Cholinergic control of cerebral blood flow in stroke, trauma and aging.

Enhancing the availability of endogenous acetylcholine by inhibition of cholinesterase with physostigmine, eptastigmine or soman at sub-toxic doses increases cerebral blood flow (CBF) and the response of this variable to changes in PaCO2. These effects are not correlated with metabolic activation, suggesting that the function of the cholinergic vasodilation is not merely to supply metabolic substrates. Since choline (Ch) can exchange between blood and the brain extracellular milieu the stage is set for possible feedback interactions between ACh synthesis and CBF. A negative feedback of CBF on ACh synthesis under conditions of a negative arteriovenous (A-V) difference for Ch across cerebral capillaries may contribute to stabilize GBF in ischemia. Eptastigmine and physostigmine significantly improve perfusion in experimental models of focal cerebral ischemia and traumatic brain injury respectively. During the short periods of time in which the A-V difference for Ch across the brain is positive, a positive feedback between cerebral free Ch and CBF may enhance the ability of the brain to recover Ch from the circulation for synthesis of membrane phospholipids. A loss of cholinergic cerebrovascular control may thus impair the survival of all cells within the CNS and contribute to the pathophysiology of dementia. Perhaps the view that the loss of cholinergic cells is the end point of Alzheimer's dementia could be modified to state that a cholinergic deficit may be the starting point of a decline in cerebral phospholipid turnover and cell membrane renewal that could lead to a generalized deterioration of cerebral function.