Effects of a Synthetic Met 5 -enkephalin Analog on Plasma Luteinizing Hormone and Prolactin Levels in Conscious Orchiectomized Rats 1

Abstract

The effects of a potent opioid peptide FK33-824, [d-Ala², MePhe⁴, Met(O)⁵-ol]enkephalin, on luteinizing hormone (LH) and prolactin (PRL) secretion were investigated in conscious castrated male rats with chronically implanted indwelling cannulae. Intraventricular injection of FK33-824 (1, 10, and 100 ng/rat) resulted in a rapid and dose-related decrease in plasma LH whereas it increased plasma PRL levels in the rat. Similar results were obtained with β-endorphin (1 μ/rat). Intravenous injection of naloxone, an opiate antagonist, blunted the LH and PRL responses to FK33-824 and β-endorphin. Pretreatment with pimozide, a dopamine antagonist, blunted the PRL but not LH responses to FK33-824. Specific destruction of the serotoninergic neural system by 5,6-DHT in combination with PCPA did not affect the LH and PRL responses to FK33-824. These results suggest that the opioid peptide inhibits LH secretion and stimulates PRL release via opiate receptors in the central nervous system in conscious orchiectomized rats and that central dopaminergic mechanisms are involved in PRL release induced by the opioid peptide.