IN VITRO EXPERIMENTS ON THE METABOLISM, ACCUMULATION AND RELEASE OF 5-HT IN THE NERVOUS SYSTEM OF THE SNAIL HELIX POMATIA

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.