IN-VIVO PRODUCTION OF NITRIC-OXIDE CORRELATES WITH NMDA-INDUCED CEREBRAL HYPEREMIA IN NEWBORN SHEEP

Stimulation of N-methyl-D-aspartate (NMDA) receptors in brain increase s nitric oxide production in vitro. We tested the hypothesis that nitr ic oxide participates in the increase in local cerebral blood flow (CB F) caused by infusion of NMDA in anesthetized newborn sheep. We used t he combined hydrogen clearance and microdialysis technique for simulta neous measurement of local CBF, infusion of drugs, and measurement of interstitial levels of L-[C-14]citrulline in the parietal cortex. Rele ase of L-[C-14]citrulline into the dialysate during continuous infusio n of L-[C-14]arginine was used as a marker of nitric oxide production in vivo. Citrulline recovery and CBF were measured hourly during a 4-h infusion of cerebrospinal fluid containing either 1) no additional dr ugs, 2) 1 mM NMDA, 3) 1 mM N-G-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor), 4) 1 mM NMDA + 1 mM L-NAME, 5) 0.1 mM 2-chloro-adenosine (adenosine receptor agonist), or 6) 0.1 mM 2-ch loroadenosine + 1 mM L-NAME. At 240 min of perfusion, CBF (ml . min(-1 ). 100 g(-1); means +/- SE) was as follows: control 52 +/- 3, NMDA 116 +/- 11, L-NAME 32 +/- 5, NMDA + L-NAME 40 +/- 4, 2-chloroadenosine 20 1 +/- 63, and 2-chloroadenosine + L-NAME 129 +/- 18. Citrulline recove ry (fmol/min) at 240 min of perfusion was as follows: control 38 +/- 1 2, NMDA 149 +/- 21, L-NAME 9 +/- 1, NMDA + L-NAME 39 +/- 5, 2-chloroad enosine 13 +/- 5, and 2-chloroadenosine + L-NAME 17 +/- 1. Infusion of NMDA increased CBF and L-[C-14]citrulline release, and these increase s were inhibited by addition of L-NAME to the dialysate. In contrast, the cerebral vasodilator, 2-chloroadenosine, with or without L-NAME. i ncreased CBF without an increase in L-[C-14]citrulline release, thereb y demonstrating that the effect of L-NAME to block NMDA-mediated incre ases in CBF was specific and not due to generalized vasoparalysis. The se results demonstrate that the local increase in CBF caused by stimul ation of cortical NMDA receptors is dependent on production of nitric oxide. This study also demonstrates that microdialysate-measured conve rsion of L-[C-14]arginine to L-[C-14]citrulline is potentially useful as a marker of nitric oxide production in vivo.