Expression of stress-evoked analgesia, connected with activity of animals in a forced swimming test]

The influence of forced swimming on the development of stress-induced analgesia was studied in 35 SHR mice, 65 NMRI mice, and 23 white outbred male rats. Mice were subjected to swimming conditions (at a temperature of 11 degrees C) for a period of 4 minutes and rats for 6 minutes. Pain thresholds were measured by a footshock. It was shown that behavioral response to acute stress is associated with a change in the pain tolerance threshold: activity of an animal under test conditions positively correlated with stress-induced analgesia. The response to stress and parameters of stress-induced analgesia depend on the genetic factor and age, however, the correlation between the activity during exposure to stress and the extent of stress-induced analgesia conserves in all cases.     

Footshock-induced changes in brain catecholamines and indoleamines are not mediated by CRF or ACTH

Stressful treatments have long been associated with increased activity of brain catecholaminergic and serotonergic neurons. An intracerebroventricular (icv) injection of the corticotropin-releasing factor (CRF) also activates brain catecholaminergic neurons. Because brain CRF-containing neurons appear to be activated during stress, it is possible that CRF mediates the catecholaminergic activation. This hypothesis has been tested by assessing the responses in brain catecholamines and indoleamines to footshock in mice pretreated icv with a CRF receptor antagonist, and in mice lacking the gene for CRF (CRFko mice). Consistent with earlier results, icv administration of CRF increased catabolites of dopamine and norepinephrine, but failed to alter tryptophan concentrations or serotonin catabolism. A brief period of footshock increased plasma corticosterone and the concentrations of tryptophan and the catabolites of dopamine, norepinephrine and serotonin in several brain regions. Mice injected icv with 25 microg alpha-helical CRF(9-41) prior to footshock had neurochemical responses that were indistinguishable from controls injected with vehicle, while the increase in plasma corticosterone was slightly attenuated in some experiments. CRFko mice exhibited neurochemical responses to footshock that were indistinguishable from wild-type mice. However, whereas wild-type mice showed the expected increase in plasma corticosterone, there was no such increase in CRFko mice. Similarly, hypophysectomized mice also showed normal neurochemical responses to footshock, but no increase in plasma corticosterone. Hypophysectomy itself elevated brain tryptophan and catecholamine and serotonin metabolism. Treatment with ACTH icv or peripherally failed to induce any changes in cerebral catecholamines and indoleamines. These results suggest that CRF and its receptors, and ACTH and other pituitary hormones, are not involved in the catecholamine and serotonin responses to a brief period of footshock.     

Neural cell adhesion molecule (NCAM-/-) null mice show impaired sensitization of the startle response

The neural cell adhesion molecule (NCAM) plays important roles in development of the nervous system and in synaptic plasticity and memory formation in the adult. The present study sought to further investigate the role of NCAM in learning by testing habituation and footshock sensitization learning of the startle response (SR) in NCAM null mutant (NCAM-/-) and wildtype littermate (NCAM+/+) mice. Whereas habituation is a form of non-associative learning, footshock sensitization is induced by rapid contextual fear conditioning. Habituation was tested by repetitive presentation of acoustic and tactile startle stimuli. Although NCAM-/- mice showed differences in sensitivity in both stimulus modalities, habituation learning was intact in NCAM-/- mice, suggesting that NCAM does not play a role in the mechanisms underlying synaptic plasticity in the startle pathway. Footshock sensitization was elicited by presentation of electric footshocks between two series of acoustic stimuli. In contrast to habituation, footshock sensitization learning was attenuated in NCAM-/- mice: the acoustic SR increase after the footshocks was lower in the mutant than in wildtype mice, indicating that NCAM plays an important role in the relevant brain areas, such as amygdala and/or the hippocampus.     

Lappaconitine and N-deacetyllappaconitine potentiate footshock-induced analgesia in rats

The effects of lappaconitine (LA) and N-deacetyllappaconitine (DLA) on footshock-induced analgesia (FSIA) were studied by the rat tail flick test. Rats subjected to 90 s nonescaping footshock had a significant increase in tail flick latency. Naloxone (4 micrograms, i.c.v.) partially antagonized the FSIA. After 5 consecutive exposures to footshock, rats developed a complete tolerance to the FSIA. The rats tolerant to FSIA showed a cross-tolerance to morphine- but not LA- and DLA-induced analgesia. Administrations of subanalgesic doses of LA and DLA potentiated the FSIA in both intact and adrenalectomized rats.     

Exposure to nuclear magnetic resonance imaging procedure attenuates morphine-induced analgesia in mice

Adult male mice exposed to a Nuclear Magnetic Resonance Imaging (NMRI) procedure during the mid-dark period and injected with morphine (10 mg/kg) failed to exhibit the normal nocturnally enhanced morphine analgesia response to a thermal stimulus that was displayed by mice exposed to a sham imaging procedure and treated with morphine (p less than .01). When tested during the mid-light period, animals exposed to the NMRI procedure and given morphine displayed attenuated analgesia levels relative to sham exposed mice (p less than .01) treated with morphine. However, the morphine induced analgesia was not totally abolished since the imaged mice still exhibited analgesia relative to saline treated mice (p less than .01). These results suggest that the magnetic and/or radio-frequency fields associated with the NMRI procedure alter both day- and night-time responses to morphine. These results may reflect magnetic field induced alterations in neuronal calcium binding and/or alterations in nocturnal pineal gland activity.     

Bi-directional effect of cholecystokinin receptor-2 overexpression on stress-triggered fear memory and anxiety in the mouse

Fear, an emotional response of animals to environmental stress/threats, plays an important role in initiating and driving adaptive response, by which the homeostasis in the body is maintained. Overwhelming/uncontrollable fear, however, represents a core symptom of anxiety disorders, and may disturb the homeostasis. Because to recall or imagine certain cue(s) of stress/threats is a compulsory inducer for the expression of anxiety, it is generally believed that the pathogenesis of anxiety is associated with higher attention (acquisition) selectively to stress or mal-enhanced fear memory, despite that the actual relationship between fear memory and anxiety is not yet really established. In this study, inducible forebrain-specific cholecystokinin receptor-2 transgenic (IF-CCKR-2 tg) mice, different stress paradigms, batteries of behavioral tests, and biochemical assays were used to evaluate how different CCKergic activities drive fear behavior and hormonal reaction in response to stresses with different intensities. We found that in IF-CCKR-2 tg mice, contextual fear was impaired following 1 trial of footshock, while overall fear behavior was enhanced following 36 trials of footshock, compared to their littermate controls. In contrast to a standard Yerkes-Dodson (inverted-U shaped) stress-fear relationship in control mice, a linearized stress-fear curve was observed in CCKR-2 tg mice following gradient stresses. Moreover, compared to 1 trial, 36 trials of footshock in these transgenic mice enhanced anxiety-like behavior in other behavioral tests, impaired spatial and recognition memories, and prolonged the activation of adrenocorticotropic hormone (ACTH) and glucocorticoids (CORT) following new acute stress. Taken together, these results indicate that stress may trigger two distinctive neurobehavioral systems, depending on both of the intensity of stress and the CCKergic tone in the brain. A "threshold theory" for this two-behavior system has been suggested.     

Stable fly, Stomoxys calcitrans, mouthpart removal influences stress and anticipatory responses in mice

Abstract Biting fly attack induces a variety of stress and anxiety related changes in the physiology and behaviour of the target animals. Significant reductions in pain, or more appropriately, nociceptive sensitivity (latency of a foot-lifting response to an aversive thermal stimulus), are evident in laboratory mice after a 1 h exposure to stable flies, Stomoxys calcitrans. The role of the various components of biting fly attack in the development of this stress-induced reduction in pain sensitivity (analgesia) is, however, unclear. This study demonstrates that fly-naive mice do not exhibit a stress-induced analgesia when exposed to stable flies whose biting mouthparts have been removed. In contrast, mice that have been previously exposed to intact stable flies exhibit significant analgesia when exposed to flies that are incapable of biting. However, the level of analgesia induced is lower than that elicited by exposure to intact stable flies. Exposure to non-biting house flies, Musca domestica , has no effect on nociceptive sensitivity. It appears that the actual bite of the stable fly is necessary for the induction of analgesia and probably other stress and anxiety associated responses in fly naive mice. However, mice rapidly learn to recognize biting flies and exhibit significant, possibly anticipatory analgesic responses to the mere presence of biting flies.     

Aversive Responses of Female Mice to the Odors of Parasitized Males: Neuromodulatory Mechanisms and Implications for Mate Choice

The detection and avoidance of parasitized conspecifics is proposed to have important consequences for the behavior of animals, especially as related to mate choice. A reduction in pain sensitivity (i.e. analgesia) is a major correlate of exposure to real or potential danger and threatening stimuli, facilitating the expression of various active (e.g. fleeing) and passive (e.g. immobilization) defense responses. The present study examined pain sensitivity (latency of a foot-lifting response to 50 ° C thermal surface) of female mice, Mus musculus, that were exposed to the urine and other odor secretions of male mice subclinically infected with the naturally occurring, enteric, sporozoan parasite, Eimeria vermiformis. A 30-min exposure to the odors of a parasitized male induced an analgesia in the female mice that was found to be mediated by the increased activity of endogenous opioid peptide systems. A brief 1-min exposure to the male odors induced a shorter duration and lower amplitude analgesia of a non-opioid (serotonergic) nature. Maximum analgesic responses were induced by the odors of pre-infective [5 days post-infection (PI)] and infective (day 10 PI) males, with significantly lower responses elicited by the odors of post-infective (day 17 PI) male mice. Exposure to the odors of unparasitized males had no significant effects on the pain sensitivity of female mice. These results indicate that female mice can distinguish between the odors of parasitized and non-parasitized male mice, and find the odors of parasitized males threatening and/or stressful. These odor-induced analgesic responses and their neurohormonal correlates may be part of an adaptive preparatory defense mechanism that facilitates the detection and avoidance of parasitized males by female mice and contributes to female mate choice.     

The analgesic effect induced by capsaicin is enhanced in inflammatory states

Agonists of the vanilloid receptor type 1 (VR1), such as capsaicin, induce an analgesic effect following an initial excitatory response. It has been demonstrated that the vanilloid system plays an important role in inflammatory hyperalgesia. In accordance, we show that the VR1 antagonist capsazepine (30 microg; i.pl.) prevented the thermal hyperalgesia induced by carrageenan or complete Freund's adjuvant (CFA) in mice. Furthermore, we studied whether this inflammation-induced activation of the vanilloid system could enhance the analgesic properties of capsaicin. A single administration of capsaicin (10 microg; i.pl.) induced in control mice an analgesic effect that lasted for 2 days. In contrast, in carrageenan-treated animals, the analgesic effect of this dose of capsaicin lasted for 6 days and in CFA-treated mice for 30 days. This prolongation of capsaicin-induced analgesia during inflammation was mediated through VR1 since it was completely blocked by coadministration of capsazepine (10 microg). Licking behavior induced by capsaicin in carrageenan- and CFA-treated mice was greater than in control animals. However, although capsaicin induced a more prolonged analgesia in CFA-treated mice, the licking behavior was greater in the carrageenan-treated group, suggesting that the prolongation of analgesia is independent of the initial nociceptive input. Overall, these results show that the analgesic effects of capsaicin are importantly enhanced during inflammation, supporting the fact that the stimulation of VR1 could perhaps constitute a suitable strategy to avoid inflammatory hyperalgesia.     

The expression of the c-fos gene in the brain of mice in the dynamic acquisition of defensive behavioral habits

Levels of c-fos mRNA expression in mouse cerebral cortex and hippocampus at different stages of footshock escape and avoidance learning were studied by Northern hydridization. In the first series of experiments a mouse was presented with 30 electric footshock daily in a chamber where it could escape from the floor by jumping on the safe platform attached to the wall. A large increase in c-fos mRNA level in the cerebral cortex and hippocampus was observed during the first day of training. Mice that were trained for 9 consecutive days and acquired a footshock escape reaction showed no elevation of c-fos expression in the brain as compared to the quiet control group. In the second series of experiments the levels of c-fos expression were compared in individual mice trained to avoid the footshock by jumping on the platform in response to an auditory conditioned stimulus. Mice which acquired avoidance behavior more rapidly had lower c-fos mRNA levels than slow learners. There was no such to difference between the corresponding yoked control groups which consisted of animals matched the rapid and slow learners by the number of footshocks received. It is concluded that achievement of adaptive results in the course of learning leads to a suppression of further c-fos induction by motivational excitation.     

Brain renin angiotensin system (RAS) in stress-induced analgesia and impaired retention.

Physiological stress is known to produce analgesia and memory disruption. Brain renin angiotensin system (RAS) has been reported to participate in stress response and plays a role in the processing of sensory information. Angiotensin receptors (AT), particularly AT1 subtypes have been reported to be distributed in brain areas that are intimately associated with stress response. The purpose of present study was to examine the modulation of AT1 receptor in the immobilization stress and angiotensin II (AngII)-induced analgesia and impaired retention, and to determine whether resultant behavioral changes involve common sensory signals. Result of present experiments showed that immobilization stress in mice and rats, and intracerebroventricular (ICV) administration of AngII (10 and 20 ng) in rats produced an increase in tail-flick latency. Similarly, post training administration of AngII or immobilization stress produced impairment of retention tested on plus-maze learning and on passive avoidance step-down task. Both these responses were sensitive to reversal by prior treatment with losartan (10 and 20 mg/kg), an AT1 AngII receptor antagonist. On the other hand, naloxone, an opiate antagonist preferentially attenuated the stress and AngII-induced analgesia and retention deficit induced by immobilization stress, but failed to reverse the AngII induced retention deficit. These results suggest immobilization stress-induced analgesia and impaired retention involves the participation of brain RAS. Further, failure of naloxone to reverse AngII-induced retention impairment shows. AngII-induced behavioral changes are under control of different sensory inputs.     

Neuromodulatory effects of TRH upon swim and cholinergic analgesia

In addition to short-acting analgesic actions by itself and modulation of analgesic responses induced by endogenous opioids and neurotensin, central administration of thyrotropin-releasing hormone (TRH) potentiates footshock analgesia. The present study evaluated the effects of TRH upon the neurohormonally-mediated though nonopioid analgesia induced by swims in rats. Intracerebroventricular TRH (10 and 50 micrograms) dose-dependently potentiated swim (21, 15, 2 degrees C baths) analgesia on the tail-flick test, an effect which was not due to the hypothermic or basal pain threshold changes. Intravenous (8 mg/kg) TRH potentiated swim (21 degrees C) analgesia; the 600:1 difference in potency between routes strongly suggests central sites of neuromodulatory action. Intracerebroventricular diketopiperazine (50 micrograms), a TRH metabolite, and RX77368 (50 micrograms), a TRH analogue, also potentiated swim (21 degrees C) analgesia, effects also independent of hypothermia and basal reactivity to pain. Finally, given the excitatory interaction between TRH and acetylcholine as well as the cholinergic involvement in swim analgesia, intracerebroventricular TRH potentiated pilocarpine (10 mg/kg, IP) analgesia.     

A role for histamine and histamine H2-receptors in non-opiate footshock-induced analgesia

Scrambled DC current applied to the hind paws of rats caused an analgesic response that was inhibited by the histamine H2-receptor antagonists cimetidine, ranitidine and oxmetidine, but not by high doses of naloxone (the opiate antagonist), or other transmitter receptor antagonists. In contrast, AC current applied to all paws produced analgesia that was blocked by naloxone, but not cimetidine, showing the independence of these systems. These findings indicate a specific role for histamine and H2-receptors as mediators of endogenous non-opiate analgesia. In addition, a combination of cimetidine and naloxone did not abolish either form of footshock analgesia, implying the existence of a non-opiate, non-H2, endogenous pain-relieving system. These results also suggest that drugs capable of penetrating the brain and stimulating H2-receptors might have analgesic properties.     

Apparent involvement of opioid peptides in stress-induced enhancement of tumor growth

Exposure to stress has been associated with alterations in both immune function and tumor development in man and laboratory animals. In the present study, we investigated the effect of a particular type of inescapable footshock stress, known to cause an opioid mediated form of analgesia, on survival time of female Fischer 344 rats injected with a mammary ascites tumor. Rats subjected to inescapable footshock manifested an enhanced tumor growth indicated by a decreased survival time and decreased percent survival. This tumor enhancing effect of stress was prevented by the opiate antagonist, naltrexone, suggesting a role for endogenous opioid peptides in this process. In the absence of stress, naltrexone did not affect tumor growth.     

Evidence for activation of endogenous opioid systems in mice following short exposure to stable flies

Biting flies influence both physiology and behaviour of domestic and wild animals. This study demonstrates that brief (30 min) exposure of male and female mice to stable flies leads to significant increases in nociceptive responses, indicative of the induction of analgesia. The biting fly-induced analgesia was mediated by endogenous opioid systems as it was blocked by the prototypic opiate antagonist naloxone. Exposure for 30 min to the bedding of biting fly-exposed mice also induced significant opioid mediated analgesic responses in mice. Exposure to either house flies or the bedding of house fly-exposed mice had no significant effects on nociception. These results indicate that brief exposure to either stable flies, or to olfactory cues associated with mice exposed to stable flies, activates endogenous opioid systems leading to the induction of analgesia and likely other opioid mediated behavioural and physiological stress responses. These results suggest the involvement of endogenous opioid systems in the mediation of the behavioural and physiological consequences of biting fly exposure in domestic and wild animals.     

Lesions of the Fasciculus Retroflexus Alter Footshock-Induced cFos Expression in the Mesopontine Rostromedial Tegmental Area of Rats

Midbrain dopamine neurons are an essential part of the circuitry underlying motivation and reinforcement. They are activated by rewards or reward-predicting cues and inhibited by reward omission. The lateral habenula (lHb), an epithalamic structure that forms reciprocal connections with midbrain dopamine neurons, shows the opposite response being activated by reward omission or aversive stimuli and inhibited by reward-predicting cues. It has been hypothesized that habenular input to midbrain dopamine neurons is conveyed via a feedforward inhibitory pathway involving the GABAergic mesopontine rostromedial tegmental area. Here, we show that exposing rats to low-intensity footshock (four, 0.5 mA shocks over 20 min) induces cFos expression in the rostromedial tegmental area and that this effect is prevented by lesions of the fasciculus retroflexus, the principal output pathway of the habenula. cFos expression is also observed in the medial portion of the lateral habenula, an area that receives dense DA innervation via the fr and the paraventricular nucleus of the thalamus, a stress sensitive area that also receives dopaminergic input. High-intensity footshock (120, 0.8 mA shocks over 40 min) also elevates cFos expression in the rostromedial tegmental area, medial and lateral aspects of the lateral habenula and the paraventricular thalamus. In contrast to low-intensity footshock, increases in cFos expression within the rostromedial tegmental area are not altered by fr lesions suggesting a role for non-habenular inputs during exposure to highly aversive stimuli. These data confirm the involvement of the lateral habenula in modulating the activity of rostromedial tegmental area neurons in response to mild aversive stimuli and suggest that dopamine input may contribute to footshock- induced activation of cFos expression in the lateral habenula.     

Dynorphin (1-13): analgesia, hypothermia, cross-tolerance with morphine and beta-endorphin

Intracerebroventricular administration of 20, 40 and 60 nmol of dynorphin (1-13) produced analgesia, as assessed by flinch/jump response to footshock, and hypothermia in the rat. Rats developed tolerance to both the analgesic and thermic effects of the 20 nmol dose of dynorphin. Dynorphin and beta-endorphin showed cross-tolerance with respect to their analgesic but not their thermic effects. Dynorphin and morphine also produced cross-tolerant analgesic effects. Naloxone (10 mg/kg, IP) completely blocked the barrel rolling produced by 20 nmol dynorphin but did not alter its analgesic or thermic effects.     

Anxiety- and depressive-like responses and c-<Emphasis Type="Italic">fos</Emphasis> activity in preproenkephalin knockout mice: Oversensitivity hypothesis of enkephalin deficit-induced posttraumatic stress disorder

The present study used the preproenkephalin knockout (ppENK) mice to test whether the endogenous enkephalins deficit could facilitate the anxiety- and depressive-like symptoms of posttraumatic stress disorder (PTSD). On Day 1, sixteen wildtype (WT) and sixteen ppENK male mice were given a 3 mA or no footshock treatment for 10 seconds in the footshock apparatus, respectively. On Days 2, 7, and 13, all mice were given situational reminders for 1 min per trial, and the freezing response was assessed. On Day 14, all mice were tested in the open field test, elevated plus maze, light/dark avoidance test, and forced swim test. Two hours after the last test, brain tissues were stained to examine c-fos expression in specific brain areas. The present results showed that the conditioned freezing response was significant for different genotypes (ppENK vs WT). The conditioned freezing effect of the ppENK mice was stronger than those of the WT mice. On Day 14, the ppENK mice showed more anxiety- and depressive-like responses than WT mice. The magnitude of Fos immunolabeling was also significantly greater in the primary motor cortex, bed nucleus of the stria terminalis-lateral division, bed nucleus of the stria terminalis-supracapsular division, paraventricular hypothalamic nucleus-lateral magnocellular part, central nucleus of the amygdala, and basolateral nucleus of the amygdala in ppENK mice compared with WT mice. In summary, animals with an endogenous deficit in enkephalins might be more sensitive to PTSD-like aversive stimuli and elicit stronger anxiety and depressive PTSD symptoms, suggesting an oversensitivity hypothesis of enkephalin deficit-induced PTSD.     

钙离子对小鼠电针镇痛和吗啡镇痛影响的相似性

本工作以小鼠为对象研究了脑 Ca~(2 )水平与吗啡及电针镇痛的相互关系。脑室内注射杆菌肽和亮-脑啡肽能增强电针镇痛,后者可被腹腔注射 Ca~(2 )对抗。用多巴胺受体激动剂阿朴吗啡和拮抗剂氟派啶预处理并不改变 Ca~(2 )对电针镇痛的对抗作用,这表示脑内多巴胺类似不直接参与 Ca~(2 )对抗电针镇痛的作用。Ca~(2 )对抗电针镇痛和吗啡镇痛的时程十分相似。所有的结果表明,吗啡镇痛与电针镇痛的机理很可能是相同的。     

Altered expression of synaptotagmin 13 mRNA in adult mouse brain after contextual fear conditioning

Contextual fear memory processing requires coordinated changes in neuronal activity and molecular networks within brain. A large number of fear memory-related genes, however, still remain to be identified. Synaptotagmin 13 (Syt13), an atypical member of synaptotagmin family, is highly expressed in brain, but its functional roles within brain have not yet been clarified. Here, we report that the expression of Syt13 mRNA in adult mouse brain was altered following contextual fear conditioning. C57BL/6 mice were exposed to a novel context and stimulated by strong electrical footshock according to a contextual fear conditioning protocol. After 24h, the mice were re-exposed to the context without electrical footshock for the retrieval of contextual fear memory. To investigate the relationship between Syt13 and contextual fear memory, we carried out in situ hybridization and analyzed gene expression patterns for Syt13 at four groups representing temporal changes in brain activity during contextual fear memory formation. Contextual fear conditioning test induced significant changes in mRNA levels for Syt13 within various brain regions, including lateral amygdala, somatosensory cortex, piriform cortex, habenula, thalamus, and hypothalamus, during both acquisition and retrieval sessions. Our data suggest that Syt13 may be involved in the process of contextual fear memory.