HPA axis responsiveness to stress: implications for healthy aging

The wound-healing response and upregulated embryonic mechanisms: brothers-in-arms forever

The cutaneous wound-healing reaction occurs in overlapping but inter-related phases, which ultimately result in fibrosis. The pathophysiological mechanisms involved in fibrotic diseases, including organ-related and even systemic diseases, such as systemic sclerosis, could represent the successive systemic upregulation of extraembryonic-like phenotypes, that is, amniotic and vitelline phenotypes. These two extraembryonic-like phenotypes act on the injured tissue to induce a process similar to gastrulation, which occurs during the early phases of embryo development. The amniotic-like phenotype plays a leading role in the development of neurogenic responses with significant hydroelectrolytic alterations that essentially represent the development of open microcirculation within the injured tissue. In turn, through the overlapping expression of a vitelline-like phenotype, a bone marrow-related response is produced. Interstitial infiltration by molecular and cellular mediators contributed by amniotic- and vitelline-like functions provides the functional and metabolic autonomy needed for inducing new tissue formation through mechanisms similar to those that act in gastrulation during the early phases of embryonic development. Thus, while a new tissue is formed, it quickly evolves into fibrotic tissue because of premature senescence. Mechanisms related to extraembryonic-like functions have been suggested in the following physiological and pathological processes: embryonic development; wound-healing reactions occurring during adult life; and senescence. The existence of this sort of basic self-organizing fractal-like functional pattern is an essential characteristic of our way of life.

The neuroendocrine control of the innate immune system in health and brain diseases

The innate immune reaction takes place in the brain during immunogenic challenges, injury, and disease. Such a response is highly regulated by numerous anti-inflammatory mechanisms that may directly affect the ultimate consequences of such a reaction within the cerebral environment. The neuroendocrine control of this innate immune system by glucocorticoids is critical for the delicate balance between cell survival and damage in the presence of inflammatory mediators. Glucocorticoids play key roles in regulating the expression of inflammatory genes, and they also have the ability to modulate numerous functions that may ultimately lead to brain damage or repair after injury. Here we review these mechanisms and discuss data supporting both neuroprotective and detrimental roles of the neuroendocrine control of innate immunity.

Corticotropin-releasing hormone stimulates mitotic kinesin-like protein 1 expression via a PLC/PKC-dependent signaling pathway in hippocampal neurons

Corticotropin-releasing hormone (CRH) has been shown to modulate dendritic development in hippocampus. Mitotic kinesin-like protein 1 (MKLP1) plays key roles in dendritic differentiation. In the present study, we examined the effects of CRH on MKLP1 expression in cultured hippocampal neurons and determine subsequent signaling pathways involved. CRH dose-dependently increased MKLP1 mRNA and protein expression. This effect can be reversed by CRHR1 antagonist but not by CRHR2 antagonist. CRHR1 knockdown impaired this effect of CRH. CRH stimulated GTP-bound Gαs protein and phosphorylated phospholipase C (PLC)-β3 expression, which were blocked by CRHR1 antagonist. Transfection of GP antagonist-2A, an inhibitory peptide of Gαq protein, blocked CRH-induced phosphorylated PLC-β3 expression. PLC and PKC inhibitors completely blocked whereas adenylyl cyclase (AC) and PKA inhibitors did not affect CRH-induced MKLP1 expression. Our results indicate that CRH act on CRHR1 to induce MKLP1 expression via PLC/PKC signaling pathway. CRH may regulate MKLP1 expression, thereby modulating dendritic development.

Impact of corticosterone treatment on spontaneous seizure frequency and epileptiform activity in mice with chronic epilepsy

Stress is the most commonly reported precipitating factor for seizures in patients with epilepsy. Despite compelling anecdotal evidence for stress-induced seizures, animal models of the phenomena are sparse and possible mechanisms are unclear. Here, we tested the hypothesis that increased levels of the stress-associated hormone corticosterone (CORT) would increase epileptiform activity and spontaneous seizure frequency in mice rendered epileptic following pilocarpine-induced status epilepticus. We monitored video-EEG activity in pilocarpine-treated mice 24/7 for a period of four or more weeks, during which animals were serially treated with CORT or vehicle. CORT increased the frequency and duration of epileptiform events within the first 24 hours of treatment, and this effect persisted for up to two weeks following termination of CORT injections. Interestingly, vehicle injection produced a transient spike in CORT levels – presumably due to the stress of injection – and a modest but significant increase in epileptiform activity. Neither CORT nor vehicle treatment significantly altered seizure frequency; although a small subset of animals did appear responsive. Taken together, our findings indicate that treatment of epileptic animals with exogenous CORT designed to mimic chronic stress can induce a persistent increase in interictal epileptiform activity.

PPARγ and stress: implications for aging

Complex interactions link psychological stress and aging - stress generally promotes aging processes, and conversely, aging can contribute to stress dysregulation. Stress and aging have remarkably similar effects on brain. Both induce neuroinflammation and alter neuronal metabolism and activity, which to varying extents are causally-linked to the development of stress and aging pathology. As such, induction of one or more of these brain disturbances by either stress or aging could predispose for the development of dysfunction in the other. Notably, peroxisome proliferator-activated receptor γ (PPARγ) is expressed in brain regions that regulate both stress and aging (e.g., hippocampus) and can act to prevent the consequences of aging and stress on the brain. In addition, PPARγ agonists reduce the physiological stress response itself. Thus, PPARγ may represent a critical mechanistic link between brain aging and stress that could hold therapeutic potential for the prevention and treatment of age-related cognitive and mood disorders.

Pre-gestational stress reduces the ratio of 5-HIAA to 5-HT and the expression of 5-HT1A receptor and serotonin transporter in the brain of foetal rat

Background

Many studies have found that stress before or during pregnancy is linked to an increased incidence of behavioural disorders in offspring. However, few studies have investigated hypothalamic-pituitary-adrenal (HPA) axis activity and the serotonergic system as a consequence of pregestational stress. In the present study, we investigated the effect of pre-gestational stress on HPA axis activity in maternal rats and their foetuses and examined whether changes in HPA axis activity of maternal rats produced functional changes in the serotonergic system in the brain of foetuses.

Results

We used the behavioural tests to assess the model of chronic unpredictable stress (CUS) in maternal rats. We found the activity in the open field and sucrose consumption was lower for rats with CUS than for the controls. Body weight but not brain weight was higher for control foetuses than those from the CUS group. Serum corticosterone and corticotrophin-releasing hormone levels were significantly higher for mothers with CUS before pregnancy and their foetuses than for the controls. Levels of 5-hydroxytryptamine (5-HT) were higher in the hippocampus and hypothalamus of foetuses in the CUS group than in the controls, and 5-hydroxyindoleacetic acid (5-HIAA) levels were lower in the hippocampus in foetuses in the CUS group than in the control group. Levels of 5-HIAA in the hypothalamus did not differ between foetuses in the CUS group and in the control group. The ratio of 5-HIAA to 5-HT was significantly lower for foetuses in the CUS group than in the control group. Levels of 5-HT1A receptor were significantly lower in the foetal hippocampus in the CUS group than in the control group, with no significant difference in the hypothalamus. The levels of serotonin transporter (SERT) were lower in both the foetal hippocampus and foetal hypothalamus in the CUS group than in the control group.

Conclusions

Our data demonstrate that pre-gestational stress alters HPA axis activity in maternal rats and their foetuses, which is associated with functional changes in 5-HT activity (5-HT, 5-HIAA and ratio of 5-HIAA to 5-HT), as well as the levels of the 5-HT1A receptor and SERT in the hippocampus and hypothalamus of foetuses.

In healthy young and elderly adults, hypothalamic-pituitary-adrenocortical axis reactivity (HPA AR) varies with increasing pharmacological challenge and with age, but not with gender

BACKGROUND

Hypothalamic-pituitary-adrenocortical axis reactivity (HPA AR) is the key indicator of the psychophysiological response to stress. The HPA AR may vary with age and gender. To investigate these factors concurrently, the aims of the present study were to observe HPA AR (plasma ACTH and plasma cortisol) in response to a pharmacological challenge (dexamethasone/corticotropin releasing hormone test: DEX/CRH-test) and as a function of age and gender.

METHOD

19 young (10 females and 9 males; mean age = 24.05 years) and 23 elderly (11 females and 12 males; mean age = 71.61 years) healthy volunteers took part in the study. To assess HPA AR, participants underwent the combined DEX/CRH test applied with the following DEX doses: 0.75, 1.5, and 3.0 mg, respectively.

RESULTS

A dose-dependent response was observed in young adult participants, but not in elderly participants. With increasing DEX doses, ACTH and cortisol values decreased in young adult participants, while the decrease was blunted among elderly compared to young adult participants. No differences were observed for gender.

CONCLUSIONS

Results point to diminished HPA axis sensitivity as an effect of normal aging, irrespective of gender. Therefore, altered HPA regulation in old age should be taken into account for developing new therapeutic approaches acting on the HPA axis and its receptor mechanisms.

Acetylation-mediated epigenetic regulation of glucocorticoid receptor activity: circadian rhythm-associated alterations of glucocorticoid actions in target tissues

Glucocorticoids influence organ functions through the glucocorticoid receptor, a protein acetylated and deacetylated by several histone acetyltransferases and deacetylases. We reported that the circadian rhythm-related transcription factor "Clock", a key component of the biological CLOCK with inherent histone acetyltransferase activity, acetylates glucocorticoid receptor lysines within its hinge region--a "lysine cluster" containing a KXKK motif--and represses its transcriptional activity. This Clock-induced repression of the glucocorticoid receptor activity is inversely phased to the diurnally circulating glucocorticoids and may act as a local counter regulatory mechanism to the actions of these hormones. Importantly, uncoupling of the central CLOCK-regulated hypothalamic-pituitary-adrenal axis and peripheral CLOCK-mediated alterations of glucocorticoid action, such as chronic stress and frequent trans-time zone travel or night-shift work, may cause functional hypercortisolism and contribute to various pathologies. Thus, acetylation-mediated epigenetic regulation of the glucocorticoid receptor may be essential for the maintenance of proper time-integrated glucocorticoid action, significantly influencing human well-being and longevity.