Differential changes in cell size and number in topographic subdivisions of human basal nucleus in normal aging

Cellular enlargement - A new hallmark of aging?

Years of important research has revealed that cells heavily invest in regulating their size. Nevertheless, it has remained unclear why accurate size control is so important. Our recent study using hematopoietic stem cells (HSCs) in vivo indicates that cellular enlargement is causally associated with aging. Here, we present an overview of these findings and their implications. Furthermore, we performed a broad literature analysis to evaluate the potential of cellular enlargement as a new aging hallmark and to examine its connection to previously described aging hallmarks. Finally, we highlight interesting work presenting a correlation between cell size and age-related diseases. Taken together, we found mounting evidence linking cellular enlargement to aging and age-related diseases. Therefore, we encourage researchers from seemingly unrelated areas to take a fresh look at their data from the perspective of cell size.

Inconsistencies in atlas-based volumetric measures of the human nucleus basalis of Meynert: A need for high-resolution alternatives

The nucleus basalis of Meynert (nbM) is the major source of cortical acetylcholine (ACh) and has been related to cognitive processes and to neurological disorders. However, spatially delineating the human nbM in MRI studies remains challenging. Due to the absence of a functional localiser for the human nbM, studies to date have localised it using nearby neuroanatomical landmarks or using probabilistic atlases. To understand the feasibility of MRI of the nbM we set our four goals; our first goal was to review current human nbM region-of-interest (ROI) selection protocols used in MRI studies, which we found have reported highly variable nbM volume estimates. Our next goal was to quantify and discuss the limitations of existing atlas-based volumetry of nbM. We found that the identified ROI volume depends heavily on the atlas used and on the probabilistic threshold set. In addition, we found large disparities even for data/studies using the same atlas and threshold. To test whether spatial resolution contributes to volume variability, as our third goal, we developed a novel nbM mask based on the normalized BigBrain dataset. We found that as long as the spatial resolution of the target data was 1.3 mm isotropic or above, our novel nbM mask offered realistic and stable volume estimates. Finally, as our last goal we tried to discern nbM using publicly available and novel high resolution structural MRI ex vivo MRI datasets. We find that, using an optimised 9.4T quantitative T₂^⁎ ex vivo dataset, the nbM can be visualised using MRI. We conclude caution is needed when applying the current methods of mapping nbM, especially for high resolution MRI data. Direct imaging of the nbM appears feasible and would eliminate the problems we identify, although further development is required to allow such imaging using standard (f)MRI scanning.

OUP accepted manuscript

Cognitive deficits in Alzheimer’s disease, specifically amnestic (memory dominant) deficits, are associated with cholinergic degeneration in the basal forebrain. The cholinergic nucleus within the basal forebrain, the nucleus basalis of Meynert, exhibits local atrophy and reduced cortical tract integrity on MRI, and reveals amyloid-β and phosphorylated-tau pathology at autopsy. To understand the pathophysiology of nucleus basalis of Meynert atrophy and its neocortical projections in Alzheimer’s disease, we used a combined post-mortem in situ MRI and histopathology approach. A total of 19 Alzheimer’s disease (10 amnestic and nine non-amnestic) and nine non-neurological control donors underwent 3 T T₁-weighted MRI for anatomical delineation and volume assessment of the nucleus basalis of Meynert, and diffusion-weighted imaging for microstructural assessment of the nucleus and its projections. At subsequent brain autopsy, tissue dissection and immunohistochemistry were performed for amyloid-β, phosphorylated-tau and choline acetyltransferase. Compared to controls, we observed an MRI-derived volume reduction and altered microstructural integrity of the nucleus basalis of Meynert in Alzheimer’s disease donors. Furthermore, decreased cholinergic cell density was associated with reduced integrity of the nucleus and its tracts to the temporal lobe, specifically to the temporal pole of the superior temporal gyrus, and the parahippocampal gyrus. Exploratory post hoc subgroup analyses indicated that cholinergic cell density could be associated with cortical tract alterations in amnestic Alzheimer’s disease donors only. Our study illustrates that in Alzheimer’s disease, cholinergic degeneration in the nucleus basalis of Meynert may contribute to damaged cortical projections, specifically to the temporal lobe, leading to cognitive deterioration.

Altered functional connectivity between the nucleus basalis of Meynert and anterior cingulate cortex is associated with declined attentional performance after total sleep deprivation

BACKGROUND

Sleep deprivation can markedly influence vigilant attention. The nucleus basalis of Meynert (NBM), the main source of cholinergic projections to the cortex, plays an important role in wakefulness maintenance and attention control. However, the involvement of NBM in attentional impairments after total sleep deprivation (TSD) has yet to be established. The purpose of this study is to investigate the alterations in NBM functional connectivity and its association with the attentional performance following TSD.

METHODS

Thirty healthy adult males were recruited in the study. Participants underwent two resting-state functional magnetic resonance imaging (rs-fMRI) scans, once in rested wakefulness (RW) and once after 36 h of TSD. Seed-based functional connectivity analysis was performed using rs-fMRI data for the left and right NBM. The vigilant attention was measured using a psychomotor vigilance test (PVT). Furthermore, Pearson correlation analysis was conducted to investigate the relationship between altered NBM functional connectivity and changed PVT performance after TSD.

RESULTS

Compared to RW, enhanced functional connectivity was observed between right NBM and bilateral thalamus and cingulate cortex, while reduced functional connectivity was observed between left NBM and right superior parietal lobule following TSD. Moreover, altered NBM functional connectivity with the left anterior cingulate cortex was negatively correlated with PVT performance after TSD.

CONCLUSION

Our results suggest that the disrupted NBM-related cholinergic circuit highlights an important role in attentional performance after TSD. The enhanced NBM functional connectivity with the anterior cingulate cortex may act as neural signatures for attentional deficits induced by sleep deprivation.

547 transcriptomes from 44 brain areas reveal features of the aging brain in non-human primates

BACKGROUND

Brain aging is a complex process that depends on the precise regulation of multiple brain regions; however, the underlying molecular mechanisms behind this process remain to be clarified in non-human primates.

RESULTS

Here, we explore non-human primate brain aging using 547 transcriptomes originating from 44 brain areas in rhesus macaques (Macaca mulatta). We show that expression connectivity between pairs of cerebral cortex areas as well as expression symmetry between the left and right hemispheres both decrease after aging. Although the aging mechanisms across different brain areas are largely convergent, changes in gene expression and alternative splicing vary at diverse genes, reinforcing the complex multifactorial basis of aging. Through gene co-expression network analysis, we identify nine modules that exhibit gain of connectivity in the aged brain and uncovered a hub gene, PGLS, underlying brain aging. We further confirm the functional significance of PGLS in mice at the gene transcription, molecular, and behavioral levels.

CONCLUSIONS

Taken together, our study provides comprehensive transcriptomes on multiple brain regions in non-human primates and provides novel insights into the molecular mechanism of healthy brain aging.

Altered Functional Connectivity of the Basal Nucleus of Meynert in Mild Cognitive Impairment: A Resting-State fMRI Study

Background: Cholinergic dysfunction plays an important role in mild cognitive impairment (MCI). The basal nucleus of Meynert (BNM) provides the main source of cortical cholinergic innervation. Previous studies have characterized structural changes of the cholinergic basal forebrain in individuals at risk of developing Alzheimer’s disease (AD). However, whether and how functional connectivity of the BNM (BNM-FC) is altered in MCI remains unknown.

Objective: The aim of this study was to identify alterations in BNM-FC in individuals with MCI as compared to healthy controls (HCs), and to examine the relationship between these alterations with neuropsychological measures in individuals with MCI.

Method: One-hundred-and-one MCI patients and 103 HCs underwent resting-state functional magnetic resonance imaging (rs-fMRI). Imaging data were processed with SPM8 and CONN software. BNM-FC was examined via correlation in low frequency fMRI signal fluctuations between the BNM and all other brain voxels. Group differences were examined with a covariance analysis with age, gender, education level, mean framewise displacement (FD) and global correlation (GCOR) as nuisance covariates. Pearson’s correlation was conducted to evaluate the relationship between the BNM-FC and clinical assessments.

Result: Compared with HCs, individuals with MCI showed significantly decreased BNM-FC in the left insula extending into claustrum (insula/claustrum). Furthermore, greater decrease in BNM-FC with insula/claustrum was associated with more severe impairment in immediate recall during Auditory Verbal Learning Test (AVLT) in MCI patients.

Conclusion: MCI is associated with changes in BNM-FC to the insula/claustrum in relation to cognitive impairments. These new findings may advance research of the cholinergic bases of cognitive dysfunction during healthy aging and in individuals at risk of developing AD.

The Effects of Methylphenidate on Resting-State Functional Connectivity of the Basal Nucleus of Meynert, Locus Coeruleus, and Ventral Tegmental Area in Healthy Adults

Background: Methylphenidate (MPH) influences catecholaminergic signaling. Extant work examined the effects of MPH on the neural circuits of attention and cognitive control, but few studies have investigated the effect of MPH on the brain's resting-state functional connectivity (rsFC).

Methods: In this observational study, we compared rsFC of a group of 24 healthy adults who were administered an oral 45 mg dose of MPH with a group of 24 age and gender matched controls who did not receive MPH. We focused on three seed regions: basal nucleus of Meynert (BNM), locus coeruleus (LC), and ventral tegmental area/substantia nigra, pars compacta (VTA/SNc), each providing cholinergic, noradrenergic and dopaminergic inputs to the cerebral cortex. Images were pre-processed and analyzed as in our recent work (Li et al., 2014; Zhang et al., 2015). We used one-sample t-test to characterize group-specific rsFC of each seed region and two-sample t-test to compare rsFC between groups.

Results: MPH reversed negative connectivity between BNM and precentral gyri. MPH reduced positive connectivity between LC and cerebellum, and induced positive connectivity between LC and right hippocampus. MPH decreased positive VTA/SNc connectivity to the cerebellum and putamen, and reduced negative connectivity to left middle occipital gyrus.

Conclusion: MPH had distinct effects on the rsFC of BNM, LC, and VTA/SNc in healthy adults. These new findings may further our understanding of the role of catecholaminergic signaling in Attention Deficit Hyperactivity Disorder (ADHD) and Parkinson's disease and provide insights into the therapeutic mechanisms of MPH in the treatment of clinical conditions that implicate catecholaminergic dysfunction.

Resting state functional connectivity of the basal nucleus of Meynert in humans: in comparison to the ventral striatum and the effects of age

The basal nucleus of Meynert (BNM) provides the primary cholinergic inputs to the cerebral cortex. Loss of neurons in the BNM is linked to cognitive deficits in Alzheimer's disease and other degenerative conditions. Numerous animal studies described cholinergic and non-cholinergic neuronal responses in the BNM; however, work in humans has been hampered by the difficulty of defining the BNM anatomically. Here, on the basis of a previous study that delineated the BNM of post-mortem human brains in a standard stereotaxic space, we sought to examine functional connectivity of the BNM, as compared to the nucleus accumbens (or ventral striatum, VS), in a large resting state functional magnetic resonance imaging data set. The BNM and VS shared but also showed a distinct pattern of cortical and subcortical connectivity. Compared to the VS, the BNM showed stronger positive connectivity with the putamen, pallidum, thalamus, amygdala and midbrain, as well as the anterior cingulate cortex, supplementary motor area and pre-supplementary motor area, a network of brain regions that respond to salient stimuli and orchestrate motor behavior. In contrast, compared to the BNM, the VS showed stronger positive connectivity with the ventral caudate and medial orbitofrontal cortex, areas implicated in reward processing and motivated behavior. Furthermore, the BNM and VS each showed extensive negative connectivity with visual and lateral prefrontal cortices. Together, the distinct cerebral functional connectivities support the role of the BNM in arousal, saliency responses and cognitive motor control and the VS in reward related behavior. Considering the importance of BNM in age-related cognitive decline, we explored the effects of age on BNM and VS connectivities. BNM connectivity to the visual and somatomotor cortices decreases while connectivity to subcortical structures including the midbrain, thalamus, and pallidum increases with age. These findings of age-related changes of cerebral functional connectivity of the BNM may facilitate research of the neural bases of cognitive decline in health and illness.

Nerve growth factor retrieves neuropeptide Y and cholinergic immunoreactivity in the nucleus accumbens of old rats

The nucleus accumbens (NAc) contains high levels of neuropeptide Y (NPY), which is involved in the regulation of functions and behaviors that deteriorate with aging. We sought to determine if aging alters NPY expression in this nucleus and, in the affirmative, if those changes are attributable to the cholinergic innervation of the NAc. The total number and the somatic volume of NPY- and choline acetyltransferase-immunoreactive neurons, and the density of cholinergic varicosities were estimated in the NAc of adult (6 months old) and aged (24 months old) rats. In aged rats, the number of NPY neurons was reduced by 20% and their size was unaltered. The number of cholinergic neurons and the density of the cholinergic varicosities were unchanged, but their somas were hypertrophied. Nerve growth factor administration to aged rats further increased the volume of cholinergic neurons, augmented the density of the cholinergic varicosities, and reversed the age-related decrease in the number of NPY neurons. Our data show that the age-related changes in NPY levels in the NAc cannot be solely ascribed to the cholinergic innervation of the nucleus.

The septo-hippocampal system, learning and recovery of function

We understand this review as an attempt to summarize recent advances in the understanding of cholinergic function in cognition. Such a role has been highlighted in the 1970s by the discovery that dementia patients have greatly reduced cholinergic activity in cortex and hippocampus. A brief anatomical description of the major cholinergic pathways focuses on the basal forebrain and its projections to cortex and hippocampus. From this distinction, compelling evidence suggests that the basal forebrain --> cortex projection regulates the excitability of principal cortical neurons and is thereby critically involved in attention, stimulus detection and memory function, although the biological conditions for these functions are still debated. Similar uncertainties remain for the septo-hippocampal cholinergic system. Although initial lesions of the septum caused memory deficits reminiscent of hippocampal ablations, recent and more refined neurotoxic lesion studies which spared non-cholinergic cells of the basal forebrain failed to confirm these memory impairments in experimental animals despite a near total loss of cholinergic labeling. Yet, a decline in cholinergic markers in aging and dementia still stands as the most central piece of evidence for a link between the cholinergic system and cognition and appear to provide valuable targets for therapeutic approaches.

Stereotaxic probabilistic maps of the magnocellular cell groups in human basal forebrain

The basal forebrain contains several interdigitating anatomical structures, including the diagonal band of Broca, the basal nucleus of Meynert, the ventral striatum, and also cell groups underneath the globus pallidus that bridge the centromedial amygdala to the bed nucleus of the stria terminalis. Among the cell populations, the magnocellular, cholinergic corticopetal projection neurons have received particular attention due to their loss in Alzheimer's disease. In MRI images, the precise delineation of these structures is difficult due to limited spatial resolution and contrast. Here, using microscopic delineations in ten human postmortem brains, we present stereotaxic probabilistic maps of the basal forebrain areas containing the magnocellular cell groups. Cytoarchitectonic mapping was performed in silver stained histological serial sections. The positions and the extent of the magnocellular cell groups within the septum (Ch1-2), the horizontal limb of the diagonal band (Ch3), and in the sublenticular part of the basal forebrain (Ch4) were traced in high-resolution digitized histological sections, 3D reconstructed, and warped to the reference space of the MNI single subject brain. The superposition of the cytoarchitectonic maps in the MNI brain shows the intersubject variability of the various Ch compartments and their stereotaxic position relative to other brain structures. Both the right and left Ch4 regions showed significantly smaller volumes when age was considered as a covariate. Probabilistic maps of compartments of the basal forebrain magnocellular system are now available as an open source reference for correlation with fMRI, PET, and structural MRI data of the living human brain.

Age-related alterations of Nestin-immunoreactive neurons in rat basal forebrain with aged memory deficit

Age-related and aged memory deficit changes in Nestin-immunoreactive (Nestin-IR) neurons were studied following recent evidence of distinct Nestin-IR neurons within adult rat basal forebrain. Morris water maze task assessed spatial learning capacity of 3- and 24-month rats (aged-impaired and aged-unimpaired groups). Nestin-IR neuron distributional and morphological features were investigated by immunohistochemistry and positive neuronal number calculation. Nestin-IR neuron number declined with aging, especially aged-impaired. Significant negative correlations existed between average escape latencies and Nestin-IR neuron number in medial septum-diagonal band of Broca (MS-DBB). Correlations of rostral portion [medial septum (MS) and vertical limb diagonal band (vDB)] were higher than caudal portion [horizontal limb diagonal band (hDB)]. Aged-impaired showed reduced complexity of Nestin-IR neuron dendrite arborization and dendritic length. Nestin-IR astrocyte-like cells appeared scattered among Nestin-IR neurons on some aged-impaired slices. In conclusion, aged-impaired rats showed worse cognitive spatial performance and less Nestin-IR neuronal number compared to aged-unimpaired. Nestin-IR neuronal loss and morphological changes are some pathological characteristics of rat aged basal forebrain and may be important in neurobiological mechanisms of brain aging and aged memory deficit.

Nerve Growth Factor Differentially Affects Spatial and Recognition Memory in Aged Rats

In rats, object discrimination depends on the integrity of the cholinergic system, thus it could be expected that nerve growth factor (NGF) can improve the behavior in aged subjects. The interactive effect of age and cholinergic improvement was assessed behaviorally in young and aged rats. Animals were injected by infusion of NGF into the lateral ventricles and they were tested in two behavioral tasks: an object-location and an object-recognition task. Spatial and recognition memory were assessed in an open field containing five different objects. Rats were submitted to six consecutive sessions. Both age-groups showed comparable habituation of exploratory response in Session 1-4. Discrimination index (DI) was calculated to assess responses to spatial change in Session 5 and object change in Session 6. Control young and aged rats were able to discriminate between familiar and novel object, however DI was lower in aged rats. Treatment with NGF induced decline of object discrimination in both age-groups. Different results were obtained in spatial displacement test. NGF was able to improve spatial memory in aged rats, but had no effect in young controls. These data confer on NGF potential role in improving spatial but not episodic memory in aged rats.

Does healthy aging affect the hemispheric activation balance during paced index-to-thumb opposition task? An fMRI study

Normal aging is generally associated with declining performance in cognitive and fine motor tasks. Previous functional imaging studies have been inconsistent regarding the effect of aging on primary motor cortex (M1) activation during finger movement, showing increased, unchanged or decreased activation contralaterally, and more consistently increased activation ipsilaterally. Furthermore, no study has addressed the effect of age on M1 hemispheric activation balance. We studied 18 optimally healthy right-handed subjects, age range 18-79 years (mean +/- SD: 47 +/- 17) using 3 T fMRI and right index finger-thumb tapping auditory-paced at 1.25 Hz. The weighted Laterality Index (wLI) for M1 was obtained according to Fernandez et al. (2001) [Fernandez, G., de Greiff, A., von Oertzen, J., Reuber, M., Lun, S., Klaver, P., et al. 2001. Language mapping in less than 15 min: real-time functional MRI during routine clinical investigation. Neuroimage 14 585-594], with some modifications. The wLI, as well as the total activation on each side, were assessed against age using non-parametric correlation. There was a highly significant negative correlation between age and wLI such that the older the subjects, the lower the wLI. Furthermore, there was a highly significant positive correlation between total activation for ipsilateral M1 and age, and a nearly significant trend for contralateral M1. This study documents that during execution of a simple paced motor task, the older the subject the less lateralized the M1 activation balance as a result of increasing amount of activation on both sides, more significantly so ipsilaterally. Thus, in aging, enhanced M1 recruitment bilaterally is required to produce the same motor performance, suggesting a compensatory process. These findings are in line with cognitive studies indicating a tendency for the aging brain to reduce its functional lateralization, perhaps from less efficient transcallosal connections.

Aging and subcellular localization of m2 muscarinic autoreceptor in basalocortical neurons in vivo

By using immunohistochemical approaches at the light and electron microscopic levels, we have shown that aging modifies the subcellular distribution of the m2 muscarinic autoreceptor (m2R) differentially at somato-dendritic postsynaptic sites and at axonal presynaptic sites in cholinergic basalocortical neurons, in vivo. In cholinergic perikarya and dendrites of the nucleus basalis magnocellularis (NBM), aging is associated with a decrease of the density of m2R at the plasma membrane and in the cytoplasm, suggesting a decrease of the total number of m2R in the somato-dendritic field. In contrast, the number of substance P receptors per somato-dendritic surface was not affected. In the frontal cortex (FC), we have shown a decrease of cytoplasmic m2R density also leading to a decrease of the number of m2R per surface of varicosities but with no change of the density of m2R at the membrane. Our results suggest that the decrease of m2R in the somato-dendritic field of the NBM, but not a modification of the number of presynaptic m2 autoreceptors at the plasma membrane in the FC, could contribute to the decrease of the efficacy of cholinergic transmission observed with aging in the rat.

Memory impairment in aged primates is associated with focal death of cortical neurons and atrophy of subcortical neurons

Mechanisms of cognitive decline with aging remain primarily unknown. We determined whether localized cell loss occurred in brain regions associated with age-related cognitive decline in primates. On a task requiring the prefrontal cortex, aged monkeys were impaired in maintaining representations in working memory. Stereological quantification in area 8A, a prefrontal region associated with working memory, demonstrated a significant 32 +/- 11% reduction in the number of Nissl-stained neurons compared with young monkeys. Furthermore, the number of immunolabeled cholinergic neurons projecting to this region of cortex from the nucleus basalis was also reduced by 50 +/- 6%. In contrast, neuronal number was strikingly preserved in an adjoining prefrontal cortical region also associated with working memory, area 46, and in the component of the nucleus basalis projecting to this region. These findings demonstrate extensive but highly localized loss of neocortical neurons in aged, cognitively impaired monkeys that likely contributes to cognitive decline. Cell degeneration, when present, extends transneuronally.

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.

Age-related vulnerability to diencephalic amnesia produced by thiamine deficiency: the role of time of insult

Age is a risk factor for the development of many neurological disorders, including alcohol-related neurological disorders. A rodent model of Wernicke-Korsakoff Syndrome (WKS), acute pyrithiamine-induced thiamine deficiency (PTD), produces diencephalic damage and impairments of memory similar to what is seen in WKS patients. Advanced age increases the vulnerability to the cascade of acute and some chronic neurological events caused by PTD treatment. Interactions between PTD treatment and age at the time of treatment (3, 10, or 21 months), in addition to the effects of an increased recovery period, were examined relative to spatial memory impairment and neuropathology in Fischer 344 rats. Although acute neurological disturbances and medial thalamic brain lesions were more prevalent in middle-aged and senescent rats exposed to PTD treatment, relative to young rats, behavioral data did not support the view that PTD and aging have synergistic effects. In addition, both advanced age and PTD treatment result in a loss of basal forebrain cholinergic neurons, though there was no interaction. Despite the fact that no convincing evidence was found for an effect of extended recovery time on neuropathology measures, young rats given an extensive recovery period displayed less working memory impairment. In summary, these data provide evidence for an increased susceptibility of the aged rat to the acute neurological consequences and diencephalic pathology associated with PTD treatment and indicated a similar vulnerability of the middle-aged rat. However, the synergistic interaction between aging and PTD treatment in thalamic tissue loss did not express behaviorally.

Human cholinergic basal forebrain: chemoanatomy and neurologic dysfunction

The human cholinergic basal forebrain (CBF) is comprised of magnocellular hyperchromic neurons within the septal/diagonal band complex and nucleus basalis (NB) of Meynert. CBF neurons provide the major cholinergic innervation to the hippocampus, amygdala and neocortex. They play a role in cognition and attentional behaviors, and are dysfunctional in Alzheimer's disease (AD). The human CBF displays a continuum of large cells that contain various cholinergic markers, nerve growth factor (NGF) and its cognate receptors, calbindin, glutamate receptors, and the estrogen receptors, ERalpha and ERbeta. Admixed with these cholinergic neuronal phenotypes are smaller interneurons containing the m2 muscarinic acetylcholine receptor (mAChRs), NADPH-diaphorase, GABA, calcium binding proteins and several inhibitory neuropeptides including galanin (GAL), which is over expressed in AD. Studies using human autopsy material indicate an age-related dissociation of calbindin and the glutamate receptor GluR2 within CBF neurons, suggesting that these molecules act synergistically to induce excitotoxic cell death during aging, and possibly during AD. Choline acetyltrasnferease (ChAT) activity and CBF neuron number is preserved in the cholinergic basocortical system and up regulated in the septohippocampal system during prodromal as compared with end stage AD. In contrast, the number of CBF neurons containing NGF receptors is reduced early in the disease process suggesting a phenotypic silence and not a frank loss of neurons. In end stage AD, there is a selective reduction in trkA mRNA but not p75(NTR) in single CBF cells suggesting a neurotrophic defect throughout the progression of AD. These observations indicate the complexity of the chemoanatomy of the human CBF and suggest that multiple factors play different roles in its dysfunction in aging and AD.

Effects of aging on neurons and glial cells from the superficial layers of the superior colliculus in rats

The aim of this study was to investigate the effect of aging on glial cells and neurons from the superficial layers of the superior colliculus in rats. We used stereological methods to estimate the volume of the superficial layers, neuron size, and the number of neurons and glial cells in Wistar male rats aged 3, 24, 26, and 28 months. A 32.6% volume increase was found in the stratum griseum superficiale between the ages of 3 and 26 months, while in the 28-month-old animals a 19% decrease was observed. The stratum opticum did not show any changes in volume with age. Also, our analysis revealed a process of somatic and nuclear atrophy in the neurons of the superficial layers in animals aged 26 and 28 months. On the other hand, no statistically significant differences were found in the numbers of neurons. The number of glial cells in the stratum griseum superficiale showed an increase between the 3rd and 26th month, while the stratum opticum suffered no change.

Loss of calbindin-D28K from aging human cholinergic basal forebrain: relation to plaques and tangles

Reports from our laboratory have indicated a substantial and specific loss of the calcium binding protein calbindin-D28K (CB) from the human basal forebrain cholinergic neurons (BFCN) in the course of normal aging. In the present set of experiments we determined the relationship between the age-related loss of CB and the presence and density of plaques and tangles in the brains of normal elderly. In 23 cases ranging in age from 20 to 93 years of age we observed plaques and tangles in the BFCN region and the cerebral cortex in a subset of cases. Plaques were seen in the basal forebrain in very few cases above 65 years. Plaque density in the basal forebrain and cortex displayed a significant negative correlation with the proportion of the BFCN, which contained CB immunoreactivity. However, the brains of 2 elderly cases that displayed a substantial loss of CB from the BFCN did not contain any plaques. Tangles were observed in the BFCN as early as 26 years of age. Only tangles in the entorhinal cortex showed a significant negative correlation with the loss of CB from the BFCN. It is likely that loss of CB from the BFCN and formation of plaques and tangles are part of general age-related processes that occur in parallel rather than being causally related.

Loss of calbindin-D28k from aging human cholinergic basal forebrain: relation to neuronal loss

Cholinergic neurons of the basal forebrain (BFCN) are selectively vulnerable in neurodegenerative disorders of the elderly, particularly in Alzheimer's disease (AD). We investigated age-related changes in the BFCN that may serve as a substrate for this vulnerability. We report a substantial and selective age-related loss of the calcium binding protein calbindin-D(28K) (CB) from the human BFCN. Unbiased stereological estimation indicated that, in individuals under age 65 years, 72% of the choline acetyltransferase (ChAT)-positive BFCN contained CB immunoreactivity. In individuals over age 65 years, only 28% of the BFCN contained CB immunoreactivity, a dramatic loss of 61%. Similar results were obtained using neuronal counts from matching single- or double-stained sections in a larger cohort. The loss of CB immunoreactivity was neurochemically specific. No age-related changes were observed in the number of ChAT- or low-affinity nerve growth factor receptor (p75(NTR))-immunoreactive profiles. The loss of CB was greatest in very old individuals, in whom a small loss of BFCN was observed. Furthermore, the loss of CB displayed the same pattern as the loss of BFCN in AD and was more substantial in the posterior compared with the anterior BFCN sector, suggesting a role for CB in the selective vulnerability of BFCN in AD. The depletion of CB from the BFCN is likely to deprive these neurons of the capacity to buffer high levels of intracellular Ca(2+) and thus to leave them vulnerable to pathological processes, such as those in neurodegenerative disorders, which can cause increased intracellular Ca(2+), thus leading to their degeneration.

Age-related alterations in responses of the nucleus basalis magnocellularis neurons to frontal cortex stimulation in rats

The present study investigated the age-related alterations in responses of the nucleus basalis magnocellularis (nbM) neurons to frontal cortex (FCX) stimulation. Single unit extracellular recording from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 137 units were located within the nbM in the three age groups (young, 3 months; adult, 12 months; old, 24 months). FCX stimulation elicited responses in 91% of the 137 neurons. Most of them were excited. The frequency of occurrence of excitatory responses in the nbM neurons was decreased with aging. The thresholds and latencies of excitatory responses evoked by FCX stimulation were increased in old rats. The mean peak-firing rate of exciting phase was gradually reduced with aging. These findings indicate that there might be some functional changes in the nbM neurons with aging.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Age-related alterations in responses of nucleus basalis magnocellularis neurons to peripheral nociceptive stimuli

The present study investigated the effects of peripheral noxious stimuli on the spontaneous activity of the nucleus basalis magnocellularis (nbM) neurons in young, adult and old rats. Single unit extracellular recordings from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 104 units were antidromically identified as nbM-cortical neurons. Noxious but not innocuous mechanical stimulation elicited responses in 72% of the 104 neurons. Most of them were excited. The receptive fields were usually very large and bilateral. Most of the neurons also responded to noxious thermal, chemical and electrical stimuli. No marked differences were observed in the incidence of neurons having different spontaneous firing rates, firing patterns and response type among the three age groups. However, the latency of responses evoked by noxious thermal or electrical stimulation and the threshold of excitatory responses evoked by electrical stimulation were increased with aging. The duration and peak-firing rate of excitatory responses evoked by noxious thermal, chemical or electrical stimulation were decreased in old rats. These findings indicate that there might be some functional changes in the nbM neurons and its projection pathway with aging, which impair their responsive ability to peripheral nociceptive stimuli.

Age- and dementia-associated impairments in divided attention: psychological constructs, animal models, and underlying neuronal mechanisms

Following a brief description of the psychological construct 'divided attention', impairments in divided attention and related executive functions are discussed as the major variable in the development and manifestation of age- and dementia-associated cognitive impairments. Neuropsychological and functional imaging studies in humans have indicated that dorsolateral and ventrolateral prefrontal, cingulate, parietal and premotor cortical areas are involved in the mediation of dual task performance. Furthermore, reduced activity in these areas has been suggested to mediate age- and dementia-associated impairments in divided attention. Experimental studies in animals have provided strong support for the hypothesis that cholinergic projections terminating in all cortical areas and layers crucially mediate the performance in tasks that tax processing capacity and/or the allocation of processing resources to competing demands. This specification of the 'cholinergic hypothesis' is evaluated in light of recent critical accounts of the role of this system in the development of age- and dementia- related cognitive disorders. The converging animal experimental and human neuropathological, as well as neuropsychological, evidence indicates that decreases in the integrity of cortical cholinergic inputs represent a necessary, possibly even sufficient, neuronal process mediating the impairments in divided attention and the resulting, broad decline in cognitive functions.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Increased expression of estrogen receptor α and β in the nucleus basalis of Meynert in Alzheimer’s disease

The human nucleus basalis of Meynert (NBM) is severely affected in Alzheimer's disease (AD). Since estrogens may reduce both the risk and severity of AD, possibly by an action on the cholinergic system, we determined whether estrogen receptors are present in the human NBM and what their changes are in normal aging and in AD. ERalpha was expressed to a higher degree than ERbeta and was localized mainly in the cell nucleus, while ERbeta was mainly confined to the cytoplasm. A significant positive correlation between the percentage of ERalpha nuclear positive neurons and age was found in men but not in women, whereas the proportion of ERbeta cytoplasm positive cells increased during aging in both sexes. In AD the proportion of neurons showing nuclear staining for both ERalpha and beta and cytoplasmic staining for ERbeta was markedly increased. The percentage of ERbeta nuclear positive neurons increased in AD only in women but not in men. The ApoE genotype had no effect on ER expression in the NBM in AD. In conclusion, whereas only minor sex- and age-related changes in both ERs were found in the human NBM, a clear upregulation of ERalpha and beta was observed in AD.

Aging potentiates the acute and chronic neurological symptoms of pyrithiamine-induced thiamine deficiency in the rodent

The present study aimed to assess the role of advanced age in the development and manifestation of thiamine deficiency using an animal model of Wernicke-Korsakoff syndrome (WKS). Interactions between pyrithiamine-induced thiamine deficiency (PTD) and age were examined relative to working memory impairment and neuropathology in Fischer 344 rats. Young (2-3 months) and aged (22-23 months) F344 rats were assigned to one of two treatment conditions: PTD or pair-fed control (PF). Rats in the former group were further divided into three groups according to duration of PTD treatment. Working memory was assessed with an operant matching-to-position (MTP) task; after testing, animals were sacrificed and both gross and immunocytochemical measures of brain pathology were obtained. Aged rats exhibited acute neurological disturbances during the PTD treatment regime earlier than did young rats, and also developed more extensive neuropathology with a shorter duration of PTD. Aged rats displayed increased brain shrinkage (smaller frontal cortical and callosal thickness) as well as enhanced astrocytic activity in the thalamus and a decrease in ChAT-positive cell numbers in the medial septum; the latter two measures of neuropathology were potentiated by PTD. In both young and aged rats, and to a greater degree in the latter group, PTD reduced thalamic volume. Behaviorally, aged rats displayed impaired choice accuracy on the delayed MTP task. Regardless of age, rats with lesions centered on the internal medullary lamina of the thalamus also displayed impaired choice accuracy. Moreover, increased PTD treatment duration led to increased response times on the delayed MTP task. These results suggest that aging does indeed potentiate the neuropathology associated with experimental thiamine deficiency, supporting an age coupling hypothesis of alcohol-related neurological disorders.

Age-related loss of the AMPA receptor subunits GluR2/3 in the human nucleus basalis of Meynert

Magnocellular cholinergic neurons in the basal forebrain have long been recognized as vulnerable to the pathology of Alzheimer's disease. Despite numerous anatomical, pharmacological, behavioral, and physiological investigations of these neurons the cellular mechanism that underlines their selective vulnerability remains unclear. As part of an ongoing investigation into the molecular mechanism(s) underlying neuronal vulnerability in Alzheimer's disease and normal aging, we employed immunocytochemical techniques and examined the cellular localization of the alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionate (AMPA) glutamate receptor subunits GluR1 and GluR2/3 in the basal forebrain of eight nondemented elderly human subjects (66-102 years). For each case we observed GluR1-positive magnocellular cells darkly labeled within all main divisions of the basal forebrain (Ch1-Ch4). Double-labeling immunohistochemical techniques confirmed that the overwhelming majority (94%) of these neurons were also positive for the p75NGFr antibody, thus substantiating the cholinergic nature of these neurons. In contrast, GluR2/3 immunolabeling upon magnocellular neurons was relatively faint or nonexistent. The latter observations were most apparent in cases of advanced age and in the posterior part of the nucleus basalis of Meynert (NBM) (i.e., Ch4). In contrast, in adjacent structures (e.g., globus pallidus), a number of robustly labeled GluR2/3-positive cells were observed. In addition to the eight elderly subjects, we examined GluR1 and GluR2/3 immunostaining in the NBM of five younger cases, 5, 33, 36, 47, and 48 years of age. Although practical considerations limited our observations to the Ch4 region, we observed both GluR1 and GluR2/3 labeling upon NBM neurons in this latter region. On average, the distribution of labeled cells and intensity of immunoreaction were comparable between GluR1 and GluR2/3. The presence of GluR2/3- and GluR1-labeled neurons in the Ch4 region of younger cases but primarily GluR1 in cases of advanced age suggests an age-related decrease in GluR2/3. Functionally, the loss of GluR2 from the AMPA receptor complex results in ion channels highly permeable to Ca(2+). These alterations in cation permeability of the AMPA receptor together with the occurrence of a number of other intrinsic and extrinsic events (i.e., decrease Ca(2+)-binding protein) likely contribute to the vulnerability of these neurons in aging and in AD.

Behavioral training-induced c-Fos expression in the rat nucleus basalis of Meynert during aging

The present study investigated the behavioral training-induced c-Fos expression in the nucleus basalis of Meynert (nbM) in differently aged rats. This study demonstrated that the c-Fos expression in nbM was significantly increased and the peak occurred at 2 h after dark-avoidance training. Although the increase of c-Fos expression was also observed after pseudotraining, the number of Fos-like immunoreactive neurons in pseudotrained rats was significantly less than that in dark-avoidance trained rats at each time-point. This result suggested that c-Fos expression might be involved in learning and memory processes. In addition, all the pseudotraining-, training- and memory arousing-induced c-Fos expression was decreased with increasing age, and the decrease was more notable in trained and memory aroused rats. This suggested that the total number of nbM neurons and/or the sensitivity of nbM neurons to experimental manipulations, especially learning and memory performance, might reduce during aging.

Cosmic ray hits in the central nervous system at solar maximum

It has been suggested that a manned mission to Mars be launched at solar maximum rather than at solar minimum to minimize the radiation exposure to galactic cosmic rays. It is true that the number of hits from highly ionizing particles to critical regions in the brain will be less at solar maximum, and it is of interest to estimate how much less. We present here calculations for several sites within the brain from iron ions (z = 26) and from particles with charge, z, greater than or equal to 15. The same shielding configurations and sites in the brain used in an earlier paper for solar minimum are employed so that direct comparison of results between the two solar activity conditions can be made. A simple pressure-vessel wall and an equipment room onboard a spacecraft are chosen as shielding examples. In the equipment room, typical results for the thalamus are that the probability of any particles with 7 greater than or equal to 15 and from 2.3 percent to 1.3 percent for iron ions. The extra shielding provided in the equipment room makes little difference in these numbers. We conclude that this decrease in hit frequency (less than a factor of two) does not provide a compelling reason to avoid solar minimum for a manned mission to Mars. This conclusion could be revised, however, if a very small number of hits is found to cause critical malfunction within the brain.

Stereological age-related changes in neurons of the rat dorsal lateral geniculate nucleus

Quantitative methods were used to compare the changes taking place in the volume of the dorsal lateral geniculate nucleus (dLGN) and corresponding neurons of young, adult and old rats. The study was carried out on male albino rats aged 3, 18, 24 and 28 months. In order to estimate the volume of the dLGN, neuronal volume density, numerical density and total number of neurons, we used serial sections stained according to the Klüver-Barrera technique and stereological methods. We found that dorsal lateral geniculate nucleus volume increases between 3 and 28 months, with a larger increase between 24 and 28 months. Neuronal volume density and numerical density of neurons are greater at 3 months and undergo a significant decrease between 24 and 28 months. Finally, the total number of neurons is shown to be smaller in adult and old animals than in younger ones, even though no significant variations are found between 18 and 28 months. Furthermore, this study confirms the need to analyze the total number of neurons and not just neuronal density if we want to correctly evaluate some of the microscopic changes occurring during senescence.

Age-related attenuation of stimulated cortical acetylcholine release in basal forebrain-lesioned rats

In vivo microdialysis was used to measure the effects of partial deafferentation of cortical cholinergic inputs on acetylcholine efflux in young (four to seven months) and aged (24-28 months) male F344/BNNIA rats. Partial deafferentation was produced by bilateral infusions of the immunotoxin 192 immunoglobulin G-saporin (0.56 microg/1.0 microl) or its vehicle solution into the ventral pallidum/substantia innominata region of the basal forebrain. The lesion produced comparable (65%) decreases in basal cortical acetylcholine efflux in young and aged rats. Presentation of a complex environmental stimulus (exposure to darkness/palatable food), in conjunction with the systemic administration of the benzodiazepine receptor weak inverse agonist ZK 93 426, increased cortical acetylcholine efflux in young shams, aged shams and young lesioned rats, but not in aged lesioned rats. Administration of the benzodiazepine receptor partial inverse agonist FG 7142, in the absence of the environmental stimulus, comparably stimulated cortical acetylcholine efflux in young and aged sham rats. FG 7142-induced increases in acetylcholine efflux were attenuated by approximately 50% following partial deafferentation in both young and aged rats. These results suggests that, under certain conditions, ageing potently interacts with the integrity of the cortical cholinergic afferent system. The effects of ageing on cortical cholinergic function may be most potently revealed by experiments assessing age-related limitations in the responsiveness of a partially deafferented cholinergic system to certain behavioral and/or pharmacological stimuli.

Reduced neuronal activity and reactivation in Alzheimer's disease

1. Alzheimer's disease is a multifactorial disease in which age and APOE-epsilon 4 are important risk factors. Various mutations and even viral infections such as herpes simplex (Itzhaki et al., 1997) may play an additional role. 2. The neuropathological hallmarks of Alzheimer's disease (AD), i.e. amorphous plaques, neuritic plaques (NPs), pretangles, neurofibrillary tangles (NFT) and cell death are not part of a single pathogenetic cascade but are basically independent phenomena. 3. Pretangles can occur in neurons from which the metabolic rate is not altered. However, in brain areas where classical AD changes, i.e. NPs and NFTs, are present, such as the CA1 area of the hippocampus, the nucleus basalis of Meynert and the tuberomamillary nucleus, a decreased metabolic rate is found. Decreased metabolic rate appears to be an independent phenomenon in Alzheimer's disease. It is not induced by the presence of pretangles, NFT or NPs. 4. Decreased metabolic rate may precede cognitive impairment and is thus an early occurring hallmark of Alzheimer's disease, which, in principle, may be reversible. The observation that the administration of glucose or insulin enhances memory in Alzheimer patients also supports the view that Alzheimer's disease is basically a metabolic disease. Moreover, several observations indicate that activated neurons are better able to withstand aging and AD, a phenomenon paraphrased by us as "use it or lose it". It is, therefore, attractive to direct the development of therapeutic strategies towards restimulation of neuronal metabolic rate in order to improve cognition and other symptoms in Alzheimer's disease. A number of pharmacological and non-pharmacological studies support the concept that activation of the brain indeed has beneficial effects on several aspects of cognition and other central functions.

Cosmic ray hit frequencies in critical sites in the central nervous system

One outstanding question to be addressed in assessing the risk of exposure to space travelers from galactic cosmic rays (GCR) outside the geomagnetosphere is to ascertain the effects of single heavy-ion hits on cells in critical regions of the central nervous system (CNS). As a first step toward this end, it is important to determine how many "hits" might be received by a neural cell in several critical CNS areas during an extended mission outside the confines of the earth's magnetic field. Critical sites in the CNS: the macula, and an interior brain point (typical of the genu, thalamus, hippocampus and nucleus basalis of Meynert) were chosen for the calculation of hit frequencies from galactic cosmic rays for a mission to Mars during solar minimum (i.e., at maximum cosmic-ray intensity). The shielding at a given position inside the body was obtained using the Computerized Anatomical Man (CAM) model, and a radiation transport code which includes nuclear fragmentation was used to calculate yearly fluences at the point of interest. Since the final Mars spacecraft shielding configuration has not yet been determined, we considered the minimum amount of aluminum required for pressure vessel-wall requirements in the living quarters of a spacecraft, and a typical duty area as a pressure vessel plus necessary equipment. The conclusions are: (1) variation of the position of the "target site" within the head plays only a small role in varying hit frequencies; (2) the average number of hits depends linearly on the cross section of the critical portion of the cell assumed in the calculation; (3) for a three-year mission to Mars at solar minimum (i.e., assuming the 1977 spectrum of galactic cosmic rays), 2% or 13% of the "critical sites" of cells in the CNS would be directly hit at least once by iron ions, depending on whether 60 micrometers2 or 471 micrometers2 is assumed as the critical cross sectional area; and (4) roughly 6 million out of some 43 million hippocampal cells and 55 thousand out of 1.8 million thalamus cell nuclei would be directly hit by iron ions at least once on such a mission for space travelers inside a simple pressure vessel. Also, roughly 20 million out of 43 million hippocampal cells and 230 thousand out of 1.8 million thalamus cell nuclei would be directly hit by one or more particles with z > or = 15 on such a mission.

Selective muscarinic antagonists differentially affect in vivo acetylcholine release and memory performances of young and aged rats

Brain acetylcholine release and memory performance were investigated in young (three- to six-months) and old (20- to 24-months) rats. Acetylcholine release was measured in vivo in the cortex and hippocampus of freely-moving animals, under basal conditions and in the presence of the following muscarinic antagonists: scopolamine, (+/-)-5,11-dihydro-11-[[(2-[2-[(dipropylamino) methyl]-1-piperidinyl]ethyl) amino] carbonyl]-6H-pyrido(2,3-b)(1,4)-benzodiazepine-6-one (AFDX 384) and pirenzepine. The amount of acetylcholine released from the cortex and hippocampus of old rats was significantly reduced. In the presence of scopolamine and AFDX 384 but not of pirenzepine, the acetylcholine release was significantly higher in the old than the young rats, suggesting that changes in presynaptic M2/M4 muscarinic receptor function occur with ageing in the two brain regions. Cognitive capacities were evaluated using two different behavioural tasks: object recognition and passive avoidance response. Old rats were unable to discriminate between familiar and novel objects and had impaired performance in the passive avoidance test. AFDX 384 restored the performance in both tests. Furthermore, in young rats AFDX 384 reversed the impairment of both object recognition and passive avoidance response induced by scopolamine. The effect of AFDX 384 on acetylcholine release and behaviour in the old rats offers further support to a relationship between the age-related cholinergic hypofunction and cognitive impairment and indicates the blockade of presynaptic muscarinic receptors as a possible selective target for therapeutic strategies aimed at improving age-associated memory deficits.

NOR activity in hippocampal areas during the postnatal development and ageing

The silver staining of the nucleolar organizer regions (Ag-NORs) was used in order to estimate the biosynthetic activity of three hippocampal areas (dentate gyrus, CA1 and CA3) during postnatal development and ageing. 32 Wistar rats were used and 4 groups were formed according to the age of the animals (14, 21, 90 days and 23 months). Several Ag-NOR parameters such as mean Ag-NOR area and the ratio between Ag-NOR and nuclear areas per neuronal cell were quantified using an image analysis system. High values of these parameters are associated with a high rate of rRNA transcription. In this way, the neural biosynthetic activity in all regions studied decreased as the older ages are reached. Differences between areas are shown with the dentate gyrus and CA1 areas decreasing faster. The different activity among these areas is discussed, taking into account the particular affect on these areas of some injuries and the ageing process. Our results support the hypothesis of NOR loss as a main cause of ageing as reported by other authors.

Age-related loss of calbindin from human basal forebrain cholinergic neurons

Loss of basal forebrain cholinergic neurons (BFCN) occurs in many age-related neurological diseases. Although age is the common risk factor in these disorders, no consistent age-related changes have been reported in the human BFCN. We investigated age-related alterations in choline acetyltransferase (ChAT), low-affinity nerve growth factor receptor (p75LNGFR) and calbindin-D28k (CalBP) immunoreactivity in the human BFCN. No significant age-related changes were observed in ChAT or p75LNGFR immunoreactivity. By contrast, normal aging was accompanied by a selective, substantial and significant loss of CalBP immunoreactivity from the BFCN. Other CalBP-positive neurons were unchanged. Loss of the calcium buffering capacity conferred by CalBP may leave the BFCN vulnerable to damage in neurodegenerative disorders.

Neuronal responses at the sight of objects in monkey basal forebrain subregions during operant visual tasks

The basal forebrain appears to be important in cognitive function. It has been suggested that this region is composed of several cholinergic cell groups, mainly the medial septum, the diagonal band of Broca, and the nucleus basalis of Meynert. To elucidate the functional differences between these subregions, we have recorded single-unit activity from the periventricular areas involving the medial septum, the diagonal band of Broca (ms/dbB), and the substantia innominata (SI), including the nucleus basalis of Meynert, of an object-discriminating monkey. Of 226 ms/dbB and 439 SI neurons analyzed, 36 (15.9%) and 115 (26.2%), respectively, responded to the sight of some complex object. Thirteen (5.8%) ms/dbB and 80 (18.2%) SI neurons responded to virtually all objects and the ratio of these neurons in the SI was higher than that in the ms/dbB. The other 23 (10.2%) ms/dbB and 35 (8.0%) SI neurons responded preferentially to one or two of three categories (rewarding, aversive, or meaningless) of familiar or to unfamiliar objects, and response selectivity to one category of the ms/dbB neurons (15; 6.6%) was higher than that to the SI neurons (14; 3.2%). The results suggest that the SI, including the nucleus basalis of Meynert, may encode visual information about objects more broadly and participate more fully in visual attention than the ms/dbB region, which may be more closely related to learning.

Quantitative morphological changes in neurons from the dorsal lateral geniculate nucleus of young and old rats

BACKGROUND

We studied the morphological changes occurring in neurons from the dorsal lateral geniculate nucleus (dLGN) during aging by analysing the size and shape of cell bodies and nuclei.

METHODS

Male albino Wistar rats, aged 3, 18, 24, and 30 months, were used. After appropriate tissue preparation and following the usual histological procedure, the profiles of 1,920 neuronal bodies and nuclei were drawn using a camera lucida. Data was later recorded and processed with a semiautomatic image analyser.

RESULTS AND CONCLUSIONS

We observed that dLGN neurons do not change in size from the age of 3-24 months. Between 24 and 30 months, the soma and nucleus of the cell undergo hypertrophy, 32.8% and 35.6%, respectively, when compared to those from 3-month-old animals (P < 0.01). Furthermore, we found a high correlation between cell body size/nucleus size, which does not disappear with age. The r values (correlation coefficient) were 0.7998, 0.8662, 0.8433 and 0.7304, and R2 (determination coefficient) was equal to 0.6397, 0.7504, 0.7112, and 0.5335. These latter values show that in 63.97%, 75.04%, 71.12%, and 53.35% of cases, respectively, modifications in somata size were accompanied by similar changes in nucleus size, and vice-versa. The study of the shape of the soma and nucleus of the cell revealed that both structures have a rounded-oval configuration that does not change in a significant way from adulthood to old age.

Functional and neurobiological similarities of aging in monkeys and humans

Aging in humans may be accompanied by alterations in several functional abilities. However, there is a great deal of individual variability in the functions that may be altered with age within and across aged people. One potential source of age-related behavioral variation may lie in a differential vulnerability of neurobiological systems to the aging process in particular individuals. Aged monkeys demonstrate behavioral and brain alterations that have many parallels with those observed in aged humans and are valuable animal models in which to investigate the interrelationships between age, behavior and neurobiological measures. This review outlines the similarities of functional and neurobiological aging in monkeys and humans, notes the variability that exists in both behavioral and neural systems in aging, and identifies some of the areas of aging that are in need of further investigation.

Increased vulnerability of septal cholinergic neurons to partial loss of target neurons in aged rats

To investigate whether the ageing process might affect neuron-target interactions which influence the phenotype of septal cholinergic neurons, we compared the response of these neurons to partial loss of target tissue in young adult and aged animals. Groups of young adult (four to six months) or aged (24-33 months) male Sprague-Dawley rats received unilateral infusions into the hippocampus of either the excitotoxic amino acid N-methyl-D-aspartate, or vehicle. The resulting excitotoxic lesions reduced the mean cross-sectional area of the hippocampus by 55-60%. Ipsilateral septal cholinergic neurons immunohistochemically stained for either choline acetyltransferase or low-affinity neurotrophin receptor (p75NTR) were morphometrically evaluated. In young adult rats with partial hippocampal lesions, the number and staining intensity of ipsilateral septal cholinergic neurons were not significantly different from age-matched control values, but these cholinergic neurons exhibited a significant 12% reduction in cross-sectional area. In aged rats with hippocampal lesions of equivalent size, ipsilateral cholinergic neurons showed a significant 29% reduction in cross-sectional area, a significant 19% reduction in choline acetyltransferase staining intensity as measured by densitometry, and a significant 21% reduction in the number of choline acetyltransferase- but not p75NTR-stained septal neurons, as compared with age-matched control animals. These findings show that in aged rats, septal cholinergic neurons atrophy more severely in response to the partial loss of their target neurons than in young adult rats, in the form of pronounced cell shrinkage and down-regulation of intracellular levels of the transmitter-synthesizing enzyme, choline acetyltransferase, in some cases to the point of the absence of detectable staining for this marker in some cells. The continued detection of p75NTR indicates that significant neuronal cell death did not take place. These findings suggest that basal forebrain cholinergic neurons have an increased vulnerability to disturbances of neuron-target interactions in aged animals, which may contribute to the degenerative changes exhibited by these cholinergic neurons in ageing and age-related conditions such as Alzheimer's disease.