Effects of specific activation of mu-, delta- and kappa-opioid receptors on the secretion of luteinizing hormone and prolactin in the ovariectomized rat.
With the recent development of highly specific ligands for the mu, delta and kappa 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 microliters/h) at various concentrations into the third cerebroventricle of unanesthetized, ovariectomized rats. The mu agonist, DAGO, at both 1 and 10 micrograms/h caused a significant suppression of LH secretion and a significant stimulation of PRL release. DPDPE, the delta agonist, had no effect on either hormone at 1 microgram/h but inhibited LH secretion at 10 micrograms/h. There was still no effect of this high dose of DPDPE on PRL release. The kappa agonist, U50,488H, had no effect on either hormone at 10 micrograms/h, but at 100 micrograms/h produced a significant suppression of LH release and a highly variable increase in PRL. Coinfusion of 100 micrograms/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: activation of the mu receptors produces an inhibition of LH secretion and a stimulation of PRL release; activation of the delta receptors produces an inhibition of LH secretion but has no effect on PRL release, and activation of the kappa receptors produces an inhibition of LH release and a variable stimulation of PRL secretion.