Steroidal hormones determine sex-related differences in opioid-induced elevation of nociceptive threshold in sheep (Ovis aries).
OBJECTIVE
As sex related differences in opioid-induced elevations of nociceptive thresholds have been observed in sheep, a study was undertaken to determine if puberty, oestrus, pregnancy or administration of the hormones oestrogen and testosterone had any effect on these differences.


METHODS
Withdrawal latency to a nociceptive thermal source was measured in Romney cross sheep consisting of lambs (n = 6) prior to and after their first oestrus, ram lambs (n = 6) prior to and after puberty, non-oestrous ewes (n = 6), oestrous ewes (n = 6), pregnant and subsequently suckling ewes (n = 12), wethers (n = 6) and rams (n = 6). The effects of a kappa opioid agonist (GR 89696), oestradiol benzoate, testosterone and saline on this withdrawal latency were tested in all groups.


RESULTS
GR 89696 increased withdrawal latency in all animals, compared to saline, but this was most marked in ewes, particularly those in oestrus or in late pregnancy. Testosterone and oestradiol had no effect on their own, or in combination, on the withdrawal latency seen with saline alone. Oestradiol increased the effects of GR 89696 on lambs pre-pubertal ram lambs, ewes and wethers but not on post-pubertal ram lambs nor rams. Co-administration of testosterone antagonised this oestradiol effect.


CONCLUSIONS
The efficacy of kappa opioids appears to be affected by animal sex, an effect that can be contributed to by both oestradiol and testosterone.
Cook
C J
CJ
Technology Development Group, Horticultural and Food Research Institute of New Zealand, Private Bag 3123, Hamilton, New Zealand.