Louise Witten

Lu AE58054, a 5-HT6 antagonist, reverses cognitive impairment induced by subchronic phencyclidine in a novel object recognition test in rats.

The in-vitro potency and selectivity, in-vivo binding affinity and effect of the 5-HT(6)R antagonist Lu AE58054 ([2-(6-fluoro-1H-indol-3-yl)-ethyl]-[3-(2,2,3,3-tetrafluoropropoxy)-benzyl]-amine) on impaired cognition were evaluated. Lu AE58054 displayed high affinity to the human 5-HT(6) receptor (5-HT(6)R) with a Ki of 0.83 nm. In a 5-HT(6) GTPgammaS efficacy assay Lu AE58054 showed no agonist activity, but demonstrated potent inhibition of 5-HT-mediated activation. Besides medium affinity to adrenergic alpha(1A)- and alpha(1B)-adrenoreceptors, Lu AE58054 demonstrated >50-fold selectivity for more than 70 targets examined. Orally administered Lu AE58054 potently inhibited striatal in-vivo binding of the 5-HT(6) antagonist radioligand [(3)H]Lu AE60157 ([(3)H]8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline), with an ED(50) of 2.7 mg/kg. Steady-state modelling of an acute pharmacokinetic/5-HT(6)R occupancy time-course experiment indicated a plasma EC(50) value of 20 ng/ml. Administration of Lu AE58054 in a dose range (5-20 mg/kg p.o.) leading to above 65% striatal 5-HT(6)R binding occupancy in vivo, reversed cognitive impairment in a rat novel object recognition task induced after subchronic treatment for 7 d with phencyclidine (PCP 2 mg/kg b.i.d., i.p. for 7 d, followed by 7 d drug free). The results indicate that Lu AE58054 is a selective antagonist of 5-HT(6)Rs with good oral bioavailability and robust efficacy in a rat model of cognitive impairment in schizophrenia. Lu AE58054 may be useful for the pharmacotherapy of cognitive dysfunction in disease states such as schizophrenia and Alzheimers disease.

Effect of sertindole on extracellular dopamine, acetylcholine, and glutamate in the medial prefrontal cortex of conscious rats: a comparison with risperidone and exploration of mechanisms involved

RationaleSecond-generation antipsychotics have some beneficial effect on cognition. Recent studies, furthermore, indicate differential effects of second-generation antipsychotics on impairment in executive cognitive function.ObjectiveWe evaluated the effect of the second-generation antipsychotic drug, sertindole, on extracellular levels of dopamine (DA), acetylcholine (ACh), and glutamate (Glu) in the rat medial prefrontal cortex (mPFC). Risperidone was studied for comparison. Moreover, selective serotonin 5-HT2A, 5-HT2C, and 5-HT6 receptor antagonists were used, given alone and in combination with the preferential DA D2 receptor antagonist, haloperidol, to further clarify the action of the two drugs.Materials and methodsRats were treated acutely with vehicle or drugs, and extracellular levels of neurotransmitters were assessed by microdialysis in freely moving animals.ResultsSertindole and risperidone significantly increased extracellular levels of DA. Haloperidol; the 5-HT2A receptor antagonist, M100907; the 5-HT2C receptor antagonist, SB242084; and the 5-HT6 receptor antagonist, GSK-742457, induced minor increases in levels of DA, but the three latter compounds raised the DA levels notably in combination with haloperidol. Sertindole and risperidone significantly increased the extracellular levels of ACh but only sertindole raised the extracellular levels of Glu. The selective 5-HT6 receptor antagonist, SB-271046, significantly increased the extracellular levels of Glu.ConclusionSertindole and risperidone markedly increased extracellular levels of DA in mPFC. The built-in 5-HT2A/5-HT2C/D2 receptor antagonism of the two drugs might be involved in this action. Both drugs increased the extracellular levels of ACh but only sertindole enhanced Glu levels. The high affinity of sertindole for the 5-HT6 receptor compared to risperidone may differentiate sertindole from risperidone.

The effects of acute treatment with escitalopram on the different stages of contextual fear conditioning are reversed by atomoxetine

RationaleAlthough the antidepressant and anxiolytic effects of selective serotonin reuptake inhibitors and serotonin–noradrenaline reuptake inhibitors are well-documented, less is known about their cognitive effects.ObjectiveEscitalopram, a selective serotonin reuptake inhibitor, and atomoxetine, a selective noradrenaline reuptake inhibitor, were used to evaluate the interaction between noradrenergic and serotonergic neurotransmission in the modulation of contextual fear conditioning in rats.MethodsContextual fear-conditioning test was used to investigate the acute effects of escitalopram, alone or in combination with atomoxetine, in different stages of learning and memory in rats. Furthermore, microdialysis in freely moving animals was used to investigate the effect of escitalopram on serotonin, dopamine, and noradrenaline levels in the rat hippocampus.ResultsEscitalopram significantly increased conditioned responses when applied before the acquisition, but decreased responses, when applied before the recall test. When administered during memory consolidation, escitalopram dose-dependently enhanced conditioned responding. These effects were blocked by atomoxetine. Escitalopram (at a dose that affects memory consolidation) increased hippocampal serotonin levels fourfold without changing dopamine or noradrenaline. Atomoxetine, at dose levels that blocked the effects of escitalopram on contextual fear conditioning, increased the extracellular levels of noradrenaline eightfold but did not change dopamine or serotonin. A combined treatment of escitalopram and atomoxetine caused a significant attenuation of escitalopram-induced increase in serotonin levels, while noradrenaline levels were not affected.ConclusionsThese findings indicate that escitalopram affects fear memory in rats, likely modulated by increases in serotonin levels in the brain. This effect is impaired by atomoxetine, probably due to a noradrenaline-mediated decrease in serotonin levels. Further studies are warranted to study the effects of potential differences among antidepressant therapies on long-term cognitive outcomes.

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

J. Neurochem. (2010) 115, 209–219.

Auditory sensory processing deficits in sensory gating and mismatch negativity-like responses in the social isolation rat model of schizophrenia.

Patients with schizophrenia exhibit disturbances in information processing. These disturbances can be investigated with different paradigms of auditory event related potentials (ERP), such as sensory gating in a double click paradigm (P50 suppression) and the mismatch negativity (MMN) component in an auditory oddball paradigm. The aim of the current study was to test if rats subjected to social isolation, which is believed to induce some changes that mimic features of schizophrenia, displays alterations in sensory gating and MMN-like response. Male Lister-Hooded rats were separated into two groups; one group socially isolated (SI) for 8 weeks and one group housed (GH). Both groups were then tested in a double click sensory gating paradigm and an auditory oddball paradigm (MMN-like) paradigm. It was observed that the SI animals showed reduced sensory gating of the cortical N1 amplitude. Furthermore, the SI animals showed significant reduction in cortical MMN-like response compared with the GH animals. No deficits in sensory gating or MMN-like response were observed in the hippocampus (CA3) of the SI animals compared with GH animals. In conclusion, the change in sensory gating of the N1 amplitude supports previous findings in SI rats and the reduced MMN-like response is similar to the deficits of MMN seen in patients with schizophrenia. Since reduced auditory MMN amplitude is believed to be more selectively associated with schizophrenia than other measures of sensory gating deficits, the current study supports the face validity of the SI reared rat model for schizophrenia.

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.

Characterization of [3H]Lu AE60157 ([3H]8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline) binding to 5-hydroxytryptamine6 (5-HT6) receptors in vivo

The serotonin6 (5-HT(6)) receptor has received attention for its proposed role in cognitive impairments associated with schizophrenia and Alzheimers disease. This has lead to a search for selective 5-HT(6) receptor ligands useful for in vivo imaging in animals and humans. The novel 5-HT(6) receptor antagonist Lu AE60157 (8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline) displays high affinity for the human (h) 5-HT(6) receptor (K(d) 0.2nM), and broad profiling in 60 additional binding and enzyme assays showed that Lu AE60157 displays 16-fold selectivity to the h5-HT(2A) receptor (K(i) 3.2nM) and >100-fold selectivity to all other evaluated targets. Lu AE60157 was labeled with tritium in the N-methyl group and evaluated as a radioligand in vitro as well as in vivo in rats and mice. Autoradiography experiments showed that [(3)H]Lu AE60157 bound preferentially to rat brain regions with expected high 5-HT(6) receptor density. Furthermore, [(3)H]Lu AE60157 showed good brain penetration after systemic administration and high (about 75%) specific in vivo binding to the striatal 5-HT(6) receptor in rats. The striatal binding of [(3)H]Lu AE60157 was fully displaced by selective 5-HT(6) receptor antagonists (SB-742457; Lu AE58054) and antipsychotics known to inhibit the binding of 5-HT(6) receptors in vitro (clozapine; olanzapine; sertindole), but was not displaced by antipsychotics lacking high 5-HT(6) receptor affinities (risperidone; haloperidol; quetiapine). No specific binding to mouse brain tissue in vivo could be obtained. In conclusion, [(3)H]Lu AE60157 is suitable for measuring in vivo occupancies of 5-HT(6) receptor ligands in rat brain regions in which 5-HT(2A) receptors do not interfere.

Comparing Pharmacological Modulation of Sensory Gating in Healthy Humans and Rats: The Effects of Reboxetine and Haloperidol

Sensory gating is the brain’s ability to filter out irrelevant information before it reaches high levels of conscious processing. In the current study we aimed to investigate the involvement of the noradrenergic and dopaminergic neurotransmitter systems in sensory gating. Furthermore, we investigated cross-species reliability by comparing effects in both healthy humans and rats, while keeping all experimental conditions as similar as possible between the species. The design of the human experiment (n=21) was a double-blind, placebo-controlled, cross-over study where sensory gating was assessed following a dose of either reboxetine (8 mg), haloperidol (2 mg), their combination or placebo at four separate visits. Similarly in the animal experiment sensory gating was assessed in rats, (n=22) following a dose of reboxetine (2 mg/kg), haloperidol (0.08 mg/kg), their combination or placebo. The sensory gating paradigms in both experiments were identical. In humans, we found significantly reduced P50 suppression following separate administration of reboxetine or haloperidol, while their combined administration did not reach statistical significance compared with placebo. In the rats, we found a similar significant reduction of sensory gating (N40) following treatment with haloperidol and the combination of haloperidol and reboxetine, but not with separate reboxetine treatment, compared with placebo. Our study indicates that even when experimental conditions are kept as similar as possible, direct human to rat cross-species translation of pharmacological effects on sensory gating is challenging, which calls for more focussed research in this important translational area.