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 activity and opioid peptides, nine sows had their piglets removed for 2 h and were treated as follows: (a) control; (b) nose-snare restraint for 20 min; (c) naloxone injections (i.v. 2 mg/kg); and (d) snare + naloxone. 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 were 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 elevated. Stress-induced activity of the hypothalamo-pituitary-adrenocortical (HPA) axis does not inhibit milk ejection in the pig, but this is not due to a protective opioid action. Endogenous opioids protect lactogenic hormones against inhibition by stress.