Adipokinetic hormone is a shortpeptide hormone that has been studied in insects. It is a lipid mobilising hormone and is responsible for regulating fuel transport in the haemolymph, for redirecting energy to other processes as required by the insect. AKH was initially discovered in the locusts Locusta migratoria and Schistocerca gregaria and is generally associated with aiding flight. Early observations of locusts showed that despite the fact that lipids are metabolised by flight muscle in order to maintain flight, which would be transported from the haemolymph, there was often still a high concentration of lipids in the haemolymph, implying that an agent may be responsible for activating lipid transport into the haemolymph and this was thought most likely to be hormonal regulation. The hormone itself is part of a larger family, often referred to as red pigment concentrating hormones discovered in crustaceans and the typical makeup of hormones in this family includes a length between 8 and 10amino acids, blocked N and C termini, phenylalanine or tyrosine at position 4 and tryptophan at position 8. RPCH was discovered in crustaceans and shown to be involved in concentrating red pigments, for sexual displays and a variety of other reasons. The similarity between AKH and RPCH is so significant that injecting insects with RPCH induces an AKH like response and vice versa AKH was first purified by an English group in 1976 and the chemical structure was determined to be a peptide hormone formed from 10 amino acids. This was the first instance where an insect peptide hormone had been identified. Later, after AKH was identified in cockroaches, locust AKH was inserted into a cockroach and a similar increase in lipid mobilisation was observed. Conversely, cockroach AKH led to similar activity within a locust. AKH has become an important area of study, particularly in insect crop pests and insects that act as intermediate or vectorhosts for parasites that can affect humans or agricultural animals. It is believed that AKH does not only aid flight in insects, but it has also been shown that an increase in AKH can lead to stronger immune responses in locusts. One group have performed experiments where locusts received an injection including AKH and lipopolysaccharide – an immune elicitor found in the cell walls of bacteria – led to a stronger immune response than locusts which only received an LPS injection. This implies that lipid mobilisation by AKH can have a variety of uses for energy dynamics within insects and may be important in a range of functions that are related to insect survival. It is now believed that parasites which infect insects may stimulate the production of AKH in order to mobilise lipids which can then be used for parasite gains.
