Established amyloid-β pathology is unaffected by chronic treatment with the selective serotonin reuptake inhibitor paroxetine

Introduction

Treatment with selective serotonin reuptake inhibitors has been suggested to mitigate amyloid-β (Aβ) pathology in Alzheimer's disease, in addition to an antidepressant mechanism of action.

Methods

We investigated whether chronic treatment with paroxetine, a selective serotonin reuptake inhibitor, mitigates Aβ pathology in plaque-bearing double-transgenic amyloid precursor protein (APP)_swe/presenilin 1 (PS1)_ΔE9 mutants. In addition, we addressed whether serotonin depletion affects Aβ pathology. Treatments were assessed by measurement of serotonin transporter occupancy and high-performance liquid chromatography. The effect of paroxetine on Aβ pathology was evaluated by stereological plaque load estimation and Aβ₄₂/Aβ₄₀ ratio by enzyme-linked immunosorbent assay.

Results

Contrary to our hypothesis, paroxetine therapy did not mitigate Aβ pathology, and depletion of brain serotonin did not exacerbate Aβ pathology. However, chronic paroxetine therapy increased mortality in APP_swe/PS1_ΔE9 transgenic mice.

Discussion

Our results question the ability of selective serotonin reuptake inhibitor therapy to ameliorate established Aβ pathology. The severe adverse effect of paroxetine may discourage its use for disease-modifying purposes in Alzheimer's disease.

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Chronic paroxetine treatment does not mitigate established amyloid-β pathology in APP_swe/PS1_ΔE9 mice.

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Chronic paroxetine treatment increases the mortality of APP_swe/PS1_ΔE9 but not littermate wild-type mice.

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Serotonergic depletion does not influence established amyloid-β pathology in APP_swe/PS1_ΔE9 mice.

Altered Volatile Organic Compound Profile in Transgenic Rats Bearing A53T Mutation of Human α-Synuclein: Comparison with Dopaminergic and Serotonergic Denervation

Early diagnosis of Parkinson's disease (PD) is of great importance due its progressive phenotype. Neuroprotective drugs could potentially slow down disease progression if used at early stages. Previously, we have reported an altered content of volatile organic compounds (VOCs) in the breath of rats following a 50% reduction in striatal dopamine (DA) content induced by 6-hydroxydopamine. We now report on the difference in the breath-print and content of VOCs between rats with mild and severe lesions of DA neurons, serotonergic neuronal lesions, and transgenic (Tg) rats carrying the PD-producing A53T mutation of the SNCA (α-synuclein) gene. The Tg rats had an increased content of 3-octen-1-ol and 4-chloro-3-methyl phenol in blood, while in brain tissue, hexanal, hexanol, and 2,3-octanedione were present in controls but absent in Tg rats. Levels of 1-heptyl-2-methyl cyclopropane were increased in brain tissue of Tg rats. The data confirm the potential of breath analysis for detection of human idiosyncratic as well as autosomal dominant PD.

Serotonergic nerve fibers in L-DOPA-derived dopamine release and dyskinesia

The 5-HT (5-hydroxytryptamine) system has been assigned a key role in the development of 3,4-dihydroxyphenyl-l-alanine (l-DOPA)-induced dyskinesia, mainly due to 5-HT neuronal ability to decarboxylate l-DOPA into dopamine. Nevertheless, knowledge of l-DOPA-induced events that could lead to development of dyskinesias are limited and therefore the present work has evaluated (i) the role of the 5-HT system in l-DOPA-derived dopamine synthesis when dopamine neurons are present, (ii) l-DOPA-induced effects on striatal dopamine release and clearance, and on 5-HT nerve fiber density, and (iii) the behavioral outcome of altered 5-HT transmission in dyskinetic rats. Chronoamperometric recordings demonstrated attenuated striatal l-DOPA-derived dopamine release (∼30%) upon removal of 5-HT nerve fibers in intact animals. Interestingly, four weeks of daily l-DOPA treatment yielded similar-sized dopamine peak amplitudes in intact animals as found after a 5-HT-lesion. Moreover, chronic l-DOPA exposure attenuated striatal 5-HT nerve fiber density in the absence of dopamine nerve terminals. Furthermore, fluoxetine-induced altered 5-HT transmission blocked dyskinetic behavior via action on 5-HT1A receptors. Taken together, the results indicate a central role for the 5-HT system in l-DOPA-derived dopamine synthesis and in dyskinesia, and therefore potential l-DOPA-induced deterioration of 5-HT function might reduce l-DOPA efficacy as well as promote the upcoming of motor side effects.

Hippocampal serotonin depletion unmasks differences in the hyperlocomotor effects of phencyclidine and MK-801: quantitative versus qualitative analyses

Antagonism of N-methyl-D-aspartate (NMDA) receptors by phencyclidine (PCP) is thought to underlie its ability to induce a schizophrenia-like syndrome in humans, yet evidence indicates it has a broader pharmacological profile. Our previous lesion studies highlighted a role for serotonergic projections from the median, but not dorsal, raphe nucleus in mediating the hyperlocomotor effects of PCP, without changing the action of the more selective NMDA receptor antagonist, MK-801. Here we compared locomotor responses to PCP and MK-801 in rats that were administered 5,7-dihydroxytryptamine (5,7-DHT) into either the dorsal or ventral hippocampus, which are preferentially innervated by median and dorsal raphe, respectively. Dorsal hippocampus lesions potentiated PCP-induced hyperlocomotion (0.5, 2.5 mg/kg), but not the effect of MK-801 (0.1 mg/kg). Ventral hippocampus lesions did not alter the hyperlocomotion elicited by either compound. Given that PCP and MK-801 may induce different spatiotemporal patterns of locomotor behavior, together with the known role of the dorsal hippocampus in spatial processing, we also assessed whether the 5,7-DHT-lesions caused any qualitative differences in locomotor responses. Treatment with PCP or MK-801 increased the smoothness of the path traveled (reduced spatial d) and decreased the predictability of locomotor patterns within the chambers (increased entropy). 5,7-DHT-lesions of the dorsal hippocampus did not alter the effects of PCP on spatial d or entropy – despite potentiating total distance moved – but caused a slight reduction in levels of MK-801-induced entropy. Taken together, serotonergic lesions targeting the dorsal hippocampus unmask a functional differentiation of the hyperlocomotor effects of PCP and MK-801. These findings have implications for studies utilizing NMDA receptor antagonists in modeling glutamatergic dysfunction in schizophrenia.

Attenuation of the effects of d-amphetamine on interval timing behavior by central 5-hydroxytryptamine depletion

RATIONALE

Interval timing in the free-operant psychophysical procedure is sensitive to the monoamine-releasing agent d-amphetamine, the D(2)-like dopamine receptor agonist quinpirole, and the D(1)-like agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzepine (SKF-81297). The effect of d-amphetamine can be antagonized by selective D(1)-like and 5-HT(2A) receptor antagonists. It is not known whether d-amphetamine's effect requires an intact 5-hydroxytryptamine (5-HT) pathway.

OBJECTIVE

The objective of this study was to examine the effects of d-amphetamine, quinpirole, and SKF-81297 on timing in intact rats and rats whose 5-hydroxytryptaminergic (5-HTergic) pathways had been ablated.

MATERIALS AND METHODS

Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data for derivation of timing indices (T(50), time corresponding to %B = 50%; Weber fraction). The effects of d-amphetamine (0.4 mg kg(-1) i.p.), quinpirole (0.08 mg kg(-1) i.p.), and SKF-81297 (0.4 mg kg(-1) s.c.) were compared between intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine.

RESULTS

Quinpirole and SKF-81297 reduced T(50) in both groups; d-amphetamine reduced T(50) only in the sham-lesioned group. The lesion reduced 5-HT levels by 80%; catecholamine levels were not affected.

CONCLUSIONS

d-Amphetamine's effect on performance in the free-operant psychophysical procedure requires an intact 5-HTergic system. 5-HT, possibly acting at 5-HT(2A) receptors, may play a 'permissive' role in dopamine release.

Effect of serotonin depletion on the neuronal, endocrine and behavioural responses to corticotropin-releasing factor in the rat

Interactions between serotonin and corticotropin-releasing factor (CRF) have been demonstrated by various studies in different parts of the brain. Both are activated by stressful stimuli. Additionally, serotoninergic fibres directly synapse with the parvocellular division of the paraventricular nucleus (PVN) that mainly synthesize CRF. The functional impact of this serotonin-CRF interaction on CRF-induced responses remains unclear. CRF infusion into the brain evokes a specific pattern of behavioural, endocrine and neuronal changes that resemble those following various forms of stress. The aim of the present study was to investigate the effects of serotonin depletion on acute CRF-induced c-fos expression, corticosterone levels and behavioural responses. Lateral ventricular (i.c.v.) infusion of CRF (250 pmol) resulted in a significant increase in grooming, corticosterone and c-fos mRNA in the PVN. Adequate and specific depletion of serotonin using 5,7-DHT did not alter these CRF-induced changes. These data suggest that acute responses induced by i.c.v. CRF are independent of basal levels of serotonin.

Serotonin depletion in vivo inhibits the branching of olfactory projection neurons in the lobster deutocerebrum

Serotonin depletion during embryogenesis has been shown previously to retard the growth of the olfactory and accessory lobes of the lobster deutocerebrum (Benton et al., 1997). The present study was undertaken to determine whether morphological changes in the interneurons innervating these lobes contribute to this growth retardation. We examined the effects of in vivo serotonin depletion using 5,7-dihydroxytryptamine (5,7-DHT) on the morphology of the olfactory projection neurons, one of two major classes of interneurons that innervate both lobes. Intracellular dye fills of olfactory projection neurons in normal embryos showed that each neuron extensively innervates either the olfactory or accessory lobe before projecting to neuropil regions in the protocerebrum. In embryos injected with 5,7-DHT, however, the deutocerebral arbors of 13.5% of the olfactory projection neurons examined were either markedly reduced compared with normal neurons or absent. Affected neurons also exhibited a number of additional aberrant morphological features suggesting that these neurons represent cells that were affected during their initial morphogenesis. Olfactory projection neurons with aberrant morphologies were also encountered, although less frequently (7.5% of the neurons examined), in control (sham-injected) embryos indicating that the sham injections can affect the development of the brain. This observation provides insights into the nature of effects seen in control embryos in previous experiments (Benton et al., 1997). The results of the present study indicate that in vivo serotonin depletion inhibits the branching of olfactory projection neurons and suggest, therefore, that one of the functions of serotonin during normal development is to promote the ingrowth of these neurons into the deutocerebral neuropils.

Pindolol-insensitive [3H]-5-hydroxytryptamine binding in the rat hypothalamus; identity with 5-hydroxytryptamine7 receptors

Pindolol-insensitive [3H]-5-hydroxytryptamine ([3H]-5-HT) binding to rat hypothalamic membranes was pharmacologically and functionally characterized to resolve whether this procedure selectively labels 5-HT7 receptors. Consistent with a previous report, 3 microM and not 100 nM pindolol was required to occupy fully 5-HT1A and 5-HT1B receptors. Remaining [3H]-5-HT binding was saturable (KD, 1.59+/-0.21 nM; Bmax, 53.8+/-3.1 fmol x mg protein(-1)). Displacement of [3H]-5-HT with metergoline and 5-CT revealed shallow Hill slopes (<0.5) but seven other compounds had slopes >0.8 and pKi values and the rank order of affinity were significantly correlated (r = 0.81 and 0.93, respectively) with published [3H]-5-HT binding to rat recombinant 5-HT7 receptors. In the presence of pindolol, 5-HT-enhanced accumulation of [32P]-cyclic AMP was unaffected by the 5-HT4 antagonist RS39604 (0.1 microM) or the 5-ht6 antagonist Ro 04-6790 (1 microM) but significantly attenuated by mesulergine (250 nM), ritanserin (450 nM) or methiothepin (200 nM) which have high affinity for the 5-HT7 receptor. Intracerebroventricular pretreatment with the serotonergic neurotoxin 5,7-dihydroxytryptamine, 5,7-DHT, elevated the [3H]-5-HT Bmax 2 fold, indicating that the hypothalamic 5-HT7 receptor is post-synaptic to 5-HT nerve terminals and regulated by synaptic 5-HT levels. These results suggest that, in the presence of 3 microM pindolol, [3H]-5-HT selectively labels hypothalamic binding sites consistent with functional 5-HT7 receptors.

Opposite change of in vivo dopamine release in the rat nucleus accumbens and striatum that follows electrical stimulation of dorsal raphe nucleus: role of 5-HT3 receptors

In the present study we investigate, using in vivo microdialysis, the involvement of central 5-HT3 receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3, 4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindole-3-acetic acid (5-HIAA) extracellular levels monitored in the nucleus accumbens and the striatum of halothane-anesthetized rats. DRN stimulation (300 microA, 1 msec at 3, 5, 10, and 20 Hz for 15 min) induced a frequency-dependent increase of accumbal DA release and a concomitant reduction of DA release in the ipsilateral striatum at 20 Hz. In both structures DOPAC and 5-HIAA dialysate contents were enhanced in a frequency-dependent manner. Central serotonin (5-HT) depletion, induced by intra-raphe injections of 5, 7-dihydroxytryptamine neurotoxin, abolished the effect of 20 Hz DRN stimulation on DA, DOPAC, and 5-HIAA extracellular levels in both regions. The 5-HT synthesis inhibitor para-chlorophenylalanine (3 x 400 mg/kg, i.p., for 3 d), although preventing the effect on DA release, failed to modify significantly the effect of 20 Hz DRN stimulation on DOPAC and 5-HIAA outflow in both structures. Ondansetron (0.1 and 1 mg/kg) and (S)-zacopride (0.1 mg/kg), two 5-HT3 antagonists, significantly impaired the increase of accumbal DA release induced by 20 Hz DRN stimulation but did not affect either the decrease of striatal DA release or the increase in DOPAC outflow in both structures. These results indicate that an enhancement of central 5-HT transmission induced by DRN stimulation differentially affects striatal and accumbal DA release and that endogenous 5-HT, via its action on 5-HT3 receptors, exerts a facilitatory control restricted to the mesoaccumbal DA pathway.

Early postnatal administration of 5,7-dihydroxytryptamine: effects on substance P and thyrotropin-releasing hormone neurons and terminals in rat brain

Substance P, thyrotropin-releasing hormone (TRH) and serotonin are putative neurotransmitters which have been proposed to co-exist in some brain neurons. Our previous immunocytochemical and biochemical studies have demonstrated that 85-100% of all serotonin neurons are destroyed following neonatal 5,7-dihydroxtryptamine (5,7-DHT) treatment. In this study, we have determined the effect of neonatal 5,7-DHT and desipramine (DMI) treatment on the biochemical content and immunocytochemical localization of substance P and TRH throughout the brain. Interestingly, we have observed that virtually all substance P- and TRH-immunoreactive cells in the ventral pons-medulla are destroyed by the neurotoxin. However, peptide-containing neurons in other regions were not affected. Additionally, we measured the peptide content and found that TRH is significantly decreased in the spinal cord (-50%) and pons-medulla (-20%), but not in other brain regions. Substance P content was not significantly altered in any region, even after a greater than 90% reduction of serotonin. These data indicate that the co-localized substance P and TRH forms a small proportion of the total peptide in brain.

Involvement of brain monoamines in the stimulant and paradoxical inhibitory effects of methylphenidate

The significance of central noradrenergic, dopaminergic and serotonergic neural systems for the locomotor stimulant effects of methylphenidate was investigated in the rat. In order to study the role of brain catecholamines, rats were pretreated with reserpine (2.5 mg/kg) followed 24 hrs later by treatment with alpha-methyltyrosine (25 mg/kg) or U-14,624 (75 mg/kg), a dopamine-beta-hydroxylase inhibitor. In these experiments, methylphenidate stimulated motor activity was antagonized by alpha-methyltyrosine and enhanced after treatment with U-14,624, suggesting that release of newly synthesized dopamine is important to a locomotor stimulant action of methylphenidate. Evidence implicating brain serotonin in the actions of methylphenidate was obtained in rats pretreated with pargyline or p-chlorophenylalanine (PCPA). Administration of pargyline 1 hr prior to methylphenidate was found to reduce the locomotor activity induced by methylphenidate and this was antagonized by pretreatment with low doses of PCPA. Higher doses of PCPA caused a significant elevation of methylphenidate induced activity which could be reduced by 5-hydroxytryptophan. Destruction of serotonergic neurons with 5,7-dihydroxytryptamine also potentiated methylphenidate induced locomotion. These latter findings suggest that serotonergic fibers have an inhibitory function in brain. These results are discussed in relation to the possible mechanism by which methylphenidate may act in hyperkinesis.