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Naloxone Administration Does Not Relieve the Inhibition of Gonadotropin Release in Food-Restricted, Lactating Rats^1,2,3,

Michelle K. McGuire^*,4, Tami R. Myers^*,5, W. Ronald Butler and Kathleen M. Rasmussen^*,6

^* Division of Nutritional Sciences, Ithaca, NY 14853 Department of Animal Science, Cornell University, Ithaca, NY 14853

Lactational anovulation is an important factor in determining birth spacing in women living in developing countries. Therefore, a more comprehensive understanding of the mechanisms involved in the relationships among lactation, nutrition and ovulation is important. This study was designed using the food-restricted, lactating rat to examine whether endogenous opioids might be involved in depressing gonadotropin release. Females were mated after 65 d of age and, beginning on d 42 of life, offered food in unrestricted amounts (control) or were food restricted to 50% of what the controls consumed. On d 15 of lactation, dams were injected with either naloxone hydrochloride (3 mg/kg body weight) or saline and killed 0, 15, 30 or 60 min later. Plasma was analyzed for luteinizing hormone, follicle-stimulating hormone and prolactin. Food restriction decreased plasma concentrations of luteininzing hormone and follicle-stimulating hormone (P < 0.005). Naloxone administration marginally influenced follicle stimulating hormone (P < 0.1), but not luteinizing hormone concentration regardless of diet group. The interaction among diet group, drug group and time of killing was significant for plasma prolactin concentration (P < 0.05). Food restriction lowered prolactin concentrations, but this effect was diminished with increasing time after injection of naloxone. Furthermore, the magnitude of the effect of food restriction was lessened and even reversed with treatment of naloxone. These results indicate that endogenous opioids are not the primary mechanism suppressing luteinizing hormone release in food-restricted lactating rats.

KEY WORDS: • rats • lactation • nutrition • ovulation • opioids

¹ Presented in part at the meeting of the International Society for Research in Human Milk and Lactation, Tlaxcala, Mexico, 1995. McGuire, M. K., Myers, T. R., Butler, W. R. & Rasmussen, K. M. Naloxone does not increase gonatropins in food-restricted, lactating rats.

² Supported in part by National Institutes of Health grant HD28663.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ Current address: Department of Animal and Veterinary Science, University of Idaho, Agricultural Science Building, Moscow, ID 83844-2330.

⁵ Current address: Department of Physiology, University of Oklahoma Health Sciences Center, Room BMSB, P.O. Box 26901, Oklahoma City, OK 73190.

⁶ To whom correspondence and reprint requests should be addressed.

Manuscript received 5 January 1996. Revision accepted 3 April 1996.