Effects of neurotransmitter receptor antagonists on sea urchin righting behavior and tube foot motility

Abstract

ABSTRACT Echinoderms, such as sea urchins, occupy an interesting position in animal phylogeny in that they are genetically closer to vertebrates than the vast majority of all other invertebrates but have a nervous system that lacks a brain or brain-like structure. Despite this, very little is known about the neurobiology of the adult sea urchin, and how the nervous system is utilized to produce behavior. Here, we investigated effects on the righting response of antagonists of ionotropic receptors for the neurotransmitters acetylcholine, GABA and glycine, and antagonists of metabotropic receptors for the amines dopamine and noradrenaline (norepinephrine). Antagonists slowed the righting response in a dose-dependent manner, with a rank order of potency of strychnine>haloperidol>propranolol>bicuculline>hexamethonium, with RT50 values (concentrations that slowed righting time by 50%) ranging from 4.3 µmol l−1 for strychnine to 7.8 mmol l−1 for hexamethonium. The results also showed that both glycine and adrenergic receptors are needed for actual tube foot movement, and this may explain the slowed righting seen when these receptors were inhibited. Conversely, inhibition of dopamine receptors slowed the righting response but had no effect on tube foot motility, indicating that these receptors play roles in the neural processing involved in the righting behavior, rather than the actual physical righting. Our results identify the first effects of inhibiting the glycinergic, dopaminergic and adrenergic neurotransmitter systems in adult sea urchins and distinguish between the ability of sea urchins to right themselves and their ability to move their tube feet.