Decreased Population Spike in CA1 Hippocampal Area of Adrenalectomized Rats after Repeated Synaptic Stimulation

Population spikes, evoked in the CA1 hippocampal area by stimulation of the Schaffer collaterals at various intensities, were recorded over a period of 70 min in slices from 7-day adrenalectomized (ADX) or sham-operated rats. Slices from sham-operated rats with intermediate plasma corticosterone levels (between 0.5 and 5μg corticosterone/100 ml plasma) at the start of the experiment displayed very stable synaptic responses. However, the responses recorded in slices from rats with lower or higher corticosterone levels gradually declined, with repeated stimulation. Similarly, a significant decline of the population spike over time was observed in slices from ADX rats, particularly with low stimulus intensities; characteristics of the compound EPSP were much more stable. The decline of the population spike amplitude was alleviated when: 1) single rather than repeated stimulation was applied; 2) the experiments were performed in the presence of 20 mM glucose; or, 3) moderately high (10^?8 or 10^?7 M) concentrations of corticosterone were administered in vitro; low (10^?9 M) or high (10^?6 M) corticosterone concentrations in vitro did not improve the stability of the synaptic response in slices from ADX rats. These data suggest that intermediate levels of corticosterone are necessary to maintain the stability of the Schaffer collateral input to CA1 neurons. With very low or high corticosterone levels, CA1 neurons apparently fail to respond to synaptic stimulation, over time. The possible mechanisms underlying this bell-shaped dose response curve for corticosterone are discussed.