CA2+-INDUCED NORADRENALINE RELEASE FROM CENTRAL NORADRENERGIC NEURONS PROMOTED BY HIGH K+ CONCENTRATION OR IONOPHORE-A23187

In slices of rat occipital cortex and hypothalamus preincubated with 3H-noradrenaline and superfused with various Ca2+-free solutions, the effect of introduction of CaCl2 on efflux of total tritium, 3H-noradrenaline and 3H-metabolites was studied. 1. When slices of both brain regions were superfused with a Ca2+-free buffer containing 30 mM K+ and 119 mM Na+, introduction of 0.65-2.6 mM CaCl2 for 2 min induced a concentration-dependent overflow of tritium. Cocaine (100 μM) doubled the overflow of tritium evoked by 1.3 mM CaCl2. When slices were superfused with physiological salt solution after 100 min of superfusion with the Ca2+-free buffer and 2 stimulation periods with 1.3 mM CaCl2, electrical field stimulation (3 Hz, 13 mA, 2 ms, 2 min duration) still elicited on overflow of tritium which contained 59% 3H-noradrenaline. The CaCl2-induced overflow was not affected by tetrodotoxin (1 μM), but was inhibited by Mg2+ ions (10 mM). 2. When cortical slices incubated with Ca2+-free solution containing 300 μM ionophore A 23187 (subsequent to preincubation with 3H-noradrenaline) were superfused with Ca2+-free solution containing 0.3 mM Mg2+ (otherwise composed like physiological salt solution), introduction of 1.3 or 2.6 mM CaCl2 for 5 min evoked a concentration-dependent overflow of tritium. The evoked overflow of 3H-noradrenaline was similar to that produced by the same Ca2+ concentrations after superfusion with K+-rich solution. The Ca2+-induced tritium overflow from slices preincubated with A 23187 was completely inhibited by 10 mM MgCl2. 3. The Ca2+-induced overflow of tritium from cortex slices superfused with 30 mM K+ or preincubated with A 23187 was mainly represented by 3H-noradrenaline; the most important degradation products were 3H-dihydroxyphenylglycol and O-methylated deaminated metabolites. Under both conditions 100 μM cocaine increased the proportional 3H-noradrenaline overflow. It is concluded that combined application of both methods of Ca2+-induced stimulation provides a tool for the determination of the site of action of drugs which alter stimulus-release coupling in noradrenergic neurons of the brain. © 1979 Springer-Verlag.