Aging in the rat olfactory bulb: quantitative changes in mitral cell organelles and somato-dendritic synapses

Age-Related Olfactory Dysfunction: Epidemiology, Pathophysiology, and Clinical Management

Like other sensory systems, olfactory function deteriorates with age. Epidemiological studies have revealed that the incidence of olfactory dysfunction increases at the age of 60 and older and males are more affected than females. Moreover, smoking, heavy alcohol use, sinonasal diseases, and Down’s syndrome are associated with an increased incidence of olfactory dysfunction. Although the pathophysiology of olfactory dysfunction in humans remains largely unknown, studies in laboratory animals have demonstrated that both the peripheral and central olfactory nervous systems are affected by aging. Aged olfactory neuroepithelium in the nasal cavity shows the loss of mature olfactory neurons, replacement of olfactory neuroepithelium by respiratory epithelium, and a decrease in basal cell proliferation both in the normal state and after injury. In the central olfactory pathway, a decrease in the turnover of interneurons in the olfactory bulb (OB) and reduced activity in the olfactory cortex under olfactory stimulation is observed. Recently, the association between olfactory impairment and neurodegenerative diseases, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), has gained attention. Evidence-based pharmacotherapy to suppress or improve age-related olfactory dysfunction has not yet been established, but preliminary results suggest that olfactory training using odorants may be useful to improve some aspects of age-related olfactory impairment.

Decreased demand for olfactory periglomerular cells impacts on neural precursor cell viability in the rostral migratory stream

The subventricular zone (SVZ) provides a constant supply of new neurons to the olfactory bulb (OB). Different studies have investigated the role of olfactory sensory input to neural precursor cell (NPC) turnover in the SVZ but it was not addressed if a reduced demand specifically for periglomerular neurons impacts on NPC-traits in the rostral migratory stream (RMS). We here report that membrane type-1 matrix metalloproteinase (MT1-MMP) deficient mice have reduced complexity of the nasal turbinates, decreased sensory innervation of the OB, reduced numbers of olfactory glomeruli and reduced OB-size without alterations in SVZ neurogenesis. Large parts of the RMS were fully preserved in MT1-MMP-deficient mice, but we detected an increase in cell death-levels and a decrease in SVZ-derived neuroblasts in the distal RMS, as compared to controls. BrdU-tracking experiments showed that homing of NPCs specifically to the glomerular layer was reduced in MT1-MMP-deficient mice in contrast to controls while numbers of tracked cells remained equal in other OB-layers throughout all experimental groups. Altogether, our data show the demand for olfactory interneurons in the glomerular layer modulates cell turnover in the RMS, but has no impact on subventricular neurogenesis.

Olfactory Dysfunction in the Elderly: Basic Circuitry and Alterations with Normal Aging and Alzheimer's Disease

Preclinical detection of Alzheimer disease is critical to determining at-risk individuals in order to improve patient and caregiver planning for their futures and to identify individuals likely to benefit from treatment as advances in therapeutics develop over time. Identification of olfactory dysfunction at the preclinical and early stages of the disease is a potentially useful method to accomplish these goals. We first review basic olfactory circuitry. We then evaluate the evidence of pathophysiological change in the olfactory processing pathways during aging and Alzheimer disease in both human and animal models. We also review olfactory behavioral studies during these processes in both types of models. In doing so, we suggest hypotheses about the localization and mechanisms of olfactory dysfunction and identify important avenues for future work.

Mitral and tufted cells are potential cellular targets of nitration in the olfactory bulb of aged mice

Olfactory sensory function declines with age; though, the underlying molecular changes that occur in the olfactory bulb (OB) are relatively unknown. An important cellular signaling molecule involved in the processing, modulation, and formation of olfactory memories is nitric oxide (NO). However, excess NO can result in the production of peroxynitrite to cause oxidative and nitrosative stress. In this study, we assessed whether changes in the expression of 3-nitrotyrosine (3-NT), a neurochemical marker of peroxynitrite and thus oxidative damage, exists in the OB of young, adult, middle-aged, and aged mice. Our results demonstrate that OB 3-NT levels increase with age in normal C57BL/6 mice. Moreover, in aged mice, 3-NT immunoreactivity was found in some blood vessels and microglia throughout the OB. Notably, large and strongly immunoreactive puncta were found in mitral and tufted cells, and these were identified as lipofuscin granules. Additionally, we found many small-labeled puncta within the glomeruli of the glomerular layer and in the external plexiform layer, and these were localized to mitochondria and discrete segments of mitral and tufted dendritic plasma membranes. These results suggest that mitral and tufted cells are potential cellular targets of nitration, along with microglia and blood vessels, in the OB during aging.

A study of mitochondria in spinal ganglion neurons during life: quantitative changes from youth to extremely advanced age

In view of the central role that mitochondria are thought to play in the ageing process, we investigated changes in mitochondria of spinal ganglion neurons in rabbits aged 1, 3.6, 6.7, and 8.8 years (the latter extremely old). Mitochondrial size increased significantly with age, while mitochondrial structure did not change. The total volume of mitochondria within the perikaryon did not change significantly during life. This indicates that in these neurons mitochondrial degradation was completely compensated by the production of new mitochondria even in the extremely advanced age. We also found that the mean volume of neuronal perikaryon increased markedly with age, so that the mean percentage of perikaryal volume occupied by mitochondria decreased significantly with a difference of about 33% between the youngest and the oldest animals. This decrease is only in very small part due to lipofuscin accumulation, so that the ratio of the total volume of mitochondria to the volume of functionally active cytoplasm decreased with age. The mitochondria of the neurons studied seem therefore unable to adapt their total volume to the volume of functionally active cytoplasm, that increases with age. This result is consistent with the observation that the neurons of old animals have a reduced ability to respond to high energy demands.

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

An ultrastructural and quantitative study of the age-related changes occurring in the relay neurons of the dorsal lateral geniculate nucleus (dLGN) was carried out using male Wistar rats aged 3, 18, 24, and 28 months. Morphometric techniques were used to obtain data regarding cellular activity including soma, nuclear, and nucleolar size. Volume fractions for rough endoplasmic reticulum (RER), mitochondria, and lipofuscin, as well as numbers and sizes of mitochondria and dense bodies (DB) was also calculated. Among the few alterations found in the perikaryon, we can highlight the redistribution and fragmentation of RER and an increase and progressive aggregation of lipofuscin. Quantitative data show a significant decrease in the volume of the soma (-42.77%) and the nucleus (-33.66%), and in the volume fraction of the RER (-18.81%) and mitochondria (-10.16%). A significant increase in lipofuscin (+213.29%), and variations in size and number of mitochondria and dense bodies were also found. Some histophysiological considerations about the findings are discussed. The findings lead to the conclusion that a relative degree of morphological stability is exhibited by relay neurons, although the quantitative data show evident intracellular changes, especially from 24 to 28 months. These changes suggest that accompanying physiological alterations may occur, with putative effects on visual function during ageing.

Comparative study of aging in the mouse olfactory bulb

Gene knockout technologies have been used to elevate the mouse as a model species. However, little work has examined age and strain differences in the mouse olfactory system. The present study compared the olfactory bulbs of mature (6 month) and aged (24 month) males of BALB/cBy, C57BL/6J, and DBA/2 strains. Volumes of the glomerular (GLM), external plexiform (EPL), and mitral/granule cell (MIG) layers varied little from strain to strain. Volume measurements increased with age even when corrected for body weight differences. Two nonoverlapping interneuron populations were examined with immunohistochemistry. Staining for the calcium binding protein calretinin varied little between strains, but age-related increases in staining were seen in EPL of C57BL/6J mice. Typical patterns of tyrosine hydroxylase immunoreactivity were observed in all subjects except for old DBA/2 mice, which evidenced considerable staining in submitral areas. Age-related increases were observed in BALB/cBy and DBA/2 mice but not in the C57BL/6J strain. Glial fibrillary acidic protein staining was similar in old BALB/cBy and DBA/2 mice, with astrocytes in all layers of the bulb, but more concentrated in the MIG. However, C57BL/6J tissue revealed very large astrocytes relatively evenly distributed in all layers. Cell proliferation dropped dramatically with age. Labeled cells could still be observed along the lateral ventricles, but very few were observed within the rostral migratory stream or subventricular zone. Although TUNEL labeling revealed many apoptotic figures in the granule cell layer of young subjects, almost no staining was seen in aged mice.

Changes with age in the Golgi apparatus of rabbit spinal ganglion neurons

We studied the Golgi apparatus in spinal ganglion neurons of rabbits aged 12, 42 and 79 months. We found no structural changes, no indications of fragmentation, no indications of peripheral displacement affecting this organelle with advancing age. The volume of the perikaryon increased significantly with age, whereas the total volume of the Golgi apparatus remained essentially constant. Hence the mean percentage of perikaryal volume occupied by the Golgi apparatus decreased with age. This decrease was only in very minor part a consequence of lipofuscin accumulation, so that the ratio between the total volume of the Golgi apparatus and the functionally active volume of cytoplasm decreased with age. This decrease could be related to the reduced neuronal metabolism that occurs during ageing. It is possible that the delivery to the axon of newly synthesized proteins destined for fast transport is one of the Golgi apparatus roles that decrease with age. Finally, we found that the age-related quantitative changes in the Golgi apparatus did not differ between large light and small dark neurons.

Age-related changes in rat cerebellar basket cells: a quantitative study using unbiased stereological methods

Cortical cerebellar basket cells are stable postmitotic cells; hence, they are liable to endure age-related changes. Since the cerebellum is a vital organ for the postural control, equilibrium and motor coordination, we aimed to determine the quantitative morphological changes in those interneurons with the ageing process, using unbiased techniques. Material from the cerebellar cortex (Crus I and Crus II) was collected from female rats aged 2, 6, 9, 12, 15, 18, 21 and 24 mo (5 animals per each age group), fixed by intracardiac perfusion, and processed for transmission electron microscopy, using conventional techniques. Serial semithin sections were obtained (5 blocks from each rat), enabling the determination of the number-weighted mean nuclear volume (by the nucleator method). On ultrathin sections, 25 cell profiles from each animal were photographed. The volume density of the nucleus, ground substance, mitochondria, Golgi apparatus (Golgi) and dense bodies (DB), and the mean surface density of the rough endoplasmic reticulum (RER) were determined, by point counting, using a morphometric grid. The mean total volumes of the soma and organelles and the mean total surface area of the RER [SN (RER)] were then calculated. The results were analysed with 1-way ANOVA; posthoc pairwise comparisons of group means were performed using the Newman-Keuls test. The relation between age and each of the parameters was studied by regression analysis. Significant age-related changes were observed for the mean volumes of the soma, ground substance, Golgi, DB, and SN (RER). Positive linear trends were found for the mean volumes of the ground substance, Golgi, and DB; a negative linear trend was found for the SN (RER). These results indicate that rat cerebellar basket cells endure important age-related changes. The significant decrease in the SN (RER) may be responsible for a reduction in the rate of protein synthesis. Additionally, it may be implicated in a cascade of events leading to cell damage due to the excitotoxic activity of glutamate, which could interfere in the functioning of the complex cerebellar neuronal network.

Quantitative changes in mitochondria of spinal ganglion neurons in aged rabbits

Within the context of our research on the age-related structural changes in spinal ganglia, we studied the mitochondria of the neuronal perikaryon in the spinal ganglia of 12-, 42-, and 79-month-old rabbits. Both the volume of the perikaryon and the total mitochondrial mass within the perikaryon increased significantly passing from young adult to old animals. Hence, there is no net loss of mitochondria in these neurons with age. Since, however, the volume of the perikaryon increased by more than 63% while the total mitochondrial mass within the perikaryon increased by only 18%, the mean percentage of perikaryal volume occupied by mitochondria decreased with age. This decrease is only in very minor part a consequence of lipofuscin accumulation, so that the ratio between the total mitochondrial mass and the functionally active volume of cytoplasm decreased with age. Possible causes of this decrease are discussed briefly. Moreover, while the mitochondrial structure did not change, mitochondrial size increased with age. Finally, in each of the three age groups both the mean percentage volume of mitochondria and the mean mitochondrial size were very similar in large light and in small dark neurons.

Implication of cysteine proteases calpain, cathepsin and caspase in ischemic neuronal death of primates

Although more than 8000 papers of apoptosis are published annually, there are very few reports concerning necrosis in the past few years. A number of recent studies using lower species animals have suggested that the cornu Ammonis (CA) 1 neuronal death after brief global cerebral ischemia occurs by apoptosis, an active and genetically controlled cell suicide process. However, the studies of monkeys and humans rather support necrosis, the calpain-mediated release of lysosomal enzyme cathepsin after ischemia conceivably contributes to the cell degeneration of CA1 neurons. This paper provides an overview of recent developments in ischemic neuronal death, presents the cascade of the primate neuronal death with particular attentions to the cysteine proteases, and also indicates selective cathepsin inhibitors as a novel neuroprotectant. Furthermore, the possible interaction of calpain, cathepsin, and caspase in the cascade of ischemic neuronal death is discussed.

Quantitative age-changes in endoplasmic reticulum and nucleus of cerebellar granule cells

A stereological study was performed on cerebellar granule cells from rats 2 to 24 months of age (eight different ages, five animals per age group) to quantify age-related alterations in the rough endoplasmic reticulum (RER). The mean surface density and the mean total surface area of the nucleus, as well as the mean absolute volume of euchromatin per cell, were also estimated to examine whether or not these had quantitative relationships with the RER. The mean surface density and the mean total surface area of RER per cell changed significantly, attaining maximum values at 24 months of 1733 microm(2)/1000 microm(3) (0.06) and 64 microm(2) (0.03), respectively, (coefficients of variation in parentheses). The corresponding values at 2 months were 706 microm(2)/1000 microm(3) (0.20) and 26 microm(2) (0.24). The mean absolute volume of the euchromatin changed significantly, with a minimum value of 57 microm(3) (0.05) occurring at 21 months. We postulate that the increase in RER may be part of a mechanism that compensates for an age-related decrease in euchromatin. An increase in the RER network may improve intracellular transport of proteins, production of which is apparently diminished with aging. The increase may also compensate for the reported decrease in calcium buffer capacity of smooth endoplasmic reticulum.

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.

Behavioural and glial changes in old rats following environmental enrichment

The effects of enriched environment on short-term memory for event durations and on astrocytes (cell density, cell area and % of GFAP immunoreactivity) in hippocampus (Hi), frontal cortex (FC) and corpus callosum (CC) were analysed in old rats housed from weaning to the end of behavioural testing (23 months) either in standard (SC) or in enriched (EC) conditions and in young adults (5 months) all housed in SC. Old SC and EC and young SC rats trained (for 2 months) or not, in a Symbolic Delayed Matching to Sample Task, had to discriminate and remember two (2- and 10-s) signals after short retention intervals. Results confirm the aging-related acquisition and memory deficit. EC reduced the slowness of acquisition, reversed the short-term memory deficit and promoted the retention of the short signal (choose short effect). Old SC naive rats had many hypertrophied astrocytes with long processes in Hi and CC while old EC rats had decreased astrocytes number and size. The behavioural testing resulted in young adult SC rats in Hi and CC, in increased astrocytes number, size and GFAP% and in their decrease in old SC rats. EC and testing have additive effects (very low astrocytes number, size and GFAP%) to compensate for the aging-induced gliosis, mostly in Hi.

Olfactory bulb and tract and temporal lobe volumes. Normative data across decades

The sense of smell shows a diminution with age as measured by the University of Pennsylvania Smell Identification Test (UPSIT). To ascertain whether the volumes of the olfactory bulbs and tracts (OBTs) and the temporal lobes (TL) declined in parallel to smell function, we examined 36 individuals from ages 22 to 78 who did not complain of any loss of the sense of smell using magnetic resonance (MR) imaging. The OBT volumes showed an initial increase to the 4th decade of life and then a decrease with increasing age, while the trend in TL volume was not as dramatic. There was no correlation between OBT or TL volumes with unilateral or total UPSIT scores. The normative data by decades can be used to assess the OBTs of cohorts of patients with neurodegenerative disorders that affect olfaction.

Suppression of cathepsins B and L causes a proliferation of lysosomes and the formation of meganeurites in hippocampus

Cultured hippocampal slices exhibited prominent ultrastructural features of brain aging after exposure to an inhibitor of cathepsins B and L. Six days of treatment with N-CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD) resulted in a dramatic increase in the number of lysosomes in the perikarya of neurons and glial cells throughout the slices. Furthermore, lysosomes in CA1 and CA3 pyramidal cells were not restricted to the soma but instead were located throughout dendritic processes. Clusters of lysosomes were commonly found within bulging segments of proximal dendrites that were notable for an absence of microtubules and neurofilaments. Although pyknotic nuclei were sometimes encountered, most of the cells in slices exposed to ZPAD for 6 d appeared relatively normal. Slices given 7 d of recovery contained several unique features, compared with those processed immediately after incubation with the inhibitor. Cell bodies of CA1 neurons were largely cleared of the excess lysosomes but had gained fusiform, somatic extensions that were filled with fused lysosomes and related complex, dense bodies. These appendages, similar in form and content to structures previously referred to as "meganeurites," were not observed in CA3 neurons or granule cells. Because meganeurites were often interposed between cell body and axon, they have the potential to interfere with processes requiring axonal transport. It is suggested that inactivation of cathepsins B and L results in a proliferation of lysosomes and that meganeurite generation provides a means of storing residual catabolic organelles. The accumulated material could be eliminated by pinching off the meganeurite but, at least in some cases, this action would result in axotomy. Reduced cathepsin L activity, increased numbers of lysosomes, and the formation of meganeurites are all reported to occur during brain aging; thus, it is possible that the infusion of ZPAD into cultured slices sets in motion a greatly accelerated gerontological sequence.

Light microscopic quantification of morphological changes during aging in neurons and glia of the rat parietal cortex

BACKGROUND

Different changes in neuronal and glial population of the aging brain have been described; however, the degree and extent of these changes are controversial. This study evaluates the quantitative and cytomorphometric effects of aging on neuronal and glial populations in the parietal cortex of the rat.

METHODS

The study was performed in two groups of rats aged 4-6 and 30-32 months. Cortical volume, neuronal density, glial density, and neuronal area, and shapes of the soma and nucleus were analyzed in cortical layers I, II-IV, V, and VI using serial sections stained with cresyl-fast-violet, and quantitative morphometric techniques.

RESULTS

No changes with age were found in volume of the cortex or neuronal density. Glial density increased significantly (mean for all layers 17%) in older rats. Layers II-IV, V, and VI showed an age-related decrease in the area of the neuronal soma. Neuronal shape, as revealed by the major/minor diameter ratio, also showed a decrease in old rats but only in layer II-IV. Nuclear area decreased with age only in layer VI.

CONCLUSIONS

The stability of neuronal density together with the increased number of glial cells and the changes in neuronal soma size suggest that aged-related cognitive impairment could be a consequence of neuronal dysfunction rather than actual neuronal losses.

Expression of tyrosine hydroxylase in the aging, rodent olfactory system

Tyrosine hydroxylase (TH) mRNA, immunoreactivity, and activity were examined as a reflection of dopamine expression in juxtaglomerular neurons intrinsic to the olfactory bulbs of young (6-month-old), middle aged (18-month-old), and aged (25- to 29-month-old) rats and mice. TH expression was maintained at levels observed in young animals in the olfactory bulbs of aged animals from two mouse strains, C57Bl/6JNia and C57Bl/6NNia, and one rat strain, an F1 hybrid between F344 and Brown Norway strains. The parental F344 rat strain exhibited reductions in TH expression of about 20% in 26- to 29-month-old animals as compared to 6- and 18 month-old rats. However, there was significant inter-animal variability. Some aged F344 rats had TH levels that were similar and others had activity levels that were 50% of those in young and middle aged animals. Neither the general condition of the animals nor the presence of adrenal tumors predicted the individuals with reduced TH expression. Olfactory bulb size, estimated from protein content, did not differ between rats and mice of different ages. In addition, expression of olfactory marker protein, a protein found primarily in mature olfactory receptor neurons, also was unchanged indicating the maintenance of afferent innervation. These data suggest that, in contrast to other brain dopamine systems, the expression of the dopamine phenotype is maintained in the aging olfactory bulb.

The lysosomal system in neuronal cell death: a review

The lysosomal system has often been considered a prominent morphologic marker of distressed or dying neurons. Lysosomes or their constituent hydrolases have been viewed in different neuropathologic states as either initiators and direct agents of cell death, agents of cellular repair and recompensation, effectors of end-stage cellular dissolution, or autolytic scavengers of cellular debris. Limited data and limitations of methodology often do not allow these potential roles to be discriminated. In all forms of neurodegeneration, it may be presumed that lysosomes ultimately rupture and release various hydrolases that promote cell autolysis during the final stages of cellular disintegration. Beyond this perhaps universal contribution to cell death, the degree to which the lysosomal system may be involved in neurodegenerative states varies considerably. In many conditions, morphologic evidence for activation of the lysosomal system is minimal or undetectable. In other cases, lysosomal activation is evident only when other morphologic signs of cell injury are also present. This level of participation may be viewed as either an attempt by the neuron to compensate for or repair the injury or a late-stage event leading to cell dissolution. The early involvement of the lysosomal system in neurodegeneration occurs most commonly in the form of intraneuronal accumulations of abnormal storage profiles or residual bodies (tertiary lysosomes). Very often the lysosomal involvement can be traced to a primary defect or dysfunction of lysosomal components or to accelerated or abnormal membrane breakdown that leads to the buildup of modified digestion-resistant substrates within lysosomes. Because they are often striking, changes in the lysosomal system are a sensitive morphologic indicator of certain types of metabolic distress; however, whether they reflect a salutary response of a compromised neuron or a mechanism to promote cell death and removal of debris from the brain remains to be established for most conditions. Factors that may influence the lysosomal response during lethal neuronal injury include species differences, stage of neuronal development, duration of injury and pace of cell death. The lysosomal system may be more closely coupled to certain forms of neuronal cell death in lower vertebrate or invertebrate systems than in mammalian systems.

Aging and the neurocytoskeleton

It has been often demonstrated that during senescence some neurons undergo atrophic changes while others add new processes and terminals. Because microtubules form a substantial component of the dendritic and axonal cytoskeleton, we have studied the amount of tubulin and acetylated alpha-tubulin in three young (6 months) and three old (24 months) rats (Fischer 344). We have used sodium dodecyl sulfate (SDS) extracts of brain homogenates and Triton solubilized fractionated brain homogenates. With the first method we did not detect any age-related differences in total brain protein, total tubulin, or in relative amounts of acetylated alpha-tubulin. With the second method, we have observed a small systematic increase in relative amount of acetylated alpha-tubulin in the Ca2+/cold insoluble fraction. These results are similar to those reported in the literature, and they indicate a possible alteration in the cytoskeletal dynamics.

Quantitation of synaptic density in the septal nuclei of young and aged Fischer 344 rats

Synaptic density in the medial and lateral septal nuclei was examined in 3 and 24-28 months of age Fischer 344 rats. The lateral nucleus had a higher synaptic density than the medial region in both age groups. There were no statistically significant differences in synapse density in either region as a function of age, but the data suggested a subpopulation of aged animals which did show an age-related decline in synaptic density in the lateral, but not medial area of the septum. These data indicate that sample size may be an important variable in assessing possible age-related differences in synaptic density, since a broad range of values, some significantly below the range of young animals, exists in the aged brain.

Dendritic hypertrophy in Purkinje neurons of old Fischer 344 rats after long-term ethanol treatment

Metric parameters of Purkinje cell dendritic networks in 24- to 26-month-old Fischer 344 rats were determined after 48 weeks of chronic ethanol intake. Measurements included the total number and length of all segments/network, the total number and length of segments within topologically defined segment categories, and the mean length of segments in each category. A main effect of ethanol was expressed as a significant increase in cumulative length within one category of terminal segments. This increase was the result of a significant increase in mean length/segment in that category. Metric changes in other segment categories were not significant, showing that changes in the networks during ethanol treatment were not distributed randomly. Recovery after ethanol treatment was associated with further nonrandom remodeling of these networks. Significant differences in lengths of terminal segments were no longer present, but internal segments in networks of both pair-fed and ethanol-fed rats were significantly longer. Only one category of internal segments showed this change during recovery. The data showed that long term ethanol treatment in old rats, at a time when effects of aging processes were prominent in Purkinje cell networks, was associated with remodeling of those networks through dendritic extension. This effect was interpreted as compensatory growth in surviving networks following ethanol-related neuronal loss and/or damage to the surrounding neuropil. Recovery from dietary treatment produced further internal remodeling of those networks that was not related specifically to ethanol. It could be shown, however, that the restructuring processes that resulted in longer internal segments after dietary recovery were different in the pair-fed and the ethanolfed rats.

125I-insulin binding is decreased in olfactory bulbs of aged rats

125I-insulin binding was studied in membrane preparations of olfactory bulb, frontal cortex, hippocampus and hypothalamus from mature (5-month-old) and aged (22-month-old) rats. In the young animals the highest level of specific insulin binding was found in the olfactory bulb, with lower values of specific insulin binding in the frontal cortex, hippocampus and hypothalamus. In the aged rats the specific insulin binding was not changed in the frontal cortex, hippocampus and hypothalamus as compared to the young ones. A significant decrease of total insulin binding was observed only in the olfactory bulbs of aged rats (0.67 +/- 0.04 pmol insulin/mg protein) as compared to the mature animals (1.3 +/- 0.08 pmol insulin/mg protein). Scatchard analysis of insulin binding data revealed that this decrease was due to changes in the number of binding sites rather than to changes in the affinity of insulin receptors. It was suggested that the decrease observed in insulin receptor number in olfactory bulbs of aged rats might be due to the atrophic changes in the structure of olfactory bulbs previously shown by electron microscopy for aged rats.

Influence of the odorization of the rearing environment on the development of odor-guided behavior in rat pups

The study describes a phenomenon in which attraction of rat pups to artificial odors disappeared after the first week of life. In experiment 1, pups were continually exposed to either: (1) the normal odors present in the litter or (2) the same odors enriched with one of four artificial odors present in the dam's food. Pups were tested daily with the odors of normal or adulterated soiled shavings from their nests. The results show that attraction to the normal test shavings lasted throughout the testing period (PN 1-7). However, pups raised on odorized shavings exhibited an attraction to the artificial odor until day 6 only, not on day 7. In experiment 2, pups were tested with dam's artificially adulterated food. The results show that the artificial odor, and not the food odor, was responsible for the lack of attraction on day 7. Experiment 3 was carried out to determine whether the date on which attraction to artificial odors changed might be specific to postnatal day 7 or whether the duration of odor exposure in test conditions was the important factor. The results suggest that the age of pup is the more important variable.

Age-related loss of synaptic terminals in the rat medial nucleus of the trapezoid body

The effect of aging on axosomatic synaptic terminals in the rat medial nucleus of the trapezoid body was studied using quantitative electron microscopy. In young adult rats (3 months of age), the mean percentage of the surface area of principal cells covered by synaptic terminals is 61.7% (S.E.M. = 4.1) while in aged animals (27-33 months of age) the per cent coverage is 43.7% (S.E.M. = 3.3). Likewise, between 3 and 27-33 months of age, the average number of synaptic terminals present along a 100 micron length of principal cell surface decreases significantly (P less than 0.001) from 28.3 (S.E.M. = 1.3) to 18.9 (S.E.M. = 1.3). Only terminals derived from calyces of Held are lost in the aged animals, displaying a 37% reduction between 3 and 27-33 months of age. The length of apposition by synaptic terminals in the medial nucleus of the trapezoid body does not change significantly with aging. We conclude that because of a significant loss of calycine synaptic endings, the structure of calyces of Held becomes less complex with advancing age in rats. This would presumably result in an age-related partial deafferentation of principal cells, causing significant alterations in the processing of auditory information in the medial nucleus of the trapezoid body.

Morphometric parameters of Purkinje dendritic networks after ethanol treatment during aging

Spine densities on terminal branches of Purkinje cell dendrites of Fischer 344 rats were significantly altered by ethanol treatment and aging processes. An effect of the control liquid diet on the lengths of terminal branches and an interaction effect of this diet with age on the numbers of terminal branches/cell also occurred, but there were no detectable interaction effects of ethanol with age on dendritic parameters. Changes in spine density on terminal branches, which accounted for 63-67% of the total dendritic length/cell, represented a major quantitative modulation of synaptic input to these neurons during age and following ethanol treatment.

Environmental conditions modulate degeneration and new dendrite growth in cerebellum of senescent rats

Spiny branchlets of cerebellar Purkinje neurons, thought to be sites of synaptic efficacy change during motor learning, degenerate during aging. To examine effects of behavioral experience on degeneration, Purkinje neurons were studied in aging rats housed for 4.5 months either under complex environment conditions promoting sensory-motor activity or in pairs in standard cages. Their data were compared with those of a baseline group of rats from standard cages sacrificed at the age of onset of differential housing for the older groups. Rats housed in the complex environment had more spiny branchlets than the other groups, indicating that new branches had formed. There was a net loss of summed total spiny branchlet material per Purkinje cell in both the laboratory cage and complex environment older groups, although the complex environment group had more spiny branchlet per cell than the laboratory cage group. Thus, dendrite loss in the aging cerebellum can be partly offset by appropriate experience. There was no net loss of Purkinje cell main branch dendrite with aging, as indicated by previous studies, and there was no effect of differential housing upon main branches in the older groups. However, changes in the pattern of branching in the main dendritic field suggested that this region undergoes reorganization with aging.

Incoming synapses and size of small granule-containing cells in a rat sympathetic ganglion after post-ganglionic axotomy

A quantitative ultrastructural study has been made of the reaction of the incoming synapses of small granule-containing cells after axotomy of the major post-ganglionic branches of the superior cervical ganglion of the young adult rat. These cells are intrinsic and interneurone-like in this ganglion, receiving a preganglionic input and giving outgoing synapses to principal post-ganglionic neurones. Unlike their outgoing synapses, which are lost after post-ganglionic axotomy (Case & Matthews, 1986), the incoming synapses of the small granule-containing cells in axotomized ganglia increased in incidence post-operatively. The increase first became clearly evident 5-7 days post-operatively and was greater, being both more sustained and progressive, after bilateral than after unilateral axotomy. After bilateral axotomy the incidence of incoming synapses rose to more than four times that of normal ganglia and was still elevated at 128 days post-operatively, but was within normal limits at 390 days. After a unilateral lesion, increases of similar extent and time course to those in the axotomized ganglia were seen in the incoming synapses of small granule-containing cells in the uninjured contralateral ganglia. The incoming synapses of the small granule-containing cells are multifocal, i.e. show several points or active foci of synaptic specialization. The increase in synapses expressed itself both through an increased incidence of these synaptic active foci per nerve terminal and through an increase in the number of presynaptic nerve terminal profiles associated with the cells. Control observations indicated that the increase in synapses was not due to surgical stress, nor was it attributable solely to post-operative ageing. The nerve terminals which were presynaptic to the small granule-containing cells post-operatively were all of preganglionic origin: no incoming synapses or presynaptic nerve terminals remained at 2 days after a preganglionic denervation of axotomized or contralateral ganglia, at whatever stage this was performed throughout the range of survival intervals. There was some evidence that the synapses had increased by sprouting, including terminal sprouting, of the preganglionic nerve fibres. In the shorter term there was an increase in the proportion of small nerve terminal profiles. In the longer term the mean size of the terminal profiles increased, and very large terminals of unusual form were seen. After post-ganglionic axotomy, and in particular after a bilateral lesion, the small granule-containing cells became hypertrophied.(ABSTRACT TRUNCATED AT 400 WORDS)

A morphometric study of the effects of maturation and aging on synaptic patterns in the spinal trigeminal nucleus of the cat

Morphometric methods have been used to study the synaptic and terminal patterns in cat trigeminal nucleus, pars interpolaris, during development and aging. Ages 1, 3, 6, 11, 16, 21, 27, 110, 600 days and 8 and 11 years were studied. Both proportions and densities (number per unit area) of certain terminals and synapses showed significant changes with age. Axoaxonic synapses especially showed two major periods of increase (3-6 days and 21-27 days). The values of most parameters increased in the 21-27 day period to peak levels and then decreased gradually with age. The results indicate two separate critical synaptogenic periods of development and a loss of synaptic elements in aging. Factors contributing to these changes are discussed as is the potential for plasticity in the different afferents at each period.

Aging in the rat olfactory system: relative stability of piriform cortex contrasts with changes in olfactory bulb and olfactory epithelium

Previous studies have quantified growth and atrophy of the olfactory bulb and olfactory epithelium of the Sprague-Dawley rat from maturity to senescence. Major events occurring in these structures include changes in the volume of mitral cells and changes in the number of septal olfactory receptors. These effects are large, consist of a growth phase followed by atrophy, and are temporally related in that events in the olfactory epithelium precede those in the mitral cells. A hypothesis of aging based on transneuronal effects would predict that these changes would be similarly transmitted to the next synaptic station in the olfactory pathway. Therefore, cells and synapses of the piriform cortex were studied in rats 3, 12, 18, 24, 27, 30, and 33 months of age. Alternate Vibratome sections through brains perfused with mixed aldehydes were processed for light and electron microscopy. No significant age effects were found for the volumes of cortical laminae Ia and Ib. Both numerical and surface density of synaptic apposition zones in layer Ia, formed primarily by mitral cell axons, were stable with age. A modest (18%) but significant decline in the proportion of layer Ia occupied by dendrites and spines was mirrored by an increase in the proportion of glial processes; no change in the proportion of axons and terminals was observed. Neither nuclear volume, nor soma volume, nor numerical density of layer II neurons changed with age. Thus, contacts made in the piriform cortex by mitral cell axons remain relatively stable in senescence, despite the marked volumetric changes in the mitral cell somata, changes which were confirmed again in this study. Age-related dendritic regression in layer II neurons may be attributable to functional deafferentation subsequent to reduced receptor input to mitral cells.

Strain comparison of synaptic density in hippocampal CA1 of aged rats

Synaptic density in hippocampal CA1 stratum radiatum was studied in two different strains of rat at 3 and 24-28 months of age. Neither Fischer 344 nor Sprague-Dawley rodents showed any age-related loss of synapses. These data support the contention that synapse loss with age is not a generalized phenomenon in the mammalian CNS.

Synaptic density and axonal sprouting in rat hippocampus: stability in adulthood and decline in late adulthood

Synaptic density and axonal sprouting remain stable in the rat dentate gyrus up to 12 months of age. At 18-24 months, an 8-10% loss in synaptic density occurs, and synaptic reinnervation could not be clearly detected at 15 days postlesion, compared to a 40% synaptic recovery in rats lesioned at 6 or 12 months postnatal. Since the amount of synaptic degeneration remaining in the denervated zone was the same at 15 days postlesion in all rats examined, synaptic reinnervation is not primarily dependent on the rate at which degeneration is removed. The results indicate that the decline in the growth response in the rat hippocampus is not a progressive trend from early adulthood into old age, but appears after the midpoint of life.

Aging-related morphological changes in the main olfactory bulb of the Fischer 344 rat

Aging-related changes in several structural characteristics of the main olfactory bulb (MOB) were evaluated using Fischer 344 rats 3, 18, 30 or 36 months of age. Histological examination of the nasal mucosa revealed no evidence of concurrent rhinitis in any of the animals studied. The internal granular layer of the MOB exhibited continual growth, increasing in volume by a factor of 63% over the range of ages studied. The sizes of MOB mitral cell perikarya and nuclei, expressed either as observed cross-sectional areas or as estimated mean volumes, did not change significantly as a function of age. The numbers of mitral cells exhibiting 2 nucleoli in the plane of section decreased from about 22% in the youngest animals to about 4% in the 18- and 30-month old animals and to nil in the oldest animals. These results are discussed in relation to findings of other investigators using Sprague-Dawley or Wistar rats. It is concluded that major structural age changes in the rat MOB are strain dependent.

Effects of complex or isolated environments on cortical dendrites of middle-aged rats

Female rats were placed into complex (EC) or isolated (IC) environments for 45 days, beginning at 450 days of age. Golgi-Cox stained layer IV stellate and layer III occipital cortical pyramidal neurons were analyzed for numbers and lengths of dendritic branches and the amount and location of dendritic material with respect to the soma. In both cell populations, neurons from EC rats had significantly more dendritic material, with differences occurring throughout the dendritic field. In the stellate population, EC neurons had significantly longer terminating (non-bifurcating) branches, and more second and fifth order branches. In pyramidal cells, EC neurons had significantly more branches at each of orders 2-5 as well as significantly longer terminating branches. This report is the first to describe substantial dendritic alterations subsequent to differential housing in middle-aged rats. These results, taken with previous work, suggest that experience may affect neuronal structure over much of an animal's lifespan.

Stability of synaptic density and spine volume in dentate gyrus of aged rats

The number of synapses per unit volume and per granule cell and the size of dendritic spines were studied in the dentate gyrus of Sprague-Dawley rats 6, 24, and 30 months of age. Neither synaptic density nor mean spine volume showed any age-related trends. An increase in granule cell packing density at 24 months and concomitant stability of the height of the granule cell layer is consistent with the idea that postnatal generation of granule cells may continue late into life. Possible explanations for the discrepancies in the literature regarding synaptic loss in this area include differences in morphometric techniques, age of animals used, regional differences within dentate gyrus, and sampling variability. Generalized synapse loss in the senescent rodent brain remains to be established.

Stability of neuron number in cortical barrels of aging mice

The question of whether age-related neuron loss occurs in the cerebral cortex of rodents, as it apparently does in humans, has not been directly answered by previous studies. The barrel, a discrete morphological and functional unit in rodent somatosensory cortex, is a favorable system in which to address the problem of neuron loss during senescence. The numerical density and absolute number of neurons as well as barrel volume were determined from a computer-assisted three-dimensional reconstruction of thick (100 microns) and semithin (1 micron) sections through a single barrel, C3, from inbred mice (C57Bl/6NNia) at 4, 12, 22, 26, 30, and 33 months of age. The number and density of neuron and glial cells and the volume of the barrel did not change significantly with age. These data indicate that neuron loss is not a universal phenomenon in senescence and that there may be significant species differences in the aging of laboratory rodents and humans.

Capillaries in aging rat olfactory bulb: a quantitative light and electron microscopic analysis

Olfactory bulbs from Charles River (Crl) rats from 3 to 36 months have been examined with light and electron microscopy. Total capillary length, surface, and volume, as well as number of endothelial cells, increases during the twofold increase in olfactory bulb volume from 3 to 18 months, but the relative density of these parameters shows no change during this time; from 18 to 36 months when neuronal cell body and dendrites are decreasing markedly in size, the relative density of capillaries shows only a modest decrease. Capillary lumen size and capillary wall thickness remain the same throughout life, but basal lamina thickness doubles from 3 to 24 months and then remains constant from 24 to 36 months. The incidence of several unusual ultrastructural features of the outer capillary basal lamina has been shown to increase with age.

Quantitative ultrastructural changes in rat cortical synapses during early-, mid- and late-adulthood

Quantitative ultrastructural analysis of rat parietal cortex was undertaken to determine the nature of the synaptic changes occurring in the molecular layer over a series of ages in early- (3 months), mid- (6 and 10 months) and late- (17 months) adulthood. The total number of synapses remained constant until 10 months of age, but decreased significantly by 17 months. Asymmetrical synapses on dendritic shafts were lost earlier (by 6 months) than asymmetrical synapses on dendritic spines (by 17 months). Symmetrical axodendritic synapses remained constant throughout adulthood. Analysis of synaptic terminal parameters revealed the following. Both individual and total presynaptic terminal areas decreased over the age range studied. Individual and total postsynaptic terminal areas, however, remained constant over the 3--17-month period. Positive correlations were obtained for the relationships between presynaptic terminal area and both age and synaptic vesicle number. The presynaptic terminal area was largest and contained the greatest number of vesicles at 3 months of age. This age was, in addition, characterized by the least numbers of mitochondria in the presynaptic terminal and spine apparatus in the postsynaptic terminal. The vacuolar and tubular cisternae of the presynaptic terminal were considerably reduced at 17 months. These data suggest that in the molecular layer of the cerebral cortex the period of adulthood is characterized by a diversity of synaptic changes. The 3-month age may reflect the end of the developmental phase and may be marked by changes in synaptic functional activity. The asymmetrical axodendritic synapses may constitute an intermediate form of synapse, capable of being transformed into axospinous synapses as dendritic spines continue to be formed in the adult.

Preservation of retinal structure in aged pigmented mice

The effects on the retina of advancing age were studied in pigmented mouse strain (C57BL/6J). The mice range in age from 65 days to 1000 days, an age well beyond the mean life span of the population (850 days). The thickness of the neuronal and plexiform layers and the planimetric density and size of the component neurons were assessed in both central (200-500 micrometers from the optic disc) and peripheral (within 200 micrometers of the retinal margin) areas. In addition, the overall size of the retina was determined by measuring its length along the horizontal meridian. Although retinas of albino rodents degenerate extensively during aging [10, 18, 31, 32, 40], in the retinas of pigmented mice neither the central nor the peripheral locus showed either marked thinning of the retinal layers or neuronal loss with advancing age. We suggest that previous findings of severe retinal degeneration in albino rodents during aging can be attributed to their lack of pigment and that pigmented animals offer a more suitable animal model for normal retinal aging.

Brain aging in normotensive and hypertensive strains of rats. II. Ultrastructural changes in neurons and glia

A variety of age-related changes occur in the structure of neurons in the cerebral cortex of Wistar-Kyoto and spontaneously hypertensive rats. The most marked alteration associated with increasing age was the deposition of lipofuscin pigment, primarily at the bases of apical dendrites of pyramidal neurons. While no strain-related differences in the amount of lipofuscin pigment were observed in the youngest (3 months) and in the aged (22-27 months) groups of rats, it appeared that hypertensive rats had larger pigment deposits at 12 months of age. At the ultrastructural level, neurons of the aged brains exhibited numerous nuclear invaginations and filamentous nuclear inclusions, increased amounts of Golgi complex and two types of cytoplasmic inclusions. The number of degenerative structures in the neuropil (membranous whorls, dystrophic axons and alterations in myelin sheaths) was also apparently increased in the aged brains. Neurofibrillary tangles were observed in dendritic processes of a 27-month-old Wistar-Kyoto rat. Glial cells accumulated distinctive pigment granules by which the three types of glia could be identified.