A COMPARISON OF GABA AND β‐ALANINE TRANSPORT AND GABA MEMBRANE BINDING IN THE RAT BRAIN

It was found that rat brain nerve endings contain a high affinity and Na^‐ dependent transport system for [³H]β‐alanine ([³H]β‐ala). As determined from Michaelis‐Menten plots, the [³H]β‐ala K_m was 2.8 × 10^‐5 M and the V_max was 0.29 nmol/mg protein/5 min. Under similar incubation conditions the [³H]GABA K_m was 3.8 x 10^‐6M and the V_max was 6.3 nmol/mg protein/5 min. The [³H]β‐ala and [³H]GABA transport systems were further characterized by determining the IC₅₀ values for a number of compounds. The compounds tested were GABA, β‐ala, l‐2,4‐diaminobutyric acid. DL‐3‐hyd‐roxy‐GABA, β‐guanidopropionic acid, strychnine, γ‐guanidobutyric acid, imidazole‐4‐acetic acid, DL‐proline, bicuculline, L‐serine, glycine, l‐α‐ala and taurine. DABA, dl‐3‐hydroxy‐GABA, β‐guanidopro‐pionic acid and γ‐guanidobutyric acid were more potent inhibitors of [³H]GABA than [³H]β‐ala transport. Strychnine, imidazole‐4‐acetic acid, proline and glycine were between 2 and 6 times more potent inhibitors of [³H]β‐ala than [³H]GABA transport. β‐Ala, bicuculline, serine, α‐alanine and taurine were all markedly more potent (12–150 times) inhibitors of [³H]β‐ala than [³H]GABA transport. IC₅₀ values were also determined for the above compounds for the sodium‐dependent and the sodium‐independent binding of [³H]GABA to both fresh and frozen brain membranes. In general, the potency of these compounds to inhibit either sodium‐independent or sodium‐dependent binding was greater in fresh tissue. It was also observed that the neurophysiologically‘glycine‐like’amino acids were more potent inhibitors in the presence of NaCl. No significant correlations were found between [³H]GABA binding under any condition and [³H]GABA or [³H]β‐ala transport into nerve endings.