Ca 2+ Mobilized by Caffeine from the Inositol 1,4,5‐Trisphosphate‐Insensitive Pool of Ca 2+ in Somatic Regions of Sympathetic Neurons Does Not Evoke [ 3 H]Norepinephrine Release

The effects of electrical stimulation, muscarinic and serotonergic agonists, and caffeine on [ 3 H]inositol 1,4,5‐trisphosphate ([ 3 H]Ins(1,4,5)P 3 ) content, intracellular free Ca 2+ concentration ([Ca 2+ ] i ), and release of [ 3 H]norepinephrine ([ 3 H]NE) were studied in cultured sympathetic neurons. Neuronal cell body [Ca 2+ ] i was unaffected by muscarinic or serotonergic receptor stimulation, which significantly increased [ 3 H]Ins(1,4,5)P 3 content. Stimulation at 2 Hz and caffeine had no effect on [ 3 H]Ins(1,4,5)P 3 , but caused greater than two‐fold increase in [Ca 2+ ] i . Only 2‐Hz stimulation released [ 3 H]NE. Caffeine had no effect on the release. When [Ca 2+ ] i was measured in growth cones, only electrical stimulation produced an increase in [Ca 2+ ] i . The other agents had no effect on Ca 2+ at the terminal regions of the neurons. We conclude that Ins(1,4,5)P 3 ‐insensitive, but caffeine‐sensitive Ca 2+ stores in sympathetic neurons are located only in the cell body and are not coupled to [ 3 H]NE release.