Genetic Evidence for Involvement of Neuronally Expressed S1P1 Receptor in Nociceptor Sensitization and Inflammatory Pain

Sphingosine-1-phosphate (S1P) is a key regulator of immune response. Immune cells, epithelia and blood cells generate high levels of S1P in inflamed tissue. However, it is not known if S1P acts on the endings of nociceptive neurons, thereby contributing to the generation of inflammatory pain. We found that the S1P1 receptor for S1P is expressed in subpopulations of sensory neurons including nociceptors. Both S1P and agonists at the S1P1 receptor induced hypersensitivity to noxious thermal stimulation in vitro and in vivo. S1P-induced hypersensitivity was strongly attenuated in mice lacking TRPV1 channels. S1P and inflammation-induced hypersensitivity was significantly reduced in mice with a conditional nociceptor-specific deletion of the S1P1 receptor. Our data show that neuronally expressed S1P1 receptors play a significant role in regulating nociceptor function and that S1P/S1P1 signaling may be a key player in the onset of thermal hypersensitivity and hyperalgesia associated with inflammation

Reduced thermal hypersensitivity in S1P₁^−/−mice.

<p>(A) Injection of the S1P₁ agonist SEW2871 induced a significant transient decrease in paw withdrawal latencies in S1P₁^fl/fl (n = 9) which was significantly less pronounced than in SNS-S1P₁^−/− mice (n = 10, *p<0.05; ANOVA). (B, C) While only a minor reduction of paw withdrawal latencies was observed in both mouse strains with local low dose S1P injection, we observed a significant decrease in paw withdrawal latencies in S1P₁^fl/fl mice (n = 7) which was similar to wt. In SNS-S1P₁^−/− mice the degree of hypersensitivity was significantly ameliorated in comparison to S1P₁^fl/fl mice (n = 9, *p<0.05, ** p<0.01; ANOVA). (D) CFA (20 µl) injection into the plantar hindpaw induced a pronounced decrease of PWL which was significantly attenuated in S1P₁-Cre mice (p<0.05, n = 4; ANOVA). (E) Paw swelling was similar in SNS-S1P₁^−/− and S1P₁^fl/fl mice (n = 4).</p

SNS-S1P₁^−/− mice are largely protected from S1P-induced hypersensitivity.

<p>(A) Deletion of exon 2 in nociceptive neurons with the SNS-Cre recombination methods in <i>SNS-Cre:S1P₁^fl/fl</i> (SNS-S1P₁^−/−) mice. (B) Taqman®-PCR analysis of DRG explants revealed an almost complete absence of S1P₁ mRNA (n = 10) in SNS-S1P₁^−/− mice in comparison to control S1P₁^fl/fl mice (n = 9, **p<0.01; Mann-Whitney U-test). (C) S1P₁ receptor immunoreactivity is expressed in a subpopulation of small size sensory neurons in DRG sections obtained from S1P₁^fl/fl but not in SNS-S1P₁^−/− mice. There is no difference in the expression profile of CGRP immunoreactivity. (D) Example of a neuron that responded to capsaicin (arrows) with calcium transients. S1P itself induced a brief transient which recovered immediately and the following response to capsaicin was strongly increased. (E, F) The percentage of neurons responding to S1P with an increase in capsaicin-induced calcium transients was significantly reduced in SNS-S1P₁^−/− mice compared to S1P₁^fl/fl mice.</p