Task- and Treatment Length-Dependent Effects of Vortioxetine on Scopolamine-Induced Cognitive Dysfunction and Hippocampal Extracellular Acetylcholine in Rats.

Major depressive disorder (MDD) is a common psychiatric disorder that often features impairments in cognitive function, and these cognitive symptoms can be important determinants of functional ability. Vortioxetine is a multimodal antidepressant that may improve some aspects of cognitive function in patients with MDD, including attention, processing speed, executive function, and memory. However, the cause of these effects is unclear, and there are several competing theories on the underlying mechanism, notably including regionally-selective downstream enhancement of glutamate neurotransmission and increased acetylcholine (ACh) neurotransmission. The current work sought to evaluate the ACh hypothesis by examining vortioxetine's ability to reverse scopolamine-induced impairments in rodent tests of memory and attention. Additionally, vortioxetine's effects on hippocampal extracellular ACh levels were examined alongside studies of vortioxetine's pharmacokinetic profile. We found that acute vortioxetine reversed scopolamine-induced impairments in social and object recognition memory, but did not alter scopolamine-induced impairments in attention. Acute vortioxetine also induced a modest and short-lived increase in hippocampal ACh levels. However, this short-term effect is at variance with vortioxetine's moderately long brain half life (5.1 hours). Interestingly, subchronic vortioxetine treatment failed to reverse scopolamine-induced social recognition memory deficits and had no effects on basal hippocampal ACh levels. These data suggest that vortioxetine has some effects on memory that could be mediated through cholinergic neurotransmission, however these effects are modest and only seen under acute dosing conditions. These limitations may argue against cholinergic mechanisms being the primary mediator of vortioxetine's cognitive effects, which are observed under chronic dosing conditions in patients with MDD.

Pharmacokinetic/Pharmacodynamic Relationship of Gabapentin in a CFA-induced Inflammatory Hyperalgesia Rat Model.

PURPOSE: Gabapentin displays non-linear drug disposition, which complicates dosing for optimal therapeutic effect. Thus, the current study was performed to elucidate the pharmacokinetic/pharmacodynamic (PKPD) relationship of gabapentin's effect on mechanical hypersensitivity in a rat model of CFA-induced inflammatory hyperalgesia. METHODS: A semi-mechanistic population-based PKPD model was developed using nonlinear mixed-effects modelling, based on gabapentin plasma and brain extracellular fluid (ECF) time-concentration data and measurements of CFA-evoked mechanical hyperalgesia following administration of a range of gabapentin doses (oral and intravenous). RESULTS: The plasma/brain ECF concentration-time profiles of gabapentin were adequately described with a two-compartment plasma model with saturable intestinal absorption rate (K m = 44.1 mg/kg, V max = 41.9 mg/h∙kg) and dose-dependent oral bioavailability linked to brain ECF concentration through a transit compartment. Brain ECF concentration was directly linked to a sigmoid E max function describing reversal of hyperalgesia (EC 50, plasma = 16.7 µg/mL, EC 50, brain = 3.3 µg/mL). CONCLUSIONS: The proposed semi-mechanistic population-based PKPD model provides further knowledge into the understanding of gabapentin's non-linear pharmacokinetics and the link between plasma/brain disposition and anti-hyperalgesic effects. The model suggests that intestinal absorption is the primary source of non-linearity and that the investigated rat model provides reasonable predictions of clinically effective plasma concentrations for gabapentin.

Brexpiprazole I: in vitro and in vivo characterization of a novel serotonin-dopamine activity modulator.

Brexpiprazole (OPC-34712, 7-{4-[4-(1-benzothiophen-4-yl)piperazin-1-yl]butoxy}quinolin-2(1H)-one) is a novel drug candidate in clinical development for psychiatric disorders with high affinity for serotonin, dopamine, and noradrenaline receptors. In particular, it bound with high affinity (Ki < 1 nM) to human serotonin 1A (h5-HT1A)-, h5-HT2A-, long form of human D2 (hD2L)-, hα1B-, and hα2C-adrenergic receptors. It displayed partial agonism at h5-HT1A and hD2 receptors in cloned receptor systems and potent antagonism of h5-HT2A receptors and hα1B/2C-adrenoceptors. Brexpiprazole also had affinity (Ki < 5 nM) for hD3-, h5-HT2B-, h5-HT7-, hα1A-, and hα1D-adrenergic receptors, moderate affinity for hH1 (Ki = 19 nM), and low affinity for hM1 receptors (Ki > 1000 nM). Brexpiprazole potently bound to rat 5-HT2A and D2 receptors in vivo, and ex vivo binding studies further confirmed high 5-HT1A receptor binding potency. Brexpiprazole inhibited DOI (2,5-dimethoxy-4-iodoamphetamine)-induced head twitches in rats, suggestive of 5-HT2A antagonism. Furthermore, in vivo D2 partial agonist activity of brexpiprazole was confirmed by its inhibitory effect on reserpine-induced DOPA accumulation in rats. In rat microdialysis studies, brexpiprazole slightly reduced extracellular dopamine in nucleus accumbens but not in prefrontal cortex, whereas moderate increases of the dopamine metabolites, homovanillic acid and DOPAC (3,4-dihydroxy-phenyl-acetic acid), in these areas also suggested in vivo D2 partial agonist activity. In particular, based on a lower intrinsic activity at D2 receptors and higher binding affinities for 5-HT1A/2A receptors than aripiprazole, brexpiprazole would have a favorable antipsychotic potential without D2 receptor agonist- and antagonist-related adverse effects. In conclusion, brexpiprazole is a serotonin-dopamine activity modulator with a unique pharmacology, which may offer novel treatment options across a broad spectrum of central nervous system disorders.

Combined In Vivo Microdialysis and PET Studies to Validate [11C]Yohimbine Binding as a Marker of Noradrenaline Release.

The noradrenaline system attracts attention for its role in mood disorders and neurodegenerative diseases but the lack of well-validated methods impairs our understanding when assessing its function and release in vivo. This study combines simultaneous positron emission tomography (PET) and microdialysis to explore if [11C]yohimbine, a selective antagonist radioligand of the α2 adrenoceptors, may be used to assess in vivo changes in synaptic noradrenaline during acute pharmacological challenges. Anesthetised Göttingen minipigs were positioned in a head holder in a PET/CT device. Microdialysis probes were placed in the thalamus, striatum and cortex and dialysis samples were collected every 10 min. Three 90 min [11C]yohimbine scans were acquired: at baseline and at two timepoints after the administration of amphetamine (1-10 mg/kg), a non-specific releaser of dopamine and noradrenaline, or nisoxetine (1 mg/kg), a specific noradrenaline transporter inhibitor. [11C]yohimbine volumes of distribution (VT) were obtained using the Logan kinetic model. Both challenges induced a significant decrease in yohimbine VT, with time courses reflecting their different mechanisms of action. Dialysis samples revealed a significant increase in noradrenaline extracellular concentrations after challenge and an inverse correlation with changes in yohimbine VT. These data suggest that [11C]yohimbine can be used to evaluate acute variations in synaptic noradrenaline concentrations after pharmacological challenges.

Involvement of a subpopulation of neuronal M4 muscarinic acetylcholine receptors in the antipsychotic-like effects of the M1/M4 preferring muscarinic receptor agonist xanomeline.

Disturbances in central dopaminergic neurotransmission are believed to be centrally involved in the pathogenesis of schizophrenia. Central dopaminergic and cholinergic systems interact and the cholinergic muscarinic agonist xanomeline has shown antipsychotic effects in clinical studies. Preclinical studies indicate that the M(4) muscarinic cholinergic receptor subtype (mAChR) modulates the activity of the dopaminergic system and that this specific mAChR subtype is involved in mediating the antipsychotic-like effects of xanomeline. A specific neuronal subpopulation that expresses M(4) mAChRs together with D(1) dopamine receptors seems to be especially important in modulating dopamine-dependent behaviors. Using mutant mice that lack the M(4) mAChR only in D(1) dopamine receptor-expressing cells (D1-M4-KO), we investigated the role of this neuronal population in the antipsychotic-like effects of xanomeline in amphetamine-induced hyperactivity and apomorphine-induced climbing. Interestingly, the antipsychotic-like effects of xanomeline in the two models were almost completely abolished in D1-M4-KO mice, suggesting that M(4) mAChRs colocalized with D(1) dopamine receptors are centrally involved in mediating the antipsychotic-like effects of xanomeline. This is consistent with the hypothesis that activation of the M(4) mAChR represents a potential target for the future medical treatment of psychosis.

A subpopulation of neuronal M4 muscarinic acetylcholine receptors plays a critical role in modulating dopamine-dependent behaviors.

Acetylcholine (ACh) regulates many key functions of the CNS by activating cell surface receptors referred to as muscarinic ACh receptors (M(1)-M(5) mAChRs). Like other mAChR subtypes, the M(4) mAChR is widely expressed in different regions of the forebrain. Interestingly, M(4) mAChRs are coexpressed with D(1) dopamine receptors in a specific subset of striatal projection neurons. To investigate the physiological relevance of this M(4) mAChR subpopulation in modulating dopamine-dependent behaviors, we used Cre/loxP technology to generate mutant mice that lack M(4) mAChRs only in D(1) dopamine receptor-expressing cells. The newly generated mutant mice displayed several striking behavioral phenotypes, including enhanced hyperlocomotor activity and increased behavioral sensitization following treatment with psychostimulants. These behavioral changes were accompanied by a lack of muscarinic inhibition of D(1) dopamine receptor-mediated cAMP stimulation in the striatum and an increase in dopamine efflux in the nucleus accumbens. These novel findings demonstrate that a distinct subpopulation of neuronal M(4) mAChRs plays a critical role in modulating several important dopamine-dependent behaviors. Since enhanced central dopaminergic neurotransmission is a hallmark of several severe disorders of the CNS, including schizophrenia and drug addiction, our findings have substantial clinical relevance.

HIF prolyl hydroxylase inhibition increases cell viability and potentiates dopamine release in dopaminergic cells.

Hypoxia-inducible factor (HIF) controls the expression of genes that adapts the cellular condition to accommodate oxidative stress. The potential beneficial effect of HIF up-regulation in ischemia has recently gained interest substantiated by the known HIF-regulation of erythropoietin and other hypoxia accommodating genes. So far the perspectives for HIF up-regulation has been focused on anemia and ischemia related diseases but little information is available about the relevance of HIF biology for neurodegenerative disease like Parkinson's disease. We therefore sought out to characterize the effect of HIF-up-regulation on survival and dopamine homeostasis in dopaminergic cells. We used a low molecular weight HIF prolyl hydroxylase (HPH) inhibitor and lentiviral based shRNA knockdown of HPH subtypes as molecular tools to increase HIF protein level and downstream HIF-regulated genes. We show that HIF induction results in protection against oxidative stress in cellular models based on PC12 cells and LUHMES cells. In addition, HPH inhibition elevates tyrosine hydroxylase expression and activity, which causes increased dopamine synthesis and release in both PC12 cells and a primary rat ventral mesencephalic cell culture. All together these findings suggest that prolyl hydroxylases may represent novel targets for therapeutic intervention in disorders characterized by dopamine homeostasis dysregulation like Parkinson's disease.

The mechanism of a high-affinity allosteric inhibitor of the serotonin transporter.

The serotonin transporter (SERT) terminates serotonin signaling by rapid presynaptic reuptake. SERT activity is modulated by antidepressants, e.g., S-citalopram and imipramine, to alleviate symptoms of depression and anxiety. SERT crystal structures reveal two S-citalopram binding pockets in the central binding (S1) site and the extracellular vestibule (S2 site). In this study, our combined in vitro and in silico analysis indicates that the bound S-citalopram or imipramine in S1 is allosterically coupled to the ligand binding to S2 through altering protein conformations. Remarkably, SERT inhibitor Lu AF60097, the first high-affinity S2-ligand reported and characterized here, allosterically couples the ligand binding to S1 through a similar mechanism. The SERT inhibition by Lu AF60097 is demonstrated by the potentiated imipramine binding and increased hippocampal serotonin level in rats. Together, we reveal a S1-S2 coupling mechanism that will facilitate rational design of high-affinity SERT allosteric inhibitors.

Reduced Brd1 expression leads to reversible depression-like behaviors and gene-expression changes in female mice.

The schizophrenia-associated gene, BRD1, encodes an epigenetic regulator in which chromatin interactome is enriched with genes implicated in mental health. Alterations in histone modifications and epigenetic regulation contribute to brain transcriptomic changes in affective disorders and preclinical data supports a role for BRD1 in psychopathology. However, the implication of BRD1 on affective pathology remains poorly understood. In this study, we assess affective behaviors and associated neurobiology in Brd1+/- mice along with their responses to Fluoxetine and Imipramine. This involves behavioral, neurostructural, and neurochemical characterizations along with regional cerebral gene expression profiling combined with integrative functional genomic analyses. We report behavioral changes in female Brd1+/- mice with translational value to depressive symptomatology that can be alleviated by the administration of antidepressant medications. Behavioral changes are accompanied by altered brain morphometry and imbalances in monoaminergic systems. In accordance, gene expression changes across brain tissues reveal altered neurotransmitter signaling and cluster in functional pathways associated with depression including 'Adrenergic-, GPCR-, cAMP-, and CREB/CREM-signaling'. Integrative gene expression analysis specifically links changes in amygdaloid intracellular signaling activity to the behavioral treatment response in Brd1+/- mice. Collectively, our study highlights the importance of BRD1 as a modulator of affective pathology and adds to our understanding of the molecular mechanisms underlying affective disorders and their treatment response.

Differential effects of gamma-secretase and BACE1 inhibition on brain Abeta levels in vitro and in vivo.

Alzheimer's disease (AD) is hypothesized to result from elevated brain levels of beta-amyloid peptide (Abeta) which is the main component of plaques found in AD brains and which cause memory impairment in mice. Therefore, there has been a major focus on the development of inhibitors of the Abeta producing enzymes gamma-secretase and beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1). In this study, we investigated the Abeta-lowering effects of the BACE1 inhibitor LY2434074 in vitro and in vivo, comparing it to the well characterized gamma-secretase inhibitor LY450139. We sampled interstitial fluid Abeta from awake APPswe/PS1dE9 AD mice by in vivo Abeta microdialysis. In addition, we measured levels of endogenous brain Abeta extracted from wildtype C57BL/6 mice. In our in vitro assays both compounds showed similar Abeta-lowering effects. However, while systemic administration of LY450139 resulted in transient reduction of Abeta in both in vivo models, we were unable to show any Abeta-lowering effect by systemic administration of the BACE1 inhibitor LY2434074 despite brain exposure exceeding the in vitro IC(50) value several fold. In contrast, significant reduction of 40-50% of interstitial fluid Abeta and wildtype cortical Abeta was observed when infusing LY2434074 directly into the brain by means of reverse microdialysis or by dosing the BACE1 inhibitor to p-glycoprotein (p-gp) mutant mice. The effects seen in p-gp mutant mice and subsequent data from our cell-based p-gp transport assay suggested that LY2434074 is a p-gp substrate. This may partly explain why BACE1 inhibition by LY2434074 has lower in vivo efficacy, with respect to decreased Abeta40 levels, compared with gamma-secretase inhibition by LY450139.

HIF prolyl hydroxylase inhibition augments dopamine release in the rat brain in vivo.

The transcription factor hypoxia-inducible factor (HIF) is essential for the activation of several genes that promote the survival of cells exposed to oxidative stress. Expression of tyrosine hydroxylase (TH), which is the rate-limiting enzyme in the dopamine (DA) synthesis, is one of the genes that are positively regulated by HIF. Accordingly, HIF induction results in elevated DA release in various cell lines in vitro. HIF prolyl hydroxylase (HPH) is critically involved in the negative regulation of HIF levels. We investigated the in vivo effects of the HPH inhibitor FG0041 on brain DA function in rats by microdialysis in freely moving rats, locomotor activity, and Western blot analysis. Administration of FG0041 (10 mg/kg i.p.), as an acute (single injection), or as subchronic (once daily for 6 days) treatment and cobalt chloride (CoCl2) (60 mg/kg s.c.) potentiated potassium (K+) induced increases in extracellular levels of DA levels in the rat striatum. The increase in extracellular DA of freely moving rats was sought in relationship to locomotor activity in rats. A significant increase in locomotor activity was observed in FG0041-treated rats compared with vehicle on a cocaine challenge. In support of these findings, protein levels of TH in the rat brain stem were increased after treatment with FG0041. These data indicate that FG0041 augments DA function in the rat brain. Inhibition of HPH enhances DA function by increasing DA release, which has implications for the use of HIF induction in the treatment of neurodegenerative diseases.

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.

Acute effects of vortioxetine and duloxetine on resting-state functional connectivity in the awake rat.

The antidepressant vortioxetine exerts its effects via modulation of several serotonin (5-HT) receptors and inhibition of the 5-HT transporter (SERT). Additionally, vortioxetine has beneficial effects on aspects of cognitive dysfunction in depressed patients. However, a global examination of the drug effect on brain network connectivity is still missing. Here we compared the effects of vortioxetine and a serotonin norepinephrine reuptake inhibitor, duloxetine, on resting-state functional connectivity (RSFC) across the whole brain in awake rats using a combination of pharmacological and awake animal resting-state functional magnetic resonance imaging (rsfMRI) techniques. Our data showed that vortioxetine and duloxetine affected different inter-areal connections with limited overlap, indicating that in addition to different primary target profiles, these two antidepressants have distinct mechanisms of action at the systems level. Further, our data suggest that vortioxetine can affect specific brain areas with distinct 5-HT receptor expression profiles. Taken together, this study demonstrates that the awake animal fMRI approach provides a powerful tool to elucidate the effects of drugs on the brain with high spatial specificity and a global field of view. This capability is valuable to understand how different drugs affect the systems-level brain function, and provides important guidance to dissect specific brain regions and connections for further detailed mechanistic studies. This study also highlights the translational opportunity of the awake animal fMRI approach between preclinical results and human studies.

Augmentation of SSRI effects on serotonin by 5-HT2C antagonists: mechanistic studies.

The treatment of depression may be improved by using an augmentation approach involving selective serotonin reuptake inhibitors (SSRIs) in combination with compounds that focus on antagonism of inhibitory serotonin receptors. Using microdialysis coupled to HPLC, it has recently been shown that the systemic co-administration of 5-HT(2C) antagonists with SSRIs augmented the acute effect of SSRIs on extracellular 5-HT. In this paper, we have investigated the mechanism through which this augmentation occurs. The increase in extracellular 5-HT was not observed when both compounds were locally infused. However, varying the route of administration for both compounds differentially revealed that an augmentation took place when the 5-HT(2C) antagonist was locally infused into ventral hippocampus and the SSRI given systemically, but not when systemic 5-HT(2C) antagonist was co-administered with the local infusion of citalopram. This suggests that the release of extracellular serotonin in ventral hippocampus may be controlled by (an)other brain area(s). As 5-HT(2C) receptors are not considered to be autoreceptors, this would implicate that other neurotransmitter systems are involved in this process. To investigate which neurotransmitter systems were involved in the interaction, systemic citalopram was challenged with several glutamatergic, GABA-ergic, noradrenergic, and dopaminergic compounds to determine their effects on serotonin release in ventral hippocampus. It was determined that the involvement of glutamate, norepinephrine, and dopamine in the augmentation did not seem likely, whereas evidence implicated a role for the GABA-ergic system in the augmentation.

An integrated microdialysis rat model for multiple pharmacokinetic/pharmacodynamic investigations of serotonergic agents.

INTRODUCTION: Integrated in vivo models applying intracerebral microdialysis in conjunction with automated serial blood sampling in conscious, freely moving rodents are an attractive approach for pharmacokinetic (PK) and simultaneous pharmacokinetic/pharmacodynamic (PK/PD) investigations of CNS active drugs within the same animal. In this work, the ability to obtain and correlate data in this manner was evaluated for the selective serotonin (5-HT) reuptake inhibitor (SSRI) escitalopram. METHODS: An instrumented rat model equipped with an intracerebral hippocampal microdialysis probe and indwelling arterial and venous catheters was applied in the studies. Concomitant with brain microdialysis, serial blood sampling was conducted by means of an automated blood sampling device. The feasibility of the rat model for simultaneous PK/PD investigations was examined by monitoring plasma and brain extracellular concentrations of escitalopram along with SSRI-associated pharmacological activity, monitored as changes in brain 5-HT levels and plasma corticosterone levels. RESULTS: Combining intracerebral microdialysis and automated blood sampling did not cause any detectable physiological changes with respect to basal levels of plasma corticosterone or brain 5-HT levels. Furthermore, the PK of escitalopram in hippocampus following intravenous injection was not influenced by the presence of vascular catheters. Conversion of escitalopram dialysate concentrations into absolute extracellular levels by means of in vivo retrodialysis was verified by the no-net-flux method, which gave similar recovery estimates. The PK of escitalopram could be characterized simultaneously in plasma and the hippocampus of conscious, freely moving rats. Concomitantly, the modulatory and functional effects of escitalopram could be monitored as increases in brain 5-HT and plasma corticosterone levels following drug administration. DISCUSSION: The applicability of intracerebral microdialysis combined with arterial blood sampling was demonstrated for simultaneous PK/PD investigations of escitalopram in individual rats under non-stressful conditions. Together, these temporal relationships provide multiple PK/PD information in individual animals, hence minimizing inter-animal variation using a reduced number of animals.

Depression and poor sleep: the effect of monoaminergic antidepressants in a pre-clinical model in rats.

The effects of five antidepressants (escitalopram, paroxetine, duloxetine, venlafaxine, and reboxetine) on the sleep architecture were investigated in freely moving rats in the light phase of a 12:12 h light:dark cycle following a single i.p. dose of antidepressant. Overall, paroxetine and escitalopram exhibited the least sleep disruptive profiles, whereas duloxetine, venlafaxine, and reboxetine increased the time spent awake and suppressed paradoxical sleep. Analysis of the EEG at 1 h intervals revealed only subtle differences from the overall picture. The effect of venlafaxine on disruption of sleep architecture could not be readily explained by its in vitro serotonin (5-HT) and noradrenaline (NA) reuptake inhibitory potencies. In vivo microdialysis experiments in the ventral hippocampus of freely moving rats revealed that venlafaxine affected the 5-HT and NA systems equally at the doses tested. Duloxetine (7.7 mg/kg) induced maximal blockade of the 5-HT transporter and duloxetine 7.7 mg/kg also modulated the noradrenaline system. Thus, in this animal model, the enhancement of noradrenergic activity is more disruptive on the sleep architecture than enhancement of serotonergic activity.

Pharmacokinetic/pharmacodynamic feedback modelling of the functional corticosterone response in rats after acute treatment with escitalopram.

The objective of this study was to characterize the pharmacokinetic/pharmacodynamic (PK/PD) relationship of the drug-induced corticosterone response after administration of escitalopram in rats. To achieve this, a mechanistic feedback turnover model mimicking the acute mechanism of action of selective serotonin reuptake inhibitors was assessed. Conscious and freely moving rats received constant rate infusions of 2.5, 5 or 10 mg/kg escitalopram or vehicle over 60 min. Automated serial blood sampling was conducted to determine escitalopram and corticosterone concentrations. The PK/PD model consisted of a turnover model of escitalopram-evoked changes in response, which included an inhibitory feedback moderator function. Accordingly, response acted linearly on the production (k(tol)) of the moderator, which acted inversely on the production (k(in)) of response. The escitalopram plasma kinetics served as input to an inhibitory function acting on the loss (k(out)) of response. The corticosterone responses were successfully described using the model by fitting responses from all doses simultaneously resulting in estimation of drug parameters (I(max), IC(50) and n) in addition to system parameters (k(in), k(out) and k(tol)) for the whole exposure range. Thus, the applicability of the model for analysis of the acute selective serotonin reuptake inhibitor-induced corticosterone response including acute auto-inhibitory feedback was demonstrated.

Antagonism of the 5-HT6 receptor - Preclinical rationale for the treatment of Alzheimer's disease.

Antagonism of the 5-HT6 receptor is a promising approach for the symptomatic treatment of Alzheimer's disease (AD). There is compelling preclinical evidence for the procognitive potential of 5-HT6 receptor antagonists and several compounds are in clinical development, as adjunct therapy to acetylcholinesterase inhibitors (AChEIs). This manuscript summarizes the scientific rationale for the use of 5-HT6 receptor antagonists as AD treatment, with some focus on the selective and high-affinity 5-HT6 receptor antagonist idalopirdine (Lu AE58054). The 5-HT6 receptor is enriched in brain regions that mediate cognition, where expression predominates on glutamatergic and GABAergic neurons and subsets of GABAergic interneurons. It is proposed that 5-HT6 receptor antagonism modulates the balance between neuronal excitation (glutamate) and inhibition (GABA), which may have widespread implications for neurotransmission and neuronal activity. This is supported by preclinical studies showing that 5-HT6 receptor antagonists increase concentrations of multiple neurotransmitters, and strengthened by recent evidence that idalopirdine facilitates neuronal oscillations and contributes to the recruitment of several neuronal networks relevant in cognition. Some of these effects are observed with idalopirdine monotherapy, whereas others require concomitant treatment with an AChEI. Several hypotheses for the mechanism underlying the synergistic actions between 5-HT6 receptor antagonists and AChEIs are discussed. Collectively, the current evidence suggests that 5-HT6 receptor antagonism adds a unique, complementary mechanism of action to that of AChEIs. The facilitation of multiple neurotransmitters and neuronal activity in brain regions that mediate cognition, and the synergy with AChEIs, are proposed to mediate the procognitive effects of 5-HT6 receptor antagonists in AD patients.

Involvement of presynaptic 5-HT1A receptors in the low propensity of brexpiprazole to induce extrapyramidal side effects in rats.

Previous studies have shown that partial and full 5-HT1A receptor agonists reduce antipsychotic-induced catalepsy. Consequently, some antipsychotics combining balanced efficacy between dopamine (DA) D2 antagonism or partial agonism and 5-HT1A receptor agonism have a low propensity to induce extrapyramidal side effects (EPS), as reflected by low cataleptogenic activity in rodents. In the present experiments, we attempted to explore the importance of pre- and postsynaptic 5-HT1A agonistic properties of brexpiprazole and aripiprazole in the context of neurological side-effect liabilities. Additional measures of prefrontal cortical serotonin (5-HT) and DA levels using microdialysis were used to support that brexpiprazole has a preferential agonist effect on presynaptic 5-HT1A receptors. Brexpiprazole (3.0 and 10mg/kg, p.o.) as well as aripiprazole (8.0 and 30mg/kg, p.o.) failed to induce catalepsy in rats. Brexpiprazole (10mg/kg, p.o.) significantly reduced the cataleptic response induced by haloperidol (0.63mg/kg, s.c.), while aripiprazole (1.0-100mg/kg, p.o.) failed to reverse the effect of haloperidol and only showed a numeric decrease at 10mg/kg, (p.o.). When 5-HT1A receptors were blocked by the selective antagonist, WAY100635 (1.0mg/kg, s.c.), cataleptogenic properties of brexpiprazole (10mg/kg; p.o), but not aripiprazole (8.0 and 30mg/kg, p.o.) were unmasked. The ("biased") 5-HT1A receptor agonists F15599 (postsynaptic preference) and F13714 (presynaptic preference) had differential effects on haloperidol-induced catalepsy: F13714 (0.16mg/kg, s.c.) counteracted catalepsy, whereas F15599 (0.040mg/kg, s.c.) had no significant effect at regionally-selective doses. These data support a role of presynaptic 5-HT1A receptors in the anticataleptic effect of brexpiprazole. The selective 5-HT2A antagonist M100907 (0.10mg/kg, s.c.) had no effect on haloperidol-induced catalepsy, arguing against a major role of 5-HT2A receptors in the cataleptogenic profile of brexpiprazole. The findings with brexpiprazole were supported using microdialysis studies: Brexpiprazole (3.0 and 10mg/kg, p.o.) decreased extracellular 5-HT levels in the medial prefrontal cortex (mPFC), while it failed to affect extracellular DA in the same samples, suggesting that the 5-HT1A agonist properties of brexpiprazole may be preferentially presynaptic. In conclusion, these results confirm that brexpiprazole and aripiprazole have low propensities to induce EPS. However, the low EPS risk of brexpiprazole is more likely dependent on its agonist properties on presynaptic 5-HT1A receptors, while that of aripiprazole is less sensitive to 5-HT1A receptor antagonism.

Effects of the 5-HT6 receptor antagonist idalopirdine on extracellular levels of monoamines, glutamate and acetylcholine in the rat medial prefrontal cortex.

Idalopirdine (Lu AE58054) is a high affinity and selective antagonist for the human serotonin 5-HT6 receptor (Ki 0.83nM) in phase III development for mild-to-moderate Alzheimer's disease as an adjunct therapy to acetylcholinesterase inhibitors (AChEIs). We have studied the effects of idalopirdine on extracellular levels of monoamines, glutamate and acetylcholine in the medial prefrontal cortex (mPFC) of freely-moving rats using microdialysis. Idalopirdine (10mg/kg p.o.) increased extracellular levels of dopamine, noradrenaline and glutamate in the mPFC and showed a trend to increase serotonin levels. No effect was observed on acetylcholine levels. The AChEI donepezil (1.3mg/kg s.c.) significantly increased the levels of acetylcholine. Pretreatment with idalopirdine 2h prior to donepezil administration potentiated the effect of donepezil on extracellular acetylcholine levels. The idalopirdine potentiation of donepezil-induced increase in acetylcholine levels was also observed during local infusion of idalopirdine (6µg/ml) into the mPFC by reverse dialysis. The data from the current study may provide a mechanistic model for the pro-cognitive effects observed with administration of idalopirdine in donepezil-treated patients with Alzheimer's disease observed in the phase 2 studies (Wilkinson et al. 2014).