CHARACTERISTICS OF ACETYLCHOLINE RELEASE BY SUPERFUSED SLICES OF RAT BRAIN1

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Abstract
—A superfusion system has been used to examine the effects of choline and the utilization of [3H]choline during resting and potassium‐stimulated release of ACh from rat cerebrum slices. The rate of ACh release from unstimulated tissue, 0·25 nmol/g per min, increased 8‐fold when the concentration of KCl in the superfusing medium was increased from 5 to 50 mm. This rate was not maintained, however, but gradually declined to one‐half the peak rate after approx. 30 min. After an initial washout period, choline was released at a rate of 2·5‐5 nmol/g per min, which was equal to 1‐2 × 10−6m in the superfusate. The addition of 1 × 10−5m‐choline to the superfusing medium was required to maintain the stimulated ACh release at near peak rates for 90 min. When hemicholinium‐3 was added to the 50 mm‐KCl medium, the release of ACh reached a peak as usual but then declined to prestimulation rates.After introducing a pulse of radioactive choline in the superfusing medium, the specific radioactivity of choline and ACh in the superfusate was determined before and during stimulation with 50 mm‐KCl. The specific radioactivity of released ACh was always greater than that of released choline; it decreased rapidly at the onset of stimulation, and then more gradually as stimulation proceeded. The specific radioactivity of ACh released in the initial minutes of stimulation was higher than that of ACh in the tissue before stimulation. In the last 10‐20 min of stimulation the specific radioactivity of the released ACh was lower than that of the tissue ACh at the end of stimulation. The relative contributions of old and newly synthesized ACh to the releasable transmitter pool are discussed.