Figure 1. Effect of drugs on luciferin–luciferase reaction.

Since the luciferin–luciferase assay is modulated by many compounds, all drugs used in this study were examined for their non-specific effect on detection of 10 nm ATP in a cell-free preparation. None of NMDA (300 μm), glutamate (300 μm, n = 8), MK-801 (30 μm, n = 8), d-AP5 (100 μm, n = 8), DCKA (30 μm, n = 8), NPPB (30 μm, n = 10), MRS2179 (100 μm, n = 6), thapsigargin (1 μm, n = 6) or 18α-glycyrrhetinic acid (1 and 10 μm, n = 5) produced a significant shift in the luminescence compared with control levels (n = 28). PPADS produced a decrease in the luciferase activity and the ecto-ATPase apyrase (1 U ml⁻¹, n = 6) produced the expected reduction in the luminescence levels by hydrolysing the ATP content of the sample, while oleamide (300 μm, n = 5) enhanced the signal. Bars show means +s.e.m. for values normalized to the corresponding control level. *P < 0.05 versus control value.