Changes in the pituitary and hypothalamic content of methionine-enkephalin during the estrous cycle of rats

Regulation of pituitary and brain enkephalin systems by estrogen

The roles of estrogen in the regulation of pituitary and brain enkephalin systems were studied. A sex-related difference in the levels of enkephalins was observed in the anterior lobe of rat pituitary. Administration of estradiol to male rats caused a marked reduction in the levels of enkephalins in the anterior lobe. Ovariectomy produced a significant increase in the enkephalin levels of the same region but this increase was completely prevented by replacement with estradiol. These results suggest a physiological role of estrogen in the regulation of pituitary enkephalin systems. Repeated injections of haloperidol elicited effects similar to those of estrogen on the pituitary and brain enkephalin levels; namely, striatal enkephalin levels were increased whereas pituitary enkephalin levels were decreased after long-term treatment with these two compounds. These results raise the possibility that the regulation of estrogen on enkephalin systems may in part be mediated through a dopaminergic mechanism.

Lack of effect of opiates on release of vasopressin from isolated rat neurohypophysis

Electrical stimulation of rat neurohypophysis in vitro results in a 150-fold increase in release of vasopressin. Incubation of the isolated lobes with endogenous opiates (Met-enkephalin, Leu-enkephalin, beta-endorphin), morphine or naloxone did not change the evoked release of vasopressin significantly. The results obtained do not confirm the hypothesis that endogenous opiates inhibit the release of vasopressin from this preparation.

Possible involvement of endorphin withdrawal or imbalance in specific premenstrual syndromes and postpartum depression

Premenstrual and postpartum dysphoric changes are very prevalent. However, their etiology is still obscure. The authors hypothesize that changes in levels of endorphins may be involved in the pathophysiology of these changes. Studies of various endorphins indicate a possible relationship between levels of endorphins and depressive symptoms. In addition, some studies of naloxone and naltrexone suggest a relationship between a blockage in the action of endorphins and the development of a syndrome of dysphoric symptoms similar to the depressive features manifested premenstrually and postpartum by many women and frequently seen in some depressed outpatients. There is also some evidence that there may be a relationship between elevated levels of endorphins and other subtypes of depressive syndromes. Endorphins and estrogen levels have been shown to covary. During the postpartum and the premenstrual period, levels of both change rapidly and substantially. Therefore the link between changes in levels of endorphins and the dysphoric changes during the periods in focus is supported from three complementary directions: (1) the characteristic psychiatric symptomatology, (2) the reported hormonal changes, and (3) the possible involvement of endorphins in neuroendocrine regulation.

Enkephalins co-exist with oxytocin and vasopressin in nerve terminals of rat neurohypophysis

Evidence is accumulating that opiates inhibit the release of oxytocin and vasopressin by acting on nerve terminals in the neurohypophysis. Extracts of neurohypophysis have been shown to contain substantial amounts of Met- and Leu-enkephalin, and Leu-enkephalin-immunoreactive (IR) nerve fibres originating in the magnocellular hypothalamic nuclei have been described in the neural lobe, where the two hormones are secreted. We have compared the distribution of oxytocin, vasopressin and enkephalin immunoreactivity (IR) in the neurohypophysis of the rat, and report here that Met-enkephalin-IR is invariably associated with nerve terminals that contain oxytocin-IR whereas the terminals that contain vasopressin-IR often, but not invariably, are Leu-enkephalin immunoreactive.

Changes in the beta-endorphin content of discrete hypothalamic nuclei during the estrous cycle of the rat

Concentrations of beta-endorphin were measured in discrete brain nuclei of 4-day cycling rats on each day of the estrous cycle. On the afternoon of proestrus beta-endorphin levels were significantly higher in median eminence and suprachiasmatic nucleus, and lower in arcuate nucleus, when compared to levels found on other days of the cycle. These changes coincided with the peak of plasma prolactin, which was blocked by prior administration of naloxone.