Sphingosine 1-phosphate enhances spontaneous transmitter release at the frog neuromuscular junction

Intracellular recordings were made from isolated frog sciatic-sartorius nerve-muscle preparations, and the effects of sphingosine I-phosphate (SI-P) on miniature endplate potentials (MEPPs) were studied. Extracellular application of S1-P (1 and 30 muM) had no significant effects on the frequency and amplitude of MEPPs. Delivery into nerve terminals by liposomes containing 10(-5), 10(-4) or 10(-3) M SI-P was associated with a concentration-dependent increase in MEPP frequency of 37, 63 and 86%. The per cent of median MEPP amplitude was not significantly changed, but there was an increase in the number of 'giant' MEPPs. Pre-exposure of the preparations to S1-P 10(-5) but not 10(-8) m entrapped in liposomes for 15 min blocked the effects of subsequent superfusion of S1-P (10(-4) m)-filled liposomes on MEPP frequency. Thus, intracellular S1-P receptors seem to undergo 'desensitization' to higher concentrations of S1-P. The result provides the first evidence that S1-P acting intracellularly but not extracellularly enhances spontaneous transmitter release at the frog neuromuscular junction.