Sphingosine-1-phosphate-induced nociceptor excitation and ongoing pain behavior in mice and humans is largely mediated by S1P3 receptor.

The biolipid sphingosine-1-phosphate (S1P) is an essential modulator of innate immunity, cell migration, and wound healing. It is released locally upon acute tissue injury from endothelial cells and activated thrombocytes and, therefore, may give rise to acute post-traumatic pain sensation via a yet elusive molecular mechanism. We have used an interdisciplinary approach to address this question, and we find that intradermal injection of S1P induced significant licking and flinching behavior in wild-type mice and a dose-dependent flare reaction in human skin as a sign of acute activation of nociceptive nerve terminals. Notably, S1P evoked a small excitatory ionic current that resulted in nociceptor depolarization and action potential firing. This ionic current was preserved in "cation-free" solution and blocked by the nonspecific Cl(-) channel inhibitor niflumic acid and by preincubation with the G-protein inhibitor GDP-ß-S. Notably, S1P(3) receptor was detected in virtually all neurons in human and mouse DRG. In line with this finding, S1P-induced neuronal responses and spontaneous pain behavior in vivo were substantially reduced in S1P(3)(-/-) mice, whereas in control S1P(1) floxed (S1P(1)(fl/fl)) mice and mice with a nociceptor-specific deletion of S1P(1)(-/-) receptor (SNS-S1P(1)(-/-)), neither the S1P-induced responses in vitro nor the S1P-evoked pain-like behavior was altered. Therefore, these findings indicate that S1P evokes significant nociception via G-protein-dependent activation of an excitatory Cl(-) conductance that is largely mediated by S1P(3) receptors present in nociceptors, and point to these receptors as valuable therapeutic targets for post-traumatic pain.

A key role for gp130 expressed on peripheral sensory nerves in pathological pain.

Interleukin-6 (IL-6) is a key mediator of inflammation. Inhibitors of IL-6 or of its signal transducing receptor gp130 constitute a novel class of anti-inflammatory drugs, which raise great hopes for improved treatments of painful inflammatory diseases such as rheumatoid arthritis. IL-6 and gp130 may enhance pain not only indirectly through their proinflammatory actions but also through a direct action on nociceptors (i.e., on neurons activated by painful stimuli). We found indeed that the IL-6/gp130 ligand-receptor complex induced heat hypersensitivity both in vitro and in vivo. This process was mediated by activation of PKC-delta via Gab1/2/PI(3)K and subsequent regulation of TRPV1, a member of the transient receptor potential (TRP) family of ion channels. To assess the relevance of this direct pain promoting effect of IL-6, we generated conditional knock-out mice, which lack gp130 specifically in nociceptors, and tested them in models of inflammatory and tumor-induced pain. These mice showed significantly reduced levels of inflammatory and tumor-induced pain but no changes in immune reactions or tumor growth. Our results uncover the significance of gp130 expressed in peripheral pain sensing neurons in the pathophysiology of major clinical pain disorders and suggest their use as novel pain relieving agents in inflammatory and tumor pain.

The CB1 receptor antagonist, SR141716A, prevents high-frequency stimulation-induced reduction of feedback inhibition in the rat dentate gyrus following perforant path stimulation in vivo.

Endocannabinoids acting through CB(1) receptors are thought to regulate GABAergic and glutamatergic neurotransmission and may modulate long-term potentiation (LTP). High-frequency stimulation (HFS) of the medial perforant path to induce LTP was studied in the dentate gyrus with or without the selective CB(1) receptor antagonist, SR141716A in isoflurane-anaesthetised rats. HFS significantly increased the slope of the field excitatory post-synaptic potential (fEPSP) and the amplitude of the population spike (PS; P<0.001 in each case; n=6). Following administration of SR141716A, HFS no longer increased fEPSP slope, whereas PS amplitude potentiation remained significant (P<0.0001; n=6). Paired-stimuli revealed that HFS significantly reduced inhibition observed at intervals of 10 ms (P<0.01; n=6), and produced a leftward shift of the interval-inhibition curve (P<0.05; n=6). Following administration of SR141716A, HFS no longer reduced inhibition at the 10 ms interval, but a leftward shift in the interval-inhibition curve was still observed (P<0.05, n=6). These results indicate that LTP in the dentate gyrus reduces local circuit inhibition, consistent with a reduction of GABA release and/or duration of the post-synaptic GABA-receptor mediated response. Selective effects of SR141716A on the degree, but not the timecourse, of paired-pulse inhibition suggest that the reduction in GABA release following LTP induction is due to CB(1) activation. Results also suggest that CB(1) receptors contribute to HFS-induced potentiation of the fEPSP, but not to the mechanism underlying potentiation of PS amplitude. We suggest that CB(1) activation during HFS of the medial perforant path increases glutamate release from perforant path synapses, but inhibits release of GABA from local circuit interneurons.

Activated platelets release sphingosine 1-phosphate and induce hypersensitivity to noxious heat stimuli in vivo.

At the site of injury activated platelets release various mediators, one of which is sphingosine 1-phosphate (S1P). It was the aim of this study to explore whether activated human platelets had a pronociceptive effect in an in vivo mouse model and whether this effect was based on the release of S1P and subsequent activation of neuronal S1P receptors 1 or 3. Human platelets were prepared in different concentrations (10(5)/µl, 10(6)/µl, 10(7)/µl) and assessed in mice with different genetic backgrounds (WT, S1P1 (fl/fl), SNS-S1P1 (-/-), S1P3 (-/-)). Intracutaneous injections of activated human platelets induced a significant, dose-dependent hypersensitivity to noxious thermal stimulation. The degree of heat hypersensitivity correlated with the platelet concentration as well as the platelet S1P content and the amount of S1P released upon platelet activation as measured with LC MS/MS. Despite the significant correlations between S1P and platelet count, no difference in paw withdrawal latency (PWL) was observed in mice with a global null mutation of the S1P3 receptor or a conditional deletion of the S1P1 receptor in nociceptive primary afferents. Furthermore, neutralization of S1P with a selective anti-S1P antibody did not abolish platelet induced heat hypersensitivity. Our results suggest that activated platelets release S1P and induce heat hypersensitivity in vivo. However, the platelet induced heat hypersensitivity was caused by mediators other than S1P.

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.