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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

TTX-sensitive and TTX-resistant Na+ currents do not contribute to neuronal excitability in the caps−lpH+ neurons. a Steady-state activation of TTX-sensitive, TTX-S, component of Na+ current. Insert: representative traces of TTX-S Na+ current b A part of a low voltage region from A is presented in a larger scale demonstrating negligible values of TTX-S current at depolarization steps in a range of −70 - -50 mV (n = 9 from three rats). c Steady-state activation curve of TTX-resistant, TTX-R, component of Na+ current demonstrating its activation threshold at −35 mV (n = 9 from three rats). Insert: representative traces of TTX-R Na+ current. Altogether these results demonstrate a lack of low-threshold TTX-R current and substantially lower density of TTX-S current compared to T-type current [39] at depolarization steps up to -40 mV
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Fig8: TTX-sensitive and TTX-resistant Na+ currents do not contribute to neuronal excitability in the caps−lpH+ neurons. a Steady-state activation of TTX-sensitive, TTX-S, component of Na+ current. Insert: representative traces of TTX-S Na+ current b A part of a low voltage region from A is presented in a larger scale demonstrating negligible values of TTX-S current at depolarization steps in a range of −70 - -50 mV (n = 9 from three rats). c Steady-state activation curve of TTX-resistant, TTX-R, component of Na+ current demonstrating its activation threshold at −35 mV (n = 9 from three rats). Insert: representative traces of TTX-R Na+ current. Altogether these results demonstrate a lack of low-threshold TTX-R current and substantially lower density of TTX-S current compared to T-type current [39] at depolarization steps up to -40 mV

Mentions: TTX-resistant sodium channels (TTX-R) along with TTX-sensitive channels (TTX-S) are widely expressed in small DRG neurons and are implicated in the molecular mechanisms of nociception and pain [33–35]. Particularly, TTX-R sodium channels encoded by Nav1.9 play a similar role to that of T-type channels in lowering the AP threshold and promoting burst discharges [36–38]. Therefore, we pharmacologically isolated TTX-S and TTX-R components of sodium current in the caps−lpH+ neurons (Fig 8) in order to check whether both components are present in these neurons and whether they may substantially contribute to neuronal excitability. To isolate the TTX-R component a subtraction procedure was applied to current traces recorded in a series of different external solutions (see Materials and Methods). TTX-R current appeared to be small even at the maximum of I-V curve (<12 pA/pF) and had an activation threshold around −35 mV (Fig 8c), lacking a low voltage-activated component conducted by Nav1.9 channels and being represented solely by high voltage-activated current conducted by Nav1.8 channels. Thus, it could not affect either the ADP area or the AP threshold in the caps−lpH+ neurons in the −80 - -40 mV range of depolarization, where the sodium current is represented by its TTX-S component. This component had a shallow slope of activation curve (from 0 to about 1.6 pA/pF) in a range of test potentials from −80 to −50 mV (Fig 8a, b) and a density substantially less than one of T-type Ca2+ current. T-type current density estimated at the level of an AP threshold of −53 mV in Tyrode’s solution was ~6 pA/pF (Fig 2b; assuming IBa:ICa = 1.2 [39])) and, therefore, TTX-S could not determine the AP threshold. Thus, we have found that voltage-operated sodium channels could not substantially influence the values of the ADP area and the APT in the caps−lpH+ neurons and were not upregulated in the longer-term diabetic rats. At the same time T-type Ca2+ channels were upregulated and definitely contributed to increased excitability of the caps−lpH+ neurons in longer-term diabetes.Fig. 8


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)

TTX-sensitive and TTX-resistant Na+ currents do not contribute to neuronal excitability in the caps−lpH+ neurons. a Steady-state activation of TTX-sensitive, TTX-S, component of Na+ current. Insert: representative traces of TTX-S Na+ current b A part of a low voltage region from A is presented in a larger scale demonstrating negligible values of TTX-S current at depolarization steps in a range of −70 - -50 mV (n = 9 from three rats). c Steady-state activation curve of TTX-resistant, TTX-R, component of Na+ current demonstrating its activation threshold at −35 mV (n = 9 from three rats). Insert: representative traces of TTX-R Na+ current. Altogether these results demonstrate a lack of low-threshold TTX-R current and substantially lower density of TTX-S current compared to T-type current [39] at depolarization steps up to -40 mV
© Copyright Policy - open-access
Related In: Results  -  Collection

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Fig8: TTX-sensitive and TTX-resistant Na+ currents do not contribute to neuronal excitability in the caps−lpH+ neurons. a Steady-state activation of TTX-sensitive, TTX-S, component of Na+ current. Insert: representative traces of TTX-S Na+ current b A part of a low voltage region from A is presented in a larger scale demonstrating negligible values of TTX-S current at depolarization steps in a range of −70 - -50 mV (n = 9 from three rats). c Steady-state activation curve of TTX-resistant, TTX-R, component of Na+ current demonstrating its activation threshold at −35 mV (n = 9 from three rats). Insert: representative traces of TTX-R Na+ current. Altogether these results demonstrate a lack of low-threshold TTX-R current and substantially lower density of TTX-S current compared to T-type current [39] at depolarization steps up to -40 mV
Mentions: TTX-resistant sodium channels (TTX-R) along with TTX-sensitive channels (TTX-S) are widely expressed in small DRG neurons and are implicated in the molecular mechanisms of nociception and pain [33–35]. Particularly, TTX-R sodium channels encoded by Nav1.9 play a similar role to that of T-type channels in lowering the AP threshold and promoting burst discharges [36–38]. Therefore, we pharmacologically isolated TTX-S and TTX-R components of sodium current in the caps−lpH+ neurons (Fig 8) in order to check whether both components are present in these neurons and whether they may substantially contribute to neuronal excitability. To isolate the TTX-R component a subtraction procedure was applied to current traces recorded in a series of different external solutions (see Materials and Methods). TTX-R current appeared to be small even at the maximum of I-V curve (<12 pA/pF) and had an activation threshold around −35 mV (Fig 8c), lacking a low voltage-activated component conducted by Nav1.9 channels and being represented solely by high voltage-activated current conducted by Nav1.8 channels. Thus, it could not affect either the ADP area or the AP threshold in the caps−lpH+ neurons in the −80 - -40 mV range of depolarization, where the sodium current is represented by its TTX-S component. This component had a shallow slope of activation curve (from 0 to about 1.6 pA/pF) in a range of test potentials from −80 to −50 mV (Fig 8a, b) and a density substantially less than one of T-type Ca2+ current. T-type current density estimated at the level of an AP threshold of −53 mV in Tyrode’s solution was ~6 pA/pF (Fig 2b; assuming IBa:ICa = 1.2 [39])) and, therefore, TTX-S could not determine the AP threshold. Thus, we have found that voltage-operated sodium channels could not substantially influence the values of the ADP area and the APT in the caps−lpH+ neurons and were not upregulated in the longer-term diabetic rats. At the same time T-type Ca2+ channels were upregulated and definitely contributed to increased excitability of the caps−lpH+ neurons in longer-term diabetes.Fig. 8

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