Fig. 3. The inhibition of catecholamine secretion by gabapentin is concentration dependent. Chromaffin cells were seeded on 24-well plates and treated with vehicle (control) or gabapentin (0.1 mM GBP or 1 mM GBP) for 18–24 h. Evoked catecholamine secretion was determined using high-performance liquid chromatography (HPLC) and expressed as a percentage of total cellular content (mean ± SEM). Both concentrations of gabapentin significantly reduced secretion compared with controls (n = 8; *P < 0.05; ***P < 0.001) (A ). Data from all HPLC experiments were pooled and percent inhibition of evoked catecholamine secretion (mean ± SEM) plotted against log¹⁰of gabapentin concentration (10 μM, n = 4; 100 μM, n = 8; 1 mM, n = 20; 3 mM, n = 4)^.A concentration response curve was generated by fitting the data to a Boltzmann function with a Hill slope = 1, and yielded an estimated EC⁵⁰of 76 μM (B ).