Electrophysiological characterization of Na⁺ and T-type Ca²⁺ channel alterations and their impact on action potential generation in rodent models of neuropathic pain

The pathomechanism underlying neuropathic pain is still unclear and for some diseases such as Guillain-Barré-Syndrome almost unexplored. For this, experimental autoimmune neuritis (EAN), a rat model of Guillain Barré-Syndrom (GBS), was established in our laboratory to detect nociceptive neurons for changes in their membrane properties that might be correlated to severe pain. EAN rats developed motor deficits, weight loss as well as mechanical allodynia, all symptoms resembling the clinical and pathological features of GBS. Current clamp recordings of small DRG neurons of EAN rats revealed a significant reduction in AP duration as well as signs of enhanced excitability such as lower rheobase and higher frequency of evoked AP discharge. These alterations have been detected in a subtype of DRG neurons that display sensitivity to capsaicin. Changes in these membrane properties were also accompanied by alterations of Na+ currents in the same subgroup of neurons. The transient TTXS Na+ current was clearly up-regulated in EAN animals, while the transient TTXR Na+ current tends to be down-regulated, leaving the total transient Na+ current unchanged. This switch in Na+ currents was again demonstrated in the significantly faster inactivation kinetics of the total transient Na+ current, reflecting the predominant role of the fast TTXS Na+ current in EAN DRG neurons. In addition, EAN induced a reduction in the magnitude of the total persistent Na+ current of cap+ DRG neurons that could be attributed to the TTXS INaP. These changes in Na+ currents can be considered as potential basis for the altered electrical properties observed in cap+ DRG neurons of EAN rats. Previous studies reported contradicting results concerning the role of T-type Ca2+ currents in supporting neuropathic pain. For this, in the second part of this study changes in T-type Ca2+ currents in nociceptive DRG neurons were analysed in a mouse model of partial nerve ligation-induced neuropathic pain. Our results show that the PNL animals suffered an increase in nociceptive sensitivity that was paralleled by a significant up-regulation of T-type Ca2+ currents in cap+ DRG neurons. Pharmacological experiments revealed that this increase was attributed to a Ni2+ resistant current component, a result that stands in contrast with a pronociceptive role of CaV3.2 in neuropathic pain suggested previously. However, mice lacking CaV3.2 still suffered pain hypersensitivity following nerve injury. Also, T-type Ca2+ currents were significantly enhanced in DRG of PNL CaV3.2 KO mice. Moreover, RT-PCR revealed a lack of up-regulation of these channel subunits on the mRNA level. Collectively, our data suggest an up-regulation of one or both of the Ni2+ insensitive T-type subunits (CaV3.1 or CaV3.3) in small cap+ DRG neurons after partial nerve ligation. These changes can explain the increased excitability, as evidenced by a considerable reduction of threshold of AP firing in the same subset of neurons. In conclusion, our present data demonstrate a role of Na+ currents as well as LVA-currents in the increased cellular excitability and nociceptive sensitivity in different animal models of neuropathic pain. Further studies are required to identify the specific subunits involved and hence allow effective treatment with minimal side effects