N-Methyl-D-aspartate (NMDA) receptors are composed of a NR1 subunit in combination with NR2 (A-D) subunits. NR2B localization on primary afferent terminals in the dorsal horn suggests that antagonists for these subunits could be used to target pain. NR2B antagonists such as ifenprodil, Ro25-6981 and CP 101,606 have anti-hyperalgesic action with few motor side effects (Boyce et al., 1999). 4-Trifluoromethoxy-N-(2-trifluoromethylbenzyl)-benzamidine (SAE 502) is an orally bioavailable NR2B-selective NMDA antagonist with efficacy in carrageenan-induced hyperalgesia assay in the rat and no locomotor dysfunction (Claiborne et al., 2003). The aim of this study was to compare the effect of peripheral and central SAE502 administration on tactile allodynia induced by NMDA, and mechanical hyperalgesia induced by capsaicin or peripheral nerve damage.

Male Wistar rats (180-200g body weight, n=6/group) were used for the study. Tactile allodynia was measured by von Frey filaments and mechanical hyperalgesia was measured as paw withdrawal threshold (PWT) to increasing pressure by an analgesimeter (Ugo Basile, Italy). Intrathecal drugs were administered via a 10 µl lumbar injection between L4 and L5 under brief enflurane anaesthesia. Naïve rat hind paws were injected with 10 µl volume of vehicle (50% DMSO and 0.9% sterile saline), NMDA (100 nmole) or capsaicin (1 nmole) and mechanical hypersensitivity assessed up to 3 hours post-injection. The left sciatic nerve was partially ligated (Seltzer et al., 1990) and the establishment of mechanical hyperalgesia assessed on day 14. SAE502 (1–100 nmole) was injected either intraplantarly (i.pl.) or intrathecally (i.t.). Data is expressed as mean ± SEM and analysed by ANOVA followed by Tukey HSD test (P< 0.05) on the raw data.

Intraplantar NMDA (100 nmole/rat) reduced left hind PWT from 12.2±1.4 to 5.7±0.9g and intrathecal NMDA (1 nmole/rat) from 12.5±1.2 to 5.3±0.7g within 15 min from injections. Co-administration of SAE502 (100 nmole) completely blocked both intraplantar and intrathecal NMDA-induced allodynia. Intraplantar capsaicin (1 nmole) induced a mechanical hyperalgesia which peaked at 15 min. This was not modified by intraplantar SAE502 but was inhibited by 43.6±2.5% following intrathecal administration. In rats with sciatic nerve partially ligated, intrathecal SAE502 (10-100 µg) reversed the established neuropathic mechanical hyperalgesia by 54.2±3.3%. In contrast, intraplantar injection of SAE502 did not produce any significant reversal of the neuropathic mechanical hyperalgesia.

These data show that doses of a NR2B antagonist that would inhibit centrally and peripherally NMDA-induced allodynia, were anti-hyperalgesic following spinal but not peripheral administration in neuropathic rat and capsaicin-induced nociceptive pain. Therefore, centrally (spinal) but not peripherally localized NR2B subunit may play a role in pain.

S Boyce et al., 1999. Neuropharmacol. 38: 611-623
C. Claiborne et al., 2003. Bioorg & Med Chem Lett . 13 : 697-700
Z Seltzer et al., 1990. Pain 43: 205-218