Modulation of anoxic injury in CNS white matter by adenosine and interaction between adenosine and GABA

1. We examined the role of adenosine in the development of anoxic injury in a CNS white matter tract, the rat optic nerve. Application of adenosine protected the rat optic nerve from anoxic injury; 2.5 μM adenosine increased compound action potential (CAP) recovery after a standard 60-min anoxic period from 28.6 ± 2.5%, mean ± SE, to 51.0 ± 3.1% (P < 0001). The protective effect of adenosine was abolished by the adenosine receptor antagonist theophylline (100 μM). 2. The protective effect of adenosine evolved slowly after adenosine application; maximum protection required 60 min of adenosine exposure before the onset of anoxia. The concentration dependence of the protective effect was parabolic, with maximum protection at 2.5 μM. Neither high nor very low adenosine concentrations protected against anoxia. These characteristics are similar to those previously found for the inhibitory neurotransmitter γ-aminobutyric acid (GABA) in the same preparation. 3. Inhibition of adenosine receptors (100 μM theophylline) reduced the level of recovery from that found under control conditions (24.3 ± 4.8% compared with 36.2 ± 2.5%, P < 0.05). The adenosine uptake inhibitor propentofylline, which potentiates release of endogenous adenosine during brain anoxia, significantly increased CAP recovery after anoxia. This effect was abolished by theophylline. It appeared therefore that release of endogenous adenosine limited injury in the optic nerve during anoxia. 4. The protective effect of adenosine was removed by pretreatment with the protein kinase C (PKC) inhibitor staurosporine (10 nM), indicating that activation of PKC was required for protection after exposure to adenosine. 5. Coadministration of low nanomolar concentrations of GABA shifted the concentration dependence of the protective effect of adenosine to lower concentrations. In the presence of 20 nM GABA, maximum protection was found at 2.5 μM adenosine; in 40 nM GABA, maximum protection was found at 1.5 μM adenosine; and in 100 nM GABA, maximum protection was observed at 900 nM adenosine. 6. The data suggest that adenosine and GABA can act synergistically at nanomolar concentrations to recruit a PKC-mediated protective mechanism during anoxia in white matter. Synergism between the two receptor types may be necessary to activate this autoprotective mechanism during small increases in the concentration of adenosine and GABA that occur in anoxic white matter.