Role of Vasopressin in Altered Pial Artery Responses to Dynorphin and β-Endorphin following Brain Injury

Pial artery constriction following fluid percussion brain injury (FPI) is associated with elevated CSF dynorphin and beta-endorphin concentration in newborn pigs. Additionally, dynorphin is a dilator under control conditions and a vasoconstrictor under decreased cerebrovascular tone conditions. Vasopressin contributes to beta-endorphin-induced pial constriction and the constrictor potential for dynorphin. Recently, it has been observed that FPI reverses vasopressin from a dilator to a constrictor. The present study was designed to characterize the effect of FPI on beta-endorphin-induced constriction and the role of vasopressin in that constriction as well as in the reversal of dynorphin's vascular response following FPI. Brain injury of moderate severity (1.9 - 2.3 atm) was produced in anesthetized newborn pigs equipped with a closed cranial window. Dynorphin in physiologic and pharmacologic concentrations (10(-10), 10(-8), 10(-6) M) was reversed from a dilator to a constrictor following FPI (7 +/- 1, 11 +/- 1, and 16 +/- 1 vs -4 +/- 1, -7 +/- 1, and -11 +/- 1% before and after FPI, respectively). Dynorphin-induced vascular changes were accompanied by increased cortical periarachnoid CSF vasopressin and these biochemical changes were potentiated following FPI (24 +/- 4 vs 134 +/- 7 and 53 +/- 7 vs 222 +/- 14 pg/mliter for control and dynorphin (10(-6) M) before and after FPI, respectively). In contrast, in animals pretreated with the vasopressin receptor antagonist [1-(beta-mercapto-beta beta-cyclopentamethylene propionic acid) 2-(O-methyl)-Tyr-AVP] (MEAVP, 5 micrograms/kg iv), dynorphin-induced constriction following FPI was attenuated (6 +/- 1, 12 +/- 1, and 16 +/- 1, vs -2 +/- 1, -4 +/- 1, and -7 +/- 1% before and after FPI, respectively). Additionally, beta-endorphin-induced pial constriction was potentiated following FPI (-7 +/- 1, -10 +/- 1, -15 +/- 1 vs -10 +/- 1 -15 +/- 2, and -21 +/- 2% for beta-endorphin (10(-10), 10(-8), 10(-6) M) before and after FPI, respectively). beta-endorphin-induced CSF vasopressin release was similarly potentiated following FPI. Further, MEAVP blunted the augmented constrictor responses to beta-endorphin observed following FPI (-5 +/- 1, -9 +/- 1, -14 +/- 1 vs -2 +/- 1, -5 +/- 1, and -8 +/- 1% before and after FPI, respectively). These data indicate that FPI potentiates beta-endorphin-induced pial construction and reverses dynorphin from a dilator to a constrictor. Additionally, these data show that vasopressin contributes to augmented beta-endorphin pial constriction and the reversal of dynorphin's vascular effects following FPI. Further, since CSF dynorphin and beta-endorphin concentrations are increased following FPI, these data suggest that these two opioids contribute to pial artery constriction observed following FPI, at least, in part, via the release of vasopressin.