Cross tolerance between morphine and the long-term analgesic reaction to inescapable shock

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Review of neuroimaging studies related to pain modulation

Background and purpose: A noxious stimulus does not necessarily cause pain. Nociceptive signals arising from a noxious stimulus are subject to modulation via endogenous inhibitory and facilitatory mechanisms as they travel from the periphery to the dorsal horn or brainstem and on to higher brain sites. Research on the neural structures underlying endogenous pain modulation has largely been restricted to animal research due to the invasiveness of such studies (e.g., spinal cord transection, brain lesioning, brain site stimulation). Neuroimaging techniques (e.g., magnetoencephalography (MEG), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)) provide non-invasive means to study neural structures in humans. The aim is to provide a narrative review of neuroimaging studies related to human pain control mechanisms.

Methods: The approach taken is to summarise specific pain modulation mechanisms within the somatosensory (diffuse noxious inhibitory controls, acupuncture, movement), affective (depression, anxiety, catastrophizing, stress) and cognitive (anticipation/placebo, attention/distraction, hypnosis)domains with emphasis on the contribution of neuroimaging studies.

Results and conclusions: Findings from imaging studies are complex reflecting activation or deactivation in numerous brain areas. Despite this, neuroimaging techniques have clarified supraspinal sites involved in a number of pain control mechanisms. The periaqueductal grey (PAG) is one area that has consistently been shown to be activated across the majority of pain mechanisms. Activity in the rostral ventromedial medulla known to relay descending modulation from the PAG, has also been observed both during acupuncture analgesia and anxiety-induced hyperalgesia. Other brain areas that appear to be involved in a number of mechanisms are the anterior cingulate cortex, prefrontal cortex, orbitofrontal cortex and nucleus accumbens, but their exact role is less clear.

Implications: Neuroimaging studies have provided essential information about the pain modulatory pathways under normal conditions, but much is still to be determined. Understanding the mechanisms of pain control is important for understanding the mechanisms that contribute to failed pain control in chronic pain. Applying fMRI outside the brain, such as in the trigeminal nucleus caudalis of the spinotrigeminal pathway and in the dorsal horn of the spinal cord, and coupling brain activity with activity at these sites may help improve our understanding of the function of brain sites and shed light on functional connectivity in the pain pathway.

© 2011 Scandinavian Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.

Spinal cord-transected mice learn to step in response to quipazine treatment and robotic training

In the present study, concurrent treatment with robotic step training and a serotonin agonist, quipazine, generated significant recovery of locomotor function in complete spinal cord-transected mice (T7-T9) that otherwise could not step. The extent of recovery achieved when these treatments were combined exceeded that obtained when either treatment was applied independently. We quantitatively analyzed the stepping characteristics of spinal mice after alternatively administering no training, manual training, robotic training, quipazine treatment, or a combination of robotic training with quipazine treatment, to examine the mechanisms by which training and quipazine treatment promote functional recovery. Using fast Fourier transform and principal components analysis, significant improvements in the step rhythm, step shape consistency, and number of weight-bearing steps were observed in robotically trained compared with manually trained or nontrained mice. In contrast, manual training had no effect on stepping performance, yielding no improvement compared with nontrained mice. Daily bolus quipazine treatment acutely improved the step shape consistency and number of steps executed by both robotically trained and nontrained mice, but these improvements did not persist after quipazine was withdrawn. At the dosage used (0.5 mg/kg body weight), quipazine appeared to facilitate, rather than directly generate, stepping, by enabling the spinal cord neural circuitry to process specific patterns of sensory information associated with weight-bearing stepping. Via this mechanism, quipazine treatment enhanced kinematically appropriate robotic training. When administered intermittently during an extended period of robotic training, quipazine revealed training-induced stepping improvements that were masked in the absence of the pharmacological treatment.

Stress-induced analgesia: prediction of posttraumatic stress symptoms in battered versus nonbattered women

BACKGROUND

Chronic and inescapable trauma is implicated in the stress-induced analgesia (SIA) response.

METHODS

A sample of 27 chronically battered women was compared with 28 trauma-exposed nonbattered women on their SIA response at 1 month postindex assault.

RESULTS

For the battered women sample, the SIA response at 1 month postindex assault was found to significantly predict an increase in posttraumatic stress disorder-related hyperarousal at 3 months postindex assault. Furthermore, the battered women showed a significant increase in depression symptoms from 1 to 3 months postindex assault compared with the nonbattered women, who showed a significant decrease.

CONCLUSIONS

The findings suggest that the chronic and inescapable nature of trauma exposure in the battered women might account for an SIA response that is qualitatively different from that seen in the nonbattered women. It is suggested that the mechanism underlying the SIA response in battered women might be opioid mediated and that it might be responsible for the significant prediction of physiologic hyperarousal. Furthermore, this hyperarousal might moderate the relationship between the SIA response and depression.

Behavioral analysis of stress controllability effects in a new swim stress paradigm

Previous animal stress studies have illustrated the marked impact of coping on subsequent behavior and physiology by using shock as the stressor. The current study evaluates the generality of shock stress controllability effects in a new swim stress paradigm on several dependent measures: behavioral despair, analgesia, shuttlebox escape, and alcohol reactivity. In this new paradigm, rats in the escape group are able to learn the behavioral response as evidenced by significant reduction in the acquisition of a lever press response. Both escape and yoked subjects showed "behavioral despair" in comparison to both restrained and home cage controls when tested 24 h later. In the standard shuttlebox escape task 24-h post-stress, no group differences emerged, although a trend for poorer performance in the yoked subjects was evident. No group differences were observed in pain sensitivity after the first or second forced swim exposure. Finally, stress controllability effects were observed in behavioral reactivity to alcohol 2-h post-stress as measured by rotarod performance. This effect is opposite to the previous observations with the tailshock stress controllability paradigm. These results suggest that (1) there are certain similarities, but some fundamental differences between the behavioral endpoints measured following intermittent swim stress in comparison to the well-established effects of the intermittent tailshock stress model and (2) the qualitative nature of a stressor may markedly influence the behavioral and physiological consequences of stress and coping.

Stress-induced analgesia in mu-opioid receptor knockout mice reveals normal function of the delta-opioid receptor system

Stress-induced analgesia (SIA) was examined in wildtype and mu-opioid receptor knockout mice. We used thermal paw withdrawal (TPW) latency following a continuous 3-min swim in 20 degrees C water, and found a significant increase in TPW latency in both wild-type and knockout mice. Pre-treatment prior to the swim with naltrindole, a selective delta-opioid receptor antagonist, blocked the increase in TPW latency in knockout mice. These results demonstrate an intact delta-receptor-mediated function of a physiologically-released endogenous agonist in the mu-opioid receptor knockout mouse. The present findings are in contrast with previous reports that analgesia induced by exogenous delta agonists is reduced in the knockout mice.

Activation of serotonin-immunoreactive cells in the dorsal raphe nucleus in rats exposed to an uncontrollable stressor

The dorsal raphe nucleus (DRN) and its serotonergic terminal regions have been suggested to be part of the neural substrate by which exposure to uncontrollable stressors produces poor escape responding and enhanced conditioned fear expression. Such stressor exposure is thought to selectively activate DRN serotonergic neurons in such a way as to render them transiently sensitized to further input. As a result of this sensitized state, behavioral testing procedures are thought to cause excess serotonergic activity in brain regions that control these behaviors. The present studies were conducted to investigate activity in the DRN following exposure to escapable and yoked, inescapable tailshock. Neural activity was characterized using immunohistochemistry to detect the immediate early gene product Fos in serotonin-immunoreactive cells in the DRN. Inescapable tailshock led to greater serotonergic neural activity than did escapable tailshock, supporting the hypothesis that uncontrollable stressors preferentially activate serotonergic neurons in the DRN.

Opioid-dependent effects of inescapable shock on escape behavior and conditioned fear responding are mediated by the dorsal raphe nucleus

Manipulations of the dorsal raphe nucleus (DRN) modulate the behavioral effects of exposure to inescapable shock (IS). Opiate agonists and antagonists also influence the impact of IS, but the role of the DRN in mediating these effects is unknown. The opiate antagonist naltrexone micro-injected into the region of the DRN immediately prior to IS prevented both the escape deficit and the enhancement of fear conditioning that occur 24 h later. Intra-DRN naltrexone administered at the time of later behavioral testing reduced, but did not eliminate, these effects of prior IS. Conversely, the opiate agonist morphine, in combination with a subthreshold number of 20 IS trials, induced an escape deficit and enhanced conditioned fear 24 h later. Microinjections of naltrexone into the dorsolateral periaqueductal gray area did not alter the effects of IS and electrolytic lesions of the DRN prevented the effect of the morphine-20 IS trial combination. The role of opioids in mediating the behavioral effects of IS is discussed.

Effects of naltrexone and cross-tolerance to morphine in a learned helplessness paradigm

Opiates have been implicated in learned helplessness (LH), a phenomenon known to be related to opiate stress-induced analgesia (SIA). In the present study, we investigated the role of opiates in the induction of LH and SIA under different conditions. Adult female Wistar rats were trained either by receiving 60 inescapable 1-mA footshocks (IS group, N = 114) or by confinement in the shock box (control or NS group, N = 92). The pain threshold of some of the animals was immediately evaluated in a tail-flick test while the rest were used 24 h later in a shuttle box experiment to examine their escape performance. The opiate antagonist naltrexone (0 or 8 mg/kg, ip) and the previous induction of cross-tolerance to morphine by the chronic administration of morphine (0 or 10 mg/kg, sc, for 13 days) were used to identify opiate involvement. Analysis of variance revealed that only animals in the IS group demonstrated antinociception and an escape deficit, both of which were resistant to the procedures applied before the training session. However, the escape deficit could be reversed if the treatments were given before the test session. We conclude that, under our conditions, induction of the LH deficit in escape performance is not opiate-mediated although its expression is opiate-modulated.

Regulation of stress-induced reduced myelopoiesis in rats

In this work we demonstrate that the stress-induced reduction in bone marrow granulocyte-macrophage colonies, reported previously from our laboratory, is prevented by both the inhibition of the hypothalamic-pituitary-adrenal axis (HPAA) and the blockage of opioid receptors. The inhibition of the HPAA was obtained through the administration of dexamethasone (1 mg/kg). The blockage of opioid receptors was done in two ways, by the administration of naltrexone (8 mg/kg) and induction of tolerance to morphine. On the other hand, no protection was observed in metyrapone treated rats. We suggest that the two physiological systems, opioid and HPAA, mediate the stress-induced myelosuppression and that these systems may function independently in this particular situation.

Stress-induced behavioral responses and multiple opioid systems in the brain

Various stressor produce a wide range of behavioral responses such as analgesia, catalepsy and motor suppression, which are sensitive to opioid receptor antagonists. These behavioral responses in stress are accompanied by changes in the contents of opioid peptides, the mRNAs encoding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesia and motor suppression is reviewed and discussed in relation to a possible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-selective antagonists have demonstrated that not only mu- but also delta- and/or kappa-opioid receptors are involved in opioid-mediated stress-induced analgesia. There are two types of stress-induced analgesia referred to as opioid-mediated and non-opioid mediated forms. It has been proposed that the intensity and temporal pattern of stressor may be a critical factor determining the nature of stress-induced analgesia. Accumulated evidence demonstrate that these two forms of pain inhibitory systems interact each other according to a collateral inhibition model. Recent studies show that parallel activation of multiple opioid receptors mediates non-opioid froms of stress-induced analgesia. Dynorphins, by acting at kappa-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalins may act to attenuate this response.

Decreased forebrain [35S]TBPS binding and increased [3H]muscimol binding in rats that do not develop stress-induced behavioral depression

Recent evidence suggests that anxiety and its biological concomitants may be involved in the pathophysiology of depression. In the present study, the in vitro radioligand binding of [3H]flunitrazepam, [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) sites on the benzodiazepine/GABA chloride ionophore receptor complex (BGRC) was examined using the learned helplessness paradigm. Only rats which did not develop the syndrome showed a significant increase in [3H]muscimol binding in cerebral cortex and a decrease in [35S]TBPS binding in cerebral cortex and hippocampus in comparison to naive controls. For both ligands, this represented a change in Bmax rather than a change in affinity. Adrenalectomy had no impact on these alterations indicating that critical endogenous factors are not manufactured by the adrenal glands. These findings suggest that the BGRC in the forebrain may be a site mediating the 'coping' ability of rats that do not develop the learned helplessness syndrome. The possible involvement of neurosteroids in this effect is discussed.

Learned helplessness inducing foot shock can exacerbate morphine responsiveness

Exposure to inescapable tail shock or foot shock has been shown to produce effects on a number of learning tasks. Tail-shock exposure is also known to influence nociception and morphine reactivity. The present series of experiments investigated the effects of foot shock known to induce learned helplessness effects in our laboratory on the subsequent reactivity to morphine. A first set of experiments investigated the hypoalgesic response to a 4 mg/kg dose morphine over 4 consecutive days following exposure to foot shock. Experiment 1A did not reveal an effect of foot shock on morphine-induced hypoalgesia when testing was conducted in a novel context. In Experiment 1B, we observed an increased hypoalgesic response to morphine when testing was conducted in the shock context. The findings of Experiment 1B were replicated in Experiment 2 and extended to assess the contribution of conditioned fear hypoalgesia to these effects. The possible mechanisms responsible for these findings are discussed.

Cross-tolerance between morphine and swim analgesia in mice selectively bred for high and low stress-induced analgesia

Mice selectively bred for high (HA) and for low analgesia (LA) induced by 3-min swimming at 20 degrees C and unselected controls (C) were injected three times daily for 3 days with 20 mg/kg morphine HCl. The analgesic effect of 10 mg/kg morphine in nontolerant mice differed between the lines in the rank order of HA > C > LA and significantly decreased after repeated treatment with morphine, as revealed by the hotplate test (56 degrees C). The tolerance to morphine analgesia was more pronounced in HA than in C mice but did not develop at all in LA mice. Similarly, the magnitude of swim-induced analgesia in morphine tolerant mice decreased to a greater degree in the HA than the C line but did not change in LA mice. Naloxone HCl (1 and 10 mg/kg) attenuated swim analgesia more in nontolerant HA than C mice but had no effect in morphine-tolerant HA and C and in all LA mice. The differential degree of morphine tolerance and cross-tolerance with swim analgesia suggests that the strategy of selective breeding toward divergent magnitudes of stress-induced analgesia has differentiated opioid involvement in endogenous pain inhibition in the selected lines.

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.

Delta opiate receptors mediate tail-shock induced antinociception at supraspinal 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 preceding companion paper and the present work were aimed at identifying the neuroanatomical loci at which opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 8 experiments included in the present paper examined the effect of microinjecting 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) either into the third ventricle or over the frontal cortex. Taken together, these experiments demonstrate that the late (80-100 shock) opiate analgesia is mediated by delta opiate receptors located within subcortical structures rostral to the 4th ventricle. No evidence for supraspinal opiate involvement in the early (2 shock) opiate analgesia was found.

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.

Sleep deprivation decreases mu and delta opioid receptor binding in the rat limbic system

Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 h) the rat displayed particular behavior characterized by wakefulness, a high degree of motor and exploratory activity, increased alertness and reactivity to environmental stimuli. Our previous results indicated that this behavior was antagonized by the administration of the opioid receptor antagonist naloxone and increased by opioid agonists. In this paper we show that concomitantly with this behavior, a decreased Bmax of mu and delta opioid receptors is present in the limbic system of these animals. These data suggest an active role of limbic mu and delta receptors in the generation of arousal and insomnia related to sleep deprivation induced stress.

Changes in mu opiate receptors following inescapable shock

Rats exposed to inescapable shock exhibit profound hypoalgesia. Pharmacological evidence has suggested that changes in endogenous opiate activity may be responsible for the hypoalgesic response. We measured the binding of [3H]DAGO, a selective mu-opiate receptor agonist, in brains of rats exposed to no shock, inescapable shock, or escapable shock. Binding of [3H]DAGO in the midbrains of rats in the inescapable shock group was decreased relative to the other two groups. The decrease in binding appeared to result from a decrease in number of mu-receptors and not a change in affinity. These results support the hypothesis that inescapable shock produces long-term changes in endogenous opiate systems.

Neuropeptides: conductors of the immune orchestra

There is increasing evidence for a bidirectional communications system between the immune system and the brain. Many of the substances involved in this communication appear to be neuropeptides. These findings have given biochemical validity to the clinical and epidemiological studies that have suggested that psychosocial factors can modulate the response to infections and neoplasms.

The involvement of endogenous opiate systems in learned helplessness and stress-induced analgesia

The participation of endogenous opiate systems in the induction and expression of learned helplessness (LH) and stress-induced analgesia (SIA) was investigated in rats exposed to escapable and inescapable footshock. Following an initial footshock, analgesia was observed only in those animals that could not control their stress exposure and this SIA was prevented by the administration of naloxone. Analgesia was no longer evident in this inescapable group after 48 h. However, exposure to a shuttlebox escape task at this time reinstated the SIA but did not produce SIA in animals previously exposed to escapable footshock. Shuttlebox escape deficits indicative of LH were also exhibited by animals that received an inescapable footshock stress 48 h prior to testing. The analgesia and LH observed in the inescapable group were not affected by the administration of naloxone (3 mg/kg, IP) 10 min before shuttlebox exposure but were prevented when the same dose of naloxone was given before the initial stress. Thus, endogenous opiates clearly participate in the initial induction of LH and SIA and, although the degree of endogenous opiate involvement in the subsequent expression of these behaviors is unclear, it seems evident that their expression can occur in the presence of opiate antagonism and may therefore require the participation of additional transmitter systems.

Conditioning and sensitisation in the longitudinal course of affective illness

Few biological theories of manic-depressive illness have focused on the longitudinal course of affective dysfunction and the mechanisms underlying its often recurrent and progressive course. The authors discuss two models for the development of progressive behavioural dysfunction--behavioural sensitisation and electrophysiological kindling--as they provide clues to important clinical and biological variables relevant to sensitisation in affective illness. The role of environmental context and conditioning in mediating behavioural and biochemical aspects of this sensitisation is emphasised. The sensitisation models provide a conceptual approach to previously inexplicable clinical phenomena in the longitudinal course of affective illness and may provide a bridge between psychoanalytic/psychosocial and neurobiological formulations of manic-depressive illness.

Effects of muscarinic receptor antagonism upon two forms of stress-induced analgesia

The present study assessed in rats the effects of muscarinic receptor antagonism upon analgesia induced by cold-water swims (CWS: 2 degrees C for 3.5 min) and 2-deoxy-D-glucose (2DG: 600 mg/kg). First, CWS analgesia was significantly reduced 30 min after the swim by scopolamine (0.01 and 0.1 mg/kg) and methylscopolamine (10 mg/kg) pretreatment, and was eliminated 60 min after the swim by scopolamine (0.01-10 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment. In contrast, scopolamine potentiated CWS hypothermia. Second, while scopolamine (1 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment prolonged 2DG analgesia, both antagonists dose-dependently reduced 2DG hyperphagia. Third, the changes in analgesic and hypothermic stress responses were not due to baseline shifts in jump thresholds or body temperatures. However the dose-dependent reductions by scopolamine and methylscopolamine in baseline food intake and 2DG hyperphagia were significantly correlated. Fourth, the dose-dependent reduction by scopolamine and methylscopolamine of pilocarpine analgesia differed in pattern from the other analgesic effects, suggesting heterogeneity in muscarinic receptor modulation of different analgesic responses.

Coping and seizure susceptibility: control over shock protects against bicuculline-induced seizures

Rats were either given 80 escapable shocks, yoked inescapable shocks, restraint or given no treatment. Two hours later all subjects received i.p. injection of bicuculline (4, 6 or 8 mg/kg) and were immediately tested for latency to initial myoclonic jerk and clonus. The latency to clonic convulsion was dramatically affected by prior shock treatment, and the direction of this change depended upon the escapability/inescapability of the shock. Subjects that were given escapable shock showed a delay of onset to seizure, while subjects inescapably shocked demonstrated a decreased latency to clonus in comparison to restrained and naive controls. It was also demonstrated that if the subjects were tested immediately following a stress experience, both the 80 escapable and inescapable shock condition protected against bicuculline-induced seizures in comparison to the control condition. Finally Experiment 2 confirmed a previous finding that less stress, i.e., 20 inescapable shocks, protects against seizures when the animals are challenged with bicuculline either immediately or 2 h later. Our suggestion is that control over stress may facilitate GABAergic transmission, and this may be the mechanism whereby coping protects against the behavioral and physiological disruption produced by exposure to a stressor.

Librium prevents the analgesia and shuttlebox escape deficit typically observed following inescapable shock

Administration of a benzodiazepine, chlordiazepoxide (CDP), prior to exposure to inescapable shock prevented both the long-term analgesia and the shuttle-escape deficit typically observed following inescapable shock. If given only prior to testing, CDP had little effect. The protective effects of CDP were determined not to be a result of state dependency or a general facilitatory effect of the drug on escape performance. It is suggested that the induction of anxiety or fear by inescapable shock is critical in mobilizing endogenous changes such as transmitter depletion which are thought to be responsible for the deficits observed.

Intrinsic mechanisms of pain inhibition: activation by stress

Portions of the brain stem seem normally to inhibit pain. In man and laboratory animals these brain areas and pathways from them to spinal sensory circuits can be activated by focal stimulation. Endogenous opioids appear to be implicated although separate nonopioid mechanisms are also evident. Stress seems to be a natural stimulus triggering pain suppression. Properties of electric footshock have been shown to determine the opioid or nonopioid basis of stress-induced analgesia. Two different opioid systems can be activated by different footshock paradigms. This dissection of stress analgesia has begun to integrate divergent findings concerning pain inhibition and also to account for some of the variance that has obscured the reliable measurement of the effects of stress on tumor growth and immune function.

Opiate and non-opiate analgesia induced by inescapable tail-shock: effects of dorsolateral funiculus lesions and decerebration

Previous studies have demonstrated that inescapable tail-shock can produce either non-opiate or opiate short-term analgesia, dependent on the number of shocks delivered. Additionally, extended exposure to inescapable tail shock can produce long-term, opiate analgesic effects. Several lines of investigation suggest that the psychological dimension of perceived controllability may powerfully influence these phenomena in that each form of opiate analgesia can only be produced following exposure to inescapable, rather than equal amounts and distribution of escapable, shock. This has suggested that these opiate analgesias result from the organism's learning that it has no control over shock. Although it has been assumed that the opiate and non-opiate analgesias induced by tail shock may be subserved by neural circuitry similar to that mediating morphine analgesia and other forms of environmentally induced analgesia, no direct evidence exists to support this assumption. The present study sought to provide an initial attempt at defining the neural circuitry involved in these phenomena by examining the effect of bilateral dorsolateral funiculus (DLF) lesions and decerebration. These experiments revealed that pathways within the spinal cord DLF are critical for the production of short-term non-opiate analgesia, short-term opiate analgesia, and long-term opiate analgesia since bilateral DLF lesions abolished all three pain inhibitory effects. Additionally, it was found that decerebration did not attenuate either the short-term non-opiate or short-term opiate analgesia induced by inescapable tail shock. Combining the observations that these non-opiate and opiate short-term effects are not reduced by decerebration yet are abolished by DLF lesions clearly delimits the source of descending pain inhibition as being within the caudal brainstem.

Prevention of stress-induced analgesia by substance P

It has been shown that a variety of stressful procedures, such as immobilization and footshock, can induce a significant degree of analgesia in mice. In addition, it has been shown that for some, but not all, stressful treatments, the analgesic effect is mediated via endogenous opioids. This report describes the effects of substance P, administered systemically, on both opioid-mediated immobilization-induced analgesia and non-opioid footshock-induced analgesia. Substance P completely blocked the opioid-mediated form of stress-induced analgesia while having no effect on the non-opioid form. Exogenous substance P appears to interact with endogenous opioid pain-suppressing systems.

Adrenal modulation of opiate induced feeding

Endogenous opioids have been shown to initiate feeding in sated animals. In the present study adrenalectomy enhanced the feeding response to the kappa opiate agonist, ethylketocyclazocine and the kappa/sigma opiate agonist, butorphanol tartrate. Adrenalectomy abolished the anorectic effect of naloxone at doses as high as 10 mg/kg. Corticosterone replacement did not alter the opiate induced feeding and adrenal demedullated rats continued to show enhancement to opiate induced feeding. These data suggest that in addition to the central nervous system, the adrenal medulla is involved in opiate related induction of feeding.

Coping and immunosuppression: inescapable but not escapable shock suppresses lymphocyte proliferation

Rats were given series of escapable shocks, identical inescapable shocks, or no shock. The subjects were reexposed to a small amount of shock 24 hours later, after which an in vitro measure of the cellular immune response was examined. Lymphocyte proliferation in response to the mitogens phytohemagglutinin and concanavalin A was suppressed in the inescapable shock group but not in the escapable shock group. This suggests that the controllability of stressors is critical in modulating immune functioning.

Opioid and non-opioid mechanisms of stress analgesia: lack of cross-tolerance between stressors

Qualitatively different analgesic responses can be evoked in rats by exposure to prolonged, intermittent or brief, continuous footshock stress. These two forms of stress analgesia appear to be mediated by opioid and nonopioid pain-inhibitory substrates, respectively. The present study confirms our previous observation that tolerance develops to only the opioid form of stress analgesia and shows that cross-tolerance does not occur between the opioid and nonopioid forms. These data provide further evidence that independent mechanisms underlie opioid and nonopioid stress analgesia.

Long-term stress-induced analgesia blocked by scopolamine

An 'opioid' form of analgesia ('long-term analgesia') was completely blocked by scopolamine. This effect of a muscarinic antagonist suggests that muscarinic cholinergic processes may play an important role in this analgesia and in other types of endogenous pain control.

Extent and control of shock affects naltrexone sensitivity of stress-induced analgesia and reactivity to morphine

Opioid and nonopioid mediated changes in pain sensitivity have been observed after exposure to various stressful conditions. A series of inescapable shocks sequentially produces an early form of analgesia which is not affected by the opiate antagonist, naltrexone, and a late antinociceptive response which is sensitive to reversal by naltrexone. Here, this is shown to be true over a wide range of doses. In a further experiment subjects given either escapable or inescapable shock were analgesic immediately after the stress session. However, the analgesia of inescapably shocked subjects was more sensitive to reversal by naltrexone. A final experiment revealed that inescapably shocked subjects, but not escapably shocked subjects, were hyperreactive to the analgesic effects of morphine 24 hr after shock. These results suggest that activation of an opiate system occurs only after extended exposure to stress and that this activation is greater when the stress is inescapable. Implications for opioid versus nonopioid mechanisms of stress-induced analgesia are discussed.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Tolerance and cross tolerance with morphine resulting from physiological release of endogenous opiates

Mice which had been exposed to a chronic schedule of warm water swimming showed the development of a significant tolerance to the antinociceptive response (tail-flick latency) and a significant, two-fold increase in the ED50 of morphine (tail-flick latency and abdominal constriction response). These results suggest the involvement of endogenous opiates during swim stress in mice and are consistent with the hypothesis that during chronic stress the opiate receptors are activated in a manner analogous to the repeated application of exogenous opiates producing tolerance, morphine cross tolerance and (as previously reported) withdrawal-like behaviour.

Corticosterone: a critical factor in an opioid form of stress-induced analgesia

The finding that some opioid-mediated forms of stress-induced analgesia are antagonized by hypophysectomy and dexamethasone has led to the suggestion that beta-endorphin, released from the pituitary, may mediate these analgesic reactions. "Long-term analgesia" (an opioid-mediated form of stress-induced analgesia), which is blocked by dexamethasone and hypophysectomy, was also blocked by adrenalectomy and reinstated with corticosterone therapy. Corticosterone is proposed to play a permissive role in long-term analgesia and to be a critical hormone mediating this phenomenon.

Opioid peptides as neuroregulators: potential areas for the study of genetic-behavioral mechanisms

The opioid peptides have been related to behavior in both animal and human studies. Further investigation can be anticipated which could lead to the elucidation of genetic controls over enzymes which process these peptides and the receptors upon which the peptides act. The enzymes, both synthetic and degradative, can lead to the formation of different forms of the opiate peptides. Differential control of these enzymes or of the multiple forms of opiate receptors could lead to discrete changes in opiate status and subsequent behavioral changes. Conversely, genetically regulated behavioral modification could also lead secondarily to opiate changes.

Controllability, coping behavior, and stress-induced analgesia in the rat

Exposure to painful or stressful stimuli produces an analgesic reaction which can persist for 1-2 h post-stress. In the typical stress-induced analgesia study the subject is not permitted to alter or exert control over the aversive event to which it is exposed. That is, its behavior affects neither the duration or intensity of the event. The experiments reported here attempted to determine whether this inability of the subject to control the aversive event is an important determinant of stress-induced analgesia, or whether simple exposure to painful events is a sufficient condition for its production. In the first experiment rats were given either escapable electric shocks (the subject's behavior could terminate the shock), equal amounts of inescapable shock, or no shock. Tail-flick to radiant heat was assessed 30 min later. The group given inescapable shock was strongly analgesic, while the group given an equal amount of escapable shock was only mildly analgesic. Thus the controllability of the shock or the availability of a coping response determined the antinociceptive reaction which followed. The second experiment revealed that this differential effect of controllability on tail-flick responding is masked, shortly after the end of the shock session, by a transient analgesic effect of shock which is not sensitive to the controllability dimension. The implications of these results for stress-induced analgesia and the activation of opioid systems are discussed.