Different roles of GABA and glycine in the modulation of chemosensory responses in Hydra vulgaris (Cnidaria, Hydrozoa)

Abstract

Phylogenetic studies suggest that GABA and glycine receptors derive, as a result of divergent evolution, from a common ancestral protoreceptor originated in a unicellular organism. This raises the possibility that members of the ligand-gated ion channels (LGIC) superfamily might be widely present in living organisms including bacteria and primitive invertebrates. High-affinity GABA receptors occur in the tissues of Hydra vulgaris whose pharmacological characteristics compare with those of mammalian ionotropic GABA receptors. Behavioural studies have shown that activation of these GABA _A -like receptors by their allosteric modulators increases the duration of response to reduced glutathione (GSH). Recently, strychnine-sensitive glycine receptors have been shown to occur in Hydra tissues. Activation of these glyR also results in increased duration of the response to GSH. In order to investigate the contribution of endogenous transmitters to the modulation of the feeding response, we studied the effects of exposing the polyps to brief depolarizing pulses prior to the GSH test. A severe inhibition of the response was observed following exposure to KCl or veratridine. Administration of GABA or muscimol counteracted the effects of the pulses in a dose-dependent manner. The effects of GABA or muscimol were suppressed by the GABA _A -specific antagonist gabazine both in pulse-untreated and treated polyps. By contrast, glycine and its agonist taurine were not able to restore the physiological duration of response in pulse-treated Hydra, while another glyR agonist, β-alanine, partially reduced the pulse-induced inhibition. We conclude that GABA appears to be the major inhibitory transmitter responsible for the regulation of the feeding response. Molecular studies aimed at identifying GABA receptor subunits are in progress.