Different approaches to study acetylcholine release: endogenous ACh versus tritium efflux.

Superfused slices of guinea-pig cerebral cortex (CC), caudate nucleus (CN) and thalamus (Th) were used to compare i) the resting and electrically-evoked release of endogenous acetylcholine (ACh) in the presence of physostigmine (Phys) and ii) the resting and electrically-evoked tritium efflux (after preloading with 3H choline) in the absence or in the presence of Phys and hemicholinium (HC-3). In addition, the effect of GABA, morphine and their antagonists on both effluxes was investigated. By matching the ACh and tritium outflow on a molar basis, an average ratio of 100: 2-4 was found. When expressed as a percentage of tissue content, the ACh release at 2 Hz (2 min) was 4.1 in CN, 0.92 in CC and 0.44 in Th. Lower percent values in the same rank order, were found for tritium outflow with Phys. Thus, CN has the highest secretory activity. Tritium evoked outflow in the presence of Phys was nearly halved in comparison with the normal values (without Phys). Therefore, the autoreceptor-mediated negative feed-back seems to be similar in the three areas. Tritium evoked outflow in the presence of HC-3 was more than doubled in Th (less so in CC and CN) in comparison with the normal values. A second stimulation at 2 Hz (2 min) gave rise to the same outflow in Th while an evident fall in radiolabel efflux was found in CN. Therefore the blockade of high affinity choline uptake plays a different role in Th and CN.(ABSTRACT TRUNCATED AT 250 WORDS)