Age-related cognitive deficits in rhesus monkeys mirror human deficits on an automated test battery.

Aged non-human primates are a valuable model for gaining insight into mechanisms underlying neural decline with aging and during the course of neurodegenerative disorders. Behavioral studies are a valuable component of aged primate models, but are difficult to perform, time consuming, and often of uncertain relevance to human cognitive measures. We now report findings from an automated cognitive test battery in aged primates using equipment that is identical, and tasks that are similar, to those employed in human aging and Alzheimer's disease (AD) studies. Young (7.1+/-0.8 years) and aged (23.0+/-0.5 years) rhesus monkeys underwent testing on a modified version of the Cambridge Automated Neuropsychological Test Battery (CANTAB), examining cognitive performance on separate tasks that sample features of visuospatial learning, spatial working memory, discrimination learning, and skilled motor performance. We find selective cognitive impairments among aged subjects in visuospatial learning and spatial working memory, but not in delayed recall of previously learned discriminations. Aged monkeys also exhibit slower speed in skilled motor function. Thus, aged monkeys behaviorally characterized on a battery of automated tests reveal patterns of age-related cognitive impairment that mirror in quality and severity those of aged humans, and differ fundamentally from more severe patterns of deficits observed in AD.

Long-term reversal of cholinergic neuronal decline in aged non-human primates by lentiviral NGF gene delivery.

Spontaneous atrophy of basal forebrain cholinergic neurons occurs with aging in the non-human primate brain. Short-term reversal of this atrophy has been reported following ex vivo nerve growth factor (NGF) gene delivery, but long-term effects of in vivo NGF gene delivery in the aged primate brain have not to date been examined. We tested the hypothesis that long-term lentiviral NGF intraparenchymal gene delivery would reverse age-related cholinergic decline, without induction of adverse effects previously observed following sustained intracerebroventricular growth factor protein exposure. Three aged rhesus monkeys underwent intraparenchymal lentiviral NGF gene delivery to the cholinergic basal forebrain. 1 year later, cholinergic neuronal numbers were quantified stereologically and compared to findings in four controls, non-treated aged monkeys and four young adult monkeys. Safety was assessed on several variables related to growth factor exposure. We now report that lentiviral gene delivery of NGF to the aged primate basal forebrain sustains gene expression for at least 1 year, and significantly restores cholinergic neuronal markers to levels of young monkeys. Aging resulted in a significant 17% reduction (p<0.05) in the number of neurons labeled for the cholinergic marker p75 among basal forebrain neurons. Lentiviral NGF gene delivery induced significant (p<0.05) and nearly complete recovery of p75-labeled neuronal numbers in aged subjects to levels observed in young monkeys. Similarly, the size of cholinergic neurons in aged monkeys was significantly reduced by 16% compared to young subjects (p<0.05), and lentiviral NGF delivery to aged subjects induced complete recovery of neuronal size. Intraparenchymal NGF gene delivery over a one-year period did not result in systemic leakage of NGF, activation of inflammatory markers in the brain, pain, weight loss, Schwann cell migration, or formation of anti-NGF antibodies. These findings indicate that extended trophic support to neurons in the non-human primate brain reverses age-related neuronal atrophy. These findings also support the safety and feasibility of lentiviral NGF gene transfer for potential testing in human clinical trials to protect degenerating cholinergic neurons in Alzheimer's disease.

Endogenous neurogenesis replaces oligodendrocytes and astrocytes after primate spinal cord injury.

Neurogenesis has been described in various regions of the CNS throughout life. We examined the extent of natural cell division and replacement from 7 weeks to 7 months after cervical spinal cord injury in four adult rhesus monkeys. Bromodeoxyuridine (BrdU) injections revealed an increase of >80-fold in the number of newly divided cells in the primate spinal cord after injury, with an average of 725,000 BrdU-labeled cells identified per monkey in the immediate injury zone. By 7 months after injury, 15% of these new cells expressed mature markers of oligodendrocytes and 12% expressed mature astrocytic markers. Newly born oligodendrocytes were present in zones of injury-induced demyelination and appeared to ensheath or remyelinate host axons. Thus, cell replacement is an extensive natural compensatory response to injury in the primate spinal cord that contributes to neural repair and is a potential target for therapeutic enhancement.