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

Duzhyy DE, Viatchenko-Karpinski VY, Khomula EV, Voitenko NV, Belan PV - Mol Pain (2015)

Bottom Line: 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.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.Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Cav3.2 subtype of T-type channels.

View Article: PubMed Central - PubMed

Affiliation: Department of General Physiology of the CNS and State Key Laboratory of Molecular and Cellular Biology, Bogomoletz Institute of Physiology of National Academy of Science of Ukraine, 4 Bogomoletz street, 01024, Kyiv, Ukraine. dduzhyy@biph.kiev.ua.

ABSTRACT

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.

No MeSH data available.


Related in: MedlinePlus

T-type channels are specifically upregulated in the caps−lpH+ neurons of longer-term diabetic rats without changes in their biophysical properties. a Representative traces of Ba2+ current recorded in the caps−lpH+ neurons of control and diabetic rats. Currents were evoked by a depolarization step from a holding potential of −100 to −50 mV. b Diabetes-induced upregulation of T-type channels is revealed at the plot of the transient current density, TCD, versus a depolarizing voltage step in the activation protocol. An insert demonstrates TCD amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −50 mV. c HVA currents were not changed under diabetic conditions as revealed at the plot of persistent Ba2+ current versus depolarizing step in the activation protocol. The averaged value of Ba2+ current during last 10 ms of depolarization step was taken as a persistent current amplitude. Current densities are not significantly different between diabetes and control in a range of voltage steps from −30 to 0 mV where HVA currents are main contributors to the persistent current (p > 0.05). An insert demonstrates persistent current density amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −20 mV. d Biophysical properties of T-type channels of the caps−lpH+ neurons are not changed in longer-term diabetes. Inactivation of normalized transient current (left curves) calculated for experimental results depicted in E; activation of normalized transient current conductance (right curves) for the experimental results depicted in B. e Diabetes-induced upregulation of T-type channels is revealed in a voltage-dependent inactivation protocol for evoking transient Ba2+ currents consisted of depolarization steps to a test potential of −40 mV (250 ms) from a holding potential ranging from −100 to −40 mV (3.5 s) with 10 mV increments. f Time to peak, TTP, for Ba2+ currents recorded in a protocol used to estimate the transient current activation. g Inactivation time constant, τ, obtained from a single-exponential fit of the decaying phase of the currents evoked in the activation protocol. Numbers of cells: 19 cells from 4 rats in control group and 17 cells from 4 rats in diabetic group. Data are expressed as Mean ± S.E.M. *p < 0.02, **p < 0.001
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Fig2: T-type channels are specifically upregulated in the caps−lpH+ neurons of longer-term diabetic rats without changes in their biophysical properties. a Representative traces of Ba2+ current recorded in the caps−lpH+ neurons of control and diabetic rats. Currents were evoked by a depolarization step from a holding potential of −100 to −50 mV. b Diabetes-induced upregulation of T-type channels is revealed at the plot of the transient current density, TCD, versus a depolarizing voltage step in the activation protocol. An insert demonstrates TCD amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −50 mV. c HVA currents were not changed under diabetic conditions as revealed at the plot of persistent Ba2+ current versus depolarizing step in the activation protocol. The averaged value of Ba2+ current during last 10 ms of depolarization step was taken as a persistent current amplitude. Current densities are not significantly different between diabetes and control in a range of voltage steps from −30 to 0 mV where HVA currents are main contributors to the persistent current (p > 0.05). An insert demonstrates persistent current density amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −20 mV. d Biophysical properties of T-type channels of the caps−lpH+ neurons are not changed in longer-term diabetes. Inactivation of normalized transient current (left curves) calculated for experimental results depicted in E; activation of normalized transient current conductance (right curves) for the experimental results depicted in B. e Diabetes-induced upregulation of T-type channels is revealed in a voltage-dependent inactivation protocol for evoking transient Ba2+ currents consisted of depolarization steps to a test potential of −40 mV (250 ms) from a holding potential ranging from −100 to −40 mV (3.5 s) with 10 mV increments. f Time to peak, TTP, for Ba2+ currents recorded in a protocol used to estimate the transient current activation. g Inactivation time constant, τ, obtained from a single-exponential fit of the decaying phase of the currents evoked in the activation protocol. Numbers of cells: 19 cells from 4 rats in control group and 17 cells from 4 rats in diabetic group. Data are expressed as Mean ± S.E.M. *p < 0.02, **p < 0.001

Mentions: The caps−lpH+ DRG neurons are likely to be peptidergic C-fiber nociceptors lacking TRPV1 receptors and expressing both ASIC channels and fast-inactivating T-type channels. It suggests their possible involvement in pain symptoms (other than thermal hyperalgesia) of longer-term (12 weeks) diabetic neuropathy [7, 8, 12]. Since T-type current upregulation in small nociceptive DRG neurons is involved in diabetes-induced mechanical hyperalgesia [4], we checked whether such upregulation occurs in the caps−lpH+ DRG neurons of longer-term (9–13 weeks) diabetic rats, for which multiple pain symptoms including mechanical hyperalgesia have been demonstrated [7, 8, 12]. Blood glucose level and body weight of diabetic animals at this stage of diabetes were significantly different from the control group (Table 1). Mean capacitance of these neurons (16.6 ± 0.4 pF, n = 17, four rats) was not significantly different compared to control (15.4 ± 0.7 pF, n = 19, four rats; P > 0.12) indicating that the caps−lpH+ neurons preserved their size in longer-term diabetes. Changes in Ca2+ current density were tracked separately for transient and persistent components (Fig 2a-c, see section Analysis in Methods for the definition). In a range of voltage steps from −100 mV to −80 through −40 mV both components are predominantly represented by T-type current. Starting from a voltage step to −30 mV N-type current (having the inactivation kinetics intermediate between T-type and L-type current) substantially contributes to the transient and persistent components of Ba2+ current [28]. L-type current (having the slowest inactivation kinetics [28]) did also contribute to the persistent component at voltage steps from −30 to 0 mV (data not shown).Table 1


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

Duzhyy DE, Viatchenko-Karpinski VY, Khomula EV, Voitenko NV, Belan PV - Mol Pain (2015)

T-type channels are specifically upregulated in the caps−lpH+ neurons of longer-term diabetic rats without changes in their biophysical properties. a Representative traces of Ba2+ current recorded in the caps−lpH+ neurons of control and diabetic rats. Currents were evoked by a depolarization step from a holding potential of −100 to −50 mV. b Diabetes-induced upregulation of T-type channels is revealed at the plot of the transient current density, TCD, versus a depolarizing voltage step in the activation protocol. An insert demonstrates TCD amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −50 mV. c HVA currents were not changed under diabetic conditions as revealed at the plot of persistent Ba2+ current versus depolarizing step in the activation protocol. The averaged value of Ba2+ current during last 10 ms of depolarization step was taken as a persistent current amplitude. Current densities are not significantly different between diabetes and control in a range of voltage steps from −30 to 0 mV where HVA currents are main contributors to the persistent current (p > 0.05). An insert demonstrates persistent current density amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −20 mV. d Biophysical properties of T-type channels of the caps−lpH+ neurons are not changed in longer-term diabetes. Inactivation of normalized transient current (left curves) calculated for experimental results depicted in E; activation of normalized transient current conductance (right curves) for the experimental results depicted in B. e Diabetes-induced upregulation of T-type channels is revealed in a voltage-dependent inactivation protocol for evoking transient Ba2+ currents consisted of depolarization steps to a test potential of −40 mV (250 ms) from a holding potential ranging from −100 to −40 mV (3.5 s) with 10 mV increments. f Time to peak, TTP, for Ba2+ currents recorded in a protocol used to estimate the transient current activation. g Inactivation time constant, τ, obtained from a single-exponential fit of the decaying phase of the currents evoked in the activation protocol. Numbers of cells: 19 cells from 4 rats in control group and 17 cells from 4 rats in diabetic group. Data are expressed as Mean ± S.E.M. *p < 0.02, **p < 0.001
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Related In: Results  -  Collection

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Show All Figures
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Fig2: T-type channels are specifically upregulated in the caps−lpH+ neurons of longer-term diabetic rats without changes in their biophysical properties. a Representative traces of Ba2+ current recorded in the caps−lpH+ neurons of control and diabetic rats. Currents were evoked by a depolarization step from a holding potential of −100 to −50 mV. b Diabetes-induced upregulation of T-type channels is revealed at the plot of the transient current density, TCD, versus a depolarizing voltage step in the activation protocol. An insert demonstrates TCD amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −50 mV. c HVA currents were not changed under diabetic conditions as revealed at the plot of persistent Ba2+ current versus depolarizing step in the activation protocol. The averaged value of Ba2+ current during last 10 ms of depolarization step was taken as a persistent current amplitude. Current densities are not significantly different between diabetes and control in a range of voltage steps from −30 to 0 mV where HVA currents are main contributors to the persistent current (p > 0.05). An insert demonstrates persistent current density amplitude for each tested neuron and their mean values (boxes) with standard errors (upper whiskers) in the control (c) and diabetic (d) neurons at a depolarization step to −20 mV. d Biophysical properties of T-type channels of the caps−lpH+ neurons are not changed in longer-term diabetes. Inactivation of normalized transient current (left curves) calculated for experimental results depicted in E; activation of normalized transient current conductance (right curves) for the experimental results depicted in B. e Diabetes-induced upregulation of T-type channels is revealed in a voltage-dependent inactivation protocol for evoking transient Ba2+ currents consisted of depolarization steps to a test potential of −40 mV (250 ms) from a holding potential ranging from −100 to −40 mV (3.5 s) with 10 mV increments. f Time to peak, TTP, for Ba2+ currents recorded in a protocol used to estimate the transient current activation. g Inactivation time constant, τ, obtained from a single-exponential fit of the decaying phase of the currents evoked in the activation protocol. Numbers of cells: 19 cells from 4 rats in control group and 17 cells from 4 rats in diabetic group. Data are expressed as Mean ± S.E.M. *p < 0.02, **p < 0.001
Mentions: The caps−lpH+ DRG neurons are likely to be peptidergic C-fiber nociceptors lacking TRPV1 receptors and expressing both ASIC channels and fast-inactivating T-type channels. It suggests their possible involvement in pain symptoms (other than thermal hyperalgesia) of longer-term (12 weeks) diabetic neuropathy [7, 8, 12]. Since T-type current upregulation in small nociceptive DRG neurons is involved in diabetes-induced mechanical hyperalgesia [4], we checked whether such upregulation occurs in the caps−lpH+ DRG neurons of longer-term (9–13 weeks) diabetic rats, for which multiple pain symptoms including mechanical hyperalgesia have been demonstrated [7, 8, 12]. Blood glucose level and body weight of diabetic animals at this stage of diabetes were significantly different from the control group (Table 1). Mean capacitance of these neurons (16.6 ± 0.4 pF, n = 17, four rats) was not significantly different compared to control (15.4 ± 0.7 pF, n = 19, four rats; P > 0.12) indicating that the caps−lpH+ neurons preserved their size in longer-term diabetes. Changes in Ca2+ current density were tracked separately for transient and persistent components (Fig 2a-c, see section Analysis in Methods for the definition). In a range of voltage steps from −100 mV to −80 through −40 mV both components are predominantly represented by T-type current. Starting from a voltage step to −30 mV N-type current (having the inactivation kinetics intermediate between T-type and L-type current) substantially contributes to the transient and persistent components of Ba2+ current [28]. L-type current (having the slowest inactivation kinetics [28]) did also contribute to the persistent component at voltage steps from −30 to 0 mV (data not shown).Table 1

Bottom Line: 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.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.Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Cav3.2 subtype of T-type channels.

View Article: PubMed Central - PubMed

Affiliation: Department of General Physiology of the CNS and State Key Laboratory of Molecular and Cellular Biology, Bogomoletz Institute of Physiology of National Academy of Science of Ukraine, 4 Bogomoletz street, 01024, Kyiv, Ukraine. dduzhyy@biph.kiev.ua.

ABSTRACT

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.

No MeSH data available.


Related in: MedlinePlus