receptors on the mouthparts. Levels of free amino acids in the haemolymph affect the responsiveness of maxillary gustatory receptors to stimulation by amino acids. The mechanisms mediating this response are investigated. Sectioning the maxillary nerve does not prevent an injection of amino acids into the haemolymph from causing reduced chemo-responsiveness, indicating that centrifugal neural feedbacks are not involved. Isolating the distal two segments of the palp by ligature and then micro-injecting amino acids into the palp tip also causes modulation of responsiveness, showing that the effect is mediated at, or close to, the sensory receptors. Radio-labelling studies indicate that amino acids injected into the abdomen are found in the haemolymph of the palp within the time necessary to cause a peripheral change in gustatory responsiveness. Possible mechanisms enabling amino acids in the blood of the palp to influence sensory responsiveness are discussed. The simplest mechanism consists of amino acids in the haemolymph reaching the sensillum liquor and adapting the receptors to further stimulation from the food.