Kv7.4 Channel Contribute to Projection-Specific Auto-Inhibition of Dopamine Neurons in the Ventral Tegmental Area

Kv7 channel opener retigabine reduces self-administration of cocaine but not sucrose in rats

The increasing rates of drug misuse highlight the urgency of identifying improved therapeutics for treatment. Most drug-seeking behaviours that can be modelled in rodents utilize the repeated intravenous self-administration (SA) of drugs. Recent studies examining the mesolimbic pathway suggest that Kv7/KCNQ channels may contribute to the transition from recreational to chronic drug use. However, to date, all such studies used noncontingent, experimenter-delivered drug model systems, and the extent to which this effect generalizes to rats trained to self-administer drugs is not known. Here, we tested the ability of retigabine (ezogabine), a Kv7 channel opener, to regulate instrumental behaviour in male Sprague Dawley rats. We first validated the ability of retigabine to target experimenter-delivered cocaine in a conditioned place preference (CPP) assay and found that retigabine reduced the acquisition of place preference. Next, we trained rats for cocaine-SA under a fixed-ratio or progressive-ratio reinforcement schedule and found that retigabine pretreatment attenuated the SA of low to moderate doses of cocaine. This was not observed in parallel experiments, with rats self-administering sucrose, a natural reward. Compared with sucrose-SA, cocaine-SA was associated with reductions in the expression of the Kv7.5 subunit in the nucleus accumbens, without alterations in Kv7.2 and Kv7.3. Therefore, these studies reveal a reward-specific reduction in SA behaviour and support the notion that Kv7 is a potential therapeutic target for human psychiatric diseases with dysfunctional reward circuitry.

Inhibition of SK Channels in VTA Affects Dopaminergic Neurons to Improve the Depression-Like Behaviors of Post-Stroke Depression Rats

Purpose

This study aimed to investigate the effect of small-conductance calcium-activated potassium channels (SK channels) on the dopaminergic (DA) neuron pathways in the ventral tegmental area (VTA) during the pathogenesis of post-stroke depression (PSD) and explore the improvement of PSD by inhibiting the SK channels.

Patients and Methods

Four groups of Sprague-Dawley rats were randomly divided: Control, PSD, SK channel inhibitor (apamin) and SK channel activator (CyPPA) groups. In both control and CyPPA groups, sham surgery was performed. In the other two groups, middle cerebral arteries were occluded. The behavioral indicators related to depression in different groups were compared. Immunofluorescence was used to measure the activity of DA neurons in the VTA, while qRT-PCR was used to assess the expression of SK channel genes.

Results

The results showed that apamin treatment improved behavioral indicators related to depression compared to the PSD group. Furthermore, the qRT-PCR analysis revealed differential expression of the KCNN1 and KCNN3 subgenes of the SK channels in each group. Immunofluorescence analysis revealed an increase in the expression of DA neurons in the VTA of the PSD group, which was subsequently reduced upon apamin intervention.

Conclusion

This study suggests that SK channel activation following stroke contributes to depression-related behaviors in PSD rats through increased expression of DA neurons in the VTA. And depression-related behavior is improved in PSD rats by inhibiting the SK channels. The results of this study provide a new understanding of PSD pathogenesis and the possibility of developing new strategies to prevent PSD by targeting SK channels.

K v 7 Channel Opener Retigabine Reduces Self-Administration of Cocaine but Not Sucrose in Rats

The increasing rates of drug misuse highlight the urgency of identifying improved therapeutics for treatment. Most drug-seeking behaviors that can be modeled in rodents utilize the repeated intravenous self-administration (SA) of drugs. Recent studies examining the mesolimbic pathway suggest that K v 7/KCNQ channels may contribute in the transition from recreational to chronic drug use. However, to date, all such studies used noncontingent, experimenter-delivered drug model systems, and the extent to which this effect generalizes to rats trained to self-administer drug is not known. Here, we tested the ability of retigabine (ezogabine), a K v 7 channel opener, to regulate instrumental behavior in male Sprague Dawley rats. We first validated the ability of retigabine to target experimenter-delivered cocaine in a CPP assay and found that retigabine reduced the acquisition of place preference. Next, we trained rats for cocaine-SA under a fixed-ratio or progressive-ratio reinforcement schedule and found that retigabine-pretreatment attenuated the self-administration of low to moderate doses of cocaine. This was not observed in parallel experiments, with rats self-administering sucrose, a natural reward. Compared to sucrose-SA, cocaine-SA was associated with reductions in the expression of the K v 7.5 subunit in the nucleus accumbens, without alterations in K v 7.2 and K v 7.3. Therefore, these studies reveal a reward specific reduction in SA behavior considered relevant for the study of long-term compulsive-like behavior and supports the notion that K v 7 is a potential therapeutic target for human psychiatric diseases with dysfunctional reward circuitry.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Potassium Channel Conductance Is Involved in Phenylephrine-Induced Spontaneous Firing of Serotonergic Neurons in the Dorsal Raphe Nucleus

The serotonergic (5-HT) network from the dorsal raphe nucleus (DRN) of the brain has been demonstrated to regulate cognition, emotion, and behaviors, including learning and the sleep-wake cycle. Dysregulation of the activity of 5-HT neurons in the DRN is thought to play an important role in emotional disorders. The activity of 5-HT neurons is regulated by norepinephrine (NE) released from the projection terminals of noradrenergic input from the locus coeruleus (LC) via activation of the α1-adrenoceptor. However, insight into the molecular mechanism underlying this NE-induced regulation of 5-HT neuron activity is not clear. In this study, using the agonist of α1-adrenoceptor phenylephrine (PE), brain slices, and patch clamp, we found that A-type, Kv7/KCNQ, and calcium-activated low-conductance K+ channels (SK) underlie PE-induced spontaneous firing in DRN 5-HT neurons. Using single-cell PCR and immunofluorescence, we also identified the isoforms of these K+ channel families that might contribute to the NE/PE-induced spontaneous firing of DRN 5-HT neurons.

Autism-associated mutations in KV7 channels induce gating pore current

Autism spectrum disorder (ASD) adversely impacts >1% of children in the United States, causing social interaction deficits, repetitive behaviors, and communication disorders. Genetic analysis of ASD has advanced dramatically through genome sequencing, which has identified >500 genes with mutations in ASD. Mutations that alter arginine gating charges in the voltage sensor of the voltage-gated potassium (K_V) channel K_V7 (KCNQ) are among those frequently associated with ASD. We hypothesized that these gating charge mutations would induce gating pore current (also termed ω-current) by causing an ionic leak through the mutant voltage sensor. Unexpectedly, we found that wild-type K_V7 conducts outward gating pore current through its native voltage sensor at positive membrane potentials, owing to a glutamine in the third gating charge position. In bacterial and human K_V7 channels, gating charge mutations at the R1 and R2 positions cause inward gating pore current through the resting voltage sensor at negative membrane potentials, whereas mutation at R4 causes outward gating pore current through the activated voltage sensor at positive potentials. Remarkably, expression of the K_V7.3/R2C ASD-associated mutation in vivo in midbrain dopamine neurons of mice disrupts action potential generation and repetitive firing. Overall, our results reveal native and mutant gating pore current in K_V7 channels and implicate altered control of action potential generation by gating pore current through mutant K_V7 channels as a potential pathogenic mechanism in autism.

Alteration of Mesopontine Cholinergic Function by the Lack of KCNQ4 Subunit

The pedunculopontine nucleus (PPN), a structure known as a cholinergic member of the reticular activating system (RAS), is source and target of cholinergic neuromodulation and contributes to the regulation of the sleep–wakefulness cycle. The M-current is a voltage-gated potassium current modulated mainly by cholinergic signaling. KCNQ subunits ensemble into ion channels responsible for the M-current. In the central nervous system, KCNQ4 expression is restricted to certain brainstem structures such as the RAS nuclei. Here, we investigated the presence and functional significance of KCNQ4 in the PPN by behavioral studies and the gene and protein expressions and slice electrophysiology using a mouse model lacking KCNQ4 expression. We found that this mouse has alterations in the adaptation to changes in light–darkness cycles, representing the potential role of KCNQ4 in the regulation of the sleep–wakefulness cycle. As cholinergic neurons from the PPN participate in the regulation of this cycle, we investigated whether the cholinergic PPN might also possess functional KCNQ4 subunits. Although the M-current is an electrophysiological hallmark of cholinergic neurons, only a subpopulation of them had KCNQ4-dependent M-current. Interestingly, the absence of the KCNQ4 subunit altered the expression patterns of the other KCNQ subunits in the PPN. We also determined that, in wild-type animals, the cholinergic inputs of the PPN modulated the M-current, and these in turn can modulate the level of synchronization between neighboring PPN neurons. Taken together, the KCNQ4 subunit is present in a subpopulation of PPN cholinergic neurons, and it may contribute to the regulation of the sleep–wakefulness cycle.

Nicotine inhibits the VTA-to-amygdala dopamine pathway to promote anxiety

Nicotine stimulates dopamine (DA) neurons of the ventral tegmental area (VTA) to establish and maintain reinforcement. Nicotine also induces anxiety through an as yet unknown circuitry. We found that nicotine injection drives opposite functional responses of two distinct populations of VTA DA neurons with anatomically segregated projections: it activates neurons that project to the nucleus accumbens (NAc), whereas it inhibits neurons that project to the amygdala nuclei (Amg). We further show that nicotine mediates anxiety-like behavior by acting on β2-subunit-containing nicotinic acetylcholine receptors of the VTA. Finally, using optogenetics, we bidirectionally manipulate the VTA-NAc and VTA-Amg pathways to dissociate their contributions to anxiety-like behavior. We show that inhibition of VTA-Amg DA neurons mediates anxiety-like behavior, while their activation prevents the anxiogenic effects of nicotine. These distinct subpopulations of VTA DA neurons with opposite responses to nicotine may differentially drive the anxiogenic and the reinforcing effects of nicotine.

Differential Impact of Inhibitory G-Protein Signaling Pathways in Ventral Tegmental Area Dopamine Neurons on Behavioral Sensitivity to Cocaine and Morphine

Drugs of abuse engage overlapping but distinct molecular and cellular mechanisms to enhance dopamine (DA) signaling in the mesocorticolimbic circuitry. DA neurons of the ventral tegmental area (VTA) are key substrates of drugs of abuse and have been implicated in addiction-related behaviors. Enhanced VTA DA neurotransmission evoked by drugs of abuse can engage inhibitory G-protein-dependent feedback pathways, mediated by GABA_B receptors (GABA_BRs) and D₂ DA receptors (D₂Rs). Chemogenetic inhibition of VTA DA neurons potently suppressed baseline motor activity, as well as the motor-stimulatory effect of cocaine and morphine, confirming the critical influence of VTA DA neurons and inhibitory G-protein signaling in these neurons on this addiction-related behavior. To resolve the relative influence of GABA_BR-dependent and D₂R-dependent signaling pathways in VTA DA neurons on behavioral sensitivity to drugs of abuse, we developed a neuron-specific viral CRISPR/Cas9 approach to ablate D₂R and GABA_BR in VTA DA neurons. Ablation of GABA_BR or D₂R did not impact baseline physiological properties or excitability of VTA DA neurons, but it did preclude the direct somatodendritic inhibitory influence of GABA_BR or D₂R activation. D₂R ablation potentiated the motor-stimulatory effect of cocaine in male and female mice, whereas GABA_BR ablation selectively potentiated cocaine-induced activity in male subjects only. Neither D₂R nor GABA_BR ablation impacted morphine-induced motor activity. Collectively, our data show that cocaine and morphine differ in the extent to which they engage inhibitory G-protein-dependent feedback pathways in VTA DA neurons and highlight key sex differences that may impact susceptibility to various facets of addiction.

Kv7 Channels and Excitability Disorders

Kv7.1-Kv7.5 (KCNQ1-5) K⁺ channels are voltage-gated K⁺ channels with major roles in neurons, muscle cells and epithelia where they underlie physiologically important K⁺ currents, such as neuronal M current and cardiac I_Ks. Specific biophysical properties of Kv7 channels make them particularly well placed to control the activity of excitable cells. Indeed, these channels often work as 'excitability breaks' and are targeted by various hormones and modulators to regulate cellular activity outputs. Genetic deficiencies in all five KCNQ genes result in human excitability disorders, including epilepsy, arrhythmias, deafness and some others. Not surprisingly, this channel family attracts considerable attention as potential drug targets. Here we will review biophysical properties and tissue expression profile of Kv7 channels, discuss recent advances in the understanding of their structure as well as their role in various neurological, cardiovascular and other diseases and pathologies. We will also consider a scope for therapeutic targeting of Kv7 channels for treatment of the above health conditions.

The Role of Kv7 Channels in Neural Plasticity and Behavior

Activity-dependent persistent changes in neuronal intrinsic excitability and synaptic strength are widely thought to underlie learning and memory. Voltage-gated KCNQ/K_v7 potassium channels have been of great interest as the potential targets for memory disorders due to the beneficial effects of their antagonists in cognition. Importantly, de novo dominant mutations in their neuronal subunits KCNQ2/K_v7.2 and KCNQ3/K_v7.3 are associated with epilepsy and neurodevelopmental disorder characterized by developmental delay and intellectual disability. The role of K_v7 channels in neuronal excitability and epilepsy has been extensively studied. However, their functional significance in neural plasticity, learning, and memory remains largely unknown. Here, we review recent studies that support the emerging roles of K_v7 channels in intrinsic and synaptic plasticity, and their contributions to cognition and behavior.