Neonatal monosodium glutamate treatment alters both the activity and the sensitivity of the rat hypothalamo-pituitary-adrenocortical axis

Abstract We have investigated the effects of monosodium glutamate (MSG) lesioning of the arcuate nucleus on both central and peripheral components of the hypothalamo-pituitary-adrenocortical (HPA) axis under basal conditions and under acute and chronic stress. Plasma ACTH levels were lower in MSG-lesioned rats (27 ± 7 pg/ml) compared with controls (71 ± 18 pg/ml) while corticosterone levels were elevated (523 ± 84 ng/ml compared with 176 ± 34 ng/ml). Quantititative in situ hybridization histochemistry revealed that corticotrophin-releasing factor mRNA levels in the medial parvocellular part of the hypothalamic paraventricular nucleus were significantly lower in MSG-treated rats. MSG lesioning resulted in an enhanced response of corticosterone to restraint stress (1309 ± 92 ng/ml compared with 628 ± 125 ng/ml in sham-lesioned animals), while ACTH responses to restraint stress in MSG-lesioned and sham-MSG groups were not significantly different (160 ± 24 pg/ml and 167 ± 24 pg/ml respectively). These data suggest that MSG-lesioned rats have an increased adrenocortical sensitivity. In rats subjected to the chronic osmotic stimulus of drinking 2% saline for 12 days, plasma ACTH levels were significantly reduced (15 ± 5 pg/ml) and the ACTH and corticosterone responses to restraint stress were eliminated. ACTH levels were also reduced in MSG-treated animals given 2% saline and the ACTH response to acute stress remained absent in these animals. However, a robust corticosterone response to restraint stress was observed in saline-treated MSG-lesioned rats. These data demonstrate that MSG lesioning results in elevated basal and stress-induced plasma corticosterone, and restores the adrenocortical response to stress which is absent in chronically osmotically stimulated rats. The evidence is consistent with the suggestion that MSG lesions a pathway involved in tonic inhibition of the HPA axis. In addition, the adrenocortical sensitivity to ACTH and other secretagogues may be increased in MSG-treated animals. Journal of Endocrinology (1994) 141, 497–503

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Vitamin A regulates hypothalamic–pituitary–adrenal axis status in LOU/C rats

Journal of Endocrinology ◽

10.1530/joe-13-0062 ◽

2013 ◽

Vol 219 (1) ◽

pp. 21-27 ◽

Cited By ~ 13

Author(s):

Nathalie Marissal-Arvy ◽

Rachel Hamiani ◽

Emmanuel Richard ◽

Marie-Pierre Moisan ◽

Véronique Pallet

Keyword(s):

Vitamin A ◽

Hpa Axis ◽

Restraint Stress ◽

Vitamin A Deficiency ◽

Plasma Corticosterone ◽

Hypothalamic Pituitary Adrenal ◽

Corticosteroid Receptors ◽

Vitamin A Status ◽

Corticosterone Response ◽

Hpa Axis Activity

The aim of this study was to explore the involvement of retinoids in the hypoactivity and hyporeactivity to stress of the hypothalamic–pituitary–adrenal (HPA) axis in LOU/C rats. We measured the effects of vitamin A deficiency administered or not with retinoic acid (RA) on plasma corticosterone in standard conditions and in response to restraint stress and on hypothalamic and hippocampal expression of corticosteroid receptors, corticotropin-releasing hormone and 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) in LOU/C rats. Interestingly, under control conditions, we measured a higher plasma concentration of retinol in LOU/C than in Wistar rats, which could contribute to the lower basal activity of the HPA axis in LOU/C rats. Vitamin A deficiency induced an increased HPA axis activity in LOU/C rats, normalized by RA administration. Compared with LOU/C control rats, vitamin A-deficient rats showed a delayed and heightened corticosterone response to restraint stress. The expression of corticosteroid receptors was strongly decreased by vitamin A deficiency in the hippocampus, which could contribute to a less efficient feedback by corticosterone on HPA axis tone. The expression of 11β-HSD1 was increased by vitamin A deficiency in the hypothalamus (+62.5%) as in the hippocampus (+104.7%), which could lead to a higher production of corticosterone locally and contribute to alteration of the hippocampus. RA supplementation treatment restored corticosterone concentrations and 11β-HSD1 expression to control levels. The high vitamin A status of LOU/C rats could contribute to their low HPA axis activity/reactivity and to a protective effect against 11β-HSD1-mediated deleterious action on cognitive performances during ageing.

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Endomorphins and activation of the hypothalamo-pituitary-adrenal axis

Journal of Endocrinology ◽

10.1677/joe.0.1690185 ◽

2001 ◽

Vol 169 (1) ◽

pp. 185-193 ◽

Cited By ~ 47

Author(s):

TL Coventry ◽

DS Jessop ◽

DP Finn ◽

MD Crabb ◽

H Kinoshita ◽

...

Keyword(s):

Opioid Receptor ◽

Hpa Axis ◽

Restraint Stress ◽

Acute Stress ◽

Plasma Corticosterone ◽

Mu Opioid Receptor ◽

Inflammatory Stress ◽

Mu Opioid ◽

Beta Endorphin ◽

Immune Tissues

Endomorphin (EM)-1 and EM-2 are opioid tetrapeptides recently located in the central nervous system and immune tissues with high selectivity and affinity for the mu-opioid receptor. Intracerebroventricular (i.c.v.) administration of morphine stimulates the hypothalamo-pituitary-adrenal (HPA) axis. The present study investigated the effect of centrally administered EM-1 and EM-2 on HPA axis activation. Rats received a single i.c.v. injection of either EM-1 (0.1, 1.0, 10 microg), EM-2 (10 microg), morphine (10 microg), or vehicle (0.9% saline). Blood samples for plasma corticosterone determinations were taken immediately prior to i.c.v. administration and at various time points up to 4 h post-injection. Trunk blood, brains and pituitaries were collected at 4 h. Intracerebroventricular morphine increased plasma corticosterone levels within 30 min, whereas EM-1 and EM-2 were without effect. In addition, pre-treatment of i.c.v. EM-1 did not block the rise in corticosterone after morphine. Corticotrophin-releasing factor (CRF) mRNA and arginine vasopressin (AVP) mRNA in the paraventricular nucleus (PVN) and POMC mRNA in the anterior pituitary were found to be unaffected by either morphine or endomorphins. Since release of other opioids are elevated in response to acute stress, we exposed rats to a range of stressors to determine whether plasma EM-1 and EM-2 can be stimulated by HPA axis activation. Plasma corticosterone, ACTH and beta-endorphin were elevated following acute restraint stress, but concentrations of plasma EM-1-immunoreactivity (ir) and EM-2-ir did not change significantly. Corticosterone, ACTH and beta-endorphin were further elevated in adjuvant-induced arthritis (AA) rats by a single injection of lipopolysaccharide (LPS), but not by restraint stress. In conclusion, neither EM-1 or EM-2 appear to influence the regulation of the HPA axis. These data suggest that endomorphins may be acting on a different subset of the mu-opioid receptor than morphine. The failure to induce changes in plasma EM-ir in response to the chronic inflammatory stress of AA, the acute immunological stress of LPS, or the psychological stress of restraint, argues against an important role for endomorphins in mediating HPA axis activity.

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Intrahypothalamic Estradiol Modulates Hypothalamus-Pituitary-Adrenal-Axis Activity in Female Rats

Endocrinology ◽

10.1210/en.2011-2176 ◽

2012 ◽

Vol 153 (7) ◽

pp. 3337-3344 ◽

Cited By ~ 27

Author(s):

J. Liu ◽

P. H. Bisschop ◽

L. Eggels ◽

E. Foppen ◽

E. Fliers ◽

...

Keyword(s):

Hpa Axis ◽

Restraint Stress ◽

Acute Stress ◽

Plasma Corticosterone ◽

Female Rats ◽

Ovariectomized Rats ◽

Stress Exposure ◽

Ici 182,780 ◽

Hpa Axis Activity

Estrogen plays an important role in the regulation of the hypothalamus-pituitary-adrenal (HPA)-axis, but the neuroendocrine pathways and the role of estrogen receptor (ER) subtypes involved in specific aspects of this interaction remain unknown. In a first set of experiments, we administered estradiol (E2) intravenously, intracerebroventricularly, and by intrahypothalamic microdialysis to ovariectomized rats to measure plasma corticosterone (CORT) concentrations from carotid artery blood. Systemic infusion of E2 did not increase plasma CORT, but intracerebroventricular E2 induced a 3-fold CORT increase (P = 0.012). Local E2 infusions in the hypothalamic paraventricular nucleus (PVN) significantly increased plasma CORT (P < 0.001). A similar CORT increase was seen after PVN infusion of the ERα agonist propylpyrazoletriol, whereas the ERβ agonist diarylpropiolnitrile had no effect. In a second set of experiments, we investigated whether E2 modulates the HPA-axis response to acute stress by administering E2 agonists or its antagonist ICI 182,780 into the PVN during restraint stress exposure. After 30 min of stress exposure, plasma CORT had increased 5.0-fold (P < 0.001). E2 and propylpyrazoletriol administration in the PVN enhanced the stress-induced plasma CORT increase (8-fold vs. baseline), whereas ICI 182,780 and diarylpropiolnitrile reduced it, as compared with both E2 and vehicle administration in the PVN. In conclusion, central E2 modulates HPA-axis activity both in the basal state and during restraint stress. In the basal condition, the stimulation is mediated by ERα-sensitive neurons, whereas during stress, it is mediated by both ERα and ERβ.

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Serotonin depletion does not alter lipopolysaccharide-induced activation of the rat paraventricular nucleus

Journal of Endocrinology ◽

10.1677/joe.0.1560245 ◽

1998 ◽

Vol 156 (2) ◽

pp. 245-251 ◽

Cited By ~ 15

Author(s):

GL Conde ◽

D Renshaw ◽

SL Lightman ◽

MS Harbuz

Keyword(s):

Hpa Axis ◽

Paraventricular Nucleus ◽

Plasma Corticosterone ◽

Mrna Levels ◽

Dependent Manner ◽

Experimental Conditions ◽

Serotonin Depletion ◽

Corticotrophin Releasing Factor ◽

Subsequent Activation ◽

High Doses

We have investigated the effects of serotonin depletion on immune-mediated activation of the hypothalamo-pituitary-adrenal (HPA) axis. Corticotrophin-releasing factor (CRF) mRNA, c-fos mRNA and Fos peptide responses in the paraventricular nucleus (PVN) together with circulating levels of corticosterone were assessed in response to i.p. injections of three doses of lipopolysaccharide (LPS) both in control animals and animals pretreated with p-chlorophenylalanine (PCPA). Conscious animals received either an i.p. injection of 0.5 ml saline or 200 mg/kg PCPA in 0.5 ml saline on 2 consecutive days. This treatment resulted in a 93% depletion of serotonin on the fourth day. On day 4, animals received i.p. injections of LPS (2.5 mg/0.5 ml saline, 250 micrograms/0.5 ml or 50 micrograms/0.5 ml; E. coli 055:B5), or saline injections as controls. Pretreatment with PCPA had no effect on the basal levels of corticosterone, or on the elevated levels induced by the three doses, of LPS. Fos peptide and c-fos mRNA were undetectable in control animals, and Fos-like immunoreactivity increased in a dose-dependent manner following i.p. LPS in both control and PCPA-pretreated animals. C-fos mRNA expression induced by LPS was unaffected by serotonin depletion. Following the lowest dose of LPS, CRF mRNA did not change above control levels, however, the medium and high doses of LPS produced a significant (P < 0.05) increase in CRF mRNA levels in both depleted and intact animals. To confirm the temporal effects of serotonin depletion on activation of the HPA axis we collected plasma at 30 min, 1, 2, 3, 4, 5, and 6 h after LPS in both intact and serotonin-depleted animals. No significant differences in plasma corticosterone levels were found at any of the time points between intact and depleted animals. It appears that, at least under these experimental conditions, serotonergic inputs do not seem to play a major role in mediating the effects of LPS on changes in mRNA levels in the PVN or on the subsequent activation of the HPA axis.

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Acute stress- or lipopolysaccharide-induced corticosterone secretion in female rats is independent of the oestrous cycle

Acta Endocrinologica ◽

10.1530/eje.0.1310535 ◽

1994 ◽

Vol 131 (5) ◽

pp. 535-539 ◽

Cited By ~ 18

Author(s):

Ai-Li Guo ◽

Felice Petraglia ◽

Mario Criscuolo ◽

Guido Ficarra ◽

Rossella E Nappi ◽

...

Keyword(s):

Acute Stress ◽

Plasma Corticosterone ◽

Oestrous Cycle ◽

Female Rats ◽

Diurnal Changes ◽

Experimental Conditions ◽

Corticotrophin Releasing Factor ◽

Swimming Stress ◽

Corticosterone Secretion ◽

Corticosterone Response

Guo A-L, Petraglia F, Criscuolo M, Ficarra G, Nappi RE, Palumbo M, Valentini A, Genazzani AR. Acute stress- or lipopolysaccharide-induced corticosterone secretion in female rats is independent of the oestrous cycle. Eur J Endocrinol 1994;131:535–9. ISSN 0804–4643 The aim of the present study was to test whether oestrous cycle is associated with the hypothalamus–pituitary–adrenal (HPA) axis function. Thus, corticosterone secretion in rats was investigated following lipopolysaccharide (LPS), acute cold-swimming or ether stress or synthetic corticotrophin-releasing factor (CRF) administration throughout the oestrous cycle. Moreover, plasma corticosterone response to cold-swimming stress or LPS administration also was studied at different times of day on pro-oestrus of di-oestrus-I. The following observations were obtained: the morning plasma corticosterone levels in control rats did not differ with the stage of the oestrous cycle; plasma corticosterone levels increased significantly following LPS administration (2 mg/kg, ip) or following acute exposure to cold (4°C)-swimming or ether stress. However, this increase in plasma corticosterone levels was not related to the stage of the oestrous cycle; synthetic CRF injection induced an increase in plasma corticosterone levels constant on di-oestrus-I and pro-oestrus; plasma corticosterone response to LPS administration or acute cold-swimming stress showed diurnal changes, with the lowest values at 18.00 h. which was independent of the oestrous cycle. By showing the unchanged corticosterone response to LPS, to acute stress and to exogenous CRF throughout the oestrous cycle, and the independence of the diurnal pattern of stress response on the oestrous cycle, the present study suggests that the oestrous cycle has no influence on the HPA activity under the present experimental conditions in rats. Andrea R Genazzani, Department of Obstetrics & Gynecology, University of Modena, Via del Pozzo 71, 41100 Modena, Italy

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The vagus nerve is critical for regulation of hypothalamic-pituitary-adrenal axis responses to acute stress

10.1101/2021.06.03.446790 ◽

2021 ◽

Author(s):

Bailey N Keller ◽

Angela E Snyder ◽

Caitlin R Coker ◽

Elizabeth A Aguilar ◽

Mary K O'Brien ◽

...

Keyword(s):

Vagus Nerve ◽

Hpa Axis ◽

Restraint Stress ◽

Acute Stress ◽

Plasma Corticosterone ◽

Action Potential Firing ◽

Acute Restraint Stress ◽

Potential Firing ◽

Crf Neurons ◽

Hpa Function

The hypothalamic pituitary adrenal (HPA) axis is a critical regulator of physiologic and psychological responses to acute and chronic stressors. HPA axis function is control by numerous feedback inhibitory mechanisms, disruptions of which can lead to various psychiatric conditions, such as depression, posttraumatic stress disorder, and schizophrenia. Vagus nerve stimulation has been shown to be efficacious in these various mental health issues potentially via modulation of HPA axis function, but the mechanisms by which the vagus nerve may regulate HPA function has not been fully elucidated. In the present studies, we sought to test the hypothesis that the vagus nerve is a critical regulator of HPA function. Neuroendocrine function and neurocircuit changes in corticotropin releasing factor (CRF) neurons in the paraventricular nucleus of the hypothalamus (PVN) was examined following acute stress after subdiaphragmatic left vagotomy (VX) in adult male Sprague-Dawley rats. We found that VX mimics HPA activation seen in sham surgery animals exposed to acute restraint stress, particularly increased plasma corticosterone levels, elevated PVN CRF mRNA, and increased action potential firing of putative CRF neurons in PVN brain slices. Furthermore, VX animals exposed to acute restraint stress showed increased elevations of plasma corticosterone and PVN CRF mRNA which may be due to lack of compensatory PVN GABAergic signaling in response to acute stress. Both Sham/Stress and VX/no stress conditions increases action potential firing in putative PVN CRF neurons, but this effect was not seen in the VX/stress condition, suggesting that not all forms of stress compensation are lost following VX. Overall, these findings suggest that the vagus nerve may play a critical role in regulating HPA axis function via modulation of local PVN neurocircuit activity.

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Pituitary-adrenal response to bacterial endotoxin in developing rats

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1988.255.4.e525 ◽

1988 ◽

Vol 255 (4) ◽

pp. E525-E530 ◽

Cited By ~ 8

Author(s):

L. Witek-Janusek

Keyword(s):

Plasma Corticosterone ◽

Neonatal Rat ◽

Bacterial Endotoxin ◽

Plasma Acth ◽

Adult Rats ◽

Age Related ◽

Corticosterone Response ◽

Adrenal Response ◽

Old Rats ◽

Related Pattern

The neonatal rat is very sensitive to the lethal effects of bacterial endotoxin. Because of the adaptive importance of pituitary-adrenal secretions to stress, this study examined the ontogeny of the plasma corticosterone and adrenocorticotropic hormone (ACTH) responses to endotoxin. The lethal sensitivity of young rats to endotoxin ranged from 0.5 to 30 mg/kg (ip) in the 1- to 21-day-old rat. After endotoxin treatment, the 1- and 2-day-old rat showed marked elevations of corticosterone similar in magnitude to that seen in 21-day-old and adult rats; however, significantly depressed corticosterone increments were observed in the 5-, 10-, and 14-day-old rats. This age-related pattern of adrenocortical secretion was correlated with the developing rat's corticosterone response to exogenous ACTH. In contrast, endotoxin administered to 5-, 10-, and 14-day-old rats resulted in increments of plasma ACTH similar to those observed in the 21-day-old and adult rats. Although plasma ACTH levels increased by 84-127% in the 1- and 2-day-old rats, these increases were significantly less than those of rats at all other ages tested. Thus the newborn rat mounts an effective corticosterone response to endotoxin, loses this ability between ages 5-14 days, and regains this response at 21 days of age. Because the hyporesponsive ages exhibit a marked increase in ACTH secretion, the loss of the adrenocortical response to endotoxin appears to be a result of a depressed responsiveness of the adrenal cortex to ACTH.

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Pituitary-adrenal function after transplantation in rats: dependence on age of the adrenal graft

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1986.250.1.e87 ◽

1986 ◽

Vol 250 (1) ◽

pp. E87-E93

Author(s):

W. C. Engeland

Keyword(s):

Plasma Corticosterone ◽

Adrenal Function ◽

Adrenocortical Function ◽

Elevated Plasma ◽

Plasma Acth ◽

Adult Rats ◽

Adrenal Cells ◽

Adrenal Tissue ◽

Corticosterone Response

Comparisons of resting plasma adrenocorticotropin (ACTH) and corticosterone in the morning and afternoon were made among adult rats bearing regenerated adult adrenal grafts, neonatal (day 1) adrenal grafts, adult adrenal capsule grafts, or intact adrenals. In the morning plasma ACTH and corticosterone were similar in all rats. In the afternoon, plasma ACTH was elevated in rats bearing neonatal adrenal grafts or adult adrenal capsule grafts, but not in rats bearing whole adult adrenal grafts. There was no difference in afternoon plasma corticosterone among rats bearing transplanted adrenals, although afternoon plasma corticosterone was decreased in rats bearing transplants compared with rats with intact adrenals. Thus the increased plasma ACTH after adrenal transplantation cannot be explained entirely by decreases in resting plasma corticosterone. Adrenal responsiveness to ACTH was tested at 5 wk after transplantation in the afternoon by measuring the plasma corticosterone response to submaximal doses of ACTH. The responsiveness was decreased in rats bearing transplants. In addition, responsiveness was inversely related to the age of the grafted adrenal tissue. Adrenals regenerated from adult adrenals were more responsive than adrenals regenerated either from neonatal adrenals or from adult adrenal capsules. The findings suggest that following adrenal transplantation reestablishment of normal pituitary-adrenocortical function does not occur in rats bearing adrenals regenerated from immature adrenal cells. In addition, comparable alterations occur after regeneration of adrenal tissue from neonatal adrenal cells and adult adrenal capsular cells. Elevated plasma ACTH associated with adequate plasma corticosterone in rats bearing adrenals regenerated from immature adrenal cells may result from chronic alteration in responsiveness to steroid feedback.(ABSTRACT TRUNCATED AT 250 WORDS)

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Mechanism of restoration of ACTH release in rats with long-term lesions of the paraventricular nuclei

Journal of Endocrinology ◽

10.1677/joe.0.1110075 ◽

1986 ◽

Vol 111 (1) ◽

pp. 75-82 ◽

Cited By ~ 20

Author(s):

J. Dohanics ◽

G. Kapócs ◽

T. Janáky ◽

J. Z. Kiss ◽

G. Rappay ◽

...

Keyword(s):

Median Eminence ◽

Plasma Concentrations ◽

Plasma Acth ◽

Corticotrophin Releasing Factor ◽

Paraventricular Nuclei ◽

Basal Plasma ◽

Corticosterone Response ◽

Intact Rats ◽

Acth Release

ABSTRACT The effects of lesions in the paraventricular nucleus (PVN) on the adrenocortical response to ether stress were investigated in neurohypophysectomized and intact rats. During the first 4 days after placement of lesions in the PVN, the corticosterone response to ether stress was almost completely inhibited. It then gradually increased and, within 4–6 weeks of surgery, was restored to about 60% of that in sham-operated rats. Basal plasma concentrations of corticosterone were low in rats after placement of lesions in the PVN and/or after neurointermediate lobectomy (NILX). Corticosterone responses to ether stress were similar in groups submitted to PVN lesions and/or NILX, and lower than those in the appropriate sham-operated groups. In all lesioned groups, plasma ACTH concentrations after a combination of stressors (ether plus laparotomy) were also lower than those in the sham-operated groups. Six weeks after lesioning of the PVN, immunoreactive rat corticotrophin-releasing factor-41 (rCRF-41) concentrations in stalk-median eminence (SME) extract fell to about 5% of that in sham-operated rats, while immunoreactive arginine vasopressin (AVP) concentrations did not change. Immunohistochemistry revealed a substantial decrease in rCRF-41 immunostaining of the median eminence 6 weeks after lesioning of the PVN, though randomly located clusters of stained terminals were still seen in the whole rostro-caudal extent of the median eminence. A mixture containing synthetic rCRF-41 and AVP, in proportions similar to those in SME extracts from sham-operated rats, caused significantly less release of ACTH from anterior pituitary cell cultures than did SME extracts from sham-operated rats. Extracts of SME from PVN-lesioned rats released as much ACTH as a mixture containing synthetic rCRF-41 and AVP in proportions similar to those in the SME extracts from PVN-lesioned rats. Extracts of SME from either PVN-lesioned or sham-operated rats did not cause a significant increase in the amount of ACTH released when preincubated with antisera to both rCRF-41 and AVP. It is suggested that (1) the restoration of the adrenocortical reponse to ether stress, evident within a few days of placement of lesions in the PVN, occurs independently of neurohypophysial function; (2) the full corticosterone and ACTH response to ether or ether plus laparotomy stress requires not only an intact PVN but also an intact neurointermediate lobe; (3) SME extracts from sham-operated rats contain a factor(s) with the ability to potentiate the ACTHreleasing effect of rCRF-41 and AVP; and (4) the ACTH-releasing activity of SME extract obtained from rats with long-term PVN lesions is probably due to its remaininJ content of rCRF-41 and AVP. J. Endocr. (1986) 111, 75–82

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Facilitation of the HPA Axis to a Novel Acute Stress Following Chronic Stress Exposure Modulates Histone Acetylation and the ERK/MAPK Pathway in the Dentate Gyrus of Male Rats

Endocrinology ◽

10.1210/en.2013-1918 ◽

2014 ◽

Vol 155 (8) ◽

pp. 2942-2952 ◽

Cited By ~ 21

Author(s):

Chantelle L. Ferland ◽

Erin P. Harris ◽

Mai Lam ◽

Laura A. Schrader

Keyword(s):

Dentate Gyrus ◽

Histone Acetylation ◽

Chronic Stress ◽

Hpa Axis ◽

Acute Stress ◽

Male Rats ◽

Stress Exposure ◽

Erk Activation ◽

Acute And Chronic Stress ◽

Acute Stressor

Evidence suggests that when presented with novel acute stress, animals previously exposed to chronic homotypic or heterotypic stressors exhibit normal or enhanced hypothalamic-pituitary-adrenal (HPA) response compared with animals exposed solely to that acute stressor. The molecular mechanisms involved in this effect remain unknown. The extracellular signal-regulated kinase (ERK) is one of the key pathways regulated in the hippocampus in both acute and chronic stress. The aim of this study was to examine the interaction of prior chronic stress, using the chronic variable stress model (CVS), with exposure to a novel acute stressor (2,5-dihydro-2,4,5-trimethyl thiazoline; TMT) on ERK activation, expression of the downstream protein BCL-2, and the glucocorticoid receptor co-chaperone BAG-1 in control and chronically stressed male rats. TMT exposure after chronic stress resulted in a significant interaction of chronic and acute stress in all 3 hippocampus subregions on ERK activation and BCL-2 expression. Significantly, acute stress increased ERK activation, BCL-2 and BAG-1 protein expression in the dentate gyrus (DG) of CVS-treated rats compared with control, CVS-treated alone, and TMT-only animals. Furthermore, CVS significantly increased ERK activation in medial prefrontal cortex, but acute stress had no significant effect. Inhibition of corticosterone synthesis with metyrapone had no significant effect on ERK activation in the hippocampus; therefore, glucocorticoids alone do not mediate the molecular effects. Finally, because post-translational modifications of histones are believed to play an important role in the stress response, we examined changes in histone acetylation. We found that, in general, chronic stress decreased K12H4 acetylation, whereas acute stress increased acetylation. These results indicate a molecular mechanism by which chronic stress-induced HPA axis plasticity can lead to neurochemical alterations in the hippocampus that influence reactivity to subsequent stress exposure. This may represent an important site of dysfunction that contributes to stress-induced pathology such as depression, anxiety disorders, and posttraumatic stress disorder.

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