ORIGIN OF TRANSMITTERS RELEASED BY ELECTRICAL-STIMULATION FROM A SMALL, METABOLICALLY VERY ACTIVE VESICULAR POOL OF CHOLINERGIC SYNAPSES IN GUINEA-PIG CEREBRAL-CORTEX

The turnover of true and false transmitters was measured to find out from which subcellular pool in guinea-pig cerebral cortex acetylcholine is released after electrical stimulation. [N-Me-14C]choline was injected along with [N-Me-3H]pyrrolidinecholine or [N-Me-3H]homocholine into cortical tissue. Both choline analogues were acetylated, although two to five times less favourably than choline. All acetylated forms were incorporated into synaptic vesicles; this vesicular uptake was enhanced by low-frequency electrical stimulation (0.1 Hz) and occurred preferentially into a metabolically very active subpopulation of synaptic vesicles closely attached to the presynaptic membrane which can be isolated at the interphase between 0.8 M and 1.0 M sucrose (fraction H), using the sucrose density gradient separation technique. Acetylhomocholine was preferentially localized within the cytoplasmic compartment and taken up into synaptic vesicles to a smaller extent than acetylpyrrolidinecholine. Locke's solution was placed in Perspex cylinders (cups) applied to the surface of the cortex and acetylcholine as well as acetylpyrrolidinecholine and acetylhomocholine were released into these cups by diffusion. High-frequency stimulation (30 Hz) approximately doubled the amount of transmitters released. The molar ratios of the transmitters ([14C]acetylcholine/[3H]acetylpyrrolidinecholine or [14C]acetylcholine/[3H]acetylhomocholine) released during stimulation closely approximated those found in fraction H but differed significantly from the molar ratios determined in the monodisperse synaptic vesicle fraction (fraction D) at the 0.4 M sucrose level and also from that in the cytoplasmic compartment. Moreover, the specific activity of the released transmitter was nearly identical with that in fraction H. It is concluded that electrical stimulation releases acetylcholine from a distinct, metabolically very active vesicular pool in the guinea-pig cortex and that synaptic activation in some way facilitates the vesicular uptake of transmitters. © 1979.