EXPOSURE TO CHRONIC PSYCHOSOCIAL STRESS AND CORTICOSTERONE IN THE RAT- EFFECTS ON SPATIAL DISCRIMINATION-LEARNING AND HIPPOCAMPAL PROTEIN-KINASE C-GAMMA IMMUNOREACTIVITY

Previous reports have demonstrated a striking increase of the immunore activity of the gamma-isoform of protein kinase C (PKC gamma-ir) in Am mon's horn and dentate gyrus (DC) of rodent hippocampus after training in a spatial orientation task. In the present study, we investigated how 8 days of psychosocial stress affects spatial discrimination learn ing in a hole board and influences PKC gamma-ir in the hippocampal for mation. The acquisition of both reference memory and working memory wa s significantly delayed in the stressed animals during the entire trai ning period. With respect to cellular plasticity, the training experie nce in both nonstressed and stressed groups yielded enhanced PKC gamma -ir in the CA1 and CA3 regions of the posterior hippocampus but not in subfields of the anterior hippocampus. Stress enhanced PKC gamma-ir i n the DG and CA3 pyramidal cells of the anterior hippocampus. In stres sed animals that were subsequently trained, the PKC gamma-ir was incre ased in the posterior CA1 region to the same level as that found in no nstressed trained animals. Stress apparently abrogated the PKC gamma-i r training response in the CA3 region. In a second experiment, the ele vation of plasma corticosterone levels to values that are found during stress did not significantly influence reference memory scores but sl ightly and temporarily affected working memory. The training-induced e nhancement of PKC gamma-ir in the CA1 region was similar in trained an d corticosterone-treated trained animals, but the learning-induced PKC gamma-ir response in the posterior CA3 area was absent after corticos terone pretreatment. These results reveal that prolonged psychosocial stress causes spatial learning deficits, whereas artificial elevation of corticosterone levels to the levels that occur during stress only m ildly affects spatial memory performance. The spatial learning deficit s following stress are reflected only in part in the redistribution of hippocampal PKC gamma-ir following training. (C) 1997 Wiley-Liss, Inc .