Evaluation of autoreceptor-mediated control of [(3)H]acetylcholine release in rat and human neocortex

In order to assess the autoinhibitory control of endogenous acetylcholine (ACh) in rat and human neocortex, slices of these tissues were prelabelled with [(3)H]choline, superfused continuously and stimulated electrically using various frequencies in the presence or absence of drugs. The autoinhibitory feedback control of [(3)H]ACh release was operative - despite the absence of blockers of ACh esterase - at stimulation frequencies >/= 3 Hz in rat and >/= 6 Hz in human neocortex tissue. At these frequencies the muscarinic antagonist atropine (0.1 microM) disinhibited the release of [(3)H]ACh in both species. Estimation of the biophase concentration of ACh near the autoreceptor in the rat neocortex from concentration-response curves of the muscarinic agonist oxotremorine revealed that at 3 Hz about 25% of the autoreceptors were activated by endogenously released ACh. This estimation is consistent with an increase in [(3)H]ACh release to about 120% of control values by complete blockade of autoreceptors with atropine. The observation that in human neocortical tissue presynaptic autoinhibition of [(3)H]ACh release is operative at stimulation frequencies >/= 6 Hz suggests that selective blockade of autoinhibition may also increase ACh release in the cortex of Alzheimer's disease patients, without additional blockade of the enzyme acetylcholinesterase.