Sensitization of human and rat nociceptors by low dose morphine is toll-like receptor 4-dependent.

While opioids remain amongst the most effective treatments for moderate-to-severe pain, their substantial side effect profile remains a major limitation to broader clinical use. One such side effect is opioid-induced hyperalgesia (OIH), which includes a transition from opioid-induced analgesia to pain enhancement. Evidence in rodents supports the suggestion that OIH may be produced by the action of opioids at Toll-like Receptor 4 (TLR4) either on immune cells that, in turn, produce pronociceptive mediators to act on nociceptors, or by a direct action at nociceptor TLR4. And, sub-analgesic doses of several opioids have been shown to induce hyperalgesia in rodents by their action as TLR4 agonists. In the present in vitro patch-clamp electrophysiology experiments, we demonstrate that low dose morphine directly sensitizes human as well as rodent dorsal root ganglion (DRG) neurons, an effect of this opioid analgesic that is antagonized by LPS-RS Ultrapure, a selective TLR4 antagonist. We found that low concentration (100 nM) of morphine reduced rheobase in human (by 36%) and rat (by 26%) putative C-type nociceptors, an effect of morphine that was markedly attenuated by preincubation with LPS-RS Ultrapure. Our findings support the suggestion that in humans, as in rodents, OIH is mediated by the direct action of opioids at TLR4 on nociceptors.

Role of pattern recognition receptors in chemotherapy-induced neuropathic pain.

Progress in the development of effective chemotherapy is producing a growing population of patients with acute and chronic painful chemotherapy-induced peripheral neuropathy (CIPN), a serious treatment-limiting side effect for which there is currently no US Food and Drug Administration-approved treatment. CIPNs induced by diverse classes of chemotherapy drugs have remarkably similar clinical presentations, leading to the suggestion they share underlying mechanisms. Sensory neurons share with immune cells the ability to detect damage associated molecular patterns (DAMPs), molecules produced by diverse cell types in response to cellular stress and injury, including by chemotherapy drugs. DAMPs, in turn, are ligands for pattern recognition receptors (PRRs), several of which are found on sensory neurons, as well as satellite cells, and cells of the immune system. In the present experiments, we evaluated the role of two PRRs, TLR4 and RAGE, present in dorsal root ganglion (DRG), in CIPN. Antisense (AS)-oligodeoxynucleotides (ODN) against TLR4 and RAGE mRNA were administered intrathecally before ('prevention protocol') or 3 days after ('reversal protocol') the last administration of each of three chemotherapy drugs that treat cancer by different mechanisms (oxaliplatin, paclitaxel and bortezomib). TLR4 and RAGE AS-ODN prevented the development of CIPN induced by all three chemotherapy drugs. In the reversal protocol, however, while TLR4 AS-ODN completely reversed oxaliplatin- and paclitaxel-induced CIPN, in rats with bortezomib-induced CIPN it only produced a temporary attenuation. RAGE AS-ODN, in contrast, reversed CIPN induced by all three chemotherapy drugs. When a TLR4 antagonist was administered intradermally to the peripheral nociceptor terminal, it did not affect CIPN induced by any of the chemotherapy drugs. However, when administered intrathecally, to the central terminal, it attenuated hyperalgesia induced by all three chemotherapy drugs, compatible with a role of TLR4 in neurotransmission at the central terminal but not sensory transduction at the peripheral terminal. Finally, since it has been established that cultured DRG neurons can be used to study direct effects of chemotherapy on nociceptors, we also evaluated the role of TLR4 in CIPN at the cellular level, using patch-clamp electrophysiology in DRG neurons cultured from control and chemotherapy-treated rats. We found that increased excitability of small-diameter DRG neurons induced by in vivo and in vitro exposure to oxaliplatin is TLR4-dependent. Our findings suggest that in addition to the established contribution of PRR-dependent neuroimmune mechanisms, PRRs in DRG cells also have an important role in CIPN.

Duloxetine prevents bortezomib and paclitaxel large-fiber chemotherapy-induced peripheral neuropathy (LF-CIPN) in sprague dawley rats.

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating, treatment-limiting, side-effect of several classes of chemotherapy drugs. While negatively impacting oncology patients' quality of life, chemotherapy-induced large-fiber (LF) neuropathy is amongst the least well understood components of CIPN, and one for which there is currently no established therapy. Preliminary clinical observations have led to the suggestion that Duloxetine, which is used for the treatment of pain associated with small-fiber CIPN (SF-CIPN), may be effective against LF-CIPN. In the present experiments we developed a model of LF-CIPN and studied the effect of Duloxetine on LF-CIPN induced by two neurotoxic chemotherapy agents: the proteasome inhibitor, Bortezomib, a first-line treatment of multiple myeloma; and, the anti-microtubule taxane, Paclitaxel, used in the treatment of solid tumors. Since there are currently no models for selective the study of LF-CIPN, our first aim was to establish a pre-clinical model in the rat. LF-CIPN was evaluated with the Current Perception Threshold (CPT) assay, which uses a high frequency (1000 Hz) electrical stimulus protocol that selectively activates large-fiber myelinated afferents. Our second aim was to use this model to test the hypothesis that Duloxetine can prevent LF-CIPN. We report that Bortezomib and Paclitaxel induce elevation of CPT, compatible with loss of large-fiber function, which are prevented by Duloxetine. Our findings support the clinical observation that Duloxetine may be an effective treatment for the large-fiber CIPN. We also suggest that CPT could be used as a biomarker for LF-CIPN in patients receiving neurotoxic chemotherapy.

Sensitization of Human and Rat Nociceptors by Low Dose Morphine is TLR4-dependent.

While opioids remain amongst the most effective treatments for moderate-to-severe pain, their substantial side effect profile remains a major limitation to broader clinical use. One such side effect is opioid-induced hyperalgesia (OIH), which includes a transition from opioid-induced analgesia to pain enhancement. Evidence in rodents supports the suggestion that OIH may be produced by the action of opioids at Toll-like Receptor 4 (TLR4) either on immune cells that, in turn, produce pronociceptive mediators to act on nociceptors, or by a direct action at nociceptor TLR4. And, sub-analgesic doses of several opioids have been shown to induce hyperalgesia in rodents by their action as TLR4 agonists. In the present in vitro patch-clamp electrophysiology experiments, we demonstrate that low dose morphine directly sensitizes human as well as rodent dorsal root ganglion (DRG) neurons, an effect of this opioid analgesic that is antagonized by LPS-RS Ultrapure, a selective TLR4 antagonist. We found that morphine (100 nM) reduced rheobase in human (by 36%) and rat (by 26%) putative C-type nociceptors, an effect of morphine that was markedly attenuated by preincubation with LPS-RS Ultrapure. Our findings support the suggestion that in humans, as well as in rodents, OIH is mediated by the direct action of opioids at TLR4 on nociceptors.

PI3Kγ/AKT Signaling in High Molecular Weight Hyaluronan (HMWH)-Induced Anti-Hyperalgesia and Reversal of Nociceptor Sensitization.

High molecular weight hyaluronan (HMWH), a well-established treatment for osteoarthritis pain, is anti-hyperalgesic in preclinical models of inflammatory and neuropathic pain. HMWH-induced anti-hyperalgesia is mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, which can signal via phosphoinositide 3-kinase (PI3K), a large family of kinases involved in diverse cell functions. We demonstrate that intrathecal administration of an oligodeoxynucleotide (ODN) antisense to mRNA for PI3Kγ (a Class I PI3K isoform) expressed in dorsal root ganglia (DRGs), and intradermal administration of a PI3Kγ-selective inhibitor (AS605240), markedly attenuates HMWH-induced anti-prostaglandin E2 (PGE2) hyperalgesia, in male and female rats. Intradermal administration of inhibitors of mammalian target of rapamycin (mTOR; rapamycin) and protein kinase B (AKT; AKT Inhibitor IV), signaling molecules downstream of PI3Kγ, also attenuates HMWH-induced anti-hyperalgesia. In vitro patch-clamp electrophysiology experiments on cultured nociceptors from male rats demonstrate that some HMWH-induced changes in generation of action potentials (APs) in nociceptors sensitized by PGE2 are PI3Kγ dependent (reduction in AP firing rate, increase in latency to first AP and increase in slope of current ramp required to induce AP) and some are PI3Kγ independent [reduction in recovery rate of AP afterhyperpolarization (AHP)]. Our demonstration of a role of PI3Kγ in HMWH-induced anti-hyperalgesia and reversal of nociceptor sensitization opens a novel line of research into molecular targets for the treatment of diverse pain syndromes.SIGNIFICANCE STATEMENT We have previously demonstrated that high molecular weight hyaluronan (HMWH) attenuates inflammatory hyperalgesia, an effect mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and activation of its downstream signaling pathway, in nociceptors. In the present study, we demonstrate that phosphoinositide 3-kinase (PI3K)γ and downstream signaling pathway, protein kinase B (AKT) and mammalian target of rapamycin (mTOR), are crucial for HMWH to induce anti-hyperalgesia.

Opioid-Induced Hyperalgesic Priming in Single Nociceptors.

Clinical µ-opioid receptor (MOR) agonists produce hyperalgesic priming, a form of maladaptive nociceptor neuroplasticity, resulting in pain chronification. We have established an in vitro model of opioid-induced hyperalgesic priming (OIHP), in male rats, to identify nociceptor populations involved and its maintenance mechanisms. OIHP was induced in vivo by systemic administration of fentanyl and confirmed by prolongation of prostaglandin E2 (PGE2) hyperalgesia. Intrathecal cordycepin, which reverses Type I priming, or the combination of Src and mitogen-activated protein kinase (MAPK) inhibitors, which reverses Type II priming, both partially attenuated OIHP. Parallel in vitro experiments were performed on small-diameter (<30 µm) dorsal root ganglion (DRG) neurons, cultured from fentanyl-primed rats, and rats with OIHP treated with agents that reverse Type I or Type II priming. Enhancement of the sensitizing effect of a low concentration of PGE2 (10 nm), another characteristic feature of priming, measured as reduction in action potential (AP) rheobase, was found in weakly isolectin B4 (IB4)-positive and IB4-negative (IB4-) neurons. In strongly IB4-positive (IB4+) neurons, only the response to a higher concentration of PGE2 (100 nm) was enhanced. The sensitizing effect of 10 nm PGE2 was attenuated in weakly IB4+ and IB4- neurons cultured from rats whose OIHP was reversed in vivo Thus, in vivo administration of fentanyl induces neuroplasticity in weakly IB4+ and IB4- nociceptors that persists in vitro and has properties of Type I and Type II priming. The mechanism underlying the enhanced sensitizing effect of 100 nm PGE2 in strongly IB4+ nociceptors, not attenuated by inhibitors of Type I and Type II priming, remains to be elucidated.SIGNIFICANCE STATEMENT Commonly used clinical opioid analgesics, such as fentanyl and morphine, can produce hyperalgesia and chronification of pain. To uncover the nociceptor population mediating opioid-induced hyperalgesic priming (OIHP), a model of pain chronification, and elucidate its underlying mechanism, at the cellular level, we established an in vitro model of OIHP. In dorsal root ganglion (DRG) neurons cultured from rats primed with fentanyl, robust nociceptor population-specific changes in sensitization by prostaglandin E2 (PGE2) were observed, when compared with nociceptors from opioid naive rats. In DRG neurons cultured from rats with OIHP, enhanced PGE2-induced sensitization was observed in vitro, with differences identified in non-peptidergic [strongly isolectin B4 (IB4)-positive] and peptidergic [weakly IB4-positive (IB4+) and IB4-negative (IB4-)] nociceptors.

Mechanisms Mediating High-Molecular-Weight Hyaluronan-Induced Antihyperalgesia.

We evaluated the mechanism by which high-molecular-weight hyaluronan (HMWH) attenuates nociceptor sensitization, in the setting of inflammation. HMWH attenuated mechanical hyperalgesia induced by the inflammatory mediator prostaglandin E2 (PGE2) in male and female rats. Intrathecal administration of an oligodeoxynucleotide antisense (AS-ODN) to mRNA for cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and intradermal administration of A5G27, a CD44 receptor antagonist, both attenuated antihyperalgesia induced by HMWH. In male rats, HMWH also signals via Toll-like receptor 4 (TLR4), and AS-ODN for TLR4 mRNA administered intrathecally, attenuated HMWH-induced antihyperalgesia. Since HMWH signaling is dependent on CD44 clustering in lipid rafts, we pretreated animals with methyl-ß-cyclodextrin (MßCD), which disrupts lipid rafts. MßCD markedly attenuated HMWH-induced antihyperalgesia. Inhibitors for components of intracellular signaling pathways activated by CD44, including phospholipase C and phosphoinositide 3-kinase (PI3K), also attenuated HMWH-induced antihyperalgesia. Furthermore, in vitro application of HMWH attenuated PGE2-induced sensitization of tetrodotoxin-resistant sodium current, in small-diameter dorsal root ganglion neurons, an effect that was attenuated by a PI3K inhibitor. Our results indicate a central role of CD44 signaling in HMWH-induced antihyperalgesia and suggest novel therapeutic targets, downstream of CD44, for the treatment of pain generated by nociceptor sensitization.SIGNIFICANCE STATEMENT High-molecular-weight-hyaluronan (HMWH) is used to treat osteoarthritis and other pain syndromes. In this study we demonstrate that attenuation of inflammatory hyperalgesia by HMWH is mediated by its action at cluster of differentiation 44 (CD44) and activation of its downstream signaling pathways, including RhoGTPases (RhoA and Rac1), phospholipases (phospholipases Cε and Cγ1), and phosphoinositide 3-kinase, in nociceptors. These findings contribute to our understanding of the antihyperalgesic effect of HMWH and support the hypothesis that CD44 and its downstream signaling pathways represent novel therapeutic targets for the treatment of inflammatory pain.

In Vitro Nociceptor Neuroplasticity Associated with In Vivo Opioid-Induced Hyperalgesia.

Opioid-induced hyperalgesia (OIH) is a serious adverse event produced by opioid analgesics. Lack of an in vitro model has hindered study of its underlying mechanisms. Recent evidence has implicated a role of nociceptors in OIH. To investigate the cellular and molecular mechanisms of OIH in nociceptors, in vitro, subcutaneous administration of an analgesic dose of fentanyl (30 µg/kg, s.c.) was performed in vivo in male rats. Two days later, when fentanyl was administered intradermally (1 µg, i.d.), in the vicinity of peripheral nociceptor terminals, it produced mechanical hyperalgesia (OIH). Additionally, 2 d after systemic fentanyl, rats had also developed hyperalgesic priming (opioid-primed rats), long-lasting nociceptor neuroplasticity manifested as prolongation of prostaglandin E2 (PGE2) hyperalgesia. OIH was reversed, in vivo, by intrathecal administration of cordycepin, a protein translation inhibitor that reverses priming. When fentanyl (0.5 nm) was applied to dorsal root ganglion (DRG) neurons, cultured from opioid-primed rats, it induced a µ-opioid receptor (MOR)-dependent increase in [Ca2+]i in 26% of small-diameter neurons and significantly sensitized (decreased action potential rheobase) weakly IB4+ and IB4- neurons. This sensitizing effect of fentanyl was reversed in weakly IB4+ DRG neurons cultured from opioid-primed rats after in vivo treatment with cordycepin, to reverse of OIH. Thus, in vivo administration of fentanyl induces nociceptor neuroplasticity, which persists in culture, providing evidence for the role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH.SIGNIFICANCE STATEMENT Clinically used µ-opioid receptor agonists such as fentanyl can produce hyperalgesia and hyperalgesic priming. We report on an in vitro model of nociceptor neuroplasticity mediating this opioid-induced hyperalgesia (OIH) and priming induced by fentanyl. Using this model, we have found qualitative and quantitative differences between cultured nociceptors from opioid-naive and opioid-primed animals, and provide evidence for the important role of nociceptor µ-opioid receptor-mediated calcium signaling and peripheral protein translation in the weakly IB4-binding population of nociceptors in OIH. These findings provide information useful for the design of therapeutic strategies to alleviate OIH, a serious adverse event of opioid analgesics.

Fentanyl Induces Rapid Onset Hyperalgesic Priming: Type I at Peripheral and Type II at Central Nociceptor Terminals.

Systemic fentanyl induces hyperalgesic priming, long-lasting neuroplasticity in nociceptor function characterized by prolongation of inflammatory mediator hyperalgesia. To evaluate priming at both nociceptor terminals, we studied, in male Sprague Dawley rats, the effect of local administration of agents that reverse type I (protein translation) or type II [combination of Src and mitogen-activated protein kinase (MAPK)] priming. At the central terminal, priming induced by systemic, intradermal, or intrathecal fentanyl was reversed by the combination of Src and MAPK inhibitors, but at the peripheral terminal, it was reversed by the protein translation inhibitor. Mu-opioid receptor (MOR) antisense prevented fentanyl hyperalgesia and priming. To determine whether type I and II priming occur in the same population of neurons, we used isolectin B4-saporin or [Sar9, Met(O2)11]-substance P-saporin to deplete nonpeptidergic or peptidergic nociceptors, respectively. Following intrathecal fentanyl, central terminal priming was prevented by both saporins, whereas that in peripheral terminal was not attenuated even by their combination. However, after intradermal fentanyl, priming in the peripheral terminal requires both peptidergic and nonpeptidergic nociceptors, whereas that in the central terminal is dependent only on peptidergic nociceptors. Pretreatment with dantrolene at either terminal prevented fentanyl-induced priming in both terminals, suggesting communication between central and peripheral terminals mediated by intracellular Ca2+ signaling. In vitro application of fentanyl increased cytoplasmic Ca2+ concentration in dorsal root ganglion neurons, which was prevented by pretreatment with dantrolene and naloxone. Therefore, acting at MOR in the nociceptor, fentanyl induces hyperalgesia and priming rapidly at both the central (type II) and peripheral (type I) terminal and this is mediated by Ca2+ signaling.SIGNIFICANCE STATEMENT Fentanyl, acting at the µ-opioid receptor (MOR), induces hyperalgesia and hyperalgesic priming at both the central and peripheral terminal of nociceptors and this is mediated by endoplasmic reticulum Ca2+ signaling. Priming in the central terminal is type II, whereas that in the peripheral terminal is type I. Our findings may provide useful information for the design of drugs with improved therapeutic profiles, selectively disrupting individual MOR signaling pathways, to maintain an adequate long-lasting control of pain.

CD44 Signaling Mediates High Molecular Weight Hyaluronan-Induced Antihyperalgesia.

We studied, in male Sprague Dawley rats, the role of the cognate hyaluronan receptor, CD44 signaling in the antihyperalgesia induced by high molecular weight hyaluronan (HMWH). Low molecular weight hyaluronan (LMWH) acts at both peptidergic and nonpeptidergic nociceptors to induce mechanical hyperalgesia that is prevented by intrathecal oligodeoxynucleotide antisense to CD44 mRNA, which also prevents hyperalgesia induced by a CD44 receptor agonist, A6. Ongoing LMWH and A6 hyperalgesia are reversed by HMWH. HMWH also reverses the hyperalgesia induced by diverse pronociceptive mediators, prostaglandin E2, epinephrine, TNFα, and interleukin-6, and the neuropathic pain induced by the cancer chemotherapy paclitaxel. Although CD44 antisense has no effect on the hyperalgesia induced by inflammatory mediators or paclitaxel, it eliminates the antihyperalgesic effect of HMWH. HMWH also reverses the hyperalgesia induced by activation of intracellular second messengers, PKA and PKCε, indicating that HMWH-induced antihyperalgesia, although dependent on CD44, is mediated by an intracellular signaling pathway rather than as a competitive receptor antagonist. Sensitization of cultured small-diameter DRG neurons by prostaglandin E2 is also prevented and reversed by HMWH. These results demonstrate the central role of CD44 signaling in HMWH-induced antihyperalgesia, and establish it as a therapeutic target against inflammatory and neuropathic pain.SIGNIFICANCE STATEMENT We demonstrate that hyaluronan (HA) with different molecular weights produces opposing nociceptive effects. While low molecular weight HA increases sensitivity to mechanical stimulation, high molecular weight HA reduces sensitization, attenuating inflammatory and neuropathic hyperalgesia. Both pronociceptive and antinociceptive effects of HA are mediated by activation of signaling pathways downstream CD44, the cognate HA receptor, in nociceptors. These results contribute to our understanding of the role of the extracellular matrix in pain, and indicate CD44 as a potential therapeutic target to alleviate inflammatory and neuropathic pain.

Sexual Dimorphism in a Reciprocal Interaction of Ryanodine and IP3 Receptors in the Induction of Hyperalgesic Priming.

Hyperalgesic priming, a model of pain chronification in the rat, is mediated by ryanodine receptor-dependent calcium release. Although ryanodine induces priming in both sexes, females are 5 orders of magnitude more sensitive, by an estrogen receptor α (EsRα)-dependent mechanism. An inositol 1,4,5-triphosphate (IP3) receptor inhibitor prevented the induction of priming by ryanodine. For IP3 induced priming, females were also more sensitive. IP3-induced priming was prevented by pretreatment with inhibitors of the sarcoendoplasmic reticulum calcium ATPase and ryanodine receptor. Antisense to EsRα prevented the induction of priming by low-dose IP3 in females. The induction of priming by an EsRα agonist was ryanodine receptor-dependent and prevented by the IP3 antagonist. Thus, an EsRα-dependent bidirectional interaction between endoplasmic reticulum IP3 and ryanodine receptor-mediated calcium signaling is present in the induction of hyperalgesic priming, in females. In cultured male DRG neurons, IP3 (100 µm) potentiated depolarization-induced transients produced by extracellular application of high-potassium solution (20 mm, K20), in nociceptors incubated with ß-estradiol. This potentiation of depolarization-induced calcium transients was blocked by the IP3 antagonist, and not observed in the absence of IP3 IP3 potentiation was also blocked by ryanodine receptor antagonist. The application of ryanodine (2 nm), instead of IP3, also potentiated K20-induced calcium transients in the presence of ß-estradiol, in an IP3 receptor-dependent manner. Our results point to an EsRα-dependent, reciprocal interaction between IP3 and ryanodine receptors that contributes to sex differences in hyperalgesic priming.SIGNIFICANCE STATEMENT The present study demonstrates a mechanism that plays a role in the marked sexual dimorphism observed in a model of the transition to chronic pain, hyperalgesic priming. This mechanism involves a reciprocal interaction between the endoplasmic reticulum receptors, IP3 and ryanodine, in the induction of priming, regulated by estrogen receptor α in the nociceptor of female rats. The presence of this signaling pathway modulating the susceptibility of nociceptors to develop plasticity may contribute to our understanding of sex differences observed clinically in chronic pain syndromes.

Marked Sexual Dimorphism in the Role of the Ryanodine Receptor in a Model of Pain Chronification in the Rat.

Hyperalgesic priming, an estrogen dependent model of the transition to chronic pain, produced by agonists at receptors that activate protein kinase C epsilon (PKCε), occurs in male but not in female rats. However, activation of second messengers downstream of PKCε, such as the ryanodine receptor, induces priming in both sexes. Since estrogen regulates intracellular calcium, we investigated the interaction between estrogen and ryanodine in the susceptibility to develop priming in females. The lowest dose of ryanodine able to induce priming in females (1 pg) is 1/100,000(th) that needed in males (100 ng), an effect dependent on the activation of ryanodine receptors. Treatment of female rats with antisense to estrogen receptor alpha (ERα), but not beta (ERß), mRNA, prevented the induction of priming by low dose ryanodine, and the ERα agonist, PPT, induced ryanodine receptor-dependent priming. In vitro application of ryanodine in low concentration (2 nM) to small DRG neurons cultured from females, significantly potentiated calcium release via ryanodine receptors induced by caffeine. This effect was only observed in IB4+ neurons, cultured in the presence of ß-estradiol or PPT. Our results demonstrate a profound regulatory role of ERα in ryanodine receptor-dependent transition to chronic pain.

Upregulation of T-type Ca2+ channels in long-term diabetes determines increased excitability of a specific type of capsaicin-insensitive DRG neurons.

BACKGROUND: Previous studies have shown that increased excitability of capsaicin-sensitive DRG neurons and thermal hyperalgesia in rats with short-term (2-4 weeks) streptozotocin-induced diabetes is mediated by upregulation of T-type Ca(2+) current. In longer-term diabetes (after the 8th week) thermal hyperalgesia is changed to hypoalgesia that is accompanied by downregulation of T-type current in capsaicin-sensitive small-sized nociceptors. At the same time pain symptoms of diabetic neuropathy other than thermal persist in STZ-diabetic animals and patients during progression of diabetes into later stages suggesting that other types of DRG neurons may be sensitized and contribute to pain. In this study, we examined functional expression of T-type Ca(2+) channels in capsaicin-insensitive DRG neurons and excitability of these neurons in longer-term diabetic rats and in thermally hypoalgesic diabetic rats. RESULTS: Here we have demonstrated that in STZ-diabetes T-type current was upregulated in capsaicin-insensitive low-pH-sensitive small-sized nociceptive DRG neurons of longer-term diabetic rats and thermally hypoalgesic diabetic rats. This upregulation was not accompanied by significant changes in biophysical properties of T-type channels suggesting that a density of functionally active channels was increased. Sensitivity of T-type current to amiloride (1 mM) and low concentration of Ni(2+) (50 µM) implicates prevalence of Cav3.2 subtype of T-type channels in the capsaicin-insensitive low-pH-sensitive neurons of both naïve and diabetic rats. The upregulation of T-type channels resulted in the increased neuronal excitability of these nociceptive neurons revealed by a lower threshold for action potential initiation, prominent afterdepolarizing potentials and burst firing. Sodium current was not significantly changed in these neurons during long-term diabetes and could not contribute to the diabetes-induced increase of neuronal excitability. CONCLUSIONS: Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Cav3.2 subtype of T-type channels. This upregulation results in the increased excitability of these neurons and may contribute to nonthermal nociception at a later-stage diabetes.

Nociceptive neurons differentially express fast and slow T-type Ca²âº currents in different types of diabetic neuropathy.

T-type Ca²âº channels are known as important participants of nociception and their remodeling contributes to diabetes-induced alterations of pain sensation. In this work we have established that about 30% of rat nonpeptidergic thermal C-type nociceptive (NTCN) neurons of segments L4-L6 express a slow T-type Ca²âº current (T-current) while a fast T-current is expressed in the other 70% of these neurons. Streptozotocin-induced diabetes in young rats resulted in thermal hyperalgesia, hypoalgesia, or normalgesia 5-6 weeks after the induction. Our results show that NTCN neurons obtained from hyperalgesic animals do not express the slow T-current. Meanwhile, the fraction of neurons expressing the slow T-current did not significantly change in the hypo- and normalgesic diabetic groups. Moreover, the peak current density of fast T-current was significantly increased only in the neurons of hyperalgesic group. In contrast, the peak current density of slow T-current was significantly decreased in the hypo- and normalgesic groups. Experimental diabetes also resulted in a depolarizing shift of steady-state inactivation of fast T-current in the hyperalgesic group and slow T-current in the hypo- and normalgesic groups. We suggest that the observed changes may contribute to expression of different types of peripheral diabetic neuropathy occurring during the development of diabetes mellitus.

Specific functioning of Cav3.2 T-type calcium and TRPV1 channels under different types of STZ-diabetic neuropathy.

Streptozotocin (STZ)-induced type 1 diabetes in rats leads to the development of peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia at early stages (4th week) followed by hypoalgesia after 8weeks of diabetes development. Here we found that 6-7 week STZ-diabetic rats developed either thermal hyper- (18%), hypo- (25%) or normalgesic (57%) types of PDN. These developmentally similar diabetic rats were studied in order to analyze mechanisms potentially underlying different thermal nociception. The proportion of IB4-positive capsaicin-sensitive small DRG neurons, strongly involved in thermal nociception, was not altered under different types of PDN implying differential changes at cellular and molecular level. We further focused on properties of T-type calcium and TRPV1 channels, which are known to be involved in Ca(2+) signaling and pathological nociception. Indeed, TRPV1-mediated signaling in these neurons was downregulated under hypo- and normalgesia and upregulated under hyperalgesia. A complex interplay between diabetes-induced changes in functional expression of Cav3.2 T-type calcium channels and depolarizing shift of their steady-state inactivation resulted in upregulation of these channels under hyper- and normalgesia and their downregulation under hypoalgesia. As a result, T-type window current was increased by several times under hyperalgesia partially underlying the increased resting [Ca(2+)]i observed in the hyperalgesic rats. At the same time Cav3.2-dependent Ca(2+) signaling was upregulated in all types of PDN. These findings indicate that alterations in functioning of Cav3.2 T-type and TRPV1 channels, specific for each type of PDN, may underlie the variety of pain syndromes induced by type 1 diabetes.