Targeting N-methyl-D-aspartate 2B receptor in painful diabetic neuropathy – mechanisms, challenges, and emerging therapeutics

Table 1 Clinical and preclinical findings of N-methyl-D-aspartate receptor-2B involvement in the pathogenesis of diabetic neuropathy

Ref.	Research	Clinical/preclinical model type	Clinical/preclinical model duration	Sample	Key outcome
Ismail et al[4], 2019	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	Increased p-NMDAR-2B and t-NMDAR-2B protein expression in the spinal cord of DPN rats with aberrant thermal hyperalgesia and formalin-induced chemical hyperalgesia responses, which were significantly suppressed with a seven-day intrathecal injection of ifenprodil (0.5 µg/µL)
Ismail et al[12], 2022	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	Painless DN rats exhibited notably less formalin-induced flinching and licking, which was linked to reduced spinal phosphorylated and total NMDAR-2B protein levels compared to DPN and control groups
Uniyal et al[21], 2022	Preclinical	STZ-induced type 1 DPN rat	23 days	Rat spinal cord tissue	A considerable increment in formalin-induced flinching during Phase 1 (represented by peripheral nociceptive changes) and early and late Phase 2 (represented by central nociceptive changes), which were associated with increased spinal p-NMDAR-2B and t-NMDAR-2B expression; minocycline (a selective microglia OX-42 inhibitor) substantially suppressed the responses and phosphorylated and total NMDAR-2B expression; nevertheless, the substance did not exert any effects on thermal hyperalgesia
Suo et al[23], 2016	Preclinical	High-fat diet-induced prediabetic mice	24 weeks	Mouse spinal cord tissue	Tactile allodynia and thermal hypoalgesia were developed in pre-diabetic wild-type mice, which correlated with elevated NMDAR-2B expression and activation and Fyn-NMDAR-2B interaction in the spinal cord; the ro 25-6981 (selective NMDAR-2B, i.t.) also demonstrated alleviated tactile allodynia but not thermal hypoalgesia in a dose-dependent manner
Lu et al[24], 2020	Preclinical	Type 2 high-fat diet (HFD)-induced DPN rat	8 weeks HFD	Rat spinal cord tissue	Levels of kalirin-7, p-NMDAR-2B, PSD-95, and PSD-95-NMDAR-2B coupling were significantly improved in kalirin-knockout mice with type-2 DPN with diminished mechanical allodynia and thermal hyperalgesia, suggesting that spinally expressed kalirin-7 could contribute to type-2 DPN by regulating PSD-95/NMDAR-2B interaction-dependent NMDAR-2B phosphorylation in the spinal cord
Li et al[25], 2019	Preclinical	STZ-induced type 2 DPN rat (in vivo); BV2 immortalised murine microglia cell line (in-vitro)	14 days; 24 hours incubation	Rat spinal cord tissue; BV2 immortalised murine microglia cell line	The p-JAK2, p-STAT3, total-CAV-1, and p-NMDAR-2B were upregulated in the spinal cord dorsal horn of the DPN rat with enhanced mechanical and thermal hyperalgesia development; intrathecal injection of the JAK2 inhibitor significantly suppressed the expression of the markers and thereby reduced pain responses; in vitro, a high glucose environment markedly induced activation of p-STAT3 in microglia and upregulated p-CAV-1 and p-NMDAR-2B in neurons
Shiers et al[26], 2024; Nakazawa et al[28], 2001	Clinical	-	-	Human DRG tissue	Histological and spatial RNA analyses of human DRGs from DPN donors showed damaged peripherin-positive axons and Nageotte nodules (support and immune cell clusters); ligand-receptor interactions in DRG neurons linked to DN mechanisms were also identified
Shiers et al[26], 2024	Preclinical	STZ-induced diabetic neuropathy rats	3 weeks	Rat spinal cord tissue	The DN rats exhibited lower paw withdrawal threshold, displaying notably higher expressions of NMDAR-2B and p-CREB in the spinal cord dorsal horn, which were abolished by intrathecal injections of baclofen (specific GABAB receptor antagonist), suggesting that the activation of spinal GABAB receptors activation normalises NMDAR-2B expression in DN
Fajrin et al[29], 2020	Preclinical	STZ-induced DPN mice model	28 days	Mouse sciatic nerve and spinal cord tissue	Spinal NMDAR-2B and TRPV1 mRNA expression was significantly downregulated in the mice treated with 6-shogaol, with improved thermal hyperalgesia, allodynia, and pain-induced mechanical pressure
Liu et al[30], 2014	Preclinical	STZ-induced diabetic neuropathic pain rats	3 weeks	Rat spinal cord tissue	Baclofen substantially improved paw withdrawal threshold and thermal withdrawal latency, with significant downregulation of mRNA and reduced protein expression of spinal NMDAR-2B and p-CREB in DPN rats; the data postulated that GABAB activation by baclofen might attenuate diabetic neuropathic pain partly via the downregulation of p-CREB and NMDAR-2B expression

CAV-1: Caveolin-1; DN: Diabetic neuropathy; DRG: Dorsal root ganglia; GABAB: Gamma-amino butyric acid B receptor; NMDAR-2B: N-methyl-D-aspartate-2B; OX-42: CD11b protein; p-CAV-1: Phosphorylated caveolin-1; p-CREB: Phosphorylated cyclic AMP-responsive element binding protein; p-JAK2: Phosphorylated Janus kinase 2; p-NMDAR-2B: Phosphorylated N-methyl-D-aspartate-2B receptor; PDN: Painful diabetic neuropathy; PSD-95: Postsynaptic density-95; STAT3: Signal transducer and activator of transcription 3; t-NMDAR-2B: Total N-methyl-D-aspartate-2B receptor; TRPV1: Transient receptor potential vanilloid-1.