Abstract
Enkephalins play a major role in reproductive physiology in crustaceans; however their role in reproductive development in insects is largely unknown. We investigated the effect of exposure to exogenous leucine-enkephalin (Leu-Enk), methionine-enkephalin (Met-Enk), and the opioid antagonist naloxone on gonad development in the Eastern lubber grasshopper, Romalea microptera. Injection of either Leu-Enk or naloxone alone significantly increased the testicular index and testicular follicular diameter in males, and the ovarian index, oocyte length, and oocyte diameter in females. In contrast, injection of Met-Enk inhibited all measures of reproductive development in both sexes. Surprisingly, co-injection of naloxone with either enkephalin enhanced the effect associated with administration of the enkephalin alone. This study clearly demonstrates the ability of enkephalins to disrupt insect sexual development and also suggests the existence of conserved enkephaline-dependent regulatory mechanisms in insects and crustaceans.
Highlights
Enkephalins, classified under endorphins, are opioid pentapeptides involved in nociception through opiate receptors [1]
Enkephalins have been widely studied in different groups of vertebrates and invertebrates [7] and are expressed in the central nervous system (CNS) of many animals including humans where they act as neurotransmitters or neuromodulators [8]
While the hypothesis is supported for Leu-Enk and Met-Enk, we find unexpectedly that naloxone enhances the effect of both enkephalins in the Eastern lubber grasshopper Romalea microptera (Insecta: Orthoptera), which has been used as a model organism for insect physiology and reproductive endocrinology [25,26,27,28]
Summary
Enkephalins, classified under endorphins, are opioid pentapeptides involved in nociception through opiate receptors [1]. Apart from nociception, opiates are involved in inducing euphoria, cardiovascular regulation, decrease in gastrointestinal motility, susceptibility to seizures, and food consumption behavior [9]. The role of exogenous enkephalins in reproductive biology has been studied in decapod crustaceans in depth [10]. In the prawn Penaeus indicus, Reddy et al [11] reported an increase in reproductive indices following Leu-Enk injection, and a decrease following Met-Enk injection. Kishori et al [12] reported increased ovarian growth and vitellogenesis in the rice field crab Oziotelphusa senex senex following Leu-Enk administration, and Sarojini et al [13] reported a dose dependent delay in ovarian maturation in the female crab Uca pugilator after exogenous Met-Enk administration
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