Acute deletion of the central MR/GR steroid receptor correlates with changes in LTP, auditory neural gain, and GC-A cGMP signaling

Abstract

The complex mechanism by which stress can affect sensory processes such as hearing is still poorly understood. In a previous study, the mineralocorticoid (MR) and/or glucocorticoid receptor (GR) were deleted in frontal brain regions but not cochlear regions using a CaMKIIα-based tamoxifen-inducible Cre ^ERT2/loxP approach. These mice exhibit either a diminished (MR^TMXcKO) or disinhibited (GR^TMXcKO) auditory nerve activity. In the present study, we observed that mice differentially were (MR^TMXcKO) or were not (GR^TMXcKO) able to compensate for altered auditory nerve activity in the central auditory pathway. As previous findings demonstrated a link between central auditory compensation and memory-dependent adaptation processes, we analyzed hippocampal paired-pulse facilitation (PPF) and long-term potentiation (LTP). To determine which molecular mechanisms may impact differences in synaptic plasticity, we analyzed Arc/Arg3.1, known to control AMPA receptor trafficking, as well as regulators of tissue perfusion and energy consumption (NO-GC and GC-A). We observed that the changes in PPF of MR^TMXcKOs mirrored the changes in their auditory nerve activity, whereas changes in the LTP of MR^TMXcKOs and GR^TMXcKOs mirrored instead the changes in their central compensation capacity. Enhanced GR expression levels in MR^TMXcKOs suggest that MRs typically suppress GR expression. We observed that hippocampal LTP, GC-A mRNA expression levels, and ABR wave IV/I ratio were all enhanced in animals with elevated GR (MR^TMXcKOs) but were all lower or not mobilized in animals with impaired GR expression levels (GR^TMXcKOs and MRGR^TMXcKOs). This suggests that GC-A may link LTP and auditory neural gain through GR-dependent processes. In addition, enhanced NO-GC expression levels in MR, GR, and MRGR^TMXcKOs suggest that both receptors suppress NO-GC; on the other hand, elevated Arc/Arg3.1 levels in MR^TMXcKOs and MRGR^TMXcKOs but not GR^TMXcKOs suggest that MR suppresses Arc/Arg3.1 expression levels. Conclusively, MR through GR inhibition may define the threshold for hemodynamic responses for LTP and auditory neural gain associated with GC-A.

Keywords: GC-A; NO-GC; cognition; glucocorticoid receptor; mineralocorticoid receptor.

Figure 1

GR and MR mRNA expression level (blue) in the CA1 region of the hippocampus of MR and GR^TMXcKO mice. (A) MR^TMXcKO (green) mice showed significantly higher GR mRNA expression levels in comparison to their WT controls (gray). (B) GR^TMXcKO (red) mice showed no difference in MR mRNA expression levels in comparison to their WT controls (gray). Mean ± SEM. ns = p > 0.08, * = p < 0.05.

Figure 2

ABR wave IV/I amplitude ratio and LTP of MR^TMXcKO, GR^TMXcKO, and MRGR^TMXcKO in comparison to their WT controls. (A) MR^TMXcKOs (green) had a significantly higher wave IV/I ratio in comparison to their WT controls (gray). The MR^TMXcKOs had significantly higher LTP in comparison to their WT controls. (B) GR^TMXcKOs (red) had a slightly, non-significantly lower wave IV/I ratio compared to their WT controls (gray). GR^TMXcKOs had a trend toward lower LTP in comparison to their WT controls. (C) MRGR^TMXcKOs (blue) did not show a significant difference of wave IV/I ratio in comparison to their WT controls (gray). Contrary to this, MRGR^TMXcKOs had significantly lower LTP in comparison to their WT controls. Mean ± SEM. ns = p > 0.08, (*) = p < 0.08, * = p < 0.05, ** = p < 0.01.

Figure 3

PPF as an indicator of short-term plasticity. (A) The analysis of PPF in MR^TMXcKOs (green) showed a significantly lower paired-pulse ratio of the EPSP2/EPSP1 slope in comparison to WT controls (gray) with a significant post-hoc test at 50 ms ISI. (B) GR^TMXcKOs (red) showed no difference in paired-pulse ratio of the EPSP2/EPSP1 slope in comparison to WT controls (gray). (C) MRGR^TMXcKOs (blue) showed no difference in paired-pulse ratio of the EPSP2/EPSP1 slope in comparison to WT controls (gray). Mean ± SEM. ns = p > 0.08, *** = p < 0.001.

Figure 4

NO-GC, Arc/Arg3.1, and GC-A expression in the CA1 region of the hippocampus in MR, GR, and MRGR^TMXcKO mice. (A) MR^TMXcKO (green) mice showed significantly higher levels of NO-GC mRNA expression, Arc/Arg3.1 mRNA expression, and GC-A mRNA expression in comparison to their WT controls (gray). (B) GR^TMXcKO (red) mice showed significantly higher NO-GC mRNA expression levels, equal Arc/Arg3.1 mRNA expression levels, and equal GC-A mRNA expression levels in comparison to their WT controls (gray). (C) MRGR^TMXcKO (blue) mice showed significantly higher levels of NO-GC mRNA expression levels and Arc/Arg3.1 mRNA expression, but significantly lower GC-A mRNA expression levels in comparison to their WT controls (gray). Mean. ns = p > 0.08, * = p < 0.05, ** = p < 0.01, *** = p < 0.001.

Figure 5

A summary of the hypothesized effects of MR and GR in a physiological state and stress state (GR↑). (A) When at a normal level, MR inhibits GR (green dashed line, upper half). However, neither MR nor GR affect PPF when at physiological levels. At elevated GR conditions (lower half), PPF is low. (B) Both stress receptors exhibit an inhibitory effect on NO-GC expression levels, though the inhibitory effect of MR (green solid line) is stronger than that of GR (red dashed line). (C) MR inhibits Arc/Arg3.1 expression levels (green dashed line, upper half); however, when GR is elevated, Arc/Arg3.1 is stimulated (lower half). (D) Basal MR stimulates GC-A expression levels and LTP (green dashed line, upper half). Elevated GR expression also leads to elevated GC-A and to elevated LTP (lower half).

Figure 6

An abstract scheme of the pathways that can be concluded from the acute deletion of MR (green). (A) MR deletion leads to elevated GR expression levels. (B) As a result of elevated GR, an increase in GC-A and Arc/Arg3.1 expression levels in the hippocampus can be observed. (C) This change in expression pattern may be functionally related to higher hippocampal LTP. (D) Elevated GC-A and Arc/Arg3.1 expression levels and elevated LTP potentially provide a means for top-down mechanisms allowing for auditory neural gain.