Effect of methamphetamine on the development of acute morphine tolerance and dependence in mice

The effect of methamphetamine on morphine analgesia (tail-flick assay) was studied in non-tolerant mice and in mice made acutely tolerant to morphine following a single injection of 100 mg/kg morphine. The analgesic potency of morphine was increased in non-tolerant and tolerant mice to the same extent by 3.2 mg/kg methamphetamine (3.3 and 4.4 fold increases, respectively). In contrast, the ED50's for morphine analgesia and naloxone-precipitated jumping in mice pretreated with either 100 mg/kg morphine or both morphine and 3.2 mg/kg methamphetamine were not significantly different, indicating that methamphetamine had no effect on the development of acute morphine tolerance and dependence. Although methamphetamine had no effect on the development of acute tolerance to morphine, 4-day pretreatment with methamphetamine produced cross-tolerance to morphine analgesia. However, cross-tolerance to morphine was not accompanied by enchanced sensitivity to naloxone.     

Environmental and intraperitoneal ethanol influences morphine antinociceptive effect in mice

The ability of acute environmental or intraperitoneal (i.p.) ethanol to influence morphine antinociceptive effect was studied in mice. In order to induce tolerance to morphine analgesia, mice received daily injections of 10 mg/Kg morphine over a period of 10 days. Mice were divided into three groups: i.p. ethanol (E), environmental ethanol (E*), and control saline (M). During the induction of tolerance these groups were treated identically except on days 1 and 11. On these days, 10 minutes prior to morphine injection, mice received either i.p. ethanol (1g/Kg), environmental ethanol (a bottle of 10% ethanol placed next to the animals cage during the experiments), or an equivalent volume of saline. Analgesia was assessed using a standard hot plate protocol and dose-response cumulative curves for morphine analgesia were obtained on days 1 and 11. On day 1, both the i.p. and environmental administration of ethanol showed similar morphine-potentiation effects [Mean Effective Dose: ED50 (M1)=4.5 mg/kg; ED50 (E1)=2.4 mg/kg; ED50 (E*1)=2.1 mg/kg]. On day 11, control group mice showed a reduction of morphine analgesia at test [ED50 (M11)=14.1 mg/kg]. Mice receiving i.p. and environmental ethanol again showed a leftward shift in dose-response cumulative curves for morphine antinociception with respect to controls [ED50 (E11)=9.1 mg/kg; ED50 (E*11)=4.7 mg/kg]. I.p. ethanol administration at non-antinociceptive doses enhances the morphine antinociception effect similarly in tolerant and non-tolerant (naive) mice. The presence of environmental ethanol can also induce a similar pattern of increase in morphine antinociception effect.     

Delayed onset of single dose tolerance to morphine analgesia

Rats received either 20 mg/Kg of morphine sulfate I.P. or 5 μgm of morphine sulfate microinjected into the periaqueductal gray area of the brain. The analgesic effect of the morphine was determined by comparing pre- and postinjection tailflick latencies. To test for tolerance following a single injection, the procedure was repeated 6, 12 or 24 hours after the first injection and tests. Tolerance was not observed 6 hours after the original injection, tolerance was observed at 12 hours and increased tolerance was present at 24 hours. Single dose tolerance to morphine appears to develop slowly over a period of several hours and during much of this time, the amount of opiate present in the brain was insufficient to produce analgesia. Similarity between central and peripheral administration suggests a central mechanism of single dose tolerance.     

Increased analgesic potency of morphine and increased brain opioid binding sites in the rat following chronic naltrexone treatment

Implantation of rats with prolonged-release naltrexone pellets increased both morphine's analgesic potency in the tailflick assay and radiolabeled opioid binding in the brain. The increases in both radiolabeled opioid binding and morphine potency were time-dependent. Implantation for 24 hours did not increase binding, whereas increases of approximately 45% were seen following 8 days of implantation. Similarly, morphine's analgesic potency, measured as ED50 values, was increased by 50% following 8 days of exposure to naltrexone while a 24 hour exposure had no significant effect.     

Persistent pain model reveals sex difference in morphine potency

Central or systemic administration of agonists directed at the mu or delta opiate receptors generally produce a greater degree of analgesia in males than in females. To date, most studies examining sex-based differences in opioid analgesia have used acute noxious stimuli (i.e., tail-flick and hot plate test); thus the potential dimorphic response of centrally acting opiates in the alleviation of persistent inflammatory pain is not well established. In the present study, right hind paw withdrawal latency (PWL) to radiant thermal stimuli was measured in intact male and cycling female Sprague-Dawley rats before and after unilateral hind paw injection of the inflammatory agent complete Freund's adjuvant (CFA). Control animals received intraplantar injection of saline. Twenty four hours after CFA or saline injection, animals received either saline or morphine bisulfate (0.5-15 mg/kg sc). Separate groups of control or inflamed animals were tested on their responsiveness to morphine at 7, 14, and 21 days post-CFA or saline. No sex differences were noted for baseline PWLs, and females displayed slightly less thermal hyperalgesia at 24 h post-CFA. At all morphine doses administered, both the antihyperalgesic effects of morphine in the inflamed animals and the antinociceptive effects of morphine in control animals were significantly greater in males compared with females. Similarly, in males, the antihyperalgesic effects of morphine increased significantly at 7-21 days post-CFA; no significant shift in morphine potency was noted for females. These studies demonstrate sex-based differences in the effects of morphine on thermal hyperalgesia in a model of persistent inflammatory pain.     

Analgesic effects of dynorphin-A and morphine in mice

To investigate whether or not dynorphin-A is analgesic, the effect of this peptide was tested in comparison with that of morphine in mice. Dynorphin-A produced a potent analgesic effect in the acetic acid writhing and tail pinch tests, but a weak effect in the tail flick test when given by intracerebroventricular injection. In contrast, morphine caused a potent analgesia in all the tests. Dynorphin-A was more effective when given by intrathecal injection than by intracerebroventricular injection, whereas morphine was equipotent by both injection routes. The results suggest that dynorphin-A is analgesic and that its analgesia may be differentiated from that of morphine.     

Analgesic properties of a systemically-administered synthetic dipeptide of 5-hydroxytryptophan

Synthetic peptides of 5-hydroxytryptophan (5-HTP), including N-acetyl-5-HTP-5-HTP amide (5-HTP-ACETYL-DP), specifically inhibit the binding of serotonin to serotonin binding protein. 5-HTP-ACETYL-DP also produces a long-lasting, opiate-sensitive analgesia following central, but not systemic administration. The present study evaluated an apolar derivative of 5-HTP dipeptide, N-hexanoyl-5-HTP-5-HTP amide (5-HTP-HEX-DP), for its analgesic properties in rats following systemic administration. 5-HTP-HEX-DP (5–50 mg/kg) significantly increased jump thresholds in a dose-dependent manner with peak analgesia occurring at 2.5 hr after injection, and lasting up to 5 hr. In the tail-flick assay, 5-HTP-HEX-DP (20 mg/kg) produced a significant antinociceptive effect at 1 hr post-injection using both high and low intensity levels of radiant heat. While 5-HTP-HEX-DP and morphine each elicited analgesia following acute administration, chronic (14 days) incremental dosing with 5-HTP-HEX-DP or morphine resulted in persistent analgesia in 5-HTP-HEX-DP-treated animals, and a loss of analgesia in morphine-treated rats. Thus, significant tolerance to morphine, but not 5-HTP-HEX-DP analgesia developed using this protocol. Hence, 5-HTP-HEX-DP is a systemically-active analgesic which fails to develop tolerance when administered daily over 14 days.     

Analgesic activity of spiradoline mesylate (U-62,066E), a kappa opioid agonist in mice

The present study was undertaken to evaluate the analgesic potency of spiradoline mesylate, a k(kappa) opioid agonist, in comparison with that of morphine, by hot plate, tail-pinch and acetic acid-induced writhing assay. The ED50 values of spiradoline in hot plate, tail-pinch and acetic acid-induced writhing assay were 0.46, 0.26 and 0.20 mg/kg, respectively. The analgesic potency of spiradoline was 1.5-7.0 times higher than that of morphine. Repeated treatment with spiradoline as well as morphine developed tolerance to the analgesic effect in hot plate assay. In mice developed tolerance to one analgesic, response to the other analgesic did not alter compared to saline-treated mice. Single administration of spiradoline (1.5 and 3 mg/kg, s.c.) did not inhibit morphine-induced analgesia. These results suggest that spiradoline has more potent analgesic activity than morphine, presumably mediated through stimulation of receptors different from morphine.     

Constraints on the tailflick assay: morphine analgesia and tolerance are dependent upon locus of tail stimulation

In three experiments, the locus of tail stimulation in the tailflick assay was found to be an important parameter in determining morphine action. Rats were intravenously infused (Experiment I), injected with morphine subcutaneously (Experiment II), or implanted subcutaneously with morphine pellets (Experiment III). Analgesia was evaluated periodically following drug administration using the tailflick test and 3 adjacent 1 in. tail areas. In all three experiments, the distal tail section was more sensitive to the analgesic effects of morphine than more proximal sections. In Experiments I and III, tolerance to the effects of morphine developed more slowly at the distal tail location. These results indicate that the locus of stimulation in the tailflick assay can profoundly affect the development of analgesia and tolerance to morphine.     

Blockade of morphine supersensitivity by an antisense oligodeoxynucleotide targeting the delta opioid receptor (DOR-1)

An antisense oligodeoxynucleotide (ODN) targeting 20 bases of the coding sequence of the cloned delta opioid receptor (DOR-1), a mismatched ODN (different from the antisense ODN at 4 bases) or saline was administered to 3 groups of CD-1 mice implanted with naltrexone pellets (7.5 mg) for 7 days. Morphine supersensitivity (i.e., increased potency as defined by decreased morphine ED₅₀ values) was observed 24 h after pellet removal (day 8) in mice treated with saline or mismatch ODN, but not in antisense ODN treated mice. Antisense ODN alone had no effect on basal nociceptive thresholds or morphine analgesia but reduced the analgesic potency of the delta₂ opioid agonist [D-Ala²]deltorphin II. These data suggest that the delta₂ opioid receptor system participates in the adaptive changes contributing to increased morphine potency following chronic naltrexone treatment.     

Tolerance to morphine-induced analgesia in mice: Magnetic fields function as environmental specific cues and reduce tolerance development

Mice receiving daily injection of morphine (10 mg/kg) developed tolerance to morphine-induced analgesia, such that after 5–7 days of treatment their thermal response (paw licking) latencies in the hot plate test were indistinguishable from those of control animals. Exposure to a rotating magnetic field for thirty minutes before the daily morphine administrations significantly reduced the development of tolerance. These magnetic exposure also significantly increased over 7–10 days the basal nociceptive thresholds and paw licking response latencies of saline treated mice. Control and sham exposed mice that were fully tolerant to the analgesic effects of morphine failed to show any tolerance to morphine-induced analgesia when exposed to the magnetic stimuli prior to injection. Likewise, the partial tolerance to morphine shown by mice exposed to the rotating magnetic field pre-injection environmental cues was eliminated when control or sham pre-injection cues lacking the magnetic stimuli were provided. In all cases tolerance to morphine-induced analgesia was evident in the subsequent re-test with the original cues. These results indicate that magnetic field exposure can reduce the development of tolerance to the analgesic effects of morphine. They also show that magnetic stimuli function as significant environmental cues for the development of tolerance to morphine-induced analgesia. This suggests that magnetic stimuli affect both the associative (classical conditioning) and non-associative (physiological, pharmacological) mechanisms involved in the development of opiate tolerance.     

Ethanol-induced analgesia

The effect of ethanol (ET) on nociceptive sensitivity was evaluated using a new tail deflection response (TDR) method. The IP injection of ET (0.5-1.5 g/kg) produced rapid dose-dependent analgesia. Near maximal effect (97% decrease in TDR) was produced with the 1.5 g/kg dose of ET ten minutes after injection. At ninety minutes post-injection there was still significant analgesia. Depression of ET-induced nociceptive sensitivity was partially reversed by a 1 mg/kg dose of naloxone. On the other hand, morphine (0.5 or 5.0 mg/kg IP) did not modify ET-induced analgesia, while 3.0 minutes of cold water swim (known to produce non-opioid mediated analgesia) potentiated ET-induced analgesic effect. The 0.5 g/kg dose of ET by itself did not depress motor activity in an open field test, but prevented partially the depression in motor activity produced by cold water swim (CWS). Thus the potentiation by ET of the depression of the TDR produced by CWS cannot be ascribed to the depressant effects of ET on motor activity.     

Modification of the response to opioid and nonopioid drugs by chronic opioid antagonist treatment

Chronic exposure to opioid antagonists increases the analgesic actions of opioids such as morphine. In the present studies, morphine's analgesic potency was increased (supersensitivity) following an 8 day subcutaneous naltrexone implant in mice, but not following a 1 day implant. Supersensitivity was maximal 24hr following the 8 day implant and declined linearly and had returned to control levels by 120hr. Implantation of naltrexone pellets for 8 days was found to increase the relative analgesic potency of methadone by 120%, while the lethal potency of cocaine was slightly (19%), but significantly, decreased. In contrast, identical treatment did not alter the potency of the benzodiazepine alprazolam to induce ataxia.     

Tolerance to morphine analgesia and immobility measured in rats by changes in log-dose-response curves

Using the bar and tailflick tests, morphine log-dose-response (LDR) curves were determined for immobility and analgesia, respectively, in male Wistar rats following 15 daily i.p. injections of 0, 20, or 200 mg/kg of morphine sulfate. The LDR curves for the two measures were qualitatively similar. Chronic morphine treatment resulted in a shift to the right and flattening of both curves. These results indicate that the flattening of the LDR curve in morphine tolerant rats is general to a number of opiate effects, and raise the possibility that both morphine-produced immobility and analgesia are subserved, in part, by a similar mechanism. In addition, after a test dose of 900 mg/kg, more rats in the 20 mg/kg than in the 200 mg/kg treatment group died of convulsions. Thus, tolerance developed to lethality produced by the convulsive effects of opiates.     

Suppression of appetitive behavior in the rat by naloxone: lack of effect of prior morphine dependence

Naloxone (0.3–10 mg/kg) produced a dose-related suppression of eating and drinking in rats that had been deprived of food for 48 hr or water for 24 hr. The suppression of water intake by naloxone was unaltered in rats that had been physically dependent upon morphine one week earlier and which were tolerant to the analgesic effect of morphine at the time naloxone was tested. These results confirm the ability of naloxone to suppress appetitive behavior in the rat but do not resolve the issue of whether or not this effect of naloxone is the consequence of an interaction with an endogenous opioid system.     

Apparent pA2 value of naltrexone is not changed in rats following continuous exposure to morphine or naloxone.

Chronic opioid antagonist administration increases opioid binding sites and potentiates behavioral responses to morphine. Conversely, chronic opioid agonist administration attenuates behavioral responses to morphine, though this is not necessarily accompanied by a parallel loss of binding sites. We examined the possibility that the in vivo affinity of the mu receptors might be altered as a consequence of the continuous administration of either naloxone or morphine. Rats were implanted sc with naloxone- or morphine-filled osmotic pumps; control animals were implanted with sham pumps. One week later, 24 hr after removing the osmotic pumps, cumulative dose-response curves for fentanyl analgesia were generated in the presence of 0.0, 0.03, 0.1, or 0.3 mg/kg naltrexone, using a tail-flick procedure. The analgesic ED50 (with 95% C. L.) of fentanyl in sham implanted animals, following saline pretreatment was 0.027 mg/kg (0.019, 0.039). The potency of fentanyl was decreased in rats infused with morphine, ED50 = 0.051 mg/kg (0.028, 0.093), and increased in rats that received naloxone, ED50 = 0.018 mg/kg (0.015, 0.022). The mean apparent pA2 value for naltrexone (with 95% C.L.) in the control group was 7.7 (7.5, 7.9). No differences were detected in animals that had received either naloxone or morphine for 7 days, pA2 = 7.8 (7.5, 8.1) and 7.4 (7.3, 7.6), respectively. Our results indicate that there is no change in the apparent affinity of the mu-receptor following continuous exposure to either an opioid agonist or antagonist, at a time when the analgesic potency of the agonist is decreased or increased, respectively.     

Differential development of acute tolerance to analgesia, respiratory depression, gastrointestinal transit and hormone release in a morphine infusion model

To determine whether the differences in development of acute tolerance to several morphine actions correlate with the mu receptor subtype mediating them, we have examined the appearance of acute tolerance to analgesia, respiratory depression, gastrointestinal transit, and hormone release in an intravenous morphine infusion model. Analgesia, a naloxonazine-sensitive mu1 action, peaked at 2 hr after initiation of the infusions. The log dose-response relationship of the infusion rate to peak tailflick latency was linear from 10 to 50 micrograms/kg/min. By 8 hr, the tailflick latencies declined nearly to baseline levels, implying the rapid development of tolerance. Tolerance to morphine-induced prolactin release, another mu1 action, also developed rapidly over 8 hr. In contrast two mu2 actions, respiratory depression measured with arterial blood gas, determinations and gastrointestinal transit, showed no significant tolerance over a similar 8 hr infusion. We also observed no tolerance to morphine-induced growth hormone release, a non-mu1 action, over the same period. Thus, these results demonstrate that mu1 actions develop tolerance in an infusion model far more rapidly than a number of naloxonazine-insensitive (non-mu1) ones and may help explain differences in the rate of tolerance development to morphine actions.     

Sex chromosome complement affects nociception in tests of acute and chronic exposure to morphine in mice

We tested the role of sex chromosome complement and gonadal hormones in sex differences in several different paradigms measuring nociception and opioid analgesia using "four core genotypes" C57BL/6J mice. The genotypes include XX and XY gonadal males, and XX and XY gonadal females. Adult mice were gonadectomized and tested 3-4 weeks later, so that differences between sexes (mice with testes vs. ovaries) were attributable mainly to organizational effects of gonadal hormones, whereas differences between XX and XY mice were attributable to their complement of sex chromosomes. In Experiment 1 (hotplate test of acute morphine analgesia), XX mice of both gonadal sexes had significantly shorter hotplate baseline latencies prior to morphine than XY mice. In Experiment 2 (test of development of tolerance to morphine), mice were injected twice daily with 10 mg/kg morphine or saline for 6 days. Saline or the competitive NMDA antagonist CPP (3-(2-carboxypiperazin-4yl) propyl-1-phosphonic acid) (10 mg/kg) was co-injected. On day 7, mice were tested for hotplate latencies before and after administration of a challenge dose of morphine (10 mg/kg). XX mice showed shorter hotplate latencies than XY mice at baseline, and the XX-XY difference was greater following morphine. In Experiment 3, mice were injected with morphine (10 mg/kg) or saline, 15 min before intraplantar injection of formalin (5%/25 microl). XX mice licked their hindpaw more than XY mice within 5 min of formalin injection. The results indicate that X- or Y-linked genes have direct effects, not mediated by gonadal secretions, on sex differences in two different types of acute nociception.     

Potentiation of opioid analgesia by cocaine: the role of spinal and supraspinal receptors.

These studies examined the effect of cocaine on the analgesia produced by systemically and centrally administered opioid agonists. Cocaine (50 mg/kg, s.c.) increased the analgesic potency of systemic, ICV and IT morphine; and the ICV and IT analgesic effects of the delta selective peptide, [D-Pen2,D-Pen5]enkephalin (DPDPE). Cocaine also increased the analgesic potency of the mu selective ligand [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) administered ICV. However, cocaine did not alter the ED50 for IT DAGO. GC-MS studies indicated that brain cocaine concentration was approximately 3.0 micrograms/g wet weight 45 min following s.c. administration. These results suggest that cocaine-induced increases in opioid analgesic potency are mediated at brain mu and delta receptors and spinal mu receptors. Furthermore, there might be functional differences between spinal and supraspinal sites at which DAGO produces analgesia.     

Circadian-Rhythm-dependent Effects of L-NG-Nitroarginine Methyl Ester (L-Name) on Morphine-Induced Analgesia

Circadian changes in the interactions between L-N^G-nitroarginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, and morphine-induced antinociception were investigated by the mouse hot-plate test. Born the basal pain sensitivity and morphine-induced analgesia undergo significant 24h variations. L-NAME (40 mg/kg, ip) alone did not show any antinociceptive activity, but potentiated morphine-induced analgesia when combined with morphine at all injection times. In terms of percentage absolute potentiation (%AP), L-NAME dramatically augmented the analgesic effect of morphine in the late dark period at 19 hours after lights on (HALO). It is concluded that nitric oxide (NO) is involved in the modulation of the analgesic effect of morphine; thus, the L-NAME and morphine combination might be beneficial in alleviating pain.