Cats produce analgesia in rats on the tail-flick test: naltrexone sensitivity is determined by the nociceptive test stimulus

Ventrolateral periaqueductal gray matter integrative system of defense and antinociception

Defensive responses are neurophysiological processes crucial for survival during threatening situations. Defensive immobility is a common adaptive response, in rodents, elaborated by ventrolateral periaqueductal gray matter (vlPAG) when threat is unavoidable. It is associated with somatosensory and autonomic reactions such as alteration in the sensation of pain and rate of respiration. In this study, defensive immobility was assessed by chemical stimulation of vlPAG with different doses of NMDA (0.1, 0.3, and 0.6 nmol). After elicitation of defensive immobility, antinociceptive and respiratory response tests were also performed. Results revealed that defensive immobility was followed by a decrease in the nociceptive perception. Furthermore, the lowest dose of NMDA induced antinociceptive response without eliciting defensive immobility. During defensive immobility, respiratory responses were also disturbed. Interestingly, respiratory rate was increased and interspersed with prolonged expiratory phase of breathing. These findings suggest that vlPAG integrates three different defensive behavioral responses, contributing to the most effective defensive strategies during threatening situations.

Effects of parasites and predators on nociception: decreases analgesia reduces overwinter survival in root voles (Rodentia: Cricetidae)

Growing evidence suggests that parasite-infected prey is more vulnerable to predation. However, the mechanism underlying this phenomenon is obscure. In small mammals, analgesia induced by environmental stressors is a fundamental component of the defensive repertoire, promoting defensive responses. Thus, the reduced analgesia may impair the defensive ability of prey and increase their predation risk. This study aimed to determine whether coccidia infection increases the vulnerability to predation in root voles, Microtus oeconomus (Pallas, 1776), by decreased analgesia. Herein, a predator stimulus and parasitic infection were simulated in the laboratory via a two-level factorial experiment, then, the vole nociceptive responses to an aversive thermal stimulus were evaluated. Further, a field experiment was performed to determine the overwinter survival of voles with different nociceptive responses via repeated live trapping. The coccidia-infected voles demonstrated reduced predator-induced analgesia following exposure to predator odor. Meanwhile, pain-sensitive voles had lower overwinter survival than pain-inhibited voles in enclosed populations throughout the duration of the experiment. Our findings suggest that coccidia infection attenuates predator-induced analgesia, resulting in an increased vulnerability to predation.

Sensorimotor developmental factors influencing the performance of laboratory rodents on learning and memory

Behavioral studies in animal models have advanced our knowledge of brain function and the neural mechanisms of human diseases. Commonly used laboratory rodents, such as mice and rats, provide a useful tool for studying the behaviors and mechanisms associated with learning and memory processes which are cooperatively regulated by multiple underlying factors, including sensory and motor performance and emotional/defense innate components. Each of these factors shows unique ontogeny and governs the sustainment of behavioral performance in learning tasks, and thus, understanding the integrative processes of behavioral development are crucial in the accurate interpretation of the functional meaning of learning and memory behaviors expressed in commonly employed behavioral test paradigms. In this review, we will summarize the major findings in the developmental processes of rodent behavior on the basis of the emergence of fundamental components for sustaining learning and memory behaviors. Briefly, most sensory modalities (except for vision) and motor abilities are functional at the juvenile stage, in which several defensive components, including active and passive defensive strategies and risk assessment behavior, emerge. Sex differences are detectable from the juvenile stage through adulthood and are considerable factors that influence behavioral tests. The test paradigms addressed in this review include associative learning (with an emphasis on fear conditioning), spatial learning, and recognition. This basic background information will aid in accurately performing behavioral studies in laboratory rodents and will therefore contribute to reducing inappropriate interpretations of behavioral data and further advance research on learning and memory in rodent models.

Age and sex differences in the innate defensive behaviors of C57BL/6 mice exhibited in a fear conditioning paradigm and upon exposure to a predatory odor

Defensive responses of mice include an array of specific behaviors that involve adaptive components based on the assessment of the threat of confrontation. The freezing response is represented by a motionless moment without any specific posture or behavioral sequence, and it is widely used in the fear conditioning paradigms and other relevant defensive situations. However, freezing measurements include fragmental components of several defensive behaviors that are exhibited during the session, such as behavioral inhibition, crouching, and a moment of risk assessment behavior. From an ethological view, behavioral analyses of C57BL/6 mice of both sexes and three different ages (postnatal days (P) 25, 35, and 65) revealed a rich variety of defensive behaviors during a fear conditioning session and in response to predatory odor exposure as a nonconditioned behavior. P-25 and 35 mice exhibited more behavioral inhibition than P-65 adult mice, and P-65 mice exhibited a crouching posture more often than younger mice. This age difference was more pronounced in males. The stretch-attend posture (SAP) increased with age, except in P-25 males, which exhibited robust SAP in response to a conditioned cue; this response indicates that P-25 males are defensive in a situation-nonmatching manner. Situation-dependent defense strategies were revealed in P-35 and 65 mice: Fear conditioning paradigm induced more robust defensive responses than predatory odor exposure, to which mice primarily exhibited SAP. A sex-based difference was revealed in adult mice. Males tended to show more passive defensive responses, such as crouching and withdrawal, and females exhibited more active responses, such as SAP. These age- and sex-based differences may stem from the ethological demands and illustrate adolescent ontogenetic processes of defense behavior.

Odour-induced analgesia mediated by hypothalamic orexin neurons in mice

Various folk remedies employ certain odorous compounds with analgesic effects. In fact, linalool, a monoterpene alcohol found in lavender extracts, has been found to attenuate pain responses via subcutaneous, intraperitoneal, intrathecal, and oral administration. However, the analgesic effects of odorous compounds mediated by olfaction have not been thoroughly examined. We performed behavioural pain tests under odourant vapour exposure in mice. Among six odourant molecules examined, linalool significantly increased the pain threshold and attenuated pain behaviours. Olfactory bulb or epithelium lesion removed these effects, indicating that olfactory sensory input triggered the effects. Furthermore, immunohistochemical analysis revealed that linalool activated hypothalamic orexin neurons, one of the key mediators for pain processing. Formalin tests in orexin neuron-ablated and orexin peptide-deficient mice showed orexinergic transmission was essential for linalool odour-induced analgesia. Together, these findings reveal central analgesic circuits triggered by olfactory input in the mammalian brain and support a potential therapeutic approach for treating pain with linalool odour stimulation.

NO/cGMP production is important for the endogenous peripheral control of hyperalgesia during inflammation

Various studies have demonstrated the role of the nitric oxide (NO)/cGMP pathway in pain processing. Our group has also shown that this system participates in opioid-induced antinociception during peripheral inflammation. We have previously observed that inflammation mobilizes an endogenous opioidergic system to control hyperalgesia. Here, we investigated whether the NO/cGMP pathway underlies peripheral endogenous nociception control during inflammation. In this study, a pharmacological approach was used in conjunction with the rat paw pressure test to assess the effects of intraplantar NO synthase inhibitor NG-Nitro-l-arginine (NOArg), guanylyl cyclase inhibitor methylene blue (MB), phosphodiesterase-5 inhibitor zaprinast (ZP), or NO precursor l-arginine injection on carrageenan-induced hyperalgesia, which mimics an inflammatory process, or by prostaglandin E(2) (PGE(2)), which directly sensitizes nociceptors. Intraplantar carrageenan (62.5, 125, 250 or 500μg) or PGE(2) (0.1, 0.5 or 2μg) administration produced hyperalgesia, which manifested as a reduction in the rat nociceptive threshold to mechanical stimuli. NOArg (25, 50 or 100μg/paw) and MB (125, 250 or 500μg/paw) induced significant and dose-dependent reductions in the nociceptive threshold of carrageenan-induced (125μg/paw) hyperalgesia, but not PGE(2)-induced (0.5μg/paw) hyperalgesia. This was a local effect because it did not produce any modifications in the contralateral paw. Both Zaprinast (100, 200 or 400μg/paw) and l-arginine (100, 200 or 400μg/paw) significantly counteracted carrageenan-induced hyperalgesia (250μg/paw), yielding an increase in the nociceptive threshold compared with the control. Zaprinast (200μg/paw) or l-arginine (400μg/paw) did not produce an antinociceptive effect in the contralateral paw, indicating local action. In addition, at the same dose that was able to modify carrageenan-induced hyperalgesia, neither zaprinast nor l-arginine modified PGE(2) (2μg) injection-induced hyperalgesia of the rat paw. Taken together, these results indicate that the l-arginine/NO/cGMP pathway functions as an endogenous modulator of peripheral inflammatory hyperalgesia.

Male risk taking, female odors, and the role of estrogen receptors

Male risk-taking and decision making are affected by sex-related cues, with men making riskier choices and decisions after exposure to either women or stimuli associated with women. In non-human species females and, or their cues can also increase male risk taking. Under the ecologically relevant condition of predation threat, brief exposure of male mice to the odors of a sexually receptive novel female reduces the avoidance of, and aversive responses to, a predator. We briefly review evidence showing that estrogen receptors (ERs), ERα and ERβ, are associated with the mediation of these risk taking responses. We show that ERs influence the production of the female odors that affect male risk taking, with the odors of wild type (ERαWT, ERβWT), oxytocin (OT) wildtype (OTWT), gene-deleted 'knock-out' ERβ (ERβKO), but not ERαKO or oxytocin (OT) OTKO or ovariectomized (OVX) female mice reducing the avoidance responses of male mice to cat odor. We further show that administration of specific ERα and ERβ agonists to OVX females results in their odors increasing male risk taking and boldness towards a predator. We also review evidence that ERs are involved in the mediation of the responses of males to female cues, with ERα being associated with the sexual and both ERβ and ERα with the sexual and social mechanisms underlying the effects of female cues on male risk taking. The implications and relations of these findings with rodents to ERs and the regulation of human risk taking are briefly considered.

Recognition and avoidance of the odors of parasitized conspecifics and predators: differential genomic correlates

In many species of animals chemical stimuli are an important source of information about the threats and dangers present in the social and non-social world. Olfactory cues play a fundamental role in modulating social recognition and interactions in a wide variety of mammals. Rodents, in particular, utilize chemical signals, to recognize and avoid conspecifics infected with parasites and other pathogens. Animals also respond to, and utilize, predator odor related information to assess and minimize their risk of predation. In this review, we briefly focus on the relations between odors, parasite recognition and avoidance and consider some of the associated hormonal, neural and genomic mechanisms. We describe how both male and female rodents distinguish between infected and uninfected males on the basis of odors, displaying aversive response to, and avoidance of, the urine odors of infected individuals. We further describe how the recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin, (OT), and estrogenic mechanisms. We show that mice with deletions of the oxytocin gene (OT knockout mice (OTKO)) and mice whose genes for estrogen receptor (ER)-alpha or ER-beta [ER knockout mice (ERKO), alphaERKO and betaERKO] have been disrupted are specifically impaired in their recognition, avoidance, and memory of the odors of infected individuals. We contrast this with the recognition and display of aversive responses to predator (cat) odor that are insensitive to these genetic manipulations. These findings reveal some of the mechanisms associated with the olfactory mediated recognition of parasitized individuals and predators.

Fear-induced hypoalgesia in humans: Effects on low intensity thermal stimulation and finger temperature

Prior research indicates that exposure to fear-inducing stimuli inhibits finger withdrawal to sudden onset and high intensity radiant heat in humans. Although withdrawal latencies to intense heat are thought to reflect changes in spinal nociceptive processing, supraspinal measures are needed to determine whether pain perception is altered. The present study used gradual onset and low intensity radiant heat to induce a finger withdrawal response that depends on supraspinal processes. After baseline pain threshold tests, 57 healthy human participants were randomly assigned to 1 of 2 groups. In the fear group, participants received 3 brief shocks. In the neutral group, participants did not receive shock. Results suggest that finger withdrawal latencies to low intensity heat were increased after shock presentation, providing additional evidence that fear reduces pain on a measure that is influenced by supraspinal processes. Both self-report and physiological (skin conductance level, heart rate, and blood pressure) measures of emotion confirmed that the intended affective states were induced. Finger temperature was unaffected by emotion manipulations; thus, skin cooling does not appear to mediate increased withdrawal latencies. These findings provide additional evidence that fear not only inhibits spinal nociceptive reflexes, it also inhibits supraspinal measures of pain.

PERSPECTIVE

From a clinical perspective, these data suggest that patients who experience intense fear in response to unpredictable threatening events will show a reduction in pain perception.

Noise stress and human pain thresholds: divergent effects in men and women

Considerable animal research suggests that exposure to noxious and nonnoxious fear-inducing stimuli can produce hypoalgesia. Although this effect is thought to generalize across species, only a few studies have examined the pain modulatory effects of nonnoxious fear-eliciting stimuli in humans. The present study examined whether exposure to a series of loud noise bursts would produce a fear-related hypoalgesia in male and female human subjects. Both subjective and physiologic measures (skin conductance level, heart rate) indicated that noise exposure resulted in fear, sympathetic arousal, and decreased pain reactivity in women (n = 20). In contrast, men (n = 20) did not experience fear or physiologic arousal, but reacted with surprise and increased pain reactivity. These findings provide additional evidence that hypoalgesia is mediated by fear and physiologic arousal. Although future studies should directly manipulate surprise, it appears that surprise without fear and physiologic arousal might enhance pain processing.

Individual differences in the emotional reaction to shock determine whether hypoalgesia is observed

OBJECTIVES

Our laboratory has shown that electrical shocks induce fear in human participants and subsequently inhibit pain on the hand receiving shocks. The present study examined whether pain modulation is bilateral, by testing pain on the hand contralateral to the one receiving shocks. We also evaluated whether individual differences in emotional response to the shocks influenced pain modulation.

DESIGN

Following baseline tests, 61 participants were randomly assigned to one of two groups. In the shock group, participants received three surprising electrical shocks. In the control group, stimulating electrodes were removed from participants' fingers, and no shocks were presented. Both groups received two more pain threshold tests 2 and 8 minutes later.

OUTCOME MEASURES

Similarly to the tail-flick test used in rodents, pain threshold was tested by measuring the latency of finger withdrawal to radiant heat. The following manipulation checks assessed the emotional state induced by shocks and the control procedure: Self-reported affect, skin conductance level (SCL), heart rate (HR), and blood pressure (BP).

RESULTS

Surprisingly, self-reported affect data indicate that some participants reacted to the shocks with humor and fear, while others reacted primarily with fear. Therefore, these groups were analyzed separately. Participants reacting with fear only exhibited hypoalgesia; however, participants reacting with mixed fear and humor showed no change in pain. This divergent effect was not mediated by arousal, because SCL, HR, and BP were similar in both shocked groups (fear only, fear + humor).

CONCLUSIONS

These data suggest that fear-induced hypoalgesia occurs bilaterally. However, humor experienced concurrent with fear appears to inhibit hypoalgesia. These findings may help explain individual differences in pain following traumatic events.

Negative affect: effects on an evaluative measure of human pain

Prior work indicates that exposure to fear-inducing shock inhibits finger-withdrawal to radiant heat in humans (hypoalgesia), whereas anxiety induced by threat of shock enhances reactivity (hyperalgesia; Pain 84 (2000) 65-75). Although finger-withdrawal latencies are thought to reflect changes in pain sensitivity, additional measures of pain are needed to determine whether pain perception is altered. The present study examined the impact of negative affect on visual analog scale (VAS) ratings of fixed duration thermal stimuli. One hundred twenty-seven male and female human subjects were randomly assigned to one of three emotion-induction conditions: (1) negative affect induced by exposure to three brief shocks; (2) negative affect elicited by the threat of shock without presentation; and (3) neutral affect, with no intervention. VAS ratings were tested before and after emotion-induction. Results suggest that both negative affect manipulations reduced pain. Manipulation checks indicated that the emotion-induction treatments induced similar levels of fear but with different arousal levels. Potential mechanisms for affect induced changes in pain are discussed.

Analgesia induced by swim stress: interaction between analgesic and thermoregulatory mechanisms

Exposure of an animal to stressful stimuli, perceived by the animal as a threatening, emergency condition, elicits a transient decrease of pain sensitivity, which often affects thermoregulatory mechanisms in the threatened organism. We studied the interaction between emergency and thermoregulatory components of swim stress in developing swim stress-induced analgesia (SSIA). The subjects were mice selectively bred for high analgesia (HA) induced by swimming in 20 degrees C water, and displaying profound swim hypothermia. The mice were acclimated to one of the following conditions: (1) ambient cold (5 degrees C, mimicking the thermal component of swim stress); (2) daily 3-min swimming at 32 degrees C (mimicking the emergency, emotional in nature, component of swim stress), or (3) daily swimming at 20 degrees C (a combination of both emergency and thermal component of swim stress). Following each of the procedures the analgesia induced by swimming in 20 degrees C water and by acute exposure to -2.5 degrees C in helium/oxygen (Helox) atmosphere was measured. Analgesia was also assessed in a group of naive mice immersed in 20 degrees C shallow water with the purpose of eliminating the emergency condition, but assuring the animal's contact with the cold water environment. Cold acclimation markedly attenuated Helox-induced analgesia (HIA) without affecting SSIA, whereas repeated swims attenuated SSIA without affecting HIA. The results suggest that hypothermia is the only stimulus eliciting HIA, while the emergency condition of swimming is essential for inducing SSIA. The significantly lower magnitude of SSIA in mice acclimated to repeated swims in 20 degrees C than in 32 degrees C water suggests that SSIA develops due to an interaction between the emergency and hypothermic components of swim stress. This is further supported by a greater hypothermia and greater analgesia in freely swimming than in immersed naive mice.

The defense system of fear: behavior and neurocircuitry

Fear can be conceived as a functional defense behavior system representing a part of the innate species-specific behavioral repertoire (ethogram), basic to the survival of individuals and species. Its function is to protect living beings against dangerous, threatening and aversive situations. A distinction is made between anticipatory defense behaviors released by potential dangers and those elicited by effective dangers, especially predators. The neural mechanisms serving the defense system constitute a hierarchical network with the amygdala as point of convergence of the various threatening stimuli. The central nucleus of the amygdala projects to the midbrain periaqueductal gray (PAG), the hypothalamus and the brainstem which coordinate various defensive responses such as flight, defensive fight, freezing, avoidance reactions, submissive postures, tonic immobilization, hypoalgesia and autonomic arousal. These circuits may be activated either by unconditioned or conditioned stimuli.

Brief exposure to female odors "emboldens" male mice by reducing predator-induced behavioral and hormonal responses

In rodents, where chemical signals play a particularly important role in determining intersexual interactions, various studies have shown that male behavior and physiology is sensitive to female odor cues. Here we examined the effects of brief (1 min) and more prolonged (60 min) preexposure to the odors of a novel estrous female on the behavioral and hormonal responses of sexually experienced and inexperienced male mice, Mus musculus, to subsequent predator (cat and weasel) odor exposure and potential predator risk. Brief, but not prolonged, preexposure to the odors of an estrous female decreased the aversion and avoidance responses of male mice to cat odor in a Y-maze preference test, with the extent of responses being affected by a males prior sexual experience. Similarly, brief, but not prolonged, preexposure to female odors markedly attenuated the analgesic responses elicited in male mice by weasel odor. Brief exposure to a novel estrous female by itself had no significant immediate effects on either corticosterone or testosterone levels in the males. However, brief, but not prolonged, preexposure to the odors of an estrous female attenuated the marked increase in corticosterone and decrease in testosterone that were induced in males by exposure to weasel odor. The decreases in aversive responses to, and effects of, predator odor exposure that are induced by brief exposure to a novel estrous female may reflect a greater risk taking and boldness in males that could directly facilitate access to an immediately, and possibly transiently, available novel sexually receptive female.

The residual effects of hemorrhagic shock on pain reaction and c-fos expression in rats

To investigate the residual effects of hemorrhagic shock on pain reaction and c-fos expression, we performed formalin tests after hemorrhage and reinfusion in rats. Twenty adult male Sprague-Dawley rats were divided into Control (n = 10) and Postshock (n = 10) groups. The mean blood pressure of the Control group was 100-120 mm Hg, and that of the Postshock group was kept at 50-60 mm Hg for 30 min by draining blood. After 15 min of returning mean blood pressure to normal levels in the Postshock group, 10% formalin (3.7% formaldehyde solution, 100 microL) was injected into the left rear paw of both groups. Nociceptive behaviors were observed for 1 h after the formalin injection. The rats were killed at 2 h after the formalin injection, and the lumbar spinal cord was then stained for c-fos immunohistochemistry by using the avidin-biotin-peroxidase method. Animals in the Postshock group showed considerably less nociceptive behavior than those in the Control group. C-fos expression in the deep layer (IV-VI) of the spinal cord was significantly less in the Postshock group. In conclusion, decreases of nociceptive behaviors and c-fos expression were observed under normotensive conditions after hemorrhagic shock. The mechanisms governing these reactions remain unclear.

IMPLICATIONS

Formalin tests were performed after hemorrhage and reinfusion in rats. A stress-induced analgesia was observed under normotensive conditions after hemorrhagic shock. The mechanisms remain unclear.

Developmental changes in responsivity to threat are stimulus-specific in rats

During early ontogeny, stimuli that pose a threat to an animal change. Unrelated adult male rats may kill young rats, but infanticide ends around weaning. Predation, on the other hand, may increase during early ontogeny when rats begin to extend their activity range. We investigated the developmental course of two defensive responses, immobility and analgesia, in young rats exposed to an adult male rat or to predator cues. Preweaning 14-day-old rats became immobile and analgesic when exposed to the male and showed immobility but not analgesia when exposed to cat odor. On Day 26, around weaning, the presence of the male rat no longer induced immobility and analgesia whereas cat odor produced higher levels of immobility and analgesia compared to control and male-exposed animals. This developmental change in responsivity may reflect the differences in the risk of being harmed by a male or a cat during different periods of ontogeny.

Effects of stress and relaxation on capsaicin-induced pain

A sizable body of research has been devoted to understanding the relationship between pain sensitivity and the psychological state of the individual. Considerable disagreement as to the direction of the association still exists. This study examines the effects of 2 experimental manipulations, cognitive/emotional stress and relaxation, on capsaicin-induced pain. Subjects were pretrained in relaxation and then randomized to experimental stress produced by a 20-minute Stroop test, relaxation (tape), or a control condition (neutral video), followed by a capsaicin injection in the forearm. Cardiovascular measures were taken at regular intervals, and cortisol, norepinephrine (NE), and self-reports of arousal (relaxation index) were taken immediately before and after the experimental task. The manipulation significantly interacted with sex to predict capsaicin-induced maximum pain. Women in the stress condition reported greater pain than both men in the stress condition and women in the relaxation condition. Pain was correlated negatively with task-induced changes in NE and cortisol and positively with self-reported arousal (decreased relaxation). However, separate analyses showed that some physiologic indexes of heightened arousal (increased blood pressure and NE) predicted lower pain only in men, whereas subjective increases in arousal predicted higher pain only in women. Multiple hierarchical regression analyses confirmed that physiologic and self-reported arousal predicted pain independently and in opposite directions, and a model including both accounted for 56% of the overall variance. These findings suggest that a unidimensional model of arousal may be insufficient to explain the effects of stress on pain and that these effects operate differently in men and women.

Immune deviation following stress odor exposure: role of endogenous opioids

Olfactory cues can alter immune function. BALB/c mice exposed to odors produced by footshock stressed donor mice have increased antibody responses and increased splenic interleukin (IL)-4 production following immunization relative to recipients of odors from unstressed animals. Here we document that exposure to stress odors results in analgesia that is blocked by the non-selective opioid receptor antagonist naltrexone. The stress odor-induced increase in antigen-driven IL-4 and antibody is also blocked by oral administration of naltrexone. Thus, we provide evidence that immune deviation can occur following a psychosocial stressor, and that the deviation appears to be mediated by endogenous opioid production.

Fear and anxiety: divergent effects on human pain thresholds

Animal studies suggest that fear inhibits pain whereas anxiety enhances it; however it is unclear whether these effects generalize to humans. The present study examined the effects of experimentally induced fear and anxiety on radiant heat pain thresholds. Sixty male and female human subjects were randomly assigned to 1 of 3 emotion induction conditions: (1) fear, induced by exposure to three brief shocks; (2) anxiety, elicited by the threat of shock; (3) neutral, with no intervention. Pain thresholds were tested before and after emotion induction. Results suggest that findings from animal studies extend to humans: fear resulted in decreased pain reactivity, while anxiety led to increased reactivity. Pain rating data indicated that participants used consistent subjective criteria to indicate pain thresholds. Both subjective and physiological indicators (skin conductance level, heart rate) confirmed that the treatment conditions produced the targeted emotional states. These results support the view that emotional states modulate human pain reactivity.

Amygdala lesions produce analgesia in a novel, ethologically relevant acute pain test

Acute pain tests using mechanical stimuli typically do not involve objects important in the evolutionary history of the subjects, and may fail to evaluate the contribution of biobehavioral defensive reactions to the total pain response. Spines are common structural defenses that protect plants and animals against predation. The present studies examined the reaction to contact with such natural, mechanical pain stimuli in the laboratory rat, utilizing a floor board with protruding pins located in the middle of a novel alley (the "fakir" test). Behavioral responses were characterized in 10-min tests (Experiment 1). Subjects showed voluntary contact with the pins followed by patterns of avoidance and risk assessment (stretch attend and stretch approach). Few subjects crossed the array of pins. The amygdala has been implicated in the perception of pain, particularly in stressful or fearful contexts. In Experiment 2, the fakir test was used to examine, concurrently, the effects of amygdala lesions on analgesiometric (frequency and duration of pin crossings) and anxiometric (risk assessment) measures. Large, bilateral, lesions of the amygdala significantly increased both the number of pin crossings and time spent on the pins without affecting the risk assessment measures. These findings suggest a possible dissociation between anxiety and pain perception with an important (nonaffective) role for the amygdala in the latter.

Acquisition and expression of conditioned hypoalgesia in morphine-naive and morphine-tolerant rats

The present study used a within-subject design to examine acquisition and expression of conditioned hypoalgesia in 50 male Wistar rats. Morphine-naive rats preexposed to a heat stressor with saline were hypoalgesic when subsequently tested for latencies to tail flick or paw lick. However, morphine-tolerant rats preexposed to the heat stressor with saline failed to display hypoalgesia when tested for latencies to tail flick, but showed hypoalgesia when tested for latencies to paw lick. Taken together, these findings suggest that expression of conditioned hypoalgesic responses in morphine-tolerant rats may depend on the nociceptive test used. Both morphine-naive and morphine-tolerant rats preexposed to the heat stressor with morphine failed to display hypoalgesia on either the tail-flick or the hot-plate test, demonstrating that morphine's ability to block acquisition of conditioned hypoalgesia is independent of the test used to assess nociceptive sensitivity.

Ontogeny of defensive behavior and analgesia in rat pups exposed to an adult male rat

Aversive situations may reduce nociception. The mechanism underlying such analgesia has been suggested to involve the interaction between the two separate but interconnected motivational systems "defense" and "pain." To determine the developmental course of defense and nociception, these processes were analyzed during early ontogeny in rats. To elicit a defensive reaction, a huddle of preweanling rat pups was exposed to an unfamiliar, unrelated adult male, or, for comparison, to the mother. On postnatal Day 7 the pups did not show a behavioral reaction to the presence of the mother or the male, and no reduction in nociceptive threshold in a thermal paw withdrawal test. On Day 14, pups in the presence of the male stopped ongoing behaviors and became immobile, and showed reduced paw withdrawal after the exposure. At Day 21, 22 pups of 32 became immobile when exposed to the male, whereas 10 pups explored the partition separating them from the male. Neither group showed reduced paw withdrawal. Immobility was considered a defensive reaction because it reduces auditory and visual cues and therefore the probability of being detected. The developmental course of immobility seems to reflect both the changes in threat imposed on the pups by a potentially infanticidal male and the ability of pups to react to that threat. The reduction in paw withdrawal that followed male exposure indicates an inhibitory mechanism. It is discussed whether the activation of the defense system results in an inhibition of nociception.

Sex differences in N-methyl-D-aspartate involvement in kappa opioid and non-opioid predator-induced analgesia in mice

There are suggestions of sex differences in N-methyl-D-aspartate (NMDA) receptor system involvement in the mediation of analgesia. The present study examined the effects of the specific, competitive NMDA antagonist, NPC 12626, on the nociceptive (50 degrees C hot plate) responses of reproductive male and female laboratory mice exposed to (i) an ethologically relevant aversive stimulus, the odor of a predator and (ii) administration of the kappa opiate agonist, U69,593. A 30-s exposure to 2-propylithietane, the major component of weasel odor, elicited a 'non-opioid' analgesia that was in both sexes insensitive to naloxone and the kappa opiate antagonist nor-binaltorphimine. In male mice this non-opioid analgesia was antagonized by NPC 1262, while in reproductive females the predator-induced analgesia was insensitive to NPC 12626. Similarly, NPC 12626 attenuated the analgesic effects of the kappa opiate agonist, U69,593, in male mice while having no significant effects on the equivalent levels of kappa opiate analgesia in females. These results show that there are sex differences in NMDA involvement in the expression and, or mediation of both non-opioid stress-induced and kappa opiate-mediated analgesia.

Opioid and non-opioid NMDA-mediated predator-induced analgesia in mice and the effects of parasitic infection

The present study examined the nociceptive responses (50 degrees C, hot-plate) of uninfected and subclinically parasitized male mice exposed to the odor of a predator, an ecologically relevant threatening stimulus. In uninfected mice a 15-min exposure to 2-propylthietane, the major component of weasel odor, induced a naloxone-reversible opioid analgesia. A 30-s exposure elicited a shorter duration and lower amplitude 'non-opioid' analgesia that was insensitive to naloxone, partially sensitive to either the serotonin-1A (5-HT1A) agonist, 8-OH-DPAT, or the GABAA antagonist, bicuculline, and blocked by the competitive N-methyl-D-aspartate (NMDA) antagonist, NPC 12626. In contrast, mice chronically (25 days) and subclinically infected with the murine nematode, Heligmosomoides polygyrus, failed to show a significant non-opioid analgesia and displayed a markedly lower level of opioid analgesia than uninfected mice. These results suggest that NMDA receptor mechanisms are potently associated with the expression of the analgesia arising from exposure to the naturally aversive stimulus of predator odor. These findings also demonstrate that parasites, and likely other subchronic infections, can have a significant impact on the display of opioid and non-opioid stress-induced analgesia arising from exposure to the ethologically relevant stimulus of predator odor.

The effects of ethanol on a measure of conditioned fear in mice

The effects of ethanol on conditioned freezing, a species-specific defensive behavior used as an assay of fear, was examined in mice. In Experiment 1, ethanol, 1.2 g/kg, significantly increased freezing compared to a saline control when the mice were reexposed to a context in which they were previously shocked. Experiment 2, which administered ethanol or saline to no-shock control animals, demonstrated that the potentiated freezing produced by ethanol in Experiment 1 was specific to the interaction of ethanol and the stress response. These results suggest that both the qualitative and quantitative dimensions of the environmental stressor, as well as the dose of ethanol used, may be critical for determining ethanol's effect on a stress response.

Descending antinociceptive mechanisms in the brainstem: their role in the animal's defensive system

The identification of specialized mechanisms in the mammalian brainstem that function to inhibit the rostral transmission of nociceptive (pain-related) information in the spinal cord led to an explosion of research into the neuroanatomical and neurochemical substrates of these antinociceptive systems. As outlined in the present paper, most attention was directed at those mechanisms in the periaqueductal grey (PAG) and rostral ventromedial medulla (RVM). However, comparatively little attention has been paid to the functional role of these mechanisms in animal behaviour. The purpose of the present paper is to review research into the behavioural significance of those antinociceptive mechanisms in the PAG and RVM. It is concluded that these mechanisms function as part of the animal's fear or defensive system, serving to make a threatened animal insensitive to noxious stimulation and thereby allowing that animal to engage in defensive responses instead of recuperative activities. Further, it is argued that the organization of these antinociceptive circuits reflects the animal's increasing capacity for early detection of danger. Specifically, nociception itself is held to signify the presence of immediate threat, and consequently, nociceptive input directly activates antinociceptive circuits at either the spinal level (during intense noxious stimulation) or RVM (following exposure to moderate noxious stimuli). In contrast, events that are themselves innocuous but which signal threat (either learned or innate danger signals) activate fear and defensive systems in the amygdala and PAG which engage the descending antinociceptive projections in the RVM.

The effects of laboratory handling procedures on naloxone-precipitated withdrawal behavior in morphine-dependent rats

Previous studies of opiate dependence in our laboratory have shown that opiate-naive (control) animals can display behaviors normally ascribed to withdrawal following naloxone challenge. The possibility arises that in these control rats, the stress of the induction of dependence, pharmacological challenge, and behavioral testing may result in the release of endogenous opioids such that on naloxone administration withdrawal behaviors are elicited. In the present study, we have utilized two differing experimental protocols for the induction of morphine dependence in rats that would represent the extremes of normal laboratory handling procedures and have then assessed withdrawal behaviors following naloxone challenge. In control animals, the results show that the stressors alone are insufficient to allow for the precipitation of naloxone-induced withdrawal. However, withdrawal behavior was generally greater in the opiate-dependent animals in the "high" rather than the "low" stress group, suggesting a summation of opiate effects. Similarly, a residual effect was noted in the control opiate-naive animals; naloxone rather than saline (control) challenged rats displayed a slightly higher incidence of withdrawal behavior. These results stress the importance of maintaining constant laboratory protocols, minimizing stressful procedures, and performing appropriate controls in assessing opiate dependence and withdrawal.

Parasite infection attenuates nonopioid mediated predator-induced analgesia in mice

Parasites have been shown to have a broad range of effects on host behavior, including alterations of host responses to predators. Response to the threat of predation consist of a number of defensive behaviors, including a reduction in pain sensitivity and the induction of analgesia. The present study examined the relationships between subclinical (i.e., nonpathological) infection with the naturally occurring, enteric, sporozoan (coccidian) parasite, Eimeria vermiformis, predator exposure, and nociceptive responses in male mice. Brief (30 s) exposure of nonparasitized mice to a predator (a cat) induced marked, relatively short-lived analgesia that was insensitive to naloxone and blocked by the serotonin-1A (5-HT1A) agonist, 8-OH-DPAT. In contrast, mice acutely infected for 6 days with E. vermiformis, failed to show a predator-induced analgesia. The parasitized mice did display a naloxone-sensitive hypoalgesia or analgesia. However, restraint-stressed mice, which displayed a naloxone-sensitive hypoalgesia similar in amplitude to that of the infected mice, still exhibited a nonopioid mediated, predator-induced analgesia. These observations indicate that parasite infection attenuates 5-HT1A-sensitive predator-induced analgesia and likely reduces the accompanying fear and anxiety related anticipatory defense reactions of the host to the predator.

Predator-induced opioid and non-opioid mediated analgesia in young meadow voles: sex differences and developmental changes

The present study examined developmental changes in the nociceptive responses of male and female meadow voles, Microtus pennsylvanicus, exposed to a garter snake, a natural predator of young voles. After 15 min of exposure to the presence of a garter snake, neonatal-juvenile voles (5-20 days of age) displayed naloxone (1.0 mg/kg)-sensitive opioid mediated analgesic responses, while after a brief 30-s exposure to the snake, voles displayed a higher amplitude, non-opioid analgesia that was insensitive to naloxone and blocked by the serotonin-1A (5-HT1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin. The levels of opioid and non-opioid mediated analgesia declined during development as the threat presented by the snake decreased. Young female voles also displayed a significantly greater non-opioid, 5-HT1A sensitive analgesia than males, with no significant sex differences in the lower amplitude opioid analgesia. These results indicate that young (neonatal) meadow voles that are exposed to a naturally threatening stimulus display sexually dimorphic analgesic responses. These findings also illustrate the need to consider the ecological context when examining environmentally-induced analgesia.

The CCK-A and CCK-B receptor antagonists, devazepide and L-365,260, enhance morphine antinociception only in non-acclimated rats exposed to a novel environment

Devazepide, a potent CCK-A receptor antagonist, and L-365,260, a selective CCK-B receptor antagonist, have been introduced as pharmacologic tools for differentiating the physiologic roles of CCK-A and CCK-B receptor subtypes. In the present study, we tested the effects of devazepide and L-365,260, on morphine antinociception in rats using the thermal sensorimotor tail flick test. Both devazepide and L-365,260 significantly enhanced the antinociceptive action of morphine, but only in rats that had not been acclimated to the laboratory environment or habituated to investigator handling. When tested with fully acclimated animals, devazepide and L-365,260 had no effect whatsoever; they neither enhanced nor attenuated morphine-induced antinociception. These observations indicate that the effects of devazepide and L-365,260, CCK antagonists, on morphine antinociception appear to be dependent on the animal's response to a new environment or to the stress induced by an unaccustomed experimental paradigm.

Sex differences in opioid and non-opioid mediated predator-induced analgesia in mice

The present study compared the nociceptive responses of male and female mice exposed to a predator, an ecologically relevant threatening stimulus. After 15 min of exposure to the presence of an experienced predatory cat, mice displayed a naloxone (1.0 mg/kg)-sensitive opioid mediated analgesic response, while after a brief 30-s exposure to the cat mice displayed a lower amplitude, relatively brief, non-opioid analgesia that was insensitive to naloxone and blocked by the serotonin-1A (5-HT1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin. Male mice displayed a significantly greater opioid mediated predator-induced analgesia than females, whereas female mice showed a significantly greater non-opioid, 5-HT1A sensitive, analgesia than males. These results indicate that there are significant sex differences in both the opioid and non-opioid analgesic responses arising from exposure to a natural aversive stimulus.

Cross-generalization between an ethologically [correction of ecologically] relevant stimulus and a pentylenetetrazole-discriminative cue

Twelve male Sprague-Dawley rats were trained in a two-choice pentylenetetrazole vs. saline drug discrimination (DD) task under an FR10 schedule of food reinforcement. Two-minute reinforced test sessions with the two training stimuli engendered exclusive injection-appropriate responding. Rats were injected with saline and then exposed for 20 min to the presence of a domestic cat pretreated with catnip. Immediately following the predator exposure, rats were tested for stimulus generalization in the DD task. The predator/prey interaction engendered 92% PTZ-appropriate responding. These data suggest that the interoceptive state associated with species-specific defense reactions in rats is similar to the cues produced by a pharmacological agent within a behavioral assay which has been suggested as an animal model of human anxiety.