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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

Electrophysiological and pharmacological properties of the caps−lpH+ small-sized DRG neurons studied for diabetes-induced changes in T-type current. a A representative trace showing absence of capsaicin-induced current in the caps−lpH+ neurons. b A representative trace demonstrating substantial low-pH-induced current in the caps−lpH+ neurons. c Representative traces of Ba2+ currents demonstrating expression of T-type channels in the caps−lpH+ neurons. The currents were evoked by voltage depolarizing steps from a holding potential of −100 mV to −80 through −10 mV in 10 mV increments. d Representative traces of total currents recorded in the caps−lpH+ neurons that included Na+, Ca2+ and K+ components. The currents were evoked by voltage steps from a holding potential of −100 to −60 through 40 mV in 20 mV increments. e At the top, a representative trace of AP evoked by a threshold 1 millisecond-long current pulse shown at the bottom. APb is an action potential duration at the base. AHP80 is the time required for the AHP to decay to 80 % of its peak value. f An example demonstrating that the caps−lpH+ neurons are IB4-negative. A fluorescent image of DRG neurons stained for IB4 is shown in the left rectangle. Two IB4-positive small DRG neurons are clearly visible in the top right corner of the image. White box in a bottom left corner of the image indicates an area where IB4-negative caps−lpH+ neuron is located. Fluorescent and transmitted light images of the boxed caps−lpH+ neuron are presented in the top and bottom right squares, respectively. The IB4-negative caps−lpH+ neuron is outlined with a white dashed circle on fluorescent images and a black dashed circle on transmitted light image. No IB4 fluorescence is visible on the plasma membrane of this caps−lpH+ neuron. A scale bar is 15 μm. This neuron had a K+ “current signature” specific for the caps−lpH+ DRG neurons
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Fig1: Electrophysiological and pharmacological properties of the caps−lpH+ small-sized DRG neurons studied for diabetes-induced changes in T-type current. a A representative trace showing absence of capsaicin-induced current in the caps−lpH+ neurons. b A representative trace demonstrating substantial low-pH-induced current in the caps−lpH+ neurons. c Representative traces of Ba2+ currents demonstrating expression of T-type channels in the caps−lpH+ neurons. The currents were evoked by voltage depolarizing steps from a holding potential of −100 mV to −80 through −10 mV in 10 mV increments. d Representative traces of total currents recorded in the caps−lpH+ neurons that included Na+, Ca2+ and K+ components. The currents were evoked by voltage steps from a holding potential of −100 to −60 through 40 mV in 20 mV increments. e At the top, a representative trace of AP evoked by a threshold 1 millisecond-long current pulse shown at the bottom. APb is an action potential duration at the base. AHP80 is the time required for the AHP to decay to 80 % of its peak value. f An example demonstrating that the caps−lpH+ neurons are IB4-negative. A fluorescent image of DRG neurons stained for IB4 is shown in the left rectangle. Two IB4-positive small DRG neurons are clearly visible in the top right corner of the image. White box in a bottom left corner of the image indicates an area where IB4-negative caps−lpH+ neuron is located. Fluorescent and transmitted light images of the boxed caps−lpH+ neuron are presented in the top and bottom right squares, respectively. The IB4-negative caps−lpH+ neuron is outlined with a white dashed circle on fluorescent images and a black dashed circle on transmitted light image. No IB4 fluorescence is visible on the plasma membrane of this caps−lpH+ neuron. A scale bar is 15 μm. This neuron had a K+ “current signature” specific for the caps−lpH+ DRG neurons

Mentions: The scope of search for the caps−lpH+ neurons was limited to a population of small-sized DRG neurons (<25 pF) that provided enrichment in C-fibre nociceptors [20], and to L4-L6 ganglia where neurons innervating hind paws are localized. Isolation of the caps−lpH+ nociceptive DRG neurons was based on classification of DRG neuronal types [21]. First, separate types of neurons were identified in the aforementioned subpopulation of DRG neurons based on size and set of K+ current characteristics [21] (activation threshold, presence of A-type fast-inactivating component, K+ current inactivation time constant (τinh)). Next, these types were tested for expression of capsaicin-activated and transient low-pH-activated currents, and for fast-inactivating T-type current involved in abnormalities of thermal and mechanical sensitivity under diabetic conditions [4, 5]. The caps−lpH+ DRG neurons (Fig 1a, b) expressing fast-inactivating T-type current (Fig 1c) selected in this way have appeared to be the members of one type with a distinct set of physical and electrical parameters. Neurons of this type had a very small size (their capacitance was in a range 10–21 pF with a mean value of 15.4 ± 0.7 pF, n = 19, four rats). They also expressed K+ current with a specific “current signature”: (i) an activation threshold of −20 mV, (ii) no obvious A-type component, and (iii) slow inactivation (τinact fell within a range of 53–419 ms with a mean value of 209 ± 31 ms, n = 19, four rats; Fig 1d).Fig. 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)

Electrophysiological and pharmacological properties of the caps−lpH+ small-sized DRG neurons studied for diabetes-induced changes in T-type current. a A representative trace showing absence of capsaicin-induced current in the caps−lpH+ neurons. b A representative trace demonstrating substantial low-pH-induced current in the caps−lpH+ neurons. c Representative traces of Ba2+ currents demonstrating expression of T-type channels in the caps−lpH+ neurons. The currents were evoked by voltage depolarizing steps from a holding potential of −100 mV to −80 through −10 mV in 10 mV increments. d Representative traces of total currents recorded in the caps−lpH+ neurons that included Na+, Ca2+ and K+ components. The currents were evoked by voltage steps from a holding potential of −100 to −60 through 40 mV in 20 mV increments. e At the top, a representative trace of AP evoked by a threshold 1 millisecond-long current pulse shown at the bottom. APb is an action potential duration at the base. AHP80 is the time required for the AHP to decay to 80 % of its peak value. f An example demonstrating that the caps−lpH+ neurons are IB4-negative. A fluorescent image of DRG neurons stained for IB4 is shown in the left rectangle. Two IB4-positive small DRG neurons are clearly visible in the top right corner of the image. White box in a bottom left corner of the image indicates an area where IB4-negative caps−lpH+ neuron is located. Fluorescent and transmitted light images of the boxed caps−lpH+ neuron are presented in the top and bottom right squares, respectively. The IB4-negative caps−lpH+ neuron is outlined with a white dashed circle on fluorescent images and a black dashed circle on transmitted light image. No IB4 fluorescence is visible on the plasma membrane of this caps−lpH+ neuron. A scale bar is 15 μm. This neuron had a K+ “current signature” specific for the caps−lpH+ DRG neurons
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4490764&req=5

Fig1: Electrophysiological and pharmacological properties of the caps−lpH+ small-sized DRG neurons studied for diabetes-induced changes in T-type current. a A representative trace showing absence of capsaicin-induced current in the caps−lpH+ neurons. b A representative trace demonstrating substantial low-pH-induced current in the caps−lpH+ neurons. c Representative traces of Ba2+ currents demonstrating expression of T-type channels in the caps−lpH+ neurons. The currents were evoked by voltage depolarizing steps from a holding potential of −100 mV to −80 through −10 mV in 10 mV increments. d Representative traces of total currents recorded in the caps−lpH+ neurons that included Na+, Ca2+ and K+ components. The currents were evoked by voltage steps from a holding potential of −100 to −60 through 40 mV in 20 mV increments. e At the top, a representative trace of AP evoked by a threshold 1 millisecond-long current pulse shown at the bottom. APb is an action potential duration at the base. AHP80 is the time required for the AHP to decay to 80 % of its peak value. f An example demonstrating that the caps−lpH+ neurons are IB4-negative. A fluorescent image of DRG neurons stained for IB4 is shown in the left rectangle. Two IB4-positive small DRG neurons are clearly visible in the top right corner of the image. White box in a bottom left corner of the image indicates an area where IB4-negative caps−lpH+ neuron is located. Fluorescent and transmitted light images of the boxed caps−lpH+ neuron are presented in the top and bottom right squares, respectively. The IB4-negative caps−lpH+ neuron is outlined with a white dashed circle on fluorescent images and a black dashed circle on transmitted light image. No IB4 fluorescence is visible on the plasma membrane of this caps−lpH+ neuron. A scale bar is 15 μm. This neuron had a K+ “current signature” specific for the caps−lpH+ DRG neurons
Mentions: The scope of search for the caps−lpH+ neurons was limited to a population of small-sized DRG neurons (<25 pF) that provided enrichment in C-fibre nociceptors [20], and to L4-L6 ganglia where neurons innervating hind paws are localized. Isolation of the caps−lpH+ nociceptive DRG neurons was based on classification of DRG neuronal types [21]. First, separate types of neurons were identified in the aforementioned subpopulation of DRG neurons based on size and set of K+ current characteristics [21] (activation threshold, presence of A-type fast-inactivating component, K+ current inactivation time constant (τinh)). Next, these types were tested for expression of capsaicin-activated and transient low-pH-activated currents, and for fast-inactivating T-type current involved in abnormalities of thermal and mechanical sensitivity under diabetic conditions [4, 5]. The caps−lpH+ DRG neurons (Fig 1a, b) expressing fast-inactivating T-type current (Fig 1c) selected in this way have appeared to be the members of one type with a distinct set of physical and electrical parameters. Neurons of this type had a very small size (their capacitance was in a range 10–21 pF with a mean value of 15.4 ± 0.7 pF, n = 19, four rats). They also expressed K+ current with a specific “current signature”: (i) an activation threshold of −20 mV, (ii) no obvious A-type component, and (iii) slow inactivation (τinact fell within a range of 53–419 ms with a mean value of 209 ± 31 ms, n = 19, four rats; Fig 1d).Fig. 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