The involvement of opioid peptides in the neural regulation of luteinizing hormone secretion in the female rat


The first experiment determined whether changes in the activity of norepinephrine (NE) or dopamine (DA) neurons modulate the suppression of luteinizing hormone (LH) secretion due to activation of $\mu$-opioid receptors in the medial preoptic-anterior hypothalamic area (MPOA-AHA) in the ovariectomized rat. Utilizing push-pull perfusion, activation of MPOA-AHA $\mu$-opioid receptors with DAGO inhibited LH release but perfusate NE levels did not change. Inhibition of NE synthesis or blockade of DA receptors had no effect on the DAGO-induced suppression of LH release. This study demonstrated that NE or DA neurons do not mediate the inhibition of LH secretion in response to activation of MPOA-AHA $\mu$-opioid receptors.^ The second experiment examined whether blockade of MPOA $\kappa$-opioid receptors on the afternoon of proestrus could prematurely evoke an ovulatory LH surge, and if so, whether NE was involved. LH surges begin in our colony between 1530-1630 h. Perfusion of the MPOA between 1030-1350 h with nor-binaltorphimine (nor-BNI) produced a large increase in LH release beginning between 1230-1330 h (an advance of 3 hours), and ovulation. NE synthesis inhibition with FLA-63 partially reduced, and blockade of $\alpha$-adrenergic receptors with PBZ completely prevented the occurrence of the nor-BNI-induced LH surge. PBZ also prevented the normal afternoon LH surge. MPOA perfusate NE levels did not increase during the nor-BNI-induced or normal afternoon LH surges. Thus blockade of MPOA $\kappa$-opioid receptors prematurely evokes an ovulatory LH surge. Furthermore, the nor-BNI-induced and normal afternoon LH surges are dependent on the proper functioning of central noradrenergic neurons, but do not involve increased NE release within the MPOA.^ The final study examined whether a decrease in MPOA $\kappa$-opioid tone normally occurs on the afternoon of proestrus. In pentobarbital-anesthetized rats, nor-BNI perfusion of the MPOA was 25% less effective in increasing LH release on the afternoon of proestrus compared to the late morning. However, this difference was not significant. Thus, this finding is only suggestive of a decrease in MPOA $\kappa$-opioid tone normally occurring on the afternoon of proestrus.

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OpenCommons at University of Connecticut

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This paper was published in OpenCommons at University of Connecticut.

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