Abnormal neuronal excitability in hippocampal slices from kindled rats.

To determine if electrophysiological properties of hippocampal pathways are altered in kindled rats, extracellular recordings were made from hippocampal slices of rats kindled in the lateral entorhinal cortex and compared with those from implanted but unstimulated controls. Studies were made either 24 h or 28 days after the last kindled seizure and done in normal (3.5 mM) or elevated (7 mM) K+. The preparation of slices, data accumulation, and data analyses were done blind. One day or 28 days after the last kindled seizure, the proportion of slices with spontaneous epileptiform bursts recorded from the CA2/3 region in elevated K+ was significantly (P less than 0.001) increased in the kindled animals. The frequency of spontaneous burst firing was also increased and reached significance (P less than 0.02) at 28 days following the last kindling stimulus. One day after the last kindling stimulus, paired-pulse (GABAergic) inhibition in the CA1 region was decreased (P less than 0.001). Several measures suggested an increased synaptic inhibition in the dentate gyrus of slices from the kindled groups 1 day after kindling. Paired-pulse inhibition was increased (P less than 0.01), the current required to evoke a near-threshold population spike was increased (P less than 0.05), and the population spike amplitude was reduced for a given field excitatory postsynaptic potential (EPSP) (P less than 0.01). Twenty-eight days after the last kindling stimulus, however, paired-pulse inhibition in the dentate was slightly less in slices from kindled rats (P less than 0.005). In other respects the CA1 and dentate regions did not differ between kindled and control groups within 24 h of the last stage V seizure. Thus the maximum amplitudes of presynaptic fiber volley, population spike, and field-excitatory postsynaptic potential (EPSP) slope, and the number of population spikes evoked by a near-maximally effective afferent stimulus, were unchanged. In the CA1 region the input-output curve of field EPSP versus population spike, and the current intensity required to evoke a near-threshold population spike were also unchanged. In addition, no spontaneous bursts were recorded from CA1 in 3.5 mM K+. We conclude that either synapses or neurons intrinsic to the hippocampus are altered by kindling stimuli applied outside this brain area. The transient increase in inhibition in the dentate gyrus suggests that it may reflect a compensatory reaction to kindled seizures. In contrast, the long-lasting (at least 28 days) increase in burst firing in CA2/3 may represent a mechanism for the initiation or propagation of kindled seizures.(ABSTRACT TRUNCATED AT 400 WORDS)