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Analogues of γ‐aminobutyric acid (GABA) incorporating an isothiouronium salt as a replacement for a protonated amino functional group have been investigated for activity on: GABA receptors in the guinea‐pig ileum; [3H]‐GABA and [3H]‐diazepam binding to rat brain membranes; and GABA uptake and transamination. For the homologous series of α‐isothiouronium alkanoic acids, maximum GABA‐mimetic activity was found at 3‐[(aminoiminomethyl)thio]propanoic acid. Introduction of unsaturation into this compound gave two isomeric conformationally restricted analogues. The trans isomer was inactive at GABA receptors while the cis compound ((Z)‐3‐[(aminoiminomethyl)thio]prop‐2‐enoic acid (ZAPA)) was more potent than muscimol and GABA as a GABA agonist with respect to low affinity GABA receptor sites. Both isomers were moderately potent at inhibiting the uptake of [3H]‐GABA into rat brain slices. Comparison of possible conformations of the two unsaturated isomers by interactive computer graphics modelling and comparison with muscimol has led to a plausible active conformation of ZAPA, which may be a selective and potent agonist for low affinity GABA binding sites.

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The (Z) and (E) pairs of 5-aminopent-2-enoic acid and 5-aminopent-3- enoic acid, as well as the related 5-aminopent-3-ynoic acid, have been prepared for structure-activity studies on GABA receptors. Only the (Z) isomers were active as GABA agonists with (Z)-5-aminopent-2-enoic acid being two- to four-fold more active than 5-aminopentanoic acid.

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