Postmortem serotoninergic correlates of cognitive decline in Alzheimer's disease

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Does serotonin augmentation have any effect on cognition and activities of daily living in Alzheimer's dementia? A double-blind, placebo-controlled clinical trial

OBJECTIVE

Recent studies suggest that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits, and other neuronal systems like monoaminergic hypofunction are involved. In several studies, selective serotonin reuptake inhibitors demonstrated promotion in neurogenesis in the hippocampus and enhanced memory and cognition. The aim of this study is to survey the effect of serotonin augmentation on cognition and activities of daily living in patients with Alzheimer's disease.

METHOD

The trial was designed as a 12-week randomized, placebo-controlled, double-blind study. One hundred twenty-two patients aged 55 to 85 years with mild-to-moderate Alzheimer's dementia were randomly allocated in 1 of the 3 treatment groups: fluoxetine plus rivastigmine, rivastigmine alone, or placebo group. Efficacy measures comprised assessments of cognition, activities of daily living, and global functioning. Hamilton Depression Scale also was used to assess changes in mood throughout the study.

RESULT

Fluoxetine plus rivastigmine and rivastigmine groups demonstrated improvement on measures of cognitive and memory without any significant difference; however, the former group did better in their activities of daily living and global functioning. Patients taking placebo had significant deterioration in all the efficacy measures. Patients taking rivastigmine or rivastigmine plus fluoxetine had improvements in Hamilton Depression Scale without significant differences.

CONCLUSIONS

Concomitant use of selective serotonin-enhancing agents and acetyl cholinesterase inhibitors can provide greater benefit in activities of daily living and global functioning in patients with cognitive impairment. Because our study is preliminary, larger double-blind studies are needed to confirm the results.

Opposing effects of AMPA and 5-HT1A receptor blockade on passive avoidance and object recognition performance: correlation with AMPA receptor subunit expression in rat hippocampus

It has been suggested that antagonists at serotonin 5-HT1A receptors may exert a procognitive effect by facilitating glutamatergic neurotransmission. Here we further explored this issue by looking for the ability of a 5-HT1A antagonist to prevent the learning deficit induced by AMPA receptor blockade in two behavioural procedures in rats, and for concomitant molecular changes presumably involved in memory formation in the hippocampus. Pretraining administration of the competitive AMPA receptor antagonist, NBQX, produced a dose-related retention impairment in a passive avoidance task 24h later, and also impaired retention in a novel object recognition test when an intertrial interval of 3h was selected. Pretreatment with the selective 5-HT1A receptor antagonist, WAY-100635, prevented the learning deficit induced by NBQX in the two behavioural procedures. In biochemical studies performed on rat hippocampus after the retention tests, we found that learning increased the membrane levels of AMPA receptor GluR1 and GluR2/3 subunits, as well as the phosphorylated forms of GluR1, effects that were abolished by NBQX administration before the training session. Pretreatment with WAY-100635 counteracted the NBQX effects and restored the initial learning-specific increase in Ca2+/calmodulin-dependent protein kinase II (CaMKII) function and the later increase in GluR2/3 and phosphorylated GluR1 surface expression. Moreover, administration of WAY-100635 before object recognition training improved recognition memory 24h later and potentiated the learning-associated increase in AMPA receptor subunits. The results support the proposed utility of 5-HT1A antagonists in the treatment of cognitive disorders.

Serotonin 5-HT1A receptor blockade enhances Ca2+/calmodulin-dependent protein kinase II function and membrane expression of AMPA receptor subunits in the rat hippocampus: implications for memory formation

Stimulation of hippocampal 5-HT(1A) receptors impairs memory retention. The highly selective 5-HT(1A) antagonist, WAY-100635, prevents the cognitive deficits induced not only by 5-HT(1A) stimulation but also by cholinergic or NMDA receptor blockade. On this basis, the effects of WAY-100635 on molecular events associated with memory storage were explored. In rat hippocampus, WAY-100635 produced a rapid increase in phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and in Ca(2+)-independent CaMKII and protein kinase A (PKA) enzyme activity. This increase was followed a few hours later by an enhanced membrane expression of AMPA receptor subunits, especially of the GluR1 subunit phosphorylated at the CaMKII site, pGluR1(Ser831). The same qualitative effects were found with the weaker 5-HT(1A) antagonist NAN-190. The effects of both antagonists were no longer apparent in rats with a previous 5-HT depletion induced by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA), suggesting that 5-HT(1A) receptor blockade removes the tonic inhibition of 5-HT through 5-HT(1A) receptor stimulation on excitatory hippocampal neurons, with the consequent increase in PKA activity. In addition, administration of WAY-100635 potentiated the learning-specific increase in the hippocampus of phospho-CaMKII, Ca(2+)-independent CaMKII activity, as well as the phosphorylation of either the CaMKII or the PKA site on the AMPA receptor GluR1 subunit. This study suggests that blockade of hippocampal 5-HT(1A) receptors favours molecular events critically involved in memory formation, and provides an in vivo molecular basis for the proposed utility of 5-HT(1A) receptor antagonists in the treatment of cognitive disorders.

Analysis of the role of 5-HT1A receptors in spatial and aversive learning in the rat

The role of the brain 5-HT1A receptor in cognition was examined in the water maze (WM) and passive avoidance (PA) tasks in the male rat. Pre-training administration of the 5-HT1A receptor agonist 8-OH-DPAT impaired WM performance and facilitated PA retention at low doses (0.01 and 0.03 mg/kg) and impaired PA retention at higher doses (0.1-1.0 mg/kg). The 5-HT1A receptor antagonist NAD-299 produced a dose-dependent facilitation of PA retention. In contrast, the 5-HT1A receptor antagonists NAD-299 and WAY-100635 failed to alter acquisition and retention in the WM. The impairments in WM and PA (but not facilitation in PA) induced by 8-OH-DPAT were blocked by NAD-299. Furthermore, NAD-299 prevented the PA impairments induced by the muscarinic antagonist scopolamine or the NMDA receptor antagonist MK-801. In contrast, NAD-299 and WAY-100635 failed to attenuate the WM impairment induced by scopolamine, probably due to the failure of 5-HT1A receptor blockade to attenuate the sensorimotor disturbances induced by scopolamine. These results indicate that 5-HT1A receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that 5-HT1A receptor blockade can facilitate some aspects of cognitive function, probably via modulation of cholinergic and glutamatergic transmissions. This suggests that 5-HT1A receptor antagonists may have a potential role in the treatment of human degenerative disorders associated with cognitive deficits.

Loss of serotonin 5-HT2A receptors in the postmortem temporal cortex correlates with rate of cognitive decline in Alzheimer's disease

RATIONALE

Previous studies have demonstrated reductions of serotonin 5-HT 2A receptors in the neocortex of Alzheimer's disease (AD) patients. However, it is unclear whether such losses play a role in the cognitive decline of AD.

OBJECTIVES

To correlate neocortical 5-HT 2A receptor alterations with cognitive decline in AD.

METHODS

Postmortem frontal and temporal cortical 5-HT 2A receptors were measured by [3H]ketanserin binding in aged controls as well as in a cohort of AD patients who had been longitudinally assessed for cognitive decline and behavioral symptoms.

RESULTS

5-HT 2A receptor densities in both regions were reduced in severely demented AD patients compared to age-matched controls. In the temporal cortex, this reduction also correlated with the rate of decline of Mini-Mental State Examination (MMSE) scores. The association between 5-HT 2A receptor loss and cognitive decline was independent of the effects of choline acetyltransferase (ChAT) activity and presence of behavioral symptoms.

CONCLUSIONS

Our data suggest that loss of neocortical 5-HT 2A receptors may predict for faster cognitive decline in AD, and point to serotomimetics as potentially useful adjuvants to cholinergic replacement therapies.

Cholinergic-serotonergic imbalance contributes to cognitive and behavioral symptoms in Alzheimer's disease

Neuropsychiatric symptoms seen in Alzheimer's disease (AD) are not simply a consequence of neurodegeneration, but probably result from differential neurotransmitter alterations, which some patients are more at risk of than others. Therefore, the hypothesis of this study is that an imbalance between the cholinergic and serotonergic systems is related to cognitive symptoms and psychological syndromes of dementia (BPSD) in patients with AD. Cholinergic and serotonergic functions were assessed in post-mortem frontal and temporal cortex from 22 AD patients who had been prospectively assessed with the Mini-Mental State examination (MMSE) for cognitive impairment and with the Present Behavioral Examination (PBE) for BPSD including aggressive behavior, overactivity, depression and psychosis. Not only cholinergic deficits, but also the cholinacetyltransferase/serotonin ratio significantly correlated with final MMSE score both in frontal and temporal cortex. In addition, decreases in cholinergic function correlated with the aggressive behavior factor, supporting a dual role for the cholinergic system in cognitive and non-cognitive disturbances associated to AD. The serotonergic system showed a significant correlation with overactivity and psychosis. The ratio of serotonin to acetylcholinesterase levels was also correlated with the psychotic factor at least in women. It is concluded that an imbalance between cholinergic-serotonergic systems may be responsible for the cognitive impairment associated to AD. Moreover, the major findings of this study are the relationships between neurochemical markers of both cholinergic and serotonergic systems and non-cognitive behavioral disturbances in patients with dementia.

Modifications of 5-HT4 receptor expression in rat brain during memory consolidation

Pharmacological evidence indicates a specific role of 5-HT(4) receptors on memory function. These receptors are members of G-protein-coupled 7-transmembrane domain receptor superfamily, are positively coupled to adenylyl cyclase, and are heterogeneously located in some structures important for memory, such as the hippocampus and cortical regions. To further clarify 5-HT(4) receptors' role in memory, the expression of these receptors in passive (P3) untrained and autoshaping (A3) trained (3 sessions) adult (3 months) and old (P9 or A9; 9 months) male rats was determined by autoradiography. Adult trained (A3) rats showed a better memory respect to old trained (A9). Using [(3)H] GR113808 as ligand (0.2 nM specific activity 81 Ci/mmol) for 5-HT(4) receptor expression, 29 brain areas were analyzed, 16 areas of A3 and 17 of A9 animals displayed significant changes. The medial mammillary nucleus of A3 group showed diminished 5-HT(4) receptor expression, and in other 15 brain areas of A3 or 10 of A9 animals, 5-HT(4) receptors were increased. Thus, for A3 rats, 5-HT(4) receptors were augmented in olfactory lobule, caudate putamen, fundus striatum, CA2, retrosplenial, frontal, temporal, occipital, and cingulate cortex. Also, 5-HT(4) receptors were increased in olfactory tubercule, hippocampal CA1, parietal, piriform, and cingulate cortex of A9. However, hippocampal CA2 and CA3 areas, and frontal, parietal, and temporal cortex of A9 rats, expressed less 5-HT(4) receptors. These findings suggest that serotonergic activity, via 5-HT(4) receptors in hippocampal, striatum, and cortical areas, mediates memory function and provides further evidence for a complex and regionally specific regulation over 5-HT receptor expression during memory formation.

Differential involvement of 5-HT(1B/1D) and 5-HT6 receptors in cognitive and non-cognitive symptoms in Alzheimer's disease

Growing evidence suggests that a compromised serotonergic system plays an important role in the pathophysiology of Alzheimer's disease (AD). We assessed the expression of 5-HT(1B/1D) and 5-HT(6) receptors and cholinacetyltransferase (ChAT) activity in post-mortem frontal and temporal cortex from AD patients who had been prospectively assessed for cognitive function using the Mini-Mental State Examination (MMSE) and behavioral changes using the Present Behavioral Examination (PBE). 5-HT(1B/1D) and 5-HT(6) receptor densities were significantly reduced in both cortical areas. 5-HT(1B/1D) receptor density was correlated to MMSE decline in the frontal cortex, supporting its implication in memory impairment. The best predictor for lowered 5-HT(6) receptor density in the temporal cortex was the PBE measure of overactivity. The 5-HT(6)/ChAT ratio was related to aggression both in the frontal and temporal cortex. Therefore, antagonists acting at 5-HT(6) receptors could be useful in the treatment of non-cognitive symptoms associated to AD.

Spatial Learning in the 5-HT1B Receptor Knockout Mouse: Selective Facilitation/Impairment Depending on the Cognitive Demand

Age-related memory decline is associated with a combined dysfunction of the cholinergic and serotonergic systems in the hippocampus and frontal cortex, in particular. The 5-HT1B receptor occupies strategic cellular and subcellular locations in these structures, where it plays a role in the modulation of ACh release. In an attempt to characterize the contribution of this receptor to memory functions, 5-HT1B receptor knockout (KO) mice were submitted to various behavioral paradigms carried out in the same experimental context (water maze), which were aimed at exposing mice to various levels of memory demand. 5-HT1BKO mice exhibited a facilitation in the acquisition of a hippocampal-dependent spatial reference memory task in the Morris water maze. This facilitation was selective of task difficulty, showing thus that the genetic inactivation of the 5-HT1B receptor is associated with facilitation when the complexity of the task is increased, and reveals a protective effect on age-related hippocampal-dependent memory decline. Young-adult and aged KO and wild-type (WT) mice were equally able to learn a delayed spatial matching-to-sample working memory task in a radial-arm water maze with short (0 or 5 min) delays. However, 5-HT1BKO mice, only, exhibited a selective memory impairment at intermediate and long (15, 30, and 60 min) delays. Treatment by scopolamine induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. Taken together, these studies revealed a beneficial effect of the mutation on the acquisition of a spatial reference memory task, but a deleterious effect on a working memory task for long delays. This 5-HT1BKO mouse story highlights the problem of the potential existence of "global memory enhancers."

A pharmacological analysis of an associative learning task: 5-HT(1) to 5-HT(7) receptor subtypes function on a pavlovian/instrumental autoshaped memory

Recent studies using both invertebrates and mammals have revealed that endogenous serotonin (5-hydroxytryptamine [5-HT]) modulates plasticity processes, including learning and memory. However, little is currently known about the mechanisms, loci, or time window of the actions of 5-HT. The aim of this review is to discuss some recent results on the effects of systemic administration of selective agonists and antagonists of 5-HT on associative learning in a Pavlovian/instrumental autoshaping (P/I-A) task in rats. The results indicate that pharmacological manipulation of 5-HT1-7 receptors or 5-HT reuptake sites might modulate memory consolidation, which is consistent with the emerging notion that 5-HT plays a key role in memory formation.

Reduced serotonin 5-HT1A receptor binding in the temporal cortex correlates with aggressive behavior in Alzheimer disease

Previous studies have implicated brain serotonin 5-HT(1A) receptors in several CNS functions, including cognition, mood and emotional states. In Alzheimer disease (AD), cognitive impairment and behavioral symptoms are the main clinical features. However, the biochemical basis of such changes is poorly understood. Results from recent in vivo studies suggest that 5-HT(1A) receptors may be related to aggressive traits in healthy subjects. The present study investigated the state of 5-HT(1A) receptors in the postmortem neocortex of 33 AD patients prospectively assessed for cognition and behavioral symptoms, together with 20 matched controls, by saturation [(3)H]8-OH-DPAT binding assays. 5-HT(1A) receptor binding affinity (K(D)) and density (B(max)) were unchanged in the overall AD group compared with controls. Within the AD group, 5-HT(1A) receptor B(max) in the temporal cortex inversely correlated with aggression and dementia severity. However, multiple regression analyses showed that 5-HT(1A) receptor B(max) remained the best predictor for aggression, while temporal cortical neurofibrillary tangle grading was the best predictor for dementia severity. This suggests that 5-HT(1A) receptor alteration is directly related to aggression in AD, while dementia severity is more strongly related to the neurodegenerative process. Our data indicate further study of 5-HT(1A) receptors as a pharmacological target for the treatment of behavioral symptoms in AD.