Opioid peptides in blood and cerebrospinal fluid during acute stress

The opioid peptides beta-endorphin and [met]enkephalin are present in the peripheral circulation. Plasma beta-endorphin originates from the pituitary gland and its cosecretion with ACTH is stimulated by a variety of noxious stimuli. Although the adrenal medulla contains high concentrations of [met]enkephalin-containing polypeptides which are costored with catecholamines, and although the adrenal gland appears to secrete [met]enkephalin into the adrenal vein, the relative adrenal contribution to plasma [met]enkephalin appears to be negligible. Plasma concentrations of immunoreactive [met]enkephalin may be increased by insulin and by endotoxic shock, but they are not significantly altered by acute haemorrhagic stress nor by surgical stress. Thus blood plasma concentrations of beta-endorphin, but not of [met]enkephalin, are generally increased during acute stress. The physiological significance of endogenous opioids in the circulation is not known. It is unlikely that transient increases in the concentrations of opioid peptides in peripherally circulating blood modulate nociception, since the peptides do not enter ventricular cerebrospinal fluid in detectable amounts under these conditions. Recent evidence has raised the possibility that circulating opioids may be involved in regulating blood glucose and in activating the immune system. It is also possible that circulating beta-endorphin and related polypeptides have non-opioid actions on a variety of peripheral tissues.