Johannes Tigges

Neurobiological Bases of Age-Related Cognitive Decline in the Rhesus Monkey

The rhesus monkey offers a useful model of normal human aging because when monkeys are tested on a battery of behavioral tasks that can also be used to evaluate cognition in humans, it is found that the monkeys undergo an age-related decline in several domains of cognitive function as do humans. In monkeys these changes begin at about 20 years of age. To determine what gives rise to this cognitive decline, we have examined several parameters in the brains of monkeys. Some parameters do not change with age. Examples of this are the numbers of neurons in the neocortex and hippocampal formation, and the numbers of synapses in the hippocampal formation. Changes in other parameters can be positively correlated with chronological age; examples of this are numbers of neuritic plaques, a decrease in the numbers of neurons in the striatally projecting pars compacta of the substantia nigra, and a decrease in the thickness of layer I in primary visual cortex. But the most interesting changes are those that correlate either with cognitive decline alone, or with both cognitive decline and chronological age. Among these are a breakdown in the integrity of myelin around axons, an overall reduction in the volume of white matter in the cerebral hemispheres, thinning of layer I in area 46 of prefrontal cortex, and decreases in the cell density in cortically projecting brain stem nuclei. To date then, our studies suggest that the cognitive declines evident in the rhesus monkey may be a consequence of changes in layer I and in the integrity of myelinated axons, rather than an agerelated loss of cortical neurons or synapses, as has long been assumed.

β-Amyloid (Aβ) deposition in the brains of aged orangutans

Abstract While aged monkeys of several species show cerebral amyloid deposition in senile plaques and blood vessels similar to that seen in human aging and Alzheimer’s disease (AD), studies of great apes have been limited. Using histological and immunohistochemical methods, we examined the brains of four orangutans aged 10, 28, 31, and 36 years. We encountered sparse β-amyloid (Aβ)-immunoreactive, silver-negative plaque-like structures in the brains of the three older apes. The 36-year-old orangutan also evidenced small Aβ-positive deposits in subcortical white matter and sparse vascular amyloid deposition, primarily in meningeal vessels. Neurofibrillary tangles were not detected on silver stains or on tau or ubiquitin immunohistochemistry. Many of the Aβ-positive plaque-like deposits in the orangutans were apolipoprotein E-immunoreactive, as we have previously reported in aged rhesus monkeys and an aged chimpanzee. Also, paralleling our earlier findings in these nonhuman primates, Aβ 40 in plaques was more prominent in the orangutan than is typically seen in human aging, AD, and Down syndrome. These intriguing species differences may provide clues to the mechanisms of amyloid deposition and the development of neuropathologic changes in AD.

Aβ40 is a major form of β-amyloid in nonhuman primates

Because aged nonhuman primates show β-amyloid (Aβ) deposition in senile plaques and blood vessels similar to that seen in human aging and AD, we used C-terminal specific antibodies to Aβ40 and Aβ42 to investigate Aβ peptide length in the brains of 11 aged rhesus monkeys and a 59-year-old chimpanzee. In contrast to AD, where the earliest and most prominent form of Aβ in senile plaques is Aβ42, in the monkey, Aβ40-positive plaques predominated. The ratio of Aβ4): Aβ42-positive plaques averaged 2.08 in the monkey, as compared to a mean ratio of 0.37 in 68 human AD subjects (p < 0.001). Aβ40 was also more prominent in the chimpanzee than in humans. Possible explanations for these findings include species differences in the cleavage of Aβ from the amyloid precursor protein or in the activity of a putative carboxy peptidase forming Aβ40 from Aβ42 in situ.

Brain weight throughout the life span of the chimpanzee

Studies on human postmortem material report lower brain weights in older than in younger cohorts, whereas there is no apparent change with age in the rhesus monkey. In view of these contrasting results, we examined the pattern of brain weight across the life span in the chimpanzee, one of the closest biological relatives of humans. To place the study in context of the empirical life expectancy of the chimpanzee, we first performed a survival analysis on data from 275 chimpanzees that were maintained in the colony of the Yerkes Primate Center. The survival analysis revealed the maximum life spans of female and male chimpanzees to be about 59 and 45 years, respectively. We examined fresh brain weights from 76 chimpanzees ranging in age from birth to 59.4 years of age. The brains were taken from 9 infants (birth to 1 year of age), 25 juveniles (1–7 years), 13 adolescents (7–15 years), 21 young adults (15–30 years), and 8 old adults (over 30 years). Adult brain weight was achieved by the age of 7 years. The adolescent and young adult chimpanzees had the largest brain weights; in these two age groups combined, the mean brain weight (± standard deviation) was 368.1 g (±37.3) for females (n = 17) and 405.6 g (±39.4) for males (n = 17). This sex difference was statistically significant (P < 0.01). Simple linear regression performed on the combined material from females and males aged 7 years and older revealed a decline in brain weight with advancing age of 1.1g/year (P < 0.05). When the effect of sex on brain weight was statistically controlled for, the loss of brain weight with age was 0.9 g/year (P = 0.07). These results suggest that brain weight declines moderately with age in the chimpanzee as it does in humans. J. Comp. Neurol. 409:567–572, 1999.

Neuronal population of area 4 during the life span of the rhesus monkey

One right or left area 4 of each of 19 rhesus monkeys, ranging in age from 1 day to 35 years, was processed (frozen sectioned at 30 or 40 microns) for light microscopic analysis to assess age-related changes in the neuronal population. All neurons were examined regardless of their size. In addition, Betz cells were analyzed separately; to be regarded as Betz cells, pyramidal somata had to display a minimum height of 38 microns. A significant loss of approximately one-third was observed in the total number of neurons in maturing monkeys (less than 5.5 years). In contrast, in maturing rhesus monkeys significant increases with age were observed in the mean number of Betz cells, and in the means of Betz cell area, height, width, perimeter, and estimated volume. In adult monkeys (greater than 4.5 years), no age-associated loss of neurons was observed. Also, no loss of Betz cells occurred, although the perimeter, area, and estimated volume of Betz cells decreased slightly, but significantly, with increasing age in adult monkeys. Lipofuscin granules were discernable in Betz cells beginning at the age of 5 years and their number increased with increasing age. In the older rhesus monkeys, the lipofuscin granules were so large and numerous that in some Betz cell somata they displaced the nucleus from its usual location in the center of the cell. No age-related change in thickness of area 4 was found.

Parvalbumin immunoreactivity of the lateral geniculate nucleus in adult rhesus monkeys after monocular eye enucleation

Immunocytochemical methods with antiserum to the calcium-binding protein parvalbumin (PV) were used to examine the effects of monocular enucleation on parvalbuminergic neurons and processes in the lateral geniculate nucleus (LGN) of adult rhesus monkeys. In the LGN of normal monkeys, numerous PV-positive neurons, including the largest neurons in the nucleus, and many PV-positive processes occur in all six laminae. After monocular enucleation, PV immunoreactivity is reduced in the neuropil of the denervated laminae compared to adjacent nondenervated and to normal laminae. PV immunoreactivity of somata in denervated laminae, however, appears to be indistinguishable from that of somata in nondenervated laminae, although neurons in the denervated laminae are smaller in size. Since LGN neurons in denervated laminae have lost their visual input, the functional role of PV in this nucleus may not relate directly to visual information processing.

Brain weight does not decrease with age in adult rhesus monkeys

Cross-sectional studies on adult human autopsy material have shown that younger cohorts have heavier brains than older groups. We sought to determine whether a similar pattern is present in the rhesus monkey, a species that serves as a useful model of human brain and cognitive aging. Data were obtained from necropsies of 399 rhesus monkeys (180 females; 219 males), of ages covering the entire adult lifespan of this species. In addition to fresh brain weight, variables considered were age, sex, body weight, heart weight, identity of the prosector, and circumstance of death. Initial bivariate analyses revealed a significant sex difference in brain weight (mean for males: 96.1 g; for females: 86.1 g; p < 0.001), as well as significant correlations of brain weight with body weight (r = 0.20, p < 0.01 for females; r = 0.27, p < 0.001 for males), and heart weight (r = 0.27, p < 0.001 for females; r = 0.38, p < 0.001 for males). Identity of prosector, circumstance of death, and age were not significantly related to brain weight in bivariate analyses. Multiple linear regression, controlling for possible confounding effects of body weight and sex, also suggested that brain weight is stable throughout adulthood in the rhesus monkey.

Mild Age-Related Changes in the Dentate Gyrus of Adult Rhesus Monkeys

Memory and cognitive performance decline with advancing age in humans. Rhesus monkeys show a similar age-related memory deficit. Since the functional neuroanatomy of the temporal lobes in the two species is similar, and since the circuits of the temporal lobes are known to be involved in memory function, we undertook a study of the anatomical characteristics of synapses in the dentate gyrus of the rhesus monkey throughout the adult life span. Light- and electron-microscopic examinations were carried out on the dentate gyrus of 10 adult rhesus monkeys (4-35 years) to determine the effect of age on the thickness of the molecular layer and on axon terminals in the outer portion of the molecular layer. The thickness measurements were made on 100-microns-thick Vibratome sections and on 1 micron-thick Araldite-embedded sections. A total of 100 electron micrographs covering a test area of 3,600 microns2 for each monkey were taken in the outer portion of the molecular layer. Counts of axon terminals synapsing with dendritic spines or shafts, measurements of the cross-sectional area of these terminals, and the length of the postsynaptic density were taken on enlarged prints. The thickness of the molecular layer remained unchanged throughout adulthood. Statistical analysis revealed no overall age-associated loss of synapsing axon terminals or shrinkage of the cross-sectional areas of their profiles. Further, there was no loss in the total number of synapses (axospinous plus axodendritic) or any change in the lengths of their postsynaptic membrane densities. However, when axodendritic (shaft) synapses (which constitute 13% of the total) were considered separately, a statistically significant age-related loss was detected. Qualitative observations revealed that older monkeys had a moderate number of dystrophic myelinated axons and corpora amylacea located in astrocytic processes in the outer portion of the molecular layer, features not present in young monkeys. Also, glial cells and pericytes showed age-associated accumulation of lipofuscin-like inclusions. A single occurrence of a structured inclusion body in a dendrite was observed in a 10-year-old monkey. In conclusion, most synaptic measures in the dentate gyrus remain stable throughout adulthood of rhesus monkey and there are relatively few other age-related changes. The small age-associated loss of axodendritic synapses is only apparent following separate statistical treatment of these synapses. The functional significance of this loss is unclear since it would result in only 3% reduction in total synapses (shaft plus spinous) from 4 to 35 years of age, the maximal life span of the rhesus monkey.

Efferents of area 4 in a South American monkey (Saimiri). I. Terminations in the spinal cord.

A single injection of either tritiated proline or a mixture of proline and leucine was made in area 4 of 8 squirrel monkeys. The locus of the injection was systematically varied from medial to lateral among animals. Autoradiographs revealed a strong contralateral lateral corticospinal tract. A sparsely labelled ipsilateral lateral tract was also present in all animals. In 2 animals, a few labelled fibers indicative of an ipsilateral anterior tract were observed; the fibers terminated at lumbar levels. Grain counts over the cervical and lumbar gray showed that area 4 efferents terminated contralaterally in laminae IV--IX with a peak in lamina VII; only sparse input was seen in the vicinity of the large alpha-motoneurons of lamina IX. On the ipsilateral side, the terminals were largely confined to lamina VIII. This pattern was in accordance with that reported in other primates. The terminal fields at sacral and coccygeal levels were radically different in that large numbers of fibers recrossed to the ipsilateral side and ended in laminae V through IX; the functional significance of this strong bilateral termination was discussed.

Ultrastructural changes in the superficial layers of the superior colliculus in Galago crassicaudatus (primates) after eye enucleation.

SummarySeveral types of terminals were found in the three superficial collicular layers of Galago. At least two axon terminals with round vesicles (R1 and R2) could be distinguished on the basis of vesicle packing and electron density of the cytoplasmic and mitochondrial matrices. R1 axon terminals were characterized by aggregations of vesicles in an electron lucent cytoplasm and mitochondria with a relatively dark matrix, while in R2 axon terminals the vesicles were more evenly distributed in an electron dense cytoplasm and the mitochondrial matrix was pale. R2 endings occurred in clusters in the stratum griseum superficiale; they were absent in the stratum zonale. R1 endings were found in all three superficial collicular layers. Both types of R terminals made asymmetrical contacts with small dendrites, dendritic spines and F profiles. Profiles containing flattened vesicles and establishing symmetrical contacts were numerous, and many could be identified as dendrites by accepting as criteria for dendrites evenly spaced microtubules, clusters of ribosomes and the fact that these F profiles were postsynaptic to other terminals. F terminals were presynaptic to other F profiles, dendrites and somata; they were postsynaptic to R terminals and took part in serial synapses. Dendrodendritic contacts were frequent, somatodendritic contacts rare. After eye enucleation most R2 axon terminals underwent the electron dense degenerative reaction. The degeneration process was a lengthy one; many degenerating boutons were found 30 days after axotomy and some persisted up to 180 days postoperatively. There was strong indication that the superior colliculus received more crossed than uncrossed retinofugal fibers. The crossed and uncrossed retinocollicular axons terminated in two different substrata of the stratum griseum superficiale.