Stimulatory Action of CHH Peptides
Although several CHH peptides show inhibitory activity on molting and reproduction, in some instances they also exhibit gonad stimulatory action. In the American lobster H. americanus, CHH exists in two isoforms, CHH-A and CHH-B (de Kleijn et al., 1995). Measurements of mRNA levels as well as CHH titers in the hemolymph indicate that CHH-B expression peaks during intense vitellogenesis, suggesting a positive influence on vitellogenic activities. Similarly, MIH of several decapod species are also reported to have gonad stimulatory functions. For example, in the penaeid shrimp, Metapenaeus ensis, MIH exists in two isoforms, MIH-A and MIH-B. MIH-B is not only expressed in the XOSG, but also in the rest of nervous system, such as ventral nerve cord, thoracic ganglia, and brain during vitellogenic activities in the ovary (Gu et al., 2002). The levels of MIH-B mRNA transcripts in the eyestalk is low during the initial phase of vitellogenesis but increase toward the end of ovarian maturation, indicating a stimulatory role for this MIH neuropeptide in the control of vitellogenesis. Injection of rMIH-B delayed the molting cycle of the maturing female and increased the levels of Vg mRNA expression and Vg synthesis in the ovary and hepatopancreas of this shrimp. Furthermore, injection of MeMIH-B dsRNA into the female shrimp caused a decrease in the MeMIH-B transcript levels in the eyestalk and thoracic ganglia.
Stimulatory activity of MIH on vitellogenesis has been reported for other decapod species too. For instance, Zmora et al. (2009) observed MIH titers of the blue crab Callinectes sapidus to increase significantly in the mid-vitellogenic period, compared to early vitellogenic stages. This crab is an anecdysic brachyuran, and hence continued secretion of MIH is a necessity to inhibit any more molting. Nevertheless, the specific elevation of MIH during the vitellogenic period is suggestive of a role in reproduction. In C. sapidus, the stimulatory role of MIH in Vg synthesis is further substantiated by the specific binding of MIH to its receptors in the hepatopancreas followed by the modulation of a cAMP pathway involved in the Vg synthesis. In addition, actinomycin D blocked the stimulatory effects of MIH on Vg mRNA and Vg synthesis, while cycloheximide lowers Vg level only, thereby confirming the role of MIH in Vg transcription and translation (Zmora et al., 2009).
Taken together, it may be suggested that in the anecdysic crab, C. sapidus and the penaeid shrimp, M. ensis, MIH achieves the coordination of somatic growth and female reproduction by stimulating Vg synthesis and, at the same time, extending the intermolt conditions, making it favorable for the rather protracted vitellogenic activities. More evidence from other decapod species is, however, required to confirm this dual role of MIH in both molt inhibition and vitellogenesis induction.