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Br J Pharmacol. 1992 Mar;105(3):675-8.

Inhibitory effect of morphine on yawning induced by cholinoceptor and dopamine D2 receptor activation in rats.

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  • 1Department of Pharmacology, Medical Faculty, University of Tehran, Iran.


1. Bromocriptine (2, 4 and 8 mg kg-1, i.p.), physostigmine (0.05, 0.1 and 0.2 mg kg-1, i.p.) and pilocarpine (1, 3 and 5 mg kg-1, i.p.) induced dose-dependent yawning in rats. 2. These responses were reduced in a dose-dependent manner by pretreatment with morphine. 3. The inhibitory effect of morphine was reversed by naloxone. 4. Naloxone alone induced slight but significant yawning. 5. The present results suggest that morphine inhibits yawning in rats at an opiate receptor downstream from the sites at which cholinoceptor and dopamine D2 activation induce yawning. The anatomical location of these sites remains to be established.

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