Pain and the bulbospinal noradrenergic system: pain-induced increase in normetanephrine content in the spinal cord and its modification by morphine.

Experiments were carried out to determine whether noxious stimulation produced biochemical changes in noradrenergic neurons in the spinal cord of rats and whether morphine induced different biochemical effects in the presence and the absence of noxious stimuli. Noxious stimuli, but not stress, significantly increased the normetanephrine (NM) concentration only in the dorsal half of the spinal cord without affecting the noradrenaline concentration. The NM increase induced by noxious stimuli remained after transection at the inter-collicular level. The NM increase induced by noxious stimuli remained after transection at the inter-collicular level. When the noxious stimulation was blocked by lidocaine at the peripheral level, the NM increase disappeared. THe NM concentration elevated by pain, was further increased by morphine. The data suggest that the bulbospinal noradrenergic system works as a pain control system: pain itself slightly activates this system, but not to the extent of producing analgesia and the addition of morphine further activates this system to a level sufficient to produce analgesia.