A common complication associated with diabetes is painful or painless diabetic peripheral neuropathy (DPN). The mechanisms and determinants responsible for these peripheral neuropathies are poorly understood. Using both streptozotocin (STZ)-induced and transgene-mediated murine models of type 1 diabetes (T1D), we demonstrate that Transient Receptor Potential Vanilloid 1 (TRPV1) expression varies with the neuropathic phenotype. We have found that both STZ- and transgene-mediated T1D are associated with two distinct phases of thermal pain sensitivity that parallel changes in TRPV1 as determined by paw withdrawal latency (PWL). An early phase of hyperalgesia and a late phase of hypoalgesia are evident. TRPV1-mediated whole cell currents are larger and smaller in dorsal root ganglion (DRG) neurons collected from hyperalgesic and hypoalgesic mice. Resiniferatoxin (RTX) binding, a measure of TRPV1 expression is increased and decreased in DRG and paw skin of hyperalgesic and hypoalgesic mice, respectively. Immunohistochemical labeling of spinal cord lamina I and II, dorsal root ganglion (DRG), and paw skin from hyperalgesic and hypoalgesic mice reveal increased and decreased TRPV1 expression, respectively. A role for TRPV1 in thermal DPN is further suggested by the failure of STZ treatment to influence thermal nociception in TRPV1 deficient mice. These findings demonstrate that altered TRPV1 expression and function contribute to diabetes-induced changes in thermal perception.

Transient receptor potential vanilloid 1 (TRPV1) is a Ca2+ permeable non-selective cation channel activated by physical and chemical stimuli. Resiniferatoxin (RTX), an ultrapotent agonist of TRPV1, is under investigation for treatment of urinary bladder hyper-reflexia and chronic pain conditions. Here, we have determined the characteristics of RTX-induced responses in cells expressing native and cloned rat TRPV1. Whole-cell currents increase with repeated application of submaximal concentrations of RTX until a maximal response is attained and do not deactivate even after prolonged washout. Interestingly, the rate of activation and block by capsazepine of RTX-induced currents are significantly slower than for capsaicin-induced currents. RTX-induced whole-cell currents are outwardly rectifying, but to a lesser extent than capsaicin-induced currents. RTX-induced single channel currents exhibit multiple conductance states and outward rectification. The open probability (P o ) of RTX-induced currents is higher at all potentials as compared to capsaicin-induced currents, which showed a strong voltage-dependent decrease at negative potentials. Single-channel kinetic analyses reveal that open-time distribution of RTX-induced currents can be fitted with three exponential components at negative and positive potentials. The areas of distribution of the longer open time constants are significantly larger than capsaicin-induced currents. The closed-time distribution of RTX-induced currents can be fitted with three exponential components as compared to capsaicin-induced currents, which require four exponential components. Current-clamp experiments reveal that low concentrations of RTX caused a slow and sustained depolarization beyond threshold while generating few action potentials. Concentrations of capsaicin required for the same extent of depolarization generated a significantly greater number of action potentials. These properties of RTX may play a role in its clinical usefulness.

Streptozotocin (STZ) is a diabetogenic agent extensively used to induce diabetes and to study complications including diabetic peripheral neuropathy (DPN). While studying the influence of transient receptor potential vanilloid 1 (TRPV1) on DPN in the STZ-induced diabetic mouse model, we found that a proportion of STZ-treated mice was nondiabetic but still exhibited hyperalgesia. To understand the mechanism underlying this phenomenon, dorsal root ganglion (DRG) neurons and stably TRPV1 expressing human embryonic kidney (HEK) 293T cells were used to study the expression and function of TRPV1. Incubation of DRG neurons with STZ resulted in a significant increase in the amplitude of capsaicin-induced TRPV1-mediated current and Ca 2ϩ influx compared with vehicle-treated sister cultures. It was also found that STZ treatment induced higher levels of reactive oxygen species, which was abolished with concomitant treatment with catalase. Treatment of cells with H 2 O 2 mimicked the effects of STZ. Western blot analysis revealed an increase in TRPV1 protein content and phospho p38 (p-p38) mitogen-activated protein kinase (MAPK) levels in DRG of STZ-injected diabetic and nondiabetic hyperalgesic mice compared with control mice. Furthermore, in stably TRPV1-expressing HEK 293T cells, STZ treatment induced an increase in TRPV1 protein content and p-p38 MAPK levels, which was abolished with concomitant treatment with catalase or p38 MAPK inhibitor. These results reveal that STZ has a direct action on neurons and modulates the expression and function of TRPV1, a nociceptive ion channel that is responsible for inflammatory thermal pain.

Transient receptor potential (TRP) ankyrin 1 (TRPA1) is a Ca(2+)-permeant, nonselective cationic channel. It is predominantly expressed in the C afferent sensory nerve fibers of trigeminal and dorsal root ganglion neurons and is highly coexpressed with the nociceptive ion channel transient receptor potential vanilloid 1 (TRPV1). Several physical and chemical stimuli have been shown to activate the channel. In this study, we have used electrophysiological techniques and behavioral models to characterize the properties of TRPA1. Whole cell TRPA1 currents induced by brief application of lower concentrations of N-methyl maleimide (NMM) or allyl isothiocyanate (AITC) can be reversed readily by washout, whereas continuous application of higher concentrations of NMM or AITC completely desensitized the currents. The deactivation and desensitization kinetics differed between NMM and AITC. TRPA1 current amplitude increased with repeated application of lower concentrations of AITC, whereas saturating concentrations of AITC induced tachyphylaxis, which was more pronounced in the presence of extracellular Ca(2+). The outward rectification exhibited by native TRPA1-mediated whole cell and single-channel currents was minimal as compared with other TRP channels. TRPA1 currents were negatively modulated by protons and polyamines, both of which activate the heat-sensitive channel, TRPV1. Interestingly, neither protein kinase C nor protein kinase A activation sensitized AITC-induced currents, but each profoundly sensitized capsaicin-induced currents. Current-clamp experiments revealed that AITC produced a slow and sustained depolarization as compared with capsaicin. TRPA1 is also expressed at the central terminals of nociceptors at the caudal spinal trigeminal nucleus. Activation of TRPA1 in this area increases the frequency and amplitude of miniature excitatory or inhibitory postsynaptic currents. In behavioral studies, intraplantar and intrathecal administration of AITC induced more pronounced and prolonged changes in nociceptive behavior than those induced by capsaicin. In conclusion, the characteristics of TRPA1 we have delineated suggest that it might play a unique role in nociception.

The prevalence of diabetes has reached epidemic proportions. There are two forms of diabetes: type 1 diabetes mellitus is due to auto-immune-mediated destruction of pancreatic β-cells resulting in absolute insulin deficiency and type 2 diabetes mellitus is due to reduced insulin secretion and or insulin resistance. Both forms of diabetes are characterized by chronic hyperglycemia, leading to the development of diabetic peripheral neuropathy (DPN) and microvascular pathology. DPN is characterized by enhanced or reduced thermal, chemical, and mechanical pain sensitivities. In the long-term, DPN results in peripheral nerve damage and accounts for a substantial number of non-traumatic lower-limb amputations. This review will address the mechanisms, especially the role of reactive oxygen and nitrogen species in the development and progression of DPN.