The KCNQ channel activator retigabine blocks haloperidol-induced c-Fos expression in the striatum of the rat

The pan-Kv7 (KCNQ) Channel Opener Retigabine Inhibits Striatal Excitability by Direct Action on Striatal Neurons In Vivo

Central Kv7 (KCNQ) channels are voltage-dependent potassium channels composed of different combinations of four Kv7 subunits, being differently expressed in the brain. Notably, striatal dopaminergic neurotransmission is strongly suppressed by systemic administration of the pan-Kv7 channel opener retigabine. The effect of retigabine likely involves the inhibition of the activity in mesencephalic dopaminergic neurons projecting to the striatum, but whether Kv7 channels expressed in the striatum may also play a role is not resolved. We therefore assessed the effect of intrastriatal retigabine administration on striatal neuronal excitability in the rat determined by c-Fos immunoreactivity, a marker of neuronal activation. When retigabine was applied locally in the striatum, this resulted in a marked reduction in the number of c-Fos-positive neurons after a strong excitatory striatal stimulus induced by acute systemic haloperidol administration in the rat. The relative mRNA levels of Kv7 subunits in the rat striatum were found to be Kv7.2 = Kv7.3 = Kv7.5 > >Kv7.4. These data suggest that intrastriatal Kv7 channels play a direct role in regulating striatal excitability in vivo.

Kcnq1-5 (Kv7.1-5) potassium channel expression in the adult zebrafish

BACKGROUND

KCNQx genes encode slowly activating-inactivating K+ channels, are linked to physiological signal transduction pathways, and mutations in them underlie diseases such as long QT syndrome (KCNQ1), epilepsy in adults (KCNQ2/3), benign familial neonatal convulsions in children (KCNQ3), and hearing loss or tinnitus in humans (KCNQ4, but not KCNQ5). Identification of kcnqx potassium channel transcripts in zebrafish (Danio rerio) remains to be fully characterized although some genes have been mapped to the genome. Using zebrafish genome resources as the source of putative kcnq sequences, we investigated the expression of kcnq1-5 in heart, brain and ear tissues.

RESULTS

Overall expression of the kcnqx channel transcripts is similar to that found in mammals. We found that kcnq1 expression was highest in the heart, and also present in the ear and brain. kcnq2 was lowest in the heart, while kcnq3 was highly expressed in the brain, heart and ear. kcnq5 expression was highest in the ear. We analyzed zebrafish genomic clones containing putative kcnq4 sequences to identify transcripts and protein for this highly conserved member of the Kcnq channel family. The zebrafish appears to have two kcnq4 genes that produce distinct mRNA species in brain, ear, and heart tissues.

CONCLUSIONS

We conclude that the zebrafish is an attractive model for the study of the KCNQ (Kv7) superfamily of genes, and are important to processes involved in neuronal excitability, cardiac anomalies, epileptic seizures, and hearing loss or tinnitus.

The pharmacological effect of positive KCNQ (Kv7) modulators on dopamine release from striatal slices

Retigabine is an anti-epileptic drug that inhibits neuronal firing by stabilizing the membrane potential through positive modulation of voltage-dependent KCNQ potassium channels in cortical neurons and in mesencephalic dopamine (DA) neurons. The purpose of this study was to compare the effect of retigabine with other positive KCNQ modulators on the KCl-induced release of DA in rat striatal slices. Retigabine was found to inhibit KCl-dependent release of DA, and the IC(50) was estimated to be 0.7 μM. The KCNQ channel blocker XE-991 enhanced striatal DA release and completely abolished the effect of retigabine. Other compounds of the same class but with some preferences for different KCNQ subtypes such as ICA-27243, BMS-204352 and S-(1) were also tested. All three compounds produced a significant effect albeit weaker than retigabine. The potency of ICA-27243 was in the range of retigabine, and with a lower potency of BMS-204352 and S-(1). This study demonstrates that KCNQ channel openers inhibit KCl-induced DA release at relevant concentrations. The equal potency of ICA-27243 and retigabine suggests that the KCNQ2/3 isoform is likely the dominant subtype mediating this effect.

Antipsychotic-like effect of retigabine [N-(2-Amino-4-(fluorobenzylamino)-phenyl)carbamic acid ester], a KCNQ potassium channel opener, via modulation of mesolimbic dopaminergic neurotransmission

Dopaminergic (DAergic) neurons in the ventral tegmental area express both KCNQ2 and KCNQ4 channels, which opening is expected to decrease neuronal excitability via neuronal hyper-polarization. Because psychotic symptoms are believed to be associated with an increased excitability of dopamine (DA) cells in the mesencephalon, KCNQ channels might represent a new potential target for the treatment of psychosis. The aim of our study was to investigate the antipsychotic-like potential of KCNQ channel opening via modulation of neuronal activity within the mesolimbic DAergic system. We report that retigabine [N-(2-amino-4-(fluorobenzylamino)-phenyl)carbamic acid ester], a KCNQ opener, dose-dependently reduced basal DA firing rate and more potently suppressed burst firing activity in the ventral tegmental area, whereas XE-991 [10,10-bis(pyridinylmethyl)-9(10H)-anthracenone], a selective KCNQ blocker, induced opposite effects. In addition, retigabine prevented d-amphetamine-induced DA efflux in the nucleus accumbens and d-amphetamine-induced locomotor hyperactivity. In contrast, XE-991 potentiated both the locomotor hyperactivity and DA efflux evoked by d-amphetamine. These data strongly suggest that the activation of KCNQ channels attenuates DAergic neurotransmission in the mesolimbic system, particularly in conditions of excessive DAergic activity. In a model predictive of antipsychotic activity, the conditioned avoidance response paradigm, retigabine was found to inhibit avoidance responses, an effect blocked by coadministration of XE-991. Furthermore, retigabine was found to significantly inhibit the hyperlocomotor response to a phencyclidine (PCP) challenge in PCP-sensitized animals, considered as a disease model for schizophrenia. Taken together, our studies provide evidence that KCNQ channel openers represent a potential new class of antipsychotics.

A schizophrenia-linked mutation in PIP5K2A fails to activate neuronal M channels

RATIONALE

Evidence for an association between phosphatidylinositol-4-phosphate 5-kinase II alpha (PIP5K2A) and schizophrenia was recently obtained and replicated in several samples. PIP5K2A controls the function of KCNQ channels via phosphatidylinositol-4,5-bisphosphate (PIP2) synthesis. Interestingly, recent data suggest that KCNQ channels suppress basal activity of dopaminergic neurons and dopaminergic firing. Activation of KCNQ accordingly attenuates the central stimulating effects of dopamine, cocaine, methylphenidate, and phenylcyclidine.

OBJECTIVE

The aim of this study was to explore the functional relevance of PIP5K2A, which might influence schizophrenic behavior.

MATERIALS AND METHODS

Here, we study the effects of the neuronal PIP5K2A on KCNQ2, KCNQ5, KCNQ2/KCNQ3, and KCNQ3/KCNQ5 in the Xenopus expression system.

RESULTS

We find that wild-type PIP5K2A but not the schizophrenia-associated mutant (N251S)-PIP5K2A activates heteromeric KCNQ2/KCNQ3 and KCNQ3/KCNQ5, the molecular correlate of neuronal M channels. Homomeric KCNQ2 and KCNQ5 channels were not activated by the kinase indicating that the presence of KCNQ3 in the channel complex is required for the kinase-mediated effects. Acute application of PI(4,5)P2 and a PIP2 scavenger indicates that the mutation N251S renders the kinase PIP5K2A inactive.

CONCLUSIONS

Our results suggest that the schizophrenia-linked mutation of the kinase results in reduced KCNQ channel function and thereby might explain the loss of dopaminergic control in schizophrenic patients. Moreover, the addictive potential of dopaminergic drugs often observed in schizophrenic patients might be explained by this mechanism. At least, the insufficiency of (N251S)-PIP5K2A to stimulate neuronal M channels may contribute to the clinical phenotype of schizophrenia.

Kv7 channels: interaction with dopaminergic and serotonergic neurotransmission in the CNS

Neuronal Kv7 channels (also termed KCNQ channels) are the molecular correlate of the M-current. The Kv7 channels activate at rather negative membrane potentials (< or = 60 mV), thereby 'fine-tuning' the resting membrane potential. The Kv7 channels are widely expressed in the brain with the Kv7.2, Kv7.3 and Kv7.5 channels being the most abundant. The Kv7.4 subunit has the most restricted brain regional expression being present in discrete nuclei of brainstem only. Kv7 channels are expressed at different subcellular locations, being on both somatodendritic, axonal and terminal sites. This complex subcellular distribution of Kv7 channels enables them to participate in both pre- and postsynaptic modulation of basal and stimulated excitatory neurotransmission. Activation of neuronal Kv7 channels limits repetitive firing thereby potentially limiting the generation of long bursts, with subsequent inhibition of monoaminergic neurotransmitter release. In this review, we focus on the influence of Kv7 channels on dopaminergic and serotonergic neurotransmission. The data suggest a novel action of Kv7 channel openers which could translate into having therapeutic value in the treatment of disease states characterized by overactivity of dopaminergic (e.g. schizophrenia and drug abuse) and serotonergic neurotransmission (e.g. anxiety).

Effect of the new antiepileptic drug retigabine in a rodent model of mania

Bipolar spectrum disorders are severe chronic mood disorders that are characterized by episodes of mania or hypomania and depression. Because patients with manic symptoms often experience clinical benefit from treatment with anticonvulsant drugs, it was hypothesized that retigabine, a novel compound with anticonvulsant efficacy, may also possess antimanic activity. The amphetamine (AMPH)+chlordiazepoxide (CDP)-induced hyperactivity model has been proposed as a suitable model for studying antimanic-like activity of novel compounds in mice and rats. The aims of the present study in rats were therefore (1) to confirm previous findings with lithium and lamotrigine, and (2) to evaluate the effect of the novel compound retigabine on AMPH+CDP-induced hyperactivity in rats. In all experiments, co-administration of AMPH and CDP induced a significant increase (191-295%) in locomotor activity. Lithium chloride (0.9 mg/kg) and lamotrigine (20 mg/kg), which are known to effectively stabilize mood in humans, both significantly decreased AMPH+CDP-induced locomotor activity without affecting basal locomotor activity. The results furthermore indicate that retigabine, like lithium and lamotrigine, significantly and dose-dependently attenuates the induced hyperactivity at a lowest effective dose of 1.0 mg/kg, whereas basal locomotor activity is reduced only at doses 4.0 mg/kg. In conclusion, retigabine was found to have an antimanic-like effect in the AMPH+CDP-induced hyperactivity model, suggesting a potential role for retigabine in the treatment of mania and possibly in the management of bipolar disorder.

The neuronal KCNQ channel opener retigabine inhibits locomotor activity and reduces forebrain excitatory responses to the psychostimulants cocaine, methylphenidate and phencyclidine

Many central stimulating drugs have a pronounced stimulatory effect on striatal and cortical activity which is associated to enhanced function of mesencephalic dopaminergic neurons. Mesencephalic KCNQ (also termed K(v)7) potassium channels suppress the basal activity of dopaminergic neurons in the substantia nigra and ventral tegmental area. These regions have extensive dopaminergic projections to the striatum and cortex, and positive modulation of KCNQ channel function may therefore potentially reduce the reinforcing impact of central stimulating drugs. We studied the effects of the principal neuronal KCNQ channel opener, retigabine, in rats exposed acutely to cocaine, methylphenidate (dopamine reuptake inhibitors) or phencyclidine (PCP, a psychotomimetic NMDA receptor antagonist). Retigabine (> or =1.0 mg/kg) inhibited cocaine, methylphenidate and PCP-stimulated locomotor activity. Also, retigabine reduced spontaneous locomotor activity. The inhibitory effect of retigabine on psychostimulant-induced locomotor activity was accompanied by a marked reduction in c-Fos expression, in particular the nucleus accumbens and primary motor cortex were responsive to retigabine pre-treatment. Notably, retigabine also reduced basal extracellular levels of striatal dopamine metabolites and partially prevented dopamine overflow in the striatum induced by dopamine reuptake blockade. In combination, these data suggest that retigabine reduces striatal and cortical excitability, thereby attenuating excitatory effects of central stimulating drugs in dopamine-rich areas of the rat forebrain. KCNQ channel openers may therefore be of potential relevance in the treatment of addiction states caused by abuse of psychostimulants.

The KCNQ Channel Opener Retigabine Inhibits the Activity of Mesencephalic Dopaminergic Systems of the Rat

Homo- and heteromeric complexes of KCNQ channel subunits are the molecular correlate of the M-current, a neuron-specific voltage-dependent K(+) current with a well established role in control of neural excitability. We investigated the effect of KCNQ channel modulators on the activity of dopaminergic neurons in vitro and in vivo in the rat ventral mesencephalon. The firing of dopaminergic neurons recorded in mesencephalic slices was robustly inhibited in a concentration-dependent manner by the KCNQ channel opener N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester (retigabine). The effect of retigabine persisted in the presence of tetrodotoxin and simultaneous blockade of GABA(A) receptors, small-conductance calcium-activated K(+) (SK) channels, and hyperpolarization-activated (I(h)) channels, and it was potently reversed by the KCNQ channel blocker 4-pyridinylmethyl-9(10H)-anthracenone (XE991), indicating a direct effect on KCNQ channels. Likewise, in vivo single unit recordings from dopaminergic neurons revealed a prominent reduction in spike activity after systemic administration of retigabine. Furthermore, retigabine inhibited dopamine synthesis and c-Fos expression in the striatum under basal conditions. Retigabine completely blocked the excitatory effect of dopamine D(2) autoreceptor antagonists. Again, the in vitro and in vivo effects of retigabine were completely reversed by preadministration of XE991. Dual immunocytochemistry revealed that KCNQ4 is the major KCNQ channel subunit expressed in all dopaminergic neurons in the mesolimbic and nigrostriatal pathways. Collectively, these observations indicate that retigabine negatively modulates dopaminergic neurotransmission, likely originating from stimulation of mesencephalic KCNQ4 channels.