A Possible Enzyme Barrier for γ-Aminobutyric Acid in the Central Nervous System

Abstract

High concentrations of y-amino butyric acid (GABA) in blood did not result in an elevation of brain GABA levels. A barrier exists which restricts the passage of GABA from blood to nervous tissue. But recent histochemical studies on the distribution of GABA-a-keto-glutarate transaminase in the mammalian central nervous system suggested that the barrier may be in part a reflection of high GABA-transaminase activity in the walls of cerebral blood vessels and in the cells lining the cerebrospinal fluid spaces. To explore this suggestion further, the paper has a study that was conducted on the distribution of GABA-transaminase in kidney and liver. GABA appears to pass freely from blood to kidney, since as much as fifty per cent of the total GABA injected can be recovered from urine. Similarly, the concentration of GABA in liver after injection parallels that of the blood. The paper consists of the histochemical procedures for the localization of GABA-transaminase. The method is based on the fact that, in the course of the metabolism of GABA to succinic acid, NADH is formed, which in turn reduces a tetrazolium salt (Nitro BT) to its formazan form. Formazan is insoluble and will precipitate out at sites in the tissue where the combined presences of GABA-a-keto glutarate transaminase and succinic semialdehyde dehydrogenase have caused the conversion of GABA to succinic acid. When sections of mammalian brain are incubated with a transaminase medium, strong formazan precipitation occurs at sites where GABA-transaminase and succinic semialdehyde dehydrogenase are present in combination. Although the presence of both enzymes is a prerequisite for the reaction, recent studies suggested that the transamination step is rate limiting with respect to the histochemical demonstration of GABA metabolism. As a conclusion, the paper states that the ependymal cell layer which lines the cerebrospinal fluid spaces is among the sites in the central nervous system which exhibit strong GABA-transaminase activity. The paper also has a brief on the specificity of the histochemical reaction for GABA. In order to determine the specificity of the histochemical reaction for GABA transaminase in kidney, hydrazinopropionic acid (1 5 mg/kg) was injected into a mouse and its kidney was sectioned two hours later. Such sections, when incubated in the transaminase medium, were identical in appearance to normal sections which had been incubated in a control medium lacking GABA.