Aging Reduces Basal Cholinergic Reserve, but the Pattern of Scopolamine-Induced Memory Impairment Remains Comparable in Rats.

BACKGROUND: Experimental models of cognitive dysfunction are essential for investigating mechanisms of memory impairment and therapeutic strategies. The scopolamine-induced memory impairment (SIMI) model is widely used for inducing acute cholinergic dysfunction and related cognitive deficits. However, using young animals to study a disorder that predominantly affects older individuals can lead to misleading results. This study investigated whether age alters the behavioral and neurochemical outcomes of the acute SIMI model. METHODS: Young and aged (5 and 22 months old) male Wistar rats received a single intraperitoneal injection of scopolamine (1mg/kg). Cognitive function was evaluated using the Morris water maze (MWM) test. Hippocampal tissues were analyzed for cholinergic biomarkers. RESULTS: Aged control rats exhibited lower memory performance and decreased hippocampal cholinergic reserve (decreased acetylcholine levels and acetylcholinesterase and butyrylcholinesterase enzyme activities) compared to young controls. While acute scopolamine severely impaired MWM performance in both age groups, it produced a remarkably similar pattern of relative cholinergic disruption. Both groups showed significant acetylcholine depletion and increased cholinesterase activity post-treatment. Notably, aged brains showed a significantly stronger acetylcholinesterase enzyme response to scopolamine than young brains; otherwise, no major differences in the rate of cholinergic impairment were observed. CONCLUSION: Although normal aging depletes cholinergic reserves, the overall pattern of acute scopolamine-induced cognitive and neurochemical deficit remains highly consistent across age groups. Young rats exhibit behavioral and cholinergic responses similar to aged rats in the acute SIMI model. However, age-related differences and age-specific responses, such as stronger AChE reactivity, should be considered.