In 1968 Bruce McEwen made a landmark discovery (McEwen et al. 1968, 2015). He discovered that 3H-corticosterone given to adrenalectomized animals was not retained as expected in the hypophysiotropic region in the hypothalamus but in the hippocampus. In subsequent studies, de Kloet et al. (1975) discovered that the potent synthetic glucocorticoid dexamethasone was not retained by these receptors and also did not compete for 3H-corticosterone or 3H-aldosterone retention, suggesting the presence of two distinct receptor populations for corticosteroids. The pattern of 3H-aldosterone retention appeared to match localization of HSD-2, particularly in NTS (Geerling and Loewy 2009).
In the mid-1980s, the existence of two types of receptors for corticosteroids in the brain was demonstrated. The receptors were cloned and identified as the miner- alocorticoid receptors (MR) and glucocorticoid receptors (GR) (Evans and Arriza 1989). Around the same time, Reul and de Kloet (1985) demonstrated with binding studies that the rat hippocampus contained two receptor populations that did bind corticosterone with different affinity. The high affinity binding sites were designated type 1 receptors and later MR, the lower affinity sites—type 2 receptors—and later GR. Since in rodents and humans the principal corticosteroid (corticosterone and cortisol, respectively) circulates in a 100-1000-fold excess over aldosterone, these steroids are the predominant MR ligands in vivo. MR is abundantly expressed in limbic brain structures, e.g., hippocampus, lateral septum, and amygdala. GR binds cortisol and corticosterone with a tenfold lower affinity than MR and is widely distributed in neurons and glial cells with highest expression in PVN, limbic structures, and neocortical regions (de Kloet et al. 1998, 2005; de Kloet 1991).
The presence of MR and GR was demonstrated in the inner ear and a very distinct expression pattern of the receptors was shown (Fig. 2.1). Stria vascularis contains predominantly MR, which is in agreement with its function, namely, recycling and regulating K+ (and Na+). The spiral ganglion neurons contain predominantly GR, whereas the rest of the cells in the cochlea (auditory hair cells, supporting cells, fibrocytes, interdigital cells, and spiral limbus cells) contains both—MR and GR (Basappa et al. 2012).
The implication of the difference in affinity of the MR and GR for corticosterone and cortisol is differential occupation of these two receptor types during circadian variation and after stress. This differential activation of MR and GR as a function of circulating steroid concentration provided for over 30 years the experimental basis for research on neuronal networks underlying stress coping, behavioral adaptation, and energy metabolism (Dallman 2010; de Kloet 1991, 2014, 2016; de Kloet and Reul 1987; Lupien et al. 2009; McEwen et al. 2015).
Since MR and GR are transcription factors regulating gene expression, they are expected to interact with the genome upon binding their ligand. Using chromatin immunoprecipitation (ChIP) followed by a deep sequencing (ChIP seq), Nicole Datson and Annelies Polman have made a complete inventory of all genomic binding sites for MR and GR in the hippocampal genome (Polman et al. 2013). They observed that 40% of the GR binding sites are within the genes. The experiment involved adrenalectomized animals injected with increasing doses of corticosterone. Also on the genomic level, two populations of genome binding sites for MR and GR were found. Already at a low dose, MR/corticosterone complex associated with DNA and this binding remained relatively constant up to 3 mg of administered corticosterone. GR did bind only at higher doses of corticosterone to DNA binding sites, thus reflecting the differential binding of MR and GR to corticosterone.
Fig. 2.1 Distribution of MR and GR in the cochlear tissues. OHC outer hair cells, IHC inner hair cells, DC Deiters cells, IP inner pillar cells, OP outer pillar cells, HC Hensen cells, SV stria vascularis, SL spiral ligament, SLi spiral limbus, ISC inner sulcus cells, OSC outer sulcus cells, SGN spiral ganglion neurons, SP spiral prominence, PC pillar cells (From Kil and Kalinec 2013, Reprinted with permission) (Kil and Kalinec 2013)
Binding of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) with differential affinity to endogenous corticosteroids enables distinct responses during circadian cycle and after stress.
Moreover, using a neurophysiological approach Marian Joels and Henk Karst discovered as yet another surprise hidden in corticosteroid receptorology. They demonstrated that pyramidal and dentate gyrus neurons of the hippocampus and neurons of basolateral amygdala harbored an MR variant that responded rapidly to corticosterone, cortisol, and aldosterone (Joels and Baram 2009; Joels and de Kloet 2012; Karst et al. 2005). This membrane MR was deleted in the MR knock animals, and the signal was maintained when the steroids were applied when penetration in the cell was prevented because of coupling of the ligand to bovine serum albumin. Activation of the receptor caused within minutes increased excitatory postsynaptic potentials (EPSP) indicating a rapidly enhanced release of the excitatory transmitter glutamate.
The membrane MR-mediated action depended on an ERK1/2 pathway (Olijslagers et al. 2008). Simultaneous with MR-induced glutamate release, the voltage dependent I(A) K current at the postsynaptic membrane was decreased. Moreover, probably as a result of increased synaptic release of glutamate, the presynaptic mGLU2/3 receptor was downregulated (Nasca et al. 2015). Also GR appeared to entertain a lower affinity GR membrane variant that mediated the release of cannabinoids for transsynaptic inhibitory action on the presynaptic release of glutamate (Di et al. 2003).