Dopamine receptor mechanism(s) and antinociception and tolerance induced by swim stress in formalin test

In the present study, involvement of D1 and D2 dopamine receptors in the antinociception and tolerance induced by water swim stress in the formalin test has been investigated. Water swim stress at 20 degrees C temperature induced antinociception in both phases of the formalin test. Intraperitoneal administration of the D2 dopamine receptor antagonist, sulpiride (25 and 50 mg/kg) reduced swim stress-induced antinociception in the second phase of the formalin test. A higher dose of the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, intraperitoneal) also reduced swim stress-induced antinociception in both phases of the test. Exposure to 3 min water swimming stress, once daily for 3 days, induced tolerance to swim stress-induced antinociception in the second phase of the formalin test. Administration of sulpiride (12.5, 25 and 50 mg/kg), during exposure to water swimming stress (once daily for 3 days), decreased tolerance in the second phase, whereas the antagonist (12.5 and 50 mg/kg) increased pain scores in the first phase of the formalin test. Sulpiride (25 mg/kg) treatment however, once daily for 3 days with no water swimming stress, did not alter swim stress-induced antinociception (0.5, 1 and 3 min tests). Similarly, repeated treatment with SCH23390 (0.05 mg/kg) and water swimming stress did not alter tolerance induced by water swimming stress. Repeated administration of the antagonist in the absence of water swimming stress also did not change swim stress-induced antinociception. The results may indicate a possible involvement of both dopamine D1 and D2 receptors in the antinociception induced by swim stress and D2 receptor mechanism in the tolerance induced by repeated swim stress.     

Baclofen-induced antinociception and nicotinic receptor mechanism(s)

In this study, the influences of nicotinic receptor agents on baclofen-induced antinociception in the tail-flick test have been studied. Intraperitoneal administration of baclofen (2.5, 5 and 10 mg/kg) to mice induced a dose-dependent antinociception in the tail-flick test. Subcutaneous injection of nicotine (0.5-2.5 mg/kg) also caused a dose-dependent antinociceptive response. Intracerebral (10 and 20 microg/mouse) but not intraperitoneal administration of hexamethonium (5 and 10 mg/kg) to mice decreased the response of both nicotine and baclofen. However, administration of the GABA(B) antagonist CGP 35348 (100 and 200 mg/kg) decreased the response induced by baclofen but not by nicotine. It is concluded that at least part of the baclofen-induced antinociception may be mediated through a nicotinic mechanism.     

Dopamine receptor knockout mice]

Dopaminergic systems are transmitted by dopamine receptors which couple to GTP binding proteins. Five subtypes of dopamine receptors were so far cloned. To study the functions of each dopamine receptors, mice lacking each of D1R, D2R, D3R and D4R dopamine receptors have been generated. Histological analyses of D1R knockout mice indicated that the expression of dynorphin is reduced in the striatum. In contrast with wild-type mice, D1R knockout mice exhibit a dose dependent decrease in locomotion. D2R knockout mice display a hypoactivity. The expression of enkephalin mRNA in the striatum is increased in the D2R knockout mice. D2R knockout mice showed hyperplastic changes of intermediate lobe of the pituitary and the increased expression of POMC in the pituitary in D2R knockout mice. D3R knockout mice are more active than wild-type mice in a novel environment and they exhibit enhanced behavioral sensitivity to cocaine and amphetamine. D4R knockout mice show a hypoactivity, but they display locomotor supersensitivity to ethanol, cocaine, and methamphetamine. Dopamine synthesis and its conversion to DOPAC are elevated in the dorsal striatum from D4R knockout mice.     

Dopamine receptor 3 (D3) knockout mice show regular emotional behaviour

The "Dopamine Deficiency Hypothesis" and a considerable number of recent pharmacological studies propose to thoroughly verify and improve the standard of knowledge regarding a possible role of dopamine in the pathogenesis and treatment of depression. To elucidate the potential role of D3 receptors in the development of a depressive-like phenotype, we subjected D3 receptor knockout mice to a series of selected behavioural paradigms particularly focussing on depressive-like traits. In our experimental design we exposed animals with a deficiency of the D3 receptor (D3-/-) to a standardised behavioural test battery, in which general changes of locomotion, exploration, anxiety and depressive-like characteristics (i.e. Learned Helplessness, Sucrose Consumption and Forced Swim test) can be detected. Thorough behavioural phenotyping, however, neither revealed behavioural consequences on the basal level (locomotion, exploration) nor depressive- or anxiety-like alterations. Hence, these results do not indicate an evident involvement of the D3 receptor in the development of a depression-like syndrome in mice for now. This does, however, not exclude the D3 receptor as a potential target for pharmacotherapy.     

Attenuation of morphine tolerance and dependence by aminoguanidine in mice

The effect of aminoguanidine, an inducible nitric oxide synthase (iNOS) inhibitor, on morphine-induced tolerance and dependence in mice was investigated in this study. Acute administration of aminoguanidine (20 mg/kg, p.o.) did not affect the antinociceptive effect of morphine (10 mg/kg, s.c.) as measured by the hot plate test. Repeated administration of aminoguanidine along with morphine attenuated the development of tolerance to the antinociceptive effect of morphine. Also, the development of morphine dependence as assessed by naloxone-precipitated withdrawal manifestations was reduced by co-administration of aminoguanidine. The effect of aminoguanidine on naloxone-precipitated withdrawal was enhanced by concurrent administration of the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, dizocilpine (0.25 mg/kg, i.p.) or the non-specific nitric oxide synthase (NOS) inhibitor, l-N(G)-nitroarginine methyl ester (l-NAME; 5 mg/kg, i.p.) and antagonized by concurrent administration of the nitric oxide (NO) precursor, l-arginine (50 mg/kg, p.o.). Concomitantly, the progressive increase in NO production, but not in brain glutamate level, induced by morphine was inhibited by repeated administration of aminoguanidine along with morphine. Similarly, co-administration of aminoguanidine inhibited naloxone-induced NO overproduction, but it did not inhibit naloxone-induced elevation of brain glutamate level in morphine-dependent mice. The effect of aminoguanidine on naloxone-induced NO overproduction was potentiated by concurrent administration of dizocilpine or l-NAME and antagonized by concurrent administration of l-arginine. These results provide evidence that blockade of NO overproduction, the consequence of NMDA receptor activation, by aminoguanidine, via inhibition of iNOS, can attenuate the development of morphine tolerance and dependence.     

The offset of morphine tolerance in rats and mice.

1. In rats and mice made tolerant to morphine by pretreatment with the drug, the shift to the right of the log dose/analgesic response line for in naive animals occurs without significant change in slope provided that sufficient time is allowed for elimination of pretreatment drug. 2. Responsiveness to the analgesic effects of morphine, given together with cycloheximide to prevent reinforcement of tolerance, was measured in rats (paw pressure method) and mice (hot plate method) at intervals during 1-23 days following cessation of a variety of regimens of tolerance-inducing drug treatments. 3. A biphasic pattern of recovery of responsiveness was observed, which was independent of the regimen or the drug (morphine, methadone or diamorphine) used to induce tolerance. Estimates of the rates of the first, fast phase are imprecise but the rate of the second phase of offset, from 4th day after cessation of pretreatment had, in rats, a mean half-time of 13.2 plus or minus 0.53 days-for all pretreatments combined, there being no significant differences between the various pretreatment regimens employed. In mice, similarly, a biphasic recovery of analgesic responsiveness was seen after morphine pretreatment, the mean half-time of the slower phase being 17.4 days. 4. Precipitation of an acute withdrawal syndrome in rats by naloxone HCl given 6 h after the final injection of a tolerance-inducing treatment with morphine did not affect the subsequent rate of recovery from tolerance. 5. During the period following a tolerance-inducing pretreatment with morphine in mice, the rate of attenuation of the naloxone-evoked jumping response was faster than the rate of offset of tolerance.     

Effects of short-acting NMDA receptor antagonist MRZ 2/576 on morphine tolerance development in mice

The present study sought to evaluate the ability of a short-acting glycineB site NMDA receptor antagonist, MRZ 2/576, to affect morphine tolerance development in mice. It was found that MRZ 2/576 (10 mg/kg, i.p.) significantly retarded development of morphine analgesic tolerance (20 mg/kg, s.c., 8 days, once a day; tail-flick test) when administered 120 min or 150 min after each daily morphine injection. MRZ 2/576 did not affect the development of morphine tolerance when administered immediately, 15, 30, 60, 90, 180, 240, 300 or 360 min after the daily morphine injections. Thus, short-acting NMDA receptor antagonists may be useful in exploring the temporal characteristics of opioid tolerance (i.e., periods after morphine injection that are critical for tolerance induction) and the present study suggests that after morphine administration there is a period of NMDA receptors activation crucial for the development of tolerance.     

Quantitative assessment of tolerance to and dependence on morphine in mice

Summary The actions of 3,4-diaminopyridine, 4-aminopyridine and tetraethylammonium were studied on the chick biventer cervicis muscle preparation. All three compounds produced a greater augmentation of indirectly elicited twitches than of directly elicited twitches. The compounds did not restore transmission in OmMCa²⁺ solutions but rather produced contractures that were inhibited by acetylcholine receptor antagonists. The compounds restored twitch height in one-tenth normal Ca²⁺ solutions and induced spontaneous muscle twitching. The compounds reversed dantrolene-induced block of directly elicited twitches.Interactions between tetraethylammonium and 3,4-diaminopyridine were also studied. In indirectly stimulated preparations, the combined effects of the two compounds were more than additive at one concentration level only. In directly stimulated preparations, the effects of 3,4-diaminopyridine were greatly enhanced by tetraethylammonium pretreatment. 3,4-Diaminopyridine pretreatment produced less synergism than tetraethylammonium pretreatment.It is concluded that the actions of the aminopyridines and tetraethylammonium on transmitter release and muscle contractility are essentially similar. These actions are postulated to arise from an inhibitory action on potassium conductance and on ability to release calcium from nerve and muscle membranes. On the basis of the interaction studies, it is suggested that the compounds possess different binding capacities for two different sites on the potassium conducting channel.     

Some relations between tolerance and physical dependence to morphine in mice

Tolerance to morphine analgesia and precipatated physical dependence were studied in mice under different conditions. There was a gradual loss of tolerance during the continous absorption of morphine from a pellet. Tolerance was decreased by nalorphine during morphine absorption. An attenuated physical dependence was observed two or four days after a single dose of morphine. In animals previously treated with pellets of morphine, single doses of morphine induced less tolerance than in mice that had never been implanted with pellets; in both cases cycloheximide prevented development of tolerance. Tolerance persisted for more than 20 days after absorption of the morphine pellet. These results reinforce the hypothesis that tolerance and physical dependence are produced by a similar mechanism and that an inhibitory process of tolerance exists.     

Protective effect of bupropion on morphine tolerance and dependence in mice

Possible reversal of morphine-induced tolerance and dependence by bupropion was studied in mice. A 10-day repeated injection regimen was followed to induce morphine tolerance and dependence. Bupropion (2 and 5 mg/kg) per se, when chronically administered for 9 days, failed to produce any significant change in tail-flick latency compared with the control mice. Chronic administration of bupropion (2 or 5 mg/kg) during the induction phase (days 1-9) delayed the development of tolerance to the antinociceptive action of morphine and also reversed naloxone- (2 mg/kg) precipitated withdrawal jumps. On the other hand, acute administration of bupropion (2 or 5 mg/kg) on day 10, i.e., during the expression phase of morphine dependence, reduced the incidence of naloxone-precipitated withdrawal jumps without affecting tolerance to the analgesic effect. In conclusion, results of the present study suggest the potential use of bupropion in tolerance and dependence.     

GABA(B) receptor mechanism and imipramine-induced antinociception in ligated and non-ligated mice

This study concerned the effects of GABA(B) receptor agents on imipramine-induced antinociception in ligated and non-ligated mice in hot-plate test. The data showed that different doses of morphine (3, 6 and 9 mg/kg) induced a dose-dependent antinociception in non-ligated or ligated mice. However, the opioid response was decreased in the ligated animals. Intracerebroventricular (i.c.v.) administration of imipramine (5, 10, 20 and 40 microg/mouse) did not induce antinociception in either non-ligated or ligated mice. However, the response induced in the ligated mice was less than that induced in the non-ligated animals. Intraperitoneal (i.p.) administration of imipramine (10, 20, 30 and 40 mg/kg) induced antinociception in both ligated and non-ligated animals. The responses to the drug were not significantly different in the two groups. Administration of baclofen either i.c.v. (0.125, 0.25 and 0. 5 microg/mouse) or i.p. (0.5, 1, 2 and 4 mg/kg) induced antinociception. The response to the drug was not significantly different in ligated and non-ligated mice. I.c.v. administration of a lower dose of baclofen (0.125 microg/mouse) with different doses of imipramine (2.5, 5 and 10 mg/kg) potentiates the response of imipramine. This effect was reduced by i.c.v. injection of GABA(B) receptor antagonist, CGP35348 [P-(3-aminopropyl)-p-diethoxymethyl-phosphinic acid] (20 microg/mouse). The higher dose of antagonist (20 microg/mouse) also decreased the response induced by baclofen or imipramine. CGP35348 itself (2.5, 5, 10 and 20 microg/mouse) induced dose-dependent antinociception with no significant difference in the ligated and non-ligated mice. It is concluded that a GABA receptor mechanism(s) may modulate the antidepressant-induced antinociception.     

Inhibition by ginseng total saponin of the development of morphine reverse tolerance and dopamine receptor supersensitivity in mice

• 1.1. Ginseng total saponin (GTS), 200 mg/kg i.p. 3 hr prior to morphine, inhibited the development of reverse tolerance to the ambulatory-accelerating effect of morphine.
• 2.2. GTS, 200 mg/kg, also prevented the development of dopamine receptor supersensitivity induced by the chronic administration of morphine, 10 mg/kg a day for 7 days.
• 3.3. These results suggest that GTS may be useful for the prevention and therapy of the adverse action of morphine.

     

The N-Methyl-D-aspartate receptor antagonist dizocilpine inhibits associative antinociceptive tolerance to morphine in mice: relation with memory

I and coauthor previously reported the memory facilitation effect of morphine. The main purpose of this study was to evaluate the involvement of the N-methyl-D-aspartate (NMDA) receptor in associative tolerance to morphine by a contextual procedure. Antinociceptive response latency was measured by the tail-pinch method during repeated morphine (5 mg/kg, s.c.) injection for four consecutive days with pretreatment by dizocilpine (0.01, 0.05, 0.1 mg/kg, i.p.) at 30 min prior to morphine injection in the training phase and before and after morphine injection in the test phase. The nociceptive response latency was shortened by the single administration of dizocilpine (0.05 to 0.25 mg/kg, i.p.). Pretreatment by dizocilpine at 0.05 or 0.1 mg/kg weakened the antinociception to morphine on Day 1, but decreased the tolerance throughout the training phase. In the test phase, the animals were allocated into the same and different contexts. In the test phase, hyperalgesia before morphine injection in the same context and antinociception after morphine injection in the different context were evident in the saline-pretreated group in the training phase, but they were not observed in those contexts in the dizocilpine-pretreated groups. These results suggest that memory dysfunction with dizocilpine inhibits the recovery of associative tolerance to morphine by contextual change.     

Attenuation of morphine tolerance development by electroconvulsive shock in mice

The development of tolerance to morphine has been proposed as being analogous to learning or memory, since the presentation of a novel stimulus (morphine) to an organism results in an altered response on subsequent presentations of the same stimulus. Electroconvulsive shock has been widely used to disrupt memory in animals and its effect on the development of tolerance to morphine was examined. Pretreatment of mice with six, progressively increasing intraperitoneal doses of morphine induced tolerance to an excitant action of morphine, shown by a marked reduction in the locomotor activity elicited by a subsequent intraperitoneal test dose of morphine. In three separate experiments, the administration of eleetroconvulsive shock 2–3 hr after each pretreatment with morphine attenuated the degree of tolerance developed. The eleetroconvulsive shock had no significant effect on the locomotor activity of non-tolerant mice tested under similar conditions in each experiment. In a single pilot study, electroconvulsive shock was found to reduce the frequency of jumping precipitated by an intraperitoneal dose of naloxone, suggesting a possible decrease in the degree of dependence. The present results are interpreted as supporting the view that the mechanisms of morphine tolerance may be similar to those involved in learning or memory.     

Dopamine receptor subsensitivity in the substantia nigra after chronic morphine treatment in rats

Several classes of 5-HT and dopamine (DA) receptor binding sites, and the levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), DA and dihydroxyphenylacetic acid (DOPAC) were examined in various brain regions 24 h after a 10-day treatment with morphine (2 X 15 mg/kg s.c. daily) in adult rats. Quantitative autoradiography of receptor binding sites revealed only a significant reduction of [3H]SCH-23390 and [3H]spiperone binding to D-1 and D-2 sites, respectively, in the substantia nigra pars compacta of morphine-treated rats. 5-HT and 5-HIAA levels remained unchanged in the substantia nigra and striatum, but the nigral levels of DA and DOPAC and the levels of DOPAC in the striatum were reduced significantly by morphine treatment. Apomorphine, at a dose (0.05 mg/kg s.c.) known to stimulate DA autoreceptors, decreased nigral and striatal DOPAC levels in controls but not in morphine-treated rats. It is concluded that chronic morphine treatment probably induces a down regulation of nigral D-1 and D-2 binding sites and reduces the negative feed-back mechanisms triggered by DA autoreceptors.     

Clinically available NMDA receptor antagonists memantine and dextromethorphan reverse existing tolerance to the antinociceptive effects of morphine in mice

The tail-flick test was used to investigate the effects of chronic administration of the N-methyl-D-aspartate (NMDA) receptor antagonists, dextromethorphan, memantine and MRZ 2/579, on the development and reversal of morphine tolerance in mice in three separate experiments. Experiment 1 investigated the effects of NMDA receptor antagonists on the development of tolerance. Morphine (10 mg/kg for 6 days, twice daily) produced a 5.9-fold rightward shift of the cumulative dose-response curves. Co-administration of dextromethorphan, memantine or MRZ 2/579 between tests 1 and 2 dose-dependently (5-10 mg/kg) inhibited the development of morphine tolerance. In experiment 2, in which the effects on the reversal were investigated, morphine-tolerant mice were treated b.i.d. for an additional 6 days (between tests 2 and 3) with vehicle+vehicle, NMDA receptor antagonist+vehicle, vehicle+morphine or NMDA receptor antagonist+morphine. Morphine-tolerant mice treated with vehicle+vehicle remained morphine tolerant, whereas this residual morphine tolerance was inhibited by administration of all three NMDA antagonists (each 10 mg/kg). Morphine-tolerant mice receiving vehicle+morphine injections demonstrated an unchanged degree of antinociceptive tolerance. In these mice, the co-administration of memantine and MRZ 2/579, but not dextromethorphan, resulted in the reversal of morphine tolerance. In experiment 3, memantine and MRZ 2/579 (10 mg/kg) inhibited the acute antinociceptive effect of morphine, but dextromethorphan did not. These data indicate that low-affinity, clinically available and/or therapeutically promising NMDA receptor antagonists may be used to inhibit ongoing morphine tolerance.     

Effects of acute and chronic morphine administration on glucose tolerance in mice

The effects of acute and chronic morphine treatment on glucose tolerance were investigated in mice. In acute experiments, a single dose of morphine (20 mg/kg i.p.) increased the serum and muscle glucose level. After glucose loading (1.5 g/kg), the rate of increase and the peak of serum glucose concentration were significantly lowered in morphine-treated mice, while the availability and the half-life of glucose were similar to those of controls. In morphine-dependent mice, the fasting serum and muscle glucose levels were similar to those of control but the liver glucose was significantly greater. After glucose loading the rate of increase in serum glucose level was faster and the availability of glucose was 10% greater than that in naive mice. Again, there was no difference in the half-life of serum glucose between naive and morphine-dependent mice.     

Peripherally acting NMDA receptor/glycineB site receptor antagonists inhibit morphine tolerance

The present study focused on the role of peripheral ionotropic N-methyl-D-aspartate (NMDA) receptors in the development of tolerance to morphine-induced antinociception. An initial experiment revealed that NMDA channel blocker memantine, and NMDA receptor/glycine(B) site antagonist MRZ 2/576 inhibited maximal electroshock-induced convulsions (MES) in female NMR mice with respective potency of 5.93 and 20.8 mg/kg, while other NMDA receptor/glycine(B) site antagonists MRZ 2/596 and MDL 105,519 were ineffective, supporting lack of CNS activity of the latter two agents. This observation was also supported by blood-brain barrier experiments in vitro. In male Swiss mice, morphine (10 mg/kg) given for 6 days twice a day (b.i.d.) produced tolerance to its antinociceptive effects in the tail-flick test. The NMDA receptor/glycine(B) site antagonists, MRZ 2/576 at 0.03, 0.1, 0.3 mg/kg and MRZ 2/596 at 0.1, 0.3, 3 and 10 mg/kg attenuated the development of morphine tolerance. Similarly, in male C57/Bl mice, morphine (10 mg/kg) given for 6 days b.i.d. produced tolerance to its antinociceptive effects in the tail-flick test. Like in Swiss mice, in C57/Bl mice morphine tolerance was attenuated by both MRZ 2/576 and MRZ 2/596. Another NMDA receptor/glycine(B) site receptor antagonist, MDL 105,519 (that very weakly penetrates to the central nervous system) also inhibited morphine tolerance at the dose of 1 but not 0.1 mg/kg. Moreover, both naloxone hydrochloride (5 and 50 mg/kg) and centrally inactive naloxone methiodide (50mg/kg) inhibited morphine tolerance suggesting the involvement of peripheral opioid receptors in this phenomenon. The present data suggest that blockade of NMDA receptor/glycine(B) sites in the periphery may attenuate tolerance to the antinociceptive effects of morphine.