Florence Sotty

Therapeutic Potential of 5-HT2C Receptor Ligands

Serotonin 2C receptors are G protein-coupled receptors expressed by GABAergic, glutamatergic, and dopaminergic neurons. Anatomically, they are present in various brain regions, including cortical areas, hippocampus, ventral midbrain, striatum, nucleus accumbens, hypothalamus, and amygdala. A large body of evidence supports a critical role of serotonin 2C receptors in mediating the interaction between serotonergic and dopaminergic systems, which is at the basis of their proposed involvement in the regulation of mood, affective behavior, and memory. In addition, their expression in specific neuronal populations in the hypothalamus would be critical for their role in the regulation of feeding behavior. Modulation of these receptors has therefore been proposed to be of interest in the search for novel pharmacological strategies for the treatment of various pathological conditions, including schizophrenia and mood disorders, as well as obesity. More precisely, blockade of serotonin 2C receptors has been suggested to provide antidepressant and anxiolytic benefit, while stimulation of these receptors may offer therapeutic benefit for the treatment of psychotic symptoms in schizophrenia and obesity. In addition, modulation of serotonin 2C receptors may offer cognitive-enhancing potential, albeit still a matter of debate. In the present review, the most compelling evidence from the literature is presented and tentative hypotheses with respect to existing controversies are outlined.

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.

Phosphodiesterase 10A inhibition modulates the sensitivity of the mesolimbic dopaminergic system to D -amphetamine: involvement of the D1-regulated feedback control of midbrain dopamine neurons.

J. Neurochem. (2009) 109, 766–775.

Increased dopaminergic activity in socially isolated rats: An electrophysiological study

The development of animal models mimicking symptoms associated with schizophrenia has been a critical step in understanding the neurobiological mechanisms underlying the disease. Long-term social isolation from weaning in rodents, a model based on the neurodevelopmental hypothesis of schizophrenia, has been suggested to mimic some of the deficits seen in schizophrenic patients. We confirm in the present study that socially isolated rats display an increase in both spontaneous and d-amphetamine-induced locomotor activity, as well as deficits in sensorimotor gating as assessed in a pre-pulse inhibition paradigm. In addition, in vivo electrophysiological studies revealed changes in dopaminergic cell firing activity in the ventral tegmental area of isolated rats when compared to group-housed controls. These alterations include an increase in the number of spontaneously active dopaminergic neurons, and a change of firing activity towards a more irregular and bursting firing pattern. Taken together, our findings suggest that the behavioral phenotype induced by social isolation may be driven by an overactive dopamine system.

Relevance of dorsal raphe nucleus firing in serotonin 5-HT2C receptor blockade-induced augmentation of SSRIs effects

Selective serotonin reuptake inhibitors are the most widely prescribed antidepressant drugs. However, they exhibit a slow onset of action, putatively due to the initial decrease in serotonin cell firing mediated via somato-dendritic autoreceptors. Interestingly, blockade of 5-HT(2C) receptors significantly potentiates the effect of citalopram, a selective serotonin reuptake inhibitor, on serotonin efflux in the hippocampus and prefrontal cortex (Cremers, T.I.F.H., Giorgetti, M., Bosker, F.J., Hogg, S., Arnt, J., Mork, A., Honig, G., Bøgesø, K.P., Westerink, B.H.C., den Boer, J.A., Wikstrøm, H.V., Tecott, L.H., 2004. Inactivation of 5-HT(2C) receptors potentiates consequences of serotonin reuptake blockade. Neuropsychopharmacology 29, 1782-1789; Cremers, T.I.F.H., Rea, K., Bosker, F.J., Wikström, H.V., Hogg, S., Mørk, A., Westerink, B.H.C., 2007. Augmentation of SSRI effects on serotonin by 5-HT(2C) antagonists: mechanistic studies. Neuropsychopharmacology 32, 1550-1557.). Using in vivo electrophysiology, we show in the present study that the purported selective 5-HT(2C) receptor antagonist, SB242,084, dose-dependently counteracts citalopram-induced inhibition of serotonin cell firing. Even though the effect of SB242,084 is significant at a dose found in vivo to also partially occupy 5-HT(2A) receptors, indicating a possible contribution of a partial blockade of 5-HT(2A) receptors together with 5-HT(2C) receptors, we suggest that high occupancy at 5-HT(2C) receptors is essential for the blockade of the inhibitory effect of citalopram on 5-HT cell firing. Using microdialysis, we also show that the potentiation by SB242,084 on serotonin efflux requires an action of citalopram outside the terminal, most likely at the somato-dendritic level (i.e., on serotonin cell firing). Further experiments using local 5-HT(2C) receptor blockade indicate a role of 5-HT(2C) receptors located in the prefrontal cortex. Modulation of short or long feedback loops originating in the prefrontal cortex by 5-HT(2C) receptors could directly inhibit serotonin efflux, or alternatively, regulate serotonin cell firing in the dorsal raphe nucleus, thereby modulating serotonin efflux indirectly.

Memantine potentiates hippocampal theta oscillations at a therapeutic dose in anesthetized mice: A mechanistic link to its cognitive-enhancing properties

Memantine is an uncompetitive, low-affinity NMDA receptor antagonist clinically used for the treatment of cognitive deficits in moderate to severe Alzheimers disease. Both neurophysiological and behavioral studies in rodents have suggested a beneficial effect of memantine on synaptic plasticity and learning performances. In the present study, we investigated the effect of memantine on pedonculopontine-elicited theta oscillations in the hippocampus of urethane anesthetized mice, a model shown to be sensitive to several pharmacological agents exhibiting cognitive-enhancing properties. We found that a low dose of memantine potentiated elicited theta power while a high dose was disruptive. The low dose of memantine used was shown to yield an unbound brain concentration well within the range of therapeutic concentrations reported in rodent brain extracellular fluid and human cerebrospinal fluid. For further comparison, the effect of another uncompetitive NMDA receptor antagonist with higher affinity, i.e. MK-801, was also investigated. MK-801 was at a low dose devoid of effect on elicited theta power, while a high dose, within the range of doses reported to induce cognitive deficits in a variety of hippocampal-dependent learning paradigms in mice, was found disruptive on elicited theta waves. Taken together, our results suggest that clinically relevant doses of memantine promote neuronal network synchronization in the hippocampus, which may represent an underlying mechanism for the reported cognitive-enhancing properties in both preclinical and clinical studies.

Linking HSP90 target occupancy to HSP70 induction and efficacy in mouse brain.

HSP90 (Heat shock protein 90) is a molecular chaperone protein ubiquitously expressed throughout all tissues in the body. HSP90 has been proposed as a target to increase turnover of pathological proteins leading to neurodegeneration in Huntingtons disease, Parkinsons disease and Alzheimers disease. The mechanism of how HSP90 inhibition leads to clearance of misfolded proteins is not fully understood. It may involve direct effects of inhibiting ATPase function, indirect effects by inducing the heat-shock-response resulting in upregulation of other chaperone proteins like HSP70 or a combination of both. In the current work we established a methodology to investigate the relationship between HSP90 target occupancy and HSP70 induction in vivo. We also characterized the acute effect of two different HSP90 inhibitors in the rTg4510 transgenic mouse model of Alzheimers disease which displays a tau-mediated synaptic dysfunction. We show that reversal of synaptic impairments in this model can be obtained with a compound which has a high HSP70 induction capacity. The current developed assay methodologies may thus be of significant use in the further elucidation of the mechanism involved in the in vivo effect of HSP90 inhibition in models of neurodegeneration. Further on, the ability of HSP90 inhibitors to normalize synaptic dysfunction in an in vivo disease model of Alzheimers disease could have therapeutic relevance and further strengthens the usefulness of this animal model to establish pharmacodynamic effect of HSP90 inhibition.

PFE-360-induced LRRK2 inhibition induces reversible, non-adverse renal changes in rats

Parkinsons disease (PD) is a progressive neurodegenerative disorder for which there is no existing therapeutic approach to delay or stop progression. Genetic, biochemical and pre-clinical studies have provided evidence that leucine-rich-repeat-kinase-2 (LRRK2) kinase is involved in the pathogenesis of PD, and small molecule LRRK2 inhibitors represent a novel potential therapeutic approach. However, potentially adverse target-related effects have been discovered in the lung and kidneys of LRRK2 knock-out (ko) mice and rats. It is unclear if the LRRK2 ko effect in the kidneys and lung is also induced by pharmacological inhibition of the LRRK2 kinase. Here, we show that treatment with the LRRK2 inhibitor PFE-360 in rats induces a morphological kidney phenotype resembling that of the LRRK2 ko rats, whereas no effects were observed in the lung. The PFE-360 treatment induced morphological changes characterised by darkened kidneys and progressive accumulation of hyaline droplets in the renal proximal tubular epithelium. However, no histopathological evidence of renal tubular injury or changes in the blood and urine parameters that would be indicative of kidney toxicity or impaired kidney function were observed after up to 12 weeks of treatment. Morphological changes were detected in the kidney after 2 weeks of treatment and were partially reversible within a 30 day treatment-free period. Our findings suggest that pharmacological LRRK2 inhibition may not have adverse consequences for kidney function.

Phencyclidine administration during neurodevelopment alters network activity in prefrontal cortex and hippocampus in adult rats

Symptoms of schizophrenia have been linked to insults during neurodevelopment such as NMDA receptor (NMDAR) antagonist exposure. In animal models, this leads to schizophrenia-like behavioral symptoms as well as molecular and functional changes within hippocampal and prefrontal regions. The aim of this study was to determine how administration of the NMDAR antagonist phencyclidine (PCP) during neurodevelopment affects functional network activity within the hippocampus and medial prefrontal cortex (mPFC). We recorded field potentials in vivo after electrical brain stem stimulation and observed a suppression of evoked theta power in ventral hippocampus, while evoked gamma power in mPFC was enhanced in rats administered with PCP neonatally. In addition, increased gamma synchrony elicited by acute administration of the NMDAR antagonist MK-801 was exaggerated in neonatal PCP animals. These data suggest that NMDAR antagonist exposure during brain development alters functional networks within hippocampus and mPFC possibly contributing to the reported behavioral symptoms of this animal model of schizophrenia.NEW & NOTEWORTHY We show that insults with a NMDA receptor antagonist during neurodevelopment lead to suppressed evoked theta oscillations in ventral hippocampus in adult rats, while evoked gamma oscillations are enhanced and hypersensitive to an acute challenge with a NMDA receptor antagonist in prefrontal cortex. These observations reveal the significance of neurodevelopmental disturbances in the evolvement of schizophrenia-like symptoms and contribute to the understanding of the functional deficits underlying aberrant behavior in this disease.

A mouse model of the schizophrenia-associated 1q21.1 microdeletion syndrome exhibits altered mesolimbic dopamine transmission

Abstract1q21.1 hemizygous microdeletion is a copy number variant leading to eightfold increased risk of schizophrenia. In order to investigate biological alterations induced by this microdeletion, we generated a novel mouse model (Df(h1q21)/+) and characterized it in a broad test battery focusing on schizophrenia-related assays. Df(h1q21)/+ mice displayed increased hyperactivity in response to amphetamine challenge and increased sensitivity to the disruptive effects of amphetamine and phencyclidine hydrochloride (PCP) on prepulse inhibition. Probing of the direct dopamine (DA) pathway using the DA D1 receptor agonist SKF-81297 revealed no differences in induced locomotor activity compared to wild-type mice, but Df(h1q21)/+ mice showed increased sensitivity to the DA D2 receptor agonist quinpirole and the D1/D2 agonist apomorphine. Electrophysiological characterization of DA neuron firing in the ventral tegmental area revealed more spontaneously active DA neurons and increased firing variability in Df(h1q21)/+ mice, and decreased feedback reduction of DA neuron firing in response to amphetamine. In a range of other assays, Df(h1q21)/+ mice showed no difference from wild-type mice: gross brain morphology and basic functions such as reflexes, ASR, thermal pain sensitivity, and motor performance were unaltered. Similarly, anxiety related measures, baseline prepulse inhibition, and seizure threshold were unaltered. In addition to the central nervous system-related phenotypes, Df(h1q21)/+ mice exhibited reduced head-to tail length, which is reminiscent of the short stature reported in humans with 1q21.1 deletion. With aspects of both construct and face validity, the Df(h1q21)/+ model may be used to gain insight into schizophrenia-relevant alterations in dopaminergic transmission.