Naloxone antagonizes theophylline-induced potentiation of morphine inhibition of a nociceptive reaction in rats.

In the rat, theophylline has been shown to potentiate the effect of morphine on the threshold for vocalisation after withdrawal of stimulation. This response to painful stimulation is considered to be integrated at the level of the thalamus-hypothalamus-rhinencephalon. Naloxone antagonized the effect of the combined treatment with morphine and theophylline, suggesting pharmacological specificity for morphine. Moreover, the theophylline-induced enhancement of this pharmacological response to morphine was attenuated after pimozide pretreatment, indicating an underlying dopaminergic mechanism.