Abstracts

RATIONALE: Intracellular and extracellular pH changes may have fundamental importance in regulating neuronal activity. In particular, acidification reduces and alkalinization increases neuronal excitability. A primary factor in the movement and buffering of hydrogen ions in the central nervous system is the enzyme carbonic anhydrase. Some of the available antiepileptic drugs are known to inhibit carbonic anhydrase activity and are therefore predicted to alter pH shifts. In this study, we tested effects of antiepileptic drugs on the biphasic pH shifts in area CA1 of the hippocampus in vivo in the rat.
METHODS: Adult male Sprague-Dawley rats (120-180 g) were anesthetized with urethane. A pH sensitive electrode was placed in the pyramidal cell body layer of the dorsal hippocampus and a bipolar stimulating electrode was placed in the contralateral CA3 region in the same anteroposterior plane. Trains of stimulation, 20Hz for 15-20 seconds every 10 minutes, were used to elicit the biphasic pH shifts before and after drug administration. The peak alkalinization, peak acidification and duration of the alkalinization were quantified and the percent change in each measurement was determined in each animal tested. Drugs were administered intraperitoneally and appropriate vehicle controls were done.
RESULTS: Initial experiments showed that the alkalinization during the stimulus train was reproducible, but that the acidification, which occurred primarily after the end of the stimulus train, was dependent on the duration of any seizure activity. Therefore, drug effects were only determined on the alkalinization. Diazepam, 3 mg/kg (n=4), and phenobarbital, 60 mg/kg (n=7), significantly increased the amplitude of the alkalinization, while acetazolamide, 50 mg/kg (n=4), and topiramate 45 mg/kg (n=6), significantly decreased the amplitude of the alkalinization. Phenytoin 80 mg/kg (n=5) had no effect on the alkalinization. None of the drugs tested had an effect on the duration of the alkalinization, but there was a trend towards an increase in the duration of alkalinization with diazepam and phenobarbital, which is consistent with the increase in amplitude of the alkalinization with these drugs.
CONCLUSIONS: Previous work has shown that both glutamate and GABA can cause an alkalinization in vitro in CA1. The GABA-induced transient is inhibited and the glutamate-induced transient is enhanced by carbonic anhydrase inhibitors. The results of the present study suggest that the alkalinization in CA1 in vivo is largely due to GABA neurotransmission, since it is enhanced by GABA agonists and decreased by drugs with carbonic anhydrase inhibitory activity. In addition, the results suggest that a change in extracellular pH regulation is not an underlying mechanisms common to the antiepileptic effect of the drugs tested.
Support: NINDS 39941