Chronic Forced Swim Stress Research Articles

Activation of CRF/CRFR1 Signaling in the Central Nucleus of the Amygdala Contributes to Chronic Stress-Induced Exacerbation of Neuropathic Pain by Enhancing GluN2B-NMDA Receptor-Mediated Synaptic Plasticity in Adult Male Rats

Exacerbation of pain by chronic stress and comorbidity of pain with stress-related disorders such as depression and post-traumatic stress disorder, represent significant clinical challenges. Previously we have documented that chronic forced swim (FS) stress exacerbates neuropathic pain in spared nerve injury (SNI) rats, associated with an up-regulation of GluN2B-containing N-methyl-D-aspartate receptors (GluN2B-NMDARs) in the central nucleus of the amygdala (CeA). However, the molecular mechanisms underlying chronic FS stress (CFSS)-mediated exacerbation of pain sensitivity in SNI rats still remain unclear. In this study, we demonstrated that exposure of CFSS to rats activated the corticotropin-releasing factor (CRF)/CRF receptor type 1 (CRFR1) signaling in the CeA, which was shown to be necessary for CFSS-induced depressive-like symptoms in stressed rats, and as well, for CFSS-induced exacerbation of pain hypersensitivity in SNI rats exposed to chronic FS stress. Furthermore, we discovered that activation of CRF/CRFR1 signaling in the CeA upregulated the phosphorylation of GluN2B-NMDARs at tyrosine 1472 (pGluN2BY1472) in the synaptosomal fraction of CeA, which is highly correlated to the enhancement of synaptic GluN2B-NMDARs expression that has been observed in the CeA in CFSS-treated SNI rats. In addition, we revealed that activation of CRF/CRFR1 signaling in the CeA facilitated the CFSS-induced reinforcement of long-term potentiation as well as the enhancement of NMDAR-mediated excitatory postsynaptic currents in the basolateral amygdala (BLA)-CeA pathway in SNI rats. These findings suggest that activation of CRF/CRFR1 signaling in the CeA contributes to chronic stress-induced exacerbation of neuropathic pain by enhancing GluN2B-NMDAR-mediated synaptic plasticity in rats subjected to nerve injury. PerspectiveOur present study provides a novel mechanism for elucidating stress-induced hyperalgesia and highlights that the CRF/CRFR1 signaling and the GluN2B-NMDAR-mediated synaptic plasticity in the CeA may be important as potential therapeutic targets for chronic stress-induced pain exacerbation in human neuropathic pain. Data AvailabilityThe data that support the findings of this study are available from the corresponding author upon reasonable request.

Thymoquinone Loaded Solid Lipid Nanoparticles Demonstrated Antidepressant-Like Activity in Rats via Indoleamine 2, 3- Dioxygenase Pathway.

The World Health Organization (WHO) has ranked depression the 4th leading cause of disability worldwide. Thymoquinone (TQ), is an active constituent of Nigella sativa having various medicinal properties but has poor solubility and bioavailability. This problem was overcome by developing nanoformulation of TQ. Previously TQ reported good antioxidant and anti-inflammatory effects. Recently TQ's anti-depressant effect was demonstrated. However, the mechanisms underlying the antidepressant effect of TQ still needs evaluation. Activation of Indoleamine-2,3-dioxygenase (IDO), (an enzyme that participates in the tryptophan metabolism), leads to a decrease of serotonin (5-HT) levels. The expression of this enzyme is associated with immune system activation, which has been proposed as a common mechanism that links depression. The present study was performed in stressed animals where hippocampal levels of pro-inflammatory cytokines (IL-6 and TNF α levels), brain derived neurotropic factor (BDNF) and hippocampal kynurenine (KYN), tryptophan (TRP) and serotonin (5-HT) levels were estimated. Treatment with TQ solid lipid nanoparticles (TQSLN 20 mg/kg p.o) and TQ suspension (20 mg/kg p.o) demonstrated antidepressant-like activity in chronic forced-swim stress model. Further, it reduced the elevated hippocampal IL-6 & TNFα and reversed the increased activity of IDO as measured by ratio of hippocampal KYN/TRP and 5HT/TRP in stressed rats. The results of the present study confirm anti-inflammatory and neuroprotective effects of TQ which may be associated with 5-HT pathway. Thus, the present study offers a newer approach to reduce symptoms of depression using TQSLN. Our results are preliminary, further research is needed for more conclusive view.

Neuroethological Study of AlCl3 and Chronic Forced Swim Stress Induced Memory and Cognitive Deficits in Albino Rats.

Neuroethological Study of AlCl3 and Chronic Forced Swim Stress Induced Memory and Cognitive Deficits in Albino Rats.

Chronic stress increases pain sensitivity via activation of the rACC–BLA pathway in rats

Exposure to chronic stress can produce maladaptive neurobiological changes in pathways associated with pain processing, which may cause stress–induced hyperalgesia (SIH). However, the underlying mechanisms still remain largely unknown. In previous studies, we have reported that the amygdala is involved in chronic forced swim (FS) stress–induced depressive-like behaviors and the exacerbation of neuropathic pain in rats, of which, the basolateral amygdala (BLA) and the central nucleus of the amygdala (CeA) are shown to play important roles in the integration of affective and sensory information including nociception. Here, using in vivo multichannel recording from rostal anterior cingulate cortex (rACC) and BLA, we found that chronic FS stress (CFSS) could increase the pain sensitivity of rats in response to low intensity innoxious stimuli (LIS) and high intensity noxious stimuli (HNS) imposed upon the hindpaw, validating the occurrence of SIH in stressed rats. Moreover, we discovered that CFSS not only induced an increased activity of rACC neuronal population but also produced an augmented field potential power (FPP) of rACC local field potential (LFP), especially in low frequency theta band as well as in high frequency low gamma band ranges, both at the baseline state and under LIS and HNS conditions. In addition, by using a cross-correlation method and a partial directed coherence (PDC) algorithm to analyze the LFP oscillating activity in rACC and BLA, we demonstrated that CFSS could substantially promote the synchronization between rACC and BLA regions, and also enhanced the neural information flow from rACC to BLA. We conclude that exposure of chronic FS stress to rats could result in an increased activity of rACC neuronal population and promote the functional connectivity and the synchronization between rACC and BLA regions, and also enhance the pain–related neural information flow from rACC to BLA, which likely underlie the pathogenesis of SIH.

Effect of Embelin in Monosodium Glutamate Induced Obesity in Male Neonatal Wistar Rats

Recent clinical studies report antipsychotics as a better option to augment the action of antidepressants in treatment resistant cases. However, the proper mechanisms underlying the antidepressant effect of antipsychotics is still not clear. Indolamine 2, 3 dioxygenase (IDO) pathway is considered to be an important pathway in pro-inflammatory cytokine associated stress-induced depression. The present study investigated the antidepressant effect of venlafaxine, olanzapine and their combinations in chronic forced-swim stress-induced depression in mice. In addition, the role of pro-inflammatory cytokines and IDO-mediated pathway was investigated. Swiss albino mice were subjected to chronic forced-swim stress and evaluation for antidepressant-like activity was performed on 7th, 14th and 21st day following which serum levels of pro-inflammatory cytokines (IL-1β and IL-6 levels) and the levels of hippocampal kynurenine (KYN), tryptophan (TRP) and serotonin (5HT) were estimated. The combination exhibited augmentation of antidepressant-like activity of venlafaxine by olanzapine in chronic forced-swim stress model. Further, it reversed the enhanced serum IL-1β and IL-6 and reverted the increased activity of IDO as measured by ratio of hippocampal KYN/TRP and 5HT/TRP in stressed mice. Augmentation of antidepressant effect of venlafaxine by olanzapine is thus mediated by normalising the shift from KYN to TRP pathway in chronic forced swim induced stressed mice.

Investigating the metabolic alterations in a depressive-like rat model of chronic forced swim stress: An in vivo proton magnetic resonance spectroscopy study at 7T

Although recent investigations of major depressive disorder (MDD) have focused on the monoaminergic system, accumulating evidences suggest that alternative pathophysiological models of MDD and treatment options for patients with MDD are needed. Animals subjected to chronic forced swim stress (CFSS) develop behavioral despair. The purpose of this study was to investigate the in vivo effects of CFSS on systems other than the monoamine system in the rat prefrontal cortex (PFC) with 7T and short-echo-time (16.3 ms) proton magnetic resonance spectroscopy (1H MRS). Ten male Wistar rats underwent 14 days of CFSS, and in vivo1H MRS and forced swim tests were performed before and after CFSS. Point-resolved spectroscopy was used to quantify metabolite levels in the rat PFC. To investigate spectral overlap in glutamate and glutamine, spectral analyses in the spectra obtained in the in vivo1H MRS, parametrically matched spectral simulation, and in vitro experiments were performed. The results of the spectral analyses showed that the glutamate/glutamine spectral overlap was not critical, which suggested that in vivo1H MRS can be used to reliably assess the glutamate system. The rats showed significantly increased immobility times and decreased climbing times in the FST after CFSS, which suggested that the rats developed behavioral despair. The pre-CFSS and post-CFSS glutamate and glutamine levels did not significantly differ (p > 0.050). The levels of myo-inositol, total choline, and N-acetylaspartate, myo-inositol/creatine, and total choline/creatine increased significantly (p < 0.050). Similar findings have been reported in patients with MDD. Taken together, these results suggest that the CFSS-induced metabolic alterations were similar to those found in patients and that high-field and short-echo-time in vivo1H MRS can be used to investigate depression-induced metabolic alterations. Such investigations might provide alternative insights into the nonmonoaminergic pathophysiology and treatment of depression.

Activation of CRF/CRFR1 signaling in the basolateral nucleus of the amygdala contributes to chronic forced swim-induced depressive-like behaviors in rats

The basolateral nucleus of the amygdala (BLA) plays a key role in processing stressful events and affective disorders. Previously we have documented that exposure of chronic forced swim (FS) to rats produces a depressive-like behavior and that sensitization of BLA neurons is involved in this process. In the present study, we demonstrated that chronic FS stress (CFSS) could activate corticotropin-releasing factor (CRF)/CRF receptor type 1 (CRFR1) signaling in the BLA, and blockade of CRF/CRFR1 signaling by intra-BLA injection of NBI27914 (NBI), a selective CRFR1 antagonist, could prevent the CFSS-induced depressive-like behaviors in rats, indicating that activation of CRF/CRFR1 signaling in the BLA is required for CFSS-induced depression. Furthermore, we discovered that exposure of chronic FS to rats could reinforce long-term potentiation (LTP) at the external capsule (EC)-BLA synapse and increase BLA neuronal excitability, and that all these alterations were inhibited by CRFR1 antagonist NBI. Moreover, we found that application of exogenous CRF also may facilitate LTP at the EC-BLA synapse and sensitize BLA neuronal excitability in normal rats via the activation of CRFR1. We conclude that activation of CRF/CRFR1 signaling in the BLA contributes to chronic FS-induced depressive-like behaviors in rats through potentiating synaptic efficiency at the EC-BLA pathway and sensitizing BLA neuronal excitability.

Activation of indoleamine 2, 3- dioxygenase pathway by olanzapine augments antidepressant effects of venlafaxine in mice

Recent clinical studies report antipsychotics as a better option to augment the action of antidepressants in treatment resistant cases. However, the proper mechanisms underlying the antidepressant effect of antipsychotics is still not clear. Indolamine 2, 3 dioxygenase (IDO) pathway is considered to be an important pathway in pro-inflammatory cytokine associated stress-induced depression. The present study investigated the antidepressant effect of venlafaxine, olanzapine and their combinations in chronic forced-swim stress-induced depression in mice. In addition, the role of pro-inflammatory cytokines and IDO-mediated pathway was investigated. Swiss albino mice were subjected to chronic forced-swim stress and evaluation for antidepressant-like activity was performed on 7th, 14th and 21st day following which serum levels of pro-inflammatory cytokines (IL-1β and IL-6 levels) and the levels of hippocampal kynurenine (KYN), tryptophan (TRP) and serotonin (5HT) were estimated. The combination exhibited augmentation of antidepressant-like activity of venlafaxine by olanzapine in chronic forced-swim stress model. Further, it reversed the enhanced serum IL-1β and IL-6 and reverted the increased activity of IDO as measured by ratio of hippocampal KYN/TRP and 5HT/TRP in stressed mice. Augmentation of antidepressant effect of venlafaxine by olanzapine is thus mediated by normalising the shift from KYN to TRP pathway in chronic forced swim induced stressed mice.

Effects of chronic forced-swim stress on behavioral properties in rats with neonatal repeated MK-801 treatment

The two-hit hypothesis has been used to explain the onset mechanism of schizophrenia. It assumes that predisposition to schizophrenia is originally attributed to vulnerability in the brain which stems from genetic or early developmental factors, and that onset is triggered by exposure to later detrimental factors such as stress in adolescence or adulthood. Based on this hypothesis, the present study examined whether rats that had received neonatal repeated treatment with an N-methyl-d-aspartate (NMDA) receptor antagonist (MK-801), an animal model of schizophrenia, were vulnerable to chronic stress. Rats were treated with MK-801 (0.2mg/kg) or saline twice daily on postnatal days 7–20, and animals in the stress subgroups were subjected to 20days (5days/week×4weeks) of forced-swim stress in adulthood. Following this, behavioral tests (prepulse inhibition, spontaneous alternation, open-field, and forced-swim tests) were carried out. The results indicate that neonatal repeated MK-801 treatment in rats inhibits an increase in immobility in the forced-swim test after they have experienced chronic forced-swim stress. This suggests that rats that have undergone chronic neonatal repeated NMDA receptor blockade could have a reduced ability to habituate or adapt to a stressful situation, and supports the hypothesis that these rats are sensitive or vulnerable to stress.

Neurometabolic alterations in a depression-like rat model of chronic forced swimming stress using in vivo proton magnetic resonance spectroscopy at 7 T

Although recent investigations of major depressive disorder (MDD) have focused on the monoaminergic system, accumulating evidence suggests that alternative pathophysiological models of MDD and treatment options for patients with MDD are needed. Animals subjected to chronic forced swim stress (CFSS) develop behavioral despair. The purpose of this study was to investigate the in vivo effects of CFSS in the rat prefrontal cortex (PFC) with 7 T and short-echo-time proton magnetic resonance spectroscopy (1H MRS). Ten male Wistar rats underwent 14 days of CFSS, and in vivo1H MRS and forced swim tests were performed before and after CFSS. Point-resolved spectroscopy was used to quantify metabolite levels in the rat PFC. The spectral analyses showed that in vivo1H MRS can be used to reliably assess the Glu system. The rats showed significantly increased immobility times and decreased climbing times in the FST after CFSS, which suggested that the rats developed behavioral despair. The pre-CFSS and post-CFSS Glu and Gln levels did not significantly differ (P &gt; 0.050). The levels of myo-inositol, total choline, and N-acetylaspartate, myo-inositol/creatine, and total choline/creatine increased significantly (P &lt; 0.050). Similar findings have been reported in patients with MDD. Taken together, these results suggested that the CFSS-induced metabolic alterations were similar to those found in patients and that high-field and short-echo-time in vivo1H MRS can be used to investigate depression-induced metabolic alterations. Such investigations might provide alternative insights into the nonmonoaminergic pathophysiology and treatment of depression.Disclosure of interestThe authors have not supplied their declaration of competing interest.

Chronic stress exacerbates neuropathic pain via the integration of stress-affect-related information with nociceptive information in the central nucleus of the amygdala.

Exacerbation of pain by chronic stress and comorbidity of pain with stress-related psychiatric disorders, including anxiety and depression, represent significant clinical challenges. However, the underlying mechanisms still remain unclear. Here, we investigated whether chronic forced swim stress (CFSS)-induced exacerbation of neuropathic pain is mediated by the integration of stress-affect-related information with nociceptive information in the central nucleus of the amygdala (CeA). We first demonstrated that CFSS indeed produces both depressive-like behaviors and exacerbation of spared nerve injury (SNI)-induced mechanical allodynia in rats. Moreover, we revealed that CFSS induces both sensitization of basolateral amygdala (BLA) neurons and augmentation of long-term potentiation (LTP) at the BLA-CeA synapse and meanwhile, exaggerates both SNI-induced sensitization of CeA neurons and LTP at the parabrachial (PB)-CeA synapse. In addition, we discovered that CFSS elevates SNI-induced functional up-regulation of GluN2B-containing NMDA (GluN2B-NMDA) receptors in the CeA, which is proved to be necessary for CFSS-induced augmentation of LTP at the PB-CeA synapse and exacerbation of pain hypersensitivity in SNI rats. Suppression of CFSS-elicited depressive-like behaviors by antidepressants imipramine or ifenprodil inhibits the CFSS-induced exacerbation of neuropathic pain. Collectively, our findings suggest that CFSS potentiates synaptic efficiency of the BLA-CeA pathway, leading to the activation of GluN2B-NMDA receptors and sensitization of CeA neurons, which subsequently facilitate pain-related synaptic plasticity of the PB-CeA pathway, thereby exacerbating SNI-induced neuropathic pain. We conclude that chronic stress exacerbates neuropathic pain via the integration of stress-affect-related information with nociceptive information in the CeA.

Anti-Apoptotic Protein Bcl-xL Expression in the Midbrain Raphe Region Is Sensitive to Stress and Glucocorticoids.

Anti-apoptotic proteins are suggested to be important for the normal health of neurons and synapses as well as for resilience to stress. In order to determine whether stressful events may influence the expression of anti-apoptotic protein Bcl-xL in the midbrain and specifically in the midbrain serotonergic (5-HT) neurons involved in neurobehavioral responses to adverse stimuli, adult male rats were subjected to short-term or chronic forced swim stress. A short-term stress rapidly increased the midbrain bcl-xl mRNA levels and significantly elevated Bcl-xL immunoreactivity in the midbrain 5-HT cells. Stress-induced increase in glucocorticoid secretion was implicated in the observed effect. The levels of bcl-xl mRNA were decreased after stress when glucocorticoid elevation was inhibited by metyrapone (MET, 150 mg/kg), and this decrease was attenuated by glucocorticoid replacement with dexamethasone (DEX; 0.2 mg/kg). Both short-term stress and acute DEX administration, in parallel with Bcl-xL, caused a significant increase in tph2 mRNA levels and slightly enhanced tryptophan hydroxylase immunoreactivity in the midbrain. The increasing effect on the bcl-xl expression was specific to the short-term stress. Forced swim repeated daily for 2 weeks led to a decrease in bcl-xl mRNA in the midbrain without any effects on the Bcl-xL protein expression in the 5-HT neurons. In chronically stressed animals, an increase in tph2 gene expression was not associated with any changes in tryptophan hydroxylase protein levels. Our findings are the first to demonstrate that both short-term stress and acute glucocorticoid exposures induce Bcl-xL protein expression in the midbrain 5-HT neurons concomitantly with the activation of the 5-HT synthesis pathway in these neurons.

Tryptophan hydroxylase 2 and Bcl-xL in the rat raphe nucleus after acute and chronic forced swim stress

Stressful events induce changes in the expression of numerous genes and their protein products in the brain. However, the mechanisms mediating these changes, as well as their significance for the development of stress-induced depression or for coping with stress, remain obscure. Evidence for the sensitivity of tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme of the serotonin (5-HT) pathway, to stress is concisely reviewed, as well as the neuroprotective function of the antiapoptotic protein Bcl-xL in the brain. The goal of our experiments was to investigate the gene and protein expression of TPH2 and Bcl-xL in the dorsal (DRN) and median (MRN) raphe nuclei during repeated stress events. Gene (RT PCR) and protein (immunohistochemistry) expression was assessed 24 hours after the second and fourteenth forced swim sessions. The increase in TPH2 protein expression observed after the second swim stress exposure might reflect the protective action of Bcl-xL. During the subsequent stressful events, the stress-induced increase in Bcl-xL expression decreased. This effect was associated with the weakening of serotonergic neuron function evidenced by the compensatory activation of the TPH2 gene expression without TPH2 protein increase. Thus, shortand long-term forced swimming resulted in qualitatively different alterations in brain expression of TPH2 and Bcl-xL, suggesting their specific roles during acute and chronic stages in the development of stress-induced psychopathology. These changes may constitute a component of the mechanisms underlying elevated tph2 gene expression in depressed patients.

Augmentation of effect of venlafaxine by folic acid in behavioral paradigms of depression in mice: Evidence of serotonergic and pro-inflammatory cytokine pathways.

Though venlafaxine is an antidepressant with similar efficacy as selective serotonin receptor inhibitors, dose dependent adverse effects limit its use. Depression is associated with increased levels of pro-inflammatory cytokines. The study investigated the effect of combining low/serotonergic dose of venlafaxine with folic acid in mice exposed to chronic forced swim stress for 21 days during which immobility and swimming time following forced swim test (FST) and immobility time in tail suspension test (TST) were measured every 7th, 14th and 21st day. The serum level of pro-inflammatory cytokines (IL-1β and IL-6) and whole brain levels of monoamines (serotonin, norepinephrine and dopamine) were estimated. An augmentation of antidepressant effect was observed in both forced swim test and tail suspension test following combination of venlafaxine (2 and 4mg/kg) with folic acid (5 and 10mg/kg) after 14 and 21 days of treatment. On brain serotonin level also, a significant augmentation was observed when venlafaxine (4mg/kg) was combined with folic acid (10mg/kg). Further, the combination significantly reversed the elevated levels of serum pro-inflammatory cytokines, IL-1β and IL-6 observed in chronic FST-induced stressed mice. Combining low dose venlafaxine with an augmenting agent like folic acid, thus, appears to be an optimum strategy to increase its therapeutic efficacy and to reduce its dose.

Hippocampal interleukin-1beta in the juvenile and middle-aged rat: response to chronic forced swim or high-light open-field stress stimulation.

It is postulated that stress differentially affects interleukin-1beta (IL-1beta) during ontogenetic life. This study examined the influence of chronic exposure to forced swim (FS) stress or high-light open-field (HL-OF) stress on interleukin-1beta (IL-1beta). The total level of IL-1beta protein was assessed by Western blot analysis of hippocampal extracts. Double immunofluorescence staining was used to reveal the percentage of IL-1beta/NeuN (NeuN - neuronal marker) cells in the CA1, CA3 and dentate gyrus (DG) hippocampal subfields. Juvenile (P28; P - postnatal day) and middle-aged (P360) rats were used in the experiment. The research showed no significant differences in IL-1beta protein levels between P28 and P360 non-stress rats. However, a substantial increase in the percentage of IL-1beta-ir neurons in the CA1, CA3 and DG in P360 rats was observed. Chronic FS had no significant influence on IL-1beta expression in the hippocampus or on the percentage of IL-1beta-ir neurons in CA1, CA3 and DG hippocampal subfields in either age group. During HL-OF, the IL-1beta level was significantly increased in the hippocampus of P28 and P360 rats, whereas a marked increase in the percentage of IL-1beta-ir neurons in the CA1, CA3 and DG hippocampal areas occurred only in P360 animals. These results indicate that chronic HL-OF stimulation was the factor inducing changes in the IL-1beta protein levels in P28 and P360 rats and in the percentage of IL-1beta/NeuN-ir cells in the hippocampus of P360 animals.

The effects of central pro-and anti-inflammatory immune challenges on depressive-like behavior induced by chronic forced swim stress in rats

Although increasing evidence demonstrates that both chronic stressors and inflammatory immune activation contribute to pathophysiology and behavioral alterations associated with major depression, little is known about the interaction effect of central inflammatory immune activation and stress on depressive-like behavior. Our previous work has shown that 14-day chronic forced swim stress induces significant depressive-like behavior. The present investigation assessed whether pro-inflammatory cytokine and anti-inflammatory cytokine challenges have differential interaction effect on depressive-like behavior induced by chronic forced swim stress in rats. The pro-inflammatory and anti-inflammatory immune challenges were achieved respectively by central administration of lipopolysaccharide (LPS), a pro-inflammatory cytokine inducer, and interleukin-10 (IL-10), an anti-inflammatory cytokine. It was found that either central LPS treatment alone or chronic forced swim stress alone significantly induced depressive-like behavior, including reduced body weight gain, reduced saccharin preference and reduced locomotor activity. However, there was no significant synergistic or additive effect of central LPS treatment and stress on depressive-like behavior. LPS treatment did not exacerbate the depressive-like behavior induced by forced swim stress. Nevertheless, IL-10 reversed depressive-like behavior induced by forced swim stress, a finding indicating that IL-10 has antidepressant effect on behavioral depression induced by stress. The present findings provide new insight into the complexity of the immunity-inflammation hypothesis of depression.

Citalopram alleviates chronic stress induced depression-like behaviors in rats by activating GSK3β signaling in dorsal hippocampus

Aside from monoamine disturbances, recent evidence has implicated particular intracellular pathways, including Wnt signaling, in the pathogenesis of major depressive disorder. In the present study, we investigated the role of Wingless (Wnt)-Dishevelled (DVL)-glycogen synthase kinase 3β (GSK3β) signaling in the depression-like behaviors exhibited by rats exposed to chronic forced swim stress. We found that the rats subjected to forced swim stress for 14 consecutive days exhibited obvious depression-like behaviors and showed decreased levels of phosphorylated GSK3β and β-catenin in the hippocampus. Chronic citalopram treatment alleviated the depression-like behaviors and reversed the disruptions of the phosphorylated GSK3β and β-catenin in stressed rats. Furthermore, when the stressed rats with citalopram treatment received bilateral, dorsal hippocampus infusions of a DVL inhibitor, sulindac, the depression-like effects induced by chronic stress reappeared. These findings suggest that the Wnt-DVL-GSK3β signaling in the hippocampus is markedly involved in the pathophysiology of depression induced by chronic stress. The Wnt-DVL-GSK3β pathway may mediate the therapeutic action of citalopram, and the manipulation of DVL could be a target for novel antidepressants.

Effects of chronic forced swim stress on hippocampal brain-derived neutrophic factor (BDNF) and its receptor (TrkB) immunoreactive cells in juvenile and aged rats.

A type of stress stimulation and age are claimed to affect the expression of brain-derived neurotrophic factor (BDNF) and its receptor - tyrosine kinase B (TrkB) in the hippocampal regions differentially. This study aimed to explore the influence of chronic (15 min daily for 21 days) forced swim stress (FS) exposure on the BDNF and TrkB containing neurons in the hippocampal CA1, CA3 pyramidal cell layers and dentate gyrus (DG) granule cell layer in juvenile (P28) and aged (P360) rats. An immunofluorescence (-ir) method was used to detect BDNF-ir and TrkB-ir cells. Under chronic FS exposure, in the group of juvenile rats a significant decrease in the density of BDNF immunoreactive neurons was observed in CA1 and DG (P less than <0.001), unlike CA3, where it remained unaltered just as the density of TrkB-ir cells in CA1 and DG, but in CA3 the number of TrkB-ir cells was found to grow (P less than 0.05) in comparison with control groups. After chronic FS exposure of aged (P360) rats, the density of BDNF-ir and TrkB-ir cells did not decline in any of the subregions of the hippocampus. In all subfields of the hippocampus, the denseness of BDNF-positive neurons was significantly higher in P360 stressed group, compared with P28 stressed group, but the density of TrkB-ir fell more markedly in P360 than in P28. In conclusion, chronic FS stress influenced the number of BDNF and TrkB immunoreactive neurons only in juvenile animals. The age of rats tested in the chronic forced swim test was a decisive factor determining changes in the density of BDNF-ir and TrkB-ir in the hippocampal structures.

Changes in NGF/c-Fos double staining in the structures of the limbic system in juvenile and aged rats exposed to forced swim test.

This study aimed to investigate the influence of acute (a single 15 min) and chronic (15 min daily for 21 days) exposure to forced swim (FS) test on nerve growth factor (NGF)/c-Fos cells in hypothalamic paraventricular (PV) and supraoptic (SO) nuclei, the central (CeA) and medial (MeA) amygdaloid nuclei and CA3-hippocampus in juvenile (P28) and aged (P360) rats. The double-immunofluorescence (-ir) method was used to detect NGF-ir and c-Fos-ir cells. The amount of NGF/c-Fosir cells in relation to all NGF-ir cells is shown as a percentage. In the acute FS test an increase in NGF/c-Fos-ir cells (P<.05) was observed in all studied structures of juvenile rats and in the PV and SO of the aged individuals. After chronic FS stress, the NGF/c-Fos-ir ratio remained unaltered (except in the SO) in P28, but it increased (P<.05) in all investigated regions in P360 compared with the controls. The findings may reflect the state of molecular plasticity within the limbic hypothalamicpituitary- adrenocortical (HPA) axis in both age groups, yet the phenomenon of habituation in NGF/c-Fos-ir after chronic FS exposure was observed only in juvenile animals.

Chronic fatigue stress leads to up‐regulation of nitric oxide synthase in the rat nucleus accumbens

In the present study, the effect of chronic fatigue stress on the neuronal nitric oxide synthase (nNOS) in the rat nucleus accumbens (NAc) was assessed in order to explore the neurobiology mechanism of central fatigue stress, especially the role of the NAc in central fatigue. In the experiment, adult male Wistar rats were forced to swim till exhaustion every day for 4 weeks. Immunohistochemistry was used for measurement of the nNOS-positive neurons associated with the image manipulation. Our results showed that a long-time exposure to chronic forced swim stress increased the number of nNOS immunoreactive-positive neurons in the rat NAc (106.7%, P<0.001), distributed area (150.2%, P<0.001) and gray degree value (11.3%, P<0.01). The results indicate that nitric oxide (NO)/nNOS may be involved in the formation of the fatigue stress, and that NO may play a role in the regulation of stress in the NAc. The up-regulation of NO/nNOS during the exposure to long-time swim stress is likely to be one of the results of brain damage and psychiatric disorder induced by NO overproduction.