Serotonergic pathways in the human brain (image: Schatzberg 2004)
Behavioral aspects in the response by rats to noxious shock and pinch as well as innocuous air puffs were recorded in the dorsal raphe, median raphe and raphe magnus nuclei of awake rats. When electrical stimulation was applied to midbrain central gray and of medial thalamus, as well as systemic administration of morphine, all behavioral and electrophysiological responses to noxious stimuli were greatly diminished (without reliably affecting responses to air puffs). At the same time that brain stimulation and morphine attenuated nociceptive responses were measured, a significant elevation was seen in the spontaneous multiple unit activity of these brain areas, particularly nucleus raphe magnus. These findings point to the importance of the bulbar raphe in mechanisms of analgesia.
Reference: Oleson T, Twombly D, Liebeskind J. Effects of pain-attenuating brain stimulation and morphine on electrical activity in the raphe nuclei of the awake rat. Pain. 1977;Oct(4): 211-230.