MRI description of cerebral atrophy in mouse lemur primates

We assessed cerebral atrophy in mouse lemur primates (Microcebus murinus) by estimating CSF volume in their brains from 4.7 Tesla T2-weighted magnetic resonance images. Thirty animals aged from 1 to 10.3 years were imaged, 14 of them were followed for up to 2 years. Seven of these animals were examined for neuropathology. In 12 out of 17 animals older than 3.5 years, CSF volumes were increased. A subgroup of six animals had severe atrophy of the temporal lobe. Another subgroup of five animals displayed diffuse atrophy in addition to the temporal atrophy. One animal had a dilation of the external part of the temporal horn of the lateral ventricle in addition to the temporal atrophy. The three animals with diffuse atrophy that could be studied for neuropathology had diffuse cerebral amyloid deposits detected by immunocytochemistry. The other animals did not display amyloid deposits. Relations between the different types of atrophy as well as their causes will have to be assessed in future studies.

Cerebral T2-weighted signal decrease during aging in the mouse lemur primate reflects iron accumulation

4.7 Tesla T2-weighted magnetic resonance images showed a highly significant signal decrease in the pallidum, substantia nigra, putamen, and a less significant decrease in the thalamus and the caudate of aging mouse lemurs (Microcebus murinus). We evaluated the contribution of iron deposits to the signal decrease comparing Perls' stained histological sections of six mouse lemurs brains aged 1 to 10 years to magnetic resonance images. In young animals, none of the brain structures was stained. A large number of iron deposits were visible in the pallidum and substantia nigra of aged animals and a moderate number in the middle aged ones. In the putamen, few iron deposits were visible in aged and middle-aged animals. The thalamus and the caudate appeared unstained with Perls' technique; iron was too low to be detected. The intensification of the reaction by diaminobenzidine revealed iron deposits in the thalamus of aging animals. This study suggests that in mouse lemurs, iron deposits are responsible for T2-weighted signal decrease in the central gray nuclei.

Age dependence of the T2-weighted MRI signal in brain structures of a prosimian primate (Microcebus murinus)

Mouse lemurs (Microcebus murinus) are prosimian primates described to be convenient models of brain aging. We observed very high correlations between the T2-weighted magnetic resonance imaging (MRI) signal decrease and the natural logarithm of age in the basal ganglia. The correlation coefficient was higher for the pallidum (r = 0.95, P < 0.0001) than for other structures. We suggest that the ratio of the pallidum intensity divided by the amygdala and temporal lobe intensity should be a valuable non-invasive marker of age and of cerebral aging. It should be particularly useful for the non-invasive assessment of interventions and drugs that affect the aging process.

T2-weighted MRI studies of mouse lemurs: a primate model of brain aging

Previous histological and behavioral studies of aging mouse lemurs have demonstrated changes similar to those observed in elderly humans and in patients with Alzheimer's disease. We explored 18 animals of ages 6 months to 9 years. Axial T2-weighted images of the brain were performed on a 4.7 Tesla Bruker Biospec 47/30 system. We estimated cerebral atrophy by adding measures of high signal areas characteristic of cerebrospinal fluid (interlobular and sylvian fissures, lateral and third ventricles) of four contiguous cortical slices. We observed a significant increase of cerebral atrophy with aging and one case of an apathetic 8-year-old animal presenting a considerably higher cerebral atrophy. We also observed high correlations between decreased signal intensities and age for the pallidum, the substantia nigra, and the putamen. These results suggest that aging mouse lemurs present similar magnetic resonance images of cerebral alterations to those encountered in aging humans and that high-field T2-weighted magnetic resonance images can help in the early detection, in vivo, of animals suspected of pathological aging.

Effects of aging upon recent memory in Microcebus murinus

Four young (2 to 4-year-old) and four aged (9 to 10-year-old) grey mouse lemurs (Microcebus murinus) were given a simple visual discrimination task, and a delayed response visual discrimination task with variable retention delay. The response to the test is a motor one, that consists of choosing one out of four corridors of an apparatus based upon the degree of illumination. The aged animals did not show any learning deficiency, and were capable of memorizing the task for several months. However, they were more sensitive to the length of the delay in the delayed response task than the young animals. It is, therefore, possible to argue that the memory for recent stimulus events is affected during aging. These results corroborate those obtained in other primates, and demonstrate the usefulness of the grey mouse lemur as a new animal model for analyzing aging.