Aims: The objective of this study was to elucidate the interaction between intrathecally administered gabapentin and clonidine on neuropathic pain associated with allodynia in the spinal nerve ligation model in the rat.
Main methods: Thresholds for hind paw responses to mechanical stimuli were determined by delivering von Frey filaments to the plantar surface. The left L5 spinal nerve was ligated and a fine catheter was intrathecally implanted at the L3-4 interspace under sevoflurane anesthesia. After confirmation of the established allodynia, gabapentin at 10, 30, 60 and 100μg or clonidine at 5, 15, 30 and 50μg was injected as a monotherapy in conscious rats through the intrathecal catheter to obtain the dose-response curve of %MPE (maximum possible effect) of the antiallodynic effect and its ED(50). Gabapentin and clonidine were concomitantly administered in a fixed-dose ratio proportional to the predetermined ED(50) of these drugs, thereby obtaining a dose-response curve for the drug combination and its ED(50). The profile of the interaction between these drugs was analyzed using an isobolographic analysis.
Key findings: The ED(50) for gabapentin and clonidine were 57.3±4.0 and 20.2±1.0μg, respectively (mean±SEM). However, the co-administration of gabapentin and clonidine at a ratio of 20:7 contributed to a much smaller experimental ED(50) values (gabapentin 10.1±1.1μg, and clonidine 3.6±0.3μg) compared with their theoretical ED(50)s on the additive line in the isobologram.
Significance: In the L5 spinal nerve-ligated rats, the intrathecal co-administration of gabapentin and clonidine exerted a synergistic action on the mechanical antiallodynic effect.
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