Lan Ji Huang, et al. surgery (preoperative control). The control study was per- formed using intravenous saline. The investigator was blind to the drug given to the experimental animals. Effect of intravenous sildenafil on neuropathic pain and hemodynamics To examine its effect, saline and sildenafil (1, 3, 10, and 30 mg/kg, n = 37) were intravenously administered, and the mechanical threshold was measured with the above- mentioned method. Additionally, blood pressure and heart rate were measured in normal rats following intravenous administration of saline and sildenafil (1, 3, 10, and 30 mg/kg, n = 25). Role of GABA receptors in the action of sildenafil To investigate the role of GABA receptors in the action of sildenafil, GABAergic antagonists were intravenously administered 10 min prior to the delivery of intravenous sildenafil, and changes of the effect of sildenafil were exam- ined after pretreatment with GABAA receptor antagonist (bicuculline, 16 mg/kg, n = 7) or GABAB receptor antago- nist (saclofen, 16 mg/kg, n = 8). The maximum doses of bicuculline and saclofen used were chosen based on pre- vious experiments22 and the pilot study was performed to examine whether the maximum dosages of bicuculline (n = 5) and saclofen (n = 6) affected the paw withdrawal thre- shold or not. Statistical analysis Data are expressed as mean ± SEM. The time response data are presented as paw withdrawal threshold to mecha- nical stimulation or percentage change from the baseline mean arterial blood pressure (MAP) and heart rate (HR). The dose-response data are presented as percentage of maximal possible effect (%MPE) according to the formula.23 Postdrug threshold - baseline threshold %MPE = Cut-off threshold (15) - baseline threshold ×100 The dose-response data were analyzed using one-way analysis of variance with Scheffe post hoc analysis. Com- parison of antagonism for the effect of sildenafil was analyzed by unpaired t-test. The baseline MAP and HR of the several groups were examined by one-way analysis of variance. The effect of sildenafil on MAP and HR was examined by repeated-measures analysis of variance. Values of p < 0.05 were considered statistically significant. RESULTS Antinociceptive effect of intravenous sildenafil In control groups, a paw withdrawal threshold was 11-12 g. After nerve ligation, a paw withdrawal threshold was significantly decreased in a pathologic site. As shown in Fig. 1A and B, intravenous sildenafil resulted in a dose- dependent increase of the paw withdrawal threshold (p < 0.05, p < 0.01). GABA receptors on the activity of sildenafil Intravenous GABAA antagonist (bicuculline, p < 0.01) and GABAB antagonist (saclofen, p < 0.05) reversed the antinociception of sildenafil (Fig. 2). Both antagonists themselves were not effective in ameliorating the control response. Hemodynamic effect of intravenous sildenafil The baseline MAP and heart rate HR were 107 ± 1 mmHg and 420 ± 3 beats/min, respectively. Furthermore, the baseline MAP and HR in the several treatment groups did not differ. A 84 B Fig. 1. Temporal effect and dose response curves of intravenous saline (control) and sildenafil (n = 37) on mechanical threshold in rats following ligation of L5/6spinal nerves. Data are presented as paw withdrawal threshold (A) and %MPE (B). Intravenous sildenafil increased the withdrawal threshold in a dose-dependent manner. Each line or bar represents mean ± SEM. BL, baseline. Compared with control, *p < 0.05, p < 0.01. Yonsei Med J http://www.eymj.org Volume 51 Number 1 January 2010