Stimulation-produced and stress-induced analgesia: cross-tolerance between opioid forms

Alternative pain management via endocannabinoids in the time of the opioid epidemic: Peripheral neuromodulation and pharmacological interventions

The use of opioids in pain management is hampered by the emergence of analgesic tolerance, which leads to increased dosing and side effects, both of which have contributed to the opioid epidemic. One promising potential approach to limit opioid analgesic tolerance is activating the endocannabinoid system in the CNS, via activation of CB1 receptors in the descending pain inhibitory pathway. In this review, we first discuss preclinical and clinical evidence revealing the potential of pharmacological activation of CB1 receptors in modulating opioid tolerance, including activation by phytocannabinoids, synthetic CB1 receptor agonists, endocannabinoid degradation enzyme inhibitors, and recently discovered positive allosteric modulators of CB1 receptors. On the other hand, as non-pharmacological pain relief is advocated by the US-NIH to combat the opioid epidemic, we also discuss contributions of peripheral neuromodulation, involving the electrostimulation of peripheral nerves, in addressing chronic pain and opioid tolerance. The involvement of supraspinal endocannabinoid systems in peripheral neuromodulation-induced analgesia is also discussed. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

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.

Acute resistance exercise induces antinociception by activation of the endocannabinoid system in rats

BACKGROUND

Resistance exercise (RE) is also known as strength training, and it is performed to increase the strength and mass of muscles, bone strength, and metabolism. RE has been increasingly prescribed for pain relief. However, the endogenous mechanisms underlying this antinociceptive effect are still largely unexplored. Thus, we investigated the involvement of the endocannabinoid system in RE-induced antinociception.

METHODS

Male Wistar rats were submitted to acute RE in a weight-lifting model. The nociceptive threshold was measured by a mechanical nociceptive test (paw pressure) before and after exercise. To investigate the involvement of cannabinoid receptors and endocannabinoids in RE-induced antinociception, cannabinoid receptor inverse agonists, endocannabinoid metabolizing enzyme inhibitors, and an anandamide reuptake inhibitor were injected before RE. After RE, CB1 cannabinoid receptors were quantified in rat brain tissue by Western blot and immunofluorescence. In addition, endocannabinoid plasma levels were measured by isotope dilution-liquid chromatography mass spectrometry.

RESULTS

RE-induced antinociception was prevented by preinjection with CB1 and CB2 cannabinoid receptor inverse agonists. By contrast, preadministration of metabolizing enzyme inhibitors and the anandamide reuptake inhibitor prolonged and enhanced this effect. RE also produced an increase in the expression and activation of CB1 cannabinoid receptors in rat brain tissue and in the dorsolateral and ventrolateral periaqueductal regions and an increase in endocannabinoid plasma levels.

CONCLUSIONS

The present study suggests that a single session of RE activates the endocannabinoid system to induce antinociception.

Characterisation of mGluRs which modulate nociception in the PAG of the mouse

The contribution of metabotropic glutamate receptors (mGluRs) to the modulation of nociception by the periaqueductal gray (PAG) matter was investigated in mice. Intra-PAG microinjection of (IS,3R)-ACPD, an agonist of groups I and II mGluRs, as well as (S)-3,5-DHPG, a selective agonist of group I mGluRs, increased the latency of the nociceptive reaction (NR) in the hot plate test. (RS)-AIDA, an antagonist of group I mGluRs, antagonized the effect of (S)-3,5-DHPG, but changed the effect induced by (1S,3R)-ACPD in that a decrease in the latency for the NR could now be observed. L-CCG-I and L-SOP, which are agonists of groups II and III mGluRs respectively, decreased the latency of the NR. (2S)-alpha-EGlu and (RS)-alpha-MSOP, which are antagonists of groups II and III mGluRs, respectively, antagonized the effect of L-CCG-I and L-SOP. (RS)-AIDA and (RS)-alpha-MSOP alone decreased and increased, respectively, the latency of the NR with the highest doses used. (2S)-alpha-EGlu alone did not change significantly the latency of the NR. Intra-PAG microinjection of LH, an agonist of ionotropic glutamate receptors, induced a dose-dependent analgesia which was blocked by pretreatment with DL-AP5, a selective antagonist of NMDA receptors. No mGluRs antagonists were able to prevent LH-induced analgesia. These results emphasize the possible involvement of mGluRs in the modulation of nociception. It seems that activation of group I mGluRs potentiates, while groups II and III mGluRs decrease, the activity of the PAG for the modulation of nociception.

Social deprivation stress induces adaptative changes of opioid mechanisms in the rat tail artery

Brief (7-14 days) social deprivation stress has been found to increase blood pressure in Wistar rats, an effect dependent on activation of opioid function. The role of central opioids in this and other responses to stress has been repeatedly determined, but the possible involvement of modifications of peripheral opioid mechanisms is poorly understood. To further increase this knowledge, we have examined the opioid sensitivity of tail arteries taken from social deprived Wistar rats by studying the effect of beta-endorphin and DADLE "in vitro". Both opioids inhibited the electrically-induced constriction of the preparations in a dose-dependent manner, but these actions were significantly attenuated after 7-14 days of social deprivation. When the rats were isolated for 30-35 days, the hypertensive response was still present but the arteries from group-housed and isolated animals no longer showed differential sensitivity to opioids. This difference with respect to 7-14 days of isolation could be related to age-dependent changes of opioid function, which were observed among group-housed animals. The results suggest that social deprivation stress induces an adaptation of the tail arteries to the opioid effects on contractility. It is suggested that this endogenous adaptation could be contributing to the hypertensive response observed after social deprivation.

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

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

Oligogenic determination of morphine analgesic magnitude: a genetic analysis of selectively bred mouse lines

Two ongoing selective breeding projects have produced mice that display divergent analgesic responses to morphine. These two projects have selected for similar phenotypes: high and low levorphanol analgesia (HAR/LAR lines; Portland, OR) and high and low swim stress-induced analgesia (HA/LA lines; Jastrzebiec, Poland). Evidence suggests genetic commonalities between mice of the two projects. Using a Mendelian breeding protocol, we have recently found that one or two genetic loci predominantly determine the high morphine analgesia exhibited by HA mice. In the present study we demonstrate that the differential morphine analgesia (5 mg/kg, i.p.) displayed by HAR and LAR mice is similarly oligogenic, predominantly determined by two unlinked loci. A complementation analysis, in which the analgesic responses to morphine of the recessive homozygotes of each project (HAR and HA) were compared to those of their hybrid offspring (HAR x HA), revealed that different genetic loci have been fixed in each project. An intriguing bimodal distribution was observed in the HAR x HA population: Some HAR x HA hybrids displayed greater morphine analgesia than either HAR or HA mice, whereas others displayed minimal analgesia. LAR x LA hybrids displayed less analgesia than either LAR or LA mice. The analgesic responses of HAR x LA and LAR x HA mice were comparable to those of their low-line parents. These findings indicate not only that different loci were responsible for producing high morphine responders in each selection project but that these distinct loci can interact synergistically to produce "superhigh" and "superlow" responders.

Descending modulation of central neural plasticity in the formalin pain test

Subcutaneous injection of formalin produces a biphasic profile of pain response: a transient early phase followed by a tonic late phase. A number of studies have indicated that the development of the late phase of formalin pain is dependent upon prolonged changes in central neural function produced by neural activity that is generated during the early phase (i.e. central sensitization). In support of this, the present demonstrates that stimulation- or morphine-produced analgesia derived from the periaqueductal grey (PAG) during the early phase prevents the development of the phase. These results suggest that descending mechanisms of pain inhibition, as reflected by PAG stimulation- and morphine-produced analgesia, can prevent the development of central neural plasticity following injury.

Antinociceptive and hypothermic crosstolerance between continuous and intermittent cold-water swims in rats

Antinociceptive responses induced by continuous (CCWS: 2 degrees C, 3 min) and intermittent (ICWS: 2 degrees C, 18 10-s swims, 18 10-s recoveries) cold-water swims differ in their sensitivity to opioid antagonists and crosstolerance with morphine. The present study examined whether CCWS and ICWS antinociception and hypothermia displayed crosstolerance in rats. Jump thresholds were significantly increased following acute exposure to CCWS (30 min) and ICWS (30-60 min). CCWS antinociception displayed tolerance (90% reduction) to CCWS after 14 days and crosstolerance (100% reduction) to ICWS on the fifteenth day. ICWS antinociception displayed tolerance (74% reduction) to ICWS and crosstolerance (81% reduction) to CCWS. Core body temperatures were significantly decreased following acute exposure to CCWS (30 min) and ICWS (30-90 min). Although CCWS and ICWS hypothermia displayed tolerance to the same stressor and crosstolerance to the other stressor, the changes in the antinociceptive and hypothermic effects failed to correlated significantly with each other.

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.

Periaqueductal gray matter stimulation-produced analgesia in diabetic rats

The effect of diabetes on periaqueductal gray matter (PAG) stimulation-produced analgesia (SPA) was examined in rats. PAG SPA was assessed using the tail-pinch test. PAG stimulation produced marked analgesia in both naive and diabetic rats. Furthermore, the degree of PAG SPA did not differ between naive and diabetic rats. PAG SPA was significantly attenuated by a low dose (0.5 mg/kg, s.c.) of naloxone in naive rats, but not in diabetic rats. However, a high dose (5 mg/kg, s.c.) of naloxone significantly and equally attenuated PAG SPA in both naive and diabetic rats. On the other hand, the analgesic potency of morphine (3 mg/kg, s.c.) was significantly reduced in diabetic rats as compared with naive rats. These results suggest that PAG SPA in diabetic rats may be mediated by different opioid receptor interactions as compared with naive rats.

NMDA receptor antagonist MK-801 blocks non-opioid stress-induced analgesia in the formalin test

The analgesic effect of a 3-min swim stress was assessed using the formalin test. Male Swiss mice were injected i.p. with naloxone (0.1 or 1.0 mg/kg), MK-801 (0.075 mg/kg) or saline 15 min prior to swimming in water maintained at 20 degrees C or 32 degrees C. The mice were then injected with 20 microliters of 5% formalin into the plantar surface of 1 hind paw and pain behaviour (time spent licking the injected paw) was continuously monitored during the subsequent 10 min. Swim stress produced a significant reduction in pain behaviour at both 20 degrees C and 32 degrees C. MK-801 completely blocked the analgesia produced by both the 20 degrees C and 32 degrees C swim. At a dose of 0.1 mg/kg, naloxone partially antagonized the analgesia produced by the 32 degrees C swim but did not affect the analgesia produced by the 20 degrees C swim. Naloxone at a dose of 1.0 mg/kg had no effect on swim stress-induced analgesia. Neither MK-801 nor 0.1 mg/kg naloxone altered baseline pain behaviour, although 1.0 mg/kg naloxone did significantly reduce it. It is unlikely that the effect of MK-801 on swim stress-induced analgesia is due to an interaction with an opioid mechanism, as MK-801 had no effect on morphine analgesia. These results suggest that the analgesia produced by the 20 degrees C swim stress in the formalin test is non-opioid in nature and mediated via the NMDA receptor, whereas the 32 degrees C swim stress-induced analgesia has both an opioid and non-opioid component.

Stress-induced analgesia prevents the development of the tonic, late phase of pain produced by subcutaneous formalin

Subcutaneous injection of formalin produces a biphasic pain response: a transient early phase followed by a tonic late phase. It has recently been suggested that development of the late phase depends upon the presence of the early one. In support of this suggestion, we now demonstrate that blocking the early phase by stress-induced analgesia prevents development of the late phase, whereas the same stressor given after the first phase does not. Both phases are manifested when stress-induced analgesia is blocked by the N-methyl-D-aspartate (NMDA) or opiate antagonists, MK-801 and naloxone.

Stimulation of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.

Stimulation-produced analgesia in the mouse: evidence for laterality of opioid mediation

The effect of naloxone on periaqueductal gray matter (PAG) stimulation-produced analgesia (SPA) was examined in pentobarbital anesthetized Swiss-Webster mice. Electrodes were placed either in dorsolateral or ventrolateral PAG, and SPA threshold was assessed using the hind paw-flick test (paw withdrawal from radiant heat). SPA threshold did not differ between dorsal and ventral PAG, and naloxone equally attenuated SPA from both areas. SPA threshold for the paw contralateral to the stimulation site was half that for the ipsilateral paw. Elevation of SPA threshold by naloxone was greater for the contralateral than ipsilateral paw. Exposure to analgesic neck pinch prior to SPA almost completely abolished the antinociceptive effect of contralateral PAG stimulation without affecting SPA on the ipsilateral paw. This effect of pinch was itself reversed by prior naloxone administration. We suggest that the substrate of opioid mediated SPA from PAG in the mouse has principally a contralateral organization.

Genetic influences on brain stimulation-produced analgesia in mice: II. Correlation with brain opiate receptor concentration

The analgesic effect of electrical stimulation of the periaqueductal gray matter (PAG) was studied in 4 strains of mice: C57BL/6By (C57), BALB/cBy (BALB), CXBH, and CXBK. These strains are known to have high (CXBH), low (CXBK), and intermediate (C57 and BALB) concentrations of brain opiate receptors. The current intensity required for stimulation-produced analgesia (SPA) did not differ among strains. Naloxone attenuated SPA in CXBH, C57 and BALB mice, but was ineffective in the opiate receptor deficient CXBK mice. The results suggest that genetic differences in opiate receptor density can influence the degree to which opioid mechanisms are involved in SPA from the PAG.

Genetic influences on brain stimulation-produced analgesia in mice. I. Correlation with stress-induced analgesia

The analgesic effect of electrical stimulation of the periaqueductal gray matter (PAG) was studied in mice selectively bred for high and low stress-induced analgesia (HA and LA lines, respectively). The current intensity required for stimulation-produced analgesia (SPA) in LA mice was 5 times that for HA mice. Naloxone produced a 4-fold increase of SPA threshold in HA mice, but was ineffective in LA mice. These findings suggest that the differential responsiveness of these two lines to the analgesic effect of stress reflects a more general genetic modification of the efficacy and mechanism of their pain-inhibitory systems.

The dorsal raphe nucleus: a re-evaluation of its proposed role in opiate analgesia systems

Previous studies have concluded that (a) electrical stimulation in the periaqueductal gray/dorsal raphe nucleus (PAG/DRN) region specifically produces either non-opiate or opiate forms of antinociception dependent upon the dorsoventral level of stimulation and (b) the 'opiate' form of stimulation-produced analgesia (SPA) arising from the ventral PAG/DRN region shows cross-tolerance with opiate forms of footshock analgesia, implying common neural substrates. This latter conclusion in turn implies that SPA elicited from the ventral PAG/DRN region would be expected to be antagonized by scopolamine, since this muscarinic cholinergic antagonist blocks opiate footshock analgesia. The present study demonstrates instead that neither 10 mg/kg naloxone nor 10 mg/kg scopolamine had any effect on SPA elicited from sites histologically verified to lie within the presumptive 'opiate' ventral PAG/DRN region. These data bring into question both the site specificity of opiate SPA and the common mediation of ventral PAG/DRN SPA and opiate forms of footshock analgesia.

Site specificity in the development of tolerance to stimulation-produced analgesia from the periaqueductal gray matter of the rat

Pentobarbital-anesthetized rats were subjected to 21 min of continuous electrical stimulation of the caudal periaqueductal gray matter (PAG) at the current threshold for analgesia. Stimulation at ventral PAG sites supported analgesia for only 1 or 2 min in most animals. Stimulation at more dorsal PAG sites supported analgesia for the entire 21 min of stimulation. This demonstration of acute tolerance with continuous ventral, but not more dorsal, PAG stimulation corresponds well with previous evidence suggesting opioid mediation of analgesia from this brain region.

Effects of morphine on two types of nucleus raphe dorsalis neurons in awake cats

Forty-nine neurons recorded within the nucleus raphe dorsalis (NRD) in awake cats were classified into 2 groups: 29 regularly firing (clock-like) and 20 irregularly firing (non-clock-like) neurons. Hardly any of the clock-like neurons were influenced either by noxious stimulation (0.1 ml of 5% formalin, s.c.) or by a single dose (1 mg/kg, i.p.) or cumulative doses (0.25, 0.5, 1 mg/kg) of morphine. In contrast, about half the non-clock-like neurons were activated both by noxious stimulation and by administration of morphine. Morphine-induced activation of non-clock-like neurons was dose-related and reversed by naloxone (0.2 mg/kg, i.p.). These findings suggest that clock-like neurons in the NRD are not involved in morphine analgesia. Non-clock-like neurons, however, may play a role in the mediation of such analgesia.

Effects of centrifugal rotation on analgesia and conditioned flavor aversions

In the first experiment, 48 female Wistar rats were water deprived and given three conditioning days with saccharin-flavored water (1.5 g/liter) followed by 0, 5, 10 or 15 min of centrifugal rotation (150 rpm). Analgesia was measured by the tail flick test immediately after rotation. Over conditioning days, taste aversions developed. In general, taste aversion strength increased with duration of rotation. Analgesia also was in proportion to duration of rotation; however, over days tolerance developed in all rotated groups. In the second experiment 12 female Wistar rats were water deprived and given naloxone (20 mg/kg) or saline prior to 15 min of rotation. Rotation-induced analgesia was not affected by naloxone. It was concluded that somatic and gastrointestinal reactions to rotation are not served by a single mechanism.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.

Two opioid forms of stress analgesia: studies of tolerance and cross-tolerance

We have previously reported that stress analgesia sensitive to and insensitive to opiate antagonists can be differentially produced in rats by varying the severity or temporal pattern of inescapable footshock. In these studies, we give further evidence for the opioid and non-opioid bases of these paradigms of stress analgesia. We find that naloxone-sensitive analgesia demonstrates tolerance with repeated stress and cross-tolerance with morphine, whereas naloxone-insensitive analgesia demonstrates neither of these characteristics. Moreover, different forms of opioid, but not non-opioid, stress analgesia manifest cross-tolerance with each other. These data are discussed in terms of the similarities and differences between two forms of opioid stress analgesia.