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

Sea urchins, as echinoderms, 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 neurobiology of the adult sea urchin, and how the nervous system, is utilized to produced behavior. Here we investigate 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 norepinephrine. Antagonists slowed the righting response in a dose-dependent manner, with a rank order of potency of strychnine>haloperidol>propranolol>bicuculline>hexamethonium, with RT50s (concentrations that slowed righting time by 50%) ranging from 4.3 µM for strychnine to 7.8 mM for hexamethonium. It is also shown that both glycine and adrenergic receptors are needed for actual tube foot movement, and this may explain the slowed righting seen when these receptors are inhibited. Conversely, inhibition of dopamine receptors slowed the righting response but had no effect on tube foot motility, indicating that these receptors play roles more in the neural processing involved in the righting behavior, rather than the actual physical righting. Our results identity 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 the ability of sea urchins to move their tube feet.