Dexamethasone and stress-induced analgesia

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.

Increased thermal and mechanical nociceptive thresholds in rats with depressive-like behaviors

Clinical observations suggest that depressed patients were less sensitive to experimental pain than healthy subjects. However, few animal studies are reported concerning the association of depression and pain. The purpose of this study was to investigate the effects of unpredictable chronic mild stress (UCMS) induced depression on the perceived intensity of painful stimulation in rats. We measured the thermal and mechanical paw withdrawal thresholds (PWT) of normal and spinal nerve ligated (SNL) rats using hot plate test and von Frey test, respectively. The results showed that rats exposed to UCMS exhibited significantly higher thermal and mechanical pain thresholds in comparison to the non-depressed controls. In particular, the PWT of the SNL group was restored to nearly normal level after three weeks of UCMS, and even comparable to that of the control group. These results strongly suggest that the depressed subjects have decreased sensitivity to externally applied noxious stimulation, which is consistent with our previous findings.

Anxiety and stress can predict pain perception following a cognitive stress

Hoeger Bement, M.K., A. Weyer, M. Keller, A. Harkins, and S.K. Hunter. Anxiety and stress can predict pain perception following a cognitive stressor. PHYSIOL BEHAV 000-000. The purpose of this study was to investigate the influence of a cognitive stressor on pain perception and determine individual characteristics that may predict the pain response. Twenty-five subjects participated in three sessions: one familiarization and two experimental. The experimental sessions involved measurement of pain perception before and after 1) mental math tasks (stressor session) and 2) quiet rest (control session). Pain threshold and ratings were assessed with a mechanical noxious stimulus. Changes in stress and anxiety were examined with self-reported and physiological measures including questionnaires, visual analogue scales, and salivary cortisol levels. During the control session, stress and anxiety decreased and pain reports remain unchanged. During the stressor session, stress and anxiety increased and pain reports were variable among subjects. Based on the pain response to mental math, subjects were divided into three groups (increase, decrease or no change in pain). The increase-pain group (n=8) had lower baseline stress and anxiety, lower baseline pain reports, and large anxiety response following the mental math. In contrast, the decrease-pain group (n=9) had higher baseline stress and anxiety levels, higher baseline pain reports, and a large increase in cortisol levels. Thus, the differential response in the changes in pain perception was related to anxiety and stress levels prior to and during the cognitive stressor, indicating that psychosocial characteristics can help determine the stress-induced pain response.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Effect of long-term acetyl-L-carnitine on stress-induced analgesia in the aging rat

Cold water swim (CWS) analgesia in the rat is mediated by the hypothalamo-pituitary-adrenocortical (HPA) axis. An age-dependent increase of CWS-induced analgesia was observed in male Sprague-Dawley young (4 months), adult (15 months) and old (26 months) rats. Acetyl-L-Carnitine (ALCAR) chronically administered (75 mg/kg/daily in drinking water for 8 months) to old rats was able to maintain the stress-dependent response at the same levels as in adult rats. This effect may be explained by ALCAR capability of retarding the age-dependent loss of glucocorticoid receptors in the hippocampus, thus maintaining the glucocorticoid competence of this structure which exerts a negative feedback control over the HPA axis activity.

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.

Adaptation of body temperature and nociception to cold stress in preweanling rats

In order to investigate the response of rat pups to stress, 10-day-old rats (N = 10) were exposed to cold water (14 degrees C) for 5 min or to a control treatment (N = 10) for 5 consecutive days. On the sixth day when the rats were 15 days of age, all rat pups were immersed in cold water for 5 min. Body temperature and nociceptive threshold were recorded as functional indexes of the stress response to cold water. Results revealed that the rat pups that had been previously exposed to the cold stressor demonstrated a smaller decline in body temperature and a smaller increase in nociceptive threshold following cold-water immersion than rat pups which had no prior experience with the stressor. Thus, as measured by thermoregulatory and nociceptive function, 15-day-old rat pups are capable of adapting to repeated stressful experiences.

Chronic immobilization stress: evidence for decreases of 5-hydroxy-tryptamine immunoreactivity and for increases of glucocorticoid receptor immunoreactivity in various brain regions of the male rat

Male rats were exposed to severe 14 day immobilization stress. Body weight, body temperature, food and water intake, behavioral parameters, and serum corticosterone levels were measured during and after the stress period. On the 7th day after cessation of stress the experimental animals together with the control rats were taken to immunocytochemical analysis involving morphometry and microdensitometry of tyrosine hydroxylase (TH), 5-hydroxytryptamine (5-HT), various neuropeptide, and glucocorticoid receptor (GR) immunoreactivities (IRs) in a large number of regions of the central nervous system. In addition, adrenocorticotropic hormone (ACTH) IR was analyzed in the pituitary gland. Seven days following cessation of the chronic stress food intake, total locomotion and forward locomotion had been restored to normal. Serum corticosterone levels appeared to remain increased even 6 days following cessation of the chronic immobilization stress, probably caused by increased release of ACTH. Paraventricular corticotropin releasing hormone (CRF) IR was negatively correlated with the pituitary ACTH IR, indicating that the increase in ACTH release was produced by an increased release of CRF from the hypothalamus. The major immunocytochemical change observed 7 days after cessation of stress was a disappearance of 5-HT IR in the 5-HT cell groups B1, B2, B3, and B7. 5-HT IR in nerve terminals was only affected in the dorsal horn, where 5-HT IR was increased in the substantia gelatinosa. GR IR was found to be significantly increased in monoaminergic cell groups: serotoninergic B7, dopaminergic A12, and noradrenergic A1, A2, and A6. A trend for a reduction of TH IR was observed in nigral DA cells associated with significant reductions in TH IR in striatal DA nerve terminals. Finally, increases in 5-HT and substance P (SP) IR were found in the nerve terminals of the substantia gelatinosa of the cervical spinal cord in the stress group. In the present experimental model evidence has been obtained for a maintained activation of the hypothalamic-pituitary-adrenal axis as evaluated 7 days after cessation of severe chronic immobilization stress. The reduction of 5-HT IR in various 5-HT cell groups indicates a reduction of 5-HT synthesis, which may also be associated with reduced 5-HT release from the nerve terminals, since no depletion was observed in terminal regions and in one case an increase in 5-HT IR was noted (substantia gelatinosa).(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental aspects of nociception

Research has documented the existence of multiple, endogenous systems that modulate nociception. Based on the effects of opioid antagonists and endocrine lesions, endogenous analgesia systems have been organized into four classes: neural-opioid, neural-nonopioid; hormonal-opioid; hormonal-nonopioid. Developmental research on the ontogeny of endogenous analgesic function has revealed differential rates of maturation. Front-paw shock, a stimulus that activates a neural-opioid analgesic response, has been shown to be functionally mature by 28 days of age in the rat. Similarly, hind-paw shock, a stimulus that elicits a neural-nonopioid analgesic response, reaches maturity after two months of age. However, the hormonal-opioid analgesic system activated by cold-water immersion reaches adult levels by 10 days of age. Food deprivation produces a hormonal-opioid analgesic response in adult rats, and food deprivation/isolation of rat pups has been found to elicit an analgesic response in 6-day-old rats. From these data it seems that the rate of development of the different endogenous analgesic systems is related to the activation of neural or hormonal components. Whether the differential rates of development and the neural-hormonal distinction are related to the ecological validity of the activating stimulus remains to be determined.

Tolerance to tobacco smoke- and nicotine-induced analgesia in rats

Acute exposure of male Sprague-Dawley rats to either nicotine or tobacco smoke results in analgesia as measured by tail-flick latencies. A second treatment, 24 hr after the first, failed to produce analgesia, thereby demonstrating the rapid development of tolerance. The restraint which was a necessary part of the tobacco smoke exposure also produced analgesia, although of a more transient nature and lesser magnitude than that resulting from tobacco smoke exposure. Tolerance also developed to restraint stress-induced analgesia. The long-term (43 weeks) daily exposure of rats to tobacco smoke or restraint stress resulted in the development of cross-tolerance, suggesting that these two procedures share, at least in part, a common mechanism. Additionally, long-term tobacco smoke exposure resulted in an increased tail-flick latency when the animals had been withdrawn from tobacco smoke for 24 hr, suggesting the development of tolerance. The data also suggest a differential time course for the development of tolerance and dependence. This is the first report that addresses the effect of acute and chronic tobacco smoke exposure on pain sensitivity.

Potentiation of opioid and nonopioid forms of swim analgesia by cimetidine

Antagonism of the H-2 receptor with cimetidine and other histaminergic receptor antagonists has been used to differentiate nonopioid and opioid forms of footshock analgesia which are mediated by neural mechanisms. Cimetidine reduces nonopioid footshock analgesia while potentiating an opioid form of this analgesia. The present study examined whether cimetidine altered the nonopioid, neurohormonal analgesia induced by either continuous cold-water swims (CCWS: 2 degrees C for 3.5 min) or the opioid analgesia induced by intermittent cold-water swims (ICWS: 2 degrees C, 18 10-sec swims, 18 10-sec recovery periods). Vehicle or cimetidine (10, 50, 100 mg/kg) injections were administered alone or paired with either CCWS or ICWS; tail-flick latencies, jump thresholds and core body temperatures were then measured. Cimetidine (100 mg/kg) significantly potentiated CCWS and ICWS analgesia and hypothermia, while having minimal effects upon basal thresholds. Lower cimetidine doses produced transitory effects on these measures. These data demonstrate dissociations between neural and neurohormonal forms of nonopioid analgesia following cimetidine treatment. The latter effect may be attributed to changes in stress responsiveness or thermoregulation rather than pain inhibition.

Ontogeny of an endogenous, nonopioid and hormonally mediated analgesic system

Rats of different ages (10-day, 28-day, and 3-month-old) were exposed to cold-water stress in order to activate an endogenous analgesic system. The effects of naltrexone (7 mg/kg) and dexamethasone (.4 mg/kg) were also studied to examine the role of the opioid and hormonal systems in cold-water-induced analgesia. Following cold-water exposure, nociception was measured with the tail-flick procedure for 2 hr. Results revealed that cold water produced a significant level of analgesia in the 10-day, 28-day, and 3-month-old age groups with no differences between age groups. In addition, in each age group naltrexone did not block the analgesia while dexamethasone attenuated the analgesia produced by cold water. The effects of naltrexone and dexamethasone confirm that cold-water immersion activates a nonopioid, hormonally mediated analgesic system in each age group. Thus, this experiment found that the endogenous, nonopioid, and hormonally mediated analgesic system activated by cold water is functional early in the development of the rat. The early development of this hormonally mediated analgesic system is in contrast to the slower development of endogenous analgesia systems that are mediated by the central nervous system.

Neuromodulatory effects of TRH upon swim and cholinergic analgesia

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

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.

Age-related decrements in the analgesic response to cold-water swims

Among the compromised physiological responses affected by aging is an impaired ability to thermoregulate in a cold environment. Since acute exposure to cold-water swims (CWS) produces both analgesic and hypothermic responses in young adult rats, the present study examined whether systematic variations in these responses occurred as a function of age. Separate cohorts of 4, 9, 14, 19 and 24-month old female rats received a no-swim condition and a 2 degrees C swim for 3.5 min with tail-flick latencies, jump thresholds and core body temperature assessed 30, 60 and 90 min later. The order of conditions was counterbalanced with an interval of four days between conditions. While the four younger cohorts displayed similar CWS analgesia on the tail-flick test, the 24-month cohort failed to display CWS analgesia on this measure. Age-related differences in CWS analgesia on the jump test appeared more gradual with the three older cohorts displaying significant attenuations relative to the 9-month group, and the oldest cohort displaying significant attenuations relative to the 4-month group. In contrast, the hypothermic effect of CWS was significantly potentiated in the three oldest cohorts, with a progressive inability to cope with thermoregulation observed as a function of age. The observed decrements in the analgesic response to CWS as a function of age appear to represent a change in (a) the ability of the animal to perceive the CWS stimulus as stressful (b) an endogenous pain-inhibitory system and/or (c) an endogenous pain transmission system that is independent of thermoregulatory mechanisms.

Naloxone injections into the periaqueductal grey area and arcuate nucleus block analgesia in defeated mice

In a situation of social conflict, mice that are defeated by an opponent exhibit a marked analgesia. Microinjections of naloxone (1 or 10 micrograms) into the periaqueductal grey area (PAG) or into the region of the arcuate nucleus prior to the defeat prevented the emergence of analgesia. Microinjections of morphine (5 micrograms) into these sites had previously been shown to produce profound analgesia. Mice whose adrenals were removed rapidly developed analgesia when attacked by a stimulus animal. Injection of naloxone into PAG also antagonized defeat-induced analgesia in adrenalectomized mice. These observations indicate that sites and processes in the brain rather than in the periphery are responsible for the development of analgesia in mice that are subjected to social defeat.

Stressors produce a concurrent decrease and increase in reflex amplitude in rats treated with naloxone

The effect of inescapable foot-shock on the tail-flick response and on the startle response to brief shocks and brief tones was studied in rats. In the first experiment, 25 minutes of inescapable foot shock (stressor) produced a significant increase in tail-flick latency which was antagonized by the opioid antagonist naloxone (2.0 mg/kg). In the second experiment, the startle response to an electric shock to the tail was significantly diminished by the stressor, and this effect was not significantly reduced by naloxone. However, the size of the startle response to a brief tone was significantly increased in rats treated with naloxone. Thus, rats injected with naloxone had a decreased startle to shock but an increased startle to tone following inescapable foot shock. Finally, tones which preceded shocks by one second produced a facilitation of the startle response to the shocks in tests that followed exposure to the stressor. This facilitation was not affected significantly by naloxone. These results indicate that the changes in the startle response following the stressor were not mediated exclusively by endogenous opioids.

Effects of naltrexone on plasma corticosterone in opiate-naive rats: a central action

We have previously reported the elevation of plasma corticosterone by i.v. naloxone HCl (NX). This work has been extended, with the current study, showing a similar effect with naltrexone HCl (NTX) and that this effect is due to a central action of the drug. Using opiate-naive male rats with chronic i.v. catheters, stereotaxically placed intracerebroventricular (ICV) cannula guides where necessary, and sound-attenuated one-way vision boxes, serial blood samples were obtained from conscious unrestrained animals. NTX (5.0, 10.0, or 20.0 mg/kg i.v.) resulted in a significant increase in plasma corticosterone 15 min following injection. I.V. administration of the methylbromide salts of each drug, which do not cross the blood-brain barrier, did not produce this same elevation in hormone level at several doses (0.4, 1.0, 2.0, 10.0 or 20.0 mg/kg). In contrast, ICV injection of either of the quaternary salts (50 micrograms/10 microliter/animal) resulted in an immediate and sustained rise in plasma corticosterone. Results obtained demonstrate that NTX has a similar effect on plasma corticosterone as NX at the appropriate doses and that the effect is a central rather than a peripheral one.