In vitro experiments on the metabolism, accumulation and release of 5-HT in the nervous system of the snail Helix pomatia.

Abstract

Abstract— The accumulation, metabolism and stimulated‐induced release of 5‐HT in the nervous system of the snail was studied. When nervous tissue was incubated at 24°C in a medium containing [14C]5‐HT or [3H]tryptophan, tissue: medium ratios of about 25:1 and 4:1 respectively were obtained after 45 min incubation.The process responsible for [14C]5‐HT accumulation showed properties of an active transport system: it was temperature sensitive and was greatly inhibited by dinitrophenol and ouabain. Furthermore, the accumulation process was inhibited by imipramine and desipramine.Of a number of analogues of indole, N‐acetyl‐5‐HT and 5‐hydroxytryptophan were the most potent in the inhibition of the accumulation of [14C]5‐HT. The presence of a large molar excess of amino acids had little effect.A small amount (less than 14 per cent) of the accumulated [14C]5‐HT was metabolized to form 5‐hydroxyindole acetic acid, even after long periods (2 h) of incubation. The accumulated [3H]tryptophan was metabolized to form 5‐hydroxytryptophan and 5‐HT; the content of formed [3H]5‐HT increased with incubation time whilst the [3H]5‐hydroxytryptophan remained more or less constant.The presence of p‐chlorophenylalanine in the incubation medium did not interfere with the accumulation of [3H]tryptophan, though it inhibited the formation of [3H]5‐hydroxytryptophan and to a greater extent [3H]5‐HT.A rapid efflux of the accumulated [14C]5‐HT from snail nervous tissue was observed on electrical stimulation. Slower release resulted when the Ca2+ ion content of the incubation medium was replaced by Mg2+ ions. There is also a slight efflux of radioactive substances following electrical stimulation in tissues previously incubated in [3H]tryptophan. Most of this radioactivity was attributed to the formed [3H]5‐HT.The data support the idea that 5‐HT is a transmitter‐substance in the snail Helix pomatia, and that re‐uptake of the substance is a method of inactivating the released amine.