Relationship between NOC/oFQ,Dynorphin,and COX-2 Activation in Impaired NMDA Cerebrovasodilation after Brain Injury

Abstract

Previous studies have observed that the recently described endogenous opioid, nociceptin/orphanin FQ (NOC/oFQ), contributes to impairment of N-methyl-D-aspartate (NMDA)-induced cerebrovasodilation following fluid percussion brain injury (FPI) via a cyclooxygenase (COX)-dependent generation of superoxide anion (O₂ ^-). This study was designed to investigate the relationship between NOC/oFQ, another opioid, dynorphin, and activation of the COX-2 isoform of the enzyme in such impaired dilation to NMDA after FPI in piglets equipped with a closed cranial window. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O^- ₂ generation. Under non-brain injury conditions, NOC/oFQ (10^-10 M), the CSF concentration observed after FPI, increased CSF dynorphin, while the NOC/oFQ antagonist [F/G] NOC/oFQ (1-13) NH₂ attenuated the stimulated release of dynorphin following FPI (34 ± 3 and 97 ± 6 vs. 36± 3 and 68 ± 8 pg/mol for CSF dynorphin before and after FPI in untreated and NOC/oFQ antagonist-pretreated animals). FPI increased SOD-inhibitable NBT reduction, but pretreatment with norbinaltorphimine, a dynorphin antagonist, or NS398, a COX-2 inhibitor, blunted such reduction (1 ± 1 vs. 19 ± 3 vs. 4 ± 1 vs. 4 ± 1 pmol/mm² for control, FPI, FPI-norbinaltorphimine and FPI-NS398, respectively). Under non-brain injury conditions, dynorphin, in a concentration observed in CSF after FPI, also increased SOD-inhibitable NBT reduction, which was blunted by NS398. NMDA-induced pial artery dilation was reversed to vasoconstriction following FPI, but pretreatment with norbinaltorphimine or NS398 partially protected such responses (9 ± 1 and 16 ± 1, control; - 8 ± 1 and - 13 ± 2, FPI; 6 ± 1 and 12 ± 1% FPI-norbinaltorphimine for NMDA 10^-8, 10^-6 M, respectively). These data show that NOC/oFQ modulates the CSF release of dynorphin after FPI. These data also show that dynorphin contributes to O₂ ^- generation after FPI via COX-2 activation. These data additionally indicate that dynorphin and COX-2 activation contribute to impairment of NMDA pial artery dilation after FPI. Finally, these data suggest that NOC/oFQ impairs NMDA dilation postinsult via the sequential release of dynorphin, activation of COX-2, and generation of O₂ ^-.