Naltrexone and the tail flick reflex

Rats who received a tail flick test following repeated injections of 5 mg/kg of morphine were significantly more tolerant than rats who were not tested after each injection. Administration of 0.5 mg/kg of naltrexone hydrochloride, prior to the morphine injections, prevented the development of tolerance, increased analgesic latencies and abolished the difference between tested and nontested animals.     

The effect of local and systemic application of dopaminergic agents on tail flick latency in the rat

Dopamine (DA) is thought to have a neurotransmitter role in the spinal cord of the rat. Intrathecal administration of the DA receptor agonist apomorphine has previously been shown to reduce nocifensive responses. The present experiments investigated the site of action of apomorphine, and the mechanisms by which DA agonists apparently produce antinociception. Small doses of apomorphine (40-80 micrograms/kg) increased the tail flick latency (TFL) in lightly anaesthetised rats when given intrathecally and intravenously but not intracerebroventricularly. This effect is probably mediated via D2 receptors since the D2 agonist LY171555 had a similar effect whereas the D1 agonist SK&F 38393 was inactive. Furthermore the D2 antagonist sulpiride blocked the effects of apomorphine and LY171555. The spinal monosynaptic reflex was not modified by 150 micrograms/kg apomorphine suggesting that sensory rather than motor processes are being influenced. Pretreatment with the serotonin receptor antagonist methysergide prevented the apomorphine induced increase in TFL. It is concluded that exogenously applied DA agonist can result in antinociception in the spinal cord and that this effect may be dependent upon activity in the spinal 5-hydroxytryptaminergic and noradrenergic systems.     

Changes in nociception after lesions of descending serotonergic pathways induced with 5,6-dihydroxytryptamine. Different effects in the formalin and tail-flick tests

Intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) in mice selectively lesioned descending serotonergic pathways. Nociception was evaluated 3 days after injection of 5,6-DHT using the tail-flick and formalin tests. In the tail-flick test shortened latencies were found in the lesioned animals. In contrast, the initial behavioural response (0-15 min) to formalin was reduced, while the late response (15-40 min) was not altered. Fourteen days after intrathecal administration of 5,6-DHT the changes in nociception, both in the tail-flick and in the formalin test, had returned to the control level. These findings support the contention that the raphe-spinal serotonergic system participates in the tonic regulation of nociception in the spinal cord. Apparently this system tonically inhibits spinal nociceptive reflexes, but tonically enhances the initial behavioural responses to noxious chemical stimulation, as measured with the formalin test.     

The apparent hyperalgesic effect of a serotonin antagonist in the tail flick test is mainly due to increased tail skin temperature

It has been suggested that reduced activity in raphe-spinal serotonergic systems induces hyperalgesia. In rats, the serotonin antagonist metergoline (0.5 mg/kg intraperitoneally) reduced tail flick latency by 0.92 sec (p less than 0.001) and increased tail skin temperature by 2.4 degrees C (p less than 0.001) when measured 50 min after injection. Multiple regression analysis with tail flick latency as dependent variable and tail skin temperature and metergoline/vehicle as independent variables revealed a highly significant effect of tail temperature on tail flick latency. The increase of tail skin temperature explained a reduction of tail flick latency of 0.64 of the 0.92 sec observed [B = -0.267 +/- 0.034, t(37)= -7.75, p less than 0.0001]. When the effect on tail skin temperature was taken into account, metergoline reduced tail flick latency by 0.28 sec [B = -0.284 +/- 0.114, t(37) = -2.50, p less than 0.05]. Metergoline (0.5 and 2.0 mg/kg) did not significantly alter plantar paw skin temperature or the response temperature in the increasing temperature hot plate test. Thus, the observed effect of metergoline on tail flick latency is primarily due to an effect on tail skin temperature. The possibility exists that the remaining effect of metergoline may be due to inadequate correction for the skin temperature change, and it is concluded that the study provide no clear evidence for a tonic inhibition of nociception by serotonergic systems.     

Effects of serine/threonine protein phosphatase inhibitors on morphine-induced antinociception in the tail flick test in mice

The aim of this study was to evaluate the effects of serine/threonine protein phosphatase (PP) inhibitors on morphine-induced antinociception in the tail flick test in mice, and on [3H]naloxone binding to the forebrain crude synaptosome fraction. Neither okadaic acid nor cantharidin (1-10000 nM) displaced [3H]naloxone from its specific binding sites, which indicates that they do not interact at the opioid receptor level. The i.c.v. administration of very low doses of okadaic acid (0.001-1 pg/mouse) and cantharidin (0.001-1 ng/mouse), which inhibit PP2A, produced a dose-dependent antagonism of the antinociception induced by morphine (s.c.). However, L-nor-okadaone (0.001 pg/mouse-1 ng/mouse, i.c.v.), an analogue of okadaic acid lacking activity against protein phosphatases, did not affect the antinociceptive effect of morphine. On the other hand, high doses of okadaic acid (10 ng/mouse, i.c.v.) and cantharidin (1 microg/mouse, i.c.v.), which also block PP1, and calyculin-A (0.1 fg/mouse-1 ng/mouse, i.c.v.), which inhibits equally both PP1 and PP2A, did not modify the morphine-induced antinociception. These results suggest that the activation of type 2A serine/threonine protein phosphatases may play a role in the antinociceptive effect of morphine, and that PP1 might counterbalace this activity.     

Endogenous opioids are involved in morphine and dipyrone analgesic potentiation in the tail flick test in rats

The combined administration of low doses of opiates with non-steroidal anti-inflammatory drugs can produce additive or supra-additive analgesic effects while reducing unwanted side effects. We have recently reported that co-administration of morphine with dipyrone (metamizol) produces analgesic potentiation both in na?ve and in morphine-tolerant rats. The purpose of this work was to determine the role of opioids on the acute potentiation observed between morphine and dipyrone i.v. in the rat tail flick test. To do this, two experiments were done. In the first one, naloxone was administered 10 min before morphine (3.1 mg/kg), dipyrone (600 mg/kg) or their combination at the same doses. Control animals received saline instead of naloxone. In the second experiment, naloxone (or saline) was given 2 min after reaching the maximal peak effect with each individual analgesic treatment. When naloxone was i.v. administered prior to analgesics, it completely blocked morphine effects, partially prevented morphine/dipyrone antinociception and delayed dipyrone-induced nociception. At 3.1 mg/kg, naloxone produced an increased nociception. When naloxone was given after analgesics, it dose-dependently blocked the effects of morphine alone and in combination with dipyrone but with different potency in each case. As to dipyrone, naloxone delayed the time to antinociceptive peak effect. Taken together, these results support the notion that endogenous opioids are involved in the analgesic potentiation observed with the combination of morphine plus dipyrone.     

Dopaminergic involvement in adenosine A1 receptor-mediated antinociception in the tail flick latency model in mice

The interaction between the adenosinergic and dopaminergic systems in nociception was assessed in the tail flick latency (TFL) test in mice. Adenosine exhibited qualitatively different responses depending on the dose: Adenosine 10 and 100 mg/kg i.p. caused antinociception as evidenced by an increase in TFL while the middle dose of 30 mg/kg decreased TFL. On the other hand, the specific adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) at doses of 0.05, 0.1, 0.25 and 0.5 mg/kg (i.p.) resulted in dose-dependent antinociception. The antinociceptive effect of CPA was reversed by classical albeit nonspecific adenosine receptor antagonist theophylline (5 mg/kg) at a dose which had no effect on TFL per se. A low dose (1 mg/kg i.p.) of the dopaminergic agonist apomorphine caused an early mild hyperalgesic response while the high dose (10 mg/kg i.p.) had no significant effect on TFL. The antinociceptive effect of CPA was attenuated by pretreatment with low dose apomorphine while pretreatment with the high dose caused mild but insignificant potentiation. Theophylline, when administered prior to apomorphine failed to modify the nociceptive response. The results suggest that an interaction between adenosine and dopamine may be involved in nociception.     

Antinociception versus serum concentration relationships following acute administration of intravenous morphine in male and female Sprague-Dawley rats: differences between the tail flick and hot plate nociceptive tests

1. Antinociception versus serum morphine concentration relationships were defined in male and female Sprague-Dawley rats administered single intravenous (i.v.) bolus doses of morphine, using the hot plate (2.1-14 mg/kg) and tail flick tests (1-8 mg/kg). 2. Serum concentrations of morphine and morphine-3-glucuronide (M3G), its major metabolite in the rat, were assayed using high-performance liquid chromatography (HPLC) with electrochemical detection. 3. Significantly higher (P < 0.05) values of peak antinociception (approximately 1.7-fold), as well as the extent and duration of antinociception (approximately fourfold), were observed in male compared with female rats administered 10 mg/kg morphine in the hot plate test. Although there were no significant sex-related differences in the area under the serum morphine concentration versus time curve (AUC) at this dose, systemic exposure to M3G (M3G AUC) was significantly higher (approximately twofold; P < 0.05) in female than male rats. 4. In contrast with most previous studies investigating sex differences in morphine antinociception in rats, the antinociceptive effects of single i.v. doses of morphine (1-8 mg/kg) in the tail flick test did not differ significantly between male and female rats. 5. Morphine ED(50) and EC(50) values (95% confidence intervals) for antinociception in the hot plate test were significantly lower (P < 0.05) in male rats (ED(50) 8.4 mg/kg (7.6-9.2); EC(50) 1.8 nmol/L (1.5-2.1)) compared with female rats (ED(50) 10.6 mg/kg (9.1-12.0); EC(50) 3.7 nmol/L (3.4-4.1)). However, in the tail flick test, there was no significant difference between male and female rats in ED(50) (1.8 (0.4-3.3) and 1.4 mg/kg (0.4-2.5), respectively) or EC(50) (0.5 (0.3-0.6) and 0.4 nmol/L (0.2-0.5), respectively) values. 6. Supraspinal attenuation of morphine antinociception by M3G may account for these differences.     

Noradrenergic and opioidergic influences on the antinociceptive effect of clomipramine in the formalin test in rats

Although tricyclic antidepressant are especially useful in the treatment of chronic pain conditions, most of the work about its mechanism of action has been made on acute pain tests. The present study was aimed at studying the role played by noradrenergic and opioidergic influences on the antinociceptive activity of subchronically administered clomipramine in the formalin test (a tonic pain model) in rats. Clomipramine produced antinociception after 7 days, administration (2.5 mg/kg/day), an effect equivalent to that obtained by acute morphine (5 mg/kg). The antinociceptive effect of clomipramine was inhibited by the following: nonspecific blocking of alpha₁-and alpha₂-adrenoceptors by phentolamine, specific blocking of alpha₁-adrenoceptors by prazosin; stimulation of alpha₂ receptors by clonidine; and blocking of the opioid receptors by naloxone. Blocking the alpha₂-receptors with yohimbine did not antagonize the effect of clomipramine. These results suggest that clomipramine produces antinociception in this test, partly via the participation of the endogenous opioid system and partly by further activating or potentiating previously activated noradrenergic pathways which are involved in the control of pain information.     

Physiological evidence of a postsynaptic inhibition of the tail flick reflex by a cannabinoid receptor agonist

Current evidence indicates that cannabinoids are antinociceptive and this effect is in part mediated by spinal mechanisms. Anatomical studies have localized cannabinoid CB₁ receptors to pre- and postsynaptic sites within the spinal cord. However, behavioural tests have not clearly indicated the relative importance of each of these sites. In this study, the tail flick test was used as a model of acute pain in the rat to determine the site of action of WIN 55,212-2 ((R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate), a synthetic cannabinoid receptor agonist. WIN 55,212-2 (3 mg/kg, i.p.) increased the latency of tail withdrawal from a noxious radiant heat source, indicating it is antinociceptive in this model. Using the same paradigm, WIN 55,212-2 was then tested against the synaptically-induced nociceptive hypersensitivity in response to noxious thermal stimulation of the tail (hot water tail immersion). WIN 55,212-2 blocked this hypersensitivity, confirming a spinal site of action of the cannabinoid receptor agonist. Further, WIN 55,212-2 blocked the nociceptive hypersensitivity induced by intrathecal administration of substance P. As substance P acts on postsynaptic tachykinin NK1 receptors in the dorsal horn of the spinal cord, the data are interpreted to suggest that WIN 55,212-2 expressed its anti-hypersensitivity effects at least partially via a postsynaptic site of action; the data do not rule out a presynaptic site of action. This study suggests that cannabinoids may induce analgesia via a postsynaptic site of action in the spinal cord, as well as the possibility that they may interact with substance P signaling.     

天元克痛方和盐酸曲马多单次给药对大鼠辐射热模型的镇痛作用研究

目的:探讨天元克痛方与盐酸曲马多单次给药后,对大鼠辐射热模型的镇痛作用。方法:63只Wistar大鼠随机分为5组,分别给予无菌蒸馏水、盐酸曲马多和天元克痛方,用辐射热甩尾法测定大鼠痛阈。结果:单次给药后,盐酸曲马多组的痛阈值与空白组相比,在60、90、120min均有显著性差异;与空白组相比,天元克痛方各组基本无显著性差异。结论:盐酸曲马多组在单次给药后60min～120min能使辐射热模型大鼠的痛阈延长(与空白组相比,P<0.05或P<0.01);天元克痛方各组虽有痛阈延长现象,但对辐射热引起的短暂锐痛无明显作用(P>0.05)。     

Activation of peripheral galanin receptors: differential effects on nociception

Numerous reports suggest a significant role of peripheral galanin (GAL) in pain transmission; however, due to the lack of selective galanin receptor agonists and antagonists, the role of GAL receptors (GalR1-3) in pain transmission remains unclear. In this study, a new agonist, M617, that preferentially binds to GalR1, a GalR2 agonist (AR-M1896), and a GalR2 antagonist (M871) were tested in the periphery to elucidate the role of peripheral GalR1 and GalR2 in nociception. Ipsilateral, but not contralateral, hindpaw injection of M617 reduced capsaicin (CAP)-induced flinching by approximately 50%, suggesting that GalR1 activation produces anti-nociception. This anti-nociceptive effect was blocked by intraplantar injection of the non-selective GalR antagonist M35. In contrast ipsilateral, but not contralateral, intraplantar injection of GalR2 agonist AR-M1896 enhanced the CAP-induced nociception (1.7-fold). The GalR2 antagonist M871 blocked the pro-nociceptive effect of AR-M1896 in a dose-dependent manner. This antagonist had no effect on nociceptive behaviors induced by CAP alone. The data demonstrate that activation of peripheral GalR1 results in anti-nociception but activation of peripheral GalR2 produces pro-nociception. Thus, the use of these pharmacological tools may help to elucidate the contribution of GalR subtypes in nociceptive processing, identifying potential drug targets for the treatment of peripheral pain.     

Morphine analgesia and tolerance in the tail-flick and formalin tests: Dose-response relationships

The dose-response relationships for morphine analgesia were studied in morphine-tolerant and non-tolerant rats using two pain tests: tail-flick test which measures the threshold for an escape response, and the formalin test which assesses the behavioral response to continuous pain generated in injured tissue. The effects of prior experience with both pain tests on tolerance were also examined. In the formalin test, effective analgesia was obtained in non-tolerant rats at doses that produce minimal depression of locomotor behavior. Morphine tolerance was produced by 20 daily injections of morphine with increments that reached 16 mg/kg, a dose over the LD₁₀₀ for barrier sustained Long Evans rats. This dose regimen produced a 1.8-fold increase in the ED₅₀ in the tail-flick test and a 2.7-fold increase in the formalin test. Daily experience of the pain test, as well as the morphine regimen produced a 4.8-fold increase in the ED₅₀ in the tail-flick test but did not effect the potency of morphine in the formalin test. The magnitude of tolerance in the absence of daily behavioral testing is consistent with recent clinical reports that little tolerance occurs after prolonged administration of morphine in cancer patients and that tolerance is not an important consideration in the management of pain.     

Animal models and peripheral nociception tests for the study of neuropathic pain

Neuropathic pain associated with abnormal tactile and thermal responses that are extraterritorial to the injured nerve is known to be difficult to diagnose and treat because of clinical observation of limited responsiveness to opioids and non-steroidal anti-inflammatory drugs. To reproduce the different pathological changes observed in neuropathic pain patients, several laboratory animal models have been proposed. Recent studies using such models suggest the involvement of neuronal plasticity in pain pathways through nociceptive neurons. Our new experimental model using specific pain-producing molecules that clearly distinguish three different nociceptive fibers from each other reproduces neuropathic pain-like hyperalgesia and less sensitivity to morphine. After nerve injury, the nociceptive responses through type I neurons, which are polymodal C-fibers and drive NK1-receptor mechanisms in spinal pain transmission, were completely lost, but without changes in type II ones, which are polymodal C-fibers and drive NMDA receptor-mechanisms, while type III ones, which are capsaicin-insensitive (possibly A-fibers) and drive NMDA-receptor mechanisms, were markedly enhanced. Such pain transmission switch mechanisms are clearly consistent with clinical effectiveness including less sensitivity to morphine and more sensitivity to NMDA-antagonists. This article also presents currently used methods for experimental neuropathic pain models.     

Effects of systemic non-steroidal anti-inflammatory drugs on nociception during tail ischaemia and on reperfusion hyperalgesia in rats.

1. We have investigated the effects of five non-steroidal anti-inflammatory drugs (NSAIDs) on nociception during ischaemia and on reperfusion hyperalgesia in rats. 2. We induced tail ischaemia in conscious rats by applying a tourniquet at the base of the tail until the rats exhibited co-ordinated escape behaviour when we released the tourniquet. 3. We assessed hyperalgesia by measuring the tail flick latency following tail immersion in water at 49 degrees C, before applying and immediately after releasing the tourniquet, and then at 30 min intervals for 2 h. 4. Intraperitoneal injection of NSAIDs prior to applying the tourniquet had no effect on the co-ordinated escape behaviour during ischaemia, nor on tail flick latency in the absence of prior ischaemia. However all the drugs attenuated reperfusion hyperalgesia in a log dose-dependent manner. Doses required to abolish hyperalgesia, were indomethacin 5 mg kg-1, diclofenac sodium 42 mg kg-1, ibuprofen 54 mg kg-1, dipyrone 168 mg kg-1 and paracetamol 170 mg kg-1. 5. We conclude that the mechanisms underlying nociception during ischaemia are not the same as those underlying reperfusion hyperalgesia. Moreover our procedure provides a rapid and more humane method for measuring the antinociceptive potency of NSAIDs.     

Amitriptyline produces multiple influences on the peripheral enhancement of nociception by P2X receptors

Peripherally administered amitriptyline exhibits potential to be a locally active analgesic, while ATP augments peripheral nociception by interacting with P2X(3) receptors on sensory afferents. The present study examined the effects of amitriptyline on flinching and biting/licking behaviours and thermal hyperalgesia produced by alphabeta-methylene-ATP (alphabeta-MeATP), a ligand for P2X(3) receptors, following intraplantar administration into the hindpaw of rats. Coadministration of low doses of amitriptyline (up to 100 nmol) with alphabeta-MeATP augmented thermal hyperalgesia and flinching behaviours. The most active dose of amitriptyline (100 nmol) had no intrinsic effect. Augmentation of alphabeta-MeATP actions appears to be due to increased tissue levels of biogenic amines resulting from inhibition of uptake, as phentolamine (alpha(1)/alpha(2)-adrenergic receptor antagonist) and methysergide (5-hydroxytryptamine or 5-HT(1)/5-HT(2) receptor antagonist) inhibit the augmented flinching produced by alphabeta-MeATP/amitriptyline. When noradrenaline and 5-HT were coadministered with alphabeta-MeATP (both increase the effect of alphabeta-MeATP), amitriptyline had no effect on flinching produced by alphabeta-MeATP/noradrenaline but inhibited flinching produced by alphabeta-MeATP/5-HT. In the presence of low concentrations of formalin (0.5%, 1%; which also increase the effect alphabeta-MeATP), amitriptyline inhibited augmented behaviours. Higher doses of amitriptyline (300-1000 nmol) increased thermal thresholds, suppressed thermal hyperalgesia produced by alphabeta-MeATP, and inhibited flinching produced by alphabeta-MeATP. Collectively, these results indicate that amitriptyline produces complex influences on peripheral pain signaling by P2X receptors. Lower doses augment nociception by alphabeta-MeATP (probably by inhibiting noradrenaline and 5-HT uptake) but inhibit alphabeta-MeATP responses in the presence of inflammatory mediators (perhaps reflecting receptor blocking properties); higher doses uniformly inhibit nociception by alphabeta-MeATP (perhaps reflecting local anesthetic properties).