Effects of biogenic amines on raphe-spinal tract cells

Stimulation in the nucleus raphe magnus produces analgesia in behavioral paradigms and inhibits spinal cord nociceptive neurons. Similar effects result from stimulation of the periaqueductal gray (PAG). Such actions may be mediated via a synaptic link between PAG and nucleus raphe magnus or the adjacent reticular formation. In this study we have examined the effects of biogenic amines applied iontophoretically in the vicinity of nucleus raphe magnus neurons that project to the spinal cord in monkeys. Raphe-spinal tract (RST) neurons were identified by antidromic activation after stimulation of the dorsolateral funiculi at an upper lumbar level. The actions of serotonin, quipazine, norepinephrine, dopamine and acetylcholine (ACh) were tested against the background activity, the activity evoked by glutamate pulses or the excitation of RST cells by stimulation in the PAG. Serotonin, quipazine, norepinephrine and dopamine produced a current-dependent inhibition of background activity and the responses to glutamate pulses in all RST cells tested. No cases of excitation were found. By contrast, ACh enhanced activity produced by glutamate pulses in all RST cells observed. ACh also enhanced the background activity of all but one of the RST cells; however, ACh did not activate cells with little or no background discharge. Serotonin and norepinephrine often inhibited PAG excitation of RST cells. No facilitation of PAG excitation was observed. We conclude that the actions of serotonergic and catecholaminergic agonists on raphe-spinal cells are inhibitory whereas the effect of ACh is facilitatory.