Release of [Met5]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat

Effects of intraventricular injection of endomorphin-1, endomorphin-2 and beta -endorphin on the release of immunoreactive [Met(5)]enkephalin from the spinal cord were studied in pentobarbital anesthetized rats. Intraventricular injection of endomorphin-2, but not endomorphin-1, caused an increased release of immunoreactive [Met(5)]enkephalin in the spinal perfusates. beta -Endorphin given intraventricularly also increased the release of immunoreactive [Met(5)]enkephalin in an amount 15-fold higher than that produced by endomorphin-2. The increase of the release of immunoreactive [Met(5)]enkephalin induced by endomorphin-2 was blocked by mu -opioid receptor antagonist CTOP. Our result suggests that endomorphin-2 stimulates another subtype of mu -opioid receptor different from that acted by endomorphin-1 at the supraspinal site and subsequently increases the release of [Met(5)]enkephalin from the spinal cord. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.