Aplysia synaptosomes. II. Release of transmitters.

A subcellular fraction (P3) from Aplysia is enriched in synaptosomes (Chin et al., 1988) and is capable of accumulating 5-HT and choline. At an external 3H-5-HT concentration of 1.8 microM, the P3 fraction took up 0.12 nmol/mg protein in 30 min. Uptake was dependent on external Na+. Electron microscopic autoradiography showed that much of the accumulated 3H-5-HT is localized to synaptosomes. At 0.5 microM 3H-choline, P3 took up 0.11 nmol/mg protein in 30 min and converted 40% to 3H-ACh. This synaptosomal fraction was also capable of releasing transmitter. After 3H-5-HT or 3H-choline was taken up, P3 released about 5% of the total radioactive transmitter in a Ca2+-dependent manner during a 30 sec exposure to a depolarizing concentration of K+ (100 mM). Identified, prelabeled synaptosomes were prepared by injecting 3H-choline into the large cholinergic neuron L10. The abdominal ganglia containing the injected cells were then fractionated, yielding synaptosomes containing radioactivity derived from L10. After this synaptosomal fraction was exposed to high K+, 2% of the radioactivity was released in a Ca2+-dependent manner. This release was completely blocked by 0.1 mM histamine, a modulatory transmitter that has previously been shown to cause presynaptic inhibition in L10.