Behavioral study of chemoreception in the sea starMarthasterias glacialis: Structure-activity relationships of lactic acid, amino acids, and acetylcholine

Summary

• 1. Behavioral responses ofMarthasterias glacialis to low molecular compounds were studied under laboratory conditions. Feeding postures, stomach eversions and locomotion of initially inactive animals can be released with very dilute solutions of lactic acid, neutral 2 and 3 carbon amino acids, L isomers of 4 to 6 carbon neutral amino acids, L-arginine, acetylcholine iodide, and several of their analogues.

• 2. Hunger was induced by temporary withdrawal of food. Responsiveness to feeding stimuli was controlled with L-cysteine and L-leucine.

• 3. The lowest behavioral thresholds for the most effective feeding stimuli were 3×10⁻¹¹ mol/l for both enantiomers of lactic acid, 10⁻⁸ mol/l for L-proline and both enantiomers of cysteine and 10⁻⁷ mol/l for acetylcholine iodide and some of the effective neutral amino acids.

• 4. The behavioral threshold values for chemical stimuli differed by a factor between 30 and 100 in different sea stars. The test concentration was 3×10⁻⁷ mol/l, the level at which L-cysteine elicited a complete feeding response from all the animals. Structure-activity comparison of substances less effective than the control stimulus was thus possible. The behavioral threshold of fully effective substances was determined later.

• 5. The independence of receptor mechanisms for different substances can be inferred as: L-cysteine controlled responsiveness is not always accompanied by responsiveness to neutral amino acids.

  AutotomizedMarthasterias arms crawled after stimulation with lactic acid, cysteine, and acetylcholine iodide but did not respond to the feeding stimuli betaine and L-proline.

  An animal became inactive if electric shocks were paired with L-proline or L-cysteine emanating from an ‘electric’ food model. It was subsequently easier to release feeding with a substance which had not been presented with shocks.

• 6. α-carboxyl andα-hydroxyl groups are pre-requisites for the attachment of lactic and glycolic acids to the receptor subsites. The methyl group of lactic acid contributes to its effectiveness.

• 7. Theα-amino and ionisedα-carboxyl groups were necessary components of neutral amino acids which stimulated the animals. L- and D-enantiomers of alanine were equally effective. Responsiveness to neutral amino acids with 4 to 6 carbons was stereoselective.

• 8. Cysteine and some of its analogues interact with two different receptor mechanisms. The sulfhydryl group releases vigorous searching for food. The crawling and the feeding postures are triggered by thioglycolic acid, L-cysteine methylester HCl, Na₂S, and reduced glutathione. These substances prevent the animals from discriminating between sea water soaked and test food models. Substances withα-carboxyl,α-amino, and sulfhydryl groups such as L- and D-cysteine, DL-homocysteine, and N-acetyl-L-cysteine release the complete sequence of feeding motions and enable the selection of flavoured food models. It is postulated that behavior is influenced via the neutral amino acid receptors.

• 9. Free dissolved acetylcholine iodide, acetyl-β-methyl-choline, and cholinemethylsulphate are effective feeding stimuli forMarthasterias glacialis.