Involvement of enkephalinergic system in the reproductive stress response of the gecko Hemidactylus frenatus

Abstract

Enkephalins are recognised for their role in regulating various physiological functions, including reproduction in vertebrates. Nonetheless, the functional importance of the two forms of enkephalins, methionine-enkephalin (M-ENK) and leucine-enkephalin (L-ENK), remains inadequately understood in the context of the reproductive stress response in reptiles. In the first experiment, we subjected the lizards to various stressors (handling, chasing, noise, and confinement) and assessed the response of enkephalinergic system in the brain and pituitary gland. In the second experiment, stressed lizards were treated with mu or delta opioid receptor antagonists. Exposure of lizards during the early recrudescence phase of the seasonal ovarian cycle resulted in a significant increase in proenkephalin mRNA levels in the brain. This was accompanied by a marked rise in the percentage area of M-ENK and L-ENK immunoreactivity in the hypothalamic regions, median eminence (ME), and pars distalis of the pituitary gland (PD), compared to controls. In the second experiment, the administration of mu or delta opioid receptor antagonists, naltrexone or naltrindole, respectively, reinstated the stress-induced reduction of gonadotropin-releasing hormone immunoreactivity in the hypothalamus, ME, and PD. These treatments also restored the presence of stage IV and V (vitellogenic) follicles, which were absent in the stress-only group. The findings for the first time indicate the activation of enkephalinergic system in the hypothalamus, ME, and PD during the stress response and suggest that the stress-induced suppression of the seasonal ovarian recrudescence phase is attenuated through the administration of mu or delta opioid receptor antagonists in reptiles.