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. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.