Role of Kv1 Potassium Channels in Regulating Dopamine Release and Presynaptic D2 Receptor Function

Dopamine (DA) release in the CNS is critical for motor control and motivated behaviors. Dysfunction of its regulation is thought to be implicated in drug abuse and in diseases such as schizophrenia and Parkinson's. Although various potassium channels located in the somatodendritic compartment of DA neurons such as G-protein-gated inward rectifying potassium channels (GIRK) have been shown to regulate cell firing and DA release, little is presently known about the role of potassium channels localized in the axon terminals of these neurons. Here we used fast-scan cyclic voltammetry to study electrically-evoked DA release in rat dorsal striatal brain slices. We find that although G-protein-gated inward rectifying (GIRK) and ATP-gated (KATP) potassium channels play only a minor role, voltage-gated potassium channels of the Kv1 family play a major role in regulating DA release. The use of Kv subtype-selective blockers confirmed a role for Kv1.2, 1.3 and 1.6, but not Kv1.1, 3.1, 3.2, 3.4 and 4.2. Interestingly, Kv1 blockers also reduced the ability of quinpirole, a D2 receptor agonist, to inhibit evoked DA overflow, thus suggesting that Kv1 channels also regulate presynaptic D2 receptor function. Our work identifies Kv1 potassium channels as key regulators of DA release in the striatum

Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by Different Classes of Antidepressants

Various antidepressants are commonly used for the treatment of depression and several other neuropsychiatric disorders. In addition to their primary effects on serotonergic or noradrenergic neurotransmitter systems, antidepressants have been shown to interact with several receptors and ion channels. However, the molecular mechanisms that underlie the effects of antidepressants have not yet been sufficiently clarified. G protein-activated inwardly rectifying K+ (GIRK, Kir3) channels play an important role in regulating neuronal excitability and heart rate, and GIRK channel modulation has been suggested to have therapeutic potential for several neuropsychiatric disorders and cardiac arrhythmias. In the present study, we investigated the effects of various classes of antidepressants on GIRK channels using the Xenopus oocyte expression assay. In oocytes injected with mRNA for GIRK1/GIRK2 or GIRK1/GIRK4 subunits, extracellular application of sertraline, duloxetine, and amoxapine effectively reduced GIRK currents, whereas nefazodone, venlafaxine, mianserin, and mirtazapine weakly inhibited GIRK currents even at toxic levels. The inhibitory effects were concentration-dependent, with various degrees of potency and effectiveness. Furthermore, the effects of sertraline were voltage-independent and time-independent during each voltage pulse, whereas the effects of duloxetine were voltage-dependent with weaker inhibition with negative membrane potentials and time-dependent with a gradual decrease in each voltage pulse. However, Kir2.1 channels were insensitive to all of the drugs. Moreover, the GIRK currents induced by ethanol were inhibited by sertraline but not by intracellularly applied sertraline. The present results suggest that GIRK channel inhibition may reveal a novel characteristic of the commonly used antidepressants, particularly sertraline, and contributes to some of the therapeutic effects and adverse effects

Receptor pharmacology of the antidyskinetic drug sarizotan

Objective: Sarizotan is in clinical development for levodopa-associated dyskinesia in Parkinson patients. Sarizotan was investigated for its properties at human 5HT1A, D2, D3 and D4 receptors in functional in vitro assays and on the extracellular glutamate levels in the motor cortex and the ipsilateral dorsolateral striatum by dual-probe microdialysis in awake rats. Background: In receptor binding assays, sarizotan exhibited affinity to 5HT1A, D2, D3 and D4 receptors. Besides dopamine D1, D2 and D3 receptors in the basal ganglia, 5HT1A receptors in the cortex, modulating the corticostriatal glutamate pathways, affect striatal GABA output neurons in basal ganglia circuitry. Methods: Human 5HT1A or D2-like receptors were stably expressed in CHO cells for [35S]GTPS binding assay or in AtT-20 mouse pituitary cells for measuring drug-activated G-protein-coupled inward rectifier potassium currents (GIRK; patch clamp technique) or changes in cAMP levels (stimulated with 10 M forskolin). Microdialysis probes were implanted into the motor cortex and ipsilateral dorsolateral striatum. Sarizotan was administered systemically (0.1-10 mg/kg s.c.) or included in the cortical perfusion medium (10 M) for 60 min. WAY100135 (10 M) was intracortically perfused 20 min prior to the application of sarizotan. Perfusates were collected every 20 min for 2 hours. Results: The following table summarizes the effect of sarizotan in the functional assays: Receptor Binding IC50 GTPS GIRK cAMP 5HT1A 0.1 3.5* 6.2* 1.5* D2S 17 38** 20* 0.6* D3 7 128** 5.6* 0.5* D4.2 3 30** 4.5* 0.5* D4.4 4 0.9* 5.4* 0.2* [nM]; * EC50 (Agonist) and ** IC50 (Antagonist) Dual-probe microdialysis revealed that sarizotan (s.c. or perfused into the motor cortex) produced a significant (20%-30%) reduction of cortical and striatal glutamate levels. The inhibitory effects of s.c. sarizotan on cortical and striatal glutamate levels were counteracted by intracortical perfusion with the 5HT1A receptor antagonist WAY100135. Conclusion: Sarizotan is an agonist at 5HT1A receptors and most likely also at D3 and D4 receptors, and a partial agonist at D2 receptors in functional assays. Its antidyskinetic effect may involve at least a reduction of the activity of the corticostriatal glutamate pathways with diminished activation of striatal GABA output neurons in basal ganglia circuitry

KCNJ6 is Associated with Adult Alcohol Dependence and Involved in Gene × Early Life Stress Interactions in Adolescent Alcohol Drinking

Alcohol abuse and dependence have proven to be complex genetic traits that are influenced by environmental factors. Primate and human studies have shown that early life stress increases the propensity for alcohol abuse in later life. The reinforcing properties of alcohol are mediated by dopaminergic signaling; however, there is little evidence to indicate how stress alters alcohol reinforcement. KCNJ6 (the gene encoding G-protein-coupled inwardly rectifying potassium channel 2 (GIRK2)) is a brain expressed potassium channel with inhibitory effects on dopaminergic tone. The properties of GIRK2 have been shown to be enhanced by the stress peptide corticotrophin-releasing hormone. Therefore, we sought to examine the role of KCNJ6 polymorphisms in adult alcohol dependence and stress-related alcohol abuse in adolescents. We selected 11 SNPs in the promoter region of KCNJ6, which were genotyped in 1152 adult alcohol dependents and 1203 controls. One SNP, rs2836016, was found to be associated with alcohol dependence (p=0.01, false discovery rate). We then assessed rs2836016 in an adolescent sample of 261 subjects, which were characterized for early life stress and adolescent hazardous drinking, defined using the Alcohol Use Disorders Identification Test (AUDIT), to examine gene–environment interactions. In the adolescent sample, the risk genotype of rs2836016 was significantly associated with increased AUDIT scores, but only in those individuals exposed to high levels of psychosocial stress in early life (p=0.01). Our findings show that KCNJ6 is associated with alcohol dependence and may moderate the effect of early psychosocial stress on risky alcohol drinking in adolescents. We have identified a candidate gene for future studies investigating a possible functional link between the response to stress and alcohol reinforcement