New Mechanisms Involved in the Antinociception and Tolerance Induced by Agonists of

Methods and Findings in Experimental and Clinical Pharmacology
Vol. 25, Suppl. A, 2003
ISSN 0379-0355
Copyright 2003 Prous Science, S.A.
CCC: 0379-0355/2003
http://www.prous.com

New Mechanisms Involved in the Antinociception and Tolerance Induced by Agonists of m-Opioid Receptors

J.M. Baeyens

Department of Pharmacology and Neuroscience Institute, Faculty of Medicine, University of Granada, Spain

Agonists of m-opioid receptors produce antinociception through the activation of different kinds of G proteins that modulate the activity of several effector mechanisms, such as ionic channels and enzymes. We evaluated the modulation of m-opioid receptors of brain Na⁺,K⁺-ATPase activity by agonists in vitro and the possible role of this effect in the antinociception induced by these drugs. Morphine and endomorphin-1 produced a concentration-dependent and naloxone-reversible enhancement of synaptosomal Na⁺,K⁺-ATPase activity (1, 2). This effect of morphine is antagonized by pretreatment with b-funaltrexamine, naloxonazine or pertussis toxin, which suggests that morphine enhances Na⁺,K⁺-ATPase activity in the brain by activating m-opioid receptors and G_i/o proteins (1).

If activation of Na⁺,K⁺-ATPase plays a role in the antinociceptive effect of morphine, Na⁺,K⁺-ATPase inhibitors (such as digitalis glycosides) might be able to antagonize such an effect. The i.c.v. administration of several Na⁺,K⁺-ATPase inhibitors (ouabain at 0.1-100 ng/mouse, digoxin at 1-1000 ng/mouse and digitoxin at 10-10000 ng/mouse) dose-dependently antagonized the antinociceptive effect of morphine in mice, with the following order of potency: ouabain > digoxin > digitoxin (3). This effect can not be explained by any interaction at opioid receptors, since none of these Na⁺,K⁺-ATPase inhibitors displaced [³H]-naloxone from its binding sites, whereas naloxone did so in a concentration-dependent manner. The antinociception induced by morphine in rats was also antagonized by the i.c.v. administration of ouabain at 10 ng/rat, whereas it was not significantly modified by intrathecally administered ouabain (10 and 100 ng/rat). These results suggest that the enhancement of neuronal Na⁺,K⁺-ATPase activity induced by morphine can be involved in its supraspinal, but not spinal, antinociceptive effect (3).

Repeated administration of morphine produced tolerance to many of its biochemical and behavioral effects, including antinociception. This prompted us to evaluate whether morphine tolerance alters the ability of this drug to modulate Na⁺,K⁺-ATPase. Administration of morphine for several consecutive days reduced the potency and efficacy of this drug to activate synaptosomal Na⁺,K⁺-ATPase activity. Moreover, i.c.v. administration of ouabain (1-10 ng/mouse) was not able to antagonize morphine-induced antinociception in morphine- tolerant mice. These results suggest that changes in the ability of morphine to modulate Na⁺,K⁺-ATPase activity occur during morphine tolerance.

Finally, since not all agonists of m-opioid receptors activate the same effector mechanisms to induce antinociception (4), we tested whether different m-opioid agonists share the ability to activate Na⁺,K⁺-ATPase. Incubation in vitro of forebrain synaptosomes with morphine (1 nM-100 mM), levorphanol (0.1 nM-100 mcM) or buprenorphine (0.1 nM-10 mcM) concentration-dependently stimulate ouabain-sensitive Na⁺,K⁺-ATPase activity. The order of efficacy (using the E_max as a measure of intrinsic efficacy) was: morphine > levorphanol > buprenorphine. On the other hand, fentanyl (0.1 nM-10 mM) produces a smaller enhancement of Na⁺,K⁺-ATPase activity and methadone (0.1 nM-10 mM) does not significantly enhance it. In agreement with these data, the antinociception induced by morphine (1-32 mg/kg, s.c.), levorphanol (0.4-6.4 mg/kg, s.c.) and buprenorphine (0.02-0.64 mg/kg, s.c.) in mice is antagonized in a dose-dependent manner by ouabain (0.001-10 ng, i.c.v.), while the antinociception produced by fentanyl (0.02-0.16 mg/kg, s.c.) and methadone (2-10 mg/kg, s.c.) is not significantly modified by ouabain (1-100 ng, i.c.v.). Thus, we can conclude that at least two subgroups can be distinguished among the m-opioid receptor agonists, taking into consideration their ability to enhance Na⁺,K⁺-ATPase activity and the role of Na⁺,K⁺-ATPase in their antinociceptive effects.

ACKNOWLEDGEMENTS

This work was supported in part by grants from CICYT (SAF 97-0173) and Junta de Andalucia (CTS109).

REFERENCES

1. Masocha, W., González, L.G., Baeyens, J.M., Agil, A. Mechanisms involved in morphine-induced activation of synaptosomal Na⁺,K⁺-ATPase. Brain Res 2002, 957: 311-9.

2. Horvath, G., Agil, A., Joo, G., Dobos, I., Benedek, G., Baeyens, J.M. Evaluation of endomorphin-1 on the activity of Na⁺,K⁺-ATPase using in vitro and in vivo studies. Eur J Pharmacol 2003, 458: 291-7.

3. Masocha, W., Horvath, G., Agil, A. et al. Role of Na⁺,K⁺-ATPase in morphine-induced antinociception. J Pharmacol Exp Ther 2003, 306: 1-7.

4. Ocaña, M., Del Pozo, E., Barrios, M., Baeyens, J.M. Subgroups among m-opioid receptor agonists distinguished by ATP-sensitive K⁺ channel-acting drugs. Br J Pharmacol 1995, 114: 1296-302.

Methods and Findings in Experimental and Clinical Pharmacology Vol. 25, Suppl. A, 2003
ISSN 0379-0355 Copyright 2003 Prous Science, S.A. CCC: 0379-0355/2003 http://www.prous.com