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Overexposure to corticosteroid hormones is harmful to hippocampal neuronal integrity, likely by perturbation of calcium homeostasis. To identify molecular mechanisms at the single-cell level, we characterized mRNA expression corresponding to voltage- and ligand-gated Ca channels in individual dissociated CA1 neurons in response to long-term corticosterone (CORT) exposure. Predominant mineralocorticoid receptor occupation (ADC-LO group) resulted in low levels of P/Q- and L-type Ca channel mRNAs, high levels of GluR-2 versus GluR-1, and a high ratio of NMDAR-2A to NMDAR-2B mRNA. Corresponding alterations in protein expression were consistent with the restriction of Ca influx. In contrast, additional glucocorticoid receptor occupation (ADC-HI group) altered the expression of these mRNAs in a manner consistent with enhanced Ca influx; interestingly, qualitatively similar alterations were seen in control ADX neurons. Electrophysiological data from the same neurons indicate that Ca current amplitudes also are modulated by CORT, although on a shorter time scale. Finally, principal components analysis (PCA) suggests that neuronal AMPA and NMDA receptor composition may be regulated by MR and GR activation in a complex manner. Therefore, our data implicate molecular events by which CORT may regulate Ca influx into CA1 hippocampal neurons.

Previously it was found that the amplitude of Ca currents in CA1 hippocampal neurons increases after adrenalectomy (ADX) of young adult rats. Preliminary data suggested that this effect of ADX is age-dependent. In the present study we therefore investigated the effect of ADX on Ca currents in three age groups: rats that were 1, 3, or 6 months old. It appeared that ADX of the youngest age group resulted in a selective enhancement of sustained, high threshold Ca currents. By contrast, ADX of the oldest rats enhanced only the low threshold, transient Ca current amplitude. The age dependency of the ADX-induced effects can be explained by developmental changes in Ca current properties in the adrenally intact rats with which ADX animals were compared. Data from acutely dissociated cells, which lack most of their dendrites, suggest that the ADX-induced changes of Ca current properties are at least partly targeted at currents generated in the distal dendrites. No significant changes were observed with age or after ADX for the mRNA expression of the alpha 1D Ca channel subunit, which forms the ionpore of the sustained high threshold L-type Ca channel. We can therefore presently not exclude that the altered amplitude of Ca current with age and ADX is due to changes in ion channel function rather than in the total number of these channels.

Consequences of corticosteroid receptor activation on voltage-dependent Na+ conductances were studied in acutely dissociated CA1 hippocampal neurons. This preparation was selected because of the compact electrotonic properties of dissociated neurons, allowing reliable voltage-clamp of the large and fast Na+ currents. The Na+ currents were studied in (i) neurons of adrenalectomized animals (no steroid receptors occupied), (ii) neurons from tissue of adrenalectomized rats treated in vitro with corticosterone and the glucocorticoid receptor antagonist RU38486 (selectively occupying the mineralocorticoid receptor), (iii) corticosterone-treated neurons of adrenalectomized animals (occupying both the mineralocorticoid and glucocorticoid receptors) and (iv) neurons of sham-operated animals. Activation and steady-state inactivation properties of the Na+ current recorded in neurons of adrenalectomized animals were slightly shifted (3-5 mV) to hyperpolarized potentials as compared to the Na+ currents from neurons of the other experimental groups. Furthermore, the removal from inactivation of the Na+ current in the group of neurons of adrenalectomized animals was relatively slow. Although small, these effects could influence neuronal properties like action potential generation and accommodation. Under the present experimental conditions, no apparent differences were seen between cells with predominant mineralocorticoid receptor activation and cells where both mineralocorticoid and glucocorticoid receptors were occupied. In contrast to Na+ currents, voltage-dependent Ca2+ currents displayed no steroid-dependent shifts in voltage-dependent properties. However, Ca2+ current amplitudes were increased by approximately 160% in CA1 neurons of adrenalectomized animals as compared to Ca2+ currents from neurons of the other experimental groups. We conclude that corticosteroid receptor activation affects various properties of voltage-dependent Na+ and Ca2+ conductances in CA1 neurons, indicating that the steroid receptors are involved in the modulation of neuronal excitability in these cells.