Glycine, inhibitor
Glycine, NH2-CH2-COOH, is an amino acid present in all tissues of the body. In the central nervous system, it plays neurotransmitter and neuromodulator roles.
Glycine is synthesized from serine under the influence of a hydroxy-methyl-transferase. Like GABA, it is released by presynaptic terminations in synaptic clefts.
It acts on two types of receptors:
- Specific pentameric receptor-channels permeable to Cl - ions in the activated state. The principal agonist is glycine and, with less efficacy, beta-alanine and taurine. It induces their opening and has a hyperpolarizing effect. The preferential order of ions is as follows: Br >Cl > I.
- NMDA receptor whose main physiological agonist is glutamate. This receptor involves a regulating site which is modulated by glycine. Glycine is necessary for the action of glutamate which has a depolarizing effect by entry of sodium and calcium into the cell.
By activating these 2 types of receptors glycine has thus opposite effects. The result of these two antagonist effects depends in particular on glycine distribution and receptors in various areas of the brain.
Strychnine is an antagonist of glycine, it is not used any more in therapeutics, but can cause poisoning because it is used against moles. This poisoning causes hyperresponsiveness to stimuli, hyperreflexia and muscular hypertonicity. Its antidote is a benzodiazepine such as diazepam which, while acting on GABA receptors and not glycine receptors, decreases exitability.
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