Background and purpose: 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.
Methods: 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).
Results: 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.
Conclusions: 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.