The depression of spinal reflex potentials during hypoxia in isolated spinal cord of neonatal rat in vitro

In order to investigate the mechanism of the hypoxia-induced inhibition of the neuronal activity, we studied the effect of hypoxia on spinal reflex potentials by exposing isolated spinal cords to hypoxic ACSF (5% CO2+ 95% N2) in vitro. The monosynaptic reflex potentials (MSR) and slow ventral root potentials (sVRP) were recorded from a ventral root (L3-L5) with the electrical stimulation of the corresponding lumbar dorsal root in rat isolated spinal cords. The MSR and sVRP were reversibly depressed by a brief exposure (10-15 min) to hypoxia. The inhibitory effects of hypoxia on spinal reflex potentials were smaller in younger rats (Day 0-4) than older rats (Day 5-8). Application of adenosine (0.1 μM～1 mM) caused a concentration-dependent depression of MSR and sVRP. The sensitivity to adenosine was a little higher in MSR than sVRP. The depression of MSR and sVRP by adenosine was attenuated by a selective adenosine A1 receptor antagonist, CPT. CPT (3 μM) potentiated MSR and sVRP by about 20%. In the presence of CPT, the hypoxia-induced depression of MSR was virtually abolished. The depression of sVRP during hypoxia was significantly attenuated, but not abolished by CPT. Inhibitors of equilibrative nucleoside transporter (ENT), NBTI (5 μM) and dipyridamole (10 μM), had no effect on MSR, but reduced sVRP by about 10%. Both NBTI and dipyridamole were ineffective in the depression of MSR and sVRP during hypoxia. A GABAA receptor antagonist, bicuculline (5 μM), did not affect MSR but markedly potentiated sVRP. In the presence of bicuculline, the hypoxia-induced depression of MSR was rather profound compared with that in the absence of bicuculline. The depression of sVRP with hypoxia was significantly attenuated, but not abolished by bicuculline. A glycine receptor antagonist, strychnine (1 μM) did not affect MSR but potentiated sVRP dramatically. There were no significant change in the hypoxia-induced depression of MSR and sVRP in the presence of strychnine, but the degree of the depression of sVRP tended to be smaller. An acetylcholine muscarinic receptor antagonist, atropine (10 μM), had no effect on MSR and sVRP, nor affected the depression of MSR and sVRP during hypoxia.The NO synthase inhibitor, L-NAME (10,100 μM), slightly increased MSR and sVRP. L-NAME were not effective in the depression of MSR and sVRP by hypoxia. These results suggest that hypoxia depresses MSR through adenosine release and subsequent activation of A1 receptors. ENTs seem to play a role