Descending control from the brain to the spinal cord shapes our pain experience, ranging from powerful analgesia to extreme sensitivity. Increasing evidence from both preclinical and clinical studies points to an imbalance toward descending facilitation as a substrate of pathological pain, but the underlying mechanisms remain unknown. We used an optogenetic approach to manipulate serotonin (5-HT) neurons of the nucleus raphe magnus that project to the dorsal horn of the spinal cord. We found that 5-HT neurons exert an analgesic action in naïve mice that becomes proalgesic in an experimental model of neuropathic pain. We show that spinal KCC2 hypofunction turns this descending inhibitory control into paradoxical facilitation; KCC2 enhancers restored 5-HT–mediated descending inhibition and analgesia. Last, combining selective serotonin reuptake inhibitors (SSRIs) with a KCC2 enhancer yields effective analgesia against nerve injury–induced pain hypersensitivity. This uncovers a previously unidentified therapeutic path for SSRIs against neuropathic pain.

In mammals, threat-related behavior is typically induced by a noxious physical stressor and is associated with a broad range of behavioral responses such as freezing and avoidance. These behavioral responses are associated with the regulation of pain responses allowing individuals to cope with noxious stimuli. Whereas the structures and mechanisms involved in pain behavior are well documented, little is known about the precise neuronal circuits mediating the emotional regulation of pain behavior. Here we used a combination of behavioral, anatomical, optogenetic, and electrophysiological approaches to show that somatostatin-expressing neurons in the ventrolateral periaqueductal gray matter (vlPAG SST cells) promote antinociceptive responses during the presentation of conditioned stimuli (CS) predicting footshocks. Whereas the optogenetic inhibition of vlPAG SST cells during CS presentation promoted analgesia, their optogenetic activation reduced analgesia by potentiating pain responses in the spinal cord through a relay in the rostral ventromedial medulla (RVM). Together these results identify a brainstem circuit composed of vlPAG SST cells specifically projecting to the RVM and mediating fear conditioned analgesia (FCA) to regulate pain responses during threatful situations.

Upon threatening situations, animals exhibit a broad range of behavioral and autonomic responses necessary for survival. Under such conditions, a crucial adaptive response is the inhibition of pain responses that would otherwise interfere with behavioral defensive responses. Whereas the structures and mechanisms involved in fear and pain behavior are well documented, little is known about the precise neuronal mechanisms mediating the emotional regulation of endogenous pain-suppression. Here we used a combination of behavioral, anatomical, optogenetic, and electrophysiological approaches to show that somatostatin-expressing cells in the ventrolateral periaqueductal gray matter (SST+ vlPAG cells) control the analgesia induced by a negative emotional state. Our data indicate that the optogenetic inhibition of SST+ vlPAG cells promotes analgesia. Conversely, the optogenetic activation of long-range SST+ vlPAG cells that project to the RVM abolishes the analgesia mediated by a negative emotional state without impacting fear behavior. Together these results identify a novel brainstem circuit mechanism composed of long-range SST+ vlPAG cells projecting to the RVM that regulate analgesia elicited by negative emotional states.