The Endogenous Redox Agent L-Cysteine Induces T-Type Ca²+ Channel-Dependent Sensitization of a Novel Subpopulation of Rat Peripheral Nociceptors

Abstract

Recent studies have demonstrated a previously unrecognized contribution of T-type Ca 2ϩ channels in peripheral sensory neurons to pain sensation (nociception). However, the cellular mechanisms underlying the functions of these channels in nociception are not known. Here, in both acutely dissociated and intact rat dorsal root ganglion neurons, we characterize a novel subpopulation of capsaicin-and isolectin B 4 -positive nociceptors that also expresses a high density of T-type Ca 2ϩ currents. Using these "T-rich" cells as a model, we demonstrate that the endogenous reducing agent L-cysteine lowers the threshold for nociceptor excitability and induces burst firing by increasing the amplitude of T-type currents and shifting the gating parameters of T-type channels. These findings, which provide the first direct evidence of T-type Ca 2ϩ channel involvement in the control of nociceptor excitability, suggest that endogenous T-type channel agonists may sensitize a unique subpopulation of peripheral nociceptors, consequently influencing pain processing under normal or pathological conditions.

Cell-Specific Alterations of T-Type Calcium Current in Painful Diabetic Neuropathy Enhance Excitability of Sensory Neurons

“…3E, right), with values almost completely overlapping in voltage-independent regions (e.g., Ϫ10 mV). Finally, Figure 3F shows that the T-type currents we recorded at V h of Ϫ40 mV had deactivation kinetics very similar to those reported previously for native rat DRG (Todorovic and Lingle, 1998;Nelson et al, 2005) and recombinant rat Ca V 3.2 T-type currents (McRory et al, 2001). These kinet- currentsevokedinrepresentativemediumDRGcellsfromcontrol(topblacktrace;R s of4.0M⍀)anddiabeticrats(bottomgraytrace;R s of 1.4 M⍀) by voltage steps from Ϫ90 mV (V h ) to V t from Ϫ80 through ϩ60 mV in 10 mV increments.…”

“…After the positive identification of a T-type channel in current-clamp mode, manifested as a prominent afterdepolarizing potential (ADP), we exchanged the external recording solution to Tyrode's solution supplemented with 10 g/ml isolectin B 4 (IB 4 ) (Stucky and Lewin, 1999;Nelson et al, 2005) conjugated to green fluorescein isothiocyanate (FITC) (Sigma). We incubated the dish in the dark for 10 -12 min and then rinsed it three times with Tyrode's solution.…”

“…3B). This is important because we previously used computational modeling to demonstrate a positive correlation between the window period and enhanced cellular excitability of DRG cells (Nelson et al, 2005). Next we tested the possibility that T-type channels might be readily available for activation in conditions of the steady depolarizations that may accompany repetitive stimulation of primary sensory neurons (Dichter and Fischbach, 1977).…”

“…Pharmacological (Dogrul et al, 2003;Flatters and Bennett, 2004;Todorovic et al, 2004a;Pathirathna et al, 2005b) and gene-silencing (Bourinet et al, 2005) studies also support the idea that T-type channels contribute to symptoms of chronic pain associated with peripheral axonal injury. Furthermore, in a recent in vitro study, we found that modulation of T-type current appears to increase the subthreshold excitability of a specific subset of isolectin B 4 (IB 4 )-and capsaicin-positive sensory neurons (Nelson et al, 2005). These studies are generating interest in exploring the possibility that T-type channels can be used as targets for therapeutic interventions in different disorders of sensory neurons characterized by intractable chronic pain.…”

Chemical Interactions between Fibrosarcoma Cancer Cells and Sensory Neurons Contribute to Cancer Pain

“…Mostly noted for increasing the function of high-voltageactivated channels involved in neurotransmission (N-and P/Qtypes), the ␣2␦ subunits are also implicated in increasing the density of T-type channels in the membrane (Perez-Reyes, 2003;Dubel et al, 2004). T-type channels contribute to nociceptor excitability Nelson et al, 2005), and inhibitors of T-type channels attenuate the hyperalgesia that accompanies neuropathic pain (Flatters and Bennett, 2004;Todorovic et al, 2004). The efficacy of gabapentin in the treatment of neuropathic pain is associated with the upregulation of the ␣2␦1 subunit in DRG neurons and the binding of gabapentin to this subunit, thereby blocking channel function (Bourinet and Zamponi, 2005).…”