Postnatal development of a new type of epileptiform activity in the rat hippocampus

Long-term application of Cs+ (5 mM) induces an epileptiform field potential (Cs-FP) in area CA1 of the rat hippocampus, which is independent of N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors and γ-aminobutyric acid (GABA)A receptors. To gain insight into possible mechanisms for the induction of the Cs-FP, we investigated the postnatal development of the response. In brain slices prepared from rats at different ages, the Cs-FP was evoked by stimulation of the Schaffer-collateral–commisural pathway. We found that expression of this potential was clearly dependent on the postnatal age. Thus, the Cs-FP was completely absent at 1 week of age. By 2 weeks, a reduced form of the response was observed, whereas slices taken from 3-week-old rats, displayed full Cs-FP, which were indistinguishable in size and shape from the adult form. In the presence of 4-aminopyridine, potentials resembling the Cs-FP were evoked. These potentials showed a similar age-dependency as the Cs-FP. The Na+/K+ pump inhibitor dihydroouabain (DHO), when present during wash-in of Cs+, gave a partial block of the Cs-FP in adult slices. This effect was not seen when DHO was applied after development of the Cs-FP. The data indicate that the processes necessary for expression of the Cs-PF are absent at birth and develop during the second postnatal week. We propose that the Cs-FP depends on Cs+ entry into presynaptic neurons, and that the Na+/K+ pump contributes to this transport of Cs+. The observed age-dependency could therefore, in part, reflect the delayed development of the Na+/K+ pump.