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 β-endorphin (Ir-β-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-β-EP at different stages of the oestrous cycle. Evidence was found that Ir-β-EP levels in the hypothalamus, anterior pituitary gland and plasma are not constant during the oestrous cycle and that the Ir-β-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-β-EP levels both under normal conditions and after exposure to stress.