An optogenetic approach for studying the cellular and molecular mechanisms of A-fibre nociceptor-evoked acute and inflammatory pain

• Applicant: Professor Dr. Stefan Lechner
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2013 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 240043454

Painful sensory stimuli impinging on our skin are detected and encoded by peripheral sensory neurons termed nociceptors, which can broadly be classified into unmyelinated C-fibre nociceptors and myelinated A-fibre nociceptors. The experiments performed in the first funding period have revealed a previously unrecognized functional heterogeneity of mouse A-fibre nociceptors. We have identified the neuropeptide Y receptor type 2 (Npy2r) as a marker for a subpopulation of particularly fast conducting A-fibre nociceptors and demonstrate that these afferents are crucial for detecting sharp and potentially tissue damaging stimuli. Furthermore we show that the integration of noxious and tactile sensory information plays an important role in acute pain signaling. Thus we show that selective optogenetic activation of Npy2r-positive nociceptors evokes abnormally exacerbated pain, which is alleviated by concurrent activation of low-threshold mechanoreceptors (LTMRs) in a frequency dependent manner. We further show that as little as a single action potential in A-fiber nociceptors is sufficient to trigger nocifensive paw withdrawal, but additional sensory input from LTMRs is required for normal well-coordinated execution of this reflex. Thus our results establish a causal link between defined neural activity in functionally identified sensory neuron subpopulations and nocifensive behavior and pain. In addition to these experiments, we also performed an RNASeq screen and identified candidate genes for the as-yet-unknown ion channel that mediates mechanotransduction in nociceptors. The following questions however remain open and will be addressed in the proposed study:1.) What is the role of Npy2r-positive A-fibre nociceptors in chronic pain?2.) How is noxious input from Npy2r-positive nociceptors and tactile information from LTMRs integrated in the spinal cord?3.) Which ion channel mediates mechanotransduction in nociceptors?Npy2r-positive nociceptors express TrkA, the receptor for nerve growth factor, which plays a central role in the development of hyperalgesia during inflammation. We will use electrophysiological techniques and the optogenetic pain assay developed in the first funding period to investigate the mechanisms that underlie A-fibre nociceptor sensitization in mouse models of inflammatory pain. To decipher the neural circuit that integrate noxious and tactile information in the spinal cord, we will utilize a novel mouse line that enables us to control the activity of Npy2r-positive nociceptors and LTMRs independent of one another. Finally, we will use gene silencing and patch-clamp recordings to study the role of the identified candidate genes in nociceptor mechanotransduction.

• DFG Programme: Research Grants