Intrinsic mechanisms of pain inhibition: activation by stress

Opioid and nonopioid interactions in two forms of stress-induced analgesia

Stressful environmental events activate endogenous mechanisms of pain inhibition. Under some circumstances the analgesia is blocked by naloxone/naltrexone ("opioid"), while under others it is not ("nonopioid"). The existence of these two categories of analgesia leads to the question of how they are related. In a collateral inhibition model proposed by Kirshgessner, Bodnar, and Pasternak (1982), opiate and nonopiate mechanisms were viewed as acting in a mutually inhibitory fashion. In the present experiments, rats were exposed to either of two environmental stressors that produce a nonopioid stress-induced analgesia (SIA) following injections of the opiate antagonist naltrexone or agonist morphine. In the presence of naltrexone, SIA produced by either cold water swim (CWS) or social defeat was enhanced. These same SIAs were found to attenuate the analgesic effect of morphine, demonstrating that an activation of opioid systems can inhibit nonopioid analgesias. These results support an inhibitory interaction of opioid and nonopioid mechanisms in some forms of stress-induced analgesia.

Chronic food restriction and weight loss produce opioid facilitation of perifornical hypothalamic self-stimulation

Electrical stimulation frequency thresholds for lateral hypothalamic (LH) self-stimulation were monitored throughout a 3 week period of food restriction and a subsequent 3 week period of re-feeding. Rats with electrodes placed in the perifornical LH were sensitive to this dietary manipulation as evidenced by a high positive correlation between body weight and self-stimulation threshold. Rats with electrodes in the zona incerta/subincertal region or ventral hypothalamus displayed little or no change in threshold. Lateral ventricular injection of naltrexone (200.0 nM) reversed the decline in threshold that was otherwise present during food restriction in rats with perifornical placements. Naltrexone had no effect on thresholds of rats with placements outside the perifornical region. These findings suggest that food restriction and weight loss activate an opioid mechanism that facilitates perifornical LH self-stimulation. The documented association of perifornical LH with the phenomenon of stimulation-induced feeding, and the reciprocal connections between this region and gustatory structures, supports the hypothesis that facilitation of self-stimulation by food restriction is related to the natural phenomenon of positive alliesthesia (i.e. the hunger-dependency of food reward).

Characterization of pituitary mediation of stress-induced antinociception in rats

Antinociception, induced by continuous cold-water swims (CCWS) and certain parameters of inescapable foot shock, is reduced in hypophysectomized rats receiving supplements of corticosterone and l-thyroxine. To assess which lobe of the pituitary gland is involved in this effect, the first experiment compared the effects of total hypophysectomy and posterior lobectomy in supplemented rats upon CCWS antinociception on the tail-flick and jump tests and upon continuous inescapable foot shock antinociception on the tail-flick test. Total hypophysectomy, but not posterior lobectomy, significantly reduced CCWS antinociception on both tests in supplemented rats relative to sham surgery. Both total and posterior hypophysectomy either reduced or potentiated foot shock antinociception as functions of shock intensity or duration of exposure in supplemented rats. To assess whether hormonal supplementation is necessary for the observed effects, the second experiment examined CCWS antinociception in sham-operated and hypophysectomized rats that received either no hormonal supplements or corticosterone and/or l-thyroxine. These regimens failed to alter CCWS antinociception in sham-operated rats. Treatment of hypophysectomized rats with corticosterone and l-thyroxine either separately or together significantly reduced CCWS antinociception. In contrast, if hypophysectomized rats did not receive supplements, CCWS antinociception was significantly potentiated relative to sham-operated controls. These effects could not be attributed to treatment-induced changes in either body weight or CCWS hypothermia. These data suggest that the anterior lobe of the pituitary gland and adrenal cortex are involved in the mediation and/or maintenance of CCWS antinociception.

Sex differences in the antagonism of swim stress-induced analgesia: effects of gonadectomy and estrogen replacement

Sex differences in the neurochemical mediation of swim stress-induced analgesia (SSIA) were examined in Swiss-Webster mice. Intact and gonadectomized adult mice of both sexes were tested for their analgesic response (hot-plate test) to 3 min of forced swimming in 15 degrees C and 20 degrees C water. SSIA resulting from 15 degrees C swim was previously shown to be naloxone-insensitive (i.e., non-opioid) whereas SSIA resulting from 20 degrees C swim produced an analgesia that was partially reversible by naloxone (i.e., mixed opioid/non-opioid). The non-opioid components of these SSIA paradigms were attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801). We now report that in males, but not females, dizocilpine (0.075 mg/kg, i.p.) and naloxone (10 mg/kg, i.p.) antagonized the non-opioid and opioid components of SSIA, respectively. After ovariectomy, females displayed a pattern of antagonism similar to males such that dizocilpine attenuated non-opioid SSIA, although naloxone remained ineffective in antagonizing 20 degrees C SSIA. Thus, SSIA in intact females was neither opioid- nor NMDA-mediated, yet it was of similar magnitude to the SSIA displayed by intact males. In separate experiments, estrogen replacement (estrogen benzoate; 5.0 micrograms/day, i.p.) administered to ovariectomized mice over a 6-8 day period reinstated the dizocilpine-insensitivity of 15 degrees C SSIA characteristic of intact females. However, a similar estrogen regimen administered to both intact and castrated males did not compromise the sensitivity to dizocilpine previously noted in male mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Some psychosomatic causal factors of restraint-in-water stress ulcers

Rats were stressed by 75 min restraint in a tube suspended vertically while dry (19 degrees C) or partially immersed in tanks of water at different temperatures (19, 27, 35 degrees C), either in a conscious state or while under pentobarbital anesthesia. Restraint was followed by 75 min rest in the home cage and then sacrificed under halothane anesthesia. Assessment of the degree of gastric erosion indicated that restraint alone, whether the animal was rendered unconscious or not, was not sufficient to induce ulceration. However, in conscious animals, the addition of partial immersion did induce ulceration that was inversely related to the temperature of the water bath. This effect was not merely the result of brain stem and spinal reflex processes, because unconscious animals exposed to the most severe conditions (19 degrees C) showed no ulceration.

Two distinct unit activity responses to morphine in the rostral ventromedial medulla of awake rats

Past research investigating the rostral ventromedial medulla (RVM) in anesthetized animals has found two distinct classes of cells, ON and OFF, noted for their distinct responses to noxious stimuli and to morphine. However, only one class (ON) has been found in the awake animal paradigm. We report in this paper that we have found both the ON and OFF cell responses to morphine in the awake rat.

Stress-induced hypoalgesia and defensive freezing are attenuated by application of diazepam to the amygdala

Recent studies have shown that lesions of the amygdala, as well as systemic administration of benzodiazepine receptor agonists, block the hypoalgesia and defensive behavior normally observed when rats are exposed to stimuli associated with shock. The present study was conducted to determine if the direct application of a small quantity of the benzodiazepine diazepam (DZP) to the amygdala would affect defensive freezing and hypoalgesia as measured by the formalin test. Independent groups of rats were prepared with chronic cannulae aimed at the basolateral or central regions of the amygdala. Bilateral injection of DZP (30 micrograms) into the basolateral amygdala attenuated both the defensive freezing behavior and the hypoalgesia seen during an 8-min period following a series of three brief foot-shocks. The same dose of DZP applied to the central amygdala attenuated the freezing response, although this effect may have been due to limited diffusion of the drug into the basolateral region. Baseline levels of formalin-induced behavior were not affected by DZP in either group. These results support the idea that hypoalgesia is one component of an integrated defensive response shown by rats in anxiety- or fear-related situations and that the amygdala represents an important forebrain component of a neural circuit that subserves the expression of this response.

Antagonism of the non-opioid component of ethanol-induced analgesia by the NMDA receptor antagonist MK-801

Recent evidence from our laboratory suggests that the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) selectively antagonizes non-opioid (i.e. naloxone-insensitive) mechanisms of stress-induced analgesia in mice. For example, we have recently demonstrated that a low dose of MK-801 (0.075 mg/kg, i.p.) antagonizes the non-opioid component of a mixed opioid/non-opioid swim stress-induced analgesia (SSIA) resulting from forced swimming for 3 min in 20 degrees C water. Since ethanol-induced analgesia (EIA) has been found to be only partially attenuated by naloxone, we hypothesized that MK-801 would similarly block the non-opioid component of EIA. The effects of MK-801 and of the opioid receptor antagonist naloxone (10 mg/kg, i.p.) on analgesia produced by ethanol (2.5 g/kg in 20% vol/vol, i.p.) were studied in control mice and in mice selectively bred for high (HA) or low (LA) SSIA. HA mice showed significantly more, and LA mice significantly less, EIA than controls. Naloxone and MK-801 significantly attenuated EIA in control and HA mice, and in these lines the combined administration of both antagonists blocked EIA completely. In LA mice, which displayed very little EIA, naloxone but not MK-801 reversed EIA completely. These findings provide additional evidence for the role of the NMDA receptor in non-opioid mechanisms of analgesia. The finding that mice selectively bred for high and low SSIA, also display high and low EIA suggests common mediation of the effects of stress and ethanol on antinociceptive processes.

Formalin-induced nociceptive responses in diabetic mice

In non-diabetic mice, s.c. injection of formalin to the hindpaw had a biphasic effect: an immediate nociceptive response (first-phase) followed by a tonic response (second-phase). However, only the immediate nociceptive response was observed in diabetic mice. The duration of the first-phase response was significantly longer in diabetic mice than in non-diabetic mice. In diabetic mice, when spantide, an antagonist of substance P, reduced the duration of the nociceptive response in the first-phase to the levels that were observed in non-diabetic mice, the second-phase response appeared. The second phase also became apparent in diabetic mice after pretreatment with naltrindole (3 mg/kg), an antagonist of delta-opioid receptors. These results suggest that a negative control system, which is mediated by delta-opioid receptors and links substance P with somatostatin-mediated nociceptive transmission, may inhibit the formalin-induced second-phase of the nociceptive response in diabetic mice.

Immune and neuroendocrine response to restraint in male and female rats

A parallel study of the modification in the opioid and immunological systems induced by acute restraint (RT) was carried out in male and female rats 24 hr after the treatment. beta-Endorphin-like immunoreactivity (beta-ELI) was measured in two brain areas (ventral hypothalamus [HYP] and periaqueductal gray matter [PAG]) and in the pituitary (anterior and neurointermediate lobes), together with plasma corticosterone (C) and ACTH. Immune function was measured as mitogen-induced Interferon-gamma (IFN-gamma) production by splenocytes. RT reduced beta-ELI levels in the PAG in males and females. Plasma levels of C and ACTH did not differ from the basal levels in restrained animals. RT reduced IFN-gamma production in both sexes, but this effect was more marked in females than in males. The possible relationship between the immune and opioid system is discussed.

Early onset of reduced morphine analgesia by ingestion of sweet solutions

Morphine analgesia can be reduced by prior exposure to food and flavored fluids. The early onset of reduced morphine-induced analgesia (RMA) was studied in 82 male Wistar rats after allowing them access to either a dextrose-saccharin solution or unflavored tap water for 6 or 3 h (Experiment 1, n = 40) or for 3 h, 90, or 45 min (Experiment 2, n = 42). Morphine (4 mg/kg) was injected subcutaneously at the end of the drinking period, and after 25 min a series of tail flick tests was conducted. Morphine produced strong analgesia in all rats that drank unflavored tap water; however, in rats that drank the flavored solution, the analgesic effect of morphine was significantly attenuated following exposures of 6 or 3 h, but not following exposures of 90 or 45 min. Similar quantities of flavored fluid were consumed by groups at all exposure durations; thus, RMA was determined by duration of exposure and not amount consumed. No analgesia attributable to flavor consumption per se was observed. The results suggest that RMA is mediated by endogenous opioid activity in the gustatory and analgesic systems by a mechanism akin to tolerance that requires about 3 h to operate.

Temperature in mice after ethanol: effect of probing and regain of righting reflex

The handling involved in rectally probing a mouse in order to measure body temperature is a stress which results in an increase in body temperature. However, after an injection of ethanol the fall in body temperature caused by ethanol is exacerbated by probing. In mice, decreases in temperature following probing are ethanol-dose dependent and can be generated on both the falling and rising phases of the ethanol induced change in temperature. The effect of probing can be observed when the mice are under the hypnotic influence of ethanol, and regain of righting reflex itself is followed by a fall in temperature. The resumption of motor activity in undisturbed mice following an hypnotic dose of ethanol also is accompanied by a fall in temperature. Therefore, the drop in temperature observed in any of these procedures which involve moving the mice may be attributable to the disruption of heat conservation rather than a stress interaction.

Low and high doses of midazolam differentially affect hypoalgesia in rats conditioned to a heat stressor

These experiments examined the effects of a benzodiazepine (midazolam) on rats' sensitivity-reactivity to the heated floor of a hot-plate apparatus. Rats were either previously exposed to the heated floor, or naive to the hot-plate apparatus, while control rats were familiarized with the apparatus in the absence of pain. A low dose (0.63 or 1.25 mg/kg) of midazolam attenuated the conditioned hypoalgesic response resulting from pre-exposure to a heated floor, but did not affect the hypoalgesic response elicited by exposure to a novel hot-plate apparatus nor the "baseline" sensitivity-reactivity among control rats. A high dose (2.5 mg/kg) of midazolam resulted in a naloxone-insensitive increase in both the conditioned and the novelty-induced hypoalgesia, and provoked a small, but naloxone-reversible increase in paw-lick latencies among control rats. The results were taken to mean that exposure to the heated floor results in hypoalgesic responses as a consequence of fear conditioning and the reinstatement of novelty. Midazolam was assumed to attenuate conditioned hypoalgesia by reducing fear but at the high dose to augment the hypoalgesic effects of novelty.

Differential effect of footshock stress on humoral and cellular immune responses of the rat

Recent, mostly "in vitro", studies indicate that certain stress protocols may affect differentially immune mechanisms. In agreement with these studies, our "in vivo" results show that 4 consecutive days of intermittent footshock, in rats, depressed the primary antibody response, to a thymic dependent antigen, but enhanced the graft-versus-host response and the capacity to reject an inocula of Walker carcinoma cells. Thus it is concluded that footshock stress is capable of oppositely affecting "in vivo" produced humoral and cellular immune responses.

Parallel activation of multiple spinal opiate systems appears to mediate 'non-opiate' stress-induced analgesias

Pain is powerfully modulated by circuitries within the CNS. Two major types of pain inhibitory systems are commonly believed to exist: opiate (those that are blocked by systemic opiate antagonists and by systemic morphine tolerance) and non-opiate (those that are not). We used intrathecal delivery of mu, delta, and kappa opiate receptor antagonists to examine 3 well-accepted non-opiate stress-induced analgesias. Combined blockade of all 3 classes of opiate receptors antagonized all of the 'non-opiate' analgesias. Further experiments demonstrated that blocking mu and delta or mu and kappa was sufficient to abolish 'non-opiate' analgesias. Combined blockade of kappa and delta receptors was without effect. The clear conclusion is that all endogenous analgesia systems may in fact be opiate at the level of the spinal cord. Phenomena previously thought to be non-opiate appear to involve parallel activation of multiple spinal opiate processes. These findings suggest the need for a fundamental shift in conceptualizations regarding the organization and function of pain modulatory systems in particular, and opiate systems in general.

Paradoxical analgesia produced by naloxone in diabetic mice is attributable to supersensitivity of delta-opioid receptors

The effects of naloxone on the analgesic response were examined using the tail-flick test, in mice with streptozotocin-induced diabetes. Subcutaneous injection of naloxone (5 mg/kg, s.c.) produced a marked analgesia in diabetic mice but not in age-matched non-diabetic mice. Naloxone-induced analgesia in diabetic mice was significantly reduced by pretreatment with naltrindole (0.1 mg/kg, s.c.), a selective antagonist of delta-opioid receptors. By contrast, no significant naloxone-induced increase in tail-flick latency in diabetic mice was observed after chronic treatment with naloxone (5 mg/kg, s.c.) for 5 days. However, the tail-flick latency was significantly increased by chronic treatment with naloxone in non-diabetic mice. Furthermore, the significant naloxone-induced increase in tail-flick latency in non-diabetic mice that had been chronically treated with naloxone was also antagonized by pretreatment with naltrindole. Chronic pretreatment with 5 mg/kg of naloxone for 5 days markedly attenuated the analgesic effect of the delta-agonist DPDPE in diabetic mice, whereas this pretreatment significantly enhanced the effect of DPDPE in non-diabetic mice. These results suggest that naloxone-induced 'paradoxical' analgesia in mice may be mediated predominantly by delta-opioid receptors.

Evidence for the involvement of serotonin in the antinociception induced by electrical or chemical stimulation of the mesencephalic tectum

A great deal of evidence has shown that electrical stimulation or microinjections of GABAA blockers, such as bicuculline, into the midbrain tectum (MT) produce escape behavior, which has been associated to fear. This study was aimed to examine the characteristics of the analgesia that follows the escape behavior induced by electrical (freezing and escape thresholds) and chemical (bicuculline microinjections) stimulation of the midbrain tectum. Immediately after the expression of the aversive responses the rats were submitted to the tail-flick test. The obtained results show that analgesia always follows aversive responses integrated at the MT level regardless of the kind of stimulation applied. The antinociceptive effects induced by either electrical or chemical stimulation of the MT were not antagonized by central microinjections of naloxone. On the other hand, the non-specific serotonin antagonist methysergide microinjected into the MT was effective in antagonizing the analgesia induced by any of the aversive stimulations. Based on these results we suggest that serotonin, but not opioid mechanisms, may be involved in the integration of antinociceptive responses to stimulation of the midbrain tectum.

Effects of an infusion of morphine into the accumbens nucleus upon acquisition of hypoalgesic and fear responses in rats exposed to a heart stressor

Four experiments examined the effects of an infusion of morphine into the accumbens nucleus upon the aversive conditioning that can occur in rats exposed to the heated floor of a hot-plate apparatus. An infusion of morphine into the accumbens nucleus but not into the caudoputamen or into the prefrontal cortex impaired the acquisition of a conditioned hypoalgesic (Experiment 1) and fear (Experiment 4) response. This impairment was dose-dependent (Experiment 2) and mediated by opioid receptors in the accumbens nucleus, because it was removed by a systemic (Experiment 3a) or by an accumbal (Experiment 3b) infusion of naloxone. The results were attributed to an antagonism between the reinforcement process for aversive conditioning and the appetitive properties of an accumbal infusion of morphine.

Differential influence of naloxone on the responses of nociceptive neurons in the superficial versus the deeper dorsal horn of the medulla in the rat

Naloxone (200 micrograms/kg, i.v.) reduced the noxious thermal stimuli-evoked responses of 16/25 nociceptive neurons in the superficial laminae whereas it enhanced the responses of 6/10 nociceptive neurons in the deeper dorsal horn. However, a different picture emerged when selectivity of neuronal responsivity (nocireceptive or multireceptive) was considered. In the superficial dorsal horn, naloxone reduced the responses of the majority of (15/18) selectively nocireceptive neurons. The reduction in responses became apparent within 60 sec following naloxone administration and returned to control level within 48 min. In contrast, the responses of the majority of multireceptive neurons in the superficial (6/7), or the deeper (6/10) dorsal horn, were enhanced. The excitatory action in the superficial dorsal horn persisted for only 6-15 min, whereas it persisted for 40-70 min in the deeper dorsal horn. The firing of the majority of cold-receptive neurons (6/8) in the superficial dorsal horn was not altered. These effects were stereoselective since (+)-naloxone, the inactive isomer of naloxone, did not affect the responses of 14/16 nociceptive neurons. It is concluded that naloxone differentially, and selectively, affects the firing of nociceptive neurons in the superficial versus the deeper dorsal horn, and the firing of selectively nocireceptive versus multireceptive neurons. The relevance of these findings to the behavioral effects of naloxone, hyperalgesia and analgesia, is discussed.

Mechanisms of stress: a dynamic overview of hormonal and behavioral homeostasis

Environmental events, both physical and emotional, can produce stress reactions to widely varying degrees. Stress can affect many aspects of physiology, and levels of stress, emotional status, and means of coping with stress can influence health and disease. The stress system consists of brain elements, of which the main components are the corticotropin-releasing hormone (CRH) and locus ceruleus (LC)-norepinephrine (NE)/autonomic systems, as well as their peripheral effectors, the pituitary-adrenal axis and the autonomic system, which function to coordinate the stress response. Activation of the stress system results in behavioral and physical changes which allow the organism to adapt. This system is closely integrated with other central nervous system elements involved in the regulation of behavior and emotion, in addition to the axes responsible for reproduction, growth and immunity. With current trends in stress research which focus on understanding the mechanisms through which the stress-response is adaptive or becomes maladaptive, there is a growing association of stress system dysfunction, characterized by hyperactivity and/or hypoactivity to various pathophysiological states. The purpose of this review is to 1) define the concepts of stress and the stress response from a historical perspective, 2) present a dynamic overview of the biobehavioral mechanisms that participate in the stress response, and 3) examine the consequences of stress on the physiologic and behavioral well-being of the organism by integrating knowledge from apparently disparate fields of science.

Kappa opiate receptors mediate tail-shock induced antinociception at spinal levels

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The present work and the following companion paper were aimed at identifying the neuroanatomical loci at which endogenous opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 3 experiments included in the present paper focus on the role of spinal opiates in tail-shock induced analgesia. The first experiment demonstrates that the tail-shock parameters used do not directly activate pain suppressive circuitry within the spinal cord, but rather activate centrifugal pain modulation circuitry originating within the brain. The last two experiments examine the effect of intrathecal microinjection of either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist). Taken together, these latter 2 experiments demonstrate that both the early (after 2 shocks) and late (after 80-100 shocks) opiate analgesias are mediated by kappa opiate receptors within the spinal cord.

Effects of diabetes on stress-induced analgesia in mice

The present studies were designed to determine whether streptozotocin-induced (STZ-induced) diabetes in mice can attenuate the development of antinociception induced by exposure to both foot shock and forced swimming stress. Foot shock stress produced significant analgesia both in control and diabetic mice. However, the extent of foot shock stress-induced analgesia (FSSIA) in diabetic mice was significantly lower than that in control mice. Naloxone (2 mg/kg, i.p.) significantly attenuated FSSIA in control mice, but was without effect on FSSIA in diabetic mice. One-minute swimming stress had no significant effect on tail-pinch latency in control mice, whereas 3-min swimming stress produced significant analgesia in these mice. Diabetic mice exhibited robust swimming stress-induced analgesia (SSIA): one-min swimming stress produced significant analgesia in diabetic mice. These analgesic effects were blocked by naltrindole, a selective antagonist of delta-opioid receptors, but not by pretreatment with beta-funaltrexamine, an irreversible and selective antagonist of mu-opioid receptors. These results suggest that the deficiency in the functioning of mu-opioid receptors caused by diabetes results in significant activation of an endogenous analgesic system, which is mediated mainly by delta-opioid receptors.

Haemodynamic and abdominal motor reflexes elicited by neurotensin in anaesthetized guinea-pigs

1. Single intraperitoneal (i.p.) injections of neurotensin (NT) (0.14- 140 nmol kg-1) in anaesthetized guinea-pigs were found to trigger transient abdominal wall contractions (TAWC) accompanied by relatively sustained increases of systemic blood pressure (BP) and heart rate (HR). The modification of the latter NT effects by various drugs and surgical manipulations was examined to obtain some insight into the nature of, and possible relationship between, these responses. 2. The abdominal motor response (i.e. TAWC) to i.p. NT (14 nmol kg-1) was inhibited by prior i.v. injection of the guinea-pigs with pancuronium (0.27 mumol kg-1), morphine (1.5 and 15 mumol kg-1), clonidine (0.34 mumol kg-1), by concomitant i.p. injection of procaine 2% w/v, or by acute spinalization. It was potentiated by naloxone (2.8 and 28 mumol kg-1), but not affected by i.v. injection of autonomic drugs (i.e. pentolinium, prazosin, yohimbine and atropine), by capsaicin desensitization, or by acute bilateral cervical vagotomy. In spinalized animals a sustained abdominal wall contraction (SAWC) was unmasked, which was resistant to i.v. morphine, clonidine or baclofen but suppressed by i.v. pancuronium or i.p. lignocaine 2% w/v. 3. Haemodynamic responses to i.p. NT were not affected by i.v. pancuronium, morphine, naloxone, atropine, or by vagotomy. They were inhibited by i.v. pentolinium or clonidine (BP, HR), i.v. prazosin (BP), i.p. procaine 2% w/v (BP, HR), capsaicin desensitization or acute spinalization (BP, HR). Yohimbine (i.v.) potentiated BP and HR increases caused by i.p. NT.4. These results suggest that: (1) the haemodynamic and TAWC responses to i.p. NT in this animal model, are two independent, neurally-mediated reflexes which are likely to originate from the abdominal cavity and require a functionally intact spinal cord for their full expression; (2) the neural pathways subserving both types of responses appear to be different from each other. The nature and time-response characteristics of the reflexes caused by i.p. NT, raise the possibility that i.p. NT is a noxious stimulus, at least in guinea-pigs.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. II. Comparison across three swim-stress paradigms in selectively bred mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (dizocilpine, 0.075 mg/kg, i.p.) on swim-stress-induced analgesia (SSIA) were studied in control (C) mice and in mice selectively bred for high (HA) or low (LA) SSIA. In three consecutive experiments, animals were subjected to forced swimming at water temperature of 20 degrees C, 32 degrees C and 15 degrees C and the resulting analgesia (hot-plate test) was found to be mixed opioid/non-opioid, opioid and non-opioid, respectively, as a function of the degree of antagonism by naloxone (10 mg/kg, i.p.). The major finding of this study is that MK-801 attenuated 15 degrees C SSIA, against which naloxone was ineffective, but had no effect on 32 degrees C SSIA, which naloxone blocked completely. A combination of naloxone and MK-801 significantly attenuated 20 degrees C SSIA in C and HA mice and in HA mice this attenuation was significantly larger than that produced by either drug alone. Morphine analgesia (10 mg/kg, i.p.) was unaffected by MK-801. It is concluded that low doses of MK-801 selectively block non-opioid mechanisms of SSIA.

Chronic treatment with MIF-1 prevents the painful stimuli threshold elevation induced by neonatal handling in mice

Chronic postnatal stressful handling results in a hyposensitivity to thermal nociceptive stimuli. This phenomenon is strongly affected by manipulations of the opioid system. In the present experiment, we report that chronic treatment with MIF-1 during the neonatal period prevents the behavioral alterations induced by handling while it is completely ineffective if injected acutely before antinociceptive testing by the tail flick test at 45 days of life.

Increases in opioid-mediated swim antinociception following endopeptidase 24.15 inhibition

The duration of action and potency of endogenous opioid peptides are limited by proteolytic enzymes such as endopeptidases 24.11 and 24.15. Whereas endopeptidase 24.11 cleaves enkephalin pentapeptides, endopeptidase 24.15 degrades longer-chained opioids including dynorphin A1-8 and met-enkephalin-Arg6-Gly7-Leu8 (MERGL). Inhibitors of endopeptidase 24.11 and 24.15 both increase basal nociceptive thresholds and respective forms of opioid antinociception. Acute exposure to certain environmental stressors can produce antinociception which is opioid mediated; inhibitors of endopeptidase 24.11 potentiate this effect. The present study evaluated whether central administration of a selective inhibitor of endopeptidase 24.15, N-[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-p-aminobenzoate (cFP-AAF-pAB) increased antinociception following intermittent cold-water swims (ICWS) in rats. cFP-AAF-pAB (0.25-25 nmol, ICV) dose-dependently increased ICWS antinociception on the tail-flick and jump tests without affecting basal nociceptive thresholds. The opioid mediation of ICWS antinociception was confirmed by significant reductions in this response following naloxone. These data indicate that longer-chained endogenous opioid peptides participate in the antinociception induced by ICWS.

Control of food delivery in food-deprived rats mediates analgesia

An experiment was carried out to investigate the effects of control of food delivery, in food-deprived rats, on analgesia. Tail flick latency was used as an index of pain sensitivity and naloxone reversibility of analgesia was used as the criterion for opioid involvement. Food-deprived rats were submitted to one of two schedules of food delivery. The 'contingent' group could control the delivery of food by lever-pressing. The 'non-contingent' group received the same number of food pellets but delivery of food was independent of lever-pressing behaviour. Animals in the 'control' group were placed in the test chamber but did not receive any pellets. Subjects were tested on 6 consecutive days, each test session being of 10 min duration. Half of the animals in each group received an intraperitoneal injection of saline (0.5 ml) prior to each test session, the other half received an intraperitoneal injection of naloxone (5 mg/kg in 0.5 ml saline) prior to each session. Both contingent and non-contingent food delivery resulted in a significant post-test analgesia. The analgesia was noticeably greater when food delivery was non-contingent and this analgesia was reversible by naloxone. The findings suggest that non-contingent food delivery, in food-deprived rats, elicits an opioid analgesia, whilst contingent food delivery elicits a non-opioid analgesia.

Inhibition of rubral neurons by noxious and non-noxious pressure

The role of the red nucleus (RN) in nociception was investigated in this study. Extracellular recordings from spontaneously active RN neurons were conducted in the rat while noxious pressure was delivered to the hindpaws or tail. Cells in the RN were predominantly inhibited by the stimuli. The units were most responsive when noxious pressure was applied to the contralateral hindpaw. Furthermore, more cells in the magnocellular division of the RN responded to the stimuli than cells in the parvocellular division. Delivery of a graded pressure stimulus to the contralateral hindpaw revealed 4 cell types in the RN: non-responsive cells; cells only responsive during the early, non-noxious portion of the stimulus; cells only responsive during the later, noxious portion of the stimulus; and cells that showed an initial response during the non-noxious part of the stimulus and a second, later response during the noxious portion of the stimulus. To further examine the putative role of the RN in nociception, oxotremorine, gamma-aminobutyric acid (GABA), serotonin, glutamate, and morphine were unilaterally microinjected into the RN and the responses of the animals in the tail flick test were assessed. Only morphine produced a significant antinociception in the animals following intrarubral microinjection. However, it is unclear whether this alteration was mediated through the RN because an antinociception of equal magnitude could be elicited from the reticular formation surrounding the RN and lesions of the RN did not alter the antinociception produced by systemic administration of morphine. Although other explanations cannot be ruled out, it appears that the RN may be involved in coordinating the motor response to pain rather than modulating sensory transmission.

Stress analgesia: the opioid analgesia of long swims suppresses the non-opioid analgesia induced by short swims in mice

In mice, room temperature swimming for as short a period as 15 sec has been found to induce a non-opioid analgesia with a time course of 10-12 min. As the duration of the swim is increased, an opioid analgesia develops with a longer persistence (25-30 min); the development of the opioid analgesia appears to suppress the expression of the non-opioid analgesia so that none of the latter is evident after 3 min swims. The characteristics of the tail-flick nociceptive test are also described.

The effect of beta blockade on stress-induced cognitive dysfunction in adolescents

Test anxiety is severely disabling to students whose fear of examinations causes cognitive dysfunction that paralyzes their thinking the way stage fright impairs actors ability to act. In studies using subjective evaluations among actors and musicians, beta-blockade relieved stage fright and has been used informally to treat test anxiety in students without objective measures of effectiveness. The Scholastic Aptitude Test (SAT) was chosen as an objective test instrument to confirm the effect of beta-blockade on test anxiety and performance. Thirty-two high school students who had already taken the SAT before enrolling in this study and who had stress-induced cognitive dysfunction on exams were given 40 mg of propranolol one hour before they retook those tests. Mean SAT scores with beta-blockade were 130 points higher than on the initial SAT done before entering the study without medication (p = less than .01). A single dose of propranolol immediately before the SAT permitted improved performance in students prone to cognitive dysfunction due to test anxiety.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. I. Comparison of opiate receptor-deficient and opiate receptor-rich strains of mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (0.075 mg/kg), and the specific opiate receptor antagonist naloxone (10 mg/kg), on swim stress-induced analgesia (SSIA) were studied in opiate receptor-deficient (CXBK) and opiate receptor-rich (CXBH) mice. Animals were subjected to forced swimming, and analgesia was assessed using the hot-plate test. In CXBK mice SSIA was blocked by MK-801 but was completely insensitive to naloxone. In CXBH mice SSIA was partially attenuated both by naloxone and MK-801, and it was nearly abolished by a combination of these drugs. Morphine analgesia (10 mg/kg) was abolished by naloxone but completely unaffected by MK-801 in CXBH mice. These findings suggest that the NMDA receptor is critically involved in the non-opioid component of SSIA.

Electroacupuncture analgesia in rats: naltrexone antagonism is dependent on previous exposure

Inhibition of pain by low frequency electroacupuncture (EA) has been thought to be mediated by endogenous opioids. We reported in a previous paper, however, that naloxone (NAL) and naltrexone (NTX) either potentiated or had no effect on analgesia in EA-naive rats, independent of origin (American or Chinese), sex, weight, geographic location (the U.S.A. or China), or numerous variations of experimental methodology. In the present study, we hypothesized that the number of exposures to EA treatment may be responsible for the discrepant effect of opiate antagonists. We found, as previously demonstrated, analgesia in EA-naive rats was potentiated by NTX. After two pre-exposures to EA, however, NTX antagonized analgesia. These results indicate that, in rats: (1) pre-exposure is a key variable for opiate antagonists to produce antagonism or potentiation of analgesia; (2) environmental cues paired with the initial analgesic manipulation may be responsible for converting analgesia from non-opioid to opioid, as already demonstrated with classically conditioned and learned helplessness induced analgesia; and (3) EA analgesia in rats is a multidimensional manipulation which can be influenced by subtle environmental changes.

Electroacupuncture analgesia in naive rats: effects of brainstem and spinal cord lesions, and role of pituitary-adrenal axis

Recent studies have shown that analgesia is potentiated by naltrexone (NTX) and naloxone (NAL) pretreatment in rats exposed for the first time to electroacupuncture (EA). In the present study, we have investigated the role of the pituitary-adrenal axis and of brainstem and spinal cord structures in EA analgesia and its potentiation by NTX. The pituitary and adrenal glands do not participate in the production of EA analgesia, but may produce a non-opioid substance which interferes with the development of EA analgesia. Spinalization or dorsolateral funiculi lesions blocked EA analgesia, and intrathecal NTX had no effect. These results indicate that supraspinal structures are necessary to produce and potentiate EA analgesia. Contrary to their critical role in morphine and other models of environmentally produced analgesia nucleus raphe alatus and raphe structures dorsal to it are not necessary for the development of EA analgesia. These structures, however, may contain opiate synapses on which NTX may act as an agonist to potentiate analgesia. The various components which appear to participate in the production of EA analgesia imply a complex circuit of pain modulation systems and indicate that an organism can adapt to distinct environmental conditions with versatile means to avoid pain.

Influence of stress on the maturity of T-cells

Stress is known to influence the immune function via an effect on the central nervous system. We previously presented data showing that stress alters the population of T-cell subsets in mice. The variations of T-cell subsets in the thymus, peripheral blood, and spleen in mice similarly stressed by immobilization or by unavoidable and opioid-dependent stress were measured by flow cytometry using the monoclonal antibodies anti-L3T4, anti-Lyt 1, anti-Lyt 2 and anti-Thy 1, 2. Immobilization stress was applied for three days and T-cell subsets were measured on the days 1, 2 and 3, as well as on day 7 after release from immobilization. Lyt 2-positive cells in the thymus were the most sensitive to stress, showing significant variations. The proportion of immature T-cells increased in the thymus, blood and spleen of the stressed mice. When diazepam or naloxone were administered 30 min before the initiation of stress, these variations tended to decrease. Thus, the ratio of T-cell subsets varied with the duration of immobilization stress. This appeared to be partly mediated by the opioid system and the central nervous system.

Effects of hypophysectomy and adrenalectomy on naloxone-induced analgesia

Experiment 1 demonstrated that pairings of the opiate antagonist, naloxone, with a heated floor came to induce analgesia, as indexed by the latencies with which rats licked their paws. This analgesia appears to be neurally mediated because it is unaffected by either hypophysectomy (experiment 2) or adrenalectomy (experiment 3). However, there was evidence for a pituitary involvement, as its removal potentiated the analgesic effect accruing from naloxone-stressor pairings.

Cimetidine does not change the effect of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on the opiate form of footshock-induced analgesia

It has been demonstrated that cimetidine blocks the effect of naloxone on footshock-induced analgesia. To study the effect of cimetidine on the antiopiate properties of an endogenous peptide Tyr-MIF-1, the opiate form of intermittent footshock-induced analgesia was elicited in the rat. The nociceptive responses were determined using the hot-plate test (52.5 degrees C). Intraperitoneal pretreatment with cimetidine (100 mg/kg) or chlorpheniramine maleate (20 mg/kg) did not affect the footshock-induced analgesia, and did not change the antagonizing effect of Tyr-MIF-1 (0.2 mg/kg) on this model of antinociception. It is concluded that cimetidine and chlorpheniramine maleate do not change the antagonizing effect of Tyr-MIF-1 on the opiate form of intermittent footshock-induced analgesia.

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.

Stimulation of the hypothalamic paraventricular nucleus produces analgesia not mediated by vasopressin or endogenous opioids

The analgesic effect of electrical stimulation of the hypothalamic paraventricular nucleus (PVN) was studied. Additionally, the involvement of vasopressin and opioid peptides in this process was examined by comparing vasopressin-deficient (Brattleboro) and Long-Evans rats and by administering the opiate antagonist naloxone. Rats were chronically implanted with a stimulating electrode in the parvocellular (PVN-Pc) and magnocellular (PVN-Mg) divisions of the PVN. At least 10 days after surgery, the analgesic effects of PVN stimulation were examined in lightly anesthetized rats, using the tail-flick method, and in unanesthetized rats, using the hot-plate test. PVN stimulation produced marked analgesia in both tests. Current threshold for analgesia was lower from PVN-Pc than from PVN-Mg. Threshold did not differ significantly between Brattleboro and Long-Evans rats and was not affected by naloxone administration. The results indicate that the PVN is part of the brain's pain inhibitory system, and show that the analgesia induced by PVN stimulation is not mediated by either vasopressin or opioid peptides.

Factors affecting restraint stress-induced potentiation of morphine analgesia

The analgesic effect of opioid drugs is potentiated in rats exposed to restraint stress as compared to unstressed rats. The purpose of the present study was to quantify how the following factors affect morphine-induced analgesia: habituation to restraint versus exposure to restraint for the first time, restraint stress duration, and interval from restraint to analgesic testing. Expts. 1 and 2 generated dose- and time course curves for morphine in rats exposed to one of 3 treatments: no restraint stress (NS), first exposure to 1 or 6 h of restraint (FS), or 5 days of restraint habituation followed by 1 or 6 h of restraint on the test day (HAB). Analgesia was measured by the tail-flick assay. Rats subjected to 1 h of restraint displayed dose- and time-dependent potentiation of morphine-induced antinociception compared to unstressed rats. Given 4.0 mg/kg morphine. FS-treated subjects showed 1.4- and 2.7-fold more potentiation of analgesia than HAB- and NS-treated rats, respectively. Rats restrained for 6 h prior to testing showed significant dose effect for morphine but failed to reveal significant treatment effects. Thus, increasing the duration of restraint from 1 to 6 h attenuated morphine antinociception in FS- and HAB-treated subjects to the level of NS subjects. In Expt. 3, several groups of rats underwent a single 1-h session of restraint at various time intervals prior to injection with morphine (4.0 mg/kg) and tail-flick testing. An unstressed group also receiving morphine served as control.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of stress on convulsive parameters in the mouse

Mice exposed to the stress of conspecific aggression for 10 min showed shorter latencies to convulsions induced by pentylenetetrazol but not by pilocarpine. This effect was short lived and was not influenced by pretreatment with naltrexone (5 mg/kg, SC). The onset of pilocarpine-induced convulsions in stressed mice was reduced by the opioid antagonist. Aggression stress did not change the incidence, duration or severity of convulsions triggered by the chemoconvulsants or electroshock. The results differ widely from those obtained using other stressogenic models such as cold-restraint or swim stress. This suggests that alterations of convulsive parameters and the involvement of opioid mechanisms in their mediation are critically dependent on the characteristics of the stressogenic procedure employed.

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

Previous research has demonstrated that exposure to a cat produces a naltrexone-reversible antinociception as assessed in the formalin test in rats. Because different neurochemical mechanisms inhibit different forms of nociception, the present study examined whether presentation of a cat would also produce a naltrexone-reversible antinociception in the tail-flick response to radiant heat and electric shock. Exposure to the cat produced antinociception in both tail-flick paradigms. Naltrexone blocked the inhibition of the thermally evoked tail-flick response, but had no effect in the electric shock tail-flick paradigm. These results indicate that opioid mediation of stress-induced analgesia is determined, in part, by the nociceptive test employed.

An opiate mechanism involved in conditioned analgesia influences forced swim-induced immobility

The modulatory effect of conditioned opiate analgesia on immobility during a forced swim situation was studied. Animals submitted to inescapable shock (IS) were exposed 6 days later to a similar or different shock application context and, immediately after, tested in either hot plate test or forced swim test. A conditioned analgesia was observed only on animals submitted to the shock context. This conditioned analgesia was blocked by naloxone administration injected either before IS or before context exposure. In the same way, animals exposed to shock context and immediately forced to swim showed an increase in the immobility time, which was sensitive to naloxone injection before IS as well as before context exposure. These results and additional data referring to naloxone effect on inactivity during IS are discussed in terms of the possible role of endogenous opiate in analogous behavior expressed during different aversive experiences.

Alpha-adrenoceptor involvement in swim stress-induced antinociception in the mouse

Three different intensities of swim stress produced stress-induced antinociception (SIA) in mice which was assessed either by the reduction in the number of abdominal constrictions produced by acetic acid or by an increase in reaction time on a hot-plate. The involvement of alpha-adrenoceptors in the three models of SIA was investigated using selective antagonists. SIA produced by the mild stress of a 30 s warm water swim was attenuated by idazoxan (0.5-1 mg kg-1), and by yohimbine at a dose (1 mg kg-1) which reduced antinociception produced by clonidine (12.5-50 micrograms kg-1). Indoramin (1-2 mg kg-1) did not affect this model of SIA, but reversed phenylephrine induced inhibition of the constrictions. A 3 min room temperature swim increased reaction times on the hot-plate and this naloxone-sensitive SIA was reduced significantly by prazosin (1-2 mg kg-1), idazoxan (0.5-1 mg kg-1) and yohimbine (0.5-1 mg kg-1) but enhanced by clonidine (0.5 mg kg-1) and noradrenaline (NA) (10 micrograms i.c.v.). Mice treated with 6-hydroxydopamine (60 + 60 micrograms i.c.v.) were hypersensitive to the hot-plate and did not develop SIA. Levels of noradrenaline in the brain (minus the cerebellum) were decreased after the room temperature swim SIA. The most severe stress of a cold water swim produced SIA on the hot-plate which was initially naloxone-insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)