Effect of loperamide and naltrexone treatment on gonadotrophin concentration in the mare

Abstract

To investigate the involvement of opioids in the regulation of the reproductive axis, the opioid agonist loperamide was given via stomach tube to oestrus mares. In order to assess whether the opioid antagonist naltrexone is as effective as naloxone in overriding the inhibition of endogenous opioid pep tides on luteinizing hormone (LH) and follicle stimulating hormone (FSH) in the mare, naltrexone (NTX) was given to three dioestrus and seven anoestrus mares. Mares were given 20 and 40 mg NTX i.v.. In a third experiment, two anoestrus mares were given a dose of 80 mg NTX. Loperamide had no significant effect on either LH or FSH plasma concentrations in jugular blood. During dioestrus, both 20 and 40 mg NTX were effective in increasing LH and FSH plasma concentrations significantly. During anoestrus, two mares responded to the 20 mg dose, while four responded to the 40 mg dose. One mare responded to the 80 mg dose. These results encourage the use of the opioid antagonist naltrexone to study opioid inhibition of LH and FSH in the mare.