Age-impaired impulse flow from nucleus basalis to cortex

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Abstract
Recent studies have renewed interest in the role of acetylcholine (ACh) in the cognitive changes associated with ageing and dementia1–4. Deficits in cortical choline acetyltransferase (ChAT) in Alzheimer's disease have been consistently demonstrated5–9, while other research has suggested a connection between deterioration of cortical ACh fibres and dementia4. However, despite clear biochemical and anatomical evidence f or a fall in ACh in dementia1–9, results of therapeutic trials with cholinergic agonists, precursors and cholinesterase inhibitors have been inconsistent6–9. Such findings suggest that cortical cholinergic disorders are not wholly a function of simple biochemical change; alterations of impulse flow along cholinergic fibres could well be as debilitating. An important extrinsic source of cortical ACh innervation derives from neurones diffusely located in rat basal forebrain10–16, denoted the nucleus basalis (NB)15,17. We have now investigated the impulse conduction properties of cortically projecting, putatively cholinergic NB axons in adult and aged rats and have found that conduction latencies from NB to frontal cortex are significantly longer (by 51%) in aged animals. In addition, systematic analysis varying cortical stimulation depth revealed that these longer latencies are due entirely to decreased conduction velocities in the subcortical fibre projections. Indeed, intracortical velocities were virtually identical in the two groups. Our results indicate that ageing occasions a decrease in the temporal fidelity of impulse flow in the cholinergic input to the cortex from the NB, a previously overlooked but potentially important element in cognitive deficits that occur with age.