Abstracts

Lowering brain carbon dioxide (CO[sub]2 [/sub]) levels is used clinically to lower seizure threshold and to induce [italic]absence[/italic] seizures. Increasing CO[sub]2[/sub] levels has profound effects on respiration, memory, and consciousness. We have used hippocampal slices to investigate how CO[sub]2[/sub] alters pH, extracellular adenosine concentration, and neuronal excitability. We examined how, via alteration of purine metabolism, CO[sub]2[/sub] levels affect a model of interictal activity. Rat hippocampal slices were cut from 4-8 week old male Sprague-Dawley rats as described in Dunwiddie [amp] Hoffer, Br. J. Pharmacol. 69, 59-68. fEPSPs were recorded from area CA1 and epileptiform activity was induced in area CA3 by LTP of the recurrent collateral pathway (Stasheff, et. al., Brain Res. 344, 296-302). Extracellular adenosine levels were monitored with an enzymatic adenosine sensor (Dale, J. Physiol. 511, 265-272) while electrophysiological recordings were made. Intracellular pH was measured using 2-photon imaging of CA1 pyramidal cells loaded with BCECF. Hypercapnic acidosis (20% CO[sub]2[/sub]) caused a 48.8 [plusmn] 2.4 % decrease in fEPSP amplitude in area CA1; extracellular adenosine rose by 1.2 [plusmn] 0.2 [mu]M which contributed significantly to this inhibition (19.9 [plusmn] 3.6 %). Mild Hypercapnic acidosis (10% CO[sub]2[/sub]) also caused adenosine release which was shown to be pH-dependent. Hypercapnic acidosis attenuated epileptiform activity in area CA3 (6 out of 6 trials) by causing adenosine release. Hypocapnic alkalosis (2% CO[sub]2[/sub]) increased CA1 fEPSP amplitude by 22.1 [plusmn] 3.6% and decreased extracellular adenosine concentration by 0.5 [plusmn] 0.1[mu]M. This also increased the frequency of epileptiform activity in area CA3 from 0.06 [plusmn] 0.01 Hz to 0.11 [plusmn] 0.02. The increase in CA1 excitability due to hypocapnic alkalosis was significantly attenuated by blockade of adenosine A[sub]1[/sub] receptors and purinergic P[sub]2[/sub] receptors. Inhibition of ecto-ATPases had no effect on inhibition caused by hypercapnic alkalosis. Based on our studies, we conclude that changes in pH caused by alterations in brain CO[sub]2[/sub] levels alter extracellular adenosine concentration. This in turn modulates excitability. During hypocapnia, decreased adenosine levels cause increased excitability, due to both increased activation of P[sub]2 [/sub]receptors and decreased activation of A[sub]1[/sub] receptors, suggesting that ecto-APTases mediate this effect. Decreased effects of adenosine increase the rate of epileptiform activity and thus are likely to contribute to hypocapnia-induced lowering of seizure threshold. During hypercapnia sufficient adenosine is released to attenuate epileptiform activity. This increase in extracellular adenosine does not depend on ecto-ATPases suggesting that there may be multiple pathways contributing to pH modulation of extracellular adenosine concentration. (Supported by the Epilepsy Foundation, American Epilepsy Society, and the NIH.)