Potentiation of cold-water swim analgesia by acute, but not chronic desipramine administration

Mechanism of exercise-induced analgesia: what we can learn from physically active animals

Physical activity has become a first-line treatment in rehabilitation settings for individuals with chronic pain. However, research has only recently begun to elucidate the mechanisms of exercise-induced analgesia. Through the study of animal models, exercise has been shown to induce changes in the brain, spinal cord, immune system, and at the site of injury to prevent and reduce pain. Animal models have also explored beneficial effects of exercise through different modes of exercise including running, swimming, and resistance training. This review will discuss the central and peripheral mechanisms of exercise-induced analgesia through different modes, intensity, and duration of exercise as well as clinical applications of exercise with suggestions for future research directions.

Endogenous analgesic action of the pontospinal noradrenergic system spatially restricts and temporally delays the progression of neuropathic pain following tibial nerve injury

Pontospinal noradrenergic neurons form part of an endogenous analgesic system that suppresses acute pain, but there is conflicting evidence about its role in neuropathic pain. We investigated the chronology of descending noradrenergic control during the development of a neuropathic pain phenotype in rats following tibial nerve transection (TNT). A lumbar intrathecal cannula was implanted at the time of nerve injury allowing administration of selective α-adrenoceptor (α-AR) antagonists to sequentially assay their effects upon the expression of allodynia and hyperalgesia. Following TNT animals progressively developed mechanical and cold allodynia (by day 10) and subsequently heat hypersensitivity (day 17). Blockade of α₂-AR with intrathecal yohimbine (30 μg) revealed earlier ipsilateral sensitization of all modalities while prazosin (30 μg, α₁-AR) was without effect. Established allodynia (by day 21) was partly reversed by the re-uptake inhibitor reboxetine (5 μg, i.t.) but yohimbine no longer had any sensitising effect. This loss of effect coincided with a reduction in the descending noradrenergic innervation of the ipsilateral lumbar dorsal horn. Yohimbine reversibly unmasked contralateral hindlimb allodynia and hyperalgesia of all modalities and increased dorsal horn c-fos expression to an innocuous brush stimulus. Contralateral thermal hyperalgesia was also reversibly uncovered by yohimbine administration in a contact heat ramp paradigm in anaesthetised TNT rats. Following TNT there is an engagement of inhibitory α₂-AR-mediated noradrenergic tone which completely masks contralateral and transiently suppresses the development of ipsilateral sensitization. This endogenous analgesic system plays a key role in shaping the spatial and temporal expression of the neuropathic pain phenotype after nerve injury.

Retrograde viral vector-mediated inhibition of pontospinal noradrenergic neurons causes hyperalgesia in rats

Pontospinal noradrenergic neurons form a component of an endogenous analgesic system and represent a potential therapeutic target. We tested the principle that genetic manipulation of their excitability can alter nociception using an adenoviral vector (AVV-PRS-hKir(2.1)) containing a catecholaminergic-selective promoter (PRS) to retrogradely transduce and inhibit the noradrenergic neurons projecting to the lumbar dorsal horn through the expression of a potassium channel (hKir(2.1)). Expression of hKir(2.1) in catecholaminergic PC12 cells hyperpolarized the membrane potential and produced a barium-sensitive inward rectification. LC neurons transduced by AVV-PRS-hKir(2.1) in slice cultures also showed barium-sensitive inward rectification and reduced spontaneous firing rate (median 0.2 Hz; n = 19 vs control 1.0 Hz; n = 18, p < 0.05). Pontospinal noradrenergic neurons were retrogradely transduced in vivo by injection of AVV into the lumbar dorsal horn (L4-5). Rats transduced with AVV-PRS-hKir(2.1) showed thermal but not mechanical hyperalgesia. Similar selective augmentation of thermal hyperalgesia was seen in the CFA-inflammatory pain model after AVV-PRS-hKir(2.1). In the formalin test, rats transduced with hKir(2.1) showed enhanced nocifensive behaviors (both Phase I and II, p < 0.05, n = 11/group) and increased c-Fos-positive cells in the lumbar dorsal horn. Transduction with AVV-PRS-hKir(2.1) before spared nerve injury produced no change in tactile or cold allodynia. Thus, the selective genetic inhibition of approximately 150 pontospinal noradrenergic neurons produces a modality-specific thermal hyperalgesia, increased nocifensive behaviors, and spinal c-Fos expression in the formalin test, but not in the spared nerve injury model of neuropathic pain, indicating that these neurons exert a selective tonic restraining influence on in vivo nociception.

Changes in anxiety levels are followed by changes in behavioral strategy in mice subjected to stress and in the extent of stress-induced analgesia

The experiments reported here demonstrated that corasol increased the extent of analgesia induced by stress and decreased the duration of immobility in mice in a forced swimming test in cold water. Administration of diazepam led to the opposite changes and counteracted the actions of the anxiogen. The effects of the anxiolytic were more apparent in NMRI than mongrel mice, while in mongrel mice the effects of the anxiogen were more marked. Changes in measures following administration of agents were reciprocal in nature. These results lead to the conclusion that that these changes are determined by the level of anxiety, and that the strain differences between mongrel and NMRI mice are also linked with this factor.

Evidence-based data from animal and human experimental studies on pain relief with antidepressants: a structured review

OBJECTIVE

It has been hypothesized that serotonin reuptake inhibitor antidepressants (ADs) are only weakly antinociceptive but augment noradrenergic (NA) antinociception. Thus, ADs with combined serotonergic (SN) and NA activity, (i.e., the serotonergic/noradrenergic (SN/NA) ADs) should have greater antinociceptive activity versus the NA ADs, which in turn should have more antinociceptive activity than the SN ADs. The objective of this structured review was to test this hypothesis by reviewing relevant basic science literature on the treatment of experimental pain with the above different types of ADs. DESIGN, SETTING, PARTICIPANTS, OUTCOME, MEASURES: Animal or human experimental AD pain treatment studies were located by the usual search methods. For animal studies only placebo-controlled studies were included for review. For human studies only double blind placebo-controlled studies were selected for review. The animal and human studies were then sorted according to the pain model represented, e.g., neuropathic pain model. Studies were then characterized according to the type of AD utilized, and the antinociceptive outcome of the AD trial.

RESULTS

Twenty-two animal studies and 5 human studies fulfilled the inclusion criteria of this structured review. Within the animal nonspecific pain model there were 10 SN/NA AD trials, 9 NA AD trials and 7 SN AD trials. Of these trials 100%, 88.9%, and 14.3% respectfully demonstrated a positive AD antinociceptive effect. Overall, for all the animal models there were 25 SN/NA, 9 NA, and 8 SN trials. Of these trials 92%, 88.9%, and 25% respectfully demonstrated a positive AD antinociceptive effect. For the human pain models, only the SN/NA ADs had been utilized in 7 trials. Here in 42.8% of the trials there was a reported antinociceptive effect.

CONCLUSIONS

Overall, the results of this structured review support the above hypothesis.

Clonidine and yohimbine modulate the effects of naloxone on novelty-induced hypoalgesia

Previous research has shown that repeated daily pretreatment with the opiate receptor blocker naloxone retards the development of habituation to novelty-induced hypoalgesia. The present experiments were conducted in order to determine whether noradrenergic substrates mediates this effect. Animals in the NAL condition were administered 10 mg/kg naloxone prior to assessment of pain sensitivity on a 48.5 degrees C hot plate. Control animals (SAL condition) were administered saline prior to pain assessment, and naloxone 2-4 h later. Paw lick latencies declined over repeated tests in SAL animals, suggesting the habituation of novelty hypoalgesia. Naloxone pretreatment attenuated this decline. The longer paw lick latencies observed in NAL condition animals were reduced by administration of 2 microgram/kg clonidine, a specific noradrenergic alpha-2 receptor agonist, and enhanced in a dose dependent (0.5-4.0 mg/kg) fashion by the alpha-2 antagonist yohimbine. Clonidine and yohimbine either failed to alter pain reactivity in control animals, or produced less marked effects than those observed in naloxone-exposed animals. These results suggest that noradrenergic substrates mediate naloxone's effects on novelty hypoalgesia.

Analgesia induced by continuous versus intermittent cold water swim in the rat: differential effects of intrathecal administration of phentolamine and methysergide

Continuous cold water swim produces analgesia that is partially mediated by a noradrenergic mechanism, but is independent of both serotonergic and opioid systems. On the other hand, intermittent cold water swim elicits analgesia which is partly mediated by an opioid mechanism; the contribution of the monoamines to the production of this analgesia is not known. Therefore, the present study was done to determine whether intermittent cold water swim is also mediated by noradrenergic and/or serotonergic substrates. Prior to either continuous (3.5 min) or intermittent (10 sec in, 10 sec out for 6 min) cold water (4 degrees C) swim, male Sprague-Dawley rats (225-250 g) were administered either the noradrenergic receptor blocker phentolamine (30 micrograms), the serotonergic blocker methysergide (30 micrograms) or artificial cerebrospinal fluid to the fifth lumbar vertebral spinal level via chronic intrathecal catheters. Phentolamine significantly attenuated the analgesia resulting from both continuous and intermittent cold water swim. Methysergide attenuated intermittent cold water swim analgesia, but was without effect on continuous cold water swim analgesia. Phentolamine, but not methysergide, also attenuated continuous footshock- (2.5 mA for 3 min) induced analgesia. The similarity between the effects of phentolamine and methysergide on continuous footshock and continuous cold water swim analgesia suggests that the effects of these drugs on cold water swim analgesia are not attributable to changes in thermoregulation. These results suggest that a spinal noradrenergic mechanism is involved in the mediation of both forms of cold water swim analgesia, whereas a spinal serotonergic mechanism is involved in only intermittent cold water swim analgesia.

Yohimbine potentiates cold-water swim analgesia: re-evaluation of a noradrenergic role

Continuous cold-water swims (CCWS) elicit a nonopioid and neurohormonal analgesia which displays adaptation. The norepinephrine (NE) system has been implicated since parallel alterations in NE occur following acute and repeated CCWS exposure, and since CCWS analgesia is reduced by locus coeruleus lesions and is potentiated by clonidine and desipramine. The present study evaluated the effects of the alpha-2 NE receptor antagonist, yohimbine upon CCWS (2 degrees C for 3.5 min) analgesia on the jump and tail-flick tests, CCWS hypothermia, and basal nociceptive and thermoregulatory measures in rats. Yohimbine (0.1-2.0 mg/kg, IP) dose-dependently increased basal jump thresholds and potentiated CCWS analgesia: these effects appeared to be additive. Yohimbine potentiated CCWS analgesia on the tail-flick test without altering basal latencies. Yohimbine failed to alter either CCWS hypothermia or basal thermoregulation. Since yohimbine and clonidine, an alpha-2 NE receptor antagonist and agonist respectively, similarly potentiate CCWS analgesia, it appears that NE effects are orthoganol to the intrinsic system mediating CCWS.

Modulation of gender-specific effects upon swim analgesia in gonadectomized rats

Gender-specific effects have been observed for continuous and intermittent cold-water swim (CCWS and ICWS respectively) analgesia: analgesic magnitudes following CCWS and ICWS are significantly smaller in female rats than in age-matched and weight-matched male rats. The present study evaluated the role of gonadal status in these gender-specific effects by examining CCWS and ICWS analgesia, hypothermia and activity in intact and gonadectomized rats. Following confirmation of the original gender-specific effects on the tail-flick and jump tests, it was found that both castration and ovariectomy significantly reduced CCWS and ICWS analgesia. Indeed, castrated males displayed similar magnitudes of analgesia as intact females. The more marked hypothermia observed in intact females indicated that this variable failed to account for the analgesic gender-specific effects. The reduced hypothermia following gonadectomy also failed to account for the analgesic changes. The increased activity during ICWS, but not CCWS following gonadectomy also did not account for the analgesic changes. These data suggest that gonadal steroids normally appear to facilitate these stress-related analgesic responses.

Reduction in cold-water swim analgesia following hypothalamic paraventricular nucleus lesions

The analgesic response following cold-water swims (CWS) has been shown to be mediated through neurohormonal mechanisms and independently of opioid systems. The hypothalamic paraventricular nucleus (PVN) appears to be important in autonomic, hypophysial and medial-basal hypothalamic function. The present study examined whether lesions placed in the PVN in rats would alter CWS analgesia on the tail-flick test. Animals with lesions placed in the PVN displayed significant reductions in analgesic magnitude 30 (66%) and 60 (54%) min following CWS relative to sham-treated rats without alterations in baseline latencies. In contrast, CWS analgesia was not altered in animals with lesions placed dorsal and/or lateral to the PVN. These data are discussed in terms of the roles of PVN projections to the median eminence and brainstem/spinal structures as well as roles for neuropeptides in the PVN.

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.