Everything Has Rhythm: Focus on Glucocorticoid Pulsatility

Everything has rhythm, and hormones are no exception. Circadian, reproductive, and ultradian rhythms in hormone secretion are well established. Ultradian rhythms in the hormones of the hypothalamic-pituitary-adrenal (HPA) axis were first demonstrated several decades ago (1) and appeared a common phenomenon in mammalian species. Typically the adrenals produce hourly secretory bursts of glucocorticoids (2), which increase in amplitude toward the circadian activity period (3) and likely serve to synchronize daily and sleep-related events (4). Superimposed on this ultradian rhythm is the HPA-driven glucocorticoid response to a stressor. The stress-induced rise in glucocorticoid hormone controls the initial stress reaction, mobilizes energy resources, and facilitates the storage of the experience in the memory for future use (5). The relationship between glucocorticoids and adaptation to stress is usually based on measuring circulating hormone levels in blood. In this issue of Endocrinology Droste et al. (6) convincingly demonstrate that the glucocorticoid stress response and the basal ultradian rhythm persist in the brain’s extracellular compartment. This observation implies that the timing of glucocorticoid exposure to the brain is ensured to exert its important actions on, for example, emotion, motivation, cognition, and the central mechanism underlying energy metabolism. Droste et al. (6) combined in vivo microdialysis with vena jugularis blood sampling from rats freely behaving in their home cage (7, 8). Sampling times of 10 min or less were used for reliable measurement of the frequency, amplitude, and area under the curve of the corticosterone pulses using data analysis according to the PULSAR algorithm (9). They found that the hourly secretory bursts from the adrenals producing the ultradian rhythm in the blood are reflected rather synchronously in the dialysate of hippocampus and other brain regions. After stress, however, the appearance of the brain corticosterone peak showed a 20-min delay in absorption from plasma. The data suggest that under equilibrium conditions, similar concentrations of free steroid would be present in blood and brain. The delay in penetration therefore rather seems to be caused by slow passive diffusion, although a role of blood corticosteroid binding globulin, blood-brain barrier multidrug resistance P glycoprotein (10), local steroid metabolism (11), or translocation to the nucleus (12) cannot be excluded. The elimination phase from the brain parallels the clearance rate of the steroid from the blood. Obviously further analysis is necessary to explain the kinetics of corticosterone uptake, retention, and clearance from the brain. These findings further bridge the gap between secretory bursts of corticosterone and the action of the hormone in the brain. This action exerted by corticosterone is mediated by two types of receptors, i.e. mineralocorticoid (MR) and glucocorticoid receptors (GR), which are both abundantly expressed in neurons of the hippocampus, and other limbic-forebrain neurons. MR has a much higher affinity than GR for corticosterone in rat and cortisol in man (13). This affinity is high enough to retain the naturally occurring glucocorticoid for at least 1 h in the nucleus (14, 15). Hence, the MR is in the nucleus sufficiently long enough to overlap the 1-h interpulse interval. GR is widely expressed in neurons and glial cells with highest abundance in key brain regions regulating adaptation to stress such as the hypothalamic paraventricular nucleus and the hippocampus (16). Using cell fractionation and Western blotting, it was discovered that the translocation of immunoreactive GR to the nucleus followed the rhythmic courses of corticosterone in the blood, albeit with some delay (15) (Fig. 1). Subsequently corticosterone dissociates from GR bound to glucocorticoid response elements in a process that destabilizes the receptor, which is then down-regulated by proteasomal activity (14). Using immunocytochemistry, the differential kinetics in uptake and retention of MR and GR in the hippocampal nuclear compartment was also demonstrated, but interestingly the patterns differed between cell types in various brain regions (Sarabdjitsingh, R. A., and E. R. de Kloet, unpublished observation). Corticosterone bursts are secreted by the adrenal cortex in approximately 60-min intervals because of rapid stimulation and inhibition involving nongenomic MR (and perhaps GR-like) actions aimed to maintain adequate tissue responsivity. In response to a pulse, GR is rapidly translocated to the nucleus and consequently cleared in waves by a proteasomal-dependent mechanism, whereas MR is retained in the nucleus. MR-mediated genomic actions are for maintenance of stability and integrity; GR-mediated genomic actions are thought to coordinate circadian events. Solid line, Hippocampal extracellular corticosterone and activation of membrane MR; dotted line, the pulsatile GR nuclear translocation, dashed line, MR retention in nucleus. Scheme is based on data from rat hippocampus (data from Ref. 15 and Sarabdjitsingh R. A., and E. R. de Kloet, unpublished results). The data on glucocorticoid pulsatility raise a number of questions on the complementary role of MR and GR in control of brain function under basal and stressful conditions. First, one may wonder whether pulsatility, the stress response, and glucocorticoid feedback are linked and if so, how are MR and GR involved? To address this question, it was demonstrated that stress-induced corticosterone secretion was enhanced during the pulsatile rise and attenuated in the descending phase (8). Application of an MR antagonist enhanced the corticosterone pulse, whereas a GR antagonist was not effective under these basal conditions (17, 18). Moreover, humans carrying an MR gene variant with reduced transactivation potential, the MR-I180V variant, also showed an enhanced stress-induced cortisol response and increased autonomic outflow (19). Collectively, these findings demonstrate rapid MR-mediated feedback modulating the corticosterone pulse and the initiation of the stress reaction, probably with the goal to prevent disturbance of basal homeostatic functions. Second, higher brain functions are also under control of rapid MR mediated actions. One example is that a surge in corticosterone can amplify a violent explosion of aggressive behavior (20). This behavior may have a basis in the rapid effects of corticosterone on limbic circuitry underlying appraisal and other cognitive processes (21). Recently rapid nongenomic MR-mediated actions have been identified in hippocampal signaling, which were eliminated by forebrain MR knockout (22). Patch clamp recordings from CA1 pyramidal neurons in hippocampus showed that presynaptically, corticosterone enhances spontaneous glutamate release, whereas postsynaptically the hormone suppresses voltage dependent A-type potassium currents; both effects result in an increased excitability of the hippocampal neurons. Interestingly, none of these effects were affected by antiglucocorticoids. These membrane MR-mediated actions in hippocampus are therefore distinct from the recently identified rapid glucocorticoid-like effects that proceed through an endocannabinoid system (23) and are involved in fast feedback on the level of the hypothalamic paraventricular nucleus (24). Third, the complementary MR- and GR-mediated actions in hippocampus that are differentially activated by the stress-induced corticosterone surge have led to a novel model to study the role of pulsatility and stress in the brain (25) (Fig. 2). In this model, the membrane MR modulates the initial stress reaction, which is then dampened by the slower genomic GR-mediated actions. The latter action via GR facilitates recovery from stress-induced disturbances of homeostasis and promotes storage of the experience in the memory. But what about the nuclear MR that is maintained in the nuclear compartment all along? Current evidence shows that activation of these receptors maintains the interpulse refractory period (3) and the stability (26) and integrity of neural circuitry (27). In response to a stressor, MR and GR are differentially activated by increasing levels of corticosterone in the extracellular brain compartment. In this model based on hippocampal CA1 neurons, the initial stress reaction is modulated by (membrane) MR, whereas recovery from the stress response to restore homeostasis is mediated by the slower GR-mediated genomic actions. Genomic MR is thought to control the sensitivity or threshold for stress responsivity. In other areas of the brain, the MR-GR mediated actions are different (28 ). Most actions of MR and GR measured in hippocampal CA1 neurons are complementary but appear distinctly different, however, in other target areas for corticosterone in the hippocampus and elsewhere in the brain such as amygdala and prefrontal cortex (28). The current findings of Droste et al. (6) provide a sound basis to further explore the pulse generator in the brain. Current evidence suggests a suprapituitary location of the control center underlying ultradian rhythmicity (3), which can be amplified at the adrenal level, possibly via autonomic inputs determining adrenal sensitivity (29). The findings also allow to further explore how pulsatility and hence stress responsiveness is modulated throughout the circadian and reproductive cycle, and during the aging process (30). Pulsatility does change during disease processes such as arthritis (31) and depression (32) in a way that compromises tissue responsivity. GH research has suggested the necessity of these hormone bursts to maintain proper tissue responsiveness and prevent receptor desensitization (33), and there is no reason to exclude a similar function of glucocorticoid pulsatility. Hence, long-term disturbances in pulsatility may severely affect the underlying corticosteroid receptor dynamics to acute challenges. Appropriate pulse frequency and amplitude may prevent dysregulations in the HPA axis by attenuating excessive or prolonged glucocorticoid actions known to impair neuroplasticity underlying cognition and emotions. Frequency encoding, therefore, may be a significant factor to boost resilience still present in the diseased brain (13). Understanding the stimulation and inhibition underlying the pulsatile secretion of corticosterone in the light of the binary MR- and GR-mediated regulations is of crucial importance for this purpose. This work was supported by the Royal Netherlands Academy of Arts and Sciences, European Union Lifespan (www.lifespannetwork.eu), Human Frontier Science Program, Top Institute Pharma No. T5-209, and Netherlands Science Organisation (NWO) Mozaïek 017.002.021. Glucocorticoid receptor hypothalamic-pituitary-adrenal mineralocorticoid receptor.