Unilateral analgesia produced by intraventricular morphine.

Morphine injected into the lateral ventricle of the rat produced unilateral analgesia in the formalin test, which involves continuous, moderate pain. In contrast, analgesia was produced bilaterally in the foot-flick test which involves brief, rapidly rising pain. In the formalin test, intraventricular morphine produced analgesia in the ipsilateral but not the contralateral hindpaw. Analgesia was achieved with relatively low doses of morphine (2.5-10.0 micrograms) in the formation test while very high doses (50-200 micrograms) were necessary to produce analgesia in the foot-flick test. These results add to other data indicating that different neural mechanisms underlie opiate analgesia in different types of pain. Moreover, they indicate that, in the formation test, the neural mechanisms of morphine analgesia are somatotopically organized and that forebrain structures are likely to be involved.