Pretreatment with morphine potentiates naloxone-conditioned place aversion in mice: effects of NMDA receptor antagonists

Acute pretreatment with opioid receptor agonists potentiates behavioral effects of opioid antagonists. This phenomenon was suggested to serve as an acute model of opioid dependence. Since antagonists acting at N-methyl-D-aspartate (NMDA) receptors were repeatedly shown to attenuate development, maintenance, and expression of opioid dependence, the present study evaluated the effects of competitive NMDA receptor antagonist, D-CPPene (SDZ EAA 494; 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid), and low-affinity channel blocker, 1-amino-3,5-dimethyl adamantane hydrochloride (memantine), on establishment of naloxone-conditioned place aversion in mice that were pre-exposed to morphine. Morphine (20 mg/kg) pretreatment significantly potentiated the ability of naloxone (0.01-0.3 mg/kg) to produce place aversion. The place aversion produced by naloxone (0.1 mg/kg) was attenuated by D-CPPene (1 and 3 mg/kg but not 0.1 or 0.3 mg/kg) when it was administered 3.5 h after morphine (0.5 h prior to conditioning trial with naloxone) but not 0.5 h prior to morphine. Memantine (1-10 mg/kg) had no effect under any treatment condition (0.5 h prior to morphine, simultaneously with morphine, 2 or 3.5 h after morphine). Thus, the ability of NMDA receptor antagonist to affect development and/or expression of morphine dependence may not be a good predictor of their effects on establishment of morphine-potentiated naloxone-conditioned place aversion.     

Effects of NMDA receptor antagonists on cocaine-conditioned motor activity in rats

NMDA receptor antagonists have been reported to affect learned behaviors conditioned with abused drugs, with the outcome dependent, in part, on the class of NMDA receptor antagonist used. The present study tested the ability of various site-selective NMDA receptor antagonists to modify cocaine-conditioned motor activity. Two procedures were used for independently assessing drug effects on spontaneous activity and expression of cocaine-conditioned behavior. In the conditioning experiments, rats were administered i.p. injections of cocaine (30 mg/kg) or saline paired with distinctive environments. Spontaneous horizontal activity was dose-dependently enhanced by dizocilpine (0.03-0.3 mg/kg) and memantine (1-30 mg/kg), but not by D-CPPene (3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid; SDZ EAA 494; 1-10 mg/kg), ACEA-1021 (5-nitro-6,7-dichloro-1,4-dihydro-2, 3-quinoxalinedione; 3-56 mg/kg), or eliprodil (3-30 mg/kg). Higher doses of memantine, D-CPPene (1-10 mg/kg), eliprodil (3-30 mg/kg), or ACEA-1021 reduced vertical activity. Following five cocaine-environment pairings, rats displayed significant increases in motor activity when exposed to the cocaine-paired environment. The following antagonists were administered prior to the conditioning test: dizocilpine (MK-801; 0.03-0.1 mg/kg), memantine (1-10 mg/kg), D-CPPene (0.3-3 mg/kg), ACEA-1021 (3-10 mg/kg), and eliprodil (1-10 mg/kg). Of these, memantine, ACEA-1021 and, to the lesser degree, eliprodil attenuated expression of cocaine-conditioned motor activity at doses that did not significantly affect spontaneous motor activity. These results show that cocaine-conditioned behaviors can be selectively modulated by some, but not all, NMDA receptor antagonists.     

Behavioural effects of glutamate receptor agonists in morphine-dependent rats

Glutamate receptors are implicated in the development and expression of drug dependence. Substantial experimental evidence suggests that antagonists acting at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors attenuate the severity of opioid withdrawal. However, it is less clear whether opioid withdrawal can be potentiated by agonists of glutamate receptors. The present study evaluated the behavioural effects of various agonists of glutamate receptors, as well as a nitric oxide (NO) donor, in morphine-dependent rats trained to discriminate 0.1 mg/kg of naloxone from saline. None of the following drugs produced appreciable levels of naloxone-like responding (substitution tests) or potentiated the discriminative stimulus effects of naloxone: NMDA (3-56 mg/kg), glycine (100-1000 mg/kg), glutamate (1000-3000 mg/kg), kainate (0.3-3 mg/kg), isosorbide dinitrate (30-300 mg/kg). Nevertheless, expression of some morphine withdrawal-like somatic and behavioural signs ('wet-dog'-like shaking, scream on touch, ptosis, tremor, chewing, weight loss) was facilitated by NMDA, glycine, and isosorbide dinitrate. These results suggest that, compared to somatic symptoms, subjective effects of opioid withdrawal (as reflected by discriminative stimulus effects) are not mimicked by direct activation of glutamate receptors.     

Lowered brain stimulation reward thresholds in rats treated with a combination of caffeine and N-methyl-D-aspartate but not alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate or metabotropic glutamate receptor-5 receptor antagonists

Previous studies suggested that adenosine A1 and A2A receptor agonists counteract behavioral effects of N-methyl-D-aspartate (NMDA) receptor antagonists while adenosine receptor antagonists may produce opposite effects enhancing the actions of NMDA receptor antagonists. To further evaluate the effects of combined administration of adenosine receptor antagonist caffeine and various NMDA and non-NMDA glutamate receptor antagonists on brain stimulation reward (discrete-trial threshold current intensity titration procedure), rats with electrodes implanted into the ventral tegmental area were tested after pretreatment with NMDA receptor channel blocker MK-801 (0.01-0.3 mg/kg), competitive antagonist D-CPPene (0.3-5.6 mg/kg), glycine site antagonist L-701,324 (1.25-5 mg/kg), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist GYKI-53655 (1-10 mg/kg), metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP (1-10 mg/kg) alone and in combination with caffeine (1-30 mg/kg). MK-801 (0.056 and 0.1 mg/kg) was the only tested glutamate antagonist that lowered self-stimulation thresholds, while D-CPPene (5.6 mg/kg) and MPEP (5.6 and 10 mg/kg) had the opposite effects. Threshold-increasing effects of D-CPPene, but not of MPEP, however, were associated with marked impairment of operant performance, reflected by longer latencies to respond and higher rates of responding during the inter-trial intervals. Operant performance was also disrupted by the highest dose of MK-801 (0.3 mg/kg). For subsequent experiments, caffeine (1-30 mg/kg) was combined with the highest doses of NMDA receptor antagonists that did not lower the brain stimulation reward thresholds and did not impair operant performance. Caffeine had no appreciable effects on self-stimulation behavior when given alone. A low dose of caffeine (3 mg/kg) significantly lowered self-stimulation thresholds only when given together with MK-801 (0.03 mg/kg) or D-CPPene (3 mg/kg). Combined with the same antagonist drugs, higher doses of caffeine (10 and 30 mg/kg) facilitated time-out responding. These results indicate that, within a limited dose range, caffeine in combination with an NMDA receptor channel blocker and a competitive antagonist significantly lowers brain stimulation reward thresholds in rats.     

Antinociceptive activity of combination of morphine and NMDA receptor antagonists depends on the inter-injection interval

The actual time-course of morphine antinociception is shorter than what would be predicted from its elimination kinetics, suggesting the presence of an acute tolerance phenomenon. Since antagonists acting at NMDA subtype of glutamate receptors were repeatedly shown to prolong acute morphine antinociception, acute tolerance may be attributed to hyperactivity of NMDA receptors. The ability of various site-selective NMDA receptor antagonists to affect morphine antinociception (tail-flick test) was assessed in mice 30 and 120 min after acute morphine challenge. Competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid (D-CPPene) (SDZ EAA 494; 0.1-1 mg/kg), low-affinity channel blockers 1-amino-3,5-dimethyl adamantane (memantine) (1-10 mg/kg) and 1-amino-1,3,3,5,5-pentamethyl-cyclohexan hydrochloride (MRZ 2/579) (1-10 mg/kg), glycine site antagonists 5-nitro-6,7-dichloro-1, 4-dihydro-2,3-quinoxalinedione (ACEA-1021) (5 or 10 mg/kg) and 8-chloro-4-hydroxy-1-oxo-1,2-dihydropyridaliono(4, 5-b)quinoline-5-oxide choline salt (MRZ 2/576) (1-10 mg/kg) were administered intraperitoneally (i.p.) 15 or 30 min prior to the tail-flick test (i.e., interval between injections of morphine and NMDA receptor antagonist was either 0-15 or 90-105 min). ACEA-1021, MRZ 2/576 and to the lesser extent, memantine and MRZ 2/579 enhanced morphine antinociception when tests were conducted 120 but not 30 min post-morphine. D-CPPene potentiated morphine antinociception irrespective of the interval between morphine administration and the tail-flick test. The results suggest that NMDA receptor antagonists may restore analgesic activity of morphine in acutely tolerant mice.     

5-HT3 receptor antagonists inhibit the response of kappa opioid receptors in the morphine-induced Straub tail [corrected

We used the morphine-induced Straub tail to examine the actions of a 5-HT₃ receptor antagonist and κ opioid receptor agonist. The κ opioid receptor agonist, U-50,488H (4–16 mg/kg i.p.), produced a dose-related inhibition of the tail elevation induced by morphine (10 mg/kg s.c.) in ICR male mice. ICS-205-930 (3 and 10 mg/kg i.p.) and zacopride (0.3 and 1 mg/kg i.p.), 5-HT₃ receptor antagonists, attenuated the inhibitory effect of U-50,488H in a dose-dependent manner. ICS-205-930 and zacopride did not inhibit the morphine-induced Straub tail. These observations suggest that the actions of κ opioid receptors may be modulated by 5-HT³ receptors in the morphine-induced Straub tail.     

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.     

Morphine-induced antinociception in the formalin test: sensitization and interactions with D1 and D2 dopamine receptors and nitric oxide agents

In this study, the effects of dopamine receptor antagonists and nitric oxide agents on morphine-induced sensitization in the formalin test in mice have been investigated. Repeated daily intraperitoneal administration of morphine (30 mg/kg for 3 days) followed by a 11-day wash out period increased morphine-induced antinociception in the formalin test, which may be due to sensitization. The antinociceptive response to higher doses of morphine (6 and 9 mg/kg) but not 3 mg/kg was significantly increased in sensitized animals compared with control groups. Pretreatment of animals with an opioid receptor antagonist, naloxone (4 mg/kg), during repeated administration of morphine, attenuated the morphine-induced sensitization. In the second part of the study, the animals received SCH23390 (D1 receptor antagonist), sulpiride (D2 receptor antagonist), L-Arg (nitric oxide precursor) and NG-nitro-L-Arg methylester (nitric oxide synthase inhibitor) during repeated morphine administration, to evaluate the role of dopamine receptor antagonists and nitric oxide agents in this phenomenon. Pretreatment of animals with NG-nitro-L-Arg methylester (20 mg/kg) and sulpiride (100 mg/kg) during morphine sensitization decreased the antinociceptive response to higher doses of morphine in the formalin test. It is concluded that D2 dopamine receptor and nitric oxide mechanisms may be involved at least partly in morphine-induced sensitization in the formalin test.     

5-HT3 receptor antagonists inhibit the response of κ oploid receptors in the morphine-reduced straub tail

We used the morphine-induced Straub tail to examine the actions of a 5-HT₃ receptor antagonist and κ opioid receptor agonist. The κ opioid receptor agonist, U-50,488H (4–16 mg/kg i.p.), produced a dose-related inhibition of the tail elevation induced by morphine (10 mg/kg s.c.) in ICR male mice. ICS-205-930 (3 and 10 mg/kg i.p.) and zacopride (0.3 and 1 mg/kg i.p.), 5-HT₃ receptor antagonists, attenuated the inhibitory effect of U-50,488H in a dose-dependent manner. ICS-205-930 and zacopride did not inhibit the morphine-induced Straub tail. These observations suggest that the actions of κ opioid receptors may be modulated by 5-HT³ receptors in the morphine-induced Straub tail.     

The importance of the number of NMDA receptors in the development of supersensitivity or tolerance to and dependence on morphine.

Opiate or NMDA receptor antagonists given during and/or after the development of tolerance and dependence have been reported to prevent these developments. In the present study, MK801 (dizolcipine) and naltrexone (NX), two antagonists of NMDA and opiate receptors, respectively were used in rats to find any correlations between changes in NMDA receptor kinetics, and the intensity of tolerance and dependence. Thus, six different groups of rats were formed. The rats in the groups were given saline (S)+S, S+morphine (M), NX+S, NX+M, MK801+S and MK801+M, respectively, once per day for 8 days. On day 9, the rats from each group were divided into four subgroups. The rats of the first subgroup were subjected to the determination of tail-flick latency. The rats of the second subgroup were administered 1 mg kg-1 naloxone (NL) 2 h after administration of 3 mg kg-1M. The rats of the third subgroup were implanted with two M pellets and after 72 h they were challenged with NL. The remaining rats received drugs also on day 9 according to the previous administration paradigm. Two hours after the administrations, their brains were utilised for the determination of NMDA receptor kinetics, employing [3H]glutamate. The measurement of tail-flick latency showed the prevention by NX or MK801 of the development of tolerance to M. The rats, which were administered 3 mg kg-1M 2 h before 1 mg kg-1 NL injection, on day 9 showed that only NX given previously along with M attenuated the intensity of the development of M dependence. NX administered alone intensified the development of dependence on a single dose of M. The development of M dependence upon the M pellet implantation was intensified by the previous administration of NX or MK801 concomitantly with M. The administration of M or MK801 alone, or NX together with M, caused significant upregulation of NMDA receptors. NX alone, and MK801 given concurrently with M led to a significant downregulation. So, in light of the previous findings and the present experimental data it can be said that: (1) supersensitivity to opioids may be a downregulation of NMDA as well as an upregulation of the opioid receptor; (2) either upregulation or downregulation of NMDA receptors may facilitate subsequent development of opioid dependence; (3) tolerance to opioid may necessitate both upregulation of NMDA receptors and downregulation of opioid receptors; and (4) beneficial effects of opioid antagonists in the treatment of opiate dependence and CNS injuries may be strongly related to the down regulation of NMDA receptors.     

Blocking of enhanced sensitivity to behavioral effects of naloxone induced by narcotic agonists in rats

The present experiment evaluated whether prior treatment with naloxone could block the sensitization to opiate antagonist induced by single dose administration of pure agonist (morphine) or mixed agonist (buprenorphine). Food deprived male Wistar rats were trained to respond for food on a multiple-trial, fixed-interval 3 min schedule. Reinforcement was contingent upon a response within a 10-s limited hold period following a fixed-interval of 3 min. A trial consisted of three fixed interval of 3 min separated by a 10 min timeout period during which responses were not reinforced. The rate decreasing effects of the opioid antagonist naloxone was determined by cumulative dosing. Pretreatment with morphine (0.3 mg/ kg, SC) and buprenorphine (0.03 mg/kg, SC) resulted in an increase sensitivity to the rate decreasing effect of naloxone compared to saline pretreatment. Administration of naloxone (0.3 mg/kg) 10 min prior to pretreatment doses of buprenorphine (0.03 mg/kg; 1.0 mg/kg) and morphine (0.3 mg/kg) increased sensitization to naloxone. However, greater sensitization was observed at low dose of buprenorphine. The increased sensitivity was partially blocked at high dose of buprenorphine (1.0 mg/ kg) by naloxone pretreatment. These results suggest that the doses of naloxone used to block opioid induced sensitization might be different from those required in animals with normal sensitivity to opioid antagonists. Further agonist-induced sensitization to behavioral effects of opioid antagonist appears to be opioid receptor specific.     

The effect of chronic administration of caffeine on morphine-induced analgesia, tolerance and dependence in mice

Morphine-induced analgesia, and the development of morphine-induced tolerance and dependence was determined in mice which had drunk caffeinated water (1 mg/ml) for 14 days or in mice which had received (-)-N6-(phenylisopropyl)-adenosine (PIA) 1 mg/kg i.p. for 14 days. Analgesia was assessed by the tail flick assay. The development of dependence was assessed by determining the ED50 of naloxone to precipitate withdrawal jumping (3 h after 100 mg/kg morphine pretreatment or 72 h after s.c. implantation of a morphine 75 mg pellet) and by determining the extent of naloxone-precipitated hypothermia in morphine-implanted animals. In mice chronically administered caffeine, the ED50 for morphine-induced analgesia was significantly decreased while the naloxone ED50 for withdrawal jumping increased by 2-fold after both types of morphine pretreatment. In control animals (tap water for 14 days), doses of 1 and 10 mg/kg of naloxone caused significant hypothermia in morphine-implanted animals. Doses of naloxone up to 100 mg/kg did not cause significant hypothermia in morphine-implanted animals which had received chronic caffeine. The development of tolerance was determined by computing the morphine potency ratio for the tail flick assay (tolerant ED50/control ED50). In mice chronically administered caffeine, the potency ratio was decreased significantly in morphine-implanted animals when compared to control. Morphine-induced analgesia, tolerance and dependence was not changed significantly in animals chronically administered PIA. Neither the distribution of morphine to the brain nor the opioid receptor binding parameters for [3H]etorphine and [3H]naltrexone were altered in mice chronically administered caffeine or PIA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Behavioural and neurochemical adaptations to nicotine in rats: influence of NMDA antagonists.

1. The repeated co-administration of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (0.1 and 0.3 mg kg-1, i.p.) with nicotine (0.4 mg kg-1, s.c.) attenuated the development of tolerance to the locomotor depressant effect of the nicotine in rats. 2. The repeated co-administration of the competitive NMDA antagonist D-CPPene (SDZ EAA 494; 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid, 2 and 8 mg kg-1, i.p.) also attenuated tolerance to the locomotor depressant effect of nicotine. 3. Dizocilpine (0.3 mg kg-1, i.p.) pretreatment attenuated sensitization to the locomotor stimulant effect of nicotine (0.4 mg kg-1, s.c.) and prevented sensitization of nicotine-induced dopamine release in the nucleus accumbens. However, pretreatment with dizocilpine alone caused a modest enhancement of the behavioural response to a subsequent acute dose of nicotine. 4. D-CPPene (2.0 mg kg-1, i.p.) pretreatment prevented sensitization to the nicotine-induced dopamine release in the nucleus accumbens. There was no enhanced locomotor response that could be attributed to nicotine pretreatment when D-CPPene was co-administered with nicotine. However, pretreatment with D-CPPene alone enhanced the locomotor response to an acute dose of nicotine. 5. The results suggest the involvement of NMDA receptors in adaptations of the behavioural and neurochemical effects of nicotine that occur as a result of repeated administration of the drug.     

NMDA receptor antagonists do not block the development of sensitization of catalepsy, but make its expression state-dependent

Dopamine (DA) receptor blockade induces catalepsy in rats which increases in strength upon retesting. This increase in catalepsy represents a form of sensitization which has been shown to be completely context dependent. Sensitization of catalepsy therefore represents a good model for studying the neurobiological mechanisms underlying the interaction between the cellular effect of a drug (DA-receptor blockade) and the context. This study investigated whether glutamatergic mechanisms are involved in the development of sensitization. Rats were treated with either haloperidol or haloperidol plus an N-methyl-D-aspartate (NMDA) receptor antagonist. Haloperidol consistently induced catalepsy which developed sensitization upon retesting. Co-administration of D-CPPene (5 mg/kg and 10 mg/kg, i.p.), eliprodil (30 mg/kg, i.p.) or Ro 25-6981 (15 mg/kg, i.p.) did not have any effect on sensitization, although all three drugs exerted some anticataleptic effects. When sensitization developed under haloperidol plus NMDA receptor antagonist, the sensitized response was expressed only in the presence of the NMDA receptor antagonist. This strongly suggests that the NMDA receptor antagonists represent contextual stimuli to which catalepsy has been conditioned, and this implies that the expression of sensitization has been rendered state-dependent.     

On the mechanism of tolerance to morphine-induced Straub tail reaction in mice

The effect of 5-HT and opioid receptor antagonists on morphine-induced Straub tail was studied in mice. Straub tail behavior was induced by subcutaneous administration of different doses (20, 30, and 40 mg/kg) of morphine hydrochloride to mice. The effect of morphine was dose-dependent. Maximum response was obtained with 40 mg/kg of the drug. The response induced by morphine (20 and 40 mg/kg) was decreased by different doses of intraperitoneal injection of naloxone (1 and 2 mg/kg) or methysergide, mianserin, and ritanserin (1 and 2 mg/kg). The effect of morphine (40 mg/kg) was also reduced by intracerebroventricular injection of naloxone (0.4-0.8 microg/animal) or mianserin (2 and 4 microg/animal). Different groups of mice received one daily dose (50 mg/kg sc) of morphine sulfate for 3 days to develop tolerance to morphine. The Straub tail reaction induced by morphine hydrochloride (40 mg/kg) was tested on the fourth day. Naloxone injection (1 and 2 mg/kg ip) on Day 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), decreased tolerance induced to morphine. Methysergide, mianserin, or ritanserin (intraperitoneal) on Days 2 and 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), also decreased tolerance induced to morphine. Intracerebroventricular injection of either naloxone or mianserin also reduced tolerance to morphine. It is concluded that 5-HT(2) and opioid receptor mechanisms are involved in morphine-induced Straub tail reaction and tolerance induced to morphine also may be mediated through these receptors.     

Analgesic effects of morphine and loperamide in the rat formalin test: interactions with NMDA receptor antagonists

To reveal peripheral components of opiate analgesia, effects of loperamide, opioid agonist which does not penetrate the blood-brain barrier, were examined in formalin and acute thermal pain tests in comparison with morphine. Formalin administration induces pain behaviour such licking/biting of injected paw expressed as two phases. The first phase is caused by C-fibre activation due to peripheral stimulation, the second phase attributed to ongoing input from peripheral site, leading to spinal hyperexcitability, which is dependent on N-methyl-D-aspartate (NMDA) receptor activation. Loperamide (3-10 mg/kg) and morphine (6 mg/kg) reduced formalin-induced nociceptive behaviours and these effects were reversed by naloxone methiodide (0.03-10 mg/kg), opioid receptor antagonist which poorly penetrates the blood-brain barrier. Loperamide action was enhanced only by centrally active NMDA receptor antagonists memantine (3 mg/kg) and CGP 37849 (3 mg/kg), but not by NMDA/glycineB receptor antagonists showing weak or no central nervous system (CNS) activity. Present results suggest that central NMDA receptor blockade may be necessary to enhance analgesia induced through peripheral opioid mechanisms in formalin-evoked nociception.     

Determining Pharmacological Selectivity of the Kappa Opioid Receptor Antagonist LY2456302 Using Pupillometry as a Translational Biomarker in Rat and Human

Background:

Selective kappa opioid receptor antagonism is a promising experimental strategy for the treatment of depression. The kappa opioid receptor antagonist, LY2456302, exhibits ~30-fold higher affinity for kappa opioid receptors over mu opioid receptors, which is the next closest identified pharmacology.

Methods:

Here, we determined kappa opioid receptor pharmacological selectivity of LY2456302 by assessing mu opioid receptor antagonism using translational pupillometry in rats and humans.

Results:

In rats, morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone (3mg/kg, which produced 90% mu opioid receptor occupancy), while 100 and 300mg/kg LY2456302 (which produced 56% and 87% mu opioid receptor occupancy, respectively) only partially blocked morphine-induced mydriasis. In humans, fentanyl-induced miosis was completely blocked by 50mg naltrexone, and LY2456302 dose-dependently blocked miosis at 25 and 60mg (minimal-to-no blockade at 4–10mg).

Conclusions:

We demonstrate, for the first time, the use of translational pupillometry in the context of receptor occupancy to identify a clinical dose of LY2456302 achieving maximal kappa opioid receptor occupancy without evidence of significant mu receptor antagonism.     

The effect of acamprosate on the development of morphine-induced behavioral sensitization in rats

Recently we have shown that the alcohol anti-craving drug acamprosate (calcium acetylhomotaurinate), a functional N-methyl-D-aspartate (NMDA) receptor antagonist which is used therapeutically to prevent relapse in weaned alcoholics, was also effective in suppressing (1) conditioned place aversion induced by morphine withdrawal, and (2) expression of morphine-induced behavioral sensitization. Here, we addressed the question of whether the development of behavioral sensitization, induced by daily injections of morphine (10 mg/kg) over a period of 10 days, could also be suppressed by repeated pretreatment with acamprosate (200 mg/kg). Repeated intermittent injections of morphine produced sensitization of locomotor activity and sniffing behavior. Acamprosate did not block the development of morphine-induced behavioral sensitization. This lack of effect on the development of this phenomenon is inconsistent with the view that NMDA receptor antagonists block the development of sensitization. We suggest that this may derive from the relatively low NMDA receptor-specific antagonism of acamprosate as compared to other NMDA receptor antagonists used in this model. In conclusion, the effect of acamprosate on morphine-induced behavioral sensitization seems to be restricted to the expression of this phenomenon.     

Acute sensitization to opioid antagonists

Acute morphine pretreatment sensitizes rats to the response rate-decreasing effects of opioid antagonists naloxone and naltrexone. The effect appears to be mu-opioid receptor specific, as pretreatment with non-mu-selective oipiod agonists results in less pronounced sensitization. In the present study, food-deprived rats were trained to respond for food reinforcement on a FI 3-min schedule (9.5 min) with multiple trials. Doses of opioid antagonists were administered cumulatively before each trial of a session following 4-h pretreatment with either vehicle or morphine (3.0 mg/kg). Morphine pretreatment sensitized rats to naltrexone, lowering its ED₅₀ from 20 to 0.03 mg/kg. It also sensitized rats to naloxone and to diprenorphine, another pure antagonist. Morphine-induced sensitization was stereoselective among the optical isomers of the benzomorphans, cyclazocine, pentazocine, and N-allylnormetzocine. In addition, acute morphine pretreatment resulted in sensitization to the mixed agonist/antagonist nalorphine, but not to buprenorphine or nalbuphine. The results extend previous findings concerning the importance of the mu-opioid receptor in the development of sensitization to opioid antagonists.     

Temporal factors affecting cocaine-opioid interactions: a cocaine drug discrimination study in rats

RATIONALE: Increasing concomitant abuse of cocaine and morphine-like opioids has prompted a number of studies aimed at understanding how these drugs interact. OBJECTIVE: The present study was designed to determine if variations in opioid pretreatment time would affect how mu opioid agonists interact with cocaine. METHODS: Rats were trained to discriminate 10 mg/kg cocaine from saline. One group of subjects (n=6) received morphine (5.6 mg/kg) 0.5 h, 1 h or 4 h prior to cumulative doses of cocaine (0.1-17.8 mg/kg). These pretreatment times were selected to overlap with states of acute opioid tolerance (approximately 1 h) or acute opioid dependence (approximately 4 h) as demonstrated by previous studies. A second group (n=6) was administered naloxone (0.3 mg/kg) 5 min prior to cumulative cocaine doses, with or without a 4-h morphine (5.6 mg/kg) or methadone (3.0 mg/kg) pretreatment. In a third procedure, the same subjects used in the second experiment were also tested for time-dependent changes in the analgesic effect of morphine using a hot-plate assay. RESULTS: Morphine pretreatment 1 h prior to assessment of the cocaine dose-response function significantly enhanced the discriminative stimulus effects of cocaine. However, neither 0.5-h or 4-h morphine pretreatment had any effect. In contrast, when naloxone was administered 4 h following either morphine or methadone and 5 min prior to assessment of the cocaine dose-response curve, the discriminative stimulus effects of cocaine were significantly attenuated. In assessing morphine-induced analgesia, paw-lick latency was significantly longer at 1 h and shorter at 4 h following morphine administration. CONCLUSIONS: The results illustrate the importance of temporal parameters for interactions between cocaine and mu opioid agonists.