The mechanism by which monoamine oxidase inhibitors give rise to a non-calcium-dependent component in the depolarization-induced release of 5-HT from rat brain synaptosomes.

1. The effects of the monoamine oxidase inhibitors pargyline and nialamide on the Ca2+-dependency of [3H]-5-hydroxytryptamine release from superfused rat brain synaptosomes has been studied in order to evaluate the discrepancies that have occasionally been observed in studying transmitter release by in vivo and in vitro techniques. 2. The application of K+ pulses of low concentration (12.5-20 mM) caused an essentially Ca2+-dependent release of [3H]-5-HT. However, at K+ concentrations above 30 mM, a small non-Ca2+-dependent component appeared. 3. At high concentrations of K+ (30-55 mM), nialamide (18 microM) or pargyline (7 microM) increased the amount of [3H]-5-HT released which could be accounted for by an increase in the non-Ca2+-dependent component of release. 4. The elevation of the non-Ca2+-dependent component of release caused by the monoamine oxidase inhibitors was totally abolished by the inhibitors of the plasma membrane 5-HT carrier, chlomipramine (500 nM), citalopram (50 nM) and fluoxetine (1 microM). 5. The results suggest that the non-Ca2+-dependent component of release seen with high depolarizing concentrations of K+, particularly in the presence of monoamine oxidase inhibitors, is caused by the efflux of [3H]-5-HT through the plasma membrane carrier which seems to be activated during depolarization. 6. The significance of these findings to the physiological in vivo situation, and to the use of in vitro preparations in the study of transmitter release is discussed.