The characteristics of acetylcholine release mechanisms in the auditory cortex.

1. The characteristics of the acetylcholine (ACh) release mechanism have been studied in the auditory cortex of rabbits on stimulation of the ipsilateral medial geniculate nucleus.2. On stimulation of the medial geniculate nucleus the mean release of ACh from the auditory receiving cortex was 6.1 times the spontaneous release; the mean release from other parts of the cortex was 2.2 times the spontaneous release.3. The frequency of stimulation most effective in evoking ACh release was found to be 10/sec.4. Both the spontaneous and evoked release of ACh were reduced by 40-65% in the absence of calcium from the solution bathing the auditory cortex, and increased by 15-25% when the calcium concentration in the bathing solution was doubled.5. The presence of low concentrations of magnesium in the fluid bathing the cortex was essential for the optimal release of ACh, but high magnesium concentrations lowered this release.6. The presence of triethylcholine (TEC) in the fluid bathing the auditory cortex reduced both the spontaneous and evoked release of ACh. This reduction was reversed in the presence of choline.7. The effects of calcium, magnesium and TEC on the ACh release mechanism in the cerebral cortex and at the neuromuscular junction are compared.