Ultrastructure of neurons in the auditory cortex of ageing rats: a morphometric study

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Affects of Aging on Receptive Fields in Rat Primary Auditory Cortex Layer V Neurons

Advanced age is commonly associated with progressive cochlear pathology and central auditory deficits, collectively known as presbycusis. The present study examined central correlates of presbycusis by measuring response properties of primary auditory cortex (AI) layer V neurons in the Fischer Brown Norway rat model. Layer V neurons represent the major output of AI to other cortical and subcortical regions (primarily the inferior colliculus). In vivo single-unit extracellular recordings were obtained from 114 neurons in aged animals (29–33 mo) and compared with 105 layer V neurons in young-adult rats (4–6 mo). Three consecutive repetitions of a pure-tone receptive field map were run for each neuron. Age was associated with fewer neurons exhibiting classic V/U-shaped receptive fields and a greater percentage of neurons with more Complex receptive fields. Receptive fields from neurons in aged rats were also less reliable on successive repetitions of the same stimulus set. Aging was also associated with less firing during the stimulus in V/U-shaped receptive field neurons and more firing during the stimulus in Complex neurons, which were generally associated with inhibited firing in young controls. Finally, neurons in aged rats with Complex receptive fields were more easily driven by current pulses delivered to the soma. Collectively, these findings provide support for the notion that age is associated with diminished signal-to-noise coding by AI layer V neurons and are consistent with other research suggesting that GABAergic neurotransmission in AI may be compromised by aging.

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.

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 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.

Age-related reductions in the activities of antioxidant enzymes in the rat inferior colliculus

The inferior colliculus (IC) is a major relay and processing center of auditory signals in the midbrain and receives inputs from most other auditory nuclei. A number of studies have indicated age-related declines in the GABAergic and excitatory amino acid systems in the IC, including losses in both GABA immunoreactive (+) and GABA immunonegative (-) synapses. The goal of this project was to identify potential biochemical and morphological changes in the IC that may contribute to deficits in the functions of these neurotransmitters, using three age groups of Fischer-344 rats. Homogenates obtained from the IC showed age-dependent reductions in activities of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), with a concomitant increase in lipid peroxidation. Dephosphorylation of IC homogenates with alkaline phosphatase reduced the activities of SOD and CAT in all age groups, which could be restored by protein kinase C (PKC)-dependent phosphorylation. Restoration of enzyme activity was specific to the PKC-alpha isozyme, but not to the beta1, beta2, delta or gamma forms. No age-dependent change in the levels of PKC isoforms (alpha, beta1, beta2 and gamma) was detectable in IC homogenates. Morphological analyses indicate decreases in mitochondrial density in the somata of both GABA+ and GABA- IC neurons in 19- and 28-month-old rats when compared to 3-month-olds, along with significantly higher matricial abnormalities. These data indicate age-related increases in oxidative stress in the IC, which could be partially restored by PKC. The progressive increase in oxidative stress with age may underlie changes in neuronal morphology and function of the IC.

Lysosomal dysfunction results in lamina-specific meganeurite formation but not apoptosis in frontal cortex

An inhibitor of cathepsins B and L was used to test if lysosomal dysfunction in cultured slices of rat frontal cortex induces pathological features that develop in the human cortex during aging and Alzheimer's disease (AD). Incubation for 6 days with N-CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD) resulted in a massive proliferation of endosomes-lysosomes in all cortical layers. Slices additionally exposed to a washout of 4 days had numerous meganeurites, blister-like structures in the region of the axon hillock, in layer III but not in other cortical laminae. Meganeurites are a characteristic feature of the human frontal cortex after age 50 and are largely restricted to layer III. Tests for apoptosis were carried out at two intervals following meganeurite formation. TUNEL-labeled neurons were confined to layers II/III on the surface of the slices but there was no evidence for a ZPAD effect. In all, 6 days of lysosomal dysfunction reproduces characteristic effects of normal aging in neocortex without generating some key features of AD.

Age-related changes in the volume of somata and organelles of cerebellar granule cells

Because cerebellar granule cells are fixed post-mitotic cells, it is expected that they undergo age-related changes like other neurons. To examine this possibility, a stereological study on granule cells of rat neocerebellar cortex was performed for an age spectrum of 2 to 24 months using eight different age groups. The nucleator method, together with point and intersection counting, was used to obtain primary data; arithmetical calculations determined the secondary data. In the soma, the absolute surface area did not change significantly; the volume did, however, exhibit a significant negative linear trend with age. Excluding dense bodies, the absolute volumes of the cytoplasmic components did not vary significantly. The absolute volume of dense bodies displayed a significant positive linear trend with age. Significant positive correlations were detected between the somatic volume and the absolute volume of either mitochondria or ground substance. It was concluded that granule cells showed a fair degree of morphological stability through 18 months. However, the observed changes warn that accompanying physiological alterations may occur, with putative effects on motor coordination.

The effect of ageing on neurones in the visual sector of the thalamic reticular nucleus

This paper studies the quantitative morphological changes occurring during ageing in neurones of the dorsocaudal or visual sector of the thalamic reticular nucleus. Male Wistar rats aged 3, 6, 18, 24 and 30 months were used in this study which applied morphometric methods. We have observed an increase in the size of neurones from this sector between the 3rd and 24th month and a decrease between the 24th and 30th month. In all the ages studied the majority of neurones are fusiform.

Ultrastructural changes in brain parenchyma during normal aging and in animal models of aging

During aging, the brain parenchyma of animals and humans share many similarities, both in the gray and the white matter. Unfortunately, until now, neither aged animals nor animal models reproduce the two hallmarks of aging of the human brain: senile plaques and tangles. Therefore, observations performed on animals are limited to some aspects of the involutive process which affects brain parenchyma during aging and their appropriateness to the human situation. One striking aspect concerns the occurrence of vacuolated necrotic cells whose number increases with advancing age. These cells can constitute markers of the brain involutive process and they characterize, both in animal and human, the more vulnerable areas of the brain affected by the neuronal rarefaction. Experimental animal models can be used to study the various conditions which sustain the cell survival and to determine, at the cellular level, the factors leading the brain parenchyma to an irreversible state of degradation.

Alterations in nuclear envelope invaginations in axotomized fetal and early postnatal hamster facial motoneurons

In this study, changes in the amount of nuclear envelope invaginations (NEI) were morphometrically assessed after axotomy during late fetal and early postnatal developmental stages in hamster facial motoneurons. These changes were expressed as boundary density or BA (length of nuclear envelope per unit area of nucleus). Axotomy-induced changes in nuclear area and perimeter were also quantitatively determined. At 17 h after axotomy in the fetal operative series, no changes in any of the parameters were seen. At 1 day postoperative (dpo) in newborn, 2 and 4 postnatal day animals, the boundary densities of the total and invaginated portion of the nuclear envelope increased significantly. No corresponding qualitative changes were observed. At 2 dpo in 4 and 7 postnatal day animals, there were significant increases in the boundary densities of both invaginated and total nuclear envelope and a decrease in nuclear area. These changes were not seen at 2 dpo in the 9-day operative series. At 4 dpo in 7 and 9 postnatal day animals, scalloping of the normally smooth nuclear profile, as well as a flattening and elongation in nuclear shape, occurred. These qualitative changes in the 7 and 9 day operated groups were also accompanied by significant changes in all the measured parameters. The boundary density of the invaginated, non-invaginated and total nuclear envelope increased; whereas, nuclear area and perimeter decreased. These results argue against the generally held hypothesis that an increase in nuclear envelope invaginations is indicative of an allied increase in cellular metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Cortical neuroanatomical correlates of behavioral deficits produced by lesion of the basal forebrain in rats

Morphological changes in the frontoparietal cortex were assessed in rats that exhibited deficits in a go/no go alternation task due to electrolytic lesion of the basal forebrain. Cortical area, laminar thickness, neuronal density, and soma area were examined in frontal, hindlimb, forelimb, and parietal areas of the cortex. Quantitative morphological analysis of the frontoparietal cortex in lesioned rats revealed a decrease in laminar thickness due to reduced soma size in particular cortical laminae. Neuronal density was not affected. These effects were present in all cortical areas examined and most pronounced in laminae II-III. Similar morphological changes were observed in the same cortical areas following lesions of the basal forebrain made with ibotenic acid, allowing a discrimination of lesion effects from those induced by damage to fibers of passage or differential behavioral testing. Lesions of the basal forebrain have previously been shown to produce both behavioral deficits and changes in cortical cholinergic activity. The cortical morphological changes observed in the present study following basal forebrain lesion provide further evidence for the importance of ascending cholinergic inputs to the cortex and their role in learning and memory.

Ageing changes in the transplants of fetal substantia nigra grafted to striatum of adult rat

Fetal nigral neurons from 16 and 17 gestational days were transplanted into the intact striatum of adult rat. On different post-transplantation days (30-360 days), the structural and immunohistochemical details of the transplants were studied. The grafted neurons matured and showed phenotypical characteristics comparable to that of normal nigral neurons in adult rats until 180 days. Tyrosine hydroxylase-positive neurons were seen not only in the transplant but also in the adjacent host striatum. Tyrosine hydroxylase-positive fibres were also seen extending for a short distance into the host striatum. A large number of synapses in the transplants were of asymmetric type, containing clear round vesicles. These synapses resembled the few intrinsic type present in the normal substantia nigra. On the other hand, the predominant type 2 synapses with pleomorphic vesicles in the normal nigra were infrequently encountered in the transplants. On the 300th day, the cytoplasm of a few of the neurons showed ageing changes in the form of clear spaces, paucity of organelles especially rough endoplasmic reticulum, membrane-bound vacuoles and increase in the lipofuscin population. In addition, localized thickening of the soma and the dendrites were seen in relation to randomly distributed neurons. By 360 days, more than one quarter (26%) of the total neurons showed these changes indicating ageing. The number per unit volume of normal neurons decreased significantly when compared to the transplants on 60 and 90 days. In the substantia nigra of age-matched control, except for an increase in the lysosomal population, other ageing changes were not detectable. The neurons of intact substantia nigra of the host rat, chronologically 4-8 months older than the transplanted neurons, also appeared normal but for lipofuscin granules. The present study provides morphological evidence for rapid ageing of neurons in the long term nigral transplants. These observations raise fresh doubts regarding permanent survival of grafted neurons in the host brain. Studies so far conducted are after prior nigral lesions. Trophic factors following lesions of the host tissue may have influenced the long term survival of the transplanted neurons. On the other hand, such changes may have been missed since no detailed morphological investigations of the long term transplants have been done so far.

Fetal nigral grafts in the anterior eye chamber of adult rats: a long-term morphological study

Substantia nigral grafts of 16 and 17 gestation days showed phenotypic characteristics in the anterior eye chamber of adult rats until the third month after transplantation. Thereafter by the sixth month a number of neurons showed somal and dendritic thickening, reduced population of endoplasmic reticulum, increase in lysosomes, and clear areas devoid of organelles, indicating age changes. These changes were progressive and affected more neurons by the end of 1 year, the longest period studied. The observations suggest that the maturation of nigral neurons is independent of specific afferent input, whereas target influence is necessary for the continued maintenance of the mature neurons. All the synapses observed in the transplant were of the asymmetric variety, reminiscent of the few intrinsic synapses of the intact nigra. This suggested establishment of mutual connectivity among the transplanted neurons in the absence of a target and the type of synapse formed may have been influenced by the local environment. Large glial processes, very prominant during the 4- to 6-month period became less significant afterward but continued to be present until the end of the period studied. Though there was no morphological evidence of lymphocytic infiltration, this might suggest an immunologic reaction.

The quantification of brain lesions with an omega 3 site ligand: a critical analysis of animal models of cerebral ischaemia and neurodegeneration

Previous investigations have indicated that the detection and quantification of omega 3 (peripheral type benzodiazepine) binding site densities that are associated with reactive astroglia and macrophages could be of widespread applicability in the localization and indirect assessment of neural tissue damage in the central nervous system. In the present study, we analyze the usefulness of this approach in a number of experimental models that are characterized by (or putatively involve) neuronal degeneration. One week after the systemic administration of the excitotoxin, kainate, a marked increase in omega 3 site densities (as assessed by [3H]PK 11195 binding) was noted, an increase that was most prominent in known regions of selective vulnerability (hippocampus and septum). However, the kainate-induced omega 3 site proliferation was not a function of the dose administered, a marked interstudy variation was observed, and the binding increase was prevented by the administration of the anticonvulsant, clonazepam. The densities of omega 3 sites were studied, by autoradiography (using [3H]PK 11195 or [3H]PK 14105 as ligands), in 4 groups of Fischer 344 rats aged 3, 12, 22 and 30 months. No age-related changes were noted except in the 30-month-old group in which discrete and focal increases (reflecting tumoral processes) were observed in various brain regions. In spontaneously hypertensive, stroke-prone rats, omega 3 binding increases were observed concomitant with the development of stroke-related neurological signs. With autoradiography, the omega 3 site increase was localized to focal increases in the boundary zones between major cerebral arteries (and corresponding to regions of ischaemic or haemorrhagic infarction). Focal cerebral ischaemia was studied in rats and mice. Subsequent to middle cerebral artery occlusion in normotensive (Wistar/Kyoto) and spontaneously hypertensive rats, the density of omega 3 sites in the ipsilateral hemisphere was markedly elevated, the increase being greater in the spontaneously hypertensive rats. The increases in omega 3 labelling in these two strains matched the absolute volumes of infarctions, determined previously. Middle cerebral artery occlusion in the mouse also increased hemispheric levels of omega 3 sites; the maximum values were obtained between 4 and 8 days following the induction of focal ischaemia. These results demonstrate the feasibility of using omega 3 sites as a marker of excitotoxic, ischaemic and proliferative damage in the rodent brain. Binding measurement in tissue homogenates is an economic and time-efficient approach, whereas the autoradiographic detection of omega 3 sites allows the localization of brain lesions with a macroscopic or microscopic level of anatomical resolution.(ABSTRACT TRUNCATED AT 400 WORDS)

Auditory brainstem responses to tonal stimuli in young and aging rats

The auditory brain stem response (ABR) was studied in young adult and aged rats using 3,8 and 40 kHz tone pips. The expected inverse relationship between frequency and latency was observed in the younger group for waves I, II and III, while the response to the highest frequency stimulus had the longest latency at wave V. Absolute latencies for waves I through V each showed age-related increments with more pronounced changes occurring to 3 and 40 kHz stimuli than to the frequency of maximum sensitivity (8 kHz). Threshold increases with age for the highest frequency approximately doubled those for the lower frequencies. Examination of interpeak intervals (IPI) I-III, III-V and I-V revealed aging effects. The largest IPI I-V increment occurred to 3 kHz stimulation which reflects changes at both I-III and III-V sub-intervals. These results demonstrate electrophysiological correlates of aging due to transformations in the peripheral auditory system coupled with alterations in brainstem auditory pathways.

Neuron numbers and sizes in aging brain: comparisons of human, monkey, and rodent data

One of the several sources of interest in aging animal brains is their potential as models of the aging human brain. In this review we examine whether neuron numbers and sizes change similarly in aging human, monkey and rodent brain regions which data are available from more than one species. The number of brain regions studied in more than one species is surprisingly limited. Some regions show correspondence in age-related changes between humans and selected animal models (primary visual cortex, CA1 of hippocampus). For the majority of regions the data are conflicting, even within one species (e.g., somatosensory cortex, frontal cortex, cerebellum, cholinergic forebrain areas, locus coeruleus). Although some of the conflicting data may be attributed to procedural differences, particularly when data are expressed as density changes, much must be attributed to real species and/or strain differences in rodents. We conclude that neuron numbers and sizes may show similar age-related changes in human and animal brains only for sharply defined brain regions, animal species and/or strains, and age ranges.

Morphometric studies on pre- and paravertebral sympathetic neurons in the rat: changes with age

Morphometric measurements have been made on rat sympathetic neurons at ages between 6 and 24 months. In neurons of the coeliac-superior mesenteric ganglion there is a marked decrease in the neuronal packing density between 12 and 18 months which is accompanied by increases in the size of the neurons and their nuclei. In the superior cervical ganglion, no changes in packing density are seen until 18 months after which a decrease occurs, accompanied by slight increases in the neuronal parameters. These post-maturation changes occurring throughout adult life reveal a continued dynamism of sympathetic neurons into old age as well as revealing further differences between populations of sympathetic neurons.

Resting and pure tone evoked metabolic responses in the inferior colliculus of young adult and senescent rats

Metabolic responses from three sectors of the inferior colliculus (IC) divided according to frequency representation were examined bilaterally in young (YA) and aged adult (AA) rats using the 2-deoxyglucose method under quiescent or pure tone stimulus conditions. Resting IC metabolism in YA rats is characterized by elevated glucose uptake in the ventromedial (high frequency) relative to intermediate and dorsolateral sectors. AA resting incorporation is not reduced in any sector and is more uniform than YA uptake. Monaural high frequency stimulation (50 kHz) in YA animals evokes discrete contralateral banding along the ventromedial IC border, while stimulation in the optimal auditory sensitivity range of the rat at 8 kHz activates the intermediate sector of the contralateral IC. Although 50 kHz evoked ventromedial uptake is slightly reduced in AA rats contralaterally, the overall uptake within the three sectors does not differ significantly between ages under either of the monaural stimulus conditions; therefore senescent stability is exhibited. However, in AA rats the focus to 8 kHz stimulation has less clearly defined borders even though uptake within the intermediate sector is undiminished compared to YA animals, while 50 kHz elicited activity is typically reduced to a discontinuous band in the ventromedial sector of AA animals. These senescent changes may represent compensation in IC metabolism to changes in cochlear processing which are reflected in the activity of ascending auditory pathways.

The effect of increased longevity, produced by dietary restriction, on the neuronal population of area 17 in rat cerebral cortex

Area 17 of the brains of Sprague-Dawley derived rats, maintained on a limited ration of food to maintain their weights at the levels attained by two months of age, was compared with area 17 in control groups of rats fed ad lib. The oldest rats in the diet restricted group were sacrificed at 46 and 48 months of age, by which time their life spans had been extended about 12 months beyond the oldest age that rats fed ad lib achieve, for only few of the latter live as long as 33 months. In this study, the rats which were compared consisted of two groups of ad lib fed rats, one 3 and 6 months of age, and the other 33 months old, and two groups of diet restricted rats, one 26 months old and the other 46 to 48 month old rats (designated as 47 month old rats). Two indices were used to assess whether age affects the volume of area 17. One, the number of clusters of apical dendrites of layer V pyramidal cells per unit area of tangential sections, was the same in all groups, indicating that the lateral spread of area 17 did not alter with age. However, the other index, the thickness of area 17, did change with age, for area 17 was significantly thinner in the 47 month old diet restricted rats than in the other three groups. It was also found that the number of neuronal profiles in strips of sections passing through the entire depth of area 17 is decreased in the 47 month old rats, indicating that neurons had been lost from their cortices. This decrease in the number of neuronal profiles in the 47 month old rats was not due to nuclear shrinkage since the sizes of neuronal nuclei were not significantly different in the older ad lib and diet restricted rats. Determinations of neuronal packing densities in layers II/III, IV, V and VIa suggest that neurons are most frequently lost from the deeper cortical layers of the 47 month old rats, and in these layers large vacuolated spaces, the sizes of neuronal cell bodies, have been encountered. It is suggested that these spaces represent places from which neurons have been lost. It is concluded, therefore, that neurons are lost from area 17 in rats whose longevity is increased by diet restriction.

A morphologic analysis of neurons and neuropil in the dorsal lateral geniculate nucleus of aged rats

Light and electron microscopy were used to investigate the morphology of neuropil and neuronal cell bodies of the dorsal lateral geniculate nucleus (LGNd) of aged rats. Light microscopic examination reveals that, despite the optic tract showing signs of degeneration, the LGNd is scarcely affected. Thus, a slight but significant reduction in the diameters of both soma and nuclei is observed in aged neurons of the LGNd. Ultrastructural analysis demonstrates a few degenerating profiles of the neuropil. Neurons resembling relay cells exhibit typical features of aged neurons. Cells showing a very infolded nucleus, most of the ER cisternae connected with the nuclear envelope, abundant free polyribosomes and subsurface cisterns associated with mitochondria are similar to interneurons of adult rats. Therefore, aging and partial loss of visual input appear to induce small changes in the morphology of most of LGNd neurons.

Structure of layer II in cat primary auditory cortex (AI)

The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150-200 micron to about 400 micron beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendritic arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included small and medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; bipolar and bitufted cells with polarized, sparse dendritic arbors; small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; large sparsely spinous multipolar cells with smooth, robust apical dendrites; tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and extraverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in nuclear envelope invaginations in axotomized immature and mature hamster facial motoneurons

In this study, changes in the amount of nuclear envelope invaginations (NEI) were quantitatively assessed after axotomy during the late nuclear maturation stages (15, 20 and 25 days postnatal age) and in the adult (100-day-old) hamster facial motoneurons. These changes were expressed as boundary density or BA (length of nuclear envelope per unit area of nucleus). Absolute nuclear areas and perimeters were also estimated after axotomy at these ages. At 1/2 and 1 day after axotomy, no differences in the above parameters were noted at any of the operative ages. At 4 days postoperative, the peak of chromatolysis for all these ages, axotomy resulted in significant decreases in BA and nuclear perimeter in the immature neurons and no changes in BA and nuclear perimeter in the adult neurons. In addition, 4 days after axotomy at 20 days postnatal and later ages, pronounced increases in nuclear area occurred. These quantitative results are interpreted as evidence that the accelerated loss of NEI after axotomy during the final stages of nuclear maturation in these neurons is related to the formation of rough endoplasmic reticulum (RER) or Nissl substance. The hypothesis that an 'excess' of RER is accumulated during the late maturation stages and may account for the lack of NEI in the adult axotomized facial motoneuron is presented.

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.

Intranuclear inclusions in rat piriform cortex: increase with age and preferential location within superficial layer II

Intranuclear inclusions have been observed in layer II neurons of rat piriform cortex. These inclusions have the form of a filamentous lattice and resemble those described by others previously. The frequency of lattice-containing nuclei shows a significant fourfold increase over a period of 3-33 months of age, with the largest increase occurring after 18 months. The incidence of these inclusions is highest in the superficial third of layer II and is significantly greater than what would be expected from the distribution of all neuronal nuclei in layer II. The presence of intranuclear lattices may be related to the high level of electrical activity in piriform cortex, and their increase with age may reflect a long-term cumulative effect of this activity.

Long term effects of callosal lesions in the auditory cortex of rats of different ages

The corpus callosum was sectioned in groups of rats 3, 12, and 24 months of age, and the auditory cortex was examined three months later to determine whether there were age-related differences in the morphological response to the partial deafferentation. Material from the three groups of long-term callosally-lesioned rats were compared with three groups of age-matched control animals. Analysis focused on those cortical layers known to receive the heaviest callosal projection (layers II and III) and those neurons known to be postsynaptic to callosal afferents (layer V pyramidal neurons). There were no age-related changes in cortical thickness or in the relative thickness of the cortical layers in the control groups. However, the apical dendrites of layer V pyramidal neurons did lose dendritic spines and became thinner with age. In all three lesion groups, the cortex became thinner without altering the relative thickness of cortical layers; there was a decrease in the relative density of apical dendrite spines in layer III, but an increase in the density of these spines in layer IV. Both effects varied with age. Spine decreases in layer III were greatest in older animals and spine increases in layer IV were greatest in younger animals. The mean diameters of apical dendrites decreased in the youngest group of lesioned animals but increased in the oldest group. The results indicate that the effects of callosal deafferentation are age dependent.

Nuclear envelope invaginations in hamster facial motor neurons during development and aging

Morphometric measurements of nuclear envelope invaginations (NEI) in hamster facial motor neurons were made. These showed that the amount of NEI increased markedly between fetal life and birth to a high level that was maintained during the period of accelerated neuronal somal growth. Subsequently, the amount of NEI decreased to a plateau of low incidence that persisted through maturity and aging. This sequence of NEI formation and subsidence differs from that previously observed in a similar developing and aging series of pyramidal neurons in the same species. This may indicate, as discussed, that the presence of NEI reflects stage-specific functions that may differ depending on neuronal type.

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.

The cytoarchitecture of the dorsal cochlear nucleus in the 3-month- and 26-month-old C57BL/6 mouse: a Golgi impregnation study

The cytoarchitecture of the dorsal cochlear nucleus (DCN) was compared in 3- and 26-month-old C57BL/6 mice. The effects of genetically controlled progressive hearing loss present in the CNS in this mouse strain were analyzed with Nissl-stained and Golgi-impregnated material. The DCN was divided into the superficial molecular, an intermediate fusiform-granule, and the deep polymorphic layers. The molecular layer (ML) consisted of many fibers and a few small ovoid to spherical, fusiform, and granule cells. The fusiform-granule layer (FL) contained large fusiform and many granule cells. Most FL fusiform cells were oriented with their long axes perpendicular to the DCN surface and were present as small aggregations or individually. Cartwheel cells were adjacent to the FL fusiform cells. The deep polymorphic layer (PL) contained spherical, fusiform, granule, and multipolar neurons. The granule cells formed a dorsal cap of the DCN. From this cap, sheets of granule cells separated the DCN from the posterior ventral cochlear nucleus (PVCN) and from the brainsteM. The internal organization, neuronal location, orientation, and morphology were similar in both age groups. The granule cells had four to five primary dendrites, varicosities, and few to no dendritic appendages. The FL fusiform cells displayed different dendritic morphology in the two ages. One or two elaborate primary ML apical dendrites in the 3-month-old mice were covered with spikelike dendritic spines. The basal one or two PL dendrites were less elaborate and had few dendrite spines. In contrast, FL fusiform neurons in 26-month-old mice had regular dendritic varicosities and fewer spines which were short and stumpy. Basal dendrites had varicosities and interruptions. Cartwheel neurons in 3-month-old mice had elaborate ML dendritic trees covered with dendritic spines. In 26-month-old mice the dendrites had many varicosities and fewer short blunted dendritic spines. Large multipolar neurons in older mice had thinner dendrites with more varicosities than were in the 3-month group. In both age groups multipolar cells had few dendritic spines limited distally. Small and large spherical cells had two to five primary dendrites with varicosities, little higher-order branching, and spines. Fusiform cells had one or two primary dendrites, little secondary branching, and few to no spines. Minor degenerative changes were noted in spherical and fusiform cells in the two age groups. These included dendritic varicosities, interruptions, and some irregularities of somata surface. Degenerative changes present in the cochlea had significant effects on a limited population of DCN neurons. Finally, the neuronal morphology and architecture of the DCN in C57BL/6 mouse is similar to other mammalian species.

Aging in the neuropil of cerebral cortex--a quantitative ultrastructural study

Six age groups, each composed of four animals from each of the following ages, were used to assess age-related ultrastructural changes in the neuropil of the III layer of the frontal cortex in rats: 3, 6, 12, 18, 24 and 30 months old. Random samplings within the neuropil were taken to produce 40 electron micrographs in each rat (totaling 960). The profiles of axon terminals, dendrites and astroglial processes in the neuropil of each micrograph were traced. Then the percentage of their areas for the area of neuropil (relative volume fraction) was examined using the image analyzer system. The relative volume fractions of both the axon terminals and dendrites for the neuropil were found to have decreased in the aged rats. On the contrary, the relative volume fraction of astroglial processes for the neuropil had progressively increased with aging.

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.

Nuclear envelope invaginations in hamster pyramidal cells during development and aging

Nuclear envelope invaginations were observed in pyramidal cell nuclei of the hamster frontal cortex during development and aging. These invaginations which began to appear at 10 day did not recede at maturity as has been observed in certain other cell types, but persisted in the adult hamster and during subsequent aging. Morphometric data showed a significant increase in the number of nuclear envelope invaginations and in their length per unit of the nucleus. This increase was positively correlated with age until 500 days and is suggestive of a continued high metabolic activity that did not subside following the rapid growth phase of the pyramidal neurons.

Ultrastructure of 6-aminonicotinamide (6-AN)-induced lesions in the central nervous system of rats. III. Alterations of the spinal gray matter lesion with aging

Following a single i.p. injection of 6-AN (10 mg/kg), the anterior horn cells of 20- and 25-month-old rats increased more in size and recovered slower from chromatolytic changes than those of 3-month-old rats. Neurofilamentous hyperplasia of the perikarya was more prominent in aged rats; proliferated neurofilaments were arranged in thick parallel bundles. In the acute stage, reactive and degenerative changes of glial and mesenchymal elements were more conspicuous in 3-month-old rats; however, they disappeared by day 14 with prominent proliferation of hypertrophic astrocytes. The older rats showed less intensity and slower progression of these changes; sponginess and swelling of the astrocytic cytoplasm were still observed at day 14. Our results suggest that these age-dependent changes in the response to neurotoxins are not only induced on the neuron without mitotic phenomena after birth, but also on neuroglial cells. Furthermore, an alteration or reduction in the support of the neuron augments its intensified and delayed susceptibility to neurotoxins.

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.

Brain aging in normotensive and hypertensive strains of rats. III. A quantitative study of cerebrovasculature

Quantitative histological techniques were used to evaluate age-related alterations in the vascularity and thickness of the cerebral cortex in normotensive and hypertensive strains of rats. In both strains, aging was accompanied by a statistically significant increase in mean minimal luminal diameter (MMLD) of capillaries in all cortical laminae. While the overall depth of the cerebral cortex was significantly decreased in aged rats of both strains, the total number of capillaries in a strip of cortex extending from the pia mater to the underlying white matter did not change between 3 months and 23-24 months. The greatest vascularity was seen in the field occupying 41-60% of the depth below the pial surface. Few differences in the laminar variations in capillary density were observed. The number and MMLD of venules were increased in aged WKY, but the aged hypertensive rats exhibited a decrease in number and MMLD of venules. No significant changes in either the number or MMLD of arterioles was observed in aged and/or hypertensive rats. The total brain weight of the hypertensive rats was significantly less than that of normotensive rats at 3 and 12 months of age but no difference was observed between the aged groups. Hypertensive rats demonstrated a significantly decreased relative brain weight (brain weight per 100 g body weight) in all age groups.