An increase in tryptophan in brain may be a general mechanism for the effect of stress on sensitivity to pain

The role of the stress-induced increase in the uptake of tryptophan in brain in opioid-induced analgesia was investigated by modifying the uptake of amino acid in brain with injections of competing amino acids. Blockade of analgesia by valine (200 mg/kg, i.p.) alone, and by valine and tyrosine (100 mg/kg, i.p.), but not by valine and tryptophan (100 mg/kg, i.p.), was taken to indicate that an increase in the uptake of tryptophan in brain was involved in opioid-induced analgesia. Morphine-induced analgesia exhibited by rats that were habituated to the laboratory and not restrained did not involve an increase in the uptake of tryptophan in brain. However, a mild form of restraint, or exposure to a novel environment interacted with morphine to induce analgesia which involved an increase in the uptake of tryptophan in brain. These stressors did not affect sensitivity to pain in the absence of morphine. Analgesia induced by 3 hr of restraint, which was preventable by naltrexone (1 mg/kg, s.c.) but not reversible by naloxone (1 mg/kg, s.c.), also involved an increase in the uptake of tryptophan in brain. It is concluded that the endogenous opioid-induced analgesia that is induced by stress alone and analgesia induced by stress interacting with morphine, both depend on an increase in the uptake of tryptophan into the brain.     

Ketamine-induced potentiation of morphine analgesia in rat tail-flick test: role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels

Ketamine is known to improve opioid efficacy, reduce postoperative opioid requirement and oppose opioid associated pain hypersensitivity and tolerance. However, the mechanisms underlying these beneficial effects are not clear. This study investigated the effects of ketamine at a non-analgesic dose (30 mg/kg, i.p.) on analgesia induced by morphine (2.5, 5.0, 7.5 mg/kg, s.c.), using rat tail-flick test as an animal model of acute pain. Further, the role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels was examined on the potentiating effect of ketamine. Male rats received morphine alone at 5.0 and 7.5 but not at 2.5 mg/kg showed a dose-related increase in tail-flick latencies. The combination of morphine and ketamine resulted in dose-related increase in morphine analgesia, both on the intensity as well as on duration. The ketamine-induced potentiation of morphine (7.5 mg/kg) analgesia was unaffected by glibenclamide (3 mg/kg, s.c.) and only partially blocked by yohimbine (2 mg/kg, i.p.), but more completely abolished by naloxone (2 mg/kg, i.p.). Both morphine (5.0 mg/kg) and ketamine (30 mg/kg) alone did not evoke catalepsy in rats but on combination produced a synergistic effect, which was however, abolished by naloxone pretreatment. In the open-field test, while morphine (5.0 mg/kg) caused a depressant effect, ketamine (30 mg/kg) enhanced the locomotor activity. Nevertheless, in combination potentiated the morphine's depressant effect on locomotion, which was also antagonized by naloxone. These results indicate that ketamine at a non-analgesic dose can potentiate morphine analgesia, induce catalepsy and cause locomotor depression, possibly involving an opioid mechanism. This potentiation, although favorable in acute pain management, may have some adverse clinical implications.     

Dose-dependent reductions by naloxone of analgesia induced by cold-water stress

Animals exposed to cold-water swims, rotation, or inescapable shocks, display analgesia comparable to that of 10 mg/kg of morphine. The present study investigated whether a narcotic antagonist would eliminate analgesia induced by cold-water swims. In one group of 12 rats, naloxone at 0, 1, 5, 10 and 20 mg/kg was administered at weekly intervals immediately preceding forced cold-water swims (2°C for 3.5 min) and alterations in flinch-jump thresholds were determined 30 min thereafter. In a second group of six rats, the effects of the same dose range of naloxone were determined upon normal flinch-jump thresholds. Naloxone dose-dependently attenuated the cold-water swim-induced analgesia up to a maximal reduction of 50% at 20 mg/kg. In contrast, all doses of naloxone had no effects upon normal flinch-jump thresholds. Since low doses of naloxone completely abolish morphine-induced analgesia, the present data suggest that the analgesia induced by stress is not identical to that of opiates.     

Interactions of ketamine, naloxone and morphine in the rat

The effect of naloxone on the ketamine-induced anesthesia and analgesia, and the development of tolerance to ketamine and the cross-tolerance to morphine (measured by an analgesic effect) were investigated in the rat. Ketamine produced a dose-dependent analgesia. Naloxone, 1 mg/kg, significantly inhibited analgesia induced by ketamine, 100 mg/kg, but even in a dose of 4 mg/kg it did not affect the duration of anesthesia. A chronic administration of ketamine (100 mg/kg twice a day (b.i.d.) for 7 days) resulted in the development of tolerance to analgesic effects of ketamine. The analgesic action of morphine was attenuated in rats receiving ketamine chronically, while the analgesic effects of ketamine were significantly potentiated in morphine-dependent rats. Ketamine, 25 mg/kg, significantly attenuated the withdrawal signs evoked by naloxone in morphine-dependent rats. The results corroborate the suggestion about the participation of the central opioid neurotransmission in the mechanism of ketamine action.     

Attenuation of place preference conditioning but not place aversion conditioning by chronic mild stress

Chronic exposure to very mild unpredictable stress has previously been found to reduce or abolish the acquisition of place preference conditioning. In the present study, chronic mild stress was found to abolish the acquisition of preferences for a distinctive environment paired with morphine (0.7 mg/kg). However, chronic mild stress did not impair the acquisition of place aversion conditioning induced by naloxone (0.7 mg/kg) or picrotoxin (2.0 mg/kg). The results demonstrate that chronic stress does not cause a general impairment of associative learning but, rather, a specific impairment of rewarded behaviour.     

Characteristics of analgesia induced by noncatecholic phenylethylamine derivatives: possible involvement of endogenous opioid peptides and serotonin in phenylethylamine analog-induced analgesia

Characteristics of the analgesic action of phenylethylamine derivatives, amphetamine, phenylethylamine (PEA), hydroxyphenylethylamine (OHPEA) and hydroxyphenylalanine (OHF), were examined. Pain threshold of mice was measured by using the hot plate method. OHPEA (50 mg/kg), amphetamine (0.5-8 mg/kg) or PEA (50 mg/kg) produced an analgesic effect in the absence of MAO inhibitor, and the analgesia was reversed by naloxone (5 mg/kg) or reserpine (2 mg/kg x 2). Ten mg/kg of PEA, 250 mg/kg of OHF and 10 mg/kg of OHPEA could not produce detectable analgesia, but they revealed analgesic activity when mice were pretreated with pargyline (100 mg/kg). Analgesia induced by a combined use of PEA, OHF or OHPEA with pargyline was inhibited by naloxone or p-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis. Amphetamine-induced analgesia was also blocked by PCPA. Analgesia induced by PEA or OHPEA was blocked by methysergide (2 mg/kg). From the above findings, it was concluded that PEA, OHPEA, OHF and amphetamine possess similar characteristics in their analgesic action, and their analgesic actions involve the participation of endogenous serotonin and endogenous opioid peptides.     

The effect of adenosine receptor agonists on analgesic effects of morphine

The effect of adenosine, S-phenylisopropyladenosine (S-PIA) and dipyridamole (an adenosine reuptake inhibitor) on the analgesic action of morphine in mice and rats was investigated in the hot-plate (56 degrees C) and tail immersion (52 degrees C) tests. Adenosine, 50 and 100 mg/kg, induced analgesia in mice and rats in the hot-plate test and potentiated the action of morphine (particularly in mice). The analgesic effects of adenosine were completely abolished by caffeine (10 mg/kg in mice and rats), and partially inhibited by naloxone (1 mg/kg, only in mice). S-PIA given alone (0.6 mg/kg) produced in mice some analgesic effect in the hot-plate test: the effect was abolished by caffeine and partially by naloxone. The effect of S-PIA on the action of morphine depends on the dose and the animal species. Dipyridamole alone did not affect the reactivity of animals in tests for analgesia, but potentiated the action of morphine.     

Thermal analgesic effects from weak, complex magnetic fields and pharmacological interactions

In several experiments, robust analgesia (equivalent to about 4 mg/kg of morphine) in male rats to thermal stimuli following exposures to weak (1 microT) complex magnetic fields was explored. The analgesia occurred when patterns of magnetic fields with burst-firing-like configurations were presented for 30 min once every approximately 4 s. The analgesic effects were intensity dependent. A different frequency-modulated pattern produced analgesia more quickly. The analgesic effects following exposure to the burst-firing magnetic fields were augmented conspicuously by preinjections of morphine (4 mg/kg) or agmatine (10 mg/kg), but blocked by naloxone (1 mg/kg). The results of these experiments suggest that rational design of the temporal structure of weak magnetic fields may be a novel, inexpensive, and reliable technique for elevating thresholds to some classes of painful stimuli.     

Effects of naloxone and its quarternary analogue on stimulation-induced feeding

Feeding was induced in rats by electrical stimulation in the lateral hypothalamus. Naloxone (0.2 and 1.0 mg/kg) produced a dose-related elevation of the frequency threshold for stimulation-induced feeding while quarternary naloxone (2.0 and 10.0 mg/kg) had no effect. Since quarternary naloxone does not readily penetrate the blood-brain barrier, we conclude that the opiate receptors at which naloxone exerts its anorectic action are located in the brain rather than in potential peripheral tissues such as gastrointestinal tract, pancreas or adrenal medulla. The threshold-elevating effect of naloxone only became marked after rats had engaged in one or two 5-sec bouts of feeding. The effect continued to increase following each subsequent bout of feeding. Naloxone therefore appears to inhibit feeding by interacting with post-ingestive factors.     

The effects of opiate receptor agonists and antagonists on the stress-induced secretion of corticosterone in mice.

1 Intraperitoneal administration of normorphine, morphine or naloxone or exposure to ether vapour for 1 min, elevated plasma corticosteroid concentrations in mice. 2 Injection of saline or exposure to ether vapour rendered mice less sensitive to a subsequent exposure to ether vapour 15 min later. 3 Treatment with normorphine (50 mg/kg) potentiated the corticosteroid response to ether stress whilst pentazocine (20 mg/kg), naltrexone (10 mg/kg), morphine (24 mg/kg), levorphanol (20 mg/kg) and naloxone (50 mg/kg) prevented the stress-induced elevation of plasma corticosteroids. 4 Both naloxone and morphine inhibited the potentiation by normorphine of the response to ether, the dose of naloxone required being higher than that for inhibition of normorphine analgesia. 5 It is concluded that endogenous opioid peptides may be involved in the control of the response to ether stress in mice.     

Evidence for the involvement of central opioidergic systems in L-tyrosine methyl ester-induced analgesia in the rat

Intraperitoneal administration of L-tyrosine (used as methyl ester HCl) produced dose-dependent analgesia in male Sprague-Dawley rats as measured by the tail-flick test. The maximal analgesic response was obtained with 200 mg/kg dose of tyrosine. Administration of morphine also produced a dose-dependent analgesic response. Tyrosine in doses of 50 mg/kg or higher potentiated morphine-induced analgesia. The analgesic response of tyrosine (200 mg/kg) was antagonized by naloxone (1 mg/kg), an opiate antagonist. Subcutaneous administration of methyl naltrexone bromide (MRZ 2663 BR, 1 and 10 mg/kg) had no effect on tyrosine-induced analgesia. Intracerebroventricular injection of MRZ 2663 BR (1 and 10 micrograms/rat) effectively blocked tyrosine-induced analgesia. It is concluded that tyrosine-induced analgesia and its potentiation of analgesic response to morphine may be mediated via either the opiate receptors or activation of endogenous opioidergic systems of central origin.     

Nitrous oxide analgesia: partial antagonism by naloxone and total reversal after periaqueductal gray lesions in the rat

Analgesia induced by nitrous oxide was examined using radiant heat tail flick and electrical evoked foot flick tests in rats. Rats exposed to 80 and 60% nitrous oxide expressed statistically significant elevations of percent analgesia (% MPE) compared to air exposed rats. Rats exposed to 30% nitrous oxide showed no significant difference in percent analgesia. Pretreatment with naloxone (10 mg/kg s.c.) produced a significant decrease in %MPE and an increase in variance of response after exposures to 80% nitrous oxide in a double blind study. Kainic acid lesions of the ventral and caudal periaqueductal grey (PAG) reversed analgesia produced by 80% nitrous oxide in a crossover blink study compared to saline lesions. In conclusion, this evidence suggests that the caudal-PAG-raphe mangus-dorsal horn pain inhibition pathway is in part involved in the analgesia induced by nitrous oxide.     

Potentiation of morphine analgesia in rats given a single exposure to restraint stress immobilization.

Rats exposed to restraint stress and injected with morphine show significantly greater increases in tail-flick latency compared to unstressed rats. However, it is not necessary for rats to be restrained at the time of testing to elicit a potentiated analgesic response to morphine. We reported recently that analgesia induced by 4.0 mg/kg morphine was significantly potentiated in rats that had been restrained for only 1 h at 24 h prior to testing. One purpose of the present study was to extend this observation by determining the ability of a single exposure to restraint stress to potentiate dose-dependently morphine (0.0, 2.0, 4.0, and 8.0 mg/kg)-induced analgesia in the tail-flick test. A second purpose was to assess the generality of the phenomenon by determining whether prior restraint would potentiate the analgesic effect of morphine in another common analgesic assay, the hot-plate test. Dose- and time-course (20-120 min) curves for morphine were generated in rats exposed to one of two treatments: no restraint stress (NS) and a single exposure to 1 h of restraint (RS). Rats subjected to 1 h of restraint and tested 24 h later displayed significant time- and dose-dependent potentiation (1.3-2.0-fold) of morphine-induced analgesia compared to unstressed rats in both the tail-flick and hot-plate tests. These results demonstrate that a single period of restraint is sufficient to activate the mechanisms responsible for potentiation of morphine-induced analgesia and that the degree to which stress modified morphine's analgesia can be demonstrated using both the tail-flick and hot-plate assays.     

Stress produces opioid-like effects on investigatory behavior

Stimulation of opioid systems with opiate agonists produce characteristic alterations in the investigatory behavior exhibited by rats in a novel environment. As numerous reports now indicate that opioid systems can be activated by exposure to stress, the following study examined whether exposure to stressors could produce opiate-like alterations of investigatory behavior. Naive rats were exposed to one of three stressors (restraint, tailpinch pressure, high intensity white noise) or to control procedures, and were observed in a novel environment. The frequency and duration of a wide range of behavioral activities were recorded. All three stressors were found to produce morphine-like alterations of investigatory behavior. The average time an animal spent per contact with stimuli in the environment was decreased significantly by stress, with greater reductions being associated with locomotor hypoactivity. The stress-induced reductions of investigatory behavior were blocked by very low doses of the opiate antagonist naloxone (0.1-0.25 mg/kg). These results are consistent with an activation of opioid systems underlying some of the changes in investigatory behavior produced by exposure to stress.     

2-Deoxy-D-glucose analgesia: influences of opiate and non-opiate factors

Acute administration of 2-deoxy-D-glucose (2-DG), an antimetabolic glucose analogue induces a powerful analgesia which adapts following repeated administration. 2-DG analgesia displays significant cross-tolerance with morphine, and like morphine analgesia, is potentiated in hypophysectomized rats. The present study examined further the role of opiates in 2-DG analgesia by examining whether the opiate antagonist, naloxone, would affect 2-DG analgesia, and whether ineffective doses of 2-DG and morphine would interact in a synergistic fashion to induce analgesia. Nociceptive thresholds were measured by the flinch-jump test. Naloxone doses of 1, 5, 10 and 20 mg/kg were all ineffective in reducing significantly 2-DG (600 mg/kg) induced pain threshold elevations. Naloxone failed to attenuate 2-DG (350 mg/kg) analgesia whether administered before or after the 2-DG injection. On the other hand, simultaneous administration of sub-analgesic doses of 2-DG (200 mg/kg) and morphine (2.5 mg/kg) summated to produce significant analgesia. This, 2-DG analgesia is similar to opiates in its tolerant and summative actions, yet dissimilar in that naloxone is ineffective in reversing its effects.     

Opioid modulation of thermal dehydration-induced thirst in rats.

Male Sprague-Dawley rats were utilized to study the effects of the opioid receptor antagonists, naloxone and naltrexone, on thirst induced by thermal dehydration. In an initial experiment, the depressant effect of naloxone (1.0 mg/kg, IP) on the water intake of rats deprived of water for 24 h was confirmed. In subsequent experiments, rats were thermally dehydrated by exposing them without water to a 40 degrees C environment for 1-4 h. Following heat exposure, rats were injected with either naloxone or naltrexone either IP or ICV. Fifteen minutes later, rats were provided with water and water intake was measured for 2 h. Both naloxone and naltrexone had dose (0.1-5.0 mg/kg, IP)-dependent effects of reducing water intake of rats thermally dehydrated for 3 h. Water intake of rats thermally dehydrated for 2 or 4 h was also attenuated by pretreatment with naloxone. Rats thermally dehydrated for 3 h exhibited decreases in water intake following ICV injection of either naloxone or naltrexone at a dose of 50 micrograms. Neither naloxone nor naltrexone had an effect on urine output in any experiment. The water intake data support the hypothesis that thirst induced by thermal dehydration in rats is modulated by an opioid mechanism.     

Clonidine and guanfacine-induced antinociception in visceral pain: possible role of alpha 2/I2 binding sites

Visceral pain is one of the most common forms of pain which is poorly understood. We now studied the influence of imidazoline/guanidinium compounds such as clonidine and guanfacine on visceral pain in the presence or absence of yohimbine and benazoline. To produce visceral pain-related behaviours, formalin (10%) was administered by inserting a fine cannula into the colon via the anus. Each experiment took 1 h. Clonidine (0.001, 0.01 and 0.1 mg/kg, i.p.) and guanfacine (2.5, 5 and 10 mg/kg, i.p.) produced analgesia dose dependently. The clonidine response was inhibited by yohimbine (0.2 mg/kg, i.p.). On the other hand, benazoline (5 mg/kg, i.p.) blocked the antinociceptive effect of guanfacine (5 mg/kg). Benazoline (2.5 and 5 mg/kg) itself also induced analgesia in inflammatory colonic pain. In this study, we used morphine to ensure that the behavioural responses were pain-related. Our results showed that morphine (2.5, 5 and 10 mg/kg, s.c.) produced a dose-dependent antinociception. The morphine (7 mg/kg, s.c.) response was reduced by naloxone (2 mg/kg, i.p.). However, we concluded that both imidazoline (I(2)) and alpha(2)-adrenoceptors may play a role in producing analgesia in visceral pain.     

Naloxone-induced analgesia: effects of the benzodiazepine antagonist Ro 15-1788

Repeated exposure to pain under the influence of the opiate antagonists naloxone and naltrexone leads to the recruitment of substantial analgesia as measured by paw-lick latency on the hot-plate test (4,11). One hypothesis to explain this naloxone-induced analgesia (NIA) is that nociceptive stimulation in the face of opiate blockade becomes stressful enough to activate an analgesic adaptation that otherwise would not occur. This hypothesis was examined in two experiments by the administration of a benzodiazepine antagonist with anxiogenic properties (Ro 15-1788, in a dose of 10 mg/kg) in conjunction with repeated administrations of naloxone (5 mg/kg). One experiment incorporated defecation as a relatively direct measure of stress. Ro 15-1788 reliably augmented NIA. Defecation was increased by naloxone alone and in combination with Ro 15-1788. Overall, the results were most consistent with the hypothesis that NIA is a form of stress-induced analgesia that is at least partly nonopiate in nature.     

Analgesic effects of S and R isomers of the novel 5-HT3 receptor antagonists ADR-851 and ADR-882 in rats.

The present study examined analgesia produced by S and R isomers of the novel 5-HT3 receptor antagonists, ADR-851 and ADR-882 (0.1-10 mg/kg s.c.) against acute thermal, mechanical and formalin-induced inflammatory pain in rats. Neither isomer of ADR-851 or ADR-882 was analgesic in the thermal or mechanical test. Similarly, neither S or R forms of ADR-882 produced significant anti-nociception in the formalin test. In contrast, ADR-851R produced significant analgesia at 3 and 10 mg/kg doses in this test, while ADR-851S produced significant analgesia only at 1 mg/kg.     

Implication of endogenous opioid mechanism in the production of the antinociceptive effect induced by psychological stress in mice

Psychological (PSY) stress using the communication box produced a short-lasting antinociceptive effect which was less potent than that induced by physical stress such as footshock (FS) and forced swimming (SW) in mice. Naloxone completely antagonized PSY-stress induced analgesia (SIA) when the analgesia was measured by the tail pinch (TP) method; however, the antagonist did not reverse the effect in the tail flick (TF) assay. On the other hand, FS-SIA was antagonized by naloxone in both methods, while naloxone failed to reverse SW-SIA in either TF or TP assessment. Daily exposure to psychological stress developed tolerance to the analgesia. One-way cross-tolerance between PSY-SIA and morphine and the naloxone antagonism of PSY-SIA by the tail pinch method lead to the suggestion that an endogenous opioid system may be involved in the underlying mechanism for its production. On the contrary, from the findings of cross-tolerance between SW- or FS-SIA and the lack of naloxone antagonism in the TF method, the involvement of a more complicated mechanism is suggested in PSY-SIA. In both tests, U-50488H, a selective kappa-agonist, produced profound analgesia; however, no appreciable antagonism of naloxone was found in the TF test, whereas the effect was completely blocked by naloxone in the TP test. From the similarity in naloxone antagonism of PSY-stress and U-50488H induced analgesia, the participation of a common mechanism which may be mediated by kappa-opioid receptors, is suggested in the production of PSY-SIA.