Inhibitors of synaptosomal gamma-hydroxybutyrate transport

Synaptosomes prepared from mouse brain possess a Na+-dependent transport system for gamma-hydroxybutyrate displaying saturation kinetics, the transport constant (Kt) for which was calculated as 31 +/- 9 micromol/l. Several gamma-hydroxybutyrate and gamma-aminobutyric acid (GABA) structural analogues were tested as potential inhibitors of gamma-hydroxybutyrate transport. The most effective inhibitor was harmaline (Ki = 94 +/- 21 micromol/l), a known competitive inhibitor of Na+ binding to certain transport proteins. 2-Hydroxycinnamic acid, 3-(2-furyl)acrylic acid and citrazinic acid also inhibited transport and were competitive with respect to gamma-hydroxybutyrate. The least effective gamma-hydroxybutyrate analogues were 3-hydroxypropane sulfonic acid (Ki = 4.1 +/- 0.8 mmol/l) 3,5-dihydroxybenzoic acid (Ki = 6.1 +/- 2. 8 mmol/l) and 3-hydroxybenzoic acid (Ki = 6.9 +/- 3.3 mmol/l), although 2-hydroxypropane sulfonic acid and kynurenic acid had no measurable effects. Four inhibitors of GABA transport - nipecotic acid, guvacine, ketamine and beta-alanine and GABA itself, were without effect on gamma-hydroxybutyrate transport. These results show that certain drugs that structurally resemble gamma-hydroxybutyrate have the capacity to compete with gamma-hydroxybutyrate at its recognition site on the transporter. By examining the structure of such inhibitors, we can learn more about the properties of the substrate binding site on the carrier protein. Moreover, the absence of inhibition by GABA uptake inhibitors shows that gamma-hydroxybutyrate transport is a separate entity from GABA transport.