Neuropeptide action in insects and crustaceans

Abstract

Physiological processes are regulated by a diverse array of neuropeptides that coordinate organ systems. The neuropeptides, many of which act through G protein-coupled receptors, affect the levels of cyclic nucleotides (cAMP and cGMP) and Ca(2+) in target tissues. In this perspective, their roles in molting, osmoregulation, metabolite utilization, and cardiovascular function are highlighted. In decapod crustaceans, inhibitory neuropeptides (molt-inhibiting hormone and crustacean hyperglycemic hormone) suppress the molting gland through cAMP- and cGMP-mediated signaling. In insects, the complex movements during ecdysis are controlled by ecdysis-triggering hormone and a cascade of downstream neuropeptides. Adipokinetic/hypertrehalosemic/hyperprolinemic hormones mobilize energy stores in response to increased locomotory activity. Crustacean cardioacceleratory (cardioactive) peptide, proctolin, and FMRFamide-related peptides act on the heart, accessory pulsatile organs, and excurrent ostia to control hemolymph distribution to tissues. The osmoregulatory challenge of blood gorging in Rhodnius prolixus requires the coordinated release of serotonin and diuretic and antidiuretic hormones acting on the midgut and Malpighian tubules. These studies illustrate how multiple neuropeptides allow for flexibility in response to physiological challenges.

Figure 1

Cyclic nucleotide action in the decapod Y-organ (YO). The neuropeptides, crustacean hyperglycemic hormone (CHH), and molt-inhibiting hormone (MIH) bind to independent receptors on the YO plasma membrane. It appears that CHH binds to a receptor guanylyl cyclase (GC-II) and induces an increase in cGMP, the primary result of which is inhibition of facultative ecdysteroidogenesis. MIH induces an elevation of cGMP and possibly cAMP, but its receptor and link(s) to cyclic nucleotide levels are not conclusively determined. The primary effect of cAMP is the inhibition of constitutive synthesis (protein synthesis and cholesterol uptake); cAMP also inhibits facultative synthesis. Thickness of lines indicates relative strength of action. From Covi et al. (2009).

Figure 2

Osmotic and ionic concentrations and fluid transport in fifth instar Rhodnius prolixus in response to a blood meal. Based on Maddrell (1976). Drawing by Zach McLaughlin.