[Opioid peptide content of the brain and blood of rats under immobilization stress]

A study was made of the influence of acute and repeated immobilization on the content of immunoreactive metenkephalin (ME), leu-enkephalin (LE) and beta-endorphine (beta-E) in different regions of rat brain and that of beta-E in rat blood. Acute immobilization for 30 min led to a decrease in the content of the enkephalins in the hypothalamus. Meanwhile 150-min immobilization caused a remarkable increase in the opioid concentration in the hypothalamus and of the enkephalins in the pituitary. At the same time the beta-E content in the pituitary dropped to 38% of the control (P less than 0.001), that in the blood was twice as increased (P less than 0.05). Repeated immobilization for 7 days abolished these changes in the hypothalamus and pituitraty. The next day following immobilization for 39 days the content of LE and beta-E in the hypothalamus, medulla oblongata, midbrain and blood plasma was noticeably lowered. However, after successive immobilization it rose to the control level. The data obtained are discussed in the light of the involvement of opiate systems in the realization of antinociceptive and emotional effects of stress.