Daily changes in the release of acetylcholine from rat primary somatosensory cortex.

Using microdialysis, acetylcholine (ACh) release was measured in the somatosensory cortex of 14 rats over a 24-h period. The release of ACh was 0.195 pmol/min during the day and 0.344 pmol/min at night. The length of exposed dialysis membrane within the cortex was an important source of variability in the absolute amounts of ACh collected. Even after rejecting some cases where the membrane contacted only the superficial cortical layers, this factor accounted for 25% of the variation of absolute amounts collected in different animals. After correcting for the length of exposed membrane, the release of ACh was shown to increase 52% at night during the time when the animals were awake, feeding and grooming. Variability in the measures of ACh release obtained during periods of activity was greater than its variability during periods of inactivity. These data were interpreted in the context of several hypothesized roles for ACh in sensory cortex.