Intracellular sphingosine 1‐phosphate mediates the increased excitability produced by nerve growth factor in rat sensory neurons

Abstract

Our previous studies found that nerve growth factor (NGF), via ceramide, enhanced the number of action potentials (APs) evoked by a ramp of depolarizing current in capsaicin-sensitive sensory neurons. Ceramide can be metabolized by ceramidase to sphingosine (Sph), and Sph to sphingosine 1-phosphate (S1P) by sphingosine kinase. It is well established that each of these products of sphingomyelin metabolism can act as intracellular signalling molecules. This raises the question as to whether the enhanced excitability produced by NGF was mediated directly by ceramide or required additional metabolism to Sph and/or S1P. Sph applied externally did not affect the neuronal excitability, whereas internally perfused Sph augmented the number of APs evoked by the depolarizing ramp. Furthermore, internally perfused S1P enhanced the number of evoked APs. This sensitizing action of NGF, ceramide and internally perfused Sph was abolished by dimethylsphingosine (DMS), an inhibitor of sphingosine kinase. In contrast, internally perfused S1P enhanced the number of evoked APs in the presence of DMS. These observations support the idea that the metabolism of ceramide/Sph to S1P is critical for the sphingolipid-induced modulation of excitability. Both internally perfused Sph and S1P inhibited the outward K + current by 25-35% for the step to +60 mV. The Sph-and S1P-sensitive currents had very similar current-voltage relations, suggesting that they were likely to be the same. In addition, the Sph-induced suppression of the K + current was blocked by pretreatment with DMS. These findings demonstrate that intracellular S1P derived from ceramide acts as an internal second messenger to regulate membrane excitability; however, the effector system whereby S1P modulates excitability remains undetermined.

Sphingosine 1-Phosphate Modulates Spinal Nociceptive Processing

“…Accordingly, while both sphingolipids exhibited identical potencies toward the receptor-mediated inhibition of the neuronal cAMP accumulation, only S1P (10 M) was able to instigate a calcium response in spinal cord neurons. Thus, the data suggest that the loss of the antinociceptive properties of S1P at high doses is based on its uptake in neurons, which then might enhance nociceptive processing (12,26). The notion that S1P receptor activation in the spinal cord has antinociceptive effects is further supported by a previous study that demonstrates that intrathecal application of the S1P receptor agonist FTY720 decreases pain-related behavior in the formalin test (43).…”

“…However, it will be necessary to identify the specific S1P receptor(s) that mediate the antinociceptive actions of S1P in the spinal cord as soon as the necessary tools such as selective S1P receptor agonists or viable conditional S1P receptor knock-out mice without behavioral abnormalities are available. Finally, since S1P facilitates the NGF-induced increases in the excitability of cultured DRG neurons (11,12), our findings point toward a multifaceted role of S1P in nociceptive processing.…”

“…3B), which may have two reasons: First, at higher doses S1P may reach the DRGs and increase the excitability of sensory neurons within the DRG (11, 12). Second, because high extracellular S1P concentrations are described to cause its uptake in cells (25), the second messenger properties of S1P could facilitate neuronal excitability (12,26). To distinguish between both possibilities, we intrathecally administered sphinganine 1-phosphate (dihydro-S1P), a S1P receptor agonist that has similar receptor affinities as S1P, but that does not mimic the intracellular second messenger properties of S1P (25,27).…”

“…For example, S1P receptor activation augments glutamate secretion in hippocampal neurons (9) and elevated intracellular S1P concentrations enhance the spontaneous neurotransmitter release at neuromuscular junctions (10). Furthermore, intra-as well as extracellularly applied S1P facilitated the NGF-induced increases in the excitability of sensory neurons from dorsal root ganglia (DRG) (11,12).However, to date little is known about physiological processes in which regulation of neuronal functions by S1P may play a role. Recently we showed that intrathecal administration of the S1P receptor agonist FTY720 reduces the nociceptive behavior in the formalin assay, demonstrating an antinociceptive effect of S1P receptor activation in the spinal cord (13).…”

“…For example, S1P receptor activation augments glutamate secretion in hippocampal neurons (9) and elevated intracellular S1P concentrations enhance the spontaneous neurotransmitter release at neuromuscular junctions (10). Furthermore, intra-as well as extracellularly applied S1P facilitated the NGF-induced increases in the excitability of sensory neurons from dorsal root ganglia (DRG) (11,12).…”