Characterization of divalent cation-induced [3H]acetylcholine release from EGTA-treated rat hippocampal synaptosomes

Calcium-naive synaptosomes were used to assess the effects of divalent cations on [3H]acetylcholine release from rat hippocampal homogenates. Following equilibration with calcium-free buffer (containing 10 microM EGTA), calcium reversibly increased [3H]acetylcholine efflux (up to five-fold) while causing no measurable efflux of lactate dehydrogenase. When substituted for calcium, strontium and barium behaved similarly although barium exhibited three-fold greater efficacy. In the presence of elevated potassium, 4-aminopyridine or tetraethylammonium, the secretagogue efficacy of calcium (but not barium) was markedly increased. The release-promoting effects of both cations were inhibited by lanthanum, magnesium, cadmium, and omega-conotoxin but were insensitive to nifedipine and cobalt (both 10 microM). In addition, stimulation of muscarinic cholinergic autoreceptors substantially inhibited both calcium and barium-evoked [3H]acetylcholine release. Taken together, these results indicate that cation-evoked transmitter release from calcium-naive synaptosomes is subject to normal neuroregulatory mechanisms and therefore should be useful for investigating presynaptic modulation of neuronal exocytosis.