Modulation of GABAergic neurotransmission in the brain by dipeptides

The effects of endogenous and synthetic peptides containing GABA or its analogues on the GABA/benzodiazepine/chloride ionophore complex. GABAB receptor, Cl fluxes, GABA release and GABA uptake were studied using synaptic membranes, crude synaptoneurosomal preparations and slices prepared from the rat and mouse brain. The sodium-independent binding of GABA was strongly inhibited by GABA-histidine, followed by gamma-glutamyl-homotaurine, GABA-glycine and gamma-glutamyl-GABA. The binding of diazepam was slightly enhanced by the same peptides. The peptides alone had no effect on the chloride fluxes, but GABA-histidine, gamma-glutamyl-GABA and GABA-glycine enhanced while gamma-glutamyl-homotaurine and GABA-taurine inhibited GABA-stimulated chloride uptake. GABA-histidine was the most effective displacer of baclofen binding, but gamma-glutamylhomotaurine was entirely ineffective. The uptake of GABA was markedly inhibited in synaptosomal preparations by GABA-histidine, while all other peptides were less effective. gamma-Glutamyl-taurine attenuated but gamma-glutamyl-homotaurine and GABA-glycine enhanced the potassium-stimulated release of GABA. The present actions of GABA-histidine in vitro may be of significance for GABAergic neurotransmission in vivo.