Differential involvement of μ-opioid receptor subtypes in endomorphin-1- and -2-induced antinociception

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Abstract

We investigated the role of μ-opioid receptor subtypes in both endomorphin-1 and endomorphin-2 induced antinociception in mice using supraspinally mediated behavior. With tail pressure as a mechanical noxious stimulus, both intracerebroventricularly (i.c.v.) and intrathecally (i.t.) injected-endomorphins produced potent and significant antinociceptive activity. Antinociception induced by i.t. and i.c.v. injection of endomorphin-1 was not reversed by pretreatment with a selective μ1-opioid receptor antagonist, naloxonazine (35 mg/kg, s.c.). By contrast, antinociception induced by i.t. and i.c.v. endomorphin-2 was significantly decreased by μ1-opioid receptor antagonist. Antinociception of both i.t. and i.c.v. endomorphin-1 and -2 was completely reversed by pretreatment with β-funaltrexamine (40 mg/kg, s.c.). The results indicate that endomorphins may produce antinociception through the distinct μ1 and μ2 subtypes of μ-opioid receptor.

Introduction

Endomorphin-1 and -2 are tetrapeptide amides isolated from bovine brain, found to have high affinity and selectivity for the μ-opioid receptor, and shown to produce potent and prolonged antinociceptive activity that is reversible by naloxone and β-funaltrexamine (Zadina et al., 1997). There is biochemical and pharmacological evidence supporting the existence of μ-opioid receptor subtypes (Wolozin and Pasternak, 1981; Nishimura et al., 1984; Goodman and Pasternak, 1985). At least two μ-opioid receptor subtypes have been proposed: μ1 and μ2 (Pasternak and Wood, 1986). β-Funaltrexamine irreversibly antagonizes both μ1- and μ2-opioid receptors and inhibits both supraspinal and spinal antinociception, whereas naloxonazine selectively antagonizes the μ1-opioid receptor. It has been suggested that these receptor subtypes have different physiological roles, with μ1-opioid receptors mediating supraspinal antinociception, whereas μ2-opioid receptors mediate spinal antinociception.

In most studies, the method used to measure the antinociceptive effects of opioids is the tail flick test, which depends on tail withdrawal from radiant heat as the noxious stimulus, a response that involves a spinal reflex. The tail flick test does not require the integrative action of the higher brain centers (Irwin et al., 1951). We therefore have studied antinociceptive effects of endomorphins after pretreatment with naloxonazine or β-funaltrexamine on behavior (licking or biting) that requires perception of the noxious stimulus, and integration of behavioral responses, by central (supraspinal) mechanisms. In the present study, the role of the μ-opioid receptor subtypes, μ1 and μ2, in the antinociceptive effects of endomorphins against the response to mechanical noxious stimuli was examined with the irreversible μ1-opioid receptor antagonist, naloxonazine and the irreversible μ1- and μ2-opioid receptor antagonist, β-funaltrexamine.

Section snippets

Materials and methods

Adult male ddY mice weighing 22–25 g were housed in a light- and temperature-controlled room (light on 0900–2100 h; 24°C) and had free access to food and water. Intracerebroventricular (i.c.v.) injections were administered about 2 mm caudal and 2 mm lateral to the bregma at a depth of 3 mm (Haley and McCormick, 1957). The procedure for intrathecal (i.t.) injections was adapted from the method of Hylden and Wilcox (1980)with a constant injection volume of 2 μl/mouse. For i.t. administration, a

Potency and time course of i.c.v. and i.t. injections of endomorphin-1 and -2

The time course of antinociceptive activity for i.c.v. endomorphin-1, -2 and DAMGO is shown in Fig. 1. Groups of mice were tested for antinociception at 1, 3, 5, 10 and 15 min after i.c.v. injection of the endomorphins. Endomorphin-1 and -2 produced dose-dependent antinociception with ED50 values of 1.20 (CL95: 0.68–2.11) nmol, and 1.35 (CL95: 0.70–2.59) nmol, respectively, with maximum effects at 5 min (Fig. 1A,B). Endomorphin-1 had a longer duration of action than endomorphin-2, but the two

Discussion

In this study we showed that both endomorphin-1 and endomorphin-2 induced dose-dependent antinociception after central (i.c.v.) and spinal (i.t.) administration in a test involving central integration of response. The peak effects occurred rapidly, with 5 min for i.c.v. and within 1 min for i.t. injection. The present results of i.t. injected endomorphins are in agreement with those of Stone et al. (1997)who reported that antinociception induced by endomorphin-1 and -2 is short-lasting and is

Acknowledgements

The authors wish to thank Miho Nakayama for her secretarial assistance in preparing the manuscript.

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