On the Mechanism(s) of Cholecystokinin (CCK): Receptor Stimulation Attenuates Morphine Dependence in Mice

Abstract: In the present study, effect of cholecystokinin (CCK) agonists and on dependence to morphine in mice has been investigated. The influence of dopaminergic, adrenergic, cholinergic and serotonergic on attenuation of naloxone-induced jumping in morphine-dependent mice by CCK agonists were also considered. Mice were treated subcutaneously with morphine (50, 50 and 75 mg/kg) three times daily (10 a.m. 1 p.m. and 4 p.m.) for 3 days, and a last dose of morphine (50 mg/kg) was administered on the 4th day. Withdrawal syndrome (jumping) was precipitated by naloxone (5 mg/kg) which was administered intraperitoneally 2 hr after the last dose of morphine. To study effects of CCK receptor agonists, 10 injection of morphine (3 administrations each day) for dependence and a dose of 5 mg/kg of naloxone for withdrawal induction were employed. The CCK agonists CCK-8 (0.001- 0.1 mg/kg), unsulfated CCK-8 (CCK-8U; 0.001-0.1 mg/kg) and caerulein (0.00001-0.01 mg/kg) were able to prevent withdrawal signs precipitated by naloxone (5 mg/kg). Sulpiride and pimozide increased response induced by CCK-8 agonists. The dopamine antagonists also attenuates jumping by themselves. SCH 23390 did not alter the CCK-8 effect, but decreased the jumping by itself. Phenoxybenzamine, propranolol, methysergide and atropine did not change the caerulein effect significantly. However, single administration of atropine increased and methysergide decreased jumping. It is concluded that CCK mechanism(s) may be involved in morphine dependence, and dopaminergic mechanism(s) may interact with CCK in attenuation of naloxone-induced jumping.     

Influence of histamine,cimetidine and pyrilamine on naloxone-induced jumping in morphine-dependent mice

In the present study, the effects of histamine on naloxone-induced jumping in the presence or absence of adrenoceptor or acetylcholine receptor antagonists in morphine-dependent mice were examined. In these experiments, the drugs were used before s.c. injection of naloxone (2 mg/kg), to test their effects on the expression of jumping. The i.c.v. administration of histamine (5-20 microg/mouse) 15 min before naloxone injection decreased the number of jumps in mice. When the histamine H(2) receptor antagonist, cimetidine (5-20 mg/kg), and the histamine H(1) receptor antagonist, pyrilamine (5-20 mg/kg), were administered i.p. to morphine-dependent mice, only cimetidine enhanced the jumping behaviour. Administration of cimetidine (20 mg/kg, i.p.), 30 min, of the beta-adrenoceptor antagonist, propranolol (2.5-10 mg/kg, i.p.), 15 min but not of pyrilamine (20 mg/kg, i.p.), 30 min before naloxone injection, decreased the histamine effect. The i.p. administration of an acetylcholine receptor antagonist, atropine (5 and 10 mg/kg, i.p.), the alpha(1)-adrenoceptor antagonist, prazosin (0.5, 1 and 2 mg/kg, i.p.), and alpha(2)-adrenoceptor antagonist, yohimbine (0.5, 1 and 2 mg/kg, i.p.), 15 min before naloxone injection, had no effect on the histamine response. Single administration of propranolol, atropine or prazosin decreased, while yohimbine increased the naloxone-induced jumping. It is concluded that the histamine H(2) receptor mechanism may be involved in the influence of histamine on the expression of naloxone-induced jumping in morphine-dependent mice.     

Nicotine attenuates place aversion induced by naloxone in single-dose,morphine-treated rats

Rationale Acute physical dependence refers to the withdrawal syndrome precipitated by an opioid antagonist administered several hours after either a single dose or a short-term infusion of an opioid agonist.Objectives We examined the mechanism of nicotine-induced attenuation of naloxone-precipitated withdrawal syndrome when used to produce an aversive motivational state in a place-conditioning paradigm.Methods The effect of nicotine was investigated through place aversion induced by naloxone in morphine-pretreated rats. Additionally, the mechanism of nicotine action in this model was explored specifically in relation to the dopaminergic system through the use of dopamine receptor antagonist and agonist.Results Place avoidance behavior was potently elicited by naloxone (0.5 mg/kg s.c.) 24 h after a single exposure to morphine (10 mg/kg s.c.). Avoidance behavior was attenuated by pretreatment with a 0.2-mg/kg dose of nicotine 15 min prior to naloxone administration. The effect of nicotine was completely blocked by mecamylamine, but not hexamethonium. The dopamine receptor antagonists haloperidol (0.05, 0.1 mg/kg, s.c.), SCH23390 (0.1 mg/kg, s.c.), raclopride (1.0 mg/kg, s.c.) and eticlopride (0.1 mg/kg, s.c.) showed effects similar to mecamylamine. Additionally, the dopamine receptor agonist apomorphine (0.03, 0.1, 0.3 mg/kg, s.c.) inhibited naloxone-induced place aversion in morphine-treated rats.Conclusion The inhibitory effect of nicotine on place aversion induced by naloxone-precipitated morphine withdrawal may involve a dopaminergic portion of the central nervous system.     

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.     

Nicotine-induced grooming: a possible dopaminergic and/or cholinergic mechanism

The ability of nicotine, to induce grooming in rats was studied. Grooming was induced by i.p. injection of different doses (0.0675-0.5 mg/kg) of nicotine to rats. The effect was dose-dependent. However, the response was decreased with increasing doses of the drug from 0.25-0.5 mg/kg. Administration of the dopamine (DA) D1/D2 receptor agonist apomorphine (0.025-5 mg/kg, i.p.) also caused grooming in a dose-dependent manner. High doses of apomorphine (0.1-0.5 mg/kg, i.p.) also induced a lower degree of response. Combination of a low dose of nicotine (0.0675 mg/kg) with different doses of apomorphine did not show any interaction. However, there was an interaction between a high dose of nicotine and apomorphine. Thus, combination of a higher dose of nicotine (0.125 mg/kg) with apomorphine, reduced apomorphine-induced grooming. The muscarinic receptor antagonist atropine (5 and 10 mg/kg), peripheral nicotinic receptor antagonist hexamethonium (5 and 10 mg/kg), central nicotinic receptor antagonist mecamylamine (1 and 3 mg/kg) and D1 DA receptor antagonist SCH23390 (0.05 and 0.1 mg/kg) all decreased the response to nicotine. Atropine, mecamylamine and SCH23390 by themselves reduced spontaneous grooming. It is concluded that nicotine elicits grooming indirectly through a possible D1 dopaminergic mechanism. However, muscarinic and nicotinic cholinergic mechanism(s) may be involved.     

Involvement of Cholinergic and Opioid Receptor Mechanisms in Nicotine-Induced Antinociception

Abstract: In this work we have studied the influences of nicotinic agents on the antinociception of morphine in formalin test. Nicotine (0.001-0.1 mg/kg) induced antinociception in mice in a dose-dependent manner in the early phase of formalin test, and also potentiated the morphine effect. The nicotinic receptor antagonist, mecamylamine (0.5 mg/kg), but not hexamethonium decreased the antinociception induced by nicotine (0.1 mg/kg) in both phases. The muscarinic receptor antagonist atropine (5 and 10 mg/kg) also decreased the response of nicotine. Mecamylamine, hexamethonium or atropine did not alter morphine antinociceptive response, while naloxone decreased responses induced by nicotine or morphine. The antagonists by themselves did not elicit any response in formalin test, however, high doses of mecamylamine tend to increase pain response. It is concluded that central cholinergic and opioid receptor mechanisms may be involved in nicotine-induced antinociception.     

Nicotine-induced hypothermia through an indirect dopaminergic mechanism

The effect of nicotine on core body temperature was studied in mice. Intraperitoneal (i.p.) injection of nicotine (0.5, 1 and 2 mg/kg) induced a dose-dependent hypothermia. The response was inhibited by reserpine (5 mg/kg), the centrally active nicotinic receptor antagonist mecamylamine (0.1-1 mg/kg) and the D-2 dopamine receptor antagonist sulpiride (25-100 mg/kg). The β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) and the serotonergic blocker methysergide (5 and 10 mg/kg) did not inhibit but increased the nicotine response. The α-adrenoceptor antagonist phenoxybenzamine, the antimuscarinic agent atropine, the D-1 dopamine receptor antagonist SCH 23390, the peripheral dopamine antagonist domperidone and the peripheral nicotinic antagonist hexamethonium did not alter the nicotine-induced hypothermia. It is concluded that nicotine may cause a fall in core body temperature through a central dopaminergic mechanism.     

Effects of varenicline and mecamylamine on the acquisition, expression, and reinstatement of nicotine-conditioned place preference by drug priming in rats

Nicotine addiction is a chronic disorder characterized by a relatively high rate of relapse even after long period of abstinence. In the present study, we used the conditioned place preference (CPP) paradigm to investigate the establishment, extinction, reinstatement, and cross-reinstatement of nicotine-induced place conditioning in rats. First, we revealed that nicotine produced a place preference to the initially less-preferred compartment paired with its injections during conditioning (0.175 mg/kg, base, intraperitoneally (i.p.)). Once established, nicotine CPP was extinguished by repeated testing. Following this extinction phase, nicotine-experienced rats were challenged with nicotine (0.175 mg/kg, i.p.) or morphine (10 mg/kg, i.p.). These priming injections of both drugs induced a marked preference for the compartment previously paired with nicotine. Furthermore, given the important role of alpha4beta2 (a4b2) nicotinic receptor subtype in the acquisition and maintenance of nicotine dependence, we evaluated and compared the efficacy of varenicline, a partial a4b2 nicotinic receptor agonist (0.5, 1, and 2 mg/kg, subcutaneously (s.c.)), and mecamylamine (0.5, 1, and 2 mg/kg, s.c.), a non-selective nicotinic receptor antagonist, in blocking nicotine-induced CPP as well as reinstatement of nicotine CPP provoked by nicotine and morphine. It was shown that both nicotinic receptor ligands attenuated the acquisition and expression of nicotine CPP as well as the expression of reinstatement of nicotine CPP provoked by both drugs. Our results indicate similar cholinergic mechanisms, probably through the a4b2 receptors involved in the rewarding effects of nicotine and morphine in rats and may suggest that nicotinic receptors could be a potential target for developing pharmacotherapeutic strategies to treat and prevent nicotine and/or opioid addiction and relapse.     

In vivo evidence for the selectivity of ICI 154129 for the delta-opioid receptor

The in vivo selectivity of the novel delta opioid-receptor antagonist N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH (ICI 154129) was examined in several opioid-selective models. Antagonism at the delta receptor was demonstrated in the striatal head-turn model in the rat. Intrapallidal injection of the relatively selective delta-receptor agonist D-Ala2,D-Leu5-enkephalin (0.5 micrograms) slowed the head-turn time and this effect was completely prevented by prior subcutaneous administration of ICI 154129 (30 mg/kg). The role of delta receptors in two classical test situations was studied using the mixed opioid agonist etorphine and the antagonists naloxone and ICI 154129. The drug ICI 154129 (30 mg/kg, s.c.) failed to prevent the antinociceptive effects and stimulation of locomotor activity produced by etorphine, whereas the relatively selective mu-opioid receptor antagonist, naloxone was effective in both test situations. The possible involvement of delta receptors in morphine-induced dependence was studied by monitoring the abstinence behaviour precipitated in rats given pellets of morphine by either ICI 154129 or naloxone. Naloxone (0.5 mg/kg, i.p.) precipitated a characteristic withdrawal syndrome in conscious rats and, at a much smaller dose (0.02 mg/kg, i.p.), induced shaking behaviour in pentobarbitone-anaesthetised rats. No withdrawal signs were observed in either model after injection of ICI 154129 (30 mg/kg, s.c.), suggesting that the delta receptors are not involved in dependence on morphine.     

Cannabinoid CB1 receptors of the rat central amygdala mediate anxiety-like behavior: interaction with the opioid system

Cannabinoids, which are the active compounds of marijuana, produce some pharmacological effects similar to the opioids. In addition, there are functional interactions between the cannabinoid and opioid systems. In this study, we investigated the effects of intraperitoneal (i.p.) injection of opioid drugs on responses induced by intracentral amygdala (intra-CeA) microinjection of cannabinoid CB1 receptor agents in rats, using the elevated plus maze test of anxiety. I.p. injection of morphine (6 and 9 mg/kg) 30 min before testing, increased the percentage open arm time (%OAT) and the percentage open arm entries (%OAE), but not locomotor activity, showing an anxiolytic-like response. I.p. administration of the opioid receptor antagonist, naloxone (1 mg/kg) 15 min before testing, significantly reduced %OAT, but not %OAE and locomotor activity. The drug, however, tended to decrease locomotor activity. Intra-CeA administration of arachidonylcyclopropylamide (ACPA, an agonist shown to selectively activate CB1 receptors; 1.25 and 5 ng/rat) increased %OAT and %OAE but not locomotor activity, indicating an anxiolytic-like response. Coadministration of morphine (6 mg/kg, i.p.) plus ACPA (0.125 ng/rat, intra-CeA) increased the anxiolytic-like response. Administration of naloxone reversed the effects of intra-CeA injection of ACPA. Intra-CeA administration of the cannabinoid CB1 receptor antagonist, AM251 (2.5, 25, and 100 ng/rat) did not alter %OAT and %OAE, but the higher doses of the drug (25 and 100 ng/rat) reduced locomotor activity. Coadministration of morphine (6 mg/kg) or naloxone (0.1 mg/kg) with AM251 showed an anxiolytic-like response. In conclusion, the results may indicate an anxiolytic-like effect for cannabinoid CB1 receptors of the CeA and the existence of an interaction between the cannabinoid and the opioid systems in the modulation of anxiety.     

Effects of intragastric agmatine on morphine-induced physiological dependence in beagle dogs and rhesus monkeys

Our previous studies demonstrated that subcutaneous injection of agmatine inhibits tolerance to and physiological dependence on morphine in mice and rats. In the present study we further evaluated the effects of intragastric (i.g.) administration of agmatine on morphine-induced physiological dependence in mice, rats, beagle dogs and rhesus monkeys. When agmatine (5-40 mg/kg, i.g.) was co-administered with morphine during the development of morphine-induced physiological dependence, it inhibited the abstinent syndrome precipitated by naloxone in mice, rats and beagle dogs. In addition, agmatine (40 mg/kg, i.g.) inhibited the abstinent syndrome precipitated by naloxone in mice when it was administered on the test day. In naloxone precipitated and naturally abstinent morphine dependent model in rhesus monkeys, agmatine (40 or 80 mg/kg, i.g.) inhibited the development of physiological dependence when it was co-administered with morphine. After the development of morphine dependence, agmatine (80 mg/kg, i.g.) inhibited the naturally abstinent syndrome during the 7-d abstinent period. All these results suggested that intragastric administration of agmatine inhibits morphine-induced physiological dependence in animal models.     

Morphine inhibits dopaminergic and cholinergic induced ejaculation in rats

1. Subcutaneous injection (s.c.) of apomorphine (0.1–0.5 mg/kg) and intraperitoneal administration (i.p.) of quinpirole (0.01–0.25 mg/kg), physostigmine (0.05–0.2 mg/kg) and philocarpine (0.75–3 mg/kg, i.p.) but not neostigmine (0.1–1 mg/kg) induced ejaculation in rats.2. The responses of drugs were reduced by morphine (1–6 mg/kg, s.c.) pretreatment.3. The inhibitory effect of morphine was reversed by naloxone (1.5 mg/kg, s.c.).4. Naloxone (0.75–3 mg/kg, s.c.) alone induced slight but significant ejaculation.5. Ejaculatory responses induced by apomorphine and quinpirole but not those by physostigmine and pilocarpine were reduced by sulpiride (100 mg/kg, i.p.) pretreatment.6. Domperidone (1–30 mg/kg, i.p.) did not change the response induced by apomorphine.7. Pretreatment of animals with the cholinergic antagonist atropine (10 mg/kg, i.p.) decreased the frequency of ejaculation induced by apomorphine, quinpirole, physostigmine or pilocarpine.8. It may be concluded that D-2 activation induces ejaculation through influence on cholinergic mechanisms and morphine inhibits the ejaculation induced by activation of both cholinergic and dopaminergic systems via opiate receptor sites.     

The role of vasopressin on the effect of U-50,488 to block the development of morphine tolerance and physical dependence

U-50,488, a selective κ-opioid receptor agonist, has been reported to inhibit the development of antinociceptive tolerance to morphine in mice, rats and guinea pigs, but the mechanism involved in this action remains unknown. Since U-50,488 has been reported to supress the plasma vasopressin level, we investigated the role of vasopressin with U-50,488 in the male Sprague Dawley rat in this study. Animals (230–270 g) were chronically treated with morphine (10 mg/kg, i.p.) twice a day for 6 days in order to induce tolerance to antinociceptive effect measured by tail-flick test. Withdrawl symptoms were precipitated by naloxone (10 mg/kg, i.p.) on day 7. U-50,488 (i.p.) or AVP (i.p. or i.c.v.) or U-50,488 and AVP was (were) coadministered with chronic morphine to investigate their effects on morphine tolerance and dependence. We found that coadministration of 8 mg/kg U-50,488 (i.p.) with morphine almost completely block morphine tolerance and partially block withdrawal symptoms. In contrast, coadministration of AVP (0.3 μg/kg, i.p., or 0.01 μg, i.c.v.) with morphine and U-50,488, the effects of U-50,488 to block morphine tolerance and dependence were reversed. In addition, treatment of AVP antagonist (dPTyr(Me)AVP, 0.5 μg/kg, i.p. or 0.5 μg, i.c.v.) has the similar effect as U-50,488 to block morphine tolerance. In summary, the effect of U-50,488 to block morphine tolerance and dependence may relate to its inhibitory effect on AVP release. Received: 20 February 1996 / Accepted: 14 October 1996     

Attenuation of morphine tolerance and withdrawal syndrome by coadministration of nalbuphine

Morphine has been used widely on the treatment of many types of chronic pain. However the development of tolerance to and dependence on morphine by repeat application is a major problem in pain therapy. The purpose of the present study was to investigate whether combined administration of nalbuphine with morphine affects the development of tolerance to and dependence on morphine. We hypothesize that the use of nalbuphine, κ-agonist may prove to be useful adjunct therapy to prevent morphine-induced undesirable effects in the management of some forms of chronic pain. Morphine (10 mg/kg) was injected to rats intraperitoneally for 5 day. The variable dose of nalbuphine (0.1, 1.0 and 5.0 mg/kg) was administered (i.p.) in combination with morphine injection. The development of morphine tolerance was assessed by measuring the antinociceptive effect with the Randall-Selitto apparatus. The development of dependence on morphine was determined by the scoring the precipitated withdrawal signs for 30 min after injection of naloxone (10 mg/kg, i.p.). Nalbuphine did not attenuate antinociceptive effect of morphine in rats. Interestingly, combined administration of morphine with nalbuphine (10∶1) significantly attenuated the development of dependence on morphine. The elevation of [³H]MK-801 binding in frontal cortex, dentate gyrus, and cerebellum after chronic morphine infusion was suppressed by the coadministration of nalbuphine. In addition, the elevation of NR1 expression by morphine was decreased by the coadministration of nalbuphine in rat cortex. These results suggest that the coadministration of nalbuphine with morphine in chronic pain treatment can be one of therapies to reduce the development of tolerance to and dependence on morphine.     

Boosting effect of morphine on alcohol drinking is suppressed not only by naloxone but also by the cannabinoid CB1 receptor antagonist,SR 141716

The present study investigated the effect of the cannabinoid CB(1) receptor antagonist, SR 141716 (N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide), on the ability of low and high doses of morphine to, respectively, augment and suppress voluntary alcohol intake in selectively bred Sardinian alcohol-preferring rats. Acute administration of a low dose of morphine (1 mg/kg, s.c.) produced a specific and marked increase in alcohol intake, which correlated with an increase in blood alcohol levels and was prevented by either SR 141716 (0.3 mg/kg, i.p.) or naloxone (0.1 mg/kg, i.p.). A higher dose (10 mg/kg, s.c.) of morphine reduced both alcohol and food intakes and produced sedation and hypomotility. The suppressant effect of morphine on alcohol intake was blocked by naloxone (0.1 mg/kg, i.p.) but not by SR 141716 (0.3 mg/kg, i.p.). These results are in agreement with those showing the ability of SR 141716 to antagonize the appetitive and positive reinforcing properties of morphine and add further support to the hypothesis of the existence of a functional link between the action of opioids and of cannabinoids.     

Fluoxetine suppresses morphine tolerance and dependence: modulation of NO-cGMP/DA/serotoninergic pathways

Although the phenomenon of opioid tolerance and dependence has been widely investigated, neither opioid nor non-opioid mechanisms are completely understood. In view of the modulation of 5-HT transport into presynaptic terminals in the brain by nitric oxide (NO) via cGMP, and the existence of a tonic 5-HTergic inhibition of dopamine release, the present study investigated the effect of fluoxetine, a selective serotonin reuptake inhibitor, and NO modulators L-N(G)-nitroarginine methyl ester (L-NAME; NO synthase inhibitor) and L-Arginine (substrate for nitric oxide synthase) alone or in combination against morphine tolerance and dependence. Animals developed tolerance to the antinociceptive effect of morphine (10 mg/kg s.c. twice daily) on day 3 and the degree of tolerance was further enhanced on days 9 and 10. The development of tolerance to the antinociceptive effect of morphine was delayed by prior administration of fluoxetine (10 mg/kg i.p, twice daily for 9 days) and L-NAME (10 mg/kg i.p. twice daily for 9 days) alone or in combination. It was accentuated by L-Arginine (50 mg/kg i.p. twice daily for 9 days) alone or in combination with fluoxetine (10 mg/kg i.p. twice daily for 9 days). Similarly, fluoxetine (10 mg/kg i.p.) or L-NAME (10 mg/kg i.p.), when administered acutely on day 10, reversed morphine-induced tolerance. L-Arginine (50 mg/kg i.p.) however, when administered acutely on day 10, accentuated morphine tolerance. Fluoxetine (10 mg/kg i.p. twice daily for 9 days) suppressed the development of morphine dependence as assessed by naloxone (2 mg/kg i.p.)-precipitated withdrawal jumps. This suppression of dependence was potentiated by L-NAME (10 mg/kg i.p. twice daily for 9 days) and reversed by L-Arginine (50 mg/kg i.p. twice daily for 9 days), respectively. Acute administration of the respective drugs on day 10 modulated morphine dependence in a similar fashion. L-Arginine also reversed fluoxetine-induced weight loss in morphine-dependent animals. The present study demonstrated that fluoxetine suppressed the dependence and development of tolerance to the antinociceptive effect of morphine. Fluoxetine-induced suppression was potentiated by L-NAME and accentuated by L-Arginine. The results therefore suggest that a complex phenomenon such as morphine tolerance and dependence might involve close interplay of the NO-c GMP/5-HT/DA receptor system. To the best of the authors' knowledge, this is the first report to suggest targeting this cascade for amelioration of opioid tolerance and withdrawal syndrome.     

Blockade of orexin receptor 1 attenuates the development of morphine tolerance and physical dependence in rats

The goals of this study were to evaluate the effects of pretreatment by orexin receptor-1 antagonist on the development of morphine tolerance and physical dependence in rat. Animals were rendered dependent on morphine by subcutaneous (SC) injection of morphine sulfate (10 mg/kg) at set intervals of 12 h for 10 days. Just before the morphine administration, the animals received SB-334867, a selective orexin receptor 1 (OXR1) antagonist. To assess morphine tolerance, the antinociceptive responses of morphine were measured using the warm-water tail immersion test before and after its administration. On day 11, naloxone was injected 2 h after morphine administration and the physical dependence evaluated by quantifying/scoring naloxone-precipitated withdrawal signs for 30 min. The effect of chronic SB-334867 on locomotion was carried out by calculating the number of grid crossings as a measure of locomotor activity. Our findings demonstrated that although morphine-tolerance tended to develop in response to repeated injections of morphine, pre-treatment of OXR1 antagonist prevented this effect, causing a delay in the development of morphine-tolerance. Moreover, co-administration of orexin receptor 1 antagonist with morphine significantly decreased the somatic signs of withdrawal including diarrhea, teeth chattering, jumping, and defecation. Administration of SB-334867 alone or in a chronic co-administration with morphine failed to change locomotor activity. These results suggest that the activation of OXR1 might be involved in the development of morphine tolerance and dependence.     

Pharmacological modulation of leukotriene D(4) attenuates the development of opioid dependence in a mouse model of naloxone-induced opioid withdrawal syndrome

The present study was designed to investigate the effect of montelukast sodium, a leukotriene D(4) receptor antagonist, and 1,2,3,4,tetrahydroisoquinoline, a leukotriene D(4) synthetic pathway inhibitor, on the development of morphine dependence in a mouse model of naloxone-induced opioid withdrawal syndrome. Morphine (5 mg/kg, i.p.) was administered twice daily for a period of 5 days following which a single injection of naloxone (8 mg/kg, i.p.) precipitated the opioid withdrawal syndrome in mice. Behavioral observations were made for a period of 30 min immediately after naloxone treatment. The withdrawal syndrome was quantitatively assessed in terms of withdrawal severity score and the frequency of jumping, rearing, fore paw licking and circling. Montelukast sodium as well as 1,2,3,4,tetrahydroisoquinoline, markedly and dose dependently (p<0.01) attenuated the morphine-naloxone-induced opioid withdrawal syndrome in mice. However, administration of montelukast sodium or 1,2,3,4,tetrahydroisoquinoline did not alter the activity of the central nervous system, assessed in terms of locomotor activity count thus ruling out any per se sedative action of montelukast sodium. Further, pretreatment with montelukast sodium or 1,2,3,4,tetrahydroisoquinoline did not alter the acute analgesic effect of morphine. Thus, leukotriene D(4) may be involved in the development of opioid dependence and the precipitation of its withdrawal syndrome and thus may serve as a viable pharmacological target to tackle the problem of opioid addiction.     

Role of central nicotinic and β-adrenergic receptors in the onset and further development of tail-tremor induced by repeated nicotine administration to rats

The effects of nicotinic and β-adrenergic receptor antagonists on tail-tremor induced by repeated nicotine administration were investigated in rats. The daily administration of nicotine (0.5mg/kg/day, s.c.) for 8 days resulted in an augmentation of tail-tremor. However, repeated administration of dimethyl phenyl piperazinium iodide (1mg/kg/day, s.c.) for 8 days did not cause tail-tremor. Mecamylamine (0.5mg/kg, i.p), administered before the nicotine injection on each day, abolished the tail-tremor. After discontinuation of the mecamylamine treatment, nicotine injections caused tail-tremor augmentation. Propranolol (20mg/kg, i.p.), administered before the nicotine on each day, suppressed the appearance of tail-tremor. After the discontinuation of propranolol treatment, the degree of tail-tremor induced by a single injection of nicotine on day 9 was much greater in the propranolol-treated group than in the saline-treated control group. Neither carteolol (20mg/kg, i.p.) nor metoprolol (20mg/kg, i.p.) treatment showed such effects. Intraspinal injection of 6-hydroxydopamine markedly enhanced the tail-tremor induced on the first day of nicotine injection. This effect became more intense on subsequent administration of nicotine. The enhanced tail-tremor following 6-hydroxydopamine treatment was abolished by mecamylamine (0.5 and 1mg/kg, i.p.), and was suppressed by propranolol (5–20mg/kg, s.c.) in a dose-dependent manner. These results suggest that central nicotinic receptors are essential for the onset and for the further development of tail-tremor induced by the repeated administration of nicotine, and that β₂-adrenoceptors are associated with the tremor mechanism. Moreover, spinal noradrenergic mechanisms may be involved in the manifestation of this phenomenon. Received: 18 June 1996 / Accepted: 15 December 1996     

Effects of GABAergic system on naloxone-induced jumping in morphine-dependent mice.

The effect of GABA (gamma-aminobutyric acid system) receptor agonists and antagonists on naloxone-induced jumping in morphine-dependent mice was examined. Intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) injection of different doses of the GABA(B) receptor agonist, baclofen (2.5, 5 and 10 mg/kg), reduced naloxone-induced jumping in morphine-dependent mice. The i.p. administration of the GABA(B) receptor antagonist, CGP35348 (P-[3-aminopropyl]-p-diethoxymethyl-phosphinic acid), but not the i.c.v. injection of the drug, increased naloxone-induced jumping. The antagonist also decreased the baclofen response. Administration of the GABA(A) receptor agonist, muscimol, but not the GABA(A) receptor antagonists bicuculline and picrotoxin, decreased the naloxone response in morphine-dependent animals. It is concluded that both GABA(A) and GABA(B) receptor subtypes may have an inhibitory influence on naloxone-induced withdrawal jumping in mice.