Background & Aims

Descending projections from the brain to the spinal cord provide top-down modulation of pain sensitivity. The rostral ventromedial medulla (RVM), which includes the nucleus raphe magnus (NRM), the nucleus paragigantocellularis pars alpha (Pa) and the lateral paragigantocellularis (LPGi), is a hindbrain region involved in descending pain modulation. The RVM innervates the spinal cord and controls pain sensitivity in a bidirectional manner. RVM provides monaminergic, glutamatergic, GABAergic and glycinergic input to the spinal dorsal horn. Previous studies in the field mainly focused on monaminergic pathway. The present report focuses on the inhibitory population of RVM spinal cord projection neurons.

Methods

To investigate the function of descending inhibitory RVM neurons an intersectional chemogenetic approach was taken. In vGATcre transgenic mice aged between 6-12 weeks one side of the dorsal horn of the lumbar spinal cord was injected with AAV2retro.flex.eGFP or AAV2retro.flex.mCherry at a depth of 200-300 µm to retrogradely label vGAT neurons with the flp recombinase. A second injection with AAV9.FRT.hM3D(q).mCherry was made into the RVM to express an activatory DREADD receptor in those neurons or an AAV9/2.FRT.eGFP.TeTxLC to silence them. Intraplantar injection of Complete Freund’s Adjuvant (CFA) was used to induce peripheral inflammation. Hargreaves test and von Frey filament stimulation was used to assess nociceptive sensitivity in the CFA model before and during chemogenetic manipulation of descending vGAT RVM neurons. The impact of chemogenetic manipulation was also test in a model of trigeminal nociception. Capsaicin droplets were applied to the eye and forepaw wipes counted.

Results

Chemogenetic activation of descending vGAT RVM neurons reduced the sensitivity of the ipsilateral paw to acute heat and cold stimuli and to von Frey mechanical stimulation. It also reversed mechanical and thermal inflammatory hyperalgesia induced by intraplantar injection of CFA. Furthermore, activation of these neurons decreased heat and cold sensitivity also in the contralateral hind- and forepaws and reduced the average number of forepaw wipes to the eye after application of capsaicin. Unexpectedly, silencing (instead of activation) of these neurons induced hypersensitivity to punctate and dynamic mechanical stimuli and led to spontaneous aversive behaviors, such as flinching of the ipsilateral hindpaw.

Conclusions

Descending inhibitory projections from the RVM to the lumbar spinal cord are able to modulate nociceptive sensitivity along the entire rostrocaudal axis suggesting a widespread spinal innervation effect of these neurons. The fact that both activation and inhibition of these neurons has a different impact on different stimuli suggests the existence of two distinct population of descending RVM vGAT neurons. One population can be recruited under certain conditions to reduce nociception. A second population that appears to be continuously active controls mechanical sensitivity and prevents the activation of nociceptive pathways upon light mechanical stimulation.

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

Marilia Sousa

Poster Authors

Marília Sousa

MSc

University of Zurich

Lead Author

E-mail

Robert P. Ganley

PhD

National Institutes of Health (NIH)

Lead Author

E-mail

Matteo Ranucci

University of Zürich

Lead Author

E-mail

Kira Werder

MSc

University of Oxford

Lead Author

E-mail

Camilla Beccarini

University of Zurich

Lead Author

E-mail

Hendrik Wildner

PhD

University of Zurich, Institute of Pharmacology and Toxicology

Lead Author

E-mail

Hanns U. Zeilhofer Prof. Dr.

Lead Author

E-mail

Topics

  • Mechanisms: Biological-Molecular and Cell Biology