POTASSIUM-INDUCED CHANGES IN EXCITABILITY IN THE HIPPOCAMPAL CA1 REGION OF IMMATURE AND ADULT-RATS

Orthodromic and spontaneous population spike activity was measured in vitro in the CA1 region of rat hippocampal slices to determine maturat ional differences in excitability and susceptibility to K+-induced sei zures. Several indices of excitability in the CA1 region changed in a non-monotonic fashion during maturation, in response to step-wise incr eases in bath [K+]. Slices from rats 18-22 days old, showed a greater probability of both spontaneous epileptiform activity and episodes of seizure-like activity followed by spreading depression, and more prolo nged durations of evoked seizure-like events. Elevation of [K+] in the bathing medium increased these indices in a similar manner in older r ats but not to the same degree as in 18- to 22-day-old rats. However, the threshold level of bath [K+] resulting in evoked bursts of populat ion spikes was lower in adult and 28- to 32-day-old rats than in 18- t o 22-day-old rats, suggesting that excitability is not uniformly great er at any given age. In 10- to 15-day-old rats, elevation of bath [K+] either produced persistent blockade of population responses, or incre ased the amplitude of the initial population spike, without producing bursts. Basal levels of [K+] in the interstitium of the slices corresp onded to the various levels of [K+] placed in the bathing medium and t here were no differences among age groups. Therefore, differences in b asal [K+]o cannot account for the maturational changes in excitability and seizure activity. The period from 18-22 days of age in the rat is a useful focal point for investigating mechanisms underlying maturati onal changes in propensity to develop seizures.