Descending serotonergic modulation from rostral ventromedial medulla to spinal trigeminal nucleus is involved in experimental occlusal interference-induced chronic orofacial hyperalgesia

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

Dissecting neural circuits from rostral ventromedial medulla to spinal trigeminal nucleus bidirectionally modulating craniofacial mechanical sensitivity

Chronic craniofacial pain is intractable and its mechanisms remain unclarified. The rostral ventromedial medulla (RVM) plays a crucial role in descending pain facilitation and inhibition. It is unclear how the descending circuits from the RVM to spinal trigeminal nucleus (Sp5) are organized to bidirectionally modulate craniofacial nociception. We used viral tracing, in vivo optogenetics, calcium signaling recording, and chemogenetic manipulations to investigate the structure and function of RVM-Sp5 circuits. We found that most RVM neurons projecting to Sp5 were GABAergic or glutamatergic and facilitated or inhibited craniofacial nociception, respectively. Both GABAergic interneurons and glutamatergic projection neurons in Sp5 received RVM inputs: the former were antinociceptive, whereas the latter were pronociceptive. Furthermore, we demonstrated activation of both GABAergic and glutamatergic Sp5 neurons receiving RVM inputs in inflammation- or dysfunction-induced masseter hyperalgesia. Activating GABAergic Sp5 neurons or inhibiting glutamatergic Sp5 neurons that receive RVM projections reversed masseter hyperalgesia. Our study identifies specific cell types and projections of RVM-Sp5 circuits involved in facilitating or inhibiting craniofacial nociception respectively. Selective manipulation of RVM-Sp5 circuits can be used as potential treatment strategy to relieve chronic craniofacial muscle pain.