The present investigation was focused on the effects induced by haloperidol and apomorphine on the antinociception induced by DAMGO (highly selective μ-agonist), U50-488H (highly selective k-agonist) and deltorphin II (highly selective δ-agonists). Haloperidol (a dopamine receptor antagonist) and apomorphine (a dopamine receptor agonist) (0.1-1.0-2.0 mg/kg/i.p.) per se did not change the pain threshold of mice both in the hot plate and in the tail flick test. The antinociception effects of DAMGO (5 mg/kg/i.p.), U50-488H (5 mg/kg/i.p.) and deltorphin II (10 ng/i.c.v./mouse) peaked by 15 min after treatment and was antagonized by haloperidol in a dose dependent manner. By contrast, the dopamine receptor agonist, apomorphine, depending on the doses used, exhibited opposite effects on opiate antinociception. Treatment of mice with low doses of apomorphine (0.1-1.0 mg/kg/i.p.) reduces the antinociception induced by μ, k and δ receptor agonists whereas a high dose (2 mg/kg/i.p.) potentiates it. Our results indicate that dopamine receptors are involved in the control of antinociception at both the μ, k and δ receptor level.
Keywords: Antinociception, Apomorphine, DAGO, Deltorphin II, Haloperidol, U50-488H
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