The interaction of gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on mechanical allodynia in rats with a spinal nerve ligation

Abstract

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 microg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia.

Fig. 1

Dose-response curves from the peak effects of percent maximal possible effect (%MPE) for antiallodynia in the gabapentin, N⁶-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), and gabapentin plus R-PIA groups. These curves show a dose-dependent antiallodynic effect. Data are expressed as mean±SEM. Doses (μg) are represented logarithmically on the x axis and peak %MPE is represented on the y axis. Asterisks indicate that the mean %MPE of each group is significant compared with the smallest dose. ^*p<0.05; unpaired t-test. I.T., intrathecal injection.

Fig. 2

Time course of antiallodynic effects by intrathecal injection of gabapentin (A), N⁶-(2-phenylisopropyl)-adenosine R-(-) isomer (R-PIA) (B), and gabapentin plus R-PIA (C). These curves show a dose-dependent antiallodynic effect in each group. Data are expressed as mean±SEM. Asterisks indicate that mean percent maximal possible effect (%MPE) of each group for antiallodynia at that time point is significant compared with baseline value. ^*p<0.05; one-way repeated-measures analysis of variance followed by multiple comparisons (Tukey's method). I.T., intrathecal injection.

Fig. 3

Isobologram for the intrathecal interaction of gabapentin and N⁶-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA). Horizontal and vertical bars indicate SEM. The diagonal line connecting both 50% effective dose (ED₅₀) points is the theoretical additive line. The ED₅₀ point A is calculated from the ED₅₀ values and 95% confidence intervals of each drug. The experimental ED₅₀ point B lies far below the line of additivity, indicating significant synergism.

Fig. 4

Antagonism by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) pretreatment. Pretreatment with DPCPX 10 μg was performed 15 min before injection of a large dose in the gabapentin plus N⁶-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) group. Graphs show the time course of the antiallodynic effect in the combination group and pretreatment groups. Data are expressed as mean ±SEM. Maximal antagonism is seen after 30 and 45 min. Asterisks indicate that mean percent maximal possible effect (%MPE) of pretreatment groups for antiallodynia at that time point is significantly less compared with the gabapentin plus R-PIA group. ^*p<0.05; unpaired t-test. I.T.; intrathecal injection.

Fig. 5

Effects of intrathecal injection of gabapentin, N⁶-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), and gabapentin plus R-PIA on rotarod performance time. Rotarod performance time was measured before (baseline) and after drug administration. Rotarod performance time was not reduced by administration of gabapentin 100 μg and R-PIA 3, 10 μg, and gabapentin 5 μg plus R-PIA 0.1 μg. Rotarod performance time was significantly decreased at 20 and 60 min after the injection of gabapentin 300 μg. The results are expressed as the mean±SEM. ^*p<0.05 vs. baseline group by repeated-measures analysis of variance and Tukey's method. I.T., intrathecal injection.