Adipokinetic hormones in neuroendocrine tissue of the larval locust: Quantification and regulation of in vitro release

The insect adipokinetic hormones I and II are synthesized and stored in the corpus cardiacum. They are released from this neuroendocrine gland during flight in adult locusts and subsequently facilitate lipid mobilization. In the adult locust (Locusta migratoria) octopaminergic neurones mediate release of these hormones. All larval stages of the locust are known to contain material with adipokinetic activity and gland extracts as well as synthetic adipokinetic hormone I mediate hyperlipemia when injected in vivo. We now confirm that both hormones are present in day-5, fifth-instar male Locusta migratoria. The total content of both hormones is lower in the larva compared to the adult. Electrical stimulation of the NCC II mediates release of material with adipokinetic activity from the glandular lobe in vitro and this response is mimicked by exogenous application of octopamine. Octopamine elevates the cyclic AMP content of the glandular lobe but has no effect on cyclic GMP levels. This response is blocked by the octopamine-receptor-antagonist phentolamine but not by the β-adrenergic-receptor-antagonist propranolol. The results indicate that the larval locust has the functional ability to release material with adipokinetic activity from the corpora cardiaca. The neuronal mechanisms involved with regulating this release appear similar to those previously described in the adult.