NMDA receptor 2B Subunit and BK Channel May Affect Mechanical Allodynia by Regulating the Activity of Enkephalinergic Neurons in the Spinal Dorsal Horn

Enkephalin (ENK)‐containing neurons in the spinal superficial dorsal horn (SDH) play an important role in regulating the transmission of nociceptive information. Using BAC transgenic mice, in which ENK‐containing neurons can be identified by green fluorescent protein expressed under the control of preproenkephalin gene promoter, we have observed that ENK‐containing neurons in the SDH express the NMDA receptor 2B (NR2B) subunit and large conductance calcium‐activated potassium (BK) channels. In experiments, we investigated the effects on nociceptive behavior of intrathecally administrating NR2B subunit antagonist and BK channel antagonist in peripheral nerve‐injured mice. The experiments were performed in 6‐ to 8‐week‐old male ICR mice. Partial sciatic nerve ligation was performed as described previously (PSL, Seltzer model). To assess the development of mechanical allodynia, we measured the frequency of withdrawal responses to 10 repetitive stimuli with von Frey filaments. Behavioral assessments were performed beginning 3 days prior to the ligation surgery. On post‐surgical day 7, effects on nociceptive behavior of the BK channel antagonist, charybdotoxin (CTX), and the NR2B antagonist, ifenprodil (IFN), were analyzed. Additionally, in order to clarify whether the actions of CTX and IFN are mediated by an alteration in the release of enkephalin, we investigated the effect of the selective δ‐opioid receptor antagonist naltrindole (NTI). CTX (1 pmol/10 μL per mouse), IFN (50 nmol/10 μL per mouse) and saline as a control (10 μL per mouse) was intrathecally injected according to the modified method originally described by Hylden. All behavioral experiments were performed in double‐blind fashion. PSL significantly increased the occurrence of withdrawal reflex to von Frey stimuli being indicative of the development of mechanical allodynia. Intrathecal CTX significantly reduced the occurrence of withdrawal reflex as compared to saline group. Additionally, intraperitoneal administration of NTI at the dose of 5 mg/kg body weight attenuated the effect on nociceptive behavior of CTX. Intrathecal IFN significantly reduced the occurrence of withdrawal reflex. This effect was also blocked by intraperitoneal NTI. These behavioral observations might suggest that NMDA receptors and BK channels inhibit the activity of enkephalin‐containing neurons in the spinal dorsal horn, which is speculated to be involved in peripheral nerve injury‐induced chronic pain state. Support or Funding Information This work was supported by Dokkyo Medical University Endowment Fund.