Effect of long-term elevated corticosteroid levels on field responses to synaptic stimulation, in the rat CA1 hippocampal area.

Daily injections with high doses of corticosterone for 3 weeks were previously found to result in atrophy of the dendritic tree of hippocampal CA3 neurons, which form a major input source to CA1 pyramidal cells. In this study we examined if exposure of rats to a similar chronic corticosterone treatment is associated with changes in field responses of CA1 neurons to stimulation of Schaffer collateral/commissural fibers. In line with earlier studies, we observed that corticosterone injections for 1 day or 1 week reduce the maximal amplitude of the population spike and synaptic potentiation observed after theta burst stimulation, respectively. Yet, animals which were exposed to high corticosterone levels during 3 weeks did not exhibit significant suppression of either the field potential amplitude or the synaptic potentiation compared with the control group. The data suggest that exposure of rats for 3 weeks to very high corticosterone levels induces adaptational changes in the CA1 hippocampal network function which partly normalize the effects seen with less prolonged corticosterone treatment.