ABSTRACT
Objective To examine whether microinjectlon of morphine into the rat thaiamle nucleus submedlus (Sin) could depress the bee venom (BV)-induced nociceptive behaviours. Methods In inflammatory pain model induced by BV subcutaneous injection into rat unilateral hind paw, the inhibitory effects of morphine microinjection into thalamic nucleus suhmedius (Sin) on the spontaneous nociecptlve behavior, heat hyperalgesia and tactile ailodynia, and the influence of naioxone on the morphine effects were observed in the rat. Results A single dose of morphine (5.0 μg, 0. 5μL) applied into the Sm ipsilaterni to the BV injected paw significantly depressed the spontaneous paw flinching response. Morphine also significantly increased the heat paw withdrawal iateneies in the bilateral hind paw and the tactile paw withdrawal threshold in the ipsilnteral hind paw 2 hours after BV injection. All these depressive effects could be effectively antagonized by pre-treatment with the opiuld receptor antagonist naloxone (1.0μg, 0. 5μL) in the Sm 5rain prior to morphine administration. Naloxone alone injected to the Sm had no effect on the BV-induecd nociceptive behavior. Conclusion These results suggest that Sm is involved in opioid receptor-mediated antt-nociception in the rat with the BV-induced inflammatory pain. Together with results from previous studies, it is likely that this effect is produced by activation of the Sm-ventrolateral orbital cortex-periaqueductal gray pathway, leading to activation of the brainstem descending inhibitory system and depression of the nodceptive inputs at the spinal cord level.