Up-regulation of HCN2 channels in a thalamocortical circuit mediates allodynia in mice

Chronic pain is a significant problem that afflicts individuals and society, and for which the current clinical treatment is inadequate. In addition, the neural circuit and molecular mechanisms subserving chronic pain remain largely uncharacterized. Herein we identified enhanced activity of a glutamatergic neuronal circuit that encompasses projections from the ventral posterolateral nucleus (VPL^Glu) to the glutamatergic neurons of the hindlimb primary somatosensory cortex (S1HL^Glu), driving allodynia in mouse models of chronic pain. Optogenetic inhibition of this VPL^Glu→S1HL^Glu circuit reversed allodynia, whereas the enhancement of its activity provoked hyperalgesia in control mice. In addition, we found that the expression and function of the HCN2 (hyperpolarization-activated cyclic nucleotide-gated channel 2) were increased in VPL^Glu neurons under conditions of chronic pain. Using in vivo calcium imaging, we demonstrated that downregulation of HCN2 channels in the VPL^Glu neurons abrogated the rise in S1HL^Glu neuronal activity while alleviating allodynia in mice with chronic pain. With these data, we propose that dysfunction in HCN2 channels in the VPL^Glu→S1HL^Glu thalamocortical circuit and their upregulation occupy essential roles in the development of chronic pain.

Gene knockdown of HCN2 ion channels in the ventral tegmental area reduces ethanol consumption in alcohol preferring rats

Background: Hyperpolarization-Activated Cyclic Nucleotide-Gated (HCN) ionic channels are known to play a key role in the control of neuron excitability and have been proposed as a molecular target of ethanol. Previous studies in rats have shown that gene-induced overexpression of the HCN2 channel in the ventral tegmental area (VTA) increases the rewarding effects of ethanol and its intake by the animals.Objective: The aim of this work was to study the effects of VTA HCN2 gene knockdown in the voluntary ethanol consumption of alcohol-preferring UChB rats.Methods: Two lentiviral vectors were generated; LV-siRNA-HCN2, coding for a siRNA that elicited >95% reduction of HCN2 protein levels in vitro, and a control vector coding for a scrambled siRNA sequence. Female UChB naïve rats (n = 14) were microinjected into the VTA with LV-siRNA-HCN2 or the scrambled control vector (n = 11). Four days after, animals were given a daily free access to 10% ethanol and water for 10 days.Results: Rats treated with the LV-siRNA-HCN2 vector showed a ~ 70% reduction (p < .001) in their ethanol preference and ethanol intake compared to control animals. No changes were observed in the total fluid intake of both groups. HCN2 levels in the VTA were measured by Western blot showing a reduction of 40% (p < .05) in the rats injected with LV-siRNA-HCN2, compared to control animals.Conclusion: These results show that knockdown of HCN2 ionic channels in the VTA of UChB rats markedly reduces their voluntary ethanol intake, supporting the idea that HCN2 channels may constitute a therapeutic target for alcohol use disorders.

Decreased HCN2 channel expression attenuates neuropathic pain by inhibiting pro-inflammatory reactions and NF-κB activation in mice

Hyperpolarization-activated cyclic nucleotide-gated 2 (HCN2) ion channel activity plays a crucial role in the progress of peripheral neuropathic pain (PNP). However, the mechanism of HCN2 channels on PNP remains unclear. Here, we investigated the effects of HCN2 channel expression on the mechanical allodynia and thermal hyperalgesia, the local inflammatory response, the activation of astrocytes, microglia and transcription factor NF-κB in mice with spared sciatic nerve injury (SNI). The present study showed that the expression of HCN2 channels was increased in L4-L5 ipsilateral spinal dorsal horns, accompanied by a decreased paw mechanical withdrawal threshold (MWT) and paw withdrawal latency (PWL) in SNI mice. After intrathecal injection of ZD-7288 and si-HCN2, both MWT and PWL were significantly increased, while the level of pro-inflammatory factors TNF-α, IL-1β and MCP-1 were decreased in L4-L5 ipsilateral spinal dorsal horn. Furthermore, the inhibition of HCN2 channels reduces the activated astrocytes and microglia, and suppressed NF-κB p65 activation and nuclear translocation. In conclusion, the present study suggests that decreased HCN2 channel expression attenuates neuropathic pain by inhibiting pro-inflammatory reactions and NF-κB activation.