Maladaptive Dendritic Spine Remodeling Contributes to Diabetic Neuropathic Pain

Abstract

Diabetic neuropathic pain imposes a huge burden on individuals and society, and represents a major public health problem. Despite aggressive efforts, diabetic neuropathic pain is generally refractory to available clinical treatments. A structure-function link between maladaptive dendritic spine plasticity and pain has been demonstrated previously in CNS and PNS injury models of neuropathic pain. Here, we reasoned that if dendritic spine remodeling contributes to diabetic neuropathic pain, then (1) the presence of malformed spines should coincide with the development of pain, and (2) disrupting maladaptive spine structure should reduce chronic pain. To determine whether dendritic spine remodeling contributes to neuropathic pain in streptozotocin (STZ)-induced diabetic rats, we analyzed dendritic spine morphology and electrophysiological and behavioral signs of neuropathic pain. Our results show changes in dendritic spine shape, distribution, and shape on wide-dynamic-range (WDR) neurons within lamina IV-V of the dorsal horn in diabetes. These diabetesinduced changes were accompanied by WDR neuron hyperexcitability and decreased pain thresholds at 4 weeks. Treatment with NSC23766 (N 6 -[2-[[4-(diethylamino)-1-methylbutyl]amino]-6-methyl-4-pyrimidinyl]-2-methyl-4,6-quinolinediamine trihydrochloride), a Rac1-specific inhibitor known to interfere with spine plasticity, decreased the presence of malformed spines in diabetes, attenuated neuronal hyperresponsiveness to peripheral stimuli, reduced spontaneous firing activity from WDR neurons, and improved nociceptive mechanical pain thresholds. At 1 week after STZ injection, animals with hyperglycemia with no evidence of pain had few or no changes in spine morphology. These results demonstrate that diabetes-induced maladaptive dendritic spine remodeling has a mechanistic role in neuropathic pain. Molecular pathways that control spine morphogenesis and plasticity may be promising future targets for treatment.

Virus-Mediated Knockdown of Nav1.3 in Dorsal Root Ganglia of STZ-Induced Diabetic Rats Alleviates Tactile Allodynia

“…All animals were injected with STZ (50 mg/kg body weight, intraperitoneally [IP]; Sigma-Aldrich), as described previously (16). STZ is a pancreatic β-cell toxin that produces type I diabetes (17).…”

“…A small laminectomy (~1 mm 2 ) was performed between lumbar levels L5 and L6 to expose the thecal sac (16,17) (Figure 1B). This allowed visual confirmation that the needle entered the intrathecal space without damaging the spinal cord.…”

“…For tactile withdrawal threshold assessments, rats were placed on an elevated wire grid and the lateral plantar surface of the paw of each animal was presented with a series of calibrated Von Frey filaments (Stoelting). The withdrawal threshold (determined as 50% response rate) was determined using the "up-down" method for rats (16,20). For thermal sensitivity, the plantar surface of the paw was exposed to a radiant heat source using a Hargreaves apparatus (ITC) (21).…”

“…Stored recordings were analyzed offline with Spike2 software (version 7.02; Cambridge Electronic Design). Extracellular single-unit recording methods and identification of wide dynamic range (WDR) neurons have been described previously (16,18). Briefly, WDR neurons were first identified by their general response to a range of low-and high-threshold peripherally evoked stimuli (that is, light brushing with a cottontipped swap and pinching with sharp forceps) applied to each neuron's cutaneous receptive field.…”

“…Our previous study demonstrated that the receptive fields of dorsal horn units expand in animal models of diabetic neuropathic pain (16). To test whether Nav1.3 knockdown induces changes in receptive fields of WDR neurons (Figure 4), we mapped cutaneous receptive fields of sampled neurons by light brushing, pinching and probing with Von Frey filaments (see Materials and Methods).…”