Opioid involvement in LH release during the negative feedback effects of oestradiol and progesterone in dairy cows

Abstract

To create a clearly defined and uniform feedback phase of oestradiol, 1 mg oestradiol benzoate (OE2) was injected i.m. either 15-43 days post partum in 7 cows (Expt 1), or 12 h after prostaglandin oestrus synchronization of 4 cyclic cows (Expt 2 and 3). An endogenous opioid peptide (EOP) antagonist (250 mg naloxone), an EOP agonist (300 mg morphine), or a combination of the two was given in Expt 1, 2 and 3 respectively as a single i.v. injection in cows 16-18 h after OE2, i.e. in the negative feedback phase and before the expected onset of an induced luteinizing hormone (LH) surge. Blood samples were collected every 15 min for 1 h before and 2 h after each treatment. In Expt 1, naloxone failed to increase LH release when given during the negative feedback phase, but caused excessive release in one cow in which the LH surge had just begun. In all cows in Expt 2, morphine suppressed LH release (P less than 0.05). In Expt 3, naloxone prevented the suppressive effects of morphine for the first hour after treatment; a transient rise in LH occurred in one cow in which treatment was given 1 h before the start of the LH surge. In Expt 4, 250 mg naloxone was injected i.v. into 4 cows that had a plasma progesterone concentration of 1.5-3.1 ng mL-1 in the luteal phase of the oestrous cycle: naloxone failed to increase LH concentrations. It is concluded that LH can be further suppressed by opiates given to cows during the negative feedback phase of oestradiol.(ABSTRACT TRUNCATED AT 250 WORDS)