Long lasting increase in nociceptive threshold induced in mice by forced swimming: involvement of an endorphinergic mechanism

Mice submitted to forced swimming session(s) displayed a long lasting modification in their nociceptive threshold, assessed through their jump latency from a hot plate (55 degrees C). Thus two forced swimming sessions (6 min each, 8h apart), in water at 33 degrees C, increased by about 50% the jump latency when the hot plate test was performed 14 hours, 3 days or 6 days thereafter. The water temperature (16 degrees C vs 33 degrees C) had no critical influence in this respect. To be clearly effective (at 33 degrees C) the swimming session had to be performed twice (when performed only once it was irregularly effective); it apparently culminated for a 6 min duration, since its effectiveness was not significantly increased by extending the swimming time to 12 min or 18 min. Performing 2 forced swimming sessions (6 min each, 8h apart), 5 consecutive days, resulted in a suppression of the increase in jump latency in the hot plate test. The two forced swimming episodes-induced analgesia was prevented by the s.c. administration of diazepam (from 0.125 mg/kg) or morphine (from 5 mg/kg) or scopolamine (1 mg/kg) before each forced swimming episode. Morphine (7.5 mg/kg) was uneffective to prevent the induction of two forced swimming episodes-induced analgesia when it was administered immediately after each forced swimming session. Finally this analgesia was dose dependently reversed by naloxone (ID(50) = 0.14 mg/kg, s.c., 30 min before the hot plate test). It is hypothesized that the handling of mice immediately before the hot plate test induces the remembrance of the stress induced by previous forced swimming episodes, triggering a fear reaction which increases the nociceptive threshold.     

Hypothalamic and hippocampal release of serotonin in rats bred for hyper- or hypo-anxiety

Microdialysis for measurement of serotonin in the paraventricular nucleus of the hypothalamus (PVN) and the dorsal hippocampus was performed under both basal and stimulated conditions, known to elicit differential behavioral and neuroendocrine responses in rats with inborn high (HAB) or low (LAB) anxiety-related behavior. We studied the release of hypothalamic and hippocampal serotonin in response to elevated platform exposure and forced swim stress, a mild emotional and a combined emotional and physical stressor, respectively. The data suggest that serotonin release patterns may depend on the inborn level of anxiety, the brain area dialyzed, and the stressor the animals were exposed to. Under basal conditions, no differences in serotonin release in either the PVN or dorsal hippocampus were observed between HAB and LAB rats. While in the PVN open platform exposure failed to change the release of serotonin, forced swim stress induced an increase in both HAB (p = 0.0001) and LAB (p = 0.01) rats with a significantly greater effect in the former (p = 0.027). In the dorsal hippocampus, only LABs, but not HABs, responded to the elevated platform exposure by enhancing the release of serotonin (p = 0.01). Also, forced swim stress increased hippocampal serotonin only in LAB (p = 0.002), but not HAB, rats probably indicating an involvement of hippocampal serotonin in locomotion and active stress coping. It remains to be shown to what extent the differences in serotonin release contribute to neuroendocrine and behavioral differences between HAB and LAB rats.     

Selective blockade of 5-HT2A receptors attenuates the increased temperature response in brown adipose tissue to restraint stress in rats

Previous studies have demonstrated that 5-HT2A receptors may be involved in the central control of thermoregulation and of the cardiovascular system. Our aim was to test whether these receptors mediate thermogenic and tachycardiac responses induced by acute psychological stress. Three groups of adult male Hooded Wistar rats were instrumented with: (i) a thermistor in the interscapular area (for recording brown adipose tissue temperature) and an ultrasound Doppler probe (to record tail blood flow); (ii) temperature dataloggers to record core body temperature; (iii) ECG electrodes. On the day of the experiment, rats were subjected to a 30-min restraint stress preceded by s.c. injection of either vehicle or SR-46349B (a serotonin 2A receptor antagonist) at doses of 0.01, 0.1 and 1.0 mg/kg. The restraint stress caused a rise in brown adipose tissue temperature (from, mean +/- s.e.m., 36.6 +/- 0.2 to 38.0 +/- 0.2 degrees C), transient cutaneous vasoconstriction (tail blood flow decreased from 12 +/- 2 to 5 +/- 1 cm/s), increase in heart rate (from 303 +/- 15 to 453 +/- 15 bpm at the peak, then reduced to 393 +/- 12 bpm at the steady state), and defaecation (6 +/- 1 pellets per restraint session). The core body temperature was not affected by the restraint. Blockade of 5-HT2A receptors attenuated the increase in brown adipose tissue temperature and transient cutaneous vasoconstriction, but not tachycardia and defaecation elicited by restraint stress. These results indicate that psychological stress causes activation of 5-HT2A receptors in neural pathways that control thermogenesis in the brown adipose tissue and facilitate cutaneous vasoconstriction.     

Strain and gender specific effects in the forced swim test: effects of previous stress exposure

The chronic mild stress (CMS) procedure was developed in rodents to target anhedonia, the core symptom of depressive melancholia. Stress exposure has been shown to induce a variety of physiological, biochemical and behavioral alterations associated with depression, although its anhedonic consequences as indexed by either sucrose intake and preference or thresholds for brain stimulation reward are less reliably observed. In the present study, we assessed the effects of six weeks of CMS on the latter measure in two strains of male and female rats subsequently challenged with an acute psychophysical stressor, forced swimming; their behavior in the swimming cylinder was evaluated on two consecutive days. While brain stimulation reward thresholds and response rates were unchanged by CMS exposure, significant differences in forced swim behaviors were observed between male control and CMS groups. In particular, male Long Evans rats with a history of CMS showed the largest decrease in the duration of active behaviors on the second test day, a pattern less evident in the Sprague-Dawley strain of rats, or in any of the female groups. The results suggest that the effects of depressogenic manipulations are strain and gender dependent, with male Long Evans rats most susceptible, as demonstrated by the selective reduction of struggling behaviors. Inclusion of multiple measures, including the forced swim test, would provide a better understanding of the psychopathological profile engendered by chronic exposure to mild stressors and its genetic specificity.     

Age-Related Changes in Plasma Catecholamine Responses to Acute Swim Stress

Young adult (3 months) and aged (22 months) Fischer 344 male rats were prepared with chronic tail artery catheters. Three days after surgery, rats were exposed acutely to swim stress at 20, 25, 30, or 35°C for 15 min. Blood samples were obtained from each rat under basal conditions, at the end of the swim stress episode, and 15, 30, and 45 min after swim stress. Basal plasma levels of norepinephrine and epinephrine (EPI) were similar for 3- and 22-month-old rats. In contrast, plasma catecholamine responses of aged rats were significantly greater than those of young adult rats following swim stress at 20 and 25°C. Plasma catecholamine responses were similar for rats of the two ages following swim stress at 30 or 35°C. These findings indicate that aged rats have exaggerated sympathetic-adrenal medullary responses to acute swim stress at the lower water temperatures. Given the modulatory effects of plasma EPI on memory, these age-related alterations in plasma catecholamine responses to acute swim stress may influence spatial memory performance of rats in the Morris water maze.     

Involvement of 5-HT1A receptors in animal tests of anxiety and depression: evidence from genetic models

Clinical studies have suggested the involvement of 5-HT1A receptors in anxiety and depressive disorders because partial 5-HT1A receptor agonists such as buspirone are therapeutic. The present review considers evidence from genetic animal models that support a role for 5-HT1A receptors in anxiety-like and depressed-like behavior in animals. Selective breeding for differential hypothermic responses to a selective 5-HT1A receptor agonist led to the development of the high DPAT sensitive (HDS) and low DPAT sensitive (LDS) lines of rats. The HDS rats differ from the LDS rats on several behavioral measures reflective of anxiety or depression, including reduced social interaction, reduced responding in a conflict task and exaggerated immobility in the forced swim test. However, they do not differ from the LDS rats in the elevated plus maze task, which is a commonly used test of anxiety. Nor do the HDS rats exhibit a typical anxiogenic response to the hippocampal administration of the 5-HT1A agonist. Although the HDS rats do exhibit elevations in 5-HT1A receptors in regions of the limbic cortex, it is not clear whether these increases account for the behavioral differences. Paradoxically, 5-HT1A receptor knockout mice also exhibit anxiety-like behavior in the plus maze, open field and conflict tests compared to wild type mice. However, the knockouts exhibited less immobility in the forced swim test than wild type control mice. Recent studies using selective regional reinstatement of the receptor have implicated the postsynaptic 5-HT1A receptors in these changes in anxiety-like behavior. Thus, preliminary evidence from two different types of genetic animal models suggests that anxiety-like behavior can arise if the 5-HT1A receptor function is eliminated or overexpressed. Further study with additional tests of anxiety are needed to confirm this intriguing relationship.     

Learning behavior,escape behavior,and depression in an ulcer susceptible rat strain

The aim of the present study was to explore the relationship between depression (helpless withdrawal behavior) and susceptibility to stress ulcer in rats. The WKY genetic strain of rats has been described as highly susceptible to stomach ulcer development during water restraint, i.e., when placed in a jar of water and forced to swim to keep their head above water, a setting in which Richter identified “giving up” behavior akin to hopelessness (Richter, 1957). Since WKY rats tended to float in the water instead of swimming in an attempt to escape, and were also found to be relatively inactive in open field tests, a series of experiments were performed to ascertain whether their diminished activity and their failure to swim reflected slowness, cognitive impairment, or something actually akin to depression. The latter interpretation was supported by evidence from tests of shock avoidance behavior, of capacity to learn discrimination in an operant setting, and by the capacity of an antidepressive drug to lessen floating time in the forced swim test and also to reduce the incidence of stomach ulcers.     

Learning behavior, escape behavior, and depression in an ulcer susceptible rat strain.

The aim of the present study was to explore the relationship between depression (helpless withdrawal behavior) and susceptibility to stress ulcer in rats. The WKY genetic strain of rats has been described as highly susceptible to stomach ulcer development during water restraint, i.e., when placed in a jar of water and forced to swim to keep their head above water, a setting in which Richter identified "giving up" behavior akin to hopelessness (Richter, 1957). Since WKY rats tended to float in the water instead of swimming in an attempt to escape, and were also found to be relatively inactive in open field tests, a series of experiments were performed to ascertain whether their diminished activity and their failure to swim reflected slowness, cognitive impairment, or something actually akin to depression. The latter interpretation was supported by evidence from tests of shock avoidance behavior, of capacity to learn discrimination in an operant setting, and by the capacity of an antidepressive drug to lessen floating time in the forced swim test and also to reduce the incidence of stomach ulcers.     

强迫性冷水游泳应激对大鼠行为和海马神经颗粒素的影响

为考察应激对海马神经颗粒素含量和磷酸化水平的影响,以及神经颗粒素是否涉及应激所致行为效应的脑机制,采用强迫性冷水游泳应激模型,选取40只Sprague-Dawley大鼠,随机分为应激组、装置对照组和正常对照组1和正常对照组2。以旷场试验法测定应激前后大鼠行为的变化,以Western blotting技术测定大鼠海马区域神经颗粒素的总含量和磷酸化水平,并分析两者之间的相互关系。结果表明：应激组动物活动增加,表现出焦虑行为;而海马区域神经颗粒素含量降低,与对照组相比差异具有显著性;且多项行为指标的变化与海马神经颗粒素含量的改变呈显著相关。这些结果提示神经颗粒素有可能在应激所致焦虑行为中起作用,可作为预测应激所致焦虑行为的较为敏感的指标之一。慢性应激过程中海马区域没有发现神经颗粒素的磷酸化反应。     

The effect of hypothermia on the rat's spatial memory in the water tank task

The effect of hypothermia on the retention of the water tank navigation task has been examined in 21 male hooded rats. After a 3-min swimming test on Day 1 the animals were trained on Days 2 and 3 (2 X 12 trials) to find a small submerged platform 1 cm below the surface of a large pool (120 cm in diameter) of opaque water. On Day 4, the rats were divided into three groups (n = 7) which were cooled to colonic temperatures of 22-24 degrees C (H1), 25-27 degrees C (H2), and 28-31 degrees C (H3), respectively, and given 12 retrieval trials in the water tank. Average escape latencies increased from 6 s in normothermic rats on Day 3 to 33, 19, and 12 s on Day 4 in the H1, H2, and H3 groups, respectively. Under the same testing conditions the performance of groups H1, H2, and H3 improved on Day 5 to 20, 8 and 6 s, respectively. It is concluded that spatial memory retrieval is resistant to mild hypothermia (30 degrees C), but that it is severely impaired at body temperatures below 25 degrees C. Reacquisition of the task is slowed down but not fully prevented in deep hypothermia.     

Learning under stress: the inverted-U-shape function revisited

Although the relationship between stress intensity and memory function is generally believed to follow an inverted-U-shaped curve, strikingly this phenomenon has not been demonstrated under the same experimental conditions. We investigated this phenomenon for rats' performance in a hippocampus-dependent learning task, the radial arm water maze (RAWM). Variations in stress intensity were induced using different water temperatures (25°C, 19°C, and 16°C), which elicited increased plasma corticosterone levels. During spatial training over three consecutive days, an inverted-U shape was found, with animals trained at 19°C making fewer errors than animals trained at either higher (16°C) or lower (25°C) stress conditions. Interestingly, this function was already observed by the last trial of day 1 and maintained on the first day trial of day 2. A long-term recall probe test administered under equal temperature conditions (20°C) revealed differences in performance according to the animals' former training conditions; i.e., platform searching for rats trained at 25°C was less accurate than for rats trained at either 16°C or 19°C. In reversal learning, groups trained at both 19°C and 25°C showed better performance than the 16°C group. We also found an interaction between anxiety and exploration traits on how individuals were affected by stressors during spatial learning. In summary, our findings confirm, for the first time, the existence of an inverted-U-shape memory function according to stressor intensity during the early learning and memory phases in a hippocampus-dependent task, and indicate the existence of individual differences related to personality-like profiles for performance at either high or low stress conditions.     

Caffeine as a performance-enhancing drug in rats: sex, dose, housing, and task considerations

Past animal studies of the performance-enhancing properties of stimulant drugs, such as caffeine, may have suffered from a number of procedural and ethical problems. For example. the housing condition of the animals was often not taken into consideration. As well, endurance tests, such as the forced swim task, sometimes involved ethically (and procedurally) questionable interference with natural swimming behaviour. Some of the manipulations, such as attaching a weight to the swimming animal's tail to increase the difficulty of the task and using mortality as a dependent variable, seem grotesque, even unnecessary. In this experiment, the performance-enhancing effects of caffeine in a modified forced swim task and a dominance task were evaluated using male and female rats as subjects (N=60), housed in either enriched or isolated environments. Analysis indicated that rats respond to caffeine as an interactive function of sex, housing, dose, and task characteristics. It was concluded that performance-enhancing properties of stimulant drugs may be the result of a complex interplay of variables, making simple generalizations questionable.     

急性生理应激对大鼠的行为及脑神经颗粒素磷酸化水平的影响

突触特异性蛋白质在应激所致行为效应的中枢机制中的可能角色日益受到关注。神经颗粒素（Neurogranin,NG）是一种新发现的突触特异性蛋白质,主要分布在前额叶、杏仁核和海马区域,参与突触结构和功能可塑性机制,可能涉及到应激所致行为效应中枢机制。但是,关于NG、应激和行为之间的关系国内外尚缺乏系统的研究报道。本研究主要是探讨急性生理应激对大鼠行为和NG的作用,以及NG的变化与应激性行为效应之间的相互关系。以急性强迫性冷水游泳应激,建立生理应激动物模型。将40只雄性SD大鼠随机分为游泳应激组1（SS1,接受游泳应激和行为测试）、游泳应激组2（SS2,接受游泳应激而不接受行为测试）、正常对照组1（C1,接受行为测试）和正常对照组（C2,不给予任何处理）（n=10）。以旷场行为和高架十字迷宫任务来评定大鼠应激后的行为变化,Western blotting方法测定海马和前脑皮层中的NG含量和磷酸化水平。结果表明：应激后SS1组的呆滞行为增加,与C1组比较,差异有显著性, P<0.01; SS1组海马的NG含量和NG磷酸化水平增高,与C1和C2组相比,差异有显著性,均为P<0.05; SS1组皮层的NG含量增高,与C1和C2组相比,差异有显著性,均为P<0.01;SS1组皮层的NG磷酸化水平增高,与C1组相比,差异具有显著性,P<0.01;前脑皮层的NG磷酸化水平与呆滞行为之间的相关达显著水平。提示该应激源能诱发动物明显的恐惧反应,呆滞行为是反映急性生理应激导致行为障碍的敏感的行为学指标,海马和前脑皮层均是对急性生理应激反应敏感的脑区。NG的磷酸化水平可能是反映急性生理应激所致行为障碍的一项新的生物学指标     

Acute stress rapidly and persistently enhances memory formation in the male rat

Previous studies, as well as the present one, report that acute exposure to intermittent tailshocks enhances classical eyeblink conditioning in male rats when trained 24 h after stressor cessation. In Experiment 1, it was determined that the facilitating effect of stress on conditioning could also be obtained in response to a stressor of acute inescapable swim stress but not inescapable noise or the unconditioned stimulus of periorbital eyelid stimulation. These selective responses arose despite comparable enhancements of the stress-related hormone corticosterone in response to tailshocks, periorbital eyelid stimulation, noise stress, and supraelevation in response to swim stress. Although corticosterone is necessary for the enhanced learning in response to stress (Beylin & Shors, 1999), these results suggest that it is not sufficient. In addition, the results suggest that the enhancement is not dependent on common characteristics between the stressor and the conditioning stimuli (stimulus generalization). In Experiment 2, it was determined that the facilitating effect of the stressor on conditioning occurs within 30 min of stressor cessation. Thus, the mechanism responsible for facilitating memory formation is rapidly induced as well as persistently expressed. In Experiment 3, it was determined that exposure to the stressor does not enhance performance of the conditioned response after the response has been acquired. Thus, exposure to the stressor enhances the formation of new associations rather than affecting retention or performance of the motor response. These studies extend the circumstances under which stress is known to enhance associative learning and implicate neural mechanisms of memory enhancement that are rapidly induced and persistently expressed.     

An automated swim alley for small animals: II. Training and procedures

Experiments were conducted to establish training procedures and testing conditions that produce uniform and reliable swimming performance of rats in an automated swim alley. Data are given on the effect of water temperature and of platform rest interval on swimming speeds. In this system, rats will swim intermittently for as long as 24 h for a distance of more than 7 km.     

The effects of aging and dorsal hippocampal lesions: performance on spatial and nonspatial comparable versions of the water maze

Aged intact and young hippocampal-lesioned rats show similar deficits on the spatial water maze. However, this does not necessitate that the source of these deficits in the aged animals is due to hippocampal damage. These water maze deficits may arise from other aging factors such as changes in thermoregulation, muscle fatigue, swim ability, and response to stress. Consequently, it is imperative to examine the performance of aged rats on a comparable nonhippocampal version of this task. Past attempts to develop a hippocampus-independent version of the water maze were confounded because these tasks were easier (i.e., the rats spent much less time swimming in the water) than the spatial versions of the task. The current study examined performance on a hippocampus-independent task comparable in difficulty to the spatial water one. Middle-aged (16-m) and old (25-m) male F344 rats were given sham or dorsal hippocampus lesions and tested on both a spatial and a nonspatial water maze. The middle-aged rats with hippocampal lesions were impaired on the spatial task but not on the nonspatial task. Conversely, aged animals showed a similar impairment on both types of water maze tasks. Additionally, hippocampal lesions exacerbated the age-related impairment on both tasks. These findings indicate that caution must be used when interpreting the results of water maze tasks for aged animals.     

Altered grooming responses to stress in rats exposed prenatally to ethanol

The effects of prenatal alcohol exposure on grooming, locomotion, and rearing in response to stress were examined in adult rats whose mothers consumed a liquid diet containing 35% ethanol-derived calories (EDC). Offspring of both pair-fed 0% EDC mothers and ad libitum chow-fed mothers were included as controls. In Experiment 1, females groomed more than males following placement into a novel test chamber, but no differences due to prenatal treatment were observed. Ethanol-exposed animals groomed more than controls following the stress of a forced 1-min swim (Experiment 2), but when rats tested in Experiment 1 were observed again after forced swim stress (Experiment 3), no differences due to prenatal treatment or sex were observed. Experiment 4 examined the effects of pretreatment with 1 mg/kg naloxone on novelty-induced grooming and as in Experiment 1 prenatal treatment did not affect grooming responses. Females again groomed significantly more than males and naloxone reduced grooming equally for all groups. The results suggest that novelty-induced grooming is a sex-influenced behavior, with females grooming more than males, and that animals exposed prenatally to alcohol and tested as adults may have altered responses to certain stressors (i.e., forced swim) under specific conditions. The altered grooming response of alcohol-exposed rats to swim stress can be eliminated by preexposing them to novelty stress.     

Intermittent exposure to social defeat and open-field test in rats: acute and long-term effects on ECG,body temperature and physical activity

This study investigated the effects of exposure to an intermittent homotypic stressor on: (i) habituation of acute autonomic responsivity (i.e. cardiac sympathovagal balance and susceptibility to arrhythmias), and (ii) circadian rhythmicity of heart rate, body temperature, and physical activity. After implantation of a transmitter for the radiotelemetric recording of electrocardiogram (ECG), body temperature and physical activity, adult male rats (Rattus norvegicus, Wild Type Groningen strain) were repeatedly exposed (10 consecutive times, on alternate days) to either a social stressor (defeat by a con-specific, n = 15) or an open-field, control challenge (transfer to a new cage; n = 8). ECGs, body temperature and physical activity were continuously recorded in baseline, test and recovery periods (each lasting 15 min), at the 1st and 10th episodes of both defeat and open-field challenge. The circadian rhythms of heart rate, body temperature and physical activity were monitored before (5 days), during (16 days) and after (21 days) the intermittent stress protocol. This study indicates that there is no clear habituation of either acute cardiac autonomic responsivity (as estimated by means of time-domain indexes of heart rate variability) or arrhythmia occurrence to a brief, intermittent, homotypic challenge, regardless of the nature of the stressor (social or non-social). On the other hand, rats exposed to social challenge also failed to show adaptation of acute temperature and activity stress responsiveness, whereas rats facing open-field challenge developed habituation of activity and sensitization of temperature responses. Repeated social challenge produced remarkable reductions of the heart rate circadian rhythm amplitude (this effect being significantly greater than that produced by intermittent open-field), but only minor changes in the daily rhythms of body temperature and physical activity.     

Thermopreferendum of the rat: inter- and intra-subject variabilities

The thermopreferendum of male hooded rats was studied in a temporal thermal gradient. The preferred ambient temperature was found to be approximately 19 degrees C. In addition to the thermopreferendum, three other physiological functions (resting body temperature, daily food intake, and daily ambulatory activity) were each measured in eight rats for 8 days. Overall variability of the thermopreferendum data was higher than that of body temperature and food intake data but lower than the variability of ambulatory activity data. Coefficients of inter- and intrasubject variabilities were computed for each of the four variables. Inter-subject variability was equivalent in all variables, but intrasubject variability was smaller for thermopreferendum and activity than for food intake and core temperature. The thermopreferendum of 19 degrees C is consistent with previously published results but inconsistent with the available data on the thermoneutral zone of the rat (27-28 degrees C). The cause of this inconsistency remains to be determined.