Opioidergic control of gonadotropin secretion in the female rabbit: divergent effects of morphine on secretion of follicle-stimulating hormone and luteinizing hormone
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abstract
The nature of secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) was followed in female rabbits on a daily basis from age 36 to 60 days by sequential 5-min blood sampling over 1- to 2-h periods each day. Both LH and FSH were found to be secreted in a pulsatile manner. The mean LH pulse amplitude over the 25 days was 0.95 ± 0.32 ng/mL and for FSH it was 10.15 ± 1.11 ng/mL. Mean plasma LH levels were significantly increased from 1.46 ± 0.08 ng/mL in 36 to 42-day-old rabbits to 1.89 ± 0.12 ng/mL in 43 to 50-day-old rabbits and remained elevated from 50 to 60 days. FSH levels during the same periods also rose significantly from 14.93 ± 0.79 to 19.57 ± 2.05 ng/mL. To examine the influence of endogenous opioid peptides on the release of LH and FSH in 36 to 60-day-old female rabbits, morphine sulfate at 0.2, 0.5, 2.0, and 5.0 mg/kg was administered subcutaneously after 30 min baseline sampling, and blood was taken for another 60–120 min. Morphine at all doses and at all ages inhibited the amplitude and frequency of LH pulses but had no effect on FSH secretion. To determine whether the effects of morphine on LH secretion could be reversed with naloxone, females aged 82–114 days were used. Naloxone administered 1 h after morphine reversed the inhibitory effects of morphine, whereas the simultaneous administration of naloxone with morphine had variable effects but seemed to delay the LH increase. These data suggest that endogenous opioid peptides may have an acute inhibitory effect on the secretion of LH but not FSH.Key words: rabbit, follicle-stimulating hormone, luteinizing hormone, morphine, naloxone.
