MR pharmacology
Brain corticosteroid concentrations are influenced by several factors including brain penetration and local enzymatic conversion in specific areas. Access of synthetic glucocorticoids and cortisol to the brain is limited by P-glycoprotein (Pgp) expression (Karssen et al. , 2001), a multidrug transporter expressed in luminal blood facing membranes of endothelial cells of the Blood Brain Barrier (BBB) (Chapman, Holmes and Seckl, 2013). Pgp is encoded by the Mdr1a gene in rodents and MDR1 in humans (Jetté et al. , 1995). A hypo-glucocorticoid brain state may be induced using low doses of dexamethasone, based on the combination of the Pgp barrier and HPA-axis feedback at the pituitary level (Karssen et al. , 2005).
The potential effects of corticosteroid hormones on neuronal activity are determined by the distribution of receptors to which they bind. In limbic-frontocortical neurons high in MR expression (figure 1), preferential binding by GCs occurs with aldosterone being outcompeted, due to a 100-1000 higher circulating concentration of the hormone, even when cortisol is partially bound to corticosteroid-binding globulin (CBG) in the blood. Whether aldosterone is locally synthesized remains to be confirmed (Gomez-Sanchez et al. , 2005), but aldosterone selective MR binding mostly occurs by inactivation of 11-OH steroids (cortisol) to their inactive keto-variants (cortisone) by 11β-hydroxysteroid dehydrogenase type 2 (11HSD-2) (Baker and Katsu, 2017). In the brain this occurs predominantly in the brain stem nuclei of the solitary tracts (NTS), and discrete subpopulations of hypothalamic neurons (Geerling and Loewy, 2009) (figure 1+2). Its counterpart, 11HSD-1 reductase is more widely present in neurons and glial cells (Wyrwoll, Holmes and Seckl, 2011), returning local GCs to their active state, further encouraging MRs binding endogenous GCs (Edwards et al. , 1996). The well described interactions of MR with GR should be limited to glucocorticoid preferring MRs, for lack of endogenous GR ligand in 11HSD-2 expressing cells. Ligand-dependent interactions with transcriptional coregulator proteins may further add to cortisol/aldosterone specific effects in a cell and gene-dependent manner (Fuller, Yang and Young, 2017).
MR affinity to endogenous glucocorticoids is about 10-fold higher than that of GR, which has led to the notion that MR is substantially occupied under basal conditions (de Kloet, Joels and Holsboer, 2005). Given that brain access of cortisol is lower than that of corticosterone, the situation in the human brain may differ slightly from laboratory rodents (Karssen et al. , 2001). MR’s high affinity for corticosterone and cortisol predicts that receptor expression levels can be limiting for its effects. MR’s higher binding affinity to glucocorticoids is demonstrated in the ultradian rhythm of the HPA axis, where MR has extended activation and DNA binding duration during the inter-pulse interval (Lightman et al. , 2008). In contrast, GRs activated at the oscillating pulse peak transiently bind and dissociate DNA, termed ‘rapid cycling’, tracking the rise and fall of ligand concentration (Stavreva et al. , 2009). Functionally, MR activation under basal conditions is in line with a ‘preparative’ role in the stress response, where it determines stressor appraisal and initial reactivity (Oitzl and de Kloet, 1992; Schwabe, Wolf and Oitzl, 2010), whereas GR becomes activated with gradual increases in stress hormones and is (for example) involved in the consolidation of stress-related memories (ter Horst et al. , 2012; de Kloetet al. , 2018).
The high MR affinity for GCs has been mainly interpreted in relation to its genomic effects. Rapid non-genomic effects have been demonstrated for membrane associated MR and GR, which require 10-fold higher corticosteroid levels, in comparison to their genomic nuclear counterparts (Karst, 2005; Nahar et al. , 2015). In rats, non-genomic effects have for example been related to regulation of territorial aggression (Haller et al. , 2000). Recent genomics data suggest some genomic MR mediated responses require high levels of corticosterone (Mifsud and Reul, 2016; van Weert et al. , 2017), which is as of yet an unexplained contrast to the original ligand binding data (Reul and de Kloet, 1985).
In terms of transactivation strength, endogenous GCs seem less potent than aldosterone, by the comparative slow dissociation of aldosterone from the receptor and different ligand induced receptor conformational changes (Grossmann et al. , 2004). In fact, synthetic glucocorticoids display a very rapid off-rate, potentially explaining why in vivo potency is much less than would be anticipated purely from steady state ligand binding affinity measures (Reul et al. , 2000). Progesterone binds MR with high affinity but with minor transactivation and is a physiological antagonist of the MR. Spironolactone, a very powerful mineralocorticoid receptor antagonist (MRA) (IC50 66 nM), is less selective than eplerenone (IC50 990 nM) (Kolkhof and Bärfacker, 2017), and both are structurally based on the progesterone molecule. Esaxerenone potently and selectively interferes with MR mediated transcription (IC50 3.7 nM) (Arai et al. , 2015). The non-steroidal MRA finerenone exerts strong and selective MR inhibitory action (IC50 18 nM) (Bärfacker et al. , 2012), with no active metabolites and short half-life ~2h (Heinig et al. , 2018). In comparison to steroidal MRA mechanisms, finerenone reduces aldosterone induced nuclear translocation of GFP-MR more than that of spironolactone (Amazit et al. , 2015) and is further characterized by a bulky substituent that alters the MR LBD conformation observed with MR agonists causing rapid dissociation from the receptor (Amazit et al. , 2015). Fludrocortisone (Fludro) is a potent synthetic and selective MR agonist (Grossmann et al. , 2004; Gesmundo et al. , 2016).
While the GR is abundantly expressed throughout the brain, MRs expression profile is typically reported as more restricted (Reul and de Kloet, 1985). The highest proportion of aldosterone selective MRs in NTS, (figure 1) controls physiology and behaviour related to sodium balance and transport in epithelial cells (Geerling and Loewy, 2009). A proportion of these NTS neurons project to the parabrachial/locus coeruleus nuclei, some innervate the ventrolateral bed nucleus of the stria terminalis (BNST), and less project to the ventral tegmental area (VTA), central amygdala and hypothalamus regulating motivation and arousal, reward pathways and cognitive functions related to salt balance (de Kloet and Joëls, 2017). These receptors may be important in human psychopathology in relation to Conn’s disease or other forms of hyperaldosteronism (discussed in detail later) (Gendreitzig et al. , 2021).
Glucocorticoid preferring MR is reported in the pre-frontal cortex, hippocampus, lateral septum thalamic nuclei, and hypothalamic nuclei and, the medial and central amygdala (Reul and de Kloet, 1985; Chao, Choo and McEwen, 1989) (figure 1). Hippocampal MR contributes to indirect negative feedback regulation of the HPA axis, and affects processes in the control of emotion, cognition, and behaviour (Vogelet al ., 2016). Hippocampal MR expression is high throughout all principle glutamatergic cell layers (Reul and de Kloet, 1985) with its highest levels in the Cornu Ammonis 2 (CA2), from embryonic through to adulthood and this may be directly linked to differentiation (McCannet al. , 2021). MR-directed cell type–specific molecular signatures, involved in cellular processes and disease states in the brain, requires further interrogation using techniques such as single cell RNA sequencing (scRNA-seq). In fact, scRNA-seq under basal conditions has identified higher expression of MR than GR also in GABAergic neurons in the hippocampus (Viho et al, JNE in revision). Models assessing fear extinction and behavioral responses to stress identified cortisol preferring MR projections from infralimbic origin to innervate the locus coeruleus (LC) and NTS, and also intercalate amygdala neurones that exert GABA-ergic control over the central amygdala (Milad and Quirk, 2012; McKlveen, Myers and Herman, 2015). What is more, the prelimbic- and infralimbic PFC were identified as important for fear expression and extinction (Milad and Quirk, 2012).