A Decline in Endogenous Opioid Influence during the Steroid-Induced Hypersecretion of Luteinizing Hormone in the Rat*

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Abstract
Studies were undertaken to evaluate the influence of endogenous opioid peptides (EOP) on the LH hypersecretion induced in ovariectomized rats by estradiol benzoate (EB) or EB plus progesterone (EBP). Naloxone (0.1-15.0 mg/kg) was injected before (1200 h) and during (1400 and 1530 h) the LH surge induced by EBP treatment and during the LH surge after EB treatment (1600 h). The opiate antagonist readily stimulated LH secretion before the LH surge in EBP-treated rats at 1200 h and during the LH surge in EB-treated rats at 1600 h, but was much less effective during the LH hypersecretion induced by EBP treatment at 1400 and 1530 h. This decline in the LH secretory response to naloxone during the EBP-induced LH surge was not due to changes in the response of pituitary to LHRH. These studies indicate that during the period of LH hypersecretion by the sequential administration of EB plus P, the influence of EOP neuronal systems on LH secretion is diminished. Thus, EOP neurons may play a role in the timing and magnitude of the LH surge in EBP-treated rats.