Corticosterone delivery to the amygdala increases corticotropin-releasing factor mRNA in the central amygdaloid nucleus and anxiety-like behavior

The present study examined the effects of stereotaxic delivery of corticosterone to the amygdala on anxiety-like behavior and corticotropin-releasing factor (CRF) mRNA level in the central nucleus of the amygdala (CeA). Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in Wistar rats. Seven days post-implantation, anxiety-like behavior was accessed using an elevated plus-maze. CRF mRNA level in the CeA was determined by in situ hybridization 4 h after being tested on the elevated plus-maze. Corticosterone implants increased indices of anxiety on the elevated plus-maze and produced a concomitant increase in both basal level of CRF mRNA per neuron and the number of neurons with CRF hybridization signal in the CeA. The plus-maze increased CRF mRNA levels in the CeA of cholesterol implanted rats to the elevated basal levels observed in corticosterone treated animals. Exposure to the plus-maze did not increase CRF mRNA level in the CeA of corticosterone implanted rats beyond elevated basal levels. Taken together, these findings support the involvement of the amygdala in anxiety-like behaviors in response to chronically elevated corticosterone and suggests that elevated glucocorticoids may increase anxiety by inducing CRF expression in the CeA.     

Gender differences in corticotropin and corticosterone secretion and corticotropin-releasing factor mRNA expression in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala in response to footshock stress or psychological stress in rats

Anorexia nervosa is mostly seen in adolescent females, although the gender-differentiation mechanism is unclear. Corticotropin-releasing factor (CRF), a key peptide for stress responses such as inhibition of food intake, increases in arousal and locomotor activity, and gonadal dysfunction, is thought to be involved in the pathophysiology of anorexia nervosa. CRF in the paraventricular nucleus of the hypothalamus (PVN) and CRF in the central nucleus of the amygdala (CeA) are involved in the regulation of stress responses, and gender differences in CRF mRNA expression in these regions in response to various stressors are controversial. We therefore examined CRF gene expression in the PVN and CeA as well as corticotropin (ACTH) and corticosterone secretion in response to a 60-min period of electric footshock (FS) or psychological stress (PS) induced by a communication box in both male and female rats in proestrus or diestrus in an effort to elucidate the mechanism underlying the gender difference in the activity of the hypothalamic-pituitary-adrenal (HPA) axis and the mechanism underlying the remarkable prevalence of anorexia nervosa in females. Female rats in proestrus showed higher basal plasma ACTH and CRF mRNA expression levels in the PVN and CeA than males. Females more rapidly showed higher plasma ACTH and corticosterone levels and a higher CRF mRNA expression level in the PVN in response to FS than males. Although females in both proestrus and diestrus showed significant increases in plasma ACTH and corticosterone and CRF mRNA expression in the PVN in response to PS, no significant responses of the HPA axis to PS were found in males. FS significantly increased CRF mRNA expression in the CeA in both females and males, with significantly higher peaks in females in proestrus than in males, while PS significantly increased CRF mRNA expression in the CeA only in males. These results suggest that gender affects differentially the function of the stress-related regions such as the PVN and CeA. The finding that CRF gene expression in the PVN responds to PS only in females may be a clue to elucidation of the neurobiological mechanism underlying the gender-differential prevalence of anorexia nervosa.     

Corticosterone differentially modulates expression of corticotropin releasing factor and arginine vasopressin mRNA in the hypothalamic paraventricular nucleus following either acute or repeated restraint stress

Exposing rats to repeated restraint stress induces well-characterized adaptations in the expression of either corticotropin-releasing factor (CRF) or arginine-vasopressin (AVP) mRNA in the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN). The effects of regulating corticosterone levels on this adaptation was studied in male rats. In intact rats, acute restraint stress increased the expression of CRF mRNA whilst AVP mRNA expression was no different to control. Repeated exposure resulted in habituation of CRF expression, whereas AVP mRNA increased above that seen in either non stressed or acutely stressed animals. In adrenalectomised rats with replacement pellets of corticosterone that replicated blood levels approximating to the daily trough (mean levels 37--65 ng/mL), basal CRF expression levels were raised, but the response to acute stress was still observed. However, the habituation seen in normal animals that had been repeatedly stressed was prevented, so that CRF mRNA levels continued to be raised after repeated stress. By contrast, the AVP response to both acute and repeated stress was unaltered in these low-dose corticosterone-treated rats compared with controls. Higher dose pellets, which resulted in blood levels around those of the daily maximum (mean 118--141 ng/mL) had the opposite effects. There was no change compared to intact rats in the expression of CRF mRNA following either acute or repeated stress, but the expected increase in AVP following repeated restraint was prevented. These experiments show that corticosterone has important modulating effects on the adaptive pattern of both CRF and AVP mRNA expression in the parvocellular PVN. The 'set-point' of corticosterone differs; for CRF, experiencing higher levels is necessary for subsequent adaptation to repeated restraint to occur, whereas for AVP a return to lower levels is necessary to allow this peptide to respond to repeated stress.     

Effects of the short chain sugar acid 2-buten-4-olide on the hypothalamo-pituitary-adrenal axis in normal and adjuvant-induced arthritic rats

The effects of intraperitoneal (i.p.) administration of 2-buten-4-olide (2-B4O), an endogenous sugar acid, on the hypothalamo-adenohypophysial system were examined in Lewis rats that were normal and in adjuvant-induced arthritic (AA) rats. In comparison with vehicle-treated rats, the plasma corticosterone and c-fos mRNA levels in the paraventricular nucleus (PVN) of normal rats increased significantly after i.p. administration of 2-B4O. Dual immunostaining revealed that almost all corticotrophin-releasing factor (CRF)-immunopositive neurones in the parvocellular division of the PVN exhibited Fos-like immunoreactivity (LI) 120 min after i.p. administration of 2-B4O (100 mg/kg). In the AA rats, repeated i.p. administration of 2-B4O (100 mg/kg) after immunisation significantly suppressed the expression of clinical symptoms and significantly increased plasma concentrations of corticosterone. Further, repeated i.p. administration of 2-B4O significantly increased CRF mRNA levels in the PVN and pro-opiomelanocortin mRNA levels in the anterior pituitary; however, they did not change arginine vasopressin mRNA levels in the parvocellular division of the PVN. These results suggest that i.p. administration of 2-B4O activates the hypothalamo-pituitary-adrenal (HPA) axis via the activation of CRF neurones in the PVN, and the activation of the HPA axis by i.p. administration of 2-B4O may be associated with the inhibition of AA in rats.     

Corticotropin-releasing factor gene expression is down-regulated in the central nucleus of the amygdala of alcohol-preferring rats which exhibit high anxiety: a comparison between rat lines selectively bred for high and low alcohol preference

The role of amygdaloid corticotropin-releasing factor (CRF) in alcoholism is not clear. Alcohol-preferring (P) rats and high alcohol-drinking (HAD) rats are selectively bred for high alcohol preference, and have been considered suitable animal models for studying alcoholism. The CRF neurons in the central nucleus of the amygdala (CeA) of P rats and HAD rats were studied in comparison with those of their respective counterparts, namely, alcohol-nonpreferring (NP) rats and low alcohol-drinking (LAD) rats. Specifically, CRF-immunoreactivity (ir) in the CeA and paraventricular hypothalamic nucleus (PVN) was assessed using radioimmunohistochemical (RIH) assay in alcohol-naive P/NP rats, and HAD/LAD rats. Furthermore, CRF mRNA was examined using in situ hybridization in the CeA of P/NP rats. Anxiety levels were also evaluated using an elevated plus maze. Results of the present study showed that CRF-ir was significantly lower in the CeA of P rats than NP rats. Moreover, CRF mRNA in the CeA was also much lower in P rats than NP rats. Such differences were not seen in the PVN. Interestingly, those P rats exhibited higher anxiety than NP rats. In contrary, there were no innate differences of CRF-ir in both the CeA and PVN between HAD and LAD rats whose anxiety levels were similar. This study is consistent with the literature showing CRF knockout (KO) induces alcohol drinking, and central administrations of CRF reduce alcohol intake. Collectively, the present study suggests that reduced CRF gene expression in the CeA of P rats is associated with their alcohol preference and anxiety.     

Stereotaxic localization of corticosterone to the amygdala enhances hypothalamo-pituitary-adrenal responses to behavioral stress

The amygdala is involved in behavioral, autonomic, and neuroendocrine responses to stressful stimuli. The goal of the current study was to determine the effect of directly elevating glucocorticoids in the amygdala on hypothalamo-pituitary-adrenocortical (HPA) responses to the elevated plus maze, a behavioral stressor known to activate the amygdala. Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in male Wistar rats; vascular catheters were also placed at this time. Five days post-surgery, blood samples were drawn at 07:00 and 19:00 h to assess diurnal rhythm of plasma corticosterone. At 7 days post-implantation, rats were subjected to behavioral stress using an elevated plus maze and blood was collected 15 min prior to stress, and at 15, 45, and 90 min after the initiation of the stressor. Corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA levels were analyzed by in situ hybridization in the medial parvocellular division of the hypothalamic paraventricular nucleus (mpPVN) in corticosterone- and cholesterol-implanted rats either not exposed to the elevated plus maze (control) or 4 h post-behavioral stress. Localization of corticosterone to the amygdala had no effect on diurnal rhythm of corticosterone secretion. Behavioral stress significantly increased peak plasma corticosterone levels in both groups to a similar level. However, in the corticosterone implanted rats, plasma corticosterone concentrations at 45 and 90 min post-stress were significantly greater compared to control rats indicating a prolonged corticosterone response to behavioral stress. In non-stressed rats, corticosterone delivery to the amygdala elevated basal CRF mRNA in the mpPVN to levels similar to those observed post-stress in control animals; no further increase was observed in CRF mRNA following stress. Behavioral stress resulted in a significant elevation in CRF mRNA in cholesterol controls. Basal AVP mRNA levels were unaffected by corticosterone implants. AVP mRNA did not increase in cholesterol implanted rats in response to behavioral stress. However, AVP mRNA levels were higher in corticosterone implanted rats post stress compared to cholesterol treated controls. In conclusion, direct administration of corticosterone to the amygdala increases plasma corticosterone in response to a behavioral stressor without altering the diurnal rhythm in plasma corticosterone. Elevated basal levels of mpPVN CRF mRNA, and the induction of a mpPVN AVP mRNA response to the behavioral stressor implicate enhanced ACTH secretagogue expression in the increased HPA response to corticosterone modulation of amygdala function.     

Analysis of the anxiolytic-like effect of TRH and the response of amygdalar TRHergic neurons in anxiety

Thyrotropin-releasing hormone (TRH) was first described for its neuroendocrine role in controlling the hypothalamus-pituitary-thyroid axis (HPT). Anatomical and pharmacological data evidence its participation as a neuromodulator in the central nervous system. Administration of TRH induces various behavioural effects including arousal, locomotion, analepsy, and in certain paradigms, it reduces fear behaviours. In this work we studied the possible involvement of TRHergic neurons in anxiety tests. We first tested whether an ICV injection of TRH had behavioural effects on anxiety in the defensive burying test (DBT). Corticosterone serum levels were quantified to evaluate the stress response and, the activity of the HPT axis to distinguish the endocrine response of TRH injection. Compared to a saline injection, TRH reduced cumulative burying, and decreased serum corticosterone levels, supporting anxiolytic-like effects of TRH administration. The response of TRH neurons was evaluated in brain regions involved in the stress circuitry of animals submitted to the DBT and to the elevated plus maze (EPM), tests that allow to correlate biochemical parameters with anxiety-like behaviour. In the DBT, the response of Wistar rats was compared with that of the stress-hypersensitive Wistar Kyoto (WKY) strain. Behavioural parameters were analysed in recorded videos. Animals were sacrificed 30 or 60min after test completion. In various limbic areas, the relative mRNA levels of TRH, its receptors TRH-R1 and -R2, and its inactivating ectoenzyme pyroglutamyl peptidase II (PPII), were determined by RT-PCR, TRH tissue content by radioimmunoassay (RIA). The extent of the stress response was evaluated by measuring the expression profile of CRH, CRH-R1 and GR mRNA in the paraventricular nucleus (PVN) of the hypothalamus and in amygdala, corticosterone levels in serum. As these tests demand increased physical activity, the response of the HPT axis was also evaluated. Both tasks increased the levels of serum corticosterone. WKY rats showed higher anxiety-like behaviour in the DBT than Wistar, as well as increased PVN mRNA levels of CRH and GR. TRH mRNA levels increased in the PVN and TSH values remained unchanged in both strains although TRH content decreased in the medial basal hypothalamus of Wistar rats only. TRH content was measured in several limbic regions but only amygdala showed specific task-related changes after DBT exposure of both strains: increased TRH content. Expression of TRH mRNA decreased in the amygdala of Wistar, suggesting inhibition of TRHergic neuronal activity in this region. The participation of amygdalar TRH neurons in anxiety was confirmed in the EPM where TRH expression and release correlated with the number of entries, and the % of time spent in open arms, supporting an anxiolytic role of these TRH-neurons. These results contribute to the understanding of the involvement of TRH during emotionally charged situations and shed light on the participation of particular circuits in related behaviours.     

Blunted HPA axis response to stress influences susceptibility to posttraumatic stress response in rats.

BACKGROUND: Posttraumatic stress disorder (PTSD) is associated with low levels of circulating cortisol, and recent studies suggest that cortisol administration may reduce PTSD symptoms. This study investigated the role of cortisol in the manifestation of anxiety- and fear-like symptoms in an animal model of PTSD. METHOD: Magnitude of changes in prevalence of anxiety-like behaviors on the elevated plus-maze and nonhabituated exaggerated startle reaction were compared in three strains of rats exposed to predator stress, with and without prior corticosterone treatment. Extreme behavioral changes in both paradigms implied an extreme behavioral response (EBR), representing PTSD-like symptoms. RESULTS: Lewis rats exhibited greater baseline anxiety-like behaviors and greater stress-induced increases in anxiety-like behaviors than Fischer F344 or Sprague-Dawley rats, with only minor corticosterone increases following stress. Prevalence of EBR was 50% among Lewis rats compared with 10% of Fischer F344 and 25% of Sprague-Dawley rats. Administering corticosterone 1 hour before stress exposure reduced the prevalence of EBR from 50% to 8% in the Lewis rats. CONCLUSIONS: These results suggest that a blunted HPA response to stress may play a causal role in this model of PTSD and that this susceptibility may be prevented by administration of cortisol before stress exposure.     

Protein synthesis inhibition before or after stress exposure results in divergent endocrine and BDNF responses disassociated from behavioral responses

This study aimed to assess the effects of anisomycin, a protein synthesis inhibitor, on behavioral responses, brain-derived neurotrophic factor (BDNF) and TrkB mRNA levels, and circulating corticosterone in rats-when administered before or after initial exposure to a predator scent stress stimulus. Magnitude of changes in prevalence of anxiety-like behaviors on the elevated plus-maze and exaggerated startle reaction as well as corticosterone levels and mRNA BDNF and TrkB were compared in rats exposed to predator stress, microinjected with anisomycin before or after stress exposure. Administration of anisomycin before or after stress exposure reduced anxiety-like behavior in the elevated plus-maze and reduced the mean startle amplitude 7 days postexposure. Although the behavioral responses were similar when anisomycin was microinjected before or after stress exposure, the levels of mRNAs for BDNF and TrkB, which play a role in modulation of synaptic plasticity and the consolidation process, showed varying responses.     

Chronic intracerebral prolactin attenuates neuronal stress circuitries in virgin rats

Prolactin (PRL) has been shown to promote maternal behaviour, and to regulate neuroendocrine and emotional stress responses. These effects appear more important in the peripartum period, when the brain PRL system is highly activated. Here, we studied the mechanisms that underlie the anti-stress effects of PRL. Ovariectomized, estradiol-substituted Wistar rats were implanted with an intracerebroventricular cannula and treated with ovine PRL (0.01, 0.1 or 1 microg/h; 5 days via osmotic minipumps) or vehicle, and their responses to acute restraint stress was assessed. Chronic PRL treatment exerted an anxiolytic effect on the elevated plus-maze, and attenuated the acute restraint-induced rise in plasma adrenocorticotropin, corticosterone and noradrenaline. At the neuronal level, in situ hybridization revealed PRL effects on the expression patterns of the immediate-early gene c-fos and corticotropin-releasing factor (CRF). Under basal conditions, PRL significantly reduced c-fos mRNA expression within the central amygdala. In response to restraint, the expression of both c-fos mRNA and protein and of CRF mRNA was decreased in the parvocellular part of the paraventricular nucleus (PVN) of PRL-treated compared with vehicle-treated animals. In conclusion, our data demonstrate that chronic elevation of PRL levels within the brain results in reduced neuronal activation within the hypothalamus, specifically within the PVN, in response to an acute stressor. Thus, PRL acting at various relevant brain regions exerts profound anxiolytic and anti-stress effects, and is likely to contribute to the attenuated stress responsiveness found in the peripartum period, when brain PRL levels are physiologically upregulated.     

Behavioral effects of environmental enrichment during gestation in WKY and Wistar rats

Effects of prenatal environmental enrichment (EE) were examined in Wistar Kyoto (WKY) "depressive- and anxious-like" rats and Wistar rats. During gestation, dams lived in standard cages or in EE cages. Their behavior during gestation and lactation was observed. On weaning day, they were tested in the forced swimming test, and corticosterone concentration was measured from their plasma. The offspring, reared in standard environment, were tested as juveniles or young adults in the elevated plus maze, open field and forced swimming tests. Corticosterone concentration in feces was analyzed. EE offspring showed more anxiety-like behaviors and less activity, compared to controls. Effects were more prominent in youth than in adulthood and in Wistar rats more than in WKY. EE lowered corticosterone concentration in young WKY rats' feces. EE induced changes in the dams' behavior during gestation and lactation. These changes in dams' behavior could be mediators of the effects on the offspring.     

Sex-dependent and differential responses to acute restraint stress of corticotropin-releasing factor-producing neurons in the rat paraventricular nucleus, central amygdala, and bed nucleus of the stria terminalis

Male and female rodents respond differently to acute stress. We tested our hypothesis that this sex difference is based on differences in stress sensitivity of forebrain areas, by determining possible effects of a single acute psychogenic stressor (1-hr restraint stress) on neuronal gene expression (c-Fos and FosB immunoreactivities), storage of corticotropin-releasing factor (CRF) immunoreactivity, and CRF production (CRF mRNA in situ hybridization) as well as the expression of genes associated with epigenetic processes (quantitative RT-PCR) in the rat paraventricular nucleus (PVN), the oval and fusiform subdivisions of the bed nucleus of the stria terminalis (BSTov and BSTfu, respectively), and the central amygdala (CeA), in both males and females. Compared with females, male rats responded to the stressor with a stronger rise in corticosterone titer and a stronger increase in neuronal contents of c-Fos, CRF mRNA, and CREB-binding protein mRNA in the PVN. In the BSTov, females but not males showed an increase in c-Fos, whereas the CRF mRNA content was increased in males only. In the BSTfu, males and females showed similar stress-induced increases in c-Fos and FosB, whereas in the CeA, both sexes revealed similar increases in c-Fos and in CRF mRNA. We conclude that male and female rats differ in their reactivity to acute stress with respect to possibly epigenetically mediated (particularly in the PVN) neuronal gene expression and neuropeptide dynamics (PVN and BSTov) and that this difference may contribute to the sex dependence of the animal's physiological and behavioral responses to an acute stressor.     

Foster litters prevent hypothalamic-pituitary-adrenal axis sensitization mediated by neonatal maternal separation

Neonatal maternal separation of rat pups has been shown to produce long-term increases in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, elevated levels of hypothalamic corticotropin releasing factor (CRF) mRNA in the hypothalamic paraventricular nucleus (PVN), and enhanced anxiety-like behavior. These effects appear to be at least partially mediated by subtle disruptions in the quality of maternal-pup interactions. This hypothesis was tested by providing half the dams with foster litters during the maternal separation paradigm, so that in those litters, only the pups and not the dams were experiencing a period of separation. The separation protocol took place daily from PND2-14 for either 15 min (HMS15, handled) or 180 min (HMS180, maternal separation). During the period of separation dams were either transferred to adjacent cages without any pups present (HMS15, HMS180) or to cages containing an age-matched foster litter (HMS15F, HMS180F). As adults, the HMS180 progeny exhibited the expected increased expression of CRF mRNA in the PVN, stress hyper-responsiveness to airpuff startle and evidence of impaired feedback both in the CORT response, as well as in response to the dexamethasone suppression test. The HMS180F rats, however, appeared to be resistant to these effects of maternal separation as they demonstrated CRF mRNA levels intermediate between HMS15 and HMS180 rats. Their stress responses and feedback regulation of the HPA axis was comparable to that of the HMS15 rats. GR mRNA was elevated in the cortex of HMS180F rats. Overall, these studies support the thesis that the long-term effects of neonatal maternal separation may largely result from alterations in the quality of maternal care rather than from direct effects of the separation per se on the pups.     

Evidence for hippocampal regulation of neuroendocrine neurons of the hypothalamo-pituitary-adrenocortical axis

Expression of mRNAs coding for the ACTH secretagogues corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) was examined in the hypothalamic paraventricular nucleus (PVN) of rats bearing hippocampal lesions. Either total hippocampectomy (HPX) or extirpation of the dorsal hippocampus (DHPX) precipitated a 4-fold increase in CRF mRNA expression relative to sham-operated controls (SHAM), as determined by semiquantitative in situ hybridization histochemistry. AVP mRNA was localized to individual parvocellular neurons of the medial parvocellular division of the PVN in only the HPX and DHPX groups, consistent with enhanced production of AVP message in this neuronal population subsequent to hippocampal damage. HPX did not affect AVP mRNA content in magnocellular divisions of PVN. Plasma beta-endorphin levels were significantly elevated in the HPX and DHPX groups relative to SHAM animals, indicating a chronic increase in release of proopiomelanocortin peptides from the anterior pituitary gland in response to hippocampal lesion. Circulating corticosterone levels were elevated in HPX rats as well. To control for effects of lesion size and location, additional animals received large ablations of cerebral cortex or cerebellum. In neither case was CRF or AVP mRNA significantly altered in the PVN. The results suggest that the hippocampus exercises a tonic inhibitory role on ACTH secretagogue production in neuroendocrine neurons promoting ACTH release.     

Allergic rhinitis induces anxiety-like behavior and altered social interaction in rodents

Epidemiological and clinical studies report higher incidences of anxiety and increased emotional reactivity in individuals suffering from respiratory allergies. To evaluate if respiratory allergies are capable of promoting anxiety-like behavior in rodents, we used models of allergic rhinitis and behavioral evaluations followed by assessment of mRNA for cytokines in relevant brain regions. Mice and rats were sensitized to ovoalbumin or pollen, respectively, following standard sensitization and challenge protocols. After challenge, the animals were evaluated in the open field, elevated plus-maze and resident-intruder tests. Cytokines and corticotropin-releasing factor expression were assessed in several brain regions by real-time RT-PCR and plasma corticosterone concentrations by radioimmunoassay. Mice and rats sensitized and exposed to allergen showed increased anxiety-like behavior and reduced social interaction without any overt behavioral signs of sickness. T-helper type 2 (T_H2) cytokines were induced in both rats and mice in the olfactory bulbs and prefrontal cortex and remained unchanged in the temporal cortex and hypothalamus. The same results were found for CRF mRNA expression. No differences were observed in corticosterone concentrations 1 h after the last behavioral test. These results show that sensitization and challenge with allergens induce anxiety across rodent species and that these effects were paralleled by an increased expression of T_H2 cytokines and CRF in the prefrontal cortex. These studies provide experimental evidence that sensitized rodents experience neuroimmune-mediated anxiety and reduced social interaction associated with allergic rhinitis.     

Characterization of the vulnerability to repeated stress in Fischer 344 rats: possible involvement of microRNA-mediated down-regulation of the glucocorticoid receptor

In the present study, we established and characterized an animal model of vulnerability to repeated stress. We found that control Sprague–Dawley (SD) rats showed a gradual decrease in the HPA axis response following 14 days of repeated restraint stress, whereas Fischer 344 (F344) rats did not show such HPA axis habituation. Similar habituation was observed in the expression of c-fos mRNA, corticotropin-releasing hormone hnRNA, and phospho-CREB and phospho-ERK proteins in the hypothalamic paraventricular nucleus (PVN) of SD rats, but not in the F344 rats. In addition, repeatedly restrained F344 rats exhibited decreased cell proliferation in the dentate gyrus of the hippocampus and increased anxiety-related behaviours, while repeatedly restrained SD rats exhibited a selective enhancement of hippocampal cell proliferation in the ventral area. Moreover, we found a lower expression of glucocorticoid receptor (GR) protein, but not mRNA, in the PVN of F344 rats compared to SD rats. We also identified that microRNA (miR)-18a inhibited translation of GR mRNA in cultured neuronal cells and that increased expression of miR-18a in the PVN was observed in F344 rats compared with SD rats. These strain differences in GR protein levels were not found in the hippocampus and prefrontal cortex, and the expression of miR-18a was much lower in these brain regions than in the PVN. Our results suggest that F344 rats could be a useful animal model for studying vulnerability to repeated stress, and that miR-18a-mediated down-regulation of GR translation may be an important factor to be considered in susceptibility to stress-related disorders.     

Role of catecholamines in mediating messenger RNA and hormonal responses to stress.

The role of catecholamines in regulating the neuroendocrine stress response is controversial. We have investigated the effects of unilateral ventral noradrenergic bundle (VNAB) lesions on corticotrophin-releasing factor (CRF) and proenkephalin A mRNA responses in the parvocellular paraventricular nucleus (pPVN) to both physical and psychological stresses. We have also determined the effects of direct bilateral PVN lesions on CRF mRNA, plasma ACTH and corticosterone responses to psychological stress. 6-OHDA lesions whether to the VNAB or direct to the PVN did not result in any change in basal levels of CRF mRNA. Depletion of endogenous noradrenaline following unilateral lesions of the VNAB did not affect the CRF mRNA or the proenkephalin A mRNA response to stress. These data suggest that noradrenergic pathways are not involved in maintaining basal levels of CRF mRNA and that the noradrenergic input through the VNAB does not mediate the accumulation of CRF and proenkephalin A mRNAs in response to these stressors. Direct bilateral lesions to the PVN prevented the accumulation of CRF mRNA but not the ACTH and corticosterone responses to restraint stress. This suggests that monoamines are involved in the regulation of CRF mRNA through a mechanism independent of CRF-41 secretion.