Acetylcholine release in the rat hippocampus as measured by the microdialysis method correlates with motor activity...

Extracellular levels of acetylcholine were measured by the microdialysis method coupled to high performance liquid chromatography in the dorsal hippocampus of freely moving rats over a period of 24 h to examine whether the acetylcholine release in the hippocampus exhibited a diurnal variation. Spontaneous motor activity was simultaneously measured with an automatic animal activity monitor. The amount of acetylcholine collected per 20-min sample varied markedly, in a range from about 5 to 90 pmol. There appeared to be variations in the amount with a 2-4 h periodicity as well as an apparent diurnal periodicity. In all five rats studied, the overall mean value for the dark cycle (11.1-34.5, average 20.9 pmol/20 min) was significantly greater than that for the light cycle (5.1-21.3, average 12.3 pmol/20 min), showing a 70% average increase. Cross-correlation analysis performed between the amount of acetylcholine and the motor activity count for the animal during the sampling revealed a significant positive correlation coefficient in four rats studied. The present study demonstrates for the first time that the acetylcholine release shows a diurnal variation.