Effects of naloxone on the oestradiol-induced LH surge and cortisol release in transported cows

Four control cows released a normal LH surge in response to 1 mg oestradiol benzoate i.m. Compared to controls, 30 min transport of 5 cows 16-18 h after oestradiol significantly delayed the onset (P less than 0.05), suppressed the amplitude (P less than 0.01) and reduced the total LH release (P less than 0.001) in 4 cows and totally blocked the surge in the 5th cow. Just before the onset of transport, 5 cows were given 250 mg naloxone i.v. In 3 cows, the LH surge was delayed and reduced in amplitude and duration and was totally blocked in the other two, i.e. naloxone did not avert the detrimental effects of transport. Transport stimulated cortisol release in cows. Cows given naloxone + transport released significantly more (P less than 0.001) cortisol than did those subjected to transport alone. In conclusion, naloxone appeared to have further stimulated the hypothalamopituitary-adrenal axis of cows under stress.