STRESS AND NURSING IN THE PIG - ROLE OF HPA AXIS AND ENDOGENOUS OPIOID-PEPTIDES

To determine the effect of stress on nursing, and the roles of HPA act ivity and opioid peptides, nine sows had their piglets removed for 2 h and were treated as follows: (a) control; (b) nose-snare restraint fo r 20 min; (c) naloxone injections (IV 2 mg/kg); and (d) snare + naloxo ne. After the treatment, the piglets were returned milk ejections were timed, and the sows' blood sampled every 10 min for cortisol, growth hormone (GH), and prolactin assays. Piglet removal increased cortisol and decreased prolactin and GH. This was reversed when the piglets wer e returned. Restraint increased cortisol and decreased GH, but did not affect prolactin. Naloxone alone increased cortisol and decreased GH but did not increase the effect of restraint. The rise in GH following the piglets' return was abolished by the combination of restraint and naloxone. Neither restraint nor naloxone delayed the latency to first milk ejection or reduced the frequency. No unsuccessful nursings were observed. First milk ejections occurred when cortisol levels were ele vated. Stress-induced activity of the hypothalamo-pituitary-adrenocort ical (HPA) axis does not inhibit milk ejection in the pig, but this is not due to a protective opioid action. Endogenous opioids protect lac togenic hormones against inhibition by stress.