The effect of sense organ deprivation on the release of adipokinetic hormone in Locusta migratoria

Adipokinetic hormones of insect: release, signal transduction, and responses

Flight activity of insects provides an attractive yet relatively simple model system for regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a well-accepted model insect. Peptide adipokinetic hormones (AKHs) are synthesized and stored by neurosecretory cells of the corpus cardiacum, a neuroendocrine gland connected with the insect brain. The actions of these hormones on their fat body target cells trigger a number of coordinated signal transduction processes which culminate in the mobilization of both carbohydrate (trehalose) and lipid (diacylglycerol). These substrates fulfill differential roles in energy metabolism of the contracting flight muscles. The molecular mechanism of diacylglycerol transport in insect blood involving a reversible conversion of lipoproteins (lipophorins) has revealed a novel concept for lipid transport in the circulatory system. In an integrative approach, recent advances are reviewed on the consecutive topics of biosynthesis, storage, and release of insect AKHs, AKH signal transduction mechanisms and metabolic responses in fat body cells, and the dynamics of reversible lipophorin conversions in the insect blood.

Multifactorial control of the release of hormones from the locust retrocerebral complex

The retrocerebral complex of locusts consists of the corpus cardiacum, the corpora allata, and the nerves that connect these glands with the central nervous system. Both corpus cardiacum and corpora allata are neuroendocrine organs and consist of a glandular part, which synthesizes adipokinetic hormones and juvenile hormone, respectively, and of a neurohemal part. The glandular adipokinetic cells in the corpus cardiacum appear to be subjected to a multitude of regulatory stimulating, inhibiting, and modulating substances. Neural influence comes from secretomotor cells in the lateral part of the protocerebrum. Up to now, only peptidergic factors have been established to be present in the neural fibres that make synaptic contact with the adipokinetic cells. Humoral factors that act on the adipokinetic cells via the hemolymph are of peptidergic and aminergic nature. In addition, high concentrations of trehalose inhibit the release of adipokinetic hormones. Although there is evidence that neurosecretory cells in the protocerebrum are involved in the control of JH biosynthesis, the nature of the factors involved remains to be resolved.

Insect adipokinetic hormones

Peptides with adipokinetic (and usually carbohydrate-mobilizing) potency have been demonstrated in various insects, including Locusta migratoria, Schistocerca gregaria, Manduca sexta, Danaus plexippus and Periplaneta americana. As far as characterized by now the adipokinetic factors are blocked peptides, consisting of eight to ten amino acid residues. In locusts the adipokinetic hormones are synthesized in the glandular lobe of the corpus cardiacum and released into the haemolymph in response to flight stimuli. This release is under direct control of neurons, the cell bodies of which are located in the lateral areas of the protocerebrum, while their axons run via the nervi corporis cardiaci II into the glandular lobe. Hormone release is modulated by axons present in the nervi corporis cardiaci I as well as by the haemolymph trehalose concentration. Trehalose apparently exerts its influence via a neuronal network present in the corpus cardiacum. The fat body is the main target organ of the adipokinetic hormones, which are involved in both mobilization and release of flight substrates from fat body stores, i.e., trehalose from glycogen and diacylglycerol from triacylglycerol. Lipid release is accompanied by haemolymph lipoprotein conversions.