Failure of spinal cord serotonin depletion to alter analgesia elicited from the periaqueductal gray

The effects of spinal cord serotonin depletion or combined serotonin/norepinephrine depletion on analgesia elicited by electrical stimulation of, or morphine microinjection into, the periaqueductal gray, were tested. Spinal cord serotonin was depleted by intrathecal injection of 5,7-dihydroxytryptamine (5,7-DHT), preceded by systemic desipramine, while 5,7-DHT alone was used to deplete both norepinephrine and serotonin. Selective serotonin depletion had no effect on analgesia induced by either method at 24 h, 1 week, or 2 weeks after treatment. Depletion of both monoamines had no effect on stimulation produced analgesia 24 h and one week after treatment, but produced a slight attenuation 2 and 3 weeks after treatment. In contrast, depletion of both monoamines drastically attenuated morphine analgesia 24 h after treatment. The results are discussed in relation to multiple pain inhibitory pathways.