Kjartan F. Herrik

Pharmacological modulation of the gating properties of small conductance Ca2+-activated K+ channels alters the firing pattern of dopamine neurons in vivo.

Dopamine (DA) neurons are autonomous pacemakers that occasionally fire bursts of action potentials, discharge patterns thought to reflect tonic and phasic DA signaling, respectively. Pacemaker activity depends on the concerted and cyclic interplay between intrinsic ion channels with small conductance Ca(2+)-activated K(+) (SK) channels playing an important role. Bursting activity is synaptically initiated but neither the transmitters nor the specific ion conductances involved have been definitively identified. Physiological and pharmacological regulation of SK channel Ca(2+) sensitivity has recently been demonstrated and could represent a powerful means of modulating the expression of tonic/phasic signaling in DA neurons in vivo. To test this premise, we characterized the effects of intravenous administration of the novel positive and negative SK channel modulators NS309 and NS8593, respectively, on the spontaneous activity of substantia nigra pars compacta DA neurons in anesthetized C57BL/6 mice. NS309, dose-dependently decreased DA cell firing rate, increased the proportion of regular firing cells, and eventually stopped spontaneous firing. By contrast, systemic administration of the negative SK channel modulator NS8593 increased firing rate and shifted the pattern toward increased irregularity/bursting; an effect similar to local application of the pore blocking peptide apamin. The altered firing patterns resulting from inhibiting SK currents persisted independently of changes in firing rates induced by administration of DA autoreceptor agonists/antagonists. We conclude that pharmacological modulation of SK channel Ca(2+)-sensitivity represents a powerful mechanism for switching DA neuron firing activity between tonic and phasic signaling modalities in vivo.

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 Ca2+-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.

The 5-HT6 receptor antagonist idalopirdine potentiates the effects of donepezil on gamma oscillations in the frontal cortex of anesthetized and awake rats without affecting sleep-wake architecture.

&NA; The 5‐HT6 receptor is a promising target for cognitive disorders, in particular for Alzheimers disease (AD). The high affinity and selective 5‐HT6 receptor antagonist idalopirdine (Lu AE58054) is currently in development for mild‐moderate AD as adjunct therapy to acetylcholinesterase inhibitors (AChEIs). We studied the effects of idalopirdine alone and in combination with the AChEI donepezil on cortical function using two in vivo electrophysiological methods. Neuronal network oscillations in the frontal cortex were measured during electrical stimulation of the brainstem nucleus pontis oralis (nPO) in the anesthetized rat and by an electroencephalogram (EEG) in the awake, freely moving rat. In conjunction with the EEG study, we investigated the effects of idalopirdine and donepezil on sleep‐wake architecture using telemetric polysomnography. Idalopirdine (2 mg/kg i.v.) increased gamma power in the medial prefrontal cortex (mPFC) during nPO stimulation. Donepezil (0.3 and 1 mg/kg i.v.) also increased cortical gamma power and pretreatment with idalopirdine (2 mg/kg i.v.) potentiated and prolonged the effects of donepezil. Similarly, donepezil (1 and 3 mg/kg s.c.) dose‐dependently increased frontal cortical gamma power in the freely moving rat and pretreatment with idalopirdine (10 mg/kg p.o.) augmented the effect of donepezil 1 mg/kg. Analysis of the sleep‐wake architecture showed that donepezil (1 and 3 mg/kg s.c.) dose‐dependently delayed sleep onset and decreased the time spent in both REM and non REM sleep stages. In contrast, idalopirdine (10 mg/kg p.o.) did not affect sleep‐wake architecture nor the effects of donepezil. In summary, we show that idalopirdine potentiates the effects of donepezil on frontal cortical gamma oscillations, a pharmacodynamic biomarker associated with cognition, without modifying the effects of donepezil on sleep. The increased cortical excitability may contribute to the procognitive effects of idalopirdine in donepezil‐treated AD patients. HighlightsIdalopirdine augments the effects of donepezil on cortical gamma oscillations.Idalopirdine does not modify sleep‐wake patterns nor the effects of donepezil.5‐HT6R antagonism augments effects of cholinesterase inhibition in the cortex.Increased cortical excitation may add to procognitive effects of idalopirdine.

Temporally dissociable effects of ketamine on neuronal discharge and gamma oscillations in rat thalamo-cortical networks

Background: Sub‐anesthetic doses of the non‐competitive N‐methyl‐D‐aspartate receptor (NMDA‐R) antagonist ketamine evoke transient psychotomimetic effects, followed by persistent antidepressant effects in treatment‐resistant depressed patients and rodents through still poorly understood mechanisms. Since phencyclidine (PCP) disinhibits thalamo‐cortical networks by blocking NMDA‐Rs on GABAergic neurons of the reticular thalamic nucleus (RtN), we examined ketamines actions in the same areas. Methods: Single units and local field potentials were recorded in chloral hydrate anesthetized male Wistar rats. The effects of cumulative ketamine doses (0.25–5 mg/kg, i.v.) on neuronal discharge and oscillatory activity were examined in RtN, mediodorsal and centromedial (MD/CM) thalamic nuclei, and layer VI of the medial prefrontal cortex (mPFC). Results: Ketamine (1, 2 and 5 mg/kg, i.v.) significantly decreased the discharge of MD/CM, RtN and layer VI mPFC pyramidal neurons. Simultaneously, ketamine decreased the power of low frequency oscillations in all areas examined and increased gamma oscillations in mPFC and MD/CM. Lower ketamine doses (0.25 and 0.5 mg/kg, i.v.) were ineffective. Conclusions: As observed for PCP, ketamine markedly inhibited the activity of RtN neurons. However, unlike PCP, this effect did not translate into a disinhibition of MD/CM and mPFC excitatory neurons, possibly due to a more potent and simultaneous blockade of NMDA‐Rs by ketamine in MD/CM and mPFC neurons. Hence, the present in vivo results show that ketamine evokes an early transient inhibition of neuronal discharge in thalamo‐cortical networks, following its rapid pharmacokinetics, which is likely associated to its psychotomimetic effects. The prolonged increase in gamma oscillations may underlie its antidepressant action. HighlightsPCP and ketamine differentially affect thalamo‐cortical activity in anesthetized rats.As PCP, ketamine inhibits GABA neurons of the reticular thalamic nucleus.Unlike PCP, ketamine does not excite MD/CM nuclei excitatory neurons.Ketamine also inhibits layer VI pyramidal neurons in medial prefrontal cortex.In parallel, ketamine enhances gamma band power and reduces delta band power.

The 5-HT3 receptor antagonist ondansetron potentiates the effects of the acetylcholinesterase inhibitor donepezil on neuronal network oscillations in the rat dorsal hippocampus

ABSTRACT Cognitive impairments in Alzheimers disease (AD) have been associated with alterations in neuronal oscillatory activity, of which hippocampal theta and gamma oscillations are essential for the coordination of neuronal networks during cognitive functions. Cognitive deterioration in AD is delayed by symptomatic treatment with donepezil and other acetylcholinesterase inhibitors (AChEIs). However, the efficacy of symptomatic monotherapy is insufficient. Combining 5‐HT receptor antagonists with AChEIs represents a promising new approach for symptomatic treatment of AD. The selective 5‐HT3 receptor antagonist ondansetron decreases the activity of interneurons with a concomitant increase in the activity of pyramidal neurons in the hippocampus of freely moving rats. Additionally, 5‐HT3 receptor antagonism modulates acetylcholine release in rat cortex and hippocampus. We investigated the effects of ondansetron alone and in combination with donepezil on hippocampal oscillations using in vivo electrophysiology. Neuronal network oscillations were recorded in the dorsal hippocampus during electrical stimulation of the brainstem pedunculopontine tegmental nucleus in urethane‐anaesthetised rats. In addition, potential pharmacokinetic interactions between donepezil and ondansetron were assessed. Ondansetron alone did not affect hippocampal network oscillations. Donepezil dose‐dependently increased hippocampal theta and gamma power during PPT stimulation. Ondansetron (0.3mg/kg, i.v.) potentiated theta and gamma responses to 0.2mg/kg donepezil and prolonged theta and gamma responses to 0.3mg/kg donepezil. These effects could not be attributed to pharmacokinetic interactions between the compounds. This study demonstrates that ondansetron potentiates the effects of donepezil on elicited neuronal oscillations and suggests that 5‐HT3 receptor antagonists may be beneficial as adjunctive therapy to AChEIs for the symptomatic treatment of cognitive deficits in AD. HIGHLIGHTSDonepezil dose‐dependently increase hippocampal theta and gamma power in rats.Ondansetron augments the effects of donepezil on hippocampal neuronal oscillations.Ondansetron augments effects of donepezil on hippocampal phase‐amplitude couplings.Ondansetron potentiates the effects of donepezil on biomarkers for cognition.5‐HT3R antagonists may augment the effects of AChEIs on cognitive deficits in AD.

P.2.010 Ketamine inhibits the activity of thalamic neurons in anesthetized rats

Trabajo presentado en las Gordon Research Conferences 2016: Thalamocortical interactions, celebrado en Ventura, California, del 14 al 19 de febrero de 2016

Ketamine modulates the activity of thalamo-cortical networks in anesthetized rats

Trabajo presentado en el 10th FENS Forum of Neuroscience (FENS 2016), celebrado en Copenhague, Dinamarca, del 2 al 6 de julio de 2016