Opioid Peptide Modulation of Stress-Induced Plasma Steroid Changes in the Frog Rana esculenta

The present paper aims to evaluate whether a neuroendocrine system including opioids accounts for the effects of stress in the frog, Rana esculenta. Using an acute-stress paradigm, the involvement of hypothalamic opioid peptides was investigated biochemically as well as by in vivo studies. HPLC and RIA investigations confirm the presence of β-endorphin-like peptides in the brain of this frog. Several immunoreactive peaks are present, two of them coeluting with β-endorphin and acetyl β-endorphin reference peptides. Quantitative evaluation revealed that the β-EP content of 24-hr captured animals was higher than that in fresh captured ones. The stress paradigm, applied here was consistent with the measurement of plasma androgens and corticosterone levels in females after short-captivity confinement. In comparison with fresh-captured animals, a sharp decrease of these levels was found within 10-24 hr after capture. These effects were reversed by naltrexone, a long-acting opioid antagonist, after 24 hr of treatment. In fact, in the captive animals injected with 100 ng of naltrexone, the plasma androgen titers remained low at the 10th hour postcapture, but rose to the control levels within 24 hr. A similar effect was found for the plasma corticosterone levels. These data demonstrate that, in this anuran, endogenous opioids could mediate the stress-induced inhibition of gonadal function, and perhaps the inhibition of the hypothalamus-pituitary-adrenal axis.