The role of endogenous opioid peptides in the effects of constant illumination on reproductive function in the rat

The present experiments assessed the involvement of endogenous opioids in the inhibition of FSH and LH release, ovulation and continuous sexual receptivity following exposure to constant illumination. In the first experiment, exposure to constant illumination resulted in persistent vaginal oestrus in all rats. The injection of naloxone resulted in marked elevations in serum FSH and LH, induced ovulation and increased the frequency of lordosis behaviour. It was concluded that endogenous opioid(s) participate in these effects. In Experiment 2, levels of β-endorphin were found to be elevated in anterior pituitary and neurointermediate lobe tissue extracts from rats exposed to constant illumination, compared to levels in pro-oestrus rats. Naloxone injection into those rats exposed to constant illumination significantly increased hypothalamic levels of β-endorphin compared to saline injected controls. This suggests that the blockade of opiate receptors increases β-endorphin production, uptake and/or decreases its release from the hypothalamus. These results, and the known inhibitory action of β-endorphin on LH release suggest that it may be this opioid, perhaps in conjunction with pineal products, which is responsible for the observed anti-reproductive effects of constant illumination.