Effects of systemic morphine on diffuse noxious inhibitory controls: role of the periaqueductal grey

The effects of systemic morphine (1 mg/kg i.v.) on diffuse noxious inhibitory controls (DNIC) were studied in both sham-operated animals and those with quinolinic acid-induced lesions of the periaqueductal grey (PAG). DNIC acting on convergent neurones in the dorsal horn of the spinal cord were similar in the sham-operated and lesioned animals. However, following morphine injection, DNIC were blocked in the sham-operated but not in the PAG-lesioned animals. It is concluded that, although the PAG is not directly involved in the supraspinal loop subserving DNIC, it can modulate these controls. In addition, as naloxone reversed the effects of morphine in the control group but reduced DNIC in the PAG-lesioned animals, it is suggested that more than one opioidergic system is involved in DNIC.