Comparative effects of Pro-Leu-Gly-NH2 and cyclo(Leu-Gly) administered orally on the development of tolerance to the analgesic effect of morphine in the rat

Comparative effects of Pro-Leu-Gly-NH2 (MIF) and cyclo(Leu-Gly) (CLG) administered orally at different stages of chronic morphine treatment on the development of tolerance to the analgesic effect of morphine in the rat were determined. Male Sprague-Dawley rats were implanted with either 6 placebo or morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of a high degree of tolerance as evidenced by a decrease in the analgesic response to morphine. Administration of CLG (8 and 16 mg/kg/day) on day 5, 6 and 7 of implantation inhibited the development of tolerance to morphine but 4 and 32 mg/kg doses had no effect. Further, CLG (2 mg/kg/day for 7 days) inhibited the development of tolerance but higher doses (4 and 8 mg/kg) had no effect. MIF (26 and 52 mg/kg) administered orally on the last three days of the implantation schedule inhibited the development of tolerance to morphine. MIF (6.5 mg/kg/day for 7 days) inhibited the development of tolerance but the higher doses had no effect. Concurrent administration of MIF (6.5 mg/kg) and CLG (2 mg/kg) for seven days failed to inhibit the development of tolerance. A single dose of MIF or CLG administered a day before the assessment of tolerance did not affect the morphine tolerance. Thus, even after a significant degree of tolerance to morphine had developed, neuropeptides like MIF and CLG given orally, in appropriate doses, can inhibit development of tolerance to morphine and restore the analgesic effect of morphine.     

Intragastric administration of cyclo(Leu-Gly) inhibits the development of tolerance to the analgesic effect of morphine in the rat

The effect of intragastric administration of cyclo(Leu-Gly), a cyclic dipeptide derived from melanotropin release inhibiting factor (Pro-Leu-Gly-NH2), on the development of tolerance to the analgesic effect of morphine in the rat was determined. The tolerance to morphine in the rat was induced by subcutaneous implantation of four morphine pellets during a 3-day period. The rats which served as controls were implanted with placebo pellets. The analgesic response to a challenge dose of morphine was determined by the tail-flick test. The tail-flick latencies were determined before and then every 30 min for 180 min. The analgesic response was computed by determining the area under the time-response curve. Implantation of morphine pellets resulted in the development of tolerance as evidenced by decreased analgesic response to morphine in morphine pellet implanted rats as compared to placebo pellet implanted rats. Chronic intragastric administration of cyclo(Leu-Gly) (4 to 16 mg/kg) inhibited the development of tolerance to morphine. A dose of 8 mg/kg of cyclo(Leu-Gly) completely blocked the tolerance to morphine. The study provides for the first time evidence that intragastric administration of a cyclic peptide can inhibit the development of tolerance to morphine, and that effective neuropeptides and their analogs can be developed as potential drugs to inhibit opiate-induced tolerance.     

Orexin receptor type-1 antagonist SB-334867 inhibits the development of morphine analgesic tolerance in rats

Herein the effect of orexin receptor type-1 antagonist SB-334867 on the development of tolerance to analgesic effects of morphine was studied in rats. To incite tolerance, morphine sulfate was injected intraperitoneally (i.p., 10mg/kg) once a day for 7 days. The tail flick test was used to evaluate antinociceptive effects of the morphine. A selective OxR1 receptor antagonist, SB-334867, was microinjected (i.c.v.) into the right cerebral ventricle (10 μg/10 μl) immediately before each morphine injection. Repeated morphine application resulted in tolerance to morphine analgesic effects as a decreasing trend during 7 days. Also, repeated administration of SB-334867 (i.c.v.) alone was without significant effect on the nociception as compared to control. Microinjection of SB-334867 prior to each morphine injection inhibited the development of tolerance, so that the analgesic effects of morphine were significantly higher in SB-334867 plus morphine treated rats than that of vehicle plus morphine treated ones on days 4-7. It is concluded that orexin receptor type-1 might be involved in the development of tolerance to morphine analgesic effects.     

Effects of fluoxetine and LY 367265 on tolerance to the analgesic effect of morphine in rats

Morphine is widely used to treat chronic pain, however its utility is hindered by the development of tolerance to its analgesic effects. The aim of this study was to investigate effects of fluoxetine, a specific serotonin (5-HT) reuptake inhibitor, and LY 367265, an inhibitor of the 5-HT transporter and 5-HT2A receptor antagonist, on tolerance induced to the analgesic effect of morphine in rats. The study was carried out on male Wistar Albino rats (weighing 170-190 g). To constitute morphine tolerance, animals received morphine (50 mg/kg; s.c.) once daily for 3 days. After last dose of morphine, injected on day 4, morphine tolerance was evaluated. The analgesic effects of fluoxetine (10 mg/ kg; i.p.), LY 367265 (3 mg/kg; i.p.) and morphine were considered at 30-min intervals by tail-flick and hot-plate tests. The results showed that fluoxetine and LY 367265 significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effects were obtained 30 min after administration of fluoxetine and 60 min after administration of LY 367265. In conclusion, we observed that co-injection of morphine with fluoxetine and LY 367265 increased the analgesic effects of morphine and delayed development of tolerance to morphine analgesia.     

Prevention of development of tolerance and dependence to opiate in mice by BR-16A (Mentat), a herbal psychotropic preparation.

Chronic treatment with BR-16A (20-500 mg/kg) followed by saline on days 1 to 9 failed to produce any significant change in tail-flick latency from the saline-pretreated group in mice. Repeated administration of BR-16A(20-500 mg/kg) for 9 days however, attenuated the development of tolerance to the analgesic effect of morphine (10 mg/kg). BR-16A (20-500 mg/kg) also suppressed, in a dose-dependent manner, the development of morphine dependence as assessed by naloxone (2 mg/kg)-precipitated withdrawal on day 10 of testing.     

The effects of a hypothalamic peptide factor,MIF and its cyclic analog on tolerance to haloperidol in the rat

The effects of the hypothalamic peptide, ProLeuGlyNH₂ (MIF) and its analog, cyclo (LeuGly) (CLG) on the development of tolerance to haloperidol were investigated in male Sprague-Dawley rats. Chronic oral administration of haloperidol (1.5 mg/kg/day) for 21 days resulted in the development of tolerance to its pharmacological effects. A dose of haloperidol (3 mg/kg ip) exhibited an absence of cataleptic as well as hypothermic response in chronically haloperidol treated rats. Subcutaneous administration of MIF or CLG (2 mg/kg each) daily one hour prior to haloperidol injection blocked the haloperidol-induced tolerance as evidenced by the appearance of both the cataleptic and hypothermic responses. It is concluded that the hypothalamic peptide hormone, MIF may be important in regulating chronic effects of neuroleptic drugs.     

Short-term tolerance to morphine: effects of diazepam, phenobarbital, and amphetamine

Short-term tolerance to morphine, which can be demonstrated in as little as 3 hours after a single administration of the opiate, was examined in animals chronically pretreated with diazepam, phenobarbital, or amphetamine. Tail-flick latency in mice and changes in plasma corticosterone in rats were the parameters tested in these experiments. Rats primed with either saline or morphine, 10 mg/kg, were injected 3 hours subsequently with morphine, 5 mg/kg. Those primed with saline showed the characteristic plasma corticosterone elevation following morphine, when serial blood samples were examined, whereas those previously treated with morphine did not. Mice were primed with saline or either of two doses of morphine, 30 or 100 mg/kg, 3.5 hours prior to estimation of tail-flick latency and ED50 determinations. Mice primed with either dose of morphine had significantly higher ED50's than those primed with saline. Chronic treatment with diazepam or amphetamine in either species did not significantly alter short-term tolerance development by either parameter. However, with phenobarbital pretreatment, the plasma corticosterone response was attenuated and short-term tolerance to morphine's analgesic effects did not occur. Further studies in morphine-pelleted mice showed that analgesic tolerance occurred similarly in all groups. This suggests that barbiturates may delay the process.     

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.     

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.     

The involvement of midbrain astrocyte in the development of morphine tolerance

Aims

Systemic administration of opiate analgesics such as morphine remains the most effective treatment for alleviating severe pain across a range of conditions including acute pain. However, chronic or repeated administration of opiate analgesics results in the development of analgesic tolerance. Glial cells such as microglia and astrocytes are known to release various inflammatory cytokines and neurotrophic factors leading to regulation of neuronal function. Recently, glial cells were reported to play important roles in the development of analgesic tolerance to morphine. Here, we focused on the involvement of midbrain glial cells, particularly astrocytes, in the development of analgesic tolerance to morphine.

Main methods

Mice were treated with morphine (10 mg/kg, s.c.) or vehicle once a day for 5 days. Pentoxifylline (an inhibitor of glial activation; 20 mg/kg, i.p. or 50 and 100 μg/mouse, i.c.v.) was administered 30 min before morphine treatment. Flavopiridol (a cyclin-dependent kinase inhibitor; 5 nmol/mouse, i.c.v.) was administered 10 min before and 10 h after morphine treatment. The analgesic effect of morphine was measured using the tail flick method.

Key findings

The development of analgesic tolerance to morphine was gradually observed during daily treatment of morphine for 5 days in mice. On days 1 and 3 after repeated morphine treatment, astrocyte marker glial fibrillary acidic protein expression levels were significantly increased, as determined by western blot analyses. These phenomena were significantly inhibited following pre-treatment with pentoxifylline or flavopiridol.

Significance

We demonstrated that midbrain astrocytes play an important role in the development of analgesic tolerance to morphine.     

Effects of morphine and beta-endorphin on basal and elevated plasma levels of alpha-MSH and vasopressin.

Morphine induced an increase of plasma α-MSH levels and a decrease of AVP levels after peripheral or intracerebroventricular administration. This increase of α-MSH levels and decrease of AVP levels after morphine treatment was observed in non-stimulated animals as well as in rats in which the hormone levels were elevated by water deprivation or by administration of hypertonic saline. These latter effects of morphine on plasma levels of α-MSH and AVP could be blocked by simultaneous administration of naltrexone.β-Endorphin also increased plasma α-MSH levels and lowered plasma AVP levels. From these effects only the increase of the plasma α-MSH level and not the decrease of plasma AVP could be blocked by naltrexone. Moreover PLG treatment was ineffective with respect to the endorphin-induced decrease in plasma AVP, but it partly blocked the increase of plasma α-MSH when this tripeptide was given in combination with β-endorphin.     

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.     

Modification of morphine-induced change in striatal (3H)-spiroperidol binding and stereotype behavior by cyclo(Leu-Gly)

Recent reports from our laboratories have indicated that the peptide cyclo(Leu-Gly), an analog of MIF (Pro-Leu-Gly-NH₂), administered prior to chronic exposure to morphine, prevents the development of both analgesic tolerance and some signs of physical dependence. The same peptide treatment also prevented the development of morphine-induced increases in certain behavioral responses to the dopamine agonist apomorphine. The present study investigated behavioral (stereotypy) and neurochemical receptor changes (specific (³H)-spiroperidol binding) occuring in the rat striatal dopamine (DA) system following chronic morphine treatment with and without prior cyclo(Leu-Gly)administration. While chronic morphine treatment (s.c. 5 pellet implant for 3 days, each pellet contained 65 mg morphine free base) did not alter the total number of high-affinity striatal (³H)-spiroperidol binding sites (28 fmol/mg tissue), it did increase the affinity of the receptor for the ligand (K_D decreased from 40 to 24 pM). Cyclo(Leu-Gly) (8 mg/kg) prevented the morphine induced increase in dopamine receptor affinity. In parallel, cyclo(Leu-Gly) prevented the increase in apomorphine-induced stereotypy which was observed in chronic morphine treated rats. The peptide alone did not alter any of the binding characteristics. These data suggest that the ability of the peptide to block the development of physical dependence induced by morphine may involve the ability of the peptide to interfere with morphine-induced changes in dopaminergic systems.     

The effects of pituitary adenylate cyclase-activating polypeptide on acute and chronic morphine actions in mice

The effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on pain sensitivity, on morphine analgesia, on morphine tolerance and withdrawal were investigated in mice. The heat-radiant tail-flick test was used to assess antinociceptive threshold. Intracerebroventricular (i.c.v.) administration of PACAP alone had no effect on pain sensitivity but in a dose of 500 ng, it significantly diminished the analgesic effect of a single dose of morphine (2.25 mg/kg, s.c.). PACAP (500 ng, i.c.v.) significantly increased the chronic tolerance to morphine and enhanced the naloxone (1 mg/kg, s.c.)-precipitated withdrawal jumping. Theophylline (1 mg/kg, i.p.) pretreatment significantly enhanced the effect of PACAP on morphine analgesia but the effects of PACAP on tolerance and withdrawal were unaffected upon theophylline administration. On the grounds of our previous studies with vasoactive intestinal polypeptide (VIP), it appears that different receptors are involved in the effects of PACAP in acute and chronic morphine actions. Our results indicate that PACAP-induced actions likely participate in acute and chronic effects of morphine and suggest a potential role of PACAP in opioid analgesia, tolerance and withdrawal.     

Long-term treatment with SR141716A, the CB1 receptor antagonist, influences morphine withdrawal syndrome

The role of the cannabinoid system in morphine withdrawal was examined through long-term CB1 receptor antagonist administration in morphine pellet implanted rats. SR141716A chronic treatment (5mg/kg i.p. twice a day for four days) did not influence the development of tolerance to the morphine analgesic effect but significantly reduced the intensity of naloxone-induced opiate withdrawal in tolerant rats: Specifically there was a significant reduction in the number of digging, teeth chattering and penile licking and the incidence of diarrhoea while other signs such as writhing, head dog shakes and rearing were unaffected. These results suggest that the pharmacological treatment with SR141716A could be of some interest in ameliorating opiate withdrawal syndrome.     

Dynorphin (1-13): analgesia, hypothermia, cross-tolerance with morphine and beta-endorphin

Intracerebroventricular administration of 20, 40 and 60 nmol of dynorphin (1-13) produced analgesia, as assessed by flinch/jump response to footshock, and hypothermia in the rat. Rats developed tolerance to both the analgesic and thermic effects of the 20 nmol dose of dynorphin. Dynorphin and beta-endorphin showed cross-tolerance with respect to their analgesic but not their thermic effects. Dynorphin and morphine also produced cross-tolerant analgesic effects. Naloxone (10 mg/kg, IP) completely blocked the barrel rolling produced by 20 nmol dynorphin but did not alter its analgesic or thermic effects.     

Finasteride, a 5alpha-reductase inhibitor, potentiates antinociceptive effects of morphine, prevents the development of morphine tolerance and attenuates abstinence behavior in the rat

It has been shown that morphine increases 5alpha-reductase enzyme activity in the rat central nervous system; however importance of this finding on morphine analgesia, tolerance and dependence has not been reported. In the present study, we investigated inhibition of 5alpha-reductase enzyme on morphine effects using finasteride. To determine whether the 5alpha-reductase enzyme interact with morphine analgesia, finasteride (5 mg/kg, i.p.) was administrated with morphine (5 and 7 mg/kg, i.p.). The tail-flick test was used to assess the nociceptive threshold, before and 15, 30, 45, 60 and 90 min after drug administration. In tolerance experiments, morphine 20 mg/kg was injected i.p., twice daily for 4 days. The development and expression of dependence were assessed in the naloxone precipitation test 5 days after the morphine (20-30 mg/kg, i.p.) administration. We found that finasteride could potentiate the antinociceptive effect of morphine. In addition, chronic finasteride administration effectively blocked development of tolerance and dependence to morphine. Following chronic morphine administration, single dose injection of finasteride failed to reverse tolerance but prevented naloxone precipitate withdrawal syndrome. Therefore, it was concluded that there is a functional relationship between 5alpha-reductase enzyme and morphine.     

Alpha melanocyte stimulating hormone (α-MSH) does not modify pentylenetetrazol- and pilocarpine-induced seizures

Aims

Alpha-melanocyte stimulating hormone (α-MSH) is a pro-opiomelanocortin (POMC)-derived peptide involved in different neurological functions that also exerts anti-inflammatory effects, including in the central nervous system (CNS). Although inflammation has been implicated in seizures and epilepsy, no study has systematically investigated whether α-MSH modifies seizures. Therefore, in the current study we determined whether α-MSH alters pentylenetetrazol (PTZ)- and pilocarpine-induced seizures.

Main methods

Adult male Swiss mice were injected with α-MSH (1.66, 5 or 15 μg/3 μL, intracerebroventricular (i.c.v.)) or systemic (0.1, 0.3 or 1 mg/kg, intraperitoneally (i.p.)). Five to sixty minutes after the injection of the peptide, animals were injected with PTZ (60 mg/kg, i.p.) or pilocarpine (370 mg/kg, i.p.). Latency to myoclonic jerks and tonic–clonic seizures, number of seizure episodes, total time spent seizing and seizure intensity, assessed by the Racine and Meurs scales were recorded. Interleukin 1 beta (IL-1β) levels in the hippocampus were measured by a commercial enzyme-linked immunoabsorbent assay (ELISA).

Key findings

Neither intracerebroventricular (1.66, 5 or 15 μg/3 μL, i.c.v.) nor systemic (0.1, 0.3 or 1 mg/kg, i.p.) administration of α-MSH altered PTZ- and pilocarpine-induced seizures. IL-1β levels in the hippocampi were not altered by α-MSH, PTZ or pilocarpine.

Significance

Although inflammation has been implicated in seizures and epilepsy and α-MSH is a potent anti-inflammatory peptide, our results do not support a role for α-MSH in seizure control.     

Bovine endozepine potentiates morphine analgesia in mice

This paper describes the influence of bovine endozepine (BEP) on the analgesic effect of morphine. The intraventricular administration of BEP between doses of 2-4 nmole in mice resulted in the potentiation of the morphine-analgesic effect. The basic pain threshold did not change when BEP was given alone, thus indicating that BEP is unable to elicit analgesic effect alone. Intravenous injection of BEP in the dose range of 5-7.5 mg/kg also potentiated the analgesic effect of morphine. These observations suggest that endozepine can act as a regulator of pain.     

Ethanol-induced hypothermia. Cross tolerance with morphine

The development of tolerance to ethanol-induced hypothermia and hypnosis, and cross-tolerance with morphine was studied in mice and rats. Ethanol significantly decreased the body temperature in rats (3.0 and 3.2 g/kg) and in mice (3.5 and 4.0 g/kg). Chronic administration of ethanol resulted in the tolerance not only to ethanol hypothermia but also to hypothermic effects of morphine in examined animals. Implantation of morphine pellets caused the development of cross tolerance to ethanol-induced hypothermia in rats but not in mice. The hypnotic effect of ethanol was significantly shorter in chronic alcoholized rats but not in morphine-implanted rats. Neither chronic ethanol administration nor implantation of morphine pellets changed the duration of ethanol-induced hypnosis in mice. These results seem to support the hypothesis on the opiate-like mechanism of ethanol action.