Release of endogenous serotonin from two identified serotonin‐containing neurones and the physiological role of serotonin re‐uptake

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Abstract
1. The amounts of endogenous serotonin (5‐HT) released into the medium by the cerebro‐buccal ganglionic ring of Aplysia californica incubated in artificial sea water (ASW) were measured. The rate of spontaneous 5‐HT release varied between 0·4 and 1·2 p‐mole per hour, which is less than 1% of the total 5‐HT present in this preparation.2. Direct stimulation of the ordinarily silent 5‐HT‐containing giant cerebral neurones resulted in a 80‐100% increase of the 5‐HT released, but only when the 5‐HT uptake was blocked by chlorimipramine (1‐10 μ M).3. High K+ media (50 m M) also caused a significant increase in the amount of 5‐HT released from the preparation provided that chlorimipramine (1‐10 μ M) was present in the incubation fluid.4. Co2+ ions (10‐30 m M) added to the incubating medium blocked the spontaneous leak of endogenous 5‐HT as well as the release, in the presence of chlorimipramine, evoked either by stimulation of the 5‐HT‐giant cerebral neurones or high K+‐media.5. In the presence of chlorimipramine or desmethylimipramine, the duration and/or the amplitude of the excitatory or the inhibitory synaptic potentials evoked in the buccal neurones by the stimulation of the 5‐HT giant cerebral neurones were markedly enhanced.6. These results strongly support the idea that 5‐HT is the synaptic transmitter released at the excitatory and inhibitory junctions established by the 5‐HT giant cerebral neurones in the ipsilateral buccal ganglia. In addition, they underline the role of amine re‐uptake in the physiological inactivation of 5‐HT as a transmitter.