Calcium changes in isolated peptidergic neurons during activation by a cAMP analog

The bag cell neurons of intact Aplysia control egg-laying by producing an approximately 30 min afterdischarge of action potentials, during which the neuropeptide egg-laying hormone is released. Previous studies have shown that cAMP, protein phosphorylation and Ca2+ are involved in the mechanisms of the afterdischarge. Spontaneous discharge and afterdischarge can be produced in dissociated bag cells by treatment with cAMP analogs. While pharmacological evidence exists for a large calcium component in the action potentials of bag cells, direct evidence by measuring (Ca2+)i has not appeared, nor has Ca-buffering during cAMP activation been examined. Our studies were directed at measuring (Ca2+)i changes in isolated bag cells in attached cell culture using the metallochromic indicator, arsenazo III, simultaneously with voltage recordings across the cell membrane. It was found that a single induced action potential causes a negligible rise in (Ca2+)i while a train of 35 spikes causes a significant rise (about 3 X 10(-4) absorbance units). Increasing external (Ca2+) enhances the action potential and Ca2+ signal while cobaltous ions block both the action potentials and the rise in (Ca2+)i. Lengthening the duration of spikes by TEA (50 mM) or 8-benzylthio cAMP (8-BTcAMP; 0.5 mM) enhances Ca2+ influx and allows the detection of Ca2+ transients due to single spikes. During spontaneous groups of action potentials in either TEA or 8-BTcAMP, (Ca2+)i increases but the bag cell's Ca-buffering systems are able to restore the Ca2+ levels with a half-time of about 20 s.(ABSTRACT TRUNCATED AT 250 WORDS)