Effects of Specific Activation of Mu-, Delta- and Kappa-Opioid Receptors on the Secretion of Luteinizing Hormone and Prolactin in the Ovariectomized Rat

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Abstract
With the recent development of highly specific ligands for the µ, δ and K opioid receptors it was of interest to define the effects of activation of each of these receptor types on LH and prolactin (PRL) secretion. The compounds were infused (10 µl/h) at various concentrations into the third cerebroventricle of unanesthetized, ovariectomized rats. The µ agonist, DAGO, at both 1 and 10 µg/h caused a significant suppression of LH secretion and a significant stimulation of PRL release. DPDPE, the δ agonist, had no effect on either hormone at 1 µg/h but inhibited LH secretion at 10 µg/h. There was still no effect of this high dose of DPDPE on PRL release. The K agonist, U50,488H, had no effect on either hormone at 10 µg/h, but at 100 µg/h produced a significant suppression of LH release and a highly variable increase in PRL. Coinfusion of 100 µg/h of naloxone with the high dose of each of the agonists completely blocked the responses of both hormones to each of the agonists with one exception: the highly variable stimulation of PRL by U50,488H was not affected, thus indicating a nonspecific effect of U50,488H on PRL secretion. These results demonstrate that: (1) activation of the µ receptors produces an inhibition of LH secretion and a stimulation of PRL release; (2) activation of the δ receptors produces an inhibition of LH secretion but has no effect on PRL release, and (3) activation of the K receptors produces an inhibition of LH release and a variable stimulation of PRL secretion.