Background & Aims

Different CNS areas send descending projections to the spinal dorsal horn (DH) to modulate nociception. The rostral ventromedial medulla (RVM), which comprises the Nucleus Raphe magnus (NRM), the Nucleus paragigantocellularis pars alpha (Pa) and the Lateral Nucleus paragigantocellularis (LPGi), harbours the somata of many descending projection neurons and provides monoaminergic, excitatory and inhibitory inputs to the DH. In this study, we aimed to delineate the anatomical projection pattern and functional connectivity of inhibitory RMV neurons in the DH.

Methods

Anatomical tracing: vGat-cre expressing mice at between 6-12 weeks of age were injected with AAV2retro.flex.FLPo.BFP into the lumbar spinal cord and with AAV8.FRT.tdTOM into the RVM (injection coordinates: RC -5.8, ML ±0.5, DV 5.9) to define the location of the synaptic boutons of descending RVM neurons in the DH.
Optogenetically assisted circuit tracing: 3-4 weeks old mice were injected with AAV2retro.flex.FLPo.mCherry in the lumbar section of the spinal cord and with AAV9.FRT.ChR2-YFP into the RVM (injection coordinates: RC -5.8, ML ±0.5, DV 5.9) to label descending RVM neurons from a specific spinal site. Transverse slices of the lumbar spinal cord were prepared from these mice for targeted whole-cell recordings. Light evoked inhibitory post-synaptic currents (leIPSCs) were elicited through a 4-ms long blue light (470nm) pulses. Bicuculline (20 ?M) and strychnine (0.5 ?M) were used to identify their GABAergic or glycinergic nature.

Results

Our viral tracing experiments identified a widespread projection pattern of inhibitory neurons descending from the RVM throughout the whole rostrocaudal extension of the spinal cord. Descending inhibitory neurons locally and retrogradely labeled from one side of the lumbar spinal cord projected to other spinal cord segments and to the contralateral side.
The electrophysiological experiments confirmed that these synapses were functional. Both in slices from the lumbar and the cervical segments, interneurons of the superficial DH consistently received bicuculline and strychnine sensitive leIPSCs, and analyses of their latency and jitter confirmed that they originated from mono-synaptic contacts.

Conclusions

Our study shows a widespread bilateral projection patter of inhibitory GABAergic and glycinergic neurons that descend from the RVM to the DH. These findings suggest that inhibitory neurons descending from the RVM to the spinal DH may function as key elements of circuits involved in wide-spread pain modulation, potentially contributing to the phenomenon of diffuse noxious inhibitory control of nociception.

References

A. I. Basbaum et al. Cellular and molecular mechanisms of pain. Cell, 139(2):267–284, October 2009. doi: 10.1016/j.cell.2009.09.028.

H. U. Zeilhofer and R. Ganley. Dorsal Horn Pain Mechanisms. In The Oxford Handbook of the Neurobiology of Pain. Oxford University Press, June 2020. doi: 10.1093/oxfordhb/ 9780190860509.013.23, https://doi.org/10.1093/oxfordhb/9780190860509.013.23

M. H. Ossipov. Pain pathways: Descending modulation. In L. R. Squire, editor, Encyclopedia of Neuroscience, pages 393–399. Academic Press, Oxford, 2009. https://doi.org/10.1016/B978-008045046-9.01928-8.

Presenting Author

Matteo Ranucci

Poster Authors

Matteo Ranucci

University of Zürich

Lead Author

Topics

  • Mechanisms: Biological-Systems (Physiology/Anatomy)