CORTICOSTEROIDS INFLUENCE THE ACTION-POTENTIAL FIRING PATTERN OF HIPPOCAMPAL SUBFIELD CA3 PYRAMIDAL CELLS

Corticosteroids regulate gene expression through the activation of min eralocorticoid and glucocorticoid receptors. The hippocampus contains the highest density of mineralocorticoid and glucocorticoid receptors in the central nervous system. The modulation of neuron excitability b y corticosteroids in hippocampal subfield CA1 is well documented. Howe ver, it is not known whether corticosteroids produce different effects across the various hippocampal subfields. Therefore, we used intracel lular recording techniques to examine the actions of chronic corticost eroid treatment (2 weeks) on the electrophysiological properties of ra t hippocampal subfield CA3 pyramidal cells. The treatment groups used in this investigation were: adrenalectomy (ADX), selective mineralocor ticoid receptor activation with aldosterone (ALD), mineralocorticoid a nd glucocorticoid receptor activation with high levels of corticostero ne (HCT), and SHAM. Corticosteroid treatment altered the percentage of nonburst and burst firing neurons. The percentages of nonbursting cel ls were 74 and 62% in tissue from ADX and HCT animals compared to 42 a nd 41% in ALD and SHAM animals, respectively. The corticosteroid-induc ed effect on the ratio of nonbursting to bursting cells does not appea r to be secondary to changes in the cell's membrane input resistance, resting potential, time constant, action potential, slow-or fast-after hyperpolarizing potential properties. Based on these results we conclu de that corticosteroids are important for maintaining the ratio of non burst and burst firing pyramidal neurons in subfield CA3. These novel results are distinct from those previously reported for subfield CA1, suggesting that corticosteroids have different effects across hippocam pal subfields.