Bhardwaj A, Northington F J, Koehler R C, Stiefel T, Hanley D F, Traystman R J
Department of Neurology, Johns Hopkins Medical Institutions, Baltimore, Md 21287-7840, USA.
Stroke. 1995 Sep;26(9):1627-33. doi: 10.1161/01.str.26.9.1627.
Adenosine acts presynaptically to inhibit release of excitatory amino acids (EAAs) and is thus considered to be neuroprotective. Because EAA-stimulated synthesis of nitric oxide (NO) may play an important role in long-term potentiation and excitotoxic-mediated injury, we tested the hypotheses that adenosine agonists attenuate basal and EAA-induced NO production in the hippocampus in vivo and that adenosine A1 receptors mediate this response.
Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 hours with artificial cerebrospinal fluid containing 3 mumol/L [14C]L-arginine. Recovery of [14C]L-citrulline in the effluent was used as a marker of NO production. In 10 groups of rats, time-dependent increases in [14C]L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of N-methyl-D-aspartate (NMDA), adenosine agonists, adenosine antagonists, and the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME).
Recovery of [14C]L-citrulline during perfusion with artificial cerebrospinal fluid progressively increased to 141 +/- 27 fmol/min (+/- SEM) over 5 hours. Contralateral perfusion with 1 mmol/L NMDA augmented [14C]L-citrulline recovery to 317 +/- 62 fmol/min. Perfusion of 1 mmol/L L-NAME with NMDA inhibited [14C]L-citrulline recovery compared with NMDA alone. Perfusion with 0.1 mmol/L 2-chloroadenosine attenuated basal as well as NMDA-enhanced [14C]L-citrulline recovery. This action of 2-chloroadenosine was reversed by infusion of 0.1 mmol/L 8-cyclopentyl-1,3-dipropylxanthine, a specific A1 receptor antagonist. Infusion of 0.1 mmol/L (2S)-N6-[2-endo-norboryl]adenosine, a specific A1 receptor agonist, also attenuated the 0.1 mmol/L and 1 mmol/L NMDA-enhanced [14C]L-citrulline recovery.
Using an indirect method of assessing NO production in vivo, these data are consistent with in vitro results showing that NMDA receptor stimulation enhances NO production. Furthermore, we conclude that stimulation of A1 receptors can attenuate the basal as well as NMDA-induced production of NO. Because NMDA receptor stimulation amplifies glutamate release, our data are consistent with presynaptic A1 receptor-mediated inhibition of EAA release and consequent downregulation of NO production.
腺苷在突触前发挥作用，抑制兴奋性氨基酸（EAA）的释放，因此被认为具有神经保护作用。由于EAA刺激的一氧化氮（NO）合成可能在长时程增强和兴奋性毒性介导的损伤中起重要作用，我们检验了以下假设：腺苷激动剂可在体内减弱海马中基础状态下以及EAA诱导的NO生成，且腺苷A1受体介导这一反应。
在戊巴比妥麻醉下，将微透析探针双侧植入成年Sprague-Dawley大鼠海马的CA3区。用含3 μmol/L [14C]L-精氨酸的人工脑脊液灌注探针5小时。流出液中[14C]L-瓜氨酸的回收量用作NO生成的标志物。在10组大鼠中，比较了用N-甲基-D-天冬氨酸（NMDA）、腺苷激动剂、腺苷拮抗剂和NO合酶抑制剂Nω-硝基-L-精氨酸甲酯（L-NAME）的各种组合灌注时，右侧和左侧探针中[14C]L-瓜氨酸回收量随时间的增加情况。
在用人工脑脊液灌注期间，[14C]L-瓜氨酸的回收量在5小时内逐渐增加至141±27 fmol/分钟（±标准误）。用1 mmol/L NMDA进行对侧灌注可使[14C]L-瓜氨酸回收量增加至317±62 fmol/分钟。与单独使用NMDA相比，用1 mmol/L L-NAME与NMDA一起灌注可抑制[14C]L-瓜氨酸的回收。用0.1 mmol/L 2-氯腺苷灌注可减弱基础状态下以及NMDA增强的[14C]L-瓜氨酸回收。2-氯腺苷的这一作用可被注入0.1 mmol/L 8-环戊基-1,3-二丙基黄嘌呤（一种特异性A1受体拮抗剂）逆转。注入0.1 mmol/L（2S）-N6-[2-内-降冰片基]腺苷（一种特异性A1受体激动剂）也可减弱0.1 mmol/L和1 mmol/L NMDA增强的[14C]L-瓜氨酸回收。
使用一种在体内评估NO生成的间接方法，这些数据与体外结果一致，即NMDA受体刺激可增强NO生成。此外，我们得出结论，刺激A1受体可减弱基础状态下以及NMDA诱导的NO生成。由于NMDA受体刺激会放大谷氨酸释放，我们的数据与突触前A1受体介导的EAA释放抑制以及随之而来的NO生成下调一致。
