Roles of purines in synaptic modulation evoked by hypercapnia in isolated spinal cord of neonatal rat in vitro.
OBJECTIVE
The purine compounds, adenosine 5'-triphosphate (ATP) and adenosine, are known to accumulate in the extracellular space and to elicit various cellular responses during hypoxia/ischemia, whereas the roles of purines during hypercapnia are poorly understood. In this study, we examined the effects of various drugs affecting purine turnover on the responses to hypercapnia in the spinal cord.


METHODS
Electrically evoked reflex potentials were measured in an in vitro preparation of the isolated spinal cord of the neonatal rat by extracellular recording. Extracellular adenosine concentrations were assayed by high performance liquid chromatography (HPLC) methods.


RESULTS
Hypercapnia (20% CO2) depressed the reflex potentials, which were partially reversed by an adenosine A1 receptor antagonist, 8-cyclopentyl theophylline, but not by a P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid. Exogenous adenosine and ATP also depressed the reflex potentials via adenosine A1 receptors. The hypercapnia-evoked depression was not reversed by inhibitors of gap junction hemichannels, anion channels, P2X7 receptors or equilibrative nucleoside transporters, all of which might be involved in purine efflux pathways. The adenosine accumulation evoked by hypercapnia was not inhibited by tetrodotoxin, ethylene glycol-bis(beta-amino ethyl ether) tetraacetic acid (EGTA) or an ecto-ATPase inhibitor, ARL 67156. Homocysteine thiolactone, used to trap intracellular adenosine, significantly reduced extracellular adenosine accumulation during hypercapnia.


CONCLUSIONS
These results suggest that hypercapnia released adenosine itself from intracellular sources, using pathways different from the conventional exocytotic mechanism, and that this adenosine depressed spinal synaptic transmission via adenosine A1 receptors.
Laboratory of Pharmacology, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Japan. otsuguro@vetmed.hokudai.ac.jp
K
K
Otsuguro
M
M
Ban
T
T
Ohta
S
S
Ito
D000241:Adenosine / Q000378:metabolism* / Q000494:pharmacology; D058916:Adenosine A1 Receptor Antagonists; D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D004594:Electrophysiology; D005071:Evoked Potentials; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D006935:Hypercapnia / Q000378:metabolism* / Q000503:physiopathology; D066298:In Vitro Techniques; D008297:Male; D058919:Purinergic P2 Receptor Antagonists; D011732:Pyridoxal Phosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012018:Reflex; D013116:Spinal Cord / Q000378:metabolism* / Q000503:physiopathology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology
D058916:Adenosine A1 Receptor Antagonists; D058919:Purinergic P2 Receptor Antagonists; C077792:pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid; C053907:8-cyclopentyl-1,3-dimethylxanthine; D011732:Pyridoxal Phosphate; D000255:Adenosine Triphosphate; D013806:Theophylline; D000241:Adenosine