Abstract
The effects of three cholinesterase inhibitors (physostigmine, E2020, and Tacrine), all of which are to be cognitive enhancers, on the functional regional cerebral blood flow (rCBF) response were studied in young (5.9±1.8 years old) and aged (18.0±3.3 years old) monkeys under awake conditions using high-resolution positron emission tomography (PET). Under control condition, vibrotactile stimulation elicited increases in the rCBF response in the contralateral somatosensory cortices of both young and aged monkeys, but the degree of increase in rCBF response was significantly lower in aged (115.8%) than that in young monkeys (139.9%). Regional cerebral metabolic rate of glucose (rCMRglc) response to the stimulation, measured using [ 18 F ]-2-fluoro-2-deoxy- d-glucose (FDG) as an index of neuronal activation, was slightly, but not significantly, lower in aged than in young monkeys. The lower rCBF response to the stimulation in aged monkeys was improved by administration of the cognitive enhancers with cholinesterase inhibition (physostigmine, E2020 and Tacrine) at a dose of 50 μg/kg. The order of improvement of rCBF response (physostigmine>E2020>Tacrine) at 0.5 h after injection and that of duration of effect (E2020>Tacrine>physostigmine) were consistent with the data obtained by microdialysis. In contrast, the cognitive enhancers did not alter rCBF response to stimulation in young monkeys. The present results demonstrated that the functional change in rCBF response to the stimulation was induced during the aging process by impairment of the coupling mechanism between the neuronal activation and rCBF response. Furthermore, the observation that cognitive enhancers partly restored the functional rCBF response suggested that the coupling mechanism might be regulated via cholinergic neuronal transmission.
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