Mineralocorticoid receptors are indispensable for nongenomic modulation of hippocampal glutamate transmission by corticosterone

Abstract

The adrenal hormone corticosterone transcriptionally regulates responsive genes in the rodent hippocampus through nuclear mineralocorticoid and glucocorticoid receptors. Via this genomic pathway the hormone alters properties of hippocampal cells slowly and for a prolonged period. Here we report that corticosterone also rapidly and reversibly changes hippocampal signaling. Stress levels of the hormone enhance the frequency of miniature excitatory postsynaptic potentials in CA1 pyramidal neurons and reduce paired-pulse facilitation, pointing to a hormone-dependent enhancement of glutamate-release probability. The rapid effect by corticosterone is accomplished through a nongenomic pathway involving membrane-located receptors. Unexpectedly, the rapid effect critically depends on the classical mineralocorticoid receptor, as evidenced by the effectiveness of agonists, antagonists, and brain-specific inactivation of the mineralocorticoid but not the glucocorticoid receptor gene. Rapid actions by corticosterone would allow the brain to change its function within minutes after stress-induced elevations of corticosteroid levels, in addition to responding later through gene-mediated signaling pathways.

Fig. 1.

Corticosterone rapidly and reversibly enhances mEPSC frequency in hippocampal cells. (A) Typical example showing enhanced mEPSC frequency in a CA1 pyramidal cell during 100 nM corticosterone (cort) application (Middle) compared with the control situation (Top). Bottom shows the typical shape of an mEPSC. (B) Example showing that mEPSC frequency is rapidly and reversibly enhanced by corticosterone but not by vehicle (0.009% ethanol). (C) Cumulative frequency histogram of the mEPSC frequency in a CA1 hippocampal cell, showing that the mEPSC frequency is enhanced in the presence of corticosterone and returns to pretreatment levels after corticosterone has been washed out. (D) Percentual increase in mEPSC frequency caused by various concentrations of corticosterone (filled circles, based on n = 3–5 cells for each concentration), showing that 10 nM corticosterone, but not lower concentrations of corticosterone, induces a significant (P < 0.05) increase in mEPSC frequency. The membrane-impermeable BSA–corticosterone conjugate (100 nM; n = 5) also resulted in a marked increase in mEPSC frequency. In the presence of the protein synthesis inhibitor cycloheximide (100 nM; n = 5), corticosterone was still able to enhance the mEPSC frequency. Symbols represent the mean + SEM values.

Fig. 2.

Increased mEPSC frequency during corticosterone application is mediated by the MR and not the GR. (A) The selective GR agonist RU 28362 (100 nM) does not increase the mEPSC frequency (n = 4). The GR antagonist RU 38486 (500 nM; n = 4) does not significantly affect the percentual increase in mEPSC frequency caused by corticosterone when compared with the response evoked by corticosterone alone (shown on the far left). By contrast, the endogenous mineralocorticoid aldosterone (10 nM; n = 5), applied in the presence of the GR antagonist RU 38486, potently increased the mEPSC frequency. Additional application of the MR antagonist spironolactone (100 nM; n = 5) completely abolished the response to aldosterone. (B) Corticosterone (100 nM) significantly enhances the mEPSC frequency in CA1 pyramidal cells of brain-specific GR knockout mice (GR^NesCre)(n = 4 cells) as well as of controls (GR^loxP/loxP)(n = 4). In cells of brain-specific MR knockout mice (MR^CamKIICre) (n = 6), however, corticosterone application was ineffective, and normal effects were seen in control mice (MR^loxP/loxP)(n = 6). All data represent the percentual increase in mEPSC frequency, expressed as the ratio of the averaged frequency determined between 5 and 10 min after administration of the agonist and the averaged frequency between 5 and 0 min before administration was started (*, P < 0.05). Details on the transgenesis are provided in Supporting Text.