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 and Tph2-Cre transgenic mice, 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 sensory perception. Our tracing experiments revealed that serotonergic neurons of the LPGi were much more susceptible to transduction with spinally injected AAV2retro vectors than medial hindbrain serotonergic neurons. Taking advantage of this difference, 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 their impact on nociceptive sensitivity. The LPGi neurons that give rise to rather global and bilateral projections throughout the rostrocaudal extent of the spinal cord appear to be ideally poised to contribute to widespread systemic pain control.

Spinally projecting noradrenergic neurons of the locus coeruleus display resistance to AAV2retro-mediated transduction.

BACKGROUND: The locus coeruleus (LC) is the principal source of noradrenaline (NA) in the central nervous system. Projection neurons in the ventral portion of the LC project to the spinal cord and are considered the main source of spinal NA. To understand the precise physiology of this pathway, it is important to have tools that allow specific genetic access to these descending projections. AAV2retro serotype vectors are a potential tool to transduce these neurons via their axon terminals in the spinal cord, and thereby limit the expression of genetic material to the spinal projections from the LC. Here, we assess the suitability of AAV2retro to target these neurons and investigate strategies to increase their labelling efficiency. RESULTS: We show that the neurons in the LC that project to the spinal dorsal horn are largely resistant to transduction with AAV2retro serotype vectors. Compared to Cholera toxin B (CTb) tracing, AAV2retro.eGFP labelled far fewer neurons within the LC and surrounding regions, particularly within neurons that express tyrosine hydroxylase (TH), the rate-limiting enzyme for NA synthesis. We also show that the sensitivity for transduction of this projection can be increased using AAV2retro.eGFP.cre in ROSA26tdTom reporter mice (23% increase), with a higher proportion of the newly revealed neurons expressing TH compared to those directly labelled with AAV2retro containing an eGFP expression sequence. CONCLUSION: These tracing studies identify limitations in AAV2retro-mediated retrograde transduction of a subset of projection neurons, specifically those that express NA and project to the spinal cord. This is likely to have implications for the study of NA-containing projections as well as other types of projection neuron in the central nervous system.