A study of the influence of hydroxyzine and diazepam on morphine antinociception in the rat

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Abstract
The influence of hydroxyzine, a minor tranquilizer devoid of hypnotic activity, on the action of morphine on nociception and locomotor activity was investigated in the rat and compared with diazepam. Antinociception was evaluated by 2 methods: the electrical stimulation of the tail and the tail-flick. Spontaneous locomotor activity was measured by an Animex apparatus. Using the electrical stimulation of the tail, hydroxyzine (12.5, 25 and 50 mg/kg i.p.) showed little direct analgesic activity but markedly potentiated only the effect of morphine on the vocalization after-discharge which represents the affective component of pain. Equimolar doses of diazepam showed antinociceptive properties which were not blocked by naloxone and also potentiated morphine action on the affective component of pain. On the tail-flick test, hydroxyzine alone had no effect; hydroxyzine potentiated morphine at the largest dose tested, however it also decreased morphine antinociception at a lower dose. Diazepam alone had little effect on the tail-flick test but it potentiated morphine antinociception only at the largest dose. Both hydroxyzine and diazepam potentiated morphine depression of the locomotor activity but diazepam was significantly more active.