Correlation between seizure threshold and 5-hydroxytryptamine (5-HT) concentration at the time of seizure manifestation. Seizure threshold was defined as the minimum amount of tramadol needed to induce seizure (mg/kg). An overall n = 18 was used in this analysis. Spearman correlation revealed a significant correlation between these factors (correlation coefficient: 0.71, P < 0.01). 5-HTP = 5-hydroxytryptophan; PCPA = para-chlorophenylalanine.