Naloxone antagonizes the morphine suppression of the oestradiol-induced surge of luteinising hormone in dairy cows

Understanding the trade-off between the environment and fertility in cows and ewes

The environment contributes to production diseases that in turn badly affect cow performance, fertility and culling. Oestrus intensity is lower in lame cows, and in all cows 26% potential oestrus events are not expressed (to avoid getting pregnant). To understand these trade-offs, we need to know how animals react to their environment and how the environment influences hypothalamus-pituitary-adrenal axis (HPA) interactions with the hypothalamus-pituitary-ovarian axis (HPO). Neurotransmitters control secretion of GnRH into hypophyseal portal blood. GnRH/LH pulse amplitude and frequency drive oestradiol production, culminating in oestrus behaviour and a precisely-timed GnRH/LH surge, all of which are disrupted by poor environments. Responses to peripheral neuronal agents give clues about mechanisms, but do these drugs alter perception of stimuli, or suppress consequent responses? In vitro studies confirm some neuronal interactions between the HPA and HPO; and immuno-histochemistry clarifies the location and sequence of inter-neurone activity within the brain. In both species, exogenous corticoids, ACTH and/or CRH act at the pituitary (reduce LH release by GnRH), and hypothalamus (lower GnRH pulse frequency and delay surge release). This requires inter-neurones as GnRH cells do not have receptors for HPA compounds. There are two (simultaneous, therefore fail-safe?) pathways for CRH suppression of GnRH release via CRH-Receptors: one being the regulation of kisspeptin/dynorphin and other cell types in the hypothalamus, and the other being the direct contact between CRH and GnRH cell terminals in the median eminence. When we domesticate animals, we must provide the best possible environment otherwise animals trade-off with lower production, less intense oestrus behaviour, and impaired fertility. Avoiding life-time peri-parturient problems by managing persistent lactations in cows may be a worthy trade-off on both welfare and economic terms – better than the camouflage use of drugs/hormones/feed additives/intricate technologies? In the long term, getting animals and environment in a more harmonious balance is the ultimate strategy.

Opioid peptides involvement in endotoxin-induced suppression of LH secretion in ovariectomized Holstein heifers

Three groups of ovariectomized Holstein heifers were used in this study. Group I heifers (n = 6) were pretreated with saline (SAL), followed 3 h later by three injections of naloxone hydrochloride (NLX) given 1 h apart. In the same sequence, group II heifers (n = 3) received lipopolysaccharide (LPS), followed by the opioid antagonist NLX. Group III heifers (n = 3) received LPS followed by SAL. Concentrations of cortisol and progesterone increased (P < 0.05) following LPS injections in both groups II and III, whereas luteinizing hormone (LH) concentrations were suppressed (P < 0.05). Administration of NLX to heifers pretreated with LPS elicited significant increases (P < 0.05) in LH concentrations, whereas SAL infusion had no effect. These results indicate that the inhibitory actions of opiate mu-delta receptors, and possibly other POMC gene products, were at least partially involved in LPS-induced suppression of the gonadotropic hormone. These results are discussed, including the possibility that feedback suppression by LPS-triggered adrenal gland steroids may have interfered with complete restoration of LH secretion by the opioid antagonist.