Further studies on the binding of gamma-aminobutyric acid by brain

Revised Ion/Substrate Coupling Stoichiometry of GABA Transporters

The purpose of this review is to highlight recent evidence in support of a 3 Na⁺: 1 Cl^-: 1 GABA coupling stoichiometry for plasma membrane GABA transporters (SLC6A1 , SLC6A11 , SLC6A12 , SLC6A13 ) and how the revised stoichiometry impacts our understanding of the contribution of GABA transporters to GABA homeostasis in synaptic and extrasynaptic regions in the brain under physiological and pathophysiological states. Recently, our laboratory probed the GABA transporter stoichiometry by analyzing the results of six independent measurements, which included the shifts in the thermodynamic transporter reversal potential caused by changes in the extracellular Na⁺, Cl^-, and GABA concentrations, as well as the ratio of charge flux to substrate flux for Na⁺, Cl^-, and GABA under voltage-clamp conditions. The shifts in the transporter reversal potential for a tenfold change in the external concentration of Na⁺, Cl^-, and GABA were 84 ± 4, 30 ± 1, and 29 ± 1 mV, respectively. Charge flux to substrate flux ratios were 0.7 ± 0.1 charges/Na⁺, 2.0 ± 0.2 charges/Cl^-, and 2.1 ± 0.1 charges/GABA. We then compared these experimental results with the predictions of 150 different transporter stoichiometry models, which included 1-5 Na⁺, 0-5 Cl^-, and 1-5 GABA per transport cycle. Only the 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry model correctly predicts the results of all six experimental measurements. Using the revised 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry, we propose that the GABA transporters mediate GABA uptake under most physiological conditions. Transporter-mediated GABA release likely takes place under pathophysiological or extreme physiological conditions.

Differential effects of inorganic lead and delta-aminolevulinic acid in vitro on synaptosomal gamma-aminobutyric acid release

Several studies have shown that inorganic lead added in vitro does not alter gamma-aminobutyric acid (GABA) release from rat brain synaptosomes. The decrease in GABA release observed following chronic neonatal in vivo lead exposure has been proposed to be an indirect effect mediated by the increase in delta-aminolevulinic acid (ALA) accompanying chronic lead exposure. In the present study the effect of both lead and ALA in vitro on several aspects of [3H]GABA release from superfused rat cortical synaptosomes are examined. The present study demonstrates that lead (1-30 microM) added in vitro induces [3H]GABA release from preloaded cortical synaptosomes in a dose-dependent manner. This lead-induced increase in spontaneous [3H]GABA release does not appear to be mediated by inhibition of the membrane Na-K AT-Pase. ALA also induces a dose-dependent [3H]GABA release, but only at concentrations equal to or greater than 30 microM. Exposure to a combination of 3 microM lead and 100 microM ALA results in an increase in spontaneous [3H]GABA release that is greater than either treatment separately. The depolarization-evoked release of [3H]GABA resulting from a 1-sec exposure to 61 mM potassium chloride is reduced by lead (3 and 10 microM), whereas ALA (30-300 microM) does not alter depolarization-evoked release. These findings indicate that an indirect action of lead (elevated ALA concentrations) need not be proposed to explain the alterations in GABA release observed following chronic lead exposure.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of lithium and other alkali metals on brain chemistry and behavior. I. Glutamic acid and GABA in brain regions

Glutamic acid and GABA concentrations were measured in brain areas of rats injected with the chloride salts of Li+, Na+, K", Rb+ or Cs+ for 5 days. Regional changes in brain glutamic acid and GABA were found in animals after lithium, rubidium or cesium, but not potassium, compared to sodium treatments. Increased glutamic acid and GABA levels, caused by lithium and rubidium, were found in brain structures (hypothalamus and amygdala) known to be involved in emotional behavior. Whether these changes are associated with the effective use of lithium and, perhaps, of rubidium in affective disorders remains obscure.

Properties of gamma-aminobutyric acid (GABA) receptor binding in rat brain synaptic membrane fractions

The binding of GABA to crude synaptic membrane fractions of the rat central nervous system in the absence of sodium appears to involve synaptic receptor sites for GABA. The sodium-independent GABA receptor binding differs in a number of features from sodium-dependent GABA binding. In fresh tissue sodium-dependent GABA binding is about 10 times greater than sodium-independent binding, while freezing and thawing the brain membranes virtually abolishes sodium-dependent binding but increases sodium-independent GABA receptor binding about 2-fold. Both sodium-dependent and -independent GABA binding are saturable processes with dissociation constants of 1.2 muM and 0.37 muM respectively. Sodium-dependent GABA binding is much more sensitive to extremes of temperature and pH than is sodium-dependent GABA binding. In addition, regional variations in synaptosomal uptake of GABA, sodium-dependent and sodium-independent GABA binding are markedly different. Also, amino acids and drugs differ in their potencies as inhibitors of sodium-dependent and sodium-independent GABA binding, with the relative affinity of several agents for sodium-independent GABA receptor sites closely paralleling their abilities to mimic neurophysiologic effects of GABA at postsynaptic receptors. Though there are some similarities between the influence of drugs and amino acid analogues on sodium-dependent binding and synaptosomal accumulation of GABA there are also marked differences such as a much greater affinity of beta-alanine for the sodium-dependent GABA binding sites than for synaptosomal GABA uptake. This suggests that the sodium-dependent GABA binding may involve uptake sites for the GABA accumulating system of glia rather than for neuronal GABA transport.

The level of GABA in the brain and locomotor behavior

GABA content was measured in the brains of animals injected with AOAA, DPA or Saline. Significant increases in GABA were found in the motor cortex and cerebellum after treatment with both drugs as compared to saline injected controls. Increased GABA levels were associated with interference with the smooth execution of locomotor acts, especially where balancing and coordination of the hind limbs were necessary.

The gamma-aminobutyric acid system in rat cerebellum during cannabinoid-induced cataleptoid state

Repeated, but not single, intraperitoneal injections of delta1,6-tetrahydrocannabinol (delta1,6-THC) 20 mg/kg to rats administered daily for two weeks, produced increased gamma-aminobutyric acid (GABA) concentration and decreased glutamic acid decarboxylase (GAD) activity in the cerebellum, as well as enhancement of [3H]-GABA uptake by cerebellar crude synaptosomes. It seems that the motor impairment elicited by delta1,6-THC was not associated with the GABA system, but presumably might be related to changes in brain excitability.

The uptake of GABA by clonal nerve glia

A number of clonal nerve and glial cell lines are able to accumulate exogenous GABA. The ability of the cells to concentrate GABA is dependent upon external calcium, while the uptake process is sodium dependent and temperature sensititive. GABA uptake by the cell lines is specific with respect to analogue inhibition, but this specificity is different from that observed in brain slices. It is, however, similar to that of glia in the rat sensory ganglia. Although clonal muscle cells can concentrate GABA to a limited extent, a number of other differentiated cells cannot.