Effects of iontophoretically applied serotonin on three classes of physiologically characterized putative pain modulating neurons in the rostral ventromedial medulla of lightly anesthetized rats.

The importance of the rostral ventromedial medulla (RVM) in nociceptive modulation is well documented, and several lines of evidence point to a role for serotonin (5HT) in regulating the activity of pain modulating neurons in this region. The aim of the present study was to examine the effect of iontophoretically applied 5HT upon the firing of three physiologically defined classes of RVM neurons with distinct roles in pain modulation. The predominant effect across all classes was a facilitation of ongoing or evoked activity. A minority of cells within each class were inhibited by 5HT itself, but agonists at 5HT1 receptor types inhibited the majority of cells tested. The results thus indicate that the behavioral effects of manipulating 5HT within the RVM cannot be attributed to a selective influence on a particular cell class.