The Effect of Naloxone on Blood Progesterone of Creole Mexican Ewes with Induced Short Luteal Phase During the Anoestrus Season

From an extensive sheep flock, 20 creole mexican ewes were selected, age fluctuated between 2 and 6 years, allocated at random in groups of 10. Group 1 (n = 10) received a subcutaneous implant with 15 mg naloxone HCL. Group 2 (n = 10) was sham treated on the may 30-2008. Total 7 days after receiving the implant they were treated with GnRH (250 ng iv at 2 h interval for 24 h) and one final GnRH iv injection of 125 mu g. Blood samples were collected at 12 h intervals since, the beginning of the experiment and continued until 15 days after the last injection of GnRH. In 2 of the control ewes it was observed that progesterone concentrations increased since 36 h after the first dose of GnRH until a maximal of 3 ng mL(-1) of plasma progesterone concentration was reached on day 7 and the remaining 8 control ewes showed short luteal phases with a maximum of 1 ng mL(-1) of plasma progesterone. In ewes treated with naloxone 3 showed short luteal phases and 7 naloxone treated ewes had plasma progesterone concentrations similar to control ewes with normal luteal phases. There was a significant effect of treatment (p<0.0019) between control and naloxone treated ewes. This results show that endogenous opioids are important modulators of reproduction in the ewe.