CyPPA, a Positive SK3/SK2 Modulator, Reduces Activity of Dopaminergic Neurons, Inhibits Dopamine Release, and Counteracts Hyperdopaminergic Behaviors Induced by Methylphenidate.

Dopamine (DA) containing midbrain neurons play critical roles in several psychiatric and neurological diseases, including schizophrenia and attention deficit hyperactivity disorder, and the substantia nigra pars compacta neurons selectively degenerate in Parkinson's disease. Pharmacological modulation of DA receptors and transporters are well established approaches for treatment of DA-related disorders. Direct modulation of the DA system by influencing the discharge pattern of these autonomously firing neurons has yet to be exploited as a potential therapeutic strategy. Small conductance Ca(2+)-activated K(+) channels (SK channels), in particular the SK3 subtype, are important in the physiology of DA neurons, and agents modifying SK channel activity could potentially affect DA signaling and DA-related behaviors. Here we show that cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA), a subtype-selective positive modulator of SK channels (SK3 > SK2 > > > SK1, IK), decreased spontaneous firing rate, increased the duration of the apamin-sensitive afterhyperpolarization, and caused an activity-dependent inhibition of current-evoked action potentials in DA neurons from both mouse and rat midbrain slices. Using an immunocytochemically and pharmacologically validated DA release assay employing cultured DA neurons from rats, we show that CyPPA repressed DA release in a concentration-dependent manner with a maximal effect equal to the D2 receptor agonist quinpirole. In vivo studies revealed that systemic administration of CyPPA attenuated methylphenidate-induced hyperactivity and stereotypic behaviors in mice. Taken together, the data accentuate the important role played by SK3 channels in the physiology of DA neurons, and indicate that their facilitation by CyPPA profoundly influences physiological as well as pharmacologically induced hyperdopaminergic behavior.

Effects of neuronal Kv7 potassium channel activators on hyperactivity in a rodent model of mania.

In an effort to investigate the potential antimanic-like activity of K(v)7 channel openers, we decided to test: (1) the subtype non-selective K(v)7 opener retigabine, (2) the K(v)7.4-K(v)7.5 (and K(v)7.5/3 heteromers) preferring channel opener BMS-204352 (Maxipost), and (3) the novel K(v)7.2/3 preferring channel opener ICA-27243, in the amphetamine (AMPH)+chlordiazepoxide (CDP)-induced hyperactivity paradigm in mice, a test often used to assess potential antimanic-like activity of novel compounds. Lithium and lamotrigine were included as positive controls. Pretreatment with lithium attenuated AMPH/CDP-induced hyperactivity, without affecting the activity of AMPH- or CDP-alone, and thus confirmed some predictive validity for the test paradigm. Pretreatment with lamotrigine significantly attenuated AMPH/CDP-induced effects, but also reduced motility when tested in the presence of CDP-alone. Pretreatment with retigabine or ICA-27243 attenuated AMPH/CDP-induced hyperactivity without affecting basal locomotor activity. In contrast, pretreatment with BMS-204352 failed to decrease AMPH/CDP-induced hyperactivity at lower doses (3 and 10 mg/kg). At higher doses BMS-204352 attenuated hyperactivity induced by the AMPH/CDP mix, but only at doses decreasing basal locomotor activity (30 and 60 mg/kg). None of the K(v)7 openers tested significantly affected AMPH-induced hyperactivity. In contrast, retigabine and ICA-27243 were shown to induce significant reductions in motility when administered in combination with CDP-alone. In conclusion, the results with lithium confirm some predictive validity for the test paradigm. However, our data highlight an important confounder for interpreting a role for K(v)7 channels in the alleviation of manic-like symptoms when employing the AMPH/CDP hyperactivity model in mice. It is imperative that relevant control studies (AMPH- and CDP-alone) be incorporated and reported routinely to enable thorough interpretation of data generated by means of this behavioural test.

The novel compound (+/-)-1-[10-((E)-3-Phenyl-allyl)-3,10-diaza-bicyclo[4.3.1]dec-3-yl]-propan-1-one (NS7051) attenuates nociceptive transmission in animal models of experimental pain; a pharmacological comparison with the combined mu-opioid receptor agonist and monoamine reuptake inhibitor tramadol.

Tramadol is an atypical analgesic with a unique dual mechanism of action. It acts on monoamine transporters to inhibit reuptake of noradrenaline (NA) and serotonin (5-HT), and consequent upon metabolism, displays potent agonist activity at micro-opioid receptors. Here, we present data for the novel compound NS7051, which was shown to have sub-micromolar affinity (Ki=0.034microM) for micro-opioid receptors and inhibited reuptake of 5-HT, NA and DA (IC(50)=4.2, 3.3 and 3.5microM in cortex, hippocampus and striatum respectively). NS7051 (1-30mg/kg, s.c.) produced a dose-dependent naloxone-reversible increase in the hot plate withdrawal latency, and was also analgesic in the tail flick test. In models of persistent and chronic inflammatory nociception, NS7051 reversed flinching behaviours during interphase and second phase of the formalin test (ED(50)=1.7 and 1.8mg/kg, s.c.), and hindpaw weight-bearing deficits induced by complete Freund's adjuvant injection (ED50=1.2mg/kg, s.c.). In the chronic constriction injury model of neuropathic pain, mechanical allodynia and hyperalgesia were both reversed by NS7051 (ED50=6.7 and 4.9mg/kg, s.c.). Tramadol was also active in all pain models although at considerably higher doses (20-160mg/kg, s.c.). No ataxia was observed at antiallodynic doses giving therapeutic indices of 19 and 3 for NS7051 and tramadol. The combined opioid receptor agonism and monoamine reuptake inhibitory properties of NS7051 inferred from behavioural studies appear to contribute to its well tolerated antinociceptive profile in rats. However, unlike tramadol this did not correlate with the ability to increase hippocampal monoamine levels measured by microdialysis in anesthetised rats.

Anti-nociception is selectively enhanced by parallel inhibition of multiple subtypes of monoamine transporters in rat models of persistent and neuropathic pain.

RATIONALE: Neuropathic pain is characterised by hyperexcitability within nociceptive pathways that manifests behaviourally as allodynia and hyperalgesia and remains difficult to treat with standard analgesics. However, antidepressants have shown reasonable preclinical and clinical anti-nociceptive efficacy against signs and symptoms of neuropathic pain. OBJECTIVES: To ascertain whether inhibition of serotonin (5-HT) and/or noradrenaline (NA) and/or dopamine (DA) re-uptake preferentially mediates superior anti-nociception in preclinical pain models. METHODS: The 5-HT re-uptake inhibitor fluoxetine (3-30 mg/kg), the NA re-uptake inhibitor reboxetine (3-30 mg/kg), the dual 5-HT and NA re-uptake inhibitor venlafaxine (3-100 mg/kg) and the dual DA and NA re-uptake inhibitor bupropion (3-30 mg/kg) were tested after intraperitoneal administration in rat models of acute, persistent and neuropathic pain. RESULTS: Reboxetine and venlafaxine dose-dependently attenuated second-phase flinching in the formalin test; fluoxetine attenuated flinching only at the highest dose tested, whereas bupropion was ineffective. In the chronic constriction injury (CCI) and spinal nerve ligation models of neuropathic pain, hindpaw mechanical allodynia was significantly attenuated by fluoxetine and particularly by bupropion. Reboxetine and venlafaxine were completely ineffective. In contrast, reboxetine and venlafaxine reversed thermal hyperalgesia in CCI rats, whereas bupropion and fluoxetine were either minimally effective or ineffective. Fluoxetine, reboxetine and venlafaxine transiently increased the tail-flick latency in uninjured animals. Anti-nociceptive doses of drugs had no effect on motor function. CONCLUSIONS: Combined re-uptake inhibition of 5-HT and NA appears to confer a greater degree of anti-nociception in animal models of experimental pain than single mechanism of action inhibitors. The selective attenuation of mechanical allodynia by bupropion suggests that the additional re-uptake of DA may further augment 5-HT/NA re-uptake mediated anti-nociception after nerve injury.