Opioid modulation of luteinizing hormone, progesterone, and cortisol secretion in the prepubertal zebu heifer
Reproductive efficiency in zebu cattle is very low. Besides genotype, poor nutrition is a major cause of this low efficiency. Poor nutrition delays puberty onset by causing a dysfunction of the hypothalamo-pituitary-gonadal axis, and may also activate the general stress syndrome to inhibit reproduction. Endogenous opioid peptides (EOP) have been implicated in the regulation of luteinizing hormone, and are also released during stress;Naloxone, a potent opioid receptor antagonist, infused to prepubertal zebu heifers maintained under sub-optimal feeding conditions, caused a significant transient, dose-dependent increase in plasma progesterone and cortisol concentrations. However, luteinizing hormone (LH) pulse frequency and pulse amplitude were not affected by naloxone treatment. Further, chronic naloxone treatment of zebu heifers did not induce puberty;In a follow-up study, the effects of dietary restriction and endogenous opioidergic tone on the regulation of progesterone, cortisol and LH were investigated. Feed restriction induced a marked decrease in all LH parameters, increased basal cortisol, and decreased progesterone concentrations. These effects were reversed by refeeding. Reducing the endogenous opioid tone in full-fed heifers tended to increase LH pulse frequency, but increased basal and LH pulse amplitude. However, nalaxone had no effect on LH parameters in the feed-restricted zebu heifer. Irrespective of the nutritional status, opioid antagonism increased progesterone secretion, but the effect on cortisol was divergent, increasing slightly in full-fed heifers, and inducing an insignificant decrease in acutely feed-restricted animals;These studies suggest that undernutrition induces a dysfunction of the reproductive endocrine axis in zebu heifers. There is little support that EOP may be involved in the nutrition-induced suppression of LH, however hypercortisolemia arising from nutritional stress may indirectly inhibit LH. Further, opioidergic restraint of progesterone secretion may prevent the normal prepubertal elevation, an important event for the establishment of high frequency LH secretion.