Do medullary serotonergic neurons tonically modulate nociceptive transmission?

Mason and colleagues have developed an analytical method to reliably identify serotonergic neurons in the rostral ventromedial medulla (RVM), including the raphe magnus, during extracellular recordings in vivo. Results of these studies strongly suggest that the serotonergic neurons do not belong to the major classes of opioid-sensitive neurons phasically activated or inhibited during noxious stimuli. However, their studies have not ruled out any involvement of serotonergic RVM neurons in supraspinal opioid antinociception, and they are at odds with detailed in vitro studies of opioid mechanisms in RVM neurons. It remains possible that tonic activity serotonergic RVM neurons plays an important role in descending opioid modulation of nociception.