Release Of Gamma-aminobutyrate Research Articles

GABA Effects During Neuronal Differentiation of Stem Cells

Gamma-amino butyrate (GABA) is the most prevalent inhibitory neurotransmitter in the adult brain. In this review, we summarize the pharmacology and regulation of GABAergic transmission components (biosynthetic enzymes, receptors and transporters) in adult non-neurogenic brain regions. The effects of targeted mutations in genes relevant for GABAergic functions and how they influence specific neuronal circuits and pathological states are presented. We then review GABA actions on neuronal differentiation. During brain development, GABA has depolarizing activity in cerebrocortical neural precursors, controlling cell division and contributing to neuronal migration and maturation. In the adult forebrain there are two neurogenic regions exposed to synaptic and non-synaptic GABA release. Neural stem cells and neuronal progenitors express GABA receptors in subventricular and subgranular zones. GABA effects in these cells are very similar to those found in embryonic cortical precursor cells, and therefore it is possible that this amino acid has important roles during adult brain plasticity.

Photochemical cleavage and release of carboxylic acids from alpha-keto amides.

Carboxylate groups incorporated at the position alpha to the keto carbonyl of alpha-keto amides 1 were photochemically cleaved in aqueous media to give carboxylic acids in 70-90% yields with quantum yields of 0.3. The cleavage coproducts were diastereomeric hemiacetals 2. Prompt release of acetate and gamma-aminobutyrate (GABA) in buffer was observed by difference FT-IR spectroscopy upon 355 nm laser flash photolysis. The time-constant for release of GABA was <30 ms. [reaction--see text]

Depolarization-evoked GABA release from myenteric plexus is partially coupled to L-, N-, and P/Q-type calcium channels.

1. There are many evidences suggesting that gamma-aminobutyrate (GABA) is an important neurotransmitter and/or neuromodulator in the gut. 2. Using the myenteric plexus-longitudinal muscle preparation from the guinea pig ileum, we investigated the evoked release of [3H] GABA from enteric neurons by electrical pulses or high KCl, which occurs in a calcium-dependent and -independent way. In addition, using selective calcium channel blockers, we report the participation of distinct subtypes of calcium channels in the evoked release, showing a minor participation of L- and Q-type calcium channels, while N- and P-type have a participation of approximately 15%, each. However, regardless of the combination of Ca2+ channel blockers, we did not observe an inhibition greater than 50% of the calcium-dependent component of [3H] GABA release. 3. Thus, while the observed Ca2+-independent release mostly probable occur via reversal of the membrane GABA transporter, in our conditions, a considerable portion of the Ca2+-dependent evoked release of [3H] GABA is not coupled to L-, N-, or P/Q-type calcium channels, suggesting the involvement of intracellular calcium stores or other ways of getting calcium across the membrane.

Adenosine modulation of calcium currents and presynaptic inhibition of GABA release in suprachiasmatic and arcuate nucleus neurons.

Adenosine modulation of calcium channel currents and synaptic gamma-aminobutyrate (GABA) release was investigated with whole cell voltage-clamp recordings in rat suprachiasmatic nucleus (SCN) and arcuate nucleus cultures (n = 94). In SCN cultures, approximately 70% of the neurons showed a reversible inhibition of whole cell barium currents on the application of adenosine or its analogues. Adenosine at 1 microM reduced the amplitude of the barium currents by approximately 27%. In contrast to the significant reduction in the amplitude, the rising and decaying phases of the barium currents, and the inverted bell shape of the current-voltage curve of the barium currents, were not changed by adenosine. The adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 100 nM) and the adenosine A2 receptor agonist N6-[2-(3,5-dimethoxyphenyl)-ethyl]adenosine (DPMA; 100 nM) inhibited the barium currents by 21% and 16%, respectively, in SCN neurons, indicating both A1 and A2 receptor actions. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (100 nM) significantly reduced the effect of CPA but did not change the effect of DPMA on the barium currents. In the presence of tetrodotoxin to block action potentials, the frequency, but not the amplitude, of miniature inhibitory postsynaptic currents was significantly reduced (46%) by 1 microM adenosine, suggesting a presynaptic mechanism of adenosine action. In support of this suggestion, the postsynaptic GABA receptor responses were not influenced by 1 microM adenosine in the majority of SCN neurons. Most solitary self-innervating SCN neurons in microisland cultures were GABAergic. In these cells, the evoked autaptic GABA release (inhibitory postsynaptic current) was significantly inhibited by adenosine (37%), CPA (27%), and DPMA (28%), indicating that both A1 and A2 receptors were present in presynaptic axons. Similar to the effect in SCN neurons, adenosine inhibited both barium currents and GABA release in arcuate neurons. The reduction of whole cell barium currents by adenosine (1 microM), CPA (100 nM), and DPMA (100 nM) was 24, 17, and 19%, respectively. In solitary self-innervating arcuate neurons, adenosine inhibited the evoked GABA release (inhibitory postsynaptic current) by approximately 48%. We conclude that both adenosine A1 and A2 receptors are present in the SCN and arcuate nucleus of the hypothalamus. Adenosine inhibits calcium currents and presynaptically reduces inhibitory GABA neurotransmission.

Release of taurine and its effects on release of neurotransmitter amino acids in rat cerebral cortex.

Taurine has been postulated to function as a neurotransmitter or neuromodulator. The possibility of depolarization-evoked release of taurine from nerve terminals, and the effects of taurine on release of endogenous glutamate (Glu), aspartate (Asp), and gamma-aminobutyrate (GABA) were examined using a superfusion of crude synaptosomes prepared from rat cerebral cortex. Taurine contents in cerebral cortex and its synaptosomes were 31.7 and 25.2 nmol/mg protein, respectively. Although the basal rate of taurine release was 35.3 pmol/min/mg protein of synaptosomes (second highest releasing rate), the 2-min stimulation with KCl (30 mM) evoked only a 1.3-fold increase in release of taurine (47.3 pmol/min/mg). The increase was largely Ca(2+)-dependent. The addition of taurine to the perfusion medium significantly reduced the depolarization-evoked increases in Glu, Asp, and GABA release. The taurine-induced reduction in GABA release was attenuated by phaclofen, a GABAB antagonist, but not by bicuculline, a GABAA antagonist. However, these antagonists did not block the effects on Glu and Asp release. These data suggest that taurine may be only partly released from nerve terminals by depolarization in the cerebral cortex, but that taurine may act upon nerve terminals to regulate the release of excitatory and inhibitory amino acid transmitters.

Formation of releasable γ-aminobutyrate from putrescine by rat adrenal slices in vitro

Rat adrenal gland slices, when incubated in vitro with [1,4-14C]putrescine, accumulate the radioactive diamine and convert it, in part, to a compound indistinguishable (in four separative systems) from [14C]gamma-aminobutyrate (GABA). Adrenal glands taken from animals that had undergone adrenal enucleation 28 days previously, so that the cortex of the tissue had regenerated, likewise formed [14C]GABA from [1,4-14C]putrescine. Putrescine-derived GABA was released from adrenal slices in vitro by 48 mmol K+/l, the release being dependent upon the presence of Ca2+ in the incubation medium. ACTH(1-24) and 8-bromocyclic AMP both provoked a dose-related release of putrescine-derived GABA, although the dose-response curve for the latter differed somewhat from that for the release of corticosterone by this secretogogue. The enzyme believed to be responsible for the first step in the metabolic transformation of putrescine into GABA, diamine oxidase (DAO), is present in extracts of adrenal tissue and its catalytic activity underwent a transient increase followed by a fall below resting levels upon stimulation of adrenal slices with ACTH(1-24). The conclusion that this enzyme initiates the formation of GABA by this pathway is indicated by the observation that adrenal slices pretreated with the DAO inhibitor, aminoguanidine, released significantly less [1,4-14C]putrescine-derived GABA in response to 48 mmol K+/l than did control tissues. The functional significance of these findings remains to be established.

Regulation of Glutamate and Aspartate Release from Slices of the Hippocampal CA1 Area: Effects of Adenosine and Baclofen

Glutamate and/or aspartate is the probable transmitter released from synaptic terminals of the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers in area CA1 of the rat hippocampal formation. Slices of the CA1 area were employed to test the effects of adenosine- and gamma-aminobutyrate (GABA)-related compounds on the release of glutamate and aspartate from this projection. Under the conditions of these experiments, the release of glutamate and aspartate evoked by 50 mM K+ was more than 90% Ca2+-dependent and originated predominantly from the CA3-derived pathways. Adenosine reduced the K+-evoked release of glutamate and aspartate by a maximum of about 60%, but did not affect the release of GABA. This action was reversed by 1 microM 8-phenyltheophylline. The order of potency for adenosine analogues was as follows: L-N6-phenylisopropyladenosine greater than N6-cyclohexyladenosine greater than D-N6-phenylisopropyladenosine approximately equal to 2-chloroadenosine greater than adenosine much greater than 5'-N-ethylcarboxamidoadenosine. 8-Phenyltheophylline (10 microM) by itself enhanced glutamate/aspartate release, whereas dipyridamole alone depressed release. These results support the view that adenosine inhibits transmission at Schaffer collateral-commissural-ipsilateral associational synapses mainly by reducing transmitter release and that these effects involve the activation of an A1 receptor. Neither adenosine, L-N6-phenylisopropyladenosine, nor 8-phenyltheophylline affected the release of glutamate or aspartate evoked by 10 microM veratridine. The differing effects of adenosine compounds on release evoked by K+ and veratridine suggest that A1 receptor activation either inhibits Ca2+ influx through the voltage-sensitive channels or interferes with a step subsequent to Ca2+ entry that is coupled to the voltage-sensitive Ca2+ channels in an obligatory fashion. Neither baclofen nor any other agent active at GABAB or GABAA receptors affected glutamate or aspartate release evoked by elevated K+ or veratridine. Therefore, either baclofen does not inhibit transmission at these synapses by depressing transmitter release or else it does so in a way that cannot be detected when a chemical depolarizing agent is employed.

Glutamine enhances glutamate release in preference to gamma-aminobutyrate release in hippocampal slices.

To see the effect of physiological concentrations of glutamine on glutamate and gamma-aminobutyrate (GABA) release, rat hippocampal slices were incubated and (or) superfused without or with 0.25 mM glutamine in the presence or absence of Ca2+. The spontaneous and high K+-evoked release of glutamine, glutamate, and GABA was measured by precolumn derivatization and reversed phase high performance liquid chromatography. The spontaneous release of glutamate was increased by superfusion with glutamine and this increase was three times greater in the absence than in the presence of Ca2+. Spontaneous GABA release was not increased by glutamine. While in the absence of glutamine, the release of glutamate and GABA evoked by 50 mM K+ was about equal, in the presence of glutamine the evoked release of glutamate was nearly three times greater than that of GABA. The large evoked release of glutamate in the presence of glutamine was Ca2+ dependent nearly to the same extent as the smaller evoked release in the absence of glutamine. Results suggest that the availability of extracellular glutamine regulates the release of glutamate but not of GABA. Extracellular Ca2+ controls the spontaneous conversion of glutamine to glutamate but the site and mechanism of this control is uncertain.