Putative NMDA receptors in Hydra: a biochemical and functional study

Abstract

The feeding behaviour of the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) is modulated by a number of molecules acting as neurotransmitters in other nervous systems. Here we present biochemical and functional evidence of the occurrence of putative NMDA receptors in Hydra tissues. Saturation experiments showed the presence of one population of binding sites with nanomolar affinity and low capacity for [3H]MK-801. Before equilibrium, [3H]MK-801 binding was increased by the agonists glutamate and glycine as well as by reduced glutathione (GSH). In vivo the glutamate receptor agonist NMDA markedly decreased the duration of the response to GSH. This effect was linearly related to ligand doses in the nanomolar concentration range and was counteracted by either the NMDAR-specific antagonist D-AP5 or by the d-serine antagonist DCKA. When NMDA concentration was increased to 10 or 100 microm, duration of the response to GSH was no longer affected unless the lectin concanavalin A, which prevents receptor desensitization in other systems, was added to the test medium. Simultaneous administration of ineffective doses of NMDA and strychnine, glycine or d-serine, an agonist at the glycine binding site of the NMDA receptor in vertebrate CNS, resulted in a strong reduction of response duration. Both D-AP5 and DCKA suppressed this effect. These results, together with the decrease in response duration produced by d-serine, support the hypothesis that NMDA-like glutamate receptors may occur in Hydra tissues where they are involved in modulation of the response to GSH with opposite actions to those of GABA and glycine.

Molecular Characterization of NMDA-Like Receptors inAplysiaandLymnaea:Relevance to Memory Mechanisms

“…We would like to stress that the lingering controversy over the universality of the role of NMDA receptors as a key factor in learning and memory across different animal phyla, as well as over evolutionary aspects of functions for NMDA receptors (e.g., Wu, 2002;Rose et al, 2003) is primarily due to insufficient comparative data-especially data from basal metazoa (e.g., Pierobon et al, 2004;Scappaticci et al, 2004). Molecular and functional characterization of nonvertebrate homologs of these receptors would probably reveal novel regulatory mechanisms and pathways.…”

Coordinated modulation of cellular signaling through ligand-gated ion channels in Hydra vulgaris (Cnidaria, Hydrozoa)

“…We reported the identification and characterisation of the first invertebrate glycine receptor, whose biochemical and pharmacological properties also compare with those of vertebrate GlyRs (Pierobon et al, 2001). Saturation experiments revealed the existence of one population of binding sites for (Mothet et al, 2000).…”

“…Following the discovery of strychnine-insensitive glycine binding sites in Hydra (Pierobon et al, 2001;see below), saturation experiments showed the presence of one population of binding sites with nanomolar affinity and low capacity for […”

“…3 H]MK-801, a potent, noncompetitive antagonist of the NMDA receptor (Pierobon et al, 2004a). Before equilibrium, […”

An Ancient Model Organism to Test In Vivo Novel Functional Nanocrystals

“…Upon stimulation this cnidocil leads to the external discharge of an intracellular venom capsule (the nematocyst or cnidocyst), involved in the prey capture. Despite the simplicity of its nervous system, organized as a mesh-like network of neurons extending throughout the animal, the complexity of the mechanisms underlying neurotransmission resemble those of higher vertebrates, including both classical and peptidergic neurotransmitters (Pierobon et al, 2001;Pierobon et al, 2004). This makes Hydra an ideal system to study the behavioural response of a whole animal to an external stimuli, i.e.…”