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).