Antinociceptive activity of morphine after injection of biogenic amines in the cerebral ventricles of the conscious rat.

1. A simple cannula and a cannula guide for making injections into the cerebral ventricles of conscious rats are described.2. Intraventricular injections of 5-hydroxytryptamine (5-HT) or of noradrenaline (NA) were without effect on the nociceptive threshold of rats.3. Intraventricular injection of 5-HT potentiated the antinociceptive effect of morphine. Reserpine pretreatment antagonized the antinociceptive effect of morphine; this effect was reversed by intraventricular injection of 5-HT.4. Intraventricular injection of NA attenuated the antinociceptive action of morphine but was without effect on the inhibition by reserpine of the antinociceptive effect of morphine.5. Subcutaneous injection or slow intravenous infusion of either 5-HT or NA (up to 300 mug/rat) were without significant effect on the antinociceptive effects of morphine.6. Intraperitoneal administration of dopa increased the nociceptive threshold above normal, but reduced the antinociceptive effect of morphine. Intraventricular injection of either dopa or dopamine had no antinociceptive effect but inhibited that of morphine.7. It is suggested that the antinociceptive effect of morphine may depend on the balance between the concentrations of 5-HT and NA in the brain.