The effects of aging on hippocampal and cortical projections of the forebrain cholinergic system

Age-related differences in long-term potentiation-like plasticity and short-latency afferent inhibition and their association with cognitive function

Background

The neurophysiological differences in cortical plasticity and cholinergic system function due to ageing and their correlation with cognitive function remain poorly understood.

Aims

To reveal the differences in long-term potentiation (LTP)-like plasticity and short-latency afferent inhibition (SAI) between older and younger individuals, alongside their correlation with cognitive function using transcranial magnetic stimulation (TMS).

Methods

The cross-sectional study involved 31 younger adults aged 18-30 and 46 older adults aged 60-80. All participants underwent comprehensive cognitive assessments and a neurophysiological evaluation based on TMS. Cognitive function assessments included evaluations of global cognitive function, language, memory and executive function. The neurophysiological assessment included LTP-like plasticity and SAI.

Results

The findings of this study revealed a decline in LTP among the older adults compared with the younger adults (wald χ2=3.98, p=0.046). Subgroup analysis further demonstrated a significant reduction in SAI level among individuals aged 70-80 years in comparison to both the younger adults (SAI(N20): (t=-3.37, p=0.018); SAI(N20+4): (t=-3.13, p=0.038)) and those aged 60-70 (SAI(N20): (t=-3.26, p=0.025); SAI(N20+4): (t=-3.69, p=0.006)). Conversely, there was no notable difference in SAI level between those aged 60-70 years and the younger group. Furthermore, after employing the Bonferroni correction, the correlation analysis revealed that only the positive correlation between LTP-like plasticity and language function (r=0.61, p<0.001) in the younger group remained statistically significant.

Conclusions

During the normal ageing process, a decline in synaptic plasticity may precede cholinergic system dysfunction. In individuals over 60 years of age, there is a reduction in LTP-like plasticity, while a decline in cholinergic system function is observed in those over 70. Thus, the cholinergic system may play a vital role in preventing cognitive decline during normal ageing. In younger individuals, LTP-like plasticity might represent a potential neurophysiological marker for language function.

Cognitive Deficits in Aging Related to Changes in Basal Forebrain Neuronal Activity

Aging is a physiological process accompanied by a decline in cognitive performance. The cholinergic neurons of the basal forebrain provide projections to the cortex that are directly engaged in many cognitive processes in mammals. In addition, basal forebrain neurons contribute to the generation of different rhythms in the EEG along the sleep/wakefulness cycle. The aim of this review is to provide an overview of recent advances grouped around the changes in basal forebrain activity during healthy aging. Elucidating the underlying mechanisms of brain function and their decline is especially relevant in today's society as an increasingly aged population faces higher risks of developing neurodegenerative diseases such as Alzheimer's disease. The profound age-related cognitive deficits and neurodegenerative diseases associated with basal forebrain dysfunction highlight the importance of investigating the aging of this brain region.

Compensatory cognition in neurological diseases and aging: A review of animal and human studies

Specialized individual circuits in the brain are recruited for specific functions. Interestingly, multiple neural circuitries continuously compete with each other to acquire the specialized function. However, the dominant among them compete and become the central neural network for that particular function. For example, the hippocampal principal neural circuitries are the dominant networks among many which are involved in learning processes. But, in the event of damage to the principal circuitry, many times, less dominant networks compensate for the primary network. This review highlights the psychopathologies of functional loss and the aspects of functional recuperation in the absence of the hippocampus.

Revisiting Preclinical Observations of Several Histamine H3 Receptor Antagonists/Inverse Agonists in Cognitive Impairment, Anxiety, Depression, and Sleep-Wake Cycle Disorder

A relationship appears to exist between dysfunction of brain histamine (HA) and various neuropsychiatric brain disorders. The possible involvement of brain HA in neuropathology has gained attention recently, and its role in many (patho)physiological brain functions including memory, cognition, and sleep-wake cycle paved the way for further research on the etiology of several brain disorders. Histamine H3 receptor (H3R) evidenced in the brains of rodents and humans remains of special interest, given its unique position as a pre- and postsynaptic receptor, controlling the synthesis and release of HA as well as different other neurotransmitters in different brain regions, respectively. Despite several disappointing outcomes for several H3R antagonists/inverse agonists in clinical studies addressing their effectiveness in Alzheimer's disease (AD), Parkinson's disease (PD), and schizophrenia (SCH), numerous H3R antagonists/inverse agonists showed great potentials in modulating memory and cognition, mood, and sleep-wake cycle, thus suggesting its potential role in neurocognitive and neurodegenerative diseases such as AD, PD, SCH, narcolepsy, and major depression in preclinical rodent models. In this review, we present preclinical applications of selected H3R antagonists/inverse agonists and their pharmacological effects on cognitive impairment, anxiety, depression, and sleep-wake cycle disorders. Collectively, the current review highlights the behavioral impact of developments of H3R antagonists/inverse agonists, aiming to further encourage researchers in the preclinical drug development field to profile the potential therapeutic role of novel antagonists/inverse agonists targeting histamine H3Rs.

Cholinergic receptor binding in unimpaired older adults, mild cognitive impairment, and Alzheimer's disease dementia

BACKGROUND

Cholinergic neurotransmitter system dysfunction contributes to cognitive impairment in Alzheimer's disease and other syndromes. However, the specific cholinergic mechanisms and brain structures involved, time course of alterations, and relationships with specific cognitive deficits are not well understood.

METHODS

This study included 102 older adults: 42 cognitively unimpaired (CU), 28 with mild cognitive impairment (MCI), and 32 with Alzheimer's disease (AD) dementia. Each participant underwent a neuropsychological assessment. Regional brain α4β2 nicotinic cholinergic receptor binding (V_T/fp) was measured using 2-[¹⁸F]fluoro-3-(2(S)azetidinylmethoxy)pyridine (2FA) and PET imaging. Voxel-wise analyses of group differences were performed. Relationships between receptor binding and cognition, age, and cholinesterase inhibitor medication use were assessed using binding values in six prespecified regions of interest.

RESULTS

SPM analysis showed the group V_T/f_p binding differences in the bilateral entorhinal cortex, hippocampus, insula, anterior cingulate, thalamus, and basal ganglia (p < .05, FWE-corrected). Pairwise comparisons revealed lower binding in the AD group compared to the CU group in similar regions. Binding in the entorhinal cortex was lower in the MCI group than in the CU group; binding in the hippocampus was lower in the AD group than in the MCI group. AD participants taking cholinesterase inhibitor medication had lower 2FA binding in the bilateral hippocampus and thalamus compared to those not taking medication. In the CU group, age was negatively associated with 2FA binding in each region of interest (r_s = - .33 to - .59, p < .05 for each, uncorrected). Attention, immediate recall, and delayed recall scores were inversely associated with 2FA binding in most regions across the full sample. In the combined group of CU and MCI participants, attention was inversely associated with 2FA binding in most regions, beyond the effect of hippocampal volume.

CONCLUSIONS

Nicotinic cholinergic receptor binding in specific limbic and subcortical regions is lower in MCI and further reduced in AD dementia, compared to CU older adults, and is related to cognitive deficits. Cognitive decline with age may be a consequence of reduced cholinergic receptor density or binding affinity that may also promote vulnerability to other Alzheimer's processes. Contemporary modification of the "cholinergic deficit" of aging and AD may reveal opportunities to prevent or improve clinical symptoms.

Age and Anterior Basal Forebrain Volume Predict the Cholinergic Deficit in Patients with Mild Cognitive Impairment due to Alzheimer’s Disease

Background: Early and severe neuronal loss in the cholinergic basal forebrain is observed in Alzheimer’s disease (AD). To date, cholinomimetics play a central role in the symptomatic treatment of AD dementia. Although basic research indicates that a cholinergic deficit is present in AD before dementia, the efficacy of cholinomimetics in mild cognitive impairment (MCI) remains controversial. Predictors of cholinergic impairment could guide individualized therapy. Objective: To investigate if the extent of the cholinergic deficit, measured using positron emission tomography (PET) and the tracer 11C-N-methyl-4-piperidyl acetate (MP4A), could be predicted from the volume of cholinergic basal forebrain nuclei in non-demented AD patients. Methods: Seventeen patients with a high likelihood of MCI due to AD and 18 age-matched cognitively healthy adults underwent MRI-scanning. Basal forebrain volume was assessed using voxel-based morphometry and a cytoarchitectonic atlas of cholinergic nuclei. Cortical acetylcholinesterase (AChE) activity was measured using MP4A-PET. Results: Cortical AChE activity and nucleus basalis of Meynert (Ch4 area) volume were significantly decreased in MCI. The extent of the cholinergic deficit varied considerably across patients. Greater volumes of anterior basal forebrain nuclei (Ch1/2 area) and younger age (Spearman’s rho (17)  = –0.596, 95% -CI [–0.905, –0.119] and 0.593, 95% -CI [0.092, 0.863])) were associated with a greater cholinergic deficit. Conclusion: Data suggest that less atrophy of the Ch1/2 area and younger age are associated with a more significant cholinergic deficit in MCI due to AD. Further investigations are warranted to determine if the individual response to cholinomimetics can be inferred from these measures.

Reduced Insulin-Like Growth Factor-I Effects in the Basal Forebrain of Aging Mouse

It is known that aging is frequently accompanied by a decline in cognition. Furthermore, aging is associated with lower serum IGF-I levels that may contribute to this deterioration. We studied the effect of IGF-I in neurons of the horizontal diagonal band of Broca (HDB) of young (≤6 months old) and old (≥20-month-old) mice to determine if changes in the response of these neurons to IGF-I occur along with aging. Local injection of IGF-I in the HDB nucleus increased their neuronal activity and induced fast oscillatory activity in the electrocorticogram (ECoG). Furthermore, IGF-I facilitated tactile responses in the primary somatosensory cortex elicited by air-puffs delivered in the whiskers. These excitatory effects decreased in old mice. Immunohistochemistry showed that cholinergic HDB neurons express IGF-I receptors and that IGF-I injection increased the expression of c-fos in young, but not in old animals. IGF-I increased the activity of optogenetically-identified cholinergic neurons in young animals, suggesting that most of the IGF-I-induced excitatory effects were mediated by activation of these neurons. Effects of aging were partially ameliorated by chronic IGF-I treatment in old mice. The present findings suggest that reduced IGF-I activity in old animals participates in age-associated changes in cortical activity.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Intranasal insulin and orexins to treat age-related cognitive decline

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.

Nicotine and Cognition in Cognitively Normal Older Adults

The cholinergic system has been shown to be the primary neurotransmitter system which is responsible for the cognitive symptoms associated with dementia; its role in healthy non-demented older adults remains a gap in the literature. Understanding the effects of age-related functional changes on the nicotinic system will address this knowledge gap. As the older adult population grows and hence the importance of understanding cognitive changes that impact functional abilities and everyday life. In this article we examine the benefits of using nicotine as a method for improving cognition in non-demented healthy older adults which may have the potential for slowing neurodegeneration in aging. Furthermore, we discuss how nicotine can play a crucial role in maintaining cognitive abilities throughout normal cognitive aging.

MAP/ERK Signaling in Developing Cognitive and Emotional Function and Its Effect on Pathological and Neurodegenerative Processes

The signaling pathway of the microtubule-associated protein kinase or extracellular regulated kinase (MAPK/ERK) is a common mechanism of extracellular information transduction from extracellular stimuli to the intracellular space. The transduction of information leads to changes in the ongoing metabolic pathways and the modification of gene expression patterns. In the central nervous system, ERK is expressed ubiquitously, both temporally and spatially. As for the temporal ubiquity, this signaling system participates in three key moments: (i) Embryonic development; (ii) the early postnatal period; and iii) adulthood. During embryonic development, the system is partly responsible for the patterning of segmentation in the encephalic vesicle through the FGF8-ERK pathway. In addition, during this period, ERK directs neurogenesis migration and the final fate of neural progenitors. During the early postnatal period, ERK participates in the maturation process of dendritic trees and synaptogenesis. During adulthood, ERK participates in social and emotional behavior and memory processes, including long-term potentiation. Alterations in mechanisms related to ERK are associated with different pathological outcomes. Genetic alterations in any component of the ERK pathway result in pathologies associated with neural crest derivatives and mental dysfunctions associated with autism spectrum disorders. The MAP-ERK pathway is a key element of the neuroinflammatory pathway triggered by glial cells during the development of neurodegenerative diseases, such as Parkinson's and Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis, as well as prionic diseases. The process triggered by MAPK/ERK activation depends on the stage of development (mature or senescence), the type of cellular element in which the pathway is activated, and the anatomic neural structure. However, extensive gaps exist with regards to the targets of the phosphorylated ERK in many of these processes.

Molecular imaging identifies age-related attenuation of acetylcholine in retrosplenial cortex in response to acetylcholinesterase inhibition

The neurotransmitter of the cholinergic system, acetylcholine plays a major role in the brain's cognitive function and is involved in neurodegenerative disorders. Here, we present age-related alterations of acetylcholine levels after administration of the acetylcholinesterase inhibitor drug tacrine in normal mice. Using a quantitative, robust and molecular-specific mass spectrometry imaging method we found that tacrine administration significantly raised acetylcholine levels in most areas of sectioned mice brains, inter alia the striatum, hippocampus and cortical areas. However, acetylcholine levels in retrosplenial cortex were significantly lower in 14-month-old than in 12-week-old animals following its administration, indicating that normal aging affects the cholinergic system's responsivity. This small brain region is interconnected with an array of brain networks and is involved in numerous cognitive tasks. Simultaneous visualization of distributions of tacrine and its hydroxylated metabolites in the brain revealed a significant decrease in levels of the metabolites in the 14-month-old mice. The results highlight strengths of the imaging technique to simultaneously investigate multiple molecular species and the drug-target effects in specific regions of the brain. The proposed approach has high potential in studies of neuropathological conditions and responses to neuroactive treatments.

Aging and spatial cues influence the updating of navigational memories

Updating navigational memories is important for everyday tasks. It was recently found that older adults are impaired in updating spatial representations in small, bi-dimensional layouts. Because performance in small-scale areas cannot predict navigational behavior, we investigated how aging affects the updating of navigational memories encoded in large, 3-dimensional environments. Moreover, since locations can be encoded relative to the observer (egocentric encoding) or relative to landmarks (allocentric encoding), we tested whether the presumed age-related spatial updating deficit depends on the available spatial cues. By combining whole-body motion tracking with immersive virtual reality, we could dissociate egocentric and allocentric spatial cues and assess navigational memory under ecologically valid conditions (i.e., providing body-based and visual cues). In the task, objects were relocated overnight, and young and older participants had to navigate to the updated locations of the objects. In addition to replicating age-related deficits in allocentric memory, we found age-related impairments in updating navigational memories following egocentric encoding. Finally, older participants depicted stronger representations of the previous navigational context that were correlated with their spatial updating deficits. Given that these effects may stem from inefficient suppression of former navigational memories, our findings propose a mechanism that helps explain the navigational decline in aging.

Spatial updating deficits in human aging are associated with traces of former memory representations

The ability to update spatial memories is important for everyday situations, such as remembering where we left our keys or parked our car. Although rodent studies have suggested that old age might impair spatial updating, direct evidence for such a deficit in humans is missing. Here, we tested whether spatial updating deficits occur in human aging, whether the learning mode influences spatial updating, and what mnemonic mechanism underlies the presumed deficits. To address these questions, younger and older participants had to indicate the latest location of relocated items, following either incidental or intentional learning. Using eye tracking, we further quantified memory traces of the original and updated locations. We found that older participants were selectively impaired in recalling locations of relocated items. Furthermore, they depicted relatively stronger representations of the original locations, which were correlated with their spatial updating deficits. The findings demonstrate that stronger representations of former spatial contexts can impair spatial updating in aging, a mechanism that can help explain the commonly observed age-related decline in spatial memory.

Astroglia in Alzheimer's Disease

Alzheimer's disease is the most common cause of dementia. Cellular changes in the brains of the patients suffering from Alzheimer's disease occur well in advance of the clinical symptoms. At the cellular level, the most dramatic is a demise of neurones. As astroglial cells carry out homeostatic functions of the brain, it is certain that these cells are at least in part a cause of Alzheimer's disease. Historically, Alois Alzheimer himself has recognised this at the dawn of the disease description. However, the role of astroglia in this disease has been understudied. In this chapter, we summarise the various aspects of glial contribution to this disease and outline the potential of using these cells in prevention (exercise and environmental enrichment) and intervention of this devastating disease.

Serotonergic and cholinergic modulation of functional brain connectivity: A comparison between young and older adults

Aging is accompanied by changes in neurotransmission. To advance our understanding of how aging modifies specific neural circuitries, we examined serotonergic and cholinergic stimulation with resting state functional magnetic resonance imaging (RS-fMRI) in young and older adults. The instant response to the selective serotonin reuptake inhibitor citalopram (30 mg) and the acetylcholinesterase inhibitor galantamine (8 mg) was measured in 12 young and 17 older volunteers during a randomized, double blind, placebo-controlled, crossover study. A powerful dataset consisting of 522 RS-fMRI scans was obtained by acquiring multiple scans per subject before and after drug administration. Group × treatment interaction effects on voxelwise connectivity with ten functional networks were investigated (p < .05, FWE-corrected) using a non-parametric multivariate analysis technique with cerebrospinal fluid, white matter, heart rate and baseline measurements as covariates. Both groups showed a decrease in sensorimotor network connectivity after citalopram administration. The comparable findings after citalopram intake are possibly due to relatively similar serotonergic systems in the young and older subjects. Galantamine altered connectivity between the occipital visual network and regions that are implicated in learning and memory in the young subjects. The lack of a cholinergic response in the elderly might relate to the well-known association between cognitive and cholinergic deterioration at older age.

Strategies for Preventing Cognitive Decline in Healthy Older Adults

OBJECTIVE

Many advances have been made in the understanding of age-related changes in cognition. As research details the cognitive and neurobiological changes that occur in aging, there is increased interest in developing and understanding methods to prevent, slow, or reverse the cognitive decline that may occur in normal healthy older adults. The Institute of Medicine has recently recognized cognitive aging as having important financial and public health implications for society with the increasing older adult population worldwide. Cognitive aging is not dementia and does not result in the loss of neurons but rather changes in neurotransmission that affect brain functioning. The fact that neurons are structurally intact but may be functionally affected by increased age implies that there is potential for remediation.

METHOD AND RESULTS

This review article presents recent work using medication-based strategies for slowing cognitive changes in aging. The primary method presented is a hormonal approach for affecting cognition in older women. In addition, a summary of the work examining modifiable lifestyle factors that have shown promise in benefiting cognition in both older men and women is described.

CONCLUSIONS

Much work remains to be done so that evidence-based recommendations can be made for slowing cognitive decline in healthy older adults. The success of some of these methods thus far indicates that the brains of healthy older adults are plastic enough to be able to respond to these cognitive decline prevention strategies, and further work is needed to define the most beneficial methods.

Developmental suppression of forebrain trkA receptors and attentional capacities in aging rats: A longitudinal study

Basal forebrain (BF) cholinergic neurons innervating the cortex regulate cognitive, specifically attentional, processes. Cholinergic atrophy and cognitive decline occur at an accelerated pace in age-related neurodegenerative disorders such as Alzheimer's disease; however, the mechanism responsible for this phenomenon remains unknown. Here we hypothesized that developmental suppression of nerve growth factor signaling, mediated via tropomyosin-related kinase A (trkA) receptors, would escalate age-related attentional vulnerability. An adeno-associated viral vector expressing trkA shRNA (AAV-trkA) was utilized to knockdown trkA receptors in postnatal rats at an ontogenetic time point when cortical cholinergic inputs mature, and the impact of this manipulation on performance was assessed in animals maintained on an operant attention task throughout adulthood and until old (24 months) age. A within-subject comparison across different time points illustrated a gradual age-related decline in attentional capacities. However, the performance under baseline and distracted conditions did not differ between the AAV-trkA-infused and animals infused with a vector expressing shRNA against the control protein luciferase at any time point. Additional analysis of cholinergic measures conducted at 24 months showed that the capacity of cholinergic terminals to release acetylcholine following a depolarizing stimulus, cortical cholinergic fiber density and BF cholinergic cell size remained comparable between the two groups. Contrary to our predictions, these data indicate that developmental BF trkA disruption does not impact age-related changes in attentional functions. It is possible that life-long engagement in cognitive activity might have potentially rescued the developmental insults on the cholinergic system, thus preserving attentional capacities in advanced age.

Impact of ageing on postsynaptic neuronal nicotinic neurotransmission in auditory thalamus

KEY POINTS

Neuronal nicotinic acetylcholine receptors (nAChRs) play a fundamental role in the attentional circuitry throughout the mammalian CNS. In the present study, we report a novel finding that ageing negatively impacts nAChR efficacy in auditory thalamus, and this is probably the result of a loss of nAChR density (Bmax ) and changes in the subunit composition of nAChRs. Our data support the hypothesis that age-related maladaptive changes involving nAChRs within thalamocortical circuits partially underpin the difficulty that elderly adults experience with respect to attending to speech and other salient acoustic signals.

ABSTRACT

The flow of auditory information through the medial geniculate body (MGB) is regulated, in part, by cholinergic projections from the pontomesencephalic tegmentum. The functional significance of these projections is not fully established, although they have been strongly implicated in the allocation of auditory attention. Using in vitro slice recordings, we have analysed postsynaptic function and pharmacology of neuronal nicotinic ACh receptors (nAChRs) in young adult and the aged rat MGB. We find that ACh produces significant excitatory postsynaptic actions on young MGB neurons, probably mediated by β2-containing heteromeric nAChRs. Radioligand binding studies show a significant age-related loss of heteromeric nAChR receptor number, which supports patch clamp data showing an age-related loss in ACh efficacy in evoking postsynaptic responses. Use of the β2-selective nAChR antagonist, dihydro-β-erythroidine, suggests that loss of cholinergic efficacy may also be the result of an age-related subunit switch from high affinity β2-containing nAChRs to low affinity β4-containing nAChRs, in addition to the loss of total nAChR number. This age-related nAChR dysfunction may partially underpin the attentional deficits that contribute to the loss of speech understanding in the elderly.

Assessment of biochemical and behavioral effects of carbaryl and methomyl in Brown-Norway rats from preweaning to senescence

Factors impacting life stage-specific sensitivity to chemicals include toxicokinetic and toxicodynamic changes. To evaluate age-related differences in the biochemical and behavioral impacts of two typical N-methyl carbamate pesticides, we systematically compared their dose-response and time-course in preweanling (postnatal day, PND, 18) and adult male Brown Norway rats (n=9-10/dose or time) ranging from adolescence to senescence (1, 4, 12, 24 mo). Carbaryl was administered orally at 3, 7.5, 15, or 22.5mg/kg and data were collected at 40 min after dosing, or else given at 3 or 15 mg/kg and data collected at 30, 60, 120, and 240 min. Methomyl was studied only in adult and senescent rat (4, 12, 24 mo) in terms of dose-response (0.25. 0.6, 1.25, 2.5mg/kg) and time-course (1.25mg/kg at 30, 60, 120, 240 min). Motor activity as well as brain and erythrocyte (RBC) cholinesterase (ChE) activity were measured in the same animals. In the carbaryl dose-response, PND18 rats were the most sensitive to the brain ChE-inhibiting effects of carbaryl, but 12- and 24-mo rats showed more motor activity depression even at similar levels of brain ChE inhibition. We have previously reported that brain ChE inhibition, but not motor activity effects, closely tracked carbaryl tissue levels. There were no age-related differences in methomyl-induced ChE inhibition across doses, but greater motor activity depression was again observed in the 12- and 24-mo rats. Carbaryl time-course data showed that motor activity depression reached a maximum later, and recovered slower, in the 12- and 24-mo rats compared to the younger ages; slowest recovery and maximal effects were seen in the 24-mo rats. Acetylcholinesterase sensitivity (concentration-inhibition curves) was measured in vitro using control tissues from each age. Inhibitory concentrations of carbaryl were somewhat lower in PND18, 12-, and 24-mo tissues compared to 1- and 4-mo, but there were no differences with methomyl-treated tissues. Thus, in the dose-response and time-course, there were dissociations between brain ChE inhibition and the magnitude as well as recovery of motor activity changes. The explanation for this dissociation is unclear, and is likely due to early development followed by aging-related decline in both kinetic parameters and neurological responsiveness.

Cognition as a therapeutic target in late-life depression: potential for nicotinic therapeutics

Depression is associated with impairments to cognition and brain function at any age, but such impairments in the elderly are particularly problematic because of the additional burden of normal cognitive aging and in some cases, structural brain pathology. Individuals with late-life depression exhibit impairments in cognition and brain structural integrity, alongside mood dysfunction. Antidepressant treatment improves symptoms in some but not all patients, and those who benefit may not return to the cognitive and functional level of nondepressed elderly. Thus, for comprehensive treatment of late-life depression, it may be necessary to address both the affective and cognitive deficits. In this review, we propose a model for the treatment of late-life depression in which nicotinic stimulation is used to improve cognitive performance and improve the efficacy of an antidepressant treatment of the syndrome of late-life depression. The cholinergic system is well-established as important to cognition. Although muscarinic stimulation may exacerbate depressive symptoms, nicotinic stimulation may improve cognition and neural functioning without a detriment to mood. While some studies of nicotinic subtype specific receptor agonists have shown promise in improving cognitive performance, less is known regarding how nicotinic receptor stimulation affects cognition in depressed elderly patients. Late-life depression thus represents a new therapeutic target for the development of nicotinic agonist drugs. Parallel treatment of cognitive dysfunction along with medical and psychological approaches to treating mood dysfunction may be necessary to ensure full resolution of depressive illness in aging.

Successful brain aging: plasticity, environmental enrichment, and lifestyle

Aging is a physiological process that can develop without the appearance of concurrent diseases. However, very frequently, older people suffer from memory loss and an accelerated cognitive decline. Studies of the neurobiology of aging are beginning to decipher the mechanisms underlying not only the physiology of aging of the brain but also the mechanisms that make people more vulnerable to cognitive dysfunction and neurodegenerative diseases. Today we know that the aging brain retains a considerable functional plasticity, and that this plasticity is positively promoted by genes activated by different lifestyle factors. In this article some of these lifestyle factors and their mechanisms of action are reviewed, including environmental enrichment and the importance of food intake and some nutrients. Aerobic physical exercise and reduction of chronic stress are also briefly reviewed. It is proposed that lifestyle factors are powerful instruments to promote healthy and successful aging of the brain and delay the appearance of age-related cognitive deficits in elderly people.

Effect of bite-raised condition on the hippocampal cholinergic system of aged SAMP8 mice

Occlusal disharmony induces chronic stress, which results in learning deficits in association with the morphologic changes in the hippocampus, e.g., neuronal degeneration and increased hypertrophied glial fibrillary acidic protein-positive cells. To investigate the mechanisms underlying impaired hippocampal function resulting from occlusal disharmony, we examined the effects of the bite-raised condition on the septohippocampal cholinergic system by assessing acetylcholine release in the hippocampus and choline acetyltransferase immunoreactivity in the medial septal nucleus in aged SAMP8 mice that underwent the bite raising procedure. Aged bite-raised mice showed decreased acetylcholine release in the hippocampus and a reduced number of choline acetyltransferase-immunopositive neurons in the medial septal nucleus compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice enhances the age-related decline in the septohippocampal cholinergic system, leading to impaired learning.

Aging-related deficits in orexin/hypocretin modulation of the septohippocampal cholinergic system

The medial septum (MS) of the basal forebrain contains cholinergic neurons that project to the hippocampus, support cognitive function, and are implicated in age-related cognitive decline. Hypothalamic orexin/hypocretin neurons innervate and modulate basal forebrain cholinergic neurons and provide direct inputs to the hippocampus. However, the precise role of orexin in modulating hippocampal cholinergic transmission--and how these interactions are altered in aging--is unknown. Here, orexin A was administered to CA1 and the MS of young (3-4 months) and aged (27-29 months) Fisher 344/Brown Norway rats, and hippocampal acetylcholine efflux was analyzed by in vivo microdialysis. At both infusion sites, orexin A dose-dependently increased hippocampal acetylcholine in young, but not aged rats. Moreover, immunohistochemical characterization of the MS revealed no change in cholinergic cell bodies in aged animals, but a significant decrease in orexin fiber innervation to cholinergic cells. These findings indicate that: (1) Orexin A modulates hippocampal cholinergic neurotransmission directly and transsynaptically in young animals, (2) Aged animals are unresponsive to orexin A, and (3) Aged animals undergo an intrinsic reduction in orexin innervation to cholinergic cells within the MS. Alterations in orexin regulation of septohippocampal cholinergic activity may contribute to age-related dysfunctions in arousal, learning, and memory.

Menopause and Mental Well-Being: Timing of Symptoms and Timing of Hormone Treatment

In the aftermath of the Women's Health Initiative studies, both the clinical and basic science communities had to sort out divergent results among experimental findings, observational data and randomized controlled trials in order to establish a shared analysis. The scientific community formally debates the role of different HRT formulations, hormone doses, time of treatment initiation since the menopause and the age of treated women. Basic scientists demonstrated that the multiple neuroprotective effects of estrogen on brain cells may induce a differential biological response according to the time of treatment. Progesterone (but not all synthetic progestins) also has pivotal neuroactive functions in animal models of reproductive aging. Additionally, epidemiological surveys provide information regarding the detrimental role of hypogonadism on mental well-being. The present article briefly summarizes current evidence supporting the neuroactive role of estrogen, with reference to the clinical finding sustaining the intriguing hypothesis of the early female brain senescence as a highly responsive period to estrogen treatment.

Carbachol-induced long-term synaptic depression is enhanced during senescence at hippocampal CA3-CA1 synapses

Dysregulation of the cholinergic transmitter system is a hallmark of Alzheimer's disease and contributes to an age-associated decline in memory performance. The current study examined the influence of carbachol, a cholinergic receptor agonist, on synaptic transmission over the course of aging. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synapses in acute hippocampal slices obtained from young adult (5-8 mo) and aged (22-24 mo) male Fischer 344 rats. Bath application of carbachol elicited a transient depression of synaptic transmission, which was followed by a long-lasting depression (CCh-LTD) observed 90 min after carbachol cessation in both age groups. However, the magnitude of CCh-LTD was significantly larger in senescent animals and was attenuated by N-methyl-D-aspartate receptor blockade in aged animals. Blockade of L-type Ca(2+) channels inhibited CCh-LTD to a greater extent in aged animals compared to young adults. Finally, the expression of CCh-LTD was dependent on protein synthesis. The results indicate that altered Ca(2+) homeostasis or muscarinic activation of Ca(2+) signaling contribute to the enhanced CCh-LTD during senescence.

Does the olfactory cue activate the same brain network during aging in the rat after taste potentiated odor aversion retrieval?

Depending on the brain networks involved, aging is not accompanied by a general decrease in learning and memory capabilities. We demonstrated previously that learning and retrieval of taste potentiated odor aversion (TPOA) is preserved, and even slightly improved, in senescent rats showing some memory deficiencies in cognitive tasks (Dardou, Datiche, & Cattarelli, 2008). TPOA is a particular behavior in which the simultaneous presentation of odor and taste cues followed by a delayed visceral illness leads to a robust aversion towards both conditioned stimuli, which permits diet selection and animal survival. The present experiment was performed in order to investigate the stability or the evolution of the brain network underlying TPOA retrieval during aging. By using immunocytochemical detection of Fos and Egr1 proteins we mapped the cerebral activation induced by TPOA retrieval elicited by the odor presentation in the young, the adult and the senescent rats. The pattern of brain activation changed and the number of activated areas decreased with age. Nevertheless, the piriform cortex and the basolateral amygdala nucleus were always activated and seemed essential for TPOA retrieval. The hippocampus and the neocortical areas could have different implications in TPOA memory in relation to age. The patterns of expression of Fos and Egr1 were different, suggesting their differential involvement in TPOA retrieval. Data are discussed according to the possible roles of the brain areas studied and a model of schematic brain network subtending TPOA retrieval induced by the odor cue is proposed.

Enhanced calcium buffering in F344 rat cholinergic basal forebrain neurons is associated with age-related cognitive impairment

Alterations in neuronal Ca(2+) homeostasis are important determinants of age-related cognitive impairment. We examined the Ca(2+) influx, buffering, and electrophysiology of basal forebrain neurons in adult, middle-aged, and aged male F344 behaviorally assessed rats. Middle-aged and aged rats were characterized as cognitively impaired or unimpaired by water maze performance relative to young cohorts. Patch-clamp experiments were conducted on neurons acutely dissociated from medial septum/nucleus of the diagonal band with post hoc identification of phenotypic marker mRNA using single-cell RT-PCR. We measured whole cell calcium and barium currents and dissected these currents using pharmacological agents. We combined Ca(2+) current recording with Ca(2+)-sensitive ratiometric microfluorimetry to measure Ca(2+) buffering. Additionally, we sought changes in neuronal firing properties using current-clamp recording. There were no age- or cognition-related changes in the amplitudes or fractional compositions of the whole cell Ca(2+) channel currents. However, Ca(2+) buffering was significantly enhanced in cholinergic neurons from aged cognitively impaired rats. Moreover, increased Ca(2+) buffering was present in middle-aged rats that were not cognitively impaired. Firing properties were largely unchanged with age or cognitive status, except for an increase in the slow afterhyperpolarization in aged cholinergic neurons, independent of cognitive status. Furthermore, acutely dissociated basal forebrain neurons in which choline acetyltransferase mRNA was detected had the electrophysiological profiles of identified cholinergic neurons. We conclude that enhanced Ca(2+) buffering by cholinergic basal forebrain neurons may be important during aging.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.

Estradiol interacts with the cholinergic system to affect verbal memory in postmenopausal women: evidence for the critical period hypothesis

Estradiol has been shown to interact with the cholinergic system to affect cognition in postmenopausal women. This study further investigated the interaction of estradiol and cholinergic system functioning on verbal memory and attention in two groups of healthy younger (ages 50-62) and older (ages 70-81) postmenopausal women. Twenty-two postmenopausal women were randomly and blindly placed on 1 mg of 17-beta estradiol orally for 1 month then 2 mg for 2 months or matching placebo pills after which they participated in three anticholinergic challenge sessions when verbal memory and attention were assessed. Subjects were administered either the antimuscarinic drug scopolamine (SCOP), the antinicotinic drug mecamylamine (MECA), or placebo. After the first challenge phase, they were crossed over to the other hormone treatment for another 3 months and repeated the challenges. Results showed that estradiol pretreatment significantly attenuated the anticholinergic drug-induced impairments on a test of episodic memory (the Buschke Selective Reminding Task) for the younger group only, while estradiol treatment impaired performance of the older group. The results suggest that younger subjects may experience more cholinergic benefit from estradiol treatment than older subjects, supporting the concept of a critical period for postmenopausal estrogen use.

Aging, plasticity and environmental enrichment: structural changes and neurotransmitter dynamics in several areas of the brain

Cajal was probably the first neurobiologist to suggest that plasticity of nerve cells almost completely disappeared during aging. However, we know today that neural plasticity is still present in the brain during aging. In this review we suggest that aging is a physiological process that occurs asynchronously in different areas of the brain and that the rate of that process is modulated by environmental factors and related to the neuronal-synaptic-molecular substrates of each area. We review here some of the most recent results on aging of the brain in relation to the plastic changes that occur in young and aged animals as a result of living in an enriched environment. We highlight the results from our own laboratory on the dynamics of neurotransmitters in different areas of the brain. Specifically we review first the effects of aging on neurons, dendrites, synapses, and also on molecular and functional plasticity. Second, the effects of environmental enrichment on the brain of young and aged animals. And third the effects of an enriched environment on the age-related changes in neurogenesis and in the extracellular concentrations of glutamate and GABA in hippocampus, and on dopamine, acetylcholine, glutamate and GABA under a situation of acute mild stress in the prefrontal cortex.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Growth Hormone Responses to Low-Dose Physostigmine in Elderly vs. Young Women and Men

BACKGROUND

Growth hormone (GH) secretion is a sensitive measure of CNS cholinergic neurotransmission, and GH decreases considerably with age. Cholinesterase inhibitors, which increase acetylcholine concentrations, have been used in elderly subjects to investigate the neuroendocrine effects of aging and Alzheimer's disease. However, there have been only a few studies of a potential sex difference in GH responses to cholinesterase inhibitors in elderly subjects, with mixed results.

OBJECTIVE

We therefore administered low-dose physostigmine (PHYSO), a cholinesterase inhibitor, to normal, non-hormone-replaced, elderly women and men, to ascertain a potential sex difference in GH response. We hypothesized: (1) elderly women and men would have similar hormone responses, because of relatively low circulating estrogen in the women, and (2) the elderly women would have significantly lower baseline GH and GH responses to cholinergic challenge than the young women we studied previously.

METHODS

Normal elderly women and men > or =65 years of age meeting stringent inclusion and exclusion criteria were studied on three test days, 4-7 days apart, by serial blood sampling for several hours for baseline GH, followed by administration of low-dose PHYSO (first and third days) or saline (second day) at 18:00 h. Frequent blood sampling was continued for several hours. Plasma GH and hypothalamo-pituitary-adrenal cortical hormones were measured in each sample.

RESULTS

PHYSO administration produced no side effects in about half the elderly subjects and mild side effects in the other half, with no significant female-male differences and no significant relationship between the presence or absence of side effects and GH response. PHYSO significantly increased GH compared to saline, to a similar degree in the elderly women and men. The elderly women had a significantly greater GH response to PHYSO than did the young women, whereas GH responses were similar in the elderly and young men.

CONCLUSIONS

These results indicate similar GH responses to low-dose PHYSO in elderly women compared to elderly men, and a significantly greater GH response in elderly women compared to young women. A likely mechanism is increased sensitivity of central cholinergic systems that inhibit somatostatin and/or enhance GHRH release from the hypothalamus.

Molarless-induced changes of spines in hippocampal region of SAMP8 mice

We examined the effect of the molarless condition on the dendritic spines of hippocampal pyramidal cells in SAMP8 mice in comparison to its effect on learning ability in a maze test. The molarless condition caused a decrease in the number of the spines of CA1 pyramidal cells only in the aged mice showing a reduced learning ability. The results suggest the involvement of the molarless condition in an attenuation of input activities in the hippocampal synapses.

ELISA methods to measure cholinergic markers and nerve growth factor receptors in cortex, hippocampus, prefrontal cortex, and basal forebrain from rat brain

The central cholinergic system has a fundamental role in normal cognitive function, and in diseases that exhibit cognitive dysfunction. The purpose of this study was to design ELISA methods to measure proteins that have essential functions in the central cholinergic system. We were particularly interested in quantifying proteins that respond directly or indirectly to nerve growth factor (NGF). ELISAs offer advantages over Western blot analyses and other methods, such as increased sensitivity, decreased assay variability, increased efficiency, and decreased cost. We developed indirect ELISA methods for: choline acetyltransferase (ChAT); the vesicular acetylcholine transporter (VAChT); the high affinity choline transporter (HACT/CHT); TrkA, the high affinity NGF receptor; the p75 neurotrophin receptor (p75(NTR)). A sandwich ELISA was developed to measure tyrosine-phosphorylated TrkA in brain lysates. We used these ELISAs to compare levels of the above proteins in important memory-related brain regions--basal forebrain, hippocampus, cortex, and prefrontal cortex--from old and young rats. We identified age-related differences in the levels of the aforementioned proteins (e.g., VAChT and HACT/CHT in hippocampus). Thus, these ELISA methods should be particularly useful for comparing the effects of age, disease, drugs, and toxicants on brain levels of key cholinergic and growth factor-related proteins.

Endocrinology of menopausal transition and its brain implications

The central nervous system is one of the main target tissues for sex steroid hormones, which act on both through genomic mechanisms, modulating synthesis, release, and metabolism of many neuropeptides and neurotransmitters, and through non-genomic mechanisms, influencing electrical excitability, synaptic function, morphological features, and neuron-glia interactions. During the climacteric period, sex steroid deficiency causes many neuroendocrine changes. At the hypothalamic level, estrogen withdrawal gives rise to vasomotor symptoms, to eating behavior disorders, and altered blood pressure control. On the other hand, at the limbic level, the changes in serotoninergic, noradrenergic, and opioidergic tones contribute to the modifications in mood, behavior, and nociception. Hormone replacement therapy (HRT) positively affects climateric depression throughout a direct effect on neural activity and on the modulation of adrenergic and serotoninergic tones and may modulate the decrease in cognitive efficiency observed in climaterium. The identification of the brain as a de novo source of neurosteroids, suggests that the modifications in mood and cognitive performances occurring in postmenopausal women may also be related to a change in the levels of neurosteroids. These findings open new perspectives in the study of the effects of sex steroids on the central nervous system and on the possible use of alternative and/or auxiliary HRT.

Effects of the metabotropic glutamate receptor agonist, ACPD, on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of the rat during the normal process of aging

The aim of the present study was to investigate the effects of activation of metabotropic glutamate receptors (mGluR) on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of freely moving rats of different groups of age. Perfusion, through the microdialysis probe, of the agonist of mGluR, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100, 500 and 1000 microM), in the prefrontal cortex of young rats produced a dose-related increase of the dialysate concentrations of GABA. The effects of perfusion of ACPD on the concentrations of GABA were attenuated in middle-aged rats. In the prefrontal cortex of aged rats, perfusion of ACPD produced no changes in dialysate concentrations of GABA at any of the doses used. Conversely, perfusion of ACPD (100, 500 and 1000 microM) in the prefrontal cortex of young, middle-aged and aged rats did not modify the dialysate concentrations of acetylcholine. Basal concentrations of acetylcholine in the prefrontal cortex of middle-aged and aged rats were significantly lower than those in young rats. In contrast, basal dialysate concentrations of GABA were not significantly different in young, middle-aged and aged rats. These results suggest that the interaction GABA-glutamate in the prefrontal cortex, mediated by mGluRs, changes with age.

Nicotine-induced FGF-2 mRNA in rat brain is preserved during aging

Indirect trophic actions of nicotine on brain during aging are suggested from observations describing nicotine as a cognitive enhancer, increasing vigilance and improving learning and memory, and both in vitro and in vivo models have demonstrated neuroprotective effects of nAChR agonists. Previously, we have reported that an acute intermittent (-)nicotine treatment significantly increases fibroblast growth factor-2 (FGF-2) mRNA and protein in several brain regions of rat brain. The present study was designed to analyse if nicotine-induced FGF-2 expression in the rat brain was preserved during aging. Using in situ hybridization and quantitative RNase protection assay the present paper reports that during aging (12- and 24-month-old rats) the response of FGF-2 gene expression in the rat brain to nAChR stimulation by (-)nicotine is fully effective and involves both neurons and glial cells. The investigation was extended to other members of the FGF family, such as FGF-5 and -20, but this expression was not influenced by the (-)nicotine treatment at any age studied. Similarly following (-)nicotine treatment no changes were observed in FGF receptors (FGFR 1-3) mRNA levels in adult and aged rats. Taken together, the present and previous data support the hypothesis that neuroprotective effects of (-)nicotine and the potential beneficial effects of (-)nicotine agonists in the treatment of Alzheimer's and Parkinson's diseases, may at least in part involve an activation of the neuronal and glial FGF-2 signalling. Work is in progress to analyse the mechanism(s) linking nAChR activation to the up-regulation of FGF-2.

Altered hippocampal transcript profile accompanies an age-related spatial memory deficit in mice

We have carried out a global survey of age-related changes in mRNA levels in the C57BL/6NIA mouse hippocampus and found a difference in the hippocampal gene expression profile between 2-month-old young mice and 15-month-old middle-aged mice correlated with an age-related cognitive deficit in hippocampal-based explicit memory formation. Middle-aged mice displayed a mild but specific deficit in spatial memory in the Morris water maze. By using Affymetrix GeneChip microarrays, we found a distinct pattern of age-related change, consisting mostly of gene overexpression in the middle-aged mice, suggesting that the induction of negative regulators in the middle-aged hippocampus could be involved in impairment of learning. Interestingly, we report changes in transcript levels for genes that could affect synaptic plasticity. Those changes could be involved in the memory deficits we observed in the 15-month-old mice. In agreement with previous reports, we also found altered expression in genes related to inflammation, protein processing, and oxidative stress.

Pretraining or previous non-spatial experience improves spatial learning in the Morris water maze of nucleus basalis lesioned rats

Previous experiments have shown that infusions of ibotenic acid in the nucleus basalis magnocellularis (NBM) induce a strong impairment in spatial navigation for a hidden platform in the Morris water maze. This effect was initially attributed to a cholinergic deficit, but later studies showed that performance level did not correlate with the degree of cholinergic denervation. Therefore, this impairment is due to a combined cholinergic and non-cholinergic deficit. However, it is not clear in which particular processes the NBM is involved. In this study we have evaluated the origin of behavioural impairment in spatial navigation in the water maze after an ibotenic acid-induced lesion of NBM. In the first experiment, Wistar rats were trained preoperatively in an allocentric navigation task. Postoperatively, they were tested in the same task. All lesioned animals showed a performance level similar to controls. Lesions did not impede the acquisition of new positions in the water maze, nor did affect the ability of animals to remember new platform positions after an intertrial interval of 20s, even if animals had received only allocentric experience with the platform position, or allocentric and path integration information concurrently. Lesions also failed to affect the ability to locate a hidden platform in a new environment. However, hippocampal infusions of scopolamine (5 microg) produced a severe impairment in NBM-damaged animals, without impairing performance of controls. In the second experiment Wistar rats with the same lesion were first trained in a visual-guided task in the water maze, and subsequently evaluated in the spatial task. In both tasks lesioned animals were not different from controls. These results suggest that the NBM played an important role during acquisition phases but not in the execution of spatial navigation. Moreover, the excessive emotional response displayed by lesioned animals is postulated as a relevant cause for the impairment observed in spatial navigation after NBM damage.