Relationship between endogenous opioids and the oestrous cycle in the rat.

The aim of this study was twofold: (1) to document changes in levels of immunoreactive beta-endorphin (Ir-beta-EP) in the hypothalamus, anterior pituitary gland, neurointermediate lobe and plasma during the oestrous cycle of the rat and (2) to investigate stress-induced changes in plasma Ir-beta-EP at different stages of the oestrous cycle. Evidence was found that Ir-beta-EP levels in the hypothalamus, anterior pituitary gland and plasma are not constant during the oestrous cycle and that the Ir-beta-EP response to stress is a function of the phase of the oestrous cycle at which stress is applied. It is suggested that fluctuations in ovarian hormones may influence oestrous Ir-beta-EP levels both under normal conditions and after exposure to stress.

Effect of stress on tissue and plasma levels of immunoreactive beta-endorphin in ovariectomized rats primed with oestrogen and progesterone.

The aim of this study was to investigate the effect of oestrogen and progesterone on levels of immunoreactive beta-endorphin (Ir-beta-EP) in the hypothalamus, anterior pituitary gland, neurointermediate lobe and plasma under normal conditions and conditions of stress. The injection of oestrogen + progesterone into ovariectomized rats increased Ir-beta-EP levels in extracts of anterior pituitary tissue and in plasma, under both normal and stressed conditions. Exposure to a stress reduced the content of anterior pituitary Ir-beta-EP in ovariectomized rats treated with oestrogen + progesterone and also with sesame oil. Such treatment only resulted in increments in plasma Ir-beta-Ep in oil-treated but not in oestrogen + progesterone-treated ovariectomized rats. It was concluded (1) that oestrogen and progesterone are involved in the regulation of resting levels of anterior pituitary Ir-beta-EP during the rat oestrous cycle, (2) that stress reduces levels of anterior pituitary Ir-beta-EP and (3) that increased levels of plasma oestrogen + progesterone inhibit plasma Ir-beta-EP increments which normally accompany exposure to stress.