Effects of chronic restraint stress on feeding behavior and on monoamine levels in different brain structures in rats

Effects of 5,7-dihydroxytryptamine lesion of the dorsal raphe nucleus on the antidepressant-like action of tramadol in the unpredictable chronic mild stress in mice

RATIONALE

Tramadol is a centrally acting clinically effective analgesic, with a weak opioid receptor affinity. It shows antidepressant-like effects in animal models such as forced swimming test, learned helplessness, and unpredictable chronic mild stress (UCMS) and enhances the concentrations of noradrenaline (NA) and serotonin (5-HT) by interfering with their reuptake and release mechanisms, like some antidepressants.

OBJECTIVES

The aim of this study was to explore whether the antidepressant-like effects of tramadol is affected by the serotonergic system. For this purpose, the effects of a lesion of the dorsal raphe nucleus (DRN) by 5,7-dihydroxytryptamine (5,7-DHT) on the action of tramadol (20 mg/kg, i.p.) on depression-related behavior and neurochemical correlates were investigated in mice. From the third week onward, we administered tramadol chronically during 4 weeks.

RESULTS

Tramadol reversed the physical and behavioral abnormalities induced by the UCMS. Furthermore, the lesion of the DRN by 5,7-DHT antagonized the antidepressant-like effects of tramadol on the coat state, in the splash test but not in the resident-intruder test. The results obtained by high-pressure liquid chromatography showed that the level of 5-HT was reduced by the lesion in some brain regions without affecting the level of NA. Moreover, while the UCMS regimen diminished the level of 5-HT, tramadol increased the level of this neurotransmitter in certain regions.

CONCLUSIONS

These results seem to indicate that the serotonergic system is critically involved in the antidepressant-like effects of tramadol in the UCMS in mice.

Effects of repeated citalopram treatments on chronic mild stress-induced growth associated protein-43 mRNA expression in rat hippocampus

Although growth associated protein-43 (GAP-43) is known to play a significant role in the regulation of axonal growth and the formation of new neuronal connections in the hippocampus, there is only a few studies on the effects of acute stress on GAP-43 mRNA expression in the hippocampus. Moreover, the effects of repeated citalopram treatment on chronic mild stress (CMS)-induced changes in GAP-43 mRNA expression in the hippocampus have not been explored before. To explore this question, male rats were exposed to acute immobilization stress or CMS. Also, citalopram was given prior to stress everyday during CMS procedures. Acute immobilization stress significantly increased GAP-43 mRNA expression in all subfields of the hippocampus, while CMS significantly decreased GAP-43 mRNA expression in the dentate granule cell layer (GCL). Repeated citalopram treatment decreased GAP-43 mRNA expression in the GCL compared with unstressed controls, but this decrease was not further potentiated by CMS exposure. Similar decreases in GAP-43 mRNA expression were observed in CA1, CA3 and CA4 areas of the hippocampus only after repeated citalopram treatment in CMS-exposed rats. This result indicates that GAP-43 mRNA expression in the hippocampus may differently respond to acute and chronic stress, and that repeated citalopram treatment does not change CMS-induced decreases in GAP-43 mRNA expression in the GCL.

Antidepressant-like behavioral and neurochemical effects of the citrus-associated chemical apigenin

Apigenin is one type of bioflavonoid widely found in citrus fruits, which possesses a variety of pharmacological actions on the central nervous system. A previous study showed that acute intraperitoneal administration of apigenin had antidepressant-like effects in the forced swimming test (FST) in ddY mice. To better understand its pharmacological activity, we investigated the behavioral effects of chronic oral apigenin treatment in the FST in male ICR mice and male Wistar rats exposed to chronic mild stress (CMS). The effects of apigenin on central monoaminergic neurotransmitter systems, the hypothalamic-pituitary-adrenal (HPA) axis and platelet adenylyl cyclase activity were simultaneously examined in the CMS rats. Apigenin reduced immobility time in the mouse FST and reversed CMS-induced decrease in sucrose intake of rats. Apigenin also attenuated CMS-induced alterations in serotonin (5-HT), its metabolite 5-hydroxyindoleacetic acid (5-HIAA), dopamine (DA) levels and 5-HIAA/5-HT ratio in distinct rat brain regions. Moreover, apigenin reversed CMS-induced elevation in serum corticosterone concentrations and reduction in platelet adenylyl cyclase activity in rats. These results suggest that the antidepressant-like actions of oral apigenin treatment could be related to a combination of multiple biochemical effects, and might help to elucidate its mechanisms of action that are involved in normalization of stress-induced changes in brain monoamine levels, the HPA axis, and the platelet adenylyl cyclase activity.

Chronic stress and neural function: accounting for sex and age

Cognitive responses to stress follow the temporally dependent pattern originally established by Selye (1) wherein short-term stressors elicit adaptive responses whereas continued stress (chronic) results in maladaptive changes--deleterious effects on physiological systems and impaired cognition. However, this pattern for cognitive effects appears to apply to only half the population (males) and, more specifically, to young, adult males. Females show different cognitive responses to stress. In contrast to impaired cognition in males after chronic stress, female rodents show enhanced performance on the same memory tasks after the same stress. Not only cognition, but anxiety, shows sex-dependent changes following chronic stress--stress is anxiolytic in males and anxiogenic in females. Moreover, behavioral responses to chronic stress are different in developing as well as aging subjects (both sexes) as compared to adults. In aged rats, chronic stress enhances recognition memory in both sexes, does not alter spatial memory, and anxiety effects are opposite to young adults. When pregnant dams are exposed to chronic stress, at adulthood the offspring display yet different consequences of stress on anxiety and cognition, and, in contrast to adulthood when the behavioral effects of stress are reversible, prenatal stress effects appear enduring. Changing levels of estradiol in the sexes over the lifespan appear to contribute to the differences in response to stress. Thus, theories of stress dependent modulations in CNS function--developed solely in male models, focused on peripheral physiological processes and tested in adults--may require revision when applied to a more diverse population (age- and sex-wise) at least in relation to the neural functions of cognition and anxiety. Moreover, these results suggest that other stressors and neural functions should be investigated to determine whether age, sex and gonadal hormones also have an impact.

Contrasting effects of bromocriptine on learning of a partially baited radial arm maze task in the presence and absence of restraint stress

RATIONALE

Severe, traumatic stress or repeated exposure to stress can result in long-term deleterious effects, including hippocampal cell atrophy and death, which, in turn, result in memory impairments and behavioural abnormalities. The dopaminergic D(2) receptor agonist, bromocriptine, has been shown to modulate learning, and chronic stress is associated with dopaminergic dysfunction.

OBJECTIVES

In the present study, we evaluated the effects of bromocriptine in the presence or absence of restraint stress.

MATERIALS AND METHODS

Adult male Wistar rats were subjected to restraint stress for 21 days (6 h/day) followed by bromocriptine treatment, and learning was assessed in the partially baited radial arm maze task. In a separate group of animals, the effects of bromocriptine per se was evaluated. Dopamine levels were estimated by high-performance liquid chromatography with electrochemical detection.

RESULTS

Stressed rats showed impairment in both acquisition and retention of the radial arm maze task, and bromocriptine treatment after stress showed a reversal of stress-induced impairment. Interestingly, in the absence of stress, bromocriptine exhibited dose-dependent differential effects on learning. While rats treated with bromocriptine 5 mg/kg, i.p., demonstrated impairment in learning, the bromocriptine 10 mg/kg and vehicle-treated groups did not differ from normal controls. To understand the neurochemical basis for the effects of bromocriptine, dopamine levels were estimated. The stress-induced decrease in dopamine levels in the hippocampus and frontal cortex were restored by bromocriptine treatment. In contrast, bromocriptine alone (5 mg/kg, i.p.) decreased dopamine levels in the frontal cortex and striatum.

CONCLUSIONS

Our study shows that amelioration of stress-induced learning impairment correlates with restoration of dopamine levels by bromocriptine treatment.

Social stress during adolescence in Wistar rats induces social anxiety in adulthood without affecting brain monoaminergic content and activity

Adolescence has been described as an important period to acquire social competences required for adult life. It has been suggested that early stress experiences could affect the development of the brain at different levels. These changes in the brain during adolescence may be related with the development of psychopathologies such as depression and social anxiety in adulthood. In the first experiment, we examined long-term effects of repeated social stress during adolescence on adult social approach-avoidance behavior. For that purpose, adolescent male Wistar rats were exposed twice at postnatal day (Pnd) 45 and Pnd48 to the resident-intruder paradigm followed by three times psychosocial threat with the same resident. Three weeks after the last psychosocial threat experience the animals were behaviorally tested in a social approach-avoidance test. Socially stressed animals spent less time in the interaction zone with an unfamiliar male adult rat. These data suggest that animals exposed to social stress during adolescence show a higher level of social anxiety in adulthood. In the second experiment, we investigated whether these long-term effects of social stress during adolescence on behavior draw a parallel with changes in brain monoamine content, biosynthesis and turnover. Using the same experimental design as in the first experiment, HPLC analysis of various brain regions showed that there were no differences in monoamine content, monoamine biosynthesis and monoamines activity in the prefrontal cortex, hippocampus, hypothalamus and striatum in adulthood. These results indicate that long-lasting changes in social behavior following social stress during adolescence are not accompanied by changes in brain monoamine content, biosynthesis and turnover.

Melanin-concentrating hormone (MCH) reverts the behavioral effects induced by inescapable stress

The aim of this work was to investigate if MCH modifies the feeding and freezing responses in rats exposed to stressful stimuli. We used a basic version of contextual fear, where one group of rats were placed in a novel environment and two different groups were exposed to footshock paradigms, one of them escapable and the other one inescapable. At the end of each treatment, freezing and feeding were measured. Only the animals exposed to inescapable footshock paradigm showed significant increase in the food intake and freezing behavior in comparison to the control animals. The MCH administration (intra-hippocampal or intra-amygdaline) reverted these effects elicited by inescapable footshock. Results presented in this paper lead us to the assumption that the anxiolytic effect of the peptide is responsible for the reversion of the IS effects.

Effects of stress on catecholamine stores in central and peripheral tissues of long-term socially isolated rats

Both the peripheral sympatho-adrenomedullary and central catecholaminergic systems are activated by various psycho-social and physical stressors. Catecholamine stores in the hypothalamus, hippocampus, adrenal glands, and heart auricles of long-term socially isolated (21 days) and control 3-month-old male Wistar rats, as well as their response to immobilization of all 4 limbs and head fixed for 2 h and cold stress (4 degrees C, 2 h), were studied. A simultaneous single isotope radioenzymatic assay based on the conversion of catecholamines to the corresponding O-methylated derivatives by catechol-O-methyl-transferase in the presence of S-adenosyl-l-(3H-methyl)-methionine was used. The O-methylated derivatives were oxidized to 3H-vanilline and the radioactivity measured. Social isolation produced depletion of hypothalamic norepinephrine (about 18%) and hippocampal dopamine (about 20%) stores and no changes in peripheral tissues. Immobilization decreased catecholamine stores (approximately 39%) in central and peripheral tissues of control animals. However, in socially isolated rats, these reductions were observed only in the hippocampus and peripheral tissues. Cold did not affect hypothalamic catecholamine stores but reduced hippocampal dopamine (about 20%) as well as norepinephrine stores in peripheral tissues both in control and socially isolated rats, while epinephrine levels were unchanged. Thus, immobilization was more efficient in reducing catecholamine stores in control and chronically isolated rats compared to cold stress. The differences in rearing conditions appear to influence the response of adult animals to additional stress. In addition, the influence of previous exposure to a stressor on catecholaminergic activity in the brainstem depends on both the particular catecholaminergic area studied and the properties of additional acute stress. Therefore, the sensitivity of the catecholaminergic system to habituation appears to be tissue-specific.

Cortical/hippocampal monoamines, HPA-axis changes and aversive behavior following stress and restress in an animal model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is characterized by monoaminergic and hypothalamic-pituitary-adrenal (HPA)-axis abnormalities. Understanding monoamine-HPA-axis responses following stress and restress may provide a greater understanding of the neurobiology of PTSD and of its treatment. Hippocampal and frontal cortex serotonin, noradrenaline and dopamine, plasma corticosterone and aversive behavior were studied in rats on day 1 and day 7 post acute stress (AS = sequential restraint stress, swim stress and halothane exposure), and on day 1 and day 7 post restress (RS = swim stress). After AS, there was an early increase in both avoidant behavior and corticosterone (1 h after stress), with subsequent normalisation (day 7), suggesting an adequate adaptive response to the stressor. However, restress (RS) evoked a significant early HPA-axis hyporesponsiveness (1 h after RS) and a later significant increase in avoidant behavior on day 7 post RS. Hippocampal serotonin, noradrenaline and dopamine concentrations were unchanged 1 h post AS, but were significantly raised on day 7 post AS. Restress, however, reduced serotonin and noradrenaline levels 1 h after and on day 7 post RS, respectively, while dopamine was unchanged. In the frontal cortex only dopamine levels were altered, being significantly elevated 1 h after AS, and reduced on day 7 post RS. AS and RS thus differently effect the HPA-axis, evoking regional-specific brain monoamine changes that underlie maladaptive behavior and other post stress-related sequelae.

Selective serotonin depletion does not regulate hippocampal neurogenesis in the adult rat brain: differential effects of p-chlorophenylalanine and 5,7-dihydroxytryptamine

Serotonin is suggested to regulate adult hippocampal neurogenesis, and previous studies with serotonin depletion reported either a decrease or no change in adult hippocampal progenitor proliferation. We have addressed the effects of serotonin depletion on distinct aspects of adult hippocampal neurogenesis, namely the proliferation, survival and terminal differentiation of hippocampal progenitors. We used the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) or the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to deplete serotonin levels. 5,7-DHT selectively decreased hippocampal serotonin levels, while PCPA resulted in a significant decline in both serotonin and norepinephrine levels. We observed a robust decline in the proliferation and survival of adult hippocampal progenitors following PCPA treatment. This was supported by a decrease in the number of doublecortin-positive cells in the neurogenic niche in the hippocampus. In striking contrast, 5,7-DHT did not alter the proliferation or survival of adult hippocampal progenitors and did not alter the number of doublecortin-positive cells. The terminal differentiation of adult hippocampal progenitors was not altered by either PCPA or 5,7-DHT treatment. An acute increase in serotonin levels also did not influence adult hippocampal progenitor proliferation. These results suggest that selective serotonin depletion or an acute induction in serotonin levels does not regulate adult hippocampal neurogenesis, whereas treatment with PCPA that induces a decline in both serotonin and norepinephrine levels results in a significant decrease in adult hippocampal neurogenesis. Our results highlight the need for future studies to examine the role of other monoamines in both the effects of stress and antidepressants on adult hippocampal neurogenesis.

Quetiapine reverses the suppression of hippocampal neurogenesis caused by repeated restraint stress

Quetiapine is an atypical antipsychotic effective in treating the positive, negative, and cognitive symptoms of patients with schizophrenia. Our previous study has shown that chronic administration of quetiapine attenuates the decrease in levels of brain-derived neurotrophic factor (BDNF) in the hippocampi of rats subjected to chronic-restraint stress. In the present study, we investigated the effects of quetiapine on hippocampal neurogenesis that had been compromised in stressed rats. Newborn cells in the hippocampus were labeled by bromodeoxyuridine (BrdU), and immature neurons were detected immunohistochemically using an antibody against phosphorylated cAMP response element-binding protein (pCREB). The restrained rats (4 h/day for 7 days) showed lower levels of hippocampal neurogenesis indicated by decreased numbers of BrdU-labeled and pCREB-positive cells. Post-stress administration of quetiapine (10 mg/kg) for 7 or 21 days reversed the stress-induced suppression of hippocampal neurogenesis, evidenced in the numbers of BrdU-labeled and pCREB-positive cells that are comparable to those in non-stressed rats but higher than those in the vehicle-treated rats. The results may help us understand the therapeutic effects of quetiapine on cognitive deficits in patients with schizophrenia and depression, in which the structure and functions of the hippocampus are implicated.

Novel stressors affected catecholamine stores in socially isolated normotensive and spontaneously hypertensive rats

Catecholamines in some central (hypothalamus and hippocampus) and peripheral tissues (adrenal glands and heart auricles) of long-term socially isolated normotensive and spontaneously hypertensive rats exposed to novel immobilization stress were determined by a simultaneous single isotope radioenzymatic assay. Long-term isolation (21 days) produced depletion of hypothalamic norepinephrine (NE) stores and hippocampal dopamine (DA) stores in both normotensive and spontaneously hypertensive rats. Acute immobilization stress (2 h) significantly decreased NE and DA stores in hypothalamus and hippocampus of naive normotensive and spontaneously hypertensive rats controls. However, novel immobilization stress applied to normotensive rats previously subjected to long-term isolation produced no changes in catecholamine levels in hypothalamus, while resulting in somewhat higher depletion of NE stores in hypothalamus of spontaneously hypertensive rats treated in the same way. Novel immobilization stress decreased NE and DA stores in hippocampus of normotensive but was without effect on NE and DA stores of spontaneously hypertensive rats. Social isolation did not affect catecholamine stores in peripheral tissues but novel immobilization stress produced a significant decrease in catecholamine content. The results suggest that some central and peripherals tissues of spontaneously hypertensive rats and normotensive rats differ with regard to catecholamine content and that there are certain differences in their responsiveness to stress.

Does Serotonin Influence Aggression? Comparing Regional Activity before and during Social Interaction

Serotonin is widely believed to exert inhibitory control over aggressive behavior and intent. In addition, a number of studies of fish, reptiles, and mammals, including the lizard Anolis carolinensis, have demonstrated that serotonergic activity is stimulated by aggressive social interaction in both dominant and subordinate males. As serotonergic activity does not appear to inhibit agonistic behavior during combative social interaction, we investigated the possibility that the negative correlation between serotonergic activity and aggression exists before aggressive behavior begins. To do this, putatively dominant and more aggressive males were determined by their speed overcoming stress (latency to feeding after capture) and their celerity to court females. Serotonergic activities before aggression are differentiated by social rank in a region-specific manner. Among aggressive males baseline serotonergic activity is lower in the septum, nucleus accumbens, striatum, medial amygdala, anterior hypothalamus, raphe, and locus ceruleus but not in the hippocampus, lateral amygdala, preoptic area, substantia nigra, or ventral tegmental area. However, in regions such as the nucleus accumbens, where low serotonergic activity may help promote aggression, agonistic behavior also stimulates the greatest rise in serotonergic activity among the most aggressive males, most likely as a result of the stress associated with social interaction.

Behavioural and neurochemical effects induced by chronic mild stress applied to two different rat strains

Chronic mild stress (CMS) has been reported to induce an anhedonic-like state in rats that resembles some of the symptoms of endogenous depression in humans. In the present study, CMS-induced behavioural responses along with neurochemical alterations in dopaminergic and serotonergic function in prefrontal cortex, striatum, hypothalamus and hippocampus were examined following treatment with imipramine in Wistar and Sprague-Dawley rats. The CMS procedure lasted 7 weeks in total. Once per week, a 1-h preference test for 1% sucrose solution was conducted. Treatment with imipramine (10mg/kg i.p., once daily) commenced after experimental week 3. CMS induced significant reductions in absolute and relative sucrose intake and sucrose preference in both rat strains but their temporal pattern was different especially during the weeks 0-3. These effects were reversed by IMI. An increase in the dopaminergic and a decrease in the serotonergic activity were observed in the prefrontal cortex in both rat strains following CMS. A decrease in the striatal dopaminergic activity and an increased hippocampal serotonergic activity were also seen in both rat strains following CMS. In Wistar rats, dopaminergic and serotonergic activities were enhanced in the hypothalamus whereas in Sprague-Dawley rats no such stress-induced changes were observed. Notably, the clear decrease in sucrose consumption observed in stressed Wistar rats could be directly associated with a respective increase in the dopaminergic hypothalamic activity. Chronic treatment with imipramine normalized all neurochemical alterations induced by CMS. Our results suggest that a specific and regionally differentiated serotonin-dopamine interaction is directly related to the observed stress-induced anhedonia.

Alterations in brain antioxidant status, protein oxidation and lipid peroxidation in response to different stress models

The aim of this study was to investigate the effects of different stress models on copper, zinc-superoxide dismutase (Cu,Zn-SOD), catalase (CAT) and selenium-dependent glutathione peroxidase (Se-GSH-Px) activities, and reduced glutathione (GSH), protein carbonyl (PC) and lipid peroxidation marker (conjugated diene (CD) and thiobarbituric acid-reactive substances (TBARS)) levels in brain of rats, and to determine the most effective stress model according to each parameter. Rats were divided into four groups as following: control group (C), immobilization stress group (IS), cold stress group (CS) and immobilization-cold stress group (ICS). All stress models increased brain Cu,Zn-SOD and CAT activities, PC, CD and TBARS levels, plasma corticosterone levels and decreased brain GSH concentrations. Se-GSH-Px activity was increased in CS and ICS groups. When all stress models were taken into consideration, the highest increases in Cu,Zn-SOD and Se-GSH-Px activities were found in CS group. The lowest GSH level was seen in IS group. The highest increases in PC and TBARS levels were found in ICS group. The highest increase of CD concentration was seen in IS and ICS groups. Our results suggest that different stress models have different degrees of influences on enzymatic and non-enzymatic antioxidant defense systems, protein oxidation and lipid peroxidation in the brain.

Neuroendocrine pharmacology of stress

Exposure to hostile conditions initiates responses organized to enhance the probability of survival. These coordinated responses, known as stress responses, are composed of alterations in behavior, autonomic function and the secretion of multiple hormones. The activation of the renin-angiotensin system and the hypothalamic-pituitary-adrenocortical axis plays a pivotal role in the stress response. Neuroendocrine components activated by stressors include the increased secretion of epinephrine and norepinephrine from the sympathetic nervous system and adrenal medulla, the release of corticotropin-releasing factor (CRF) and vasopressin from parvicellular neurons into the portal circulation, and seconds later, the secretion of pituitary adrenocorticotropin (ACTH), leading to secretion of glucocorticoids by the adrenal gland. Corticotropin-releasing factor coordinates the endocrine, autonomic, behavioral and immune responses to stress and also acts as a neurotransmitter or neuromodulator in the amygdala, dorsal raphe nucleus, hippocampus and locus coeruleus, to integrate brain multi-system responses to stress. This review discussed the role of classical mediators of the stress response, such as corticotropin-releasing factor, vasopressin, serotonin (5-hydroxytryptamine or 5-HT) and catecholamines. Also discussed are the roles of other neuropeptides/neuromodulators involved in the stress response that have previously received little attention, such as substance P, vasoactive intestinal polypeptide, neuropeptide Y and cholecystokinin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABA(A), histamine and serotonin receptors have been used to attenuate the neuroendocrine response to stressors. The neuroendocrine information for these drugs is still incomplete; however, they are a new class of potential antidepressant and anxiolytic drugs that offer new therapeutic approaches to treating anxiety disorders. The studies described in this review suggest that multiple brain mechanisms are responsible for the regulation of each hormone and that not all hormones are regulated by the same neural circuits. In particular, the renin-angiotensin system seems to be regulated by different brain mechanisms than the hypothalamic-pituitary-adrenal system. This could be an important survival mechanism to ensure that dysfunction of one neurotransmitter system will not endanger the appropriate secretion of hormones during exposure to adverse conditions. The measurement of several hormones to examine the mechanisms underlying the stress response and the effects of drugs and lesions on these responses can provide insight into the nature and location of brain circuits and neurotransmitter receptors involved in anxiety and stress.

Interaction between estradiol replacement and chronic stress on feeding behavior and on serum leptin

Exposure to stress may cause either an increase or a decrease in food intake. Behavioral and physiological responses to stress, including alterations in feeding behavior, are sexually dimorphic. This study aimed to evaluate the interaction between estradiol levels and chronic variate stress on the intake of sweet food and on serum levels of leptin, a hormone secreted by the adipose cells with a role in the regulation of body weight. Adult female Wistar rats were used. After ovariectomy, the animals received estradiol replacement (or oil) subcutaneously. Rats were then divided in controls and stressed (submitted to 30 days of variate stress). Consumption of sweet food and of serum leptin was measured. Although animals receiving estradiol replacement presented smaller weight gain, they showed an increased consumption of sweet food. Chronic variate stress decreased sweet food intake at 30, but not at 20, days of treatment. Estradiol replacement in the stressed group prevented both the reduction observed in sweet food intake and the increase in leptin levels. These results suggest that there is an interaction between chronic stress and estradiol replacement in feeding behavior concerning sweet food consumption, and this interaction may be related to altered leptin levels.

Temporal effects of stress by immobilization and sensitivity of the isolated rat pacemaker to isoproterenol: roles of corticosterone, neuronal uptake, and beta-adrenergic homogeneity

A number of diseases and pathological conditions are related to the long-term adaptive response to stress, in particular under conditions of chronic stress when allostasis can shift from a healthy toward a pathological state. Although a vast number of studies have focused on the effects of chronic stress on brain and the immune system, fewer studies have been performed in peripheral tissues. Here, we used the intact isolated right atrium (pacemaker) from the rat to investigate the temporal effects of stress induced by immobilization (restraint stress) on the sensitivity of the pacemaker to the chronotropic response to isoproterenol (i.e., the effect of isoproterenol to increase the frequency of contractions of pacemakers). Immobilization sessions were conducted a specific number of times (1, 3, 7, 9, 11, and 14). We found that the response to stress over time approximates a Gaussian distribution (i.e., normal standard distribution) with no significant effects being detected after either 1 or 14 immobilization sessions, whereas supersensitivity to the chronotropic effect of isoproterenol occurred after 3, 7, 9, and 11 immobilization sessions, with a peak effect occurring after seven immobilization sessions. At a cellular level, we determined that both corticosterone and neuronal uptake of catecholamines were directly involved with the observed effects, whereas no alterations in the homogeneity of beta-adrenoceptors were detected in pacemakers of stressed animals. We hypothesize that these adaptations are essentially beneficial in nature, as they should allow the animals to more promptly respond to the demands imposed by the stressful conditions.