Extracellular calcium alters frequency modulation of [3H]acetylcholine release from rat hippocampal slices.

The concentration of extracellular Ca2+ has been shown to enhance or attenuate [3H]acetylcholine (ACh) release subsequent to a conditioning stimulus in rat brain hippocampal slices. Slices were incubated in vitro in [3H]choline solution. Subsequently the slices were subjected to two consecutive electrical stimulations separated by 15 or 30 min at 0.25, 1, 4 and 16 Hz and [3H]ACh release was assessed. It was found that a conditioning stimulus may reduce [3H]ACh release during a second stimulation. This phenomenon is frequency related and disappears when the two stimulations are 30 min apart. High extracellular Ca2+ (4.0 mM) further attenuated [3H]ACh release during the second stimulation, whereas low Ca2+ (0.32 mM) abolished the decrease in [3H]ACh release following the second stimulation in all frequencies tested.