Influence of enkephalins on gonad development in eastern lubber grasshopper, Romalea microptera (Orthoptera: Romaleidae)

The endogenous opioid peptides, enkephalins (Leu-enk and Met-enk), are involved in crustacean reproduction and have been documented to be present in insect gonads. Naloxone, an opioid antagonist, has been shown to raise the ovarian index in crustaceans and to bind with retinoid X receptors (RXR) which are associated with crustacean reproduction. However, there is no direct report demonstrating the effects of exogenous injection of opioids on reproductive growth in insects. In this study, the influence of enkephalins and naloxone in the regulation of gonad development in the grasshopper, Romalea microptera was investigated for the first time. Injection of either Leu-enk or naloxone alone, or the two combined, into male and female grasshoppers significantly increased mean gonad indices, testicular follicle diameter and oocyte diameter. Injection of either Met-enk alone or combined with naloxone into male and female grasshoppers significantly decreased mean testicular follicle diameter and mean oocyte diameter as well as mean gonad indices. These results provide strong evidence that Leu-enk and naloxone are involved in gonad stimulatory function in grasshoppers, whereas Met-enk alone or in combination with naloxone acts as gonad inhibitory function in these animals. Further, naloxone played a dual role depending on availability of enkephalins in R. microptera. This study represents the first direct demonstration of regulatory function of enkaphalins in insect sexual development.