Interaction between intrathecal morphine and glutamate receptor antagonists in formalin test

doi:10.1016/S0014-2999(00)00268-5

European Journal of Pharmacology

Volume 395, Issue 3, 3 May 2000, Pages 203-210

https://doi.org/10.1016/S0014-2999(00)00268-5 Get rights and content

Abstract

The analgesic interaction between intrathecally administered morphine and the NMDA receptor antagonist, ((±)-2-amino-5-phosphonopentanoic acid; AP-5), the NMDA receptor glycine site antagonist, (5-nitro-6,7-dichloro-2,3-quinoxaline dion; ACEA 1021), or the AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor antagonist (ACEA 2752) in the formalin test was investigated with a rat model of chronic lumbar intrathecal catheterization. After obtaining dose–response curves for each agent, combinations of morphine and AP-5, ACEA 1021 or ACEA 2752 were tested for their effect on the number of flinches in the formalin test and for associated side-effects, such as motor disturbances, flaccidity, and agitation/allodynia. Using isobolographic analyses, a potent analgesic synergy was observed with decreased side-effects between morphine and ACEA 2752 or AP-5. ACEA 1021 increased the analgesic effect of low-dose morphine. Spinal μ-opioid receptor activation and NMDA or AMPA receptor antagonism showed a synergistic antinociception against tonic pain. These results suggest an important direction in the management of inflammatory pain.

Introduction

The excitatory amino acid neurotransmitters have been demonstrated to play a prominent role in spinal nociceptive transmission Raigorodsky and Urca, 1987, Dickenson and Aydar, 1991. The excitatory effect of glutamate is thought to be mediated by NMDA receptors (Mori and Mishina, 1995) and non-NMDA receptors [mainly AMPA receptors] Honoré et al., 1988, Long et al., 1990. The NMDA receptor is not postsynaptic to the primary afferent. It is postsynaptic to an interneuron mediating a polysynaptic excitation that is responsible for the spinal nociceptive processing that results in a central facilitatory state induced by ongoing C fiber input (Yaksh, 1993). The AMPA receptor mediates monosynaptic fast excitatory transmission involving both innocuous and acute nociceptive input (Headley and Grillner, 1990). The putative strychnine-insensitive glycine site (glycine-2-type receptor) on the NMDA receptor complex is believed to facilitate excitatory transmission.

The use of selective antagonists has suggested that these receptors have a different role in the processing of nociception. NMDA receptor antagonists have minimal effects upon acute nociception, but appear to block tonic pain processing Haley et al., 1990, Näsström et al., 1992. In contrast, AMPA receptor antagonists have an analgesic effect on acute nociception Dougherty et al., 1992, Hunter and Singh, 1994. This is supported by the finding that in the formalin test in rats, NMDA receptor antagonists and non-strychnine-sensitive glycine site antagonists attenuated only the tonic second phase, while AMPA receptor antagonists inhibited the acute phase (Hunter and Singh, 1994).

Morphine, a μ-opioid receptor agonist, can exert an action both pre- and post synaptic to the primary afferent and is well known to have an antinociceptive effect on acute pain (Yaksh and Rudy, 1976) and on tonic pain (Yamamoto and Yaksh, 1992).

Combining the two agents acting on the different receptors, we hypothesized that intrathecal morphine might show a potent interaction with spinal glutamate receptor antagonists. In our previous series of experiments, spinal interaction between morphine and AMPA receptor antagonists was synergistic, whereas there was no such interaction between morphine and NMDA receptor antagonists in a rat model of acute thermal nociception (Nishiyama et al., 1998). The purpose of this study was to investigate the hypothesis that intrathecal morphine might have synergistic antinociceptive effects with both NMDA and AMPA receptor antagonists on tonic pain, using a rat model and the formalin test.

A biphasic nociceptive response to formalin has been widely accepted as a model of prolonged noxious stimulation and is characterized by both behavioral (Wheeler-Aceto and Cowan, 1991) and electrophysiological measures (Haley et al., 1990). It consists of an acute phase (0–5 min) involving direct activation of monosynaptic primary afferent input into the superficial laminae of the spinal cord which, following a quiescent period of 5–10 min, appears to trigger a subsequent polysynaptic response involving sensitization of the deeper laminae dorsal horn neurons and manifested in the behavioral model as a more prolonged, tonic phase of activity, lasting up to approximately 60 min. The tonic phase is also accompanied by the onset of a persistent inflammatory response that lasts beyond the cessation of the nociceptive response (Tjolsen et al., 1992).

Section snippets

Animal preparations

Experiments were carried out according to a protocol approved by the Institutional Animal Care Committee of the University of California, San Diego. Male Sprague–Dawley rats (250–300 g; Harlan Industries, Indianapolis, IN) were implanted with chronic lumbar intrathecal catheters under halothane (2%) anesthesia according to a modification of the method described by Yaksh and Rudy (1976). Briefly, an 8.5 cm polyethylene (PE-10; Clay Adams, Parsippany, NJ) catheter was advanced caudally through an

Effects of morphine and glutamate antagonists

Intrathecal administration of morphine, AP-5 (NMDA receptor antagonist), and ACEA 2752 (AMPA receptor antagonist) resulted in a dose-dependent decrease in the number of flinches in both phases 1 and 2 of the formalin test, although ACEA 1021 (NMDA receptor glycine site antagonist) did not decrease the flinches Fig. 1, Fig. 2. The ED₅₀s were: 7.1 μg [95% confidence interval (CI): 4.6–13.5 μg] in phase 1, 3.6 μg (95% CI: 1.3–9.5 μg) in phase 2 for morphine, 7.6 μg (95% CI: 5.3–12.5 μg) in phase

Discussion

In the present study, the opioid receptor agonist, (morphine), NMDA receptor antagonist, (AP-5) and AMPA receptor antagonist, (ACEA 2752), but not the NMDA receptor glycine site antagonist, (ACEA 1021), produced a dose dependent decrease in the number of flinches in both phases in the formalin test. The NMDA receptor antagonist and AMPA receptor antagonist showed a potent synergistic antinociception with morphine in both phases of the formalin test. This enhancement occurred without an

Acknowledgements

I would like to thank Professor Tony L. Yaksh, Anesthesia Research Laboratory, University of California San Diego for his support of this study.

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¹: This work was done at Anesthesia Research Laboratory, University of California, San Diego, CA, USA

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Interaction between intrathecal morphine and glutamate receptor antagonists in formalin test1

Abstract

Introduction

Section snippets

Animal preparations

Effects of morphine and glutamate antagonists

Discussion

Acknowledgements

Pharmacol. Biochem. Behav.

Brain Res.

Neurosci. Lett.

Pain

Neuropharmacology

Brain Res.

Trends Pharmacol. Sci.

Neurosci. Lett.

Neuropharmacology

Pain

Neurosci. Lett.

Eur. J. Pharmacol.

Brain Res.

Life Sci.

Pain

Physiol. Behav.

Pain

The contribution of excitatory amino acids to central sensitization and persistent nociception after formalin-induced tissue injury

J. Neurosci.

Interaction between intrathecal morphine and glutamate receptor antagonists in formalin test¹