Putative glycine receptors in Hydra: a biochemical and behavioural study

Abstract

Glycine acts as an inhibitory transmitter in the lower brain stem and spinal cord of vertebrate species, while very few data are yet available to support a similar role in invertebrate nervous systems. Here we report the identification and characterization of glycine receptors in the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) by biochemical and behavioural studies. Saturation experiments revealed the occurrence of one population of binding sites of nanomolar affinity (KD = 33 nm) and low capacity (Bmax = 79 fmol/mg protein) for [(3)H]strychnine. The addition of glycine or taurine (0.1 microm-1 mm) produced a dose-dependent inhibition of [(3)H]strychnine binding. Beta-alanine (0.1-1 mm) did not significantly affect [(3)H]strychnine binding. The pharmacological properties of these receptors compare with those of vertebrate glycine receptors. Stimulation of Hydra polyps by reduced glutathione resulted in a significant increase in the duration of mouth opening in the presence of glycine, taurine or beta-alanine. The enhancement of the response was related both to amino acid (10-100 microm) and to glutathione concentration (1-10 microm). The effects of glycine or its agonists were suppressed by strychnine (1-10 microm). D-serine, a glycine agonist at the vertebrate NMDA receptor, produced opposite effects to those of glycine. The effects of d-serine were suppressed by 5,7-dichlorokynurenic acid but not by strychnine. In vitro, [(3)H]strychnine binding was not displaced by d-serine. These results indicate a dual action of glycine in Hydra tissues. The hypothesis that NMDA receptors may also be present in this elementary nervous system is proposed.

Regional modulation of the response to glutathione in Hydra vulgaris (Pallas, 1766).

“…The GlyR agonists taurine and β-alanine also decrease response duration, with taurine being more potent than glycine and β-alanine. Again, the pharmacological findings are in keeping with previous results (Pierobon et al, 2001), but the effects of glycine administration are reversed in footless with respect to whole animals. In heads, glycine administration does not significantly modify response duration.…”

“…In Hydra, the classical amino acid neurotransmitters GABA and glutamate exert an inhibitory and excitatory action, respectively, on the pacemaker systems (Kass-Simon et al, 2003). In previous papers, we have shown that the response to GSH is finely tuned by inhibitory and excitatory amino acid neurotransmitters, indicating that the cellular components leading to mouth closure are modulated by the nerve net (Concas et al, 1998;Pierobon et al, 1995Pierobon et al, , 2001Pierobon et al, , 2004aPierobon et al, , 2004b. However, GABA and glycine prolong response duration, while NMDA reduces it; this finding was tentatively explained with the hypothesis of potentiation or inhibition, respectively, of a chain of multiple sequential inhibitory loci, which modulate contraction and relaxation of ectodermal and endodermal myofibrils (reviewed in Pierobon, 2012).…”

“…By using the GSH assay, we have shown that amino acid neurotransmitters, GABA, glycine, NMDA and related ligands, acting through their ionotropic receptors, are able to modulate the duration of the response to GSH by delaying or anticipating, respectively, the time of mouth closure (Concas et al, 1998;Pierobon et al, 1995Pierobon et al, , 2001Pierobon et al, , 2004a. In contrast to glutamate (Bellis et al, 1991), these ligands do not modify times of mouth opening, suggesting that their effect is exerted on the neuromuscular circuitry underlying the feeding behaviour, rather than on the GSH receptor itself (reviewed in Pierobon, 2012).…”

Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system

“…Recently, calcium and potassium channels were characterised in jellyfish with functional features similar to those measured in vertebrate counterparts Jeziorski et al (1999), Grigoriev et al (1999), while GABAreceptors were pharmacologically identified Pierobon et al (2001). Neurotransmitters like glycine Pierobon et al (2001), nitric oxide Colasanti et al (1997), endocannabinoid De Petrocellis et al (1999 and glutamate Bellis et al (1991) are likely playing a physiological role in the feeding response.…”

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.…”