Reduced effect of morphine in midbrain raphe lesioned rats

The reaction of painful stimulation was studied in rats by means of three different techniques.

Lesions of the midbrain raphe in rats, which reduced the levels of brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) decreased the analgesic activity of morphine. In some tests, an increase in sensitivity to painful stimulation was noted in lesioned animals.

The results obtained support the view that a decrease of brain 5-HT concentration antagonizes morphine analgesia. The possible role of the other brain monoamines in the analgesic activity of morphine is also discussed.