CHRONIC STRESS INCREASES THE OPIOID-MEDIATED INHIBITION OF THE PITUITARY-ADRENOCORTICAL RESPONSE TO ACUTE STRESS IN PIGS

The role of endogenous opioid mechanisms in the pituitary-adrenocortic al response to acute stress was investigated in a longitudinal study i n cyclic female pigs before and after exposure to chronic stress (long term tethered housing). Challenge of loose-housed pigs with acute nos e-sling stress for 15 min induced an activation of the hypothalamic-pi tuitary-adrenocortical axis, evidenced by a transient increase in plas ma ACTH (peak height above basal, 98 +/- 12 pg/ml; mean +/- SEM) and c ortisol(54 +/- 3 ng/ml) concentrations. Pretreatment with the opioid r eceptor antagonist naloxone (0.5 mg/kg BW, iv bolus) increased the cha llenge-induced ACTH and cortisol responses to 244 +/- 36 pg/ml and 65 +/- 5 ng/ml, respectively. This indicates that during acute nose-sling stress, endogenous opioid systems are activated that inhibit the pitu itary-adrenocortical response. After exposure of the pigs to chronic s tress (10-11 weeks of tethered housing), the challenge-induced ACTH re sponse was attenuated, whereas the cortisol response remained unchange d, suggesting an increased adrenocortical sensitivity to circulating A CTH. In addition, pretreatment with naloxone induced a greater increme nt in the ACTH and cortisol responses in tethered pigs than in loose-h oused pigs. As no such changes were found in control animals housed lo ose during the entire experimental period, this indicates that the imp act of opioid systems had increased due to chronic stress. The increas ed impact of opioid sys tems during chronic stress may prevent excessi ve hypothalamic-pituitary-adrenocortical responses to acute stressors and, thus, may be of adaptive value.