Targeted Anatomical and Functional Identification of Antinociceptive and Pronociceptive Serotonergic Neurons that Project to the Spinal Dorsal Horn

Spinally-projecting serotonergic neurons play a key role in controlling pain sensitivity and can either increase or decrease nociception depending on physiological context. It is currently unknown how serotonergic neurons mediate these opposing effects. Utilizing virus-based strategies, we identified two anatomically separated populations of serotonergic hindbrain neurons located in the lateral paragigantocellularis (LPGi) and the medial hindbrain, which respectively innervate the superficial and deep spinal dorsal horn and have contrasting effects on pain perception. Our tracing experiments revealed an unexpected high selectivity of serotonergic neurons of the LPGi for transduction with spinally injected AAV2retro vectors while medial hindbrain serotonergic neurons were largely resistant to AAV2retro transduction. Taking advantage of this selectivity, we employed intersectional chemogenetic approaches to demonstrate that activation of the LPGi serotonergic projections decreases thermal sensitivity, whereas activation of medial serotonergic neurons increases sensitivity to mechanical von Frey stimulation. Together these results suggest that there are functionally distinct classes of serotonergic hindbrain neurons that differ in their anatomical location in the hindbrain, their postsynaptic targets in the spinal cord, and impact on nociceptive sensitivity. At least the LPGi neurons give rise to rather global and bilateral projections throughout the rostrocaudal extent of the spinal cord suggesting that they contribute to widespread systemic pain control.