The neuronal potassium current IA is a potential target for pain during chronic inflammation

A novel isoquinoline alkaloid HJ-69 isolated from Zanthoxylum bungeanum attenuates inflammatory pain by inhibiting voltage-gated sodium and potassium channels

ETHNOPHARMACOLOGY RELEVANCE

Zanthoxylum bungeanum Maxim. (Z. bungeanum), a member of the Rutaceae family, has a rich history of traditional use in Asia for treating arthritis and toothache conditions. As characteristic chemical components, numerous kinds of alkaloids have been extracted from plants and their diverse biological activities have been reported. However, research on the isoquinoline alkaloid, a specific type of alkaloids, in Z. bungeanum was scarce.

AIM OF THE STUDY

The study aimed to isolate a novel isoquinoline alkaloid from Z. bungeanum and explore its pharmacological activity in vitro and analgesic activity in vivo.

MATERIALS AND METHODS

Isoquinoline alkaloid isolation and identification from Z. bungeanum were conducted using chromatographic and spectroscopic methods. The whole-cell patch-clamp technique was applied to assess its impact on neuronal excitability, and endogenous voltage-gated potassium (Kv) and sodium (Nav) currents in acutely isolated mouse small-diameter dorsal root ganglion (DRG) neurons. Its inhibitory impacts on channels were further validated with HEK293 cells stably expressing Nav1.7 and Nav1.8, and Chinese hamster ovary (CHO) cells transiently expressing Kv2.1. The formalin inflammatory pain model was utilized to evaluate the potential analgesic activity in vivo.

RESULTS

A novel isoquinoline alkaloid named HJ-69 (N-13-(3-methoxyprop-1-yl)rutaecarpine) was isolated and identified from Z. bungeanum for the first time. HJ-69 significantly suppressed the firing frequency and amplitudes of action potentials in DRG neurons. Consistently, it state-dependently inhibited endogenous Nav currents of DRG neurons, with half maximal inhibitory concentration (IC50) values of 13.06 ± 2.06 μM and 30.19 ± 2.07 μM for the inactivated and resting states, respectively. HJ-69 significantly suppressed potassium currents in DRG neurons, which notably inhibited the delayed rectifier potassium (IK) currents (IC50 = 6.95 ± 1.29 μM) and slightly affected the transient outward potassium (IA) currents (IC50 = 523.50 ± 39.16 μM). Furtherly, HJ-69 exhibited similar potencies on heterologously expressed Nav1.7, Nav1.8, and Kv2.1 channels, which correspondingly represent the main components in neurons. Notably, intraperitoneal administration of 30 mg/kg and 100 mg/kg HJ-69 significantly alleviated pain behaviors in the mouse inflammatory pain model induced by formalin.

CONCLUSION

The study concluded that HJ-69 is a novel and active isoquinoline alkaloid, and the inhibition of Nav and Kv channels contributes to its analgesic activity. HJ-69 may be a promising prototype for future analgesic drug discovery based on the isoquinoline alkaloid.

Differential expression of slow and fast-repriming tetrodotoxin-sensitive sodium currents in dorsal root ganglion neurons

Sodium channel Nav1.7 triggers the generation of nociceptive action potentials and is important in sending pain signals under physiological and pathological conditions. However, studying endogenous Nav1.7 currents has been confounded by co-expression of multiple sodium channel isoforms in dorsal root ganglion (DRG) neurons. In the current study, slow-repriming (SR) and fast-repriming (FR) tetrodotoxin-sensitive (TTX-S) currents were dissected electrophysiologically in small DRG neurons of both rats and mice. Three subgroups of small DRG neurons were identified based on the expression pattern of SR and FR TTX-S currents. A majority of rat neurons only expressed SR TTX-S currents, while a majority of mouse neurons expressed additional FR TTX-S currents. ProTx-II inhibited SR TTX-S currents with variable efficacy among DRG neurons. The expression of both types of TTX-S currents was higher in Isolectin B4-negative (IB4-) compared to Isolectin B4-positive (IB4+) neurons. Paclitaxel selectively increased SR TTX-S currents in IB4- neurons. In simulation experiments, the Nav1.7-expressing small DRG neuron displayed lower rheobase and higher frequency of action potentials upon threshold current injections compared to Nav1.6. The results suggested a successful dissection of endogenous Nav1.7 currents through electrophysiological manipulation that may provide a useful way to study the functional expression and pharmacology of endogenous Nav1.7 channels in DRG neurons.

Research progress on the mechanism of chronic neuropathic pain

Chronic neuropathic pain (CNP) refers to pain that lasts for more than three months due to a disease or an injury to the somatosensory nervous system. The incidence of CNP has been increasing in the world, causing it to become a global concern and patients often experience spontaneous pain, hyperalgesia, abnormal pain or even abnormal sensation as some of its main symptoms. In addition to serious pain and poor physical health, CNP also negatively affects patients' mental health, thus impacting the overall quality of their lives. The pathogenesis of CNP is not clear, but some studies have proved that central sensitization, peripheral sensitization, neuroinflammation, dysfunction in descending nociceptive modulatory systems, oxidative stress reaction, activation of glial cells and psychological factors play an important role in the occurrence and development of CNP. In this context, this article summarizes the current research progress on the mechanism of CNP to provide a basis for further research in preventing and treating the disease.