Opioid peptides and the regulation of gonadotrophin release in post-partum beef cows and ewes
Nutrition and suckling are known to be the most important factors affecting the length of the anovulatory post-partum period in beef cows. In sheep, nutrition and suckling are also important factors when ewes lamb during the season breeding, in more intensive production systems. A number of neurotransmitters have been proposed to be implicated in the activation of the GnRH pulse generator, which controls LH release. The aim of the present study was to investigate the role of the opioid peptides in the mediation of the effects of nutrition and suckling in both cows and ewes, during the post-partum period. Experiment 1 was designed to quantify the effects of post-partum energy intake on post-partum cows. Sixteen Aberdeen Angus and Simmental cows were arranged in a 2x2 factorial, with breed and energy intake as main factors. An opioid peptide antagonist receptor (naloxone) was administered in all cows in two periods of the post-partum (week 4 and 7) and in two consecutive days, in a high (0.8 mg/kg) or a low (0.4 mg/kg) dose. The overall response in LH to naloxone was low, and there was no significant treatment effects. However, a higher opiodergic tone at early, compared with at late, post-partum period was observed. Naloxone failed to evoke any prolactin response. Experiment 2 was designed to investigate the effects of nutrition and suckling intensity on the post-partum of ewes lambing during their breeding season, and to quantify these effects on the opioidergic tone. The experiment involved 40 ewes, in which 32 were assigned to two different diets and all of them suckled, and 80 ewes were assigned to one of these two diets and were weaned. All ewes were challenged with naloxone at day 12 post-lambing, with either a low (0.7 mg/kg) or a high (1.4 mg/kg) dose. Naloxone evoke an LH response in most of the animals, irrespective of the diet in which they were assigned. Naloxone also evoked an LH response in either suckled or weaned ewes. In the experiment 3, the interactive effect of nutrition and suckling on the post-partum interval and opioidergic tone was investigated. Sixteen Simmental cows were involved in a 2x2x2 factorial, with body condition at calving, post-partum energy intake and suckling intensity as main factors. At week 4, naloxone was administered (0.8 mg/kg) when a new follicular wave was detected and the follicle was in a growing stage, and it was also administered using the same dose, when that follicle achieved the dominant phase.