Dynamics of in Vivo Release of Molt-Inhibiting Hormone and Crustacean Hyperglycemic Hormone in the Shore Crab, Carcinus maenas

Abstract

Very little is known regarding the release patterns or circulating titers of neuropeptides in crustaceans, in particular those concerned with regulation of molting hormone (ecdysteroid) synthesis, molt-inhibiting hormone (MIH), and crustacean hyperglycemic hormone (CHH), which is also an adaptive hormone, centrally important in carbohydrate metabolism. Furthermore, the currently accepted model of molt control is founded on an untested hypothesis suggesting that molting can proceed only after decline in MIH titer. Accordingly, we measured simultaneous circulating neuropeptide profiles for both MIH and CHH by RIA of purified hemolymph during the molt cycle at fine temporal scale during day/night cycles and seasonally. For CHH we additionally determined release patterns after physiologically relevant stress. Results show that both hormones are released exclusively and episodically, rather than continuously, with notably short half-lives in circulation, suggesting dynamic and short-lived variations in levels of both hormones. During the molt cycle, there are no overt changes in MIH titer, except a massive and unprecedented increase in MIH during late premolt, just before ecdysis. The function of this hormone surge is unknown. Treatment with various stressors (hypoxia, temperature shock) showed that CHH release occurs extremely rapidly, within minutes of stress. Release of CHH after stressful episodes during premolt (when gut endocrine cells synthesize large quantities of CHH) is exclusively from the sinus gland: CHH from the gut is never involved in the stress response. The results show a hitherto unsuspected dynamism in release of MIH and CHH and suggest that currently accepted models of molt control must be reconsidered.

The adaptive significance of crustacean hyperglycaemic hormone (CHH) in daily and seasonal migratory activities of the Christmas Island red crab Gecarcoidea natalis

“…Although the half-time of injected CHH was not measured in this study, it is likely to be brief, in the order of ca. 10min, as we have observed for other crabs (Webster, 1996;Chung and Webster, 2005), thus the rapid decline in CHH levels observed following exercise is likely to reflect a dramatic reduction or cessation of CHH release within 10min of the end of this period. Although a significant increase in haemolymph glucose (ca.…”

“…To perform accurate measurements of circulating CHH levels, development of an ultrasensitive and specific immunoassay was mandatory. Previously we have used 125 I-based radioimmunoassays (RIA) to measure CHH in the haemolymph of various crabs (Webster, 1996;Chung and Webster, 2005), but these, whilst being reasonably sensitive, require relatively large (1-2ml) haemolymph samples to be taken, and possibly preclude repeated sampling from individuals, as this might involve undue stress. Furthermore, we previously knew nothing regarding the basal circulating levels of CHH in G. natalis, which might have been very small.…”

“…Clearly these can only be obtained following extreme stress, and it seems likely that CHH is released tonically, or more likely as short-lived pulses, as we have suggested previously from our studies on CHH and MIH release in Carcinus (Chung and Webster, 2005). To determine still finer temporal resolution of CHH levels, we decided to investigate changes in CHH levels during the night-today transition.…”

“…Functions related to energy metabolism might be readily defined by experimental manipulation and careful measurement of hormone titres, together with the measurement of changes in glucose levels, yet in relatively few instances has this been done with sufficient precision and temporal resolution to enable unequivocal conclusions to be drawn. CHH release occurs after stressful episodes (Keller and Orth, 1990;Webster, 1996;Chang et al, 1998;Wilcockson et al, 2002;Zou et al, 2003;Chung and Webster, 2005;Chung and Zmora, 2008), exposure to heavy metal pollutants (Lorenzon et al, 2004) or parasite load (Stentiford et al, 2001). Nevertheless, we still know relatively little about the adaptive roles of CHH in the life history of crustaceans.…”

Roles of crustacean hyperglycaemic hormone in ionic and metabolic homeostasis in the Christmas Island Blue crabDiscoplax celeste

“…In recent years, a variety of other functions for these hormones have been established, involving (for example) inhibition of ecdysteroid (Webster and Keller, 1986;Chang et al, 1990;Chung and Webster, 2003;Chung and Webster, 2005), methyl farnesoate (Liu et al, 1997) and ovarian protein synthesis (Khayat et al, 1998;Avarre et al, 2001). A subject of topical interest is the involvement of CHH in ionoregulatory processes (Charmantier-Daures et al, 1994;Charmantier et al, 1999;Chung et al, 1999;Townsend et al, 2001;Serrano et al, 2003;Chung and Webster, 2006).…”