GABAergic pain control and analgesia- the role of spinal gamma1-containing GABAA receptors.
Final Report Abstract
The loss of synaptic inhibition in the spinal dorsal horn plays an important role in the development of different chronic pain forms. Pharmacological enhancement of GABAergic inhibition through targeting specific GABAA receptors (GABAARs) in the spinal cord has been shown to alleviate neuropathic and inflammatory pain in rodents. Most GABAARs are composed of two α subunits, two β subunits and one γ subunit. Classical benzodiazepines (BDZs) bind to an interface between one α subunit and the γ2 subunit. So far, most studies focused on the functional role of the different α subunits of GABAARs demonstrating that antihyperalgesic actions of BDZs occur mainly through spinal GABAARs containing α2 subunits (α2GABAARs) while typical undesired effects of classical BDZs such as sedation, addiction and motor impairment depend on the activation of α1GABAARs. In addition, we have recently discovered that targeting α2/α3GABAARs in the spinal cord is highly effective against acute and chronic pruritus (itch) in mice and dogs. However, little attention has been paid to the diversity of the γ subunit of GABAARs (γ1- γ3) mainly because the γ1 and γ3 subunits are expressed in the brain only in small amounts and most clinically used BDZs act much stronger on γ2 than on γ1 (or γ3) containing GABAARs. In this project, I addressed the role of γ1GABAARs in spinal pain and itch processing. Therefore, we have generated mice that lack the γ2 subunit specifically from the spinal cord (hoxB8γ2-/- mice) as well as global compound-heterozygous γ1-deficient mice (gabrg1-/- mice) using the CRISPR-Cas9 technology. Immunohistochemical analyses in hoxBγ2-/- mice demonstrated preserved clusters of GABAARs in the superficial dorsal horn indicating that the γ2 subunit is not required for synaptic clustering of GABAARs at this site, thereby pointing at a potential role of spinal non-γ2GABAARs in spinal pain and itch control. This finding is in line with the absence of a (pro-)nociceptive phenotype in hoxBγ2-/- mice. However, despite the presence of GABAAR clusters in the superficial layers of the spinal cord of hoxBγ2-/- mice, data from combined electrophysiology and optogenetics revealed strongly reduced amplitudes of GABAergic inhibitory postsynaptic currents in these mice, which requires further investigation. To determine to which extent antihyperalgesic and antipruritic actions of BDZ site ligands depend on γ1GABAARs, I used a heterologous expression system and identified two compounds with activity at γ1GABAARs. The antihyperalgesic and antipruritic effects of these compounds remained highly significant in both, hoxB8γ2-/- mice and in γ1- deficient mice. This finding provides further evidence that non-γ2GABAARs play an important role in spinal pain and itch processing. Taken together, we have identified an additional way to specifically target subtypes of GABAARs in the spinal cord, which may benefit the development of innovative BDZ site ligands as powerful antihyperalgesic and antipruritic drugs.
Publications
- Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla. J Neurosci. 38, 322-334 (2017)
Klinger-Gratz PP, Ralvenius WT, Neumann E, Kato A, Nyilas R, Lele Z, Katona I, Zeilhofer HU
(See online at https://doi.org/10.1523/JNEUROSCI.1945-17.2017) - Itch suppression in mice and dogs by modulation of spinal α2 and α3GABAA receptors. Nat commun. 9, 3230 (2018)
Ralvenius WT, Neumann E, Pagani M, Acuña M, Wildner H, Benke D, Fischer N, Rostaher A, Schwager S, Detmar M, Frauenknecht K, Aguzzi A, Hubbs J, Rudolph U, Favrot C, Zeilhofer HU
(See online at https://doi.org/10.1038/s41467-018-05709-0) - Spinal GABAA receptors for pain control-Lost in translation? Pain
Zeilhofer HU, Neumann E, Munro G
(See online at https://doi.org/10.1097/j.pain.0000000000001268)
