The Dynamics of β-Endorphin Release by the Hypothalamic Nuclei and Pituitary Gland in Sheep under Physiological and Stress Condition

Summary

The secretion of β-endorphin (β-End) by the infundibular nuclei — median eminence (IN-ME) and paraventricular nuclei (PVN) of the hypothalamus and the anterior lobe (AL) of the pituitary gland was determined in 18 anestrous ewes by RIA assay of this opioid in perfusates collected from these formations. Perfusion with Ringer-Locke solution was carried out on the animals under resting and stress-full conditions, using a pushpull cannula method. Electrical footshocks were used as stress stimuli. Two series of perfusion experiments were performed. In the first, the perfusions were carried out over two hours and perfusates were collected before and during stress for 1 h. In the second series of experiments the dynamics of the release of the opioid by the IN-ME and the pituitary gland were followed by collecting five or six perfusates in 20 min fractions before and during stress, respectively. The first series of experiments showed that the concentrations of β-End in perfusates from the pituitary gland were much higher than those from the hypothalamic nuclei and that the concentrations of this opioid during stressing rose significantly only in perfusates from the IN-ME nuclei. In the second series of experiments the release of β-End from the IN-ME and pituitary gland was altered by stress. The shift was very characteristic in that the concentrations of the opioid during stimulation initially significantly rose and then gradually declined. The concentration of the opioid in the peripheral blood also rose at the beginning of stimulation and declined as stimulation continued. The observations indicate that a single determination of the release of β-End whether performed at the beginning or the end period of stress is inappropriate for the evaluation of the release of the opioid, and fails to characterize the activity of the organ examined over the stressful phase.

Key words

Dynamics - β-Endorphin release - Hypothalamic nuclei - Pituitary gland - Sheep