Kathryn J Elliott

The NMDA Receptor antagonists, LY274614 and MK-801, and the nitric oxide synthase inhibitor, NG-nitro-l-arginine, attenuate analgesic tolerance to the mu-opioid morphine but not to kappa opioids

&NA; Once daily s.c. administration of 5 mg/kg morphine, a mu‐opioid agonist, or U50488H (U50), a kappa1‐opioid agonist, for 5 days in male CD‐1 mice results in a 2–3‐fold shift to the right of the respective analgesic (tail flick) dose‐response curves, indicating the development of tolerance. Concurrent s.c. administration of the competitive NMDA receptor antagonist, LY274614 (LY), at 24 mg/kg/24 h infusion (osmotic pump) or 6 mg/kg i.p. once daily attenuates the development of morphine tolerance, when the response to saline plus morphine is compared on day 5 with LY plus morphine. Using this paradigm, once daily administration of either the non‐competitive NMDA antagonist, MK‐801, at 0.3 mg/kg i.p. or the nitric oxide synthase inhibitor, NG‐nitro‐L‐arginine (NorArg), at 1 mg/kg i.p. twice daily attenuated the development of morphine tolerance. None of these drugs modify the tail‐flick response or alter the ED50 for morphine. In contrast, co‐administration of LY, MK‐801 or NorArg, as above, failed to attenuate the development of tolerance to U50 or to the kappa3‐opioid agonist, naloxone benzoylhydrazone (NalBzoH). These results suggest that mu‐opioid tolerance but not kappa1‐ or kappa3‐opioid tolerance involves the mediation of NMDA receptors and the nitric oxide system.

The d- and l-isomers of methadone bind to the non-competitive site on the N-methyl-D-aspartate (NMDA) receptor in rat forebrain and spinal cord.

Racemic (dl)-methadone has antagonist activity at the N-methyl-D-aspartate (NMDA) receptor. We evaluated dl-methadone, the opioid active (l-) and the opioid inactive (d-) isomers in competition binding assays. dl-Methadone and its d- and l- isomers exhibited low micromolar affinities for the [3H]MK-801-labeled non-competitive site of the NMDA receptor in both rat forebrain and spinal cord synaptic membranes, with Ki values and displacement curves similar to those of dextromethorphan, an established NMDA receptor antagonist. They lacked affinity at the [3H]CGS-19755-labeled competitive site of the NMDA receptor. Therefore, both methadone and its the d- and l- isomers differ from morphine, hydromorphone, and naltrexone in that they have non-competitive antagonist activity at the NMDA receptor. A non-opioid NMDA receptor antagonist, such as d-methadone, may improve the efficacy of morphine by attenuating the development of tolerance.

N-Methyl-D-Aspartate (NMDA) Receptors, Mu and Kappa Opioid Tolerance, and Perspectives on New Analgesic Drug Development

This laboratory perspective reviews the pharmagologic approaches that have been used in preclinical animal models to demonstrate the ability of competitive (LY274614) and noncompetitive (MK801 and dextromethorphan) N-methyl-D-aspartate (NMDA) receptor antagonists to attenuate or reverse the development of morphine tolerance. We provide additional data to support previous observations that these NMDA antagonists modulate morphine (mu) opioid tolerance but do not affect U50488H (kappa1) opioid tolerance. A strategy, which utilizes efficacy as an NMDA receptor antagonist and clinical safety, provides the basis for a discussion of the clinical potential of dextromethorphan, ketamine, and felbamate as modulators of opioid tolerance in pain patients or opioid addicts. The potential use of NMDA receptor antagonists and nitric oxide synthase (NOS) inhibitors in neuropathic pain is also discussed.

Dextromethorphan attenuates and reverses analgesic tolerance to morphine

&NA; Tolerance to the antinociceptive (analgesic) effect of morphine, a mu‐opioid agonist, was developed in male CD‐1 mice as assessed by a shift to the right of the analgesic (tail‐flick) dose‐response curves and an increase in the ED50 values. Administration of dextromethorphan at 30 mg/kg s.c., but not saline, 30 min prior to an escalating 3 times per day (t.i.d.) morphine dosing schedule prevented a 5‐fold increase in the morphine ED50 value observed on treatment day 4. Concurrent administration of dextromethorphan at 12 mg/kg/24 h by s.c. infusion prevented the 6‐fold increase in the morphine ED50 value that was observed in control mice that received morphine at 30 mg/kg/24 h by s.c. infusion. Implantation of two 25 mg morphine pellets resulted in a 10‐fold increase in the morphine ED50 value on treatment day 4. Administration of dextromethorphan at 30 mg/kg s.c. t.i.d., but not saline, resulted in a reversal of morphine tolerance with the almost complete return of the morphine ED50 value to the control (opioid naive) value. These results demonstrate that dextromethorphan, an NMDA receptor antagonist can modulate morphine (mu‐receptor)‐mediated tolerance. Symbol. No caption available

Spinal gabapentin is antinociceptive in the rat formalin test

Gabapentin is a novel anticonvulsant that may be of value for the relief of clinical pain. To determine whether gabapentin is antinociceptive after spinal administration, the drug was given via an intrathecal catheter in doses from 6 to 200 micrograms/rat 10 min prior to intraplantar formalin. Five percent formalin injected subcutaneously in the right hind paw produced a biphasic reaction consisting of flinching and licking behaviors (phase 1, 0-10 min; phase 2, 10-60 min). Gabapentin dose-dependently reduced the numbers of flinches and the duration of licking during phase 2 of the formalin test. The highest dose of gabapentin (200 micrograms/rat) did not affect the tail-flick response. These results demonstrate that spinal gabapentin is antinociceptive in the formalin test.

Gabapentin enhances the antinociceptive effects of spinal morphine in the rat tail-flick test

Abstract The antinociceptive effects of the combination of spinal morphine and gabapentin were evaluated in the tail‐flick test in rats. The intrathecal coadministration of a subantinociceptive dose of morphine at 0.2 &mgr;g and gabapentin at 300 &mgr;g produced significant antinociception. Pretreatment with spinal gabapentin at 300 &mgr;g shifted the dose‐response curve of spinal morphine to the left with a decrease in morphine ED50 value from 1.06 &mgr;g to 0.34 &mgr;g. The antinociceptive effects produced by the combination of a subantinociceptive dose of morphine and gabapentin were reversed by spinal naloxone at 30 &mgr;g but were not reversed by spinal bicuculline at 0.3 &mgr;g. Furthermore, the concurrent administration of spinal naloxone at 30 &mgr;g with the combination of morphine and gabapentin blocked antinociception, while the concurrent administration of spinal bicuculline at 0.3 &mgr;g failed to prevent antinociception. These results indicate that the combination of spinal gabapentin and morphine produces an enhancement of antinociception that appears to involve the spinal mu opioid receptors. Furthermore, repeated administration of gabapentin for 3 days did not affect the enhancing effect of gabapentin on the antinociceptive effect of morphine, indicating that tolerance did not develop to gabapentins ability to enhance morphine antinociception.

Oral ketamine is antinociceptive in the rat formalin test : role of the metabolite, norketamine

The present study was designed to evaluate the oral efficacy and bioavailability of ketamine. Antinociceptive efficacy was determined with the rat formalin test and oral bioavailability by the measurement of plasma and brain concentrations of ketamine and its major metabolite, norketamine. Oral ketamine in a dose range from 30 to 180 mg/kg or saline was given prior to intraplantar formalin and the flinching behavior was measured. Oral ketamine dose-dependently reduced the flinching during phase 2, while flinching during phase 1 was reduced only with the highest dose given. Following oral ketamine at 100 mg/kg, blood and brain samples were obtained and plasma and brain ketamine and norketamine levels were measured using high-performance liquid chromatography (HPLC). The average concentration ratio of norketamine/ketamine, as expressed by the area under the curve (AUC) value, was 6.4 for plasma and 2.9 for brain. These results demonstrate that a significant amount of norketamine is formed by first pass biotransformation of ketamine and is distributed to the brain. Competition binding assays for the [3H]MK-801-labeled non-competitive site of the N-methyl-D-aspartate receptor (NMDA) receptor revealed that both norketamine and ketamine displaced [3H]MK-801 at low micromolar concentrations with Ki values of 2.5 and 0.3 mM in the forebrain, and 4.2 and 1.0 mM in the spinal cord, respectively. Spinal norketamine was approximately equipotent to ketamine in producing antinociceptive effects during phase 2 of the formalin test. Thus, norketamine appears to contribute to the antinociceptive effects of oral ketamine through its NMDA receptor antagonist activity.

Dextromethorphan suppresses both formalin-induced nociceptive behavior and the formalin-induced increase in spinal cord c-fos mRNA.

&NA; The injection of dilute formalin results in a stereotyped nociceptive behavioral response. Administration of dextromethorphan (s.c.) but not saline, 30 min prior to intraplantar formalin injection prevents this nociceptive response in a dose‐dependent manner. In addition, intraplantar formalin reliably induces c fos mRNA in the ipsilateral spinal dorsal horn as assessed with quantitative solution hybridization at 30 min postinjection. No change in c‐fos mRNA was detected in the contralateral spinal dorsal horn, nucleus raphe magnus, periaqueductal grey, medial thalamus, or sensorimotor cortex. Pretreatment with dextromethorphan at 60 mg/ kg s.c., 30 min prior to formalin resulted in a suppression of c fos induction, so that c fos mRNA levels in the ipsilateral spinal dorsal horn of animals receiving dextromethorphan prior to formalin did not differ from controls. These data indicate that dextromethorphan suppresses formalin nociceptive behavior and one of the biochemical consequences of formalin nociception, i.e., induction of c‐fos mRNA.

Oral ketamine produces a dose-dependent CNS depression in the rat

The oral to parenteral potency of ketamine to produce central nervous system (CNS) depression was estimated in rats using a behavioral scale. As a function of dose, ketamine produces CNS depression that ranges from ataxia to anesthesia by either route of administration. The oral ED50 value for ketamine was 120 mg/kg (82.2-159.2, 95% CI) while the intravenous ED50 value was 3.7 mg/kg (2.4-5.1, 95% CI). Oral ketamine produces CNS effects comparable to intravenous ketamine, although the drug is approximately 32 fold less potent by the oral route.