Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of α₂ adrenoceptors and GABA_B but not κ-opioid receptors in the tail flick test in mice

Abstract 

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the μ-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25–4mg/kg), baclofen (2–32mg/kg) or U50,488H (2–16mg/kg) (agonists of α₂ adrenoceptors, GABA_B and κ-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001–10pg/mouse, i.c.v.) and cantharidin (0.001–10ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001–1000pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of κ-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Keywords: Antinociception, G protein-coupled receptors, Serine/threonine protein phosphatases, Okadaic acid, Cantharidin, Tail flick test