Stress-Induced Changes in Plasma, Pituitary and Hypothalamic Immunoreactive β-Endorphin: Effects of Diurnal Variation, Adrenalectomy, Corticosteroids, and Opiate Agonists and Antagonists

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Abstract
Immunoreactive β-endorphin (ir-βEP) levels were determined in plasma, anterior pituitary (AP), neuro-intermediate lobe (N-IL) and hypothalamus (HT) of intact rats and adrenalectomized rats subjected to swimming stress. In intact rats, plasma ir-βEP was elevated fourfold, and ir-βEP levels in AP and N-IL significantly reduced after 30 min swimming; in contrast, HT ir-βEP in these animals was not affected. In adrenalectomized animals, similar but even more marked changes were found in plasma and tissue ir-βEP following stress. The effect of diurnal variation, morphine and naloxone on stress-induced changes in tissue ir-βEP was also examined; in addition, the influence of acute and chronic treatment of adrenalectomized animals with dexamethasone or deoxycorticosterone was explored. Plasma ir-βEP of rats swum in the afternoon was significantly higher than of those swum in the morning; no differences were detected in AP, N-IL or HT ir-βEP. Acute morphine administration (10 mg/kg) did not affect levels of ir-βEP in any tissue, baseline or post-stress; however, naloxone (10 mg/kg) consistently and significantly prevented the reduction in N-IL ir-βEP otherwise found following prolonged stress. Both acute and chronic dexamethasone (2 and 20 µg) attenuated the stress-induced changes found in AP and plasma ir-βEP in a dose-related manner; equivalent effects, however, were not seen in deoxycorticosterone (20, 200 µg) treated animals. In contrast, the stress-induced reduction found in N-IL ir-βEP was influenced neither by acute nor chronic treatment with dexamethasone or deoxycorticosterone. These results suggest that changes found in AP and plasma ir-βEP after swimming stress are glucocorticoid-sensitive, but do not involve pathways including opiate receptors; in contrast, though baseline levels of N-IL ir-βEP are mineralocorticoid-sensitive, the changes after swimming stress are steroid-independent and involve the action of endogenous opioids.