Analgesia induced by morphine microinjection into the lateral hypothalamus of the rat.

Earlier studies have demonstrated that electrical stimulation of the lateral hypothalamus (LH) produces analgesia in tests of brief, phasic pain and more prolonged tonic pain. The present experiment examined the effect of morphine microinjection into the LH on formalin-induced pain. The results show that a 10-micrograms morphine microinjection significantly decreases pain responses. Moreover, the decrease persists for the duration of the 25-min test period, which began 25 min after the formalin injection. These findings suggest that electrical stimulation of the LH engages an endogenous opiate system which is capable of producing prolonged analgesia.