INHIBITORS OF SYNAPTOSOMAL GAMMA-HYDROXYBUTYRATE TRANSPORT

Synaptosomes prepared from mouse brain possess a Na+-dependent transpo rt system for gamma-hydroxybutyrate displaying saturation kinetics, th e transport constant (K-t) for which was calculated as 31 +/- 9 mu mol /l. Several gamma-hydroxybutyrate and gamma-aminobutyric acid (GABA) s tructural analogues were tested as potential inhibitors of gamma-hydro xybutyrate transport. The most effective inhibitor was harmaline (K-i = 94 +/- 21 mu mol/l), a known competitive inhibitor of Na+ binding to certain transport proteins. 2-Hydroxycinnamic acid, 3-(2-furyl)acryli c acid and citrazinic acid also inhibited transport and were competiti ve with respect to gamma-hydroxybutyrate. The least effective gamma-hy droxybutyrate analogues were gamma-hydroxypropane sulfonic acid (K-i = 4.1 +/- 0.8 mmol/l) 3,5-dihydroxybenzoic acid (K-i = 6.1 +/- 2.8 mmol /l) and 3-hydroxybenzoic acid (K-i = 6.9 +/- 3.3 mmol/l), although 2-h ydroxypropane sulfonic acid and kynurenic acid had no measurable effec ts. Four inhibitors of GABA transport - nipecotic acid, guvacine, keta mine and beta-alanine and GABA itself, were without effect on gamma-hy droxybutyrate transport. These results show that certain drugs that st ructurally 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 abo ut 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 tr ansport.