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Lowering barometric pressure aggravates depression-like behavior in rats
Behavioural Brain Research 218 (2011) 190–193
Contents lists available at ScienceDirect
Behavioural Brain Research journal homepage: www.elsevier.com/locate/bbr
Research report
Lowering barometric pressure aggravates depression-like behavior in rats Hiroyuki Mizoguchi a,1, Kanoko Fukaya a,1, Rarami Mori a,b, Mariko Itoh a,b, Megumi Funakubo a, Jun Sato a,∗ a b
Futuristic Environmental Simulation Center, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan Technical Center of Nagoya University, Nagoya 464-8601, Japan
a r t i c l e
i n f o
Article history: Received 26 October 2010 Received in revised form 25 November 2010 Accepted 28 November 2010 Keywords: Lowering barometric pressure Depression-like behavior Forced swim stress Helplessness
a b s t r a c t Weather change has been known to influence the condition of patients with mood disorder. However, no animal studies have tested the influence of climatic factor on emotional impairment. In this study, we examined the effect of lowering barometric pressure (LP) in a climate-controlled room on immobility time in the forced swim test in rats, which is considered to be an index of behavioral despair (helplessness). When the rats were exposed to daily repeated forced swim, the immobility time gradually increased. This increment was inhibited by repeated administration of the antidepressant imipramine, suggesting that the immobility is an anxiety/depression-like behavior. LP exposure (20 hPa below the natural atmospheric pressure) further increased immobility time in rats submitted to repeated forced swim. In another series of experiments, we examined the effect of daily repeated LP exposure on the maintenance of immobility after withdrawal from 6-day repeated forced swim. When the rats were challenged with forced swim under natural atmospheric pressure on day 14 after the withdrawal, immobility time was significantly longer than in non-conditioned rats. These findings demonstrated that LP in the range of natural weather change augmented the depression-like behavior in rats. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mood disorders such as depression, anxiety, obsessive– compulsive disorder and post-traumatic stress disorder are serious problems in today’s society. Various animal models have been developed based on the hypothesis that mood disorders are caused by stressors (e.g., the chronic variable stress model) or neuronal deficits (e.g., the olfactory bulbectomy model), and utilized widely for evaluating novel compounds with respect to mood disorders in preclinical settings [1]. Chronic stress can accelerate disease processes, cause neural degeneration, and lead to depression or other mood disorders [2–4]. It has been reported that repeated forced swim stress, which is one of the models of human mood disorders, causes the loss of adult neural stem cells and that antidepressant drugs reverse this disorders [4]. We have demonstrated that changes in social environment, such as social isolation stress, induced behavioral abnormality, and animals reared in social isolation exhibited increased vulnerability to swim stress and increased anxiety to elevated open-arm [5].
Abbreviation: LP, lowering barometric pressure. ∗ Corresponding author. Tel.: +81 52 789 3929; fax: +81 52 789 3999. E-mail address: [email protected] (J. Sato). 1 These authors contributed equally to this work. 0166-4328/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2010.11.057
Climate has been long suspected to play a role in the onset and development of depression, and the seasonality in the evolvement of depressive symptoms [which are not part of seasonal affective disorder] has been reported for both unipolar and bipolar patients [6–8]. In fact, the admission rates of bipolar depressed patients increase during spring/summer and correlate with maximal environmental temperature [9] and cold, darkness and low barometric pressure have been associated with increased onset rates of depressive episodes in patients [10], suggesting climatic relationships with depression. Moreover, it has been reported that physiological disorders such as headaches [11] and neuropathic pain [12,13], are aggravated by changes of the weather, and a variety of causes have been claimed, e.g., barometric pressure, humidity and temperature. We have demonstrated that experimentally lowering barometric pressure (LP) intensifies pain-related behaviors in rats with chronic constriction injury on the sciatic nerve [12,13]. These reports suggest a climatic relationship with physiological and psychological disorders, and we hypothesized that climatic variables may be associated with onset rates of some specific clinical subtypes of depression. However, there are no data from basic research on the relationship between climatic factor and depression-like behavior in animals. In the present study, we examined whether LP in a climatecontrolled room affects the immobility time in the forced swim test in rats, which is considered to be an index of behavioral despair (helplessness).
H. Mizoguchi et al. / Behavioural Brain Research 218 (2011) 190–193
A
191
350
Immobility time (sec/6 min)
300
vehicle imipramine
250
#
200 150 100 50
Immobility time (sec/6 min)
B
1 2 3 4 5 6 7 8 9 1011
C
vehicle imipramine 400 350 300 250 200 150 100 50 0
*
day14 7 days 14 days
Head dips (times)
0
control FST FST+imipramine
60 40
*
20 0
Pre Post Pre Post Pre Post
after withdrawal Fig. 1. Effects on behavior of repeated forced swim as chronic stress. A Rats were subjected to repeated forced swim for 11 days. Rats were given imipramine (10 mg/kg, i.p.) or vehicle (saline) after each forced swim for 11 days. Values indicate the mean ± S.E.M. (n = 6). #p < 0.05 vs the vehicle-treated group. B Rats were subjected to repeated forced swim for 14 days. At 7 and 14 days after the withdrawal of repeated forced swim, rats were exposed to forced swim for 6 min again. Values indicate the mean ± S.E.M. (n = 5–6). *p < 0.05 vs day 14 on the imipramine-treated group. C Head dip activity was measured as helplessness in response to mild stress after repeated forced swim for 14 days, compared with that before forced swim. Values indicate the mean ± S.E.M. (n = 4–6). *p < 0.05 vs pre-value in the imipramine-treated group. 2. Materials and methods
2.4. Low-pressure exposure
2.1. Animals
The barometric pressure of the climate-controlled room was lowered by 20 hPa below the atmospheric pressure, which is the type of change often observed when a typhoon passes. This was accomplished over 10 min. The pressure was maintained at this level for 6 min, and then returned to the baseline pressure over 10 min. The ambient temperature was controlled at 24 ◦ C [12,13].
Male Sprague–Dawley rats, 250–300 g (Japan SLC), were used. The animals were housed two or three per cage under controlled temperature (24 ◦ C) and a 12 h light/dark cycle, and were given free access to food and water. All the experiments were conducted according to the Regulations for Animal Experiments in Nagoya University, and the Fundamental Guidelines for Proper Conduct of Animal Experiments and Related Activities in Academic Research Institutions in Japan. 2.2. Repeated forced swim stress The forced swim test (FST) was carried out as described previously with minor modifications [4,5]. Rats were subjected to daily forced swim stress for 6–14 days. Rats were placed in a glass cylinder (35 cm high × 24 cm diameter) filled to a depth of 25 cm with water (25 ± 1 ◦ C). A 3- or 6-min test was repeated each day. Immobility time (floating) was measured. A rat was judged to be immobile if it ceased struggling and remained floating motionless in water making only those movements necessary to keep its head above water. Rats were given imipramine (10 mg/kg) or vehicle (saline) intraperitoneally after each forced swim for 11–14 days. After the withdrawal of repeated forced swim, imipramine was no longer applied. At 7 and 14 days after the withdrawal of repeated forced swim, the rats were exposed to forced swim again. 2.3. Elevated open-platform test An elevated open-platform test was performed in accordance with previous reports [14], with minor modifications. The experiment was conducted in a room illuminated by white fluorescent light (100 lx). A transparent acrylic cylinder (56.5 cm high × 14 cm diameter) was placed upside-down and rats were placed individually on the top (open-platform) for 6 min. When a rat slipped off the platform, it was immediately replaced on the open-platform and the measurement was continued. Head dips (exploratory movement of head/shoulders beyond the edge of the open-platform), a risk assessment behavior, were defined as behavioral activity in response to mild stress, and measured. The total number of times that an animal showed this behavior was recorded.
2.5. Statistical analysis All data were expressed as the mean ± standard error of means (S.E.M.). Statistical significance was determined by Mann–Whitney U-test for two group comparisons, and one or two-way analysis of variance (ANOVA) for multigroup comparisons in the experiments, and by a repeated measures ANOVA. Fisher’s LSD test was used for post hoc comparisons when the F value was significant (p < 0.05).
3. Results 3.1. Effects of repeated forced swim as chronic stress on behavior Rats were subjected to the forced swim stress repeatedly. They exhibited longer immobility in a time-dependent manner (Fig. 1A), and high levels remained even after 14 days (Fig. 1B) which may be explained by either helplessness or habituation to the stress. Imipramine treatment significantly inhibited the increase in immobility time induced by the forced swim stress, although its inhibitory effect was not evident on day 1–5 (Fig. 1A, drug, F(1,10) = 6.46, p < 0.05; day, F(13,130) = 5.71, p < 0.05; interaction, F(13,130) = 4.64, p < 0.05 by repeated two-way ANOVA). Next, at 7 and 14 days after the withdrawal of repeated forced swim for 14 days, rats were exposed to forced swim again. Immobility in the vehicle-treated group remained for at least 14 days, even after the forced swim withdrawal (F(2,8) = 0.18, p > 0.05 by repeated one-way
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H. Mizoguchi et al. / Behavioural Brain Research 218 (2011) 190–193
200 180 160 140 120 100 80 60 40 20 0
B * *
* * * * * * * * *
* *
* * *
* *
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Immobility time (sec/3 min)
Immobility time (sec/3 min)
A
*
#
#
200
*
*
150
LP(-) LP (+)
100 50 0
day1 day6
1 2 3 4 5 6 7 8 9 101112 day
7
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after withdrawal (day)
Fig. 2. Effect of LP on depression-like behavior. A Rats were subjected to forced swim for 3 min, once a day for 12 days under LP condition. Values indicate the mean ± S.E.M. (n = 8). *p < 0.05 vs day 1 in each group. #p < 0.05 vs control group. B Rats were subjected to forced swim for 3 min, once a day for 6 days. Rats were then divided into an LP(+) group, which was exposed to LP condition, and an LP(−) groups with no exposure. At 7 and 14 days after the withdrawal of repeated forced swim, rats were subjected to forced swim for 3 min again. Values indicate the mean ± S.E.M. (n = 7). *p < 0.05 vs day 6 in LP(+) group. #p < 0.05 vs day 14 in LP(−) group.
ANOVA), while that in the imipramine-treated group was increased until it reached the same level as in vehicle-treated groups (Fig. 1B, F(2,10) = 8.65, p < 0.05 by repeated one-way ANOVA, followed by p < 0.05 by U-test). We also examined the mild stress-induced inhibition of activity using the elevated open-platform test. Firstly, there was no difference in head dip activity before swim stress (pre-value) between control, vehicle-treated and imipramine-treated groups (Fig. 1C, F(2,13) = 0.654, p > 0.05). After repeated forced swim stress, head dip activity was decreased compared with that before (Fig. 1C, p < 0.05 by U-test). However, the imipramine-treated group exhibited the same high levels even after swim stress (Fig. 1C). These results suggest that rats experience helplessness in response to mild stress, which is equivalent to depression-like behavior in human. 3.2. Effect of LP on depression-like behavior Next, we examined the effect of LP on the immobility time in stress-exposed rats. Rats were subjected to forced swim for 3 min, once a day for 12 days under an LP condition. In the control group, swim stress for 3 min increased the immobility time in a time-dependent manner, and after 5 days, the immobility time was significantly increased compared with that on day 1 (Fig. 2A, F(11,84) = 5.34, p < 0.05 by one-way ANOVA). Interestingly, swim stress-induced increase in immobility time was aggravated under LP environment compared with that in the control group (Fig. 2A, group, F(1,14) = 6.63, p < 0.05; day, F(11,154) = 53.6, p < 0.05; interaction, F(11,154) = 2.56, p > 0.05 by repeated two-way ANOVA), and the immobility time was significantly increased after 2 days compared with that on day 1 (F(11,84) = 19.8, p < 0.05 by one-way ANOVA). This result indicates that LP exposure augments the development of helplessness. In another series of experiments, we examined the effect of daily repeated LP exposure on the maintenance of immobility after withdrawal from 6-day repeated forced swim. Rats were subjected to forced swim for 3 min, once a day for 6 days. They were then, divided into an LP(+) group, which was exposed to LP conditions, and an LP(−) group with no exposure. At 7 and 14 days after the withdrawal of repeated forced swim, the rats were subjected to forced swim for 3 min again. In the LP(−) group previously subjected to repeated forced swim for 6 days, forced swim-induced immobility remained for at least 14 days even after the forced swim withdrawal (Fig. 2B). On the other hand, immobility time in the LP(+) group was further increased on day 7 and 14 after the forced swim withdrawal compared with that on day 6 (F(2,18) = 3.96,
p < 0.05 by one-way ANOVA), and the value on day 14 was significantly larger than that in the LP(−) group (p < 0.05 by U-test). This result suggests that LP was also involved in maintaining helplessness, not only in the development of helplessness. 4. Discussion This is the first study to investigate the relationship between low barometric pressure and depression-like behavior. The forced swim test is often used to assess behavioral despair and antidepressants through their effects on immobility [15]. Although the behavioral effects of chronic forced swim have not been established, it appears that the conditions were indeed stressful for the rats for the entire experimental paradigm. In this study, daily forced-swim increased the immobility time timedependently, and head dip activity decreased compared with that before. Also, these abnormal behaviors were ameliorated with imipramine treatment, which may indicate increased despair (Fig. 1). Alternatively, longer immobility may be interpreted as habituation to the stressor. However, the latter interpretation is unlikely, because plasma corticosterone levels were quite elevated in mice that had undergone repeated forced swim even after the last swim [4]. Many clinical studies have suggested a critical role of climatic factors in psychiatric disorder. For example, increased environmental temperature may be a risk factor for evolvement of major depressive episode in patients with bipolar disorder with psychiatric co-morbidity [9]. Cold and darkness were associated with increased onset rates of depressive episodes with melancholic features within the following month, while low barometric pressure was associated with increased onset rates of depressive episodes with psychotic features within the following month and increased admission rates for these episodes 2 months thereafter [10]. The authors of that study emphasized that climate, rather than seasonality per se, appeared to have a direct relationship with depressive episodes. Additionally, LP was previously found to be associated with increased acts of violence and emergency psychiatry visits [16], and it is one of the most important predictors of low tryptophan availability [17]. In this study, we also showed that the forced swim stress-induced increase in immobility time was aggravated under an LP environment (Fig. 2A and B). To our knowledge, this is the first study to show such a relationship in animal studies. Considering this together with the studies above, the change in barometric pressure may play a critical part in expression/development of clinical depression, although the relationship between LP and psychotic depression remains poorly understood.
H. Mizoguchi et al. / Behavioural Brain Research 218 (2011) 190–193
How does LP modulate depression-like behavior? Previously, we showed experimentally that LP intensifies pain-related behaviors in rats with chronic constriction injury through inner ear activity, and inner ear activation is a critical part in sensing the changes in barometric pressure [12,13]. Thus, some mechanism involved in psychosis might be influenced by inner ear activities, as a result of which LP-induced aggravation of depression was observed in animals in this study. Most depression occurs idiopathically, and understanding of its etiology is limited, with stressful life events, endocrine abnormalities (hypothyroidism and hypercortisolism), side effects of drugs, and other factors considered to have etiological roles. Of these many factors and mechanisms, hormonal changes and sympathetic activities can be modified within several minutes of LP, while others might require too much time to explain LP-induced aggravation of depression-like behavior. One possible way in which LP could aggravate depression is through hormonal changes. This is based on reports that neurons in vestibular nuclei project to the hypothalamus, thus possibly modulating the environment of hormones such as adrenaline. Another possibility is that the profound influence of vestibular stimulation on autonomic functions, which occurs through modification of autonomic centers in brain stem, also affects sympathetic and vagal nerve activities. In fact, the majority of evidence has pointed to hypothalamic–pituitary–adrenal and sympathetic nervous system abnormalities in depression [18]. However, further research is needed before we can understand clearly how LP modulates depression-like behavior. In conclusion, LP in the range of natural weather change augmented the depression-like behavior in rats. Climatic changes may be critical in physiological and psychological disorder. Conflict of interest statement There are no conflicts of interest in this study. Acknowledgments This study was supported in part by a Grant-in-Aid for Scientific Research (Nos. 20390415 and 21790068) from the Japan Society for the Promotion of Science.
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References [1] Krishnan V, Nestler EJ. The molecular neurobiology of depression. Nature 2008;455(7215):894–902. [2] Heim C, Nemeroff CB. The impact of early adverse experiences on brain systems involved in the pathophysiology of anxiety and affective disorders. Biol Psychiatry 1999;46(11):1509–22. [3] Nestler EJ, Gould E, Manji H, Buncan M, Duman RS, Greshenfeld HK, et al. Preclinical models: status of basic research in depression. Biol Psychiatry 2002;52:503–28. [4] Hitoshi S, Maruta N, Higashi M, Kumar A, Kato N, Ikenaka K. Antidepressant drugs reverse the loss of adult neural stem cells following chronic stress. J Neurosci Res 2007;85:3574–85. [5] Koike H, Ibi D, Mizoguchi H, Nagai T, Nitta A, Takuma K, et al. Behavioral abnormality and pharmacologic response in social isolation-reared mice. Behav Brain Res 2009;202:114–21. [6] Daniels BA, Kirkby KC, Mitchell P, Hay D, Mowry B. Seasonal variation in hospital admission for bipolar disorder, depression and schizophrenia in Tasmania. Acta Psychiatr Scand 2000;102:38–43. [7] Silverstone T, Romans S, Hunt N, McPherson H. Is there a seasonal pattern of relapse in bipolar affective disorders? A dual northern and southern hemisphere cohort study. Br J Psychiatry 1995;167:58–60. [8] Chand PK, Murthy P. Climate change and mental health. Reg Health Forum 2008;12:43–8. [9] Shapira A, Shiloh R, Potchter O, Hermesh H, Popper M, Weizman A. Admission rates of bipolar depressed patients increase during spring/summer and correlate with maximal environmental temperature. Bipolar Disord 2004;6:90–3. [10] Radua J, Pertusa A, Cardoner N. Climatic relationships with specific clinical subtypes of depression. Psychiatry Res 2010;175:217–20. [11] Messlinger K, Funakubo M, Sato J, Mizumura K. Increases in neuronal activity in rat spinal trigeminal nucleus following changes in barometric pressurerelevance for weather-associated headaches? Headache 2010;50:1449–63. [12] Funakubo M, Sato J, Honda T, Mizumura K. The inner ear is involved in the aggravation of nociceptive behavior induced by lowering barometric pressure of nerve injured rats. Eur J Pain 2010;14:32–9. [13] Funakubo M, Sato J, Obata K, Mizumura K. The rate and magnitude of atmospheric pressure change that aggravate pain-related behavior of nerve injured rats. Int J Biometeorol; doi:10.1007/s000484-010-0339-8. [14] Miyata S, Hirano S, Ohsawa M, Kamei J. Chlorpheniramine exerts anxiolytic-like effects and activates prefrontal 5-HT systems in mice. Psychopharmacology (Berl) 2009:13. [15] Porsolt RD, Le Pichon M, Jalfre M. Depression: a new animal model sensitive to antidepressant treatments. Nature 1977;266:730–2. [16] Schory TJ, Piecznski N, Nair S, el-Mallakh RS. Barometric pressure, emergency psychiatric visits, and violent acts. Can J Psychiatry 2003;48:624–7. [17] Maes M, Scharpé S, Verkerk R, D’Hondt P, Peeters D, Cosyns P, et al. Seasonal variation in plasma l-tryptophan availability in healthy volunteers. Relationships to violent suicide occurrence. Arch Gen Psychiatry 1995;52:937–46. [18] Maletic V, Raison CL. Neurobiology of depression, fibromyalgia and neuropathic pain. Front Biosci 2009;14:5291–338.
Contents lists available at ScienceDirect
Behavioural Brain Research journal homepage: www.elsevier.com/locate/bbr
Research report
Lowering barometric pressure aggravates depression-like behavior in rats Hiroyuki Mizoguchi a,1, Kanoko Fukaya a,1, Rarami Mori a,b, Mariko Itoh a,b, Megumi Funakubo a, Jun Sato a,∗ a b
Futuristic Environmental Simulation Center, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan Technical Center of Nagoya University, Nagoya 464-8601, Japan
a r t i c l e
i n f o
Article history: Received 26 October 2010 Received in revised form 25 November 2010 Accepted 28 November 2010 Keywords: Lowering barometric pressure Depression-like behavior Forced swim stress Helplessness
a b s t r a c t Weather change has been known to influence the condition of patients with mood disorder. However, no animal studies have tested the influence of climatic factor on emotional impairment. In this study, we examined the effect of lowering barometric pressure (LP) in a climate-controlled room on immobility time in the forced swim test in rats, which is considered to be an index of behavioral despair (helplessness). When the rats were exposed to daily repeated forced swim, the immobility time gradually increased. This increment was inhibited by repeated administration of the antidepressant imipramine, suggesting that the immobility is an anxiety/depression-like behavior. LP exposure (20 hPa below the natural atmospheric pressure) further increased immobility time in rats submitted to repeated forced swim. In another series of experiments, we examined the effect of daily repeated LP exposure on the maintenance of immobility after withdrawal from 6-day repeated forced swim. When the rats were challenged with forced swim under natural atmospheric pressure on day 14 after the withdrawal, immobility time was significantly longer than in non-conditioned rats. These findings demonstrated that LP in the range of natural weather change augmented the depression-like behavior in rats. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mood disorders such as depression, anxiety, obsessive– compulsive disorder and post-traumatic stress disorder are serious problems in today’s society. Various animal models have been developed based on the hypothesis that mood disorders are caused by stressors (e.g., the chronic variable stress model) or neuronal deficits (e.g., the olfactory bulbectomy model), and utilized widely for evaluating novel compounds with respect to mood disorders in preclinical settings [1]. Chronic stress can accelerate disease processes, cause neural degeneration, and lead to depression or other mood disorders [2–4]. It has been reported that repeated forced swim stress, which is one of the models of human mood disorders, causes the loss of adult neural stem cells and that antidepressant drugs reverse this disorders [4]. We have demonstrated that changes in social environment, such as social isolation stress, induced behavioral abnormality, and animals reared in social isolation exhibited increased vulnerability to swim stress and increased anxiety to elevated open-arm [5].
Abbreviation: LP, lowering barometric pressure. ∗ Corresponding author. Tel.: +81 52 789 3929; fax: +81 52 789 3999. E-mail address: [email protected] (J. Sato). 1 These authors contributed equally to this work. 0166-4328/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2010.11.057
Climate has been long suspected to play a role in the onset and development of depression, and the seasonality in the evolvement of depressive symptoms [which are not part of seasonal affective disorder] has been reported for both unipolar and bipolar patients [6–8]. In fact, the admission rates of bipolar depressed patients increase during spring/summer and correlate with maximal environmental temperature [9] and cold, darkness and low barometric pressure have been associated with increased onset rates of depressive episodes in patients [10], suggesting climatic relationships with depression. Moreover, it has been reported that physiological disorders such as headaches [11] and neuropathic pain [12,13], are aggravated by changes of the weather, and a variety of causes have been claimed, e.g., barometric pressure, humidity and temperature. We have demonstrated that experimentally lowering barometric pressure (LP) intensifies pain-related behaviors in rats with chronic constriction injury on the sciatic nerve [12,13]. These reports suggest a climatic relationship with physiological and psychological disorders, and we hypothesized that climatic variables may be associated with onset rates of some specific clinical subtypes of depression. However, there are no data from basic research on the relationship between climatic factor and depression-like behavior in animals. In the present study, we examined whether LP in a climatecontrolled room affects the immobility time in the forced swim test in rats, which is considered to be an index of behavioral despair (helplessness).
H. Mizoguchi et al. / Behavioural Brain Research 218 (2011) 190–193
A
191
350
Immobility time (sec/6 min)
300
vehicle imipramine
250
#
200 150 100 50
Immobility time (sec/6 min)
B
1 2 3 4 5 6 7 8 9 1011
C
vehicle imipramine 400 350 300 250 200 150 100 50 0
*
day14 7 days 14 days
Head dips (times)
0
control FST FST+imipramine
60 40
*
20 0
Pre Post Pre Post Pre Post
after withdrawal Fig. 1. Effects on behavior of repeated forced swim as chronic stress. A Rats were subjected to repeated forced swim for 11 days. Rats were given imipramine (10 mg/kg, i.p.) or vehicle (saline) after each forced swim for 11 days. Values indicate the mean ± S.E.M. (n = 6). #p < 0.05 vs the vehicle-treated group. B Rats were subjected to repeated forced swim for 14 days. At 7 and 14 days after the withdrawal of repeated forced swim, rats were exposed to forced swim for 6 min again. Values indicate the mean ± S.E.M. (n = 5–6). *p < 0.05 vs day 14 on the imipramine-treated group. C Head dip activity was measured as helplessness in response to mild stress after repeated forced swim for 14 days, compared with that before forced swim. Values indicate the mean ± S.E.M. (n = 4–6). *p < 0.05 vs pre-value in the imipramine-treated group. 2. Materials and methods
2.4. Low-pressure exposure
2.1. Animals
The barometric pressure of the climate-controlled room was lowered by 20 hPa below the atmospheric pressure, which is the type of change often observed when a typhoon passes. This was accomplished over 10 min. The pressure was maintained at this level for 6 min, and then returned to the baseline pressure over 10 min. The ambient temperature was controlled at 24 ◦ C [12,13].
Male Sprague–Dawley rats, 250–300 g (Japan SLC), were used. The animals were housed two or three per cage under controlled temperature (24 ◦ C) and a 12 h light/dark cycle, and were given free access to food and water. All the experiments were conducted according to the Regulations for Animal Experiments in Nagoya University, and the Fundamental Guidelines for Proper Conduct of Animal Experiments and Related Activities in Academic Research Institutions in Japan. 2.2. Repeated forced swim stress The forced swim test (FST) was carried out as described previously with minor modifications [4,5]. Rats were subjected to daily forced swim stress for 6–14 days. Rats were placed in a glass cylinder (35 cm high × 24 cm diameter) filled to a depth of 25 cm with water (25 ± 1 ◦ C). A 3- or 6-min test was repeated each day. Immobility time (floating) was measured. A rat was judged to be immobile if it ceased struggling and remained floating motionless in water making only those movements necessary to keep its head above water. Rats were given imipramine (10 mg/kg) or vehicle (saline) intraperitoneally after each forced swim for 11–14 days. After the withdrawal of repeated forced swim, imipramine was no longer applied. At 7 and 14 days after the withdrawal of repeated forced swim, the rats were exposed to forced swim again. 2.3. Elevated open-platform test An elevated open-platform test was performed in accordance with previous reports [14], with minor modifications. The experiment was conducted in a room illuminated by white fluorescent light (100 lx). A transparent acrylic cylinder (56.5 cm high × 14 cm diameter) was placed upside-down and rats were placed individually on the top (open-platform) for 6 min. When a rat slipped off the platform, it was immediately replaced on the open-platform and the measurement was continued. Head dips (exploratory movement of head/shoulders beyond the edge of the open-platform), a risk assessment behavior, were defined as behavioral activity in response to mild stress, and measured. The total number of times that an animal showed this behavior was recorded.
2.5. Statistical analysis All data were expressed as the mean ± standard error of means (S.E.M.). Statistical significance was determined by Mann–Whitney U-test for two group comparisons, and one or two-way analysis of variance (ANOVA) for multigroup comparisons in the experiments, and by a repeated measures ANOVA. Fisher’s LSD test was used for post hoc comparisons when the F value was significant (p < 0.05).
3. Results 3.1. Effects of repeated forced swim as chronic stress on behavior Rats were subjected to the forced swim stress repeatedly. They exhibited longer immobility in a time-dependent manner (Fig. 1A), and high levels remained even after 14 days (Fig. 1B) which may be explained by either helplessness or habituation to the stress. Imipramine treatment significantly inhibited the increase in immobility time induced by the forced swim stress, although its inhibitory effect was not evident on day 1–5 (Fig. 1A, drug, F(1,10) = 6.46, p < 0.05; day, F(13,130) = 5.71, p < 0.05; interaction, F(13,130) = 4.64, p < 0.05 by repeated two-way ANOVA). Next, at 7 and 14 days after the withdrawal of repeated forced swim for 14 days, rats were exposed to forced swim again. Immobility in the vehicle-treated group remained for at least 14 days, even after the forced swim withdrawal (F(2,8) = 0.18, p > 0.05 by repeated one-way
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H. Mizoguchi et al. / Behavioural Brain Research 218 (2011) 190–193
200 180 160 140 120 100 80 60 40 20 0
B * *
* * * * * * * * *
* *
* * *
* *
control LP
Immobility time (sec/3 min)
Immobility time (sec/3 min)
A
*
#
#
200
*
*
150
LP(-) LP (+)
100 50 0
day1 day6
1 2 3 4 5 6 7 8 9 101112 day
7
14
after withdrawal (day)
Fig. 2. Effect of LP on depression-like behavior. A Rats were subjected to forced swim for 3 min, once a day for 12 days under LP condition. Values indicate the mean ± S.E.M. (n = 8). *p < 0.05 vs day 1 in each group. #p < 0.05 vs control group. B Rats were subjected to forced swim for 3 min, once a day for 6 days. Rats were then divided into an LP(+) group, which was exposed to LP condition, and an LP(−) groups with no exposure. At 7 and 14 days after the withdrawal of repeated forced swim, rats were subjected to forced swim for 3 min again. Values indicate the mean ± S.E.M. (n = 7). *p < 0.05 vs day 6 in LP(+) group. #p < 0.05 vs day 14 in LP(−) group.
ANOVA), while that in the imipramine-treated group was increased until it reached the same level as in vehicle-treated groups (Fig. 1B, F(2,10) = 8.65, p < 0.05 by repeated one-way ANOVA, followed by p < 0.05 by U-test). We also examined the mild stress-induced inhibition of activity using the elevated open-platform test. Firstly, there was no difference in head dip activity before swim stress (pre-value) between control, vehicle-treated and imipramine-treated groups (Fig. 1C, F(2,13) = 0.654, p > 0.05). After repeated forced swim stress, head dip activity was decreased compared with that before (Fig. 1C, p < 0.05 by U-test). However, the imipramine-treated group exhibited the same high levels even after swim stress (Fig. 1C). These results suggest that rats experience helplessness in response to mild stress, which is equivalent to depression-like behavior in human. 3.2. Effect of LP on depression-like behavior Next, we examined the effect of LP on the immobility time in stress-exposed rats. Rats were subjected to forced swim for 3 min, once a day for 12 days under an LP condition. In the control group, swim stress for 3 min increased the immobility time in a time-dependent manner, and after 5 days, the immobility time was significantly increased compared with that on day 1 (Fig. 2A, F(11,84) = 5.34, p < 0.05 by one-way ANOVA). Interestingly, swim stress-induced increase in immobility time was aggravated under LP environment compared with that in the control group (Fig. 2A, group, F(1,14) = 6.63, p < 0.05; day, F(11,154) = 53.6, p < 0.05; interaction, F(11,154) = 2.56, p > 0.05 by repeated two-way ANOVA), and the immobility time was significantly increased after 2 days compared with that on day 1 (F(11,84) = 19.8, p < 0.05 by one-way ANOVA). This result indicates that LP exposure augments the development of helplessness. In another series of experiments, we examined the effect of daily repeated LP exposure on the maintenance of immobility after withdrawal from 6-day repeated forced swim. Rats were subjected to forced swim for 3 min, once a day for 6 days. They were then, divided into an LP(+) group, which was exposed to LP conditions, and an LP(−) group with no exposure. At 7 and 14 days after the withdrawal of repeated forced swim, the rats were subjected to forced swim for 3 min again. In the LP(−) group previously subjected to repeated forced swim for 6 days, forced swim-induced immobility remained for at least 14 days even after the forced swim withdrawal (Fig. 2B). On the other hand, immobility time in the LP(+) group was further increased on day 7 and 14 after the forced swim withdrawal compared with that on day 6 (F(2,18) = 3.96,
p < 0.05 by one-way ANOVA), and the value on day 14 was significantly larger than that in the LP(−) group (p < 0.05 by U-test). This result suggests that LP was also involved in maintaining helplessness, not only in the development of helplessness. 4. Discussion This is the first study to investigate the relationship between low barometric pressure and depression-like behavior. The forced swim test is often used to assess behavioral despair and antidepressants through their effects on immobility [15]. Although the behavioral effects of chronic forced swim have not been established, it appears that the conditions were indeed stressful for the rats for the entire experimental paradigm. In this study, daily forced-swim increased the immobility time timedependently, and head dip activity decreased compared with that before. Also, these abnormal behaviors were ameliorated with imipramine treatment, which may indicate increased despair (Fig. 1). Alternatively, longer immobility may be interpreted as habituation to the stressor. However, the latter interpretation is unlikely, because plasma corticosterone levels were quite elevated in mice that had undergone repeated forced swim even after the last swim [4]. Many clinical studies have suggested a critical role of climatic factors in psychiatric disorder. For example, increased environmental temperature may be a risk factor for evolvement of major depressive episode in patients with bipolar disorder with psychiatric co-morbidity [9]. Cold and darkness were associated with increased onset rates of depressive episodes with melancholic features within the following month, while low barometric pressure was associated with increased onset rates of depressive episodes with psychotic features within the following month and increased admission rates for these episodes 2 months thereafter [10]. The authors of that study emphasized that climate, rather than seasonality per se, appeared to have a direct relationship with depressive episodes. Additionally, LP was previously found to be associated with increased acts of violence and emergency psychiatry visits [16], and it is one of the most important predictors of low tryptophan availability [17]. In this study, we also showed that the forced swim stress-induced increase in immobility time was aggravated under an LP environment (Fig. 2A and B). To our knowledge, this is the first study to show such a relationship in animal studies. Considering this together with the studies above, the change in barometric pressure may play a critical part in expression/development of clinical depression, although the relationship between LP and psychotic depression remains poorly understood.
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How does LP modulate depression-like behavior? Previously, we showed experimentally that LP intensifies pain-related behaviors in rats with chronic constriction injury through inner ear activity, and inner ear activation is a critical part in sensing the changes in barometric pressure [12,13]. Thus, some mechanism involved in psychosis might be influenced by inner ear activities, as a result of which LP-induced aggravation of depression was observed in animals in this study. Most depression occurs idiopathically, and understanding of its etiology is limited, with stressful life events, endocrine abnormalities (hypothyroidism and hypercortisolism), side effects of drugs, and other factors considered to have etiological roles. Of these many factors and mechanisms, hormonal changes and sympathetic activities can be modified within several minutes of LP, while others might require too much time to explain LP-induced aggravation of depression-like behavior. One possible way in which LP could aggravate depression is through hormonal changes. This is based on reports that neurons in vestibular nuclei project to the hypothalamus, thus possibly modulating the environment of hormones such as adrenaline. Another possibility is that the profound influence of vestibular stimulation on autonomic functions, which occurs through modification of autonomic centers in brain stem, also affects sympathetic and vagal nerve activities. In fact, the majority of evidence has pointed to hypothalamic–pituitary–adrenal and sympathetic nervous system abnormalities in depression [18]. However, further research is needed before we can understand clearly how LP modulates depression-like behavior. In conclusion, LP in the range of natural weather change augmented the depression-like behavior in rats. Climatic changes may be critical in physiological and psychological disorder. Conflict of interest statement There are no conflicts of interest in this study. Acknowledgments This study was supported in part by a Grant-in-Aid for Scientific Research (Nos. 20390415 and 21790068) from the Japan Society for the Promotion of Science.
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