Amygdala kindling and associated changes of entorhinal responses in suckling rats

Abstract

Entorhinal field potential with amygdala stimulation in suckling (16–18 days old) and adult rats was recorded with a tungsten wire electrode (tip diameter 2–5 μm) to study the developmental changes in behavioral seizures and long-term potentiation (LTP) in the responses to amygdala kindling stimulations. Stimulating (twisted enamel-coated wires) and recording electrodes were implanted in anesthetized rats 2–3 days before kindling. The mean amplitude of the responses to test pulses (600 μA, 0.3 Hz) in the sucklings (0.58 mV) was smaller than in the adults (1.32 mV), and latency was about 3.3 ms longer. Kindling stimulations consisted of 0.5-ms monophasic rectangular pulses of 10 Hz with a 10-s train duration; the intensity was the afterdischarge (AD) threshold. Kindling stimulation in the sucklings usually increased the amplitude of the test responses evoked 10 min or 1 h after the kindling stimulation. The increased amplitude persisted for at least 24 h, showing LTP in the synaptic transmission. The LTP was especially prominent in the first kindling stimulation, and the LTP gradually increased with successive stimulations, with gradual progression of AD and the behavioral seizure stage as well. The mean number of kindling stimulations to cause generalized seizures in the suckling rats (10.5) was less than that for adults (12.5), and the continued evolution of LTP over the course of kindling was more or less easier in the sucklings than in the adults. The LTP in kindled suckling rats was observed for all intensities of test pulses and the input—output (I/O) curve in the kindled rats was significantly higher (P < 0.01) than the control I/O curve for the rats which received low frequency (0.3 Hz) stimuli only. However, the mean amplitude of the field potentials enhanced by the LTP in kindled sucklings did not reach the mean amplitude of control responses without kindling stimulations in the adults. The initial strong LTP in the monosynaptic entorhinal responses with amygdala kindling may contribute to the emergence of epileptogenesis by kindling from the pathway, and the easier evolution of LTP over the course of kindling in suckling rats may contribute to the earlier acquisition of kindling phenomena in suckling rats than in adult rats.