Effects of adrenergic agents on stress-induced brain microstructural and immunochemical changes in adult male Wistar rats

This preliminary study aims to explore how adrenergic agents modulate stress response and affect stress-induced behavioral and brain changes in rodents. A total of 40 adult male Wistar rats were subjected to chronic unpredictable mild stress (CUMS) and randomly divided into five groups. At 30 min before daily stress exposure, the rats were intraperitoneally injected with phentolamine (5 mg/kg), noradrenalin (1.0 mg/kg), propranolol (10 mg/kg), isoproterenol (0.05 mg/kg) or saline, respectively. Another 8 rats served as normal control and received daily saline injection without stress exposure. Open-field behaviors were tested at 1 day after the end of the 21 days of stress exposure. Blood samples were collected for serum corticosterone measurement. Brain sections containing hippocampus were stained with hematoxylin and eosin (H&E) as well as by immunohistochemistry for heat shock protein 70 (hsp70) and nitric oxide synthase type 2 (nos2) analyses. The experimental results demonstrated that repetitive dosing of noradrenalin, phentolamine, and propranolol during chronic stress might region-dependently attenuate stress-induced microstructural and biochemical changes in the hippocampus, although propranolol intensified stress-induced behavioral changes.     

Anti-stress effects of cilnidipine and nimodipine in immobilization subjected mice

The present study was designed to investigate the ameliorative role of cilnidipine and nimodipine in immobilization stress-induced behavioral alterations and memory defects in the mice. Acute stress was induced by immobilizing the mice for 150 min and stress-induced behavioral changes were assessed using actophotometer, hole board, open field and social interaction tests. The learning and memory was evaluated using elevated plus maze tests and biochemically, the corticosterone levels were measured in the blood serum. Acute immobilization stress resulted in decrease in locomotor activity, frequency of head dips and rearings in hole board; line crossing and rearing in the open field; increase in avoidance in social behavior along with development of memory deficits assessed by an increased transfer latency time and elevation of the corticosterone levels. Administration of cilnidipine (10 mg/kg), an L and N-type dual calcium channel blocker, and nimodipine (10 mg/kg), an L-type calcium channel blocker, significantly attenuated the immobilized stress-induced behavioral changes and restored memory deficits along with normalization of the corticosterone levels. Cilnidipine and nimodipine produced comparable beneficial effects in restoring immobilization stress subjected mice. It may be concluded that cilnidipine and nimodipine mediated attenuation of corticosterone release by blockage of calcium channels (both L and N-type) on the HPA-axis is responsible for beneficial effects in restoration of behavioral alterations and memory deficits in immobilization-induced acute stress in mice.     

5-HT1A receptor activation counteracts c-Fos immunoreactivity induced in serotonin neurons of the raphe nuclei after immobilization stress in the male rat

The serotoninergic system and the 5-HT_1A receptors have been involved in the brain response to acute stress. The aim of our study was evaluate the role of the 5-HT_1A receptors in serotoninergic cells of rostral and caudal raphe nuclei under acute immobilization in rats. Double immunocytochemical staining of 5-hydroxy-tryptamine and c-Fos protein and stereology techniques were used to study the specific cell activation in the raphe nuclei neurons in five groups (control group, immobilization group (immobilization lasting 1 h), DPAT group (8-OH-DPAT 0.3 mg/kg, s.c.), DPAT + IMMO group (8-OH-DPAT 0.3 mg/kg, s.c., 30′ prior acute immobilization) and WAY + DPAT + IMMO group (WAY-100635 0.3 mg/kg, s.c. and 8-OH-DPAT 0.3 mg/kg, s.c., 45′ and 30′, respectively, before immobilization). Our results showed an increase in the number of c-Fos immunoreactive nuclei in serotoninergic cells in both dorsal and median raphe nuclei in the immobilized group. The 8-OH-DPAT pretreatment counteracted the excitatory effects of the acute immobilization in these brain regions. In addition, WAY-100635 administration reduced the effect of 8-OH-DPAT injection, suggesting a selective 5-HT_1A receptor role. Raphe pallidus and raphe obscurus did not show any differences among experimental groups. We suggest that somatodendritic 5-HT_1A receptors in rostral raphe nuclei may play a crucial role in both mediating the consequences of uncontrollable stress and the possible beneficial effects of treatment with 5-HT_1A receptor agonists.     

The mGlu2/3 receptor agonist LY354740 suppresses immobilization stress-induced increase in rat prefrontal cortical BDNF mRNA expression

Both a 5-hydroxytryptamine2A (5-HT2A) agonist and immobilization stress previously have been shown to differentially alter brain-derived neurotrophic factor (BDNF) mRNA expression in the neocortex and hippocampus. Both 5-HT2A receptor activation and immobilization stress also increase glutamate release in the rat prefrontal cortex. Given that the metabotropic glutamate2/3 receptor (mGluR2/3) agonist (1S,2S,5R,6S)-2-aminobicyclo[3.1.0] hexane-2,6-dicarboxylate monohydrate (LY354740) suppressed electrophysiological, behavioral and biochemical effects of 5-HT2A receptor activation in the medial prefrontal cortex (mPFC), we assessed the efficacy of the mGluR2/3 agonist in suppressing the stress-induced increase in BDNF mRNA expression. LY35740 (10 mg/kg, i.p.) attenuated the immobilization stress-induced increase in BDNF mRNA expression in the rat mPFC. This result is consistent with the hypothesis that mGlu2/3 agonists may be an efficacious treatment for stress-induced neuropsychiatric syndromes.     

Sub-chronic administration of rimonabant causes loss of antidepressive activity and decreases doublecortin immunoreactivity in the mouse hippocampus

Rimonabant is a cannabinoid receptor 1 antagonist, and is used to treat anorexia and obesity. However, it has been suggested that rimonabant may act as a depressant. In the present study, we investigated the depressive effects of rimonabant using behavioral and biochemical methods. A single treatment with rimonabant (10 mg/kg, p.o.) reduced immobility duration in the forced swimming test (FST) to a level similar to that observed for the tricyclic antidepressant, imipramine (15 mg/kg, i.p.). However, mice treated with rimonabant for 2 weeks did not show any significant reductions in immobility duration versus vehicle-treated controls. To investigate why the antidepressant effect of rimonabant disappeared after extended treatment, we carried out 5-bromo-2-deoxyuridine (BrdU) and doublecortin (DCX) immunohistochemistry assay. Numbers of BrdU-immunoreactive cells were not significantly changed after administering rimonabant (10 mg/kg, p.o.) for 2 weeks in the hippocampal dentate gyrus (DG), but interestingly, numbers of DCX-immunopositive cells in the DG were significantly reduced after 2 weeks of rimonabant treatment at doses of 1 or 10 mg/(kg day) compared with vehicle-treated controls (P < 0.05). These results suggest that sub-chronic treatments with rimonabant inhibit cell proliferation in DG, and that a lack of antidepressive activity may be related to a reduction in cell proliferation in this region.     

Interaction between behavioral despair and addictive behaviors in rats

It has been suggested that depression can be either a cause or a consequence of drug abuse, providing a possible explanation for the fact that the prevalence of depression is almost 3-fold higher in drug abusers than in the general population. However, the interaction between depression and drug abuse has not been fully elucidated. To examine the interaction between behavioral despair and addictive behaviors, we used the Porsolt's forced swim test (FST) as a model of behavioral despair, and we used morphine conditioned place preference (CPP) and repeated morphine exposure as models of addictive behaviors. We found that rats exposed to a standard FST (15 min on day 0 training) rather than a weak FST (10 min on day 0 training) exhibited behavioral despair, which selectively potentiated morphine CPP (mCPP) but not food CPP (fCPP). The antidepressant imipramine (15 mg/kg, i.p.), which blocked the behavioral despair, prevented the standard FST potentiated morphine CPP. Conversely, repeated exposure to morphine (10 mg/kg, s.c.) for 6, 12 or 20 days decreased, had no effect on, or increased the immobility time, respectively, in the subsequent standard FST. Furthermore, repeated morphine exposure for 20 days exacerbated the pre-existing behavioral despair. Thus, our findings suggest that behavioral despair may increase the vulnerability of individuals to opiate abuse, which may in turn enhance behavioral despair.     

Cognitive dysfunction and glutamate reuptake: effect of EAAT2 polymorphism in schizophrenia

Spinal noradrenaline is thought to play an important role in descending pain inhibitory pathways and the modulation of nociceptive information at the spinal level. Tapentadol is a μ-opioid receptor (MOR) agonist and noradrenaline reuptake inhibitor (NRI). We showed previously that tapentadol, in contrast to morphine, elevates levels of noradrenaline, but not serotonin, in the ventral hippocampus of rats. The aim of this study was to examine the effects of tapentadol, morphine and venlafaxine on spinal monoamine levels.Rats were implanted with spinal microdialysis probes. Drugs were administered intraperitoneally, and samples were collected for 3 h in isoflurane-anesthetized animals and analysed for monoamine content using HPLC-MS/MS.In terms of area-under-curve (AUC, 0-180 min), tapentadol (4.64-21.5 mg/kg) produced a dose-dependent, significant increase in extracellular spinal noradrenaline levels (9275 ± 4346 min % at the highest dose versus −1047 ± 889 min % for vehicle). A maximum increase of 182 ± 32% of baseline was reached 60 min after administration of 10 mg/kg tapentadol. Venlafaxine (10 mg/kg) produced an effect of similar magnitude. In contrast, tapentadol decreased extracellular spinal serotonin levels (non-significantly compared to vehicle), while venlafaxine increased spinal serotonin to 267 ± 74% of baseline.In contrast to tapentadol and venlafaxine, morphine slightly decreased levels of noradrenaline and serotonin.This study demonstrates that analgesic doses of tapentadol (and venlafaxine), but not morphine, increase spinal noradrenaline levels and that tapentadol is devoid of a relevant serotonergic effect. It supports the suggestion that the NRI component of tapentadol is functionally relevant and contributes to its mechanism of action.     

Orally administered L-ornithine reduces restraint stress-induced activation of the hypothalamic-pituitary-adrenal axis in mice

In a previous study, we confirmed that orally administered l-ornithine can be transported into the brain of mice. In addition, orally administered l-ornithine, within a limited dose range, had an anxiolytic-like effect in the elevated plus-maze test. However, the mechanism by which orally administered l-ornithine reduced the stress response in mice is still unclear. Experiment 1 determined whether orally administered l-ornithine could reduce the stress-induced activation of hypothalamic-pituitary-adrenal axis. Mice were orally administered l-ornithine (0, 0.75, 1.5 and 3 mmol/10 ml/kg, p.o.), and restrained for 30 min from 30 min post administration. There was a significant decrease in the corticosterone levels in the group receiving 0.75 mmol of l-ornithine compared to the control group. In Experiment 2, the effect of orally administered l-ornithine (0 and 0.75 mmol/10 ml/kg, p.o.) on endogenous monoamine release was investigated using in vivo microdialysis. Only the monoamines metabolites 5-hydroxyindoleacetic acid (5-HIAA), dihydroxyphenylacetic acid (DOPAC) and homovallinic acids (HVA) were detected in the present study. Dialysate concentrations of 5-HIAA, DOPAC and HVA were not significantly changed immediately after administration of l-ornithine and restraint stress. In conclusion, changes of corticosterone concentrations by orally administered l-ornithine were not related to alterations in brain monoamine metabolisms.     

Prenatal stress alters presynaptic marker proteins in the hippocampus of rat pups

Exposure to stress during critical periods of an organism's maturation can result in permanent behavioral changes and induced hyper-responsive to aversive stimuli as adult. Hippocampus is a plastic and vulnerable brain structure that is susceptible to damage during aging and repeated stress. The present study examines the effect of maternal restraint stress on the level of GAP-43, pGAP-43 and synaptophysin in the hippocampus of rat pups. Prenatal stress (PS) causes a significant increase of GAP-43 and pGAP-43 (p ≤ 0.001) in the pup's hippocampus during postnatal days 7 and 14, but not at later ages. Up-regulation of GAP-43 and pGAP-43 may alter the pattern of axonal growth and synapses’ formation in the pup's brain since the first two postnatal weeks are correlated with peak period of synaptogenesis in the rat brain. We also examined the level of synaptophysin, a synaptic vesicle membrane protein, in the pup's brain. Our finding revealed that, PS causes a significant decrease of synaptophysin in the pup's hippocampus as compared to control (p ≤ 0.001). These changes are due to the direct effects of maternal stress hormone since repeated injection with corticosterone (CORT, 40 mg/kg) to pregnant rat during gestation days (GDs) 14–21 also gave the same results. Abnormal axonal sprouting and reorganization together with the alterations in synaptic vesicle membrane protein during the critical period of synaptogenesis may lead to a defect in synapse formation and axonal pruning in the hippocampus. These changes may be associated with stress-induced impairment of hippocampal function that occurs in later life of the offspring.     

Involvement of nitric oxide system in enhancement of morphine-induced conditioned place preference by agmatine in male mice

Agmatine recently has been suggested as a neurotransmitter, is able to interact with various effects of morphine like analgesia and dependence. In this study, the effects of agmatine on rewarding properties of morphine, and the possible involvement of nitric oxide (NO) system has been evaluated in an unbiased conditioned place preference (CPP) paradigm. Agmatine (1, 5 and 10 mg/kg, i.p.) alone induced neither CPP nor conditioned place aversion (CPA). Morphine (0.01, 0.05, 0.1 and 0.5 mg/kg, s.c.), while unable to show CPP or CPA, induced CPP in mice pretreated with agmatine. L-arginine (200 mg/kg, i.p.), a NO precursor, significantly enhanced the effect of agmatine (5 mg/kg) on morphine (0.5 mg/kg)-induced place preference. N^G-nitro-l-arginine methyl ester (l-NAME; 2.5 mg/kg, i.p.), a non specific nitric oxide synthase (NOS) inhibitor, and aminoguanidine (50 and 100 mg/kg, i.p.), a specific inducible NOS inhibitor, significantly reduced the effect of agmatine (5 mg/kg) on morphine (0.5 mg/kg)-induced place preference. These results suggest the possible involvement of inducible nitric oxide system in potentiating effects of agmatine on morphine-induced place preference.     

Effect of nitric oxide synthase inhibitor and NMDA receptor antagonist on the development of nicotine sensitization of nucleus accumbens dopamine release: An in vivo microdialysis study

We have previously found that the neuronal nitric oxide synthase inhibitor N-nitro-l-arginine (l-NNA) and the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 prevent behavioral sensitization to nicotine. This study aimed to investigate the effect of l-NNA and MK-801 on a neurochemical component of nicotine sensitization by evaluating the effect of the drugs on nicotine sensitization of nucleus accumbens dopamine (DA) release. Sprague–Dawley rats were pretreated with l-NNA (15 mg/kg, i.p.), MK-801 (0.3 mg/kg, i.p.), or saline 30 min before injection of nicotine (0.4 mg/kg, s.c., once daily) for seven consecutive days. Twenty-four hours after the last drug injection, animals were challenged with local perfusion of 5 mM nicotine into the shell of nucleus accumbens for 60 min and DA release was monitored using in vivo microdialysis. In rats treated with repeated nicotine, acute nicotine challenge induced a greater increase of accumbal DA release than in saline-treated animals (maximal DA response = 969 ± 235% (mean ± S.E.M.) of basal level versus 520 ± 93%, p = 0.042). Co-administration of l-NNA or MK-801 with nicotine attenuated an increase of DA release elicited by acute nicotine challenge, compared with nicotine alone (maximal DA response = 293 ± 58% and 445 ± 90% of basal level, respectively versus 969 ± 235%, p = 0.004 and p = 0.013, respectively). These data demonstrate that l-NNA and MK-801 block the development of nicotine sensitization of nucleus accumbens DA release, further supporting the involvement of nitric oxide and NMDA receptors in the development of behavioral sensitization to nicotine.     

Nimesulide,a cyclooxygenase-2 (COX-2) inhibitor counteracts immobilization stress-induced alterations in different behavioral and biochemical parameters in mice

There are reports regarding the up-regulation of cyclooxygenase isoenzyme particularly inducible isoform i.e. COX-2 in brain during neurodegenerative or neuropsychiatric disorders. In the present study, we examined the effect of nimesulide (a preferential COX-2 inhibitor) in subchronic immobilization stress. Mice were subjected to immobilization stress for 6 hrs daily for a period of seven days. Nimesulide (2.5 mg/kg, i.p.) was administered daily for 7 days before challenging them to immobilization stress. Behavioral analysis revealed the hyperlocomotor activity and increased anxious response. Subchronic stress decreased % retention of memory and also caused hyperalgesic response in mice. Biochemical analysis revealed that chronic immobilization stress significantly increased lipid peroxidation and nitrite levels and decreased the reduced glutathione and adrenal ascorbic acid levels. Chronic treatment with nimesulide significantly attenuated the immobilization stress-induced behavioral and biochemical alterations. These results suggested that the use of nimesulide could be a useful neuroprotective strategy in the treatment of stress.     

Effect of salidroside, active principle of Rhodiola rosea extract, on binge eating

Stress is a key determinant of binge eating (BE). Since Rhodiola rosea is known to modulate stress responses, its effect in a model of BE was investigated.BE for highly palatable food (HPF) was evoked in female rats by three 8-day cycles of food restriction/re-feeding (for 4 days 66% of the usual chow intake; for 4 days food ad libitum) and acute stress on the test day (day 25). R. rosea dry extract (3% rosavin, 3.12% salidroside) or its active principles were given by gavage 1 h before access to HPF. Only rats exposed to both food restrictions and stress exhibited BE in the first 15-60 min after the stressful procedure. R. rosea extract 10 mg/kg significantly reduced and 20 mg/kg abolished the BE episode. R. rosea extract 20 mg/kg abolished also stress-induced increase in serum corticosterone levels. The R. rosea active principle salidroside, but not rosavin, at doses present in the extract, dose-dependently reduced or abolished BE for the period in which it was elicited.In conclusion results indicate that R. rosea extracts may have therapeutic properties in bingeing-related eating disorders and that salidroside is the active principle responsible for this effect.     

Effects of glyprolines on stress-induced behavioral disorders in rats

We report here studies on the antistress protective actions of three peptides of the glyproline family: Pro-Gly-Pro, Pro-Gly, and Gly-Pro. Stress (10 min forced swimming) evoked typical changes in the behavioral activity of rats in the elevated cross maze and hole board tests, providing evidence of a significant increase in anxiety and a decrease in the level of orientational-investigative activity. Prior (15 min before stress) i.p. administration of Pro-Gly-Pro and Gly-Pro at a dose of 3.7 μM/kg significantly decreased the stress-induced behavioral abnormalities. This demonstrates the possibility that peptides Pro-Gly-Pro and Gly-Pro may affect CNS structures involved in forming the body’s responses to stress-inducing factors. Peptide Pro-Gly, at an equimolar dose, had no marked protective effect and only slightly decreased the stress-induced abnormalities in the behavior of rats. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 5, pp. 543–550, May, 2205.     

An ERP study on the time course of facial trustworthiness appraisal

The neuropeptide galanin has been recognized as a possible neurotransmitter/neuromodulator, and in addition has been implicated in anxiety- and depression-related behaviors. The present study demonstrates increased locomotion and rearing after galanin (0.3 mg/kg) that was given intraperitoneally (i.p.) to intact Wistar rats which were tested 1 h later in the open field (OF). These effects, which suggest an anxiolytic-like action, were blocked by i.p. administered peptidic galanin antagonist M40. Further, the locomotion increase caused by galanin and the inhibitory effect of M40 persisted for 48 h without additional treatment. Rats exposed to restraint stress (lasting 60 min) for three consecutive days and tested 1 h after stress termination exhibited reduced locomotion and exploration in the OF. Galanin (0.3 and 1.0 mg/kg) given immediately after each stress exposure prevented the decrease of locomotion and exploration induced by stress in all trials. When the test was repeated 6 days later without stress and galanin treatment the reduction of locomotion produced by stress persisted; the anti-stress behavioral effects of both galanin doses were also present. Testing performed on the 12th day after the last stress and galanin treatment with 0.3 mg/kg revealed an increased locomotion compared with unstressed and stress-exposed rats. Our results demonstrate that behavioral effects of the peptide galanin are evident even after i.p. administration. These results also suggest that galanin elicits stress-modulatory action, and support the notion that the galaninergic system may serve as a drug target in stress-related conditions.     

Protective effect of alprazolam against sleep deprivation-induced behavior alterations and oxidative damage in mice

Sleep deprivation is considered as a risk factor for various diseases. Sleep deprivation leads to behavioral, hormonal, neurochemical and biochemical alterations in the animals. The present study was designed to explore the possible involvement of GABAergic mechanism in protective effect of alprazolam against 72 h sleep deprivation-induced behavior alterations and oxidative damage in mice. In the present study, sleep deprivation caused anxiety-like behavior, weight loss, impaired ambulatory movements and oxidative damage as indicated by increase in lipid peroxidation, nitrite level and depletion of reduced glutathione and catalase activity in sleep-deprived mice brain. Treatment with alprazolam (0.25 and 0.5 mg/kg, ip) significantly improved behavioral alterations. Biochemically, alprazolam treatment significantly restored depleted reduced glutathione, catalase activity, reversed raised lipid peroxidation and nitrite level. Combination of flumazenil (0.5 mg/kg) and picrotoxin (0.5 mg/kg) with lower dose of alprazolam (0.25 mg/kg) significantly antagonized protective effect of alprazolam. However, combination of muscimol (0.05 mg/kg) with alprazolam (0.25 mg/kg, ip) potentiated protective effect of alprazolam. On the basis of these results, it might be suggested that alprazolam might produce protective effect by involving GABAergic system against sleep deprivation-induced behavior alterations and related oxidative damage.     

Assessment of endogenous opioid mediation in stress-induced hypercholesterolemia in the rat

Stressful stimuli are known to elevate total plasma cholesterol levels and activate endogenous opioid systems. In cholesterol-cholic, acid-fed female rats, a 5-day unpredictable immobilization stress paradigm increased levels of low plus very low density lipoprotein cholesterol and reduced levels of high density lipoprotein cholesterol. Pretreatment with the opiate antagonist, naltrexone (1.0 mg/kg, sc) prevented the stress-induced changes. Five-day morphine pellet implants (75 mg) duplicated the cholesterol alterations seen in immobilized rats. In addition, plasma triglyceride levels were elevated in morphine pelleted rats, but hepatic and aortic cholesterol levels were unchanged. Unchanged plasma sorbitol dehydrogenase activity and hepatic triglyceride levels indicated that both regimens were not hepatotoxic. These findings support the possible role for endogenous opioid systems in stress-induced hypercholesterolemia.     

Effects of antipsychotic drugs on BDNF, GSK-3β, and β-catenin expression in rats subjected to immobilization stress

Brain-derived neurotrophic factor (BDNF), glycogen synthase kinase-3β (GSK-3β), and β-catenin have been reported to be altered in patients with schizophrenia and have been targeted by antipsychotic drugs. Atypical antipsychotics, but not typical antipsychotics, exert neuroprotective effects by regulating these proteins. In this study, we analyzed the effects of the atypical antipsychotic drugs olanzapine and aripiprazole and a typical antipsychotic drug, haloperidol, on the expression of BDNF, phosphorylated GSK-3β, and β-catenin in the hippocampus of rats subjected to immobilization stress. Rats were subjected to immobilization stress 6 h/day for 3 weeks. The effects of olanzapine (2 mg/kg), aripiprazole (1.5 mg/kg), and haloperidol (1.0 mg/kg) were determined on BDNF, serine⁹-phosphorylated GSK-3β, and β-catenin expression by Western blotting. Immobilization stress significantly decreased the expression of BDNF, phosphorylated GSK-3β, and β-catenin in the hippocampus. Chronic administration of olanzapine and aripiprazole significantly attenuated the decreased expression of these proteins in the hippocampus of rats caused by immobilization stress, and significantly increased the levels of these proteins even without the immobilization stress. However, chronic haloperidol had no such effect. These results suggest that olanzapine and aripiprazole may exert beneficial effects by upregulating BDNF, phosphorylated GSK-3β, and β-catenin in patients with schizophrenia.     

Juvenile stress impairs body temperature regulation and augments anticipatory stress-induced hyperthermia responses in rats

Clinical studies have implicated adolescence as an important and vulnerable period during which traumatic experiences can predispose individuals to anxiety and mood disorders. As such, a stress model in juvenile rats (age 27-29 d) was previously developed to investigate the long-term effects of stress exposure during adolescence on behavior and physiology. This paradigm involves exposing rats to different stressors on consecutive days over a 3-day period. Here, we studied the effects of juvenile stress on long-term core body temperature regulation and acute stress-induced hyperthermia (SIH) responses using telemetry. We found no differences between control and juvenile stress rats in anxiety-related behavior on the elevated plus maze, which we attribute to stress associated with surgical implantation of telemetry devices. This highlights the severe impact of surgical stress on the results of subsequent behavioral measurements. Nonetheless, juvenile stress disrupted the circadian rhythmicity of body temperature and decreased circadian amplitude. It also induced chronic hypothermia during the dark phase of the day, when rats are most active. When subjected to acute social defeat stress as adults, juvenile stress had no impact on the SIH response relative to controls. However, 24 h later, juvenile stress rats displayed an elevated SIH response in anticipation of social defeat when re-exposed to the social defeat environment. Taken together, our findings indicate that juvenile stress can induce long-term alterations in body temperature regulation and heighten the increase in temperature associated with anticipation of social defeat. The outcomes of behavioral measurements in these experiments, however, are severely affected by surgical stress.     

Opioid modulation of reflex versus operant responses following stress in the rat

In pre-clinical models intended to evaluate nociceptive processing, acute stress suppresses reflex responses to thermal stimulation, an effect previously described as stress-induced "analgesia." Suggestions that endogenous opioids mediate this effect are based on demonstrations that stress-induced hyporeflexia is enhanced by high dose morphine (>5 mg/kg) and is reversed by naloxone. However, reflexes and pain sensations can be modulated differentially. Therefore, in the present study direct comparisons were made of opioid agonist and antagonist actions, independently and in combination with acute restraint stress in Long Evans rats, on reflex lick-guard (L/G) and operant escape responses to nociceptive thermal stimulation (44.5 degrees C). A high dose of morphine (>8 mg/kg) was required to reduce reflex responding, but a moderate dose of morphine (1 mg/kg) significantly reduced escape responding. The same moderate dose (and also 5 mg/kg) of morphine significantly enhanced reflex responding. Naloxone (3 mg/kg) significantly enhanced escape responding but did not affect L/G responding. Restraint stress significantly suppressed L/G reflexes (hyporeflexia) but enhanced escape responses (hyperalgesia). Stress-induced hyperalgesia was significantly reduced by morphine and enhanced by naloxone. In contrast, stress-induced hyporeflexia was blocked by both naloxone and 1 mg/kg of morphine. Thus, stress-induced hyperalgesia was opposed by endogenous opioid release and by administration of morphine. Stress-induced hyporeflexia was dependent upon endogenous opioid release but was counteracted by a moderate dose of morphine. These data demonstrate a differential modulation of reflex and operant outcome measures by stress and by separate or combined opioid antagonism or administration of morphine.