Effect of nitric oxide donors on GABA uptake by rat brain synaptosomes

P2Y1 receptor inhibits GABA transport through a calcium signalling-dependent mechanism in rat cortical astrocytes

Astrocytes express a variety of purinergic (P2) receptors, involved in astrocytic communication through fast increases in [Ca(2+) ]i . Of these, the metabotropic ATP receptors (P2Y) regulate cytoplasmic Ca(2+) levels through the PLC-PKC pathway. GABA transporters are a substrate for a number of Ca(2+) -related kinases, raising the possibility that calcium signalling in astrocytes impact the control of extracellular levels of the major inhibitory transmitter in the brain. To access this possibility we tested the influence of P2Y receptors upon GABA transport into astrocytes. Mature primary cortical astroglial-enriched cultures expressed functional P2Y receptors, as evaluated through Ca(2+) imaging, being P2Y1 the predominant P2Y receptor subtype. ATP (100 μM, for 1 min) caused an inhibition of GABA transport through either GAT-1 or GAT-3 transporters, decreasing the Vmax kinetic constant. ATP-induced inhibition of GATs activity was still evident in the presence of adenosine deaminase, precluding an adenosine-mediated effect. This, was mimicked by a specific agonist for the P2Y1,12,13 receptor (2-MeSADP). The effect of 2-MeSADP on GABA transport was blocked by the P2 (PPADS) and P2Y1 selective (MRS2179) receptor antagonists, as well as by the PLC inhibitor (U73122). 2-MeSADP failed to inhibit GABA transport in astrocytes where intracellular calcium had been chelated (BAPTA-AM) or where calcium stores were depleted (α-cyclopiazonic acid, CPA). In conclusion, P2Y1 receptors in astrocytes inhibit GABA transport through a mechanism dependent of P2Y1 -mediated calcium signalling, suggesting that astrocytic calcium signalling, which occurs as a consequence of neuronal firing, may operate a negative feedback loop to enhance extracellular levels of GABA.

SNAP-25/syntaxin 1A complex functionally modulates neurotransmitter gamma-aminobutyric acid reuptake

Neurotransmitter gamma-aminobutyric acid (GABA) release to the synaptic clefts is mediated by the formation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which includes two target SNAREs syntaxin 1A and SNAP-25 and one vesicle SNARE VAMP-2. The target SNAREs syntaxin 1A and SNAP-25 form a heterodimer, the putative intermediate of the SNARE complex. Neurotransmitter GABA clearance from synaptic clefts is carried out by the reuptake function of its transporters to terminate the postsynaptic signaling. Syntaxin 1A directly binds to the neuronal GABA transporter GAT-1 and inhibits its reuptake function. However, whether other SNARE proteins or SNARE complex regulates GABA reuptake remains unknown. Here we demonstrate that SNAP-25 efficiently inhibits GAT-1 reuptake function in the presence of syntaxin 1A. This inhibition depends on SNAP-25/syntaxin 1A complex formation. The H3 domain of syntaxin 1A is identified as the binding sites for both SNAP-25 and GAT-1. SNAP-25 binding to syntaxin 1A greatly potentiates the physical interaction of syntaxin 1A with GAT-1 and significantly enhances the syntaxin 1A-mediated inhibition of GAT-1 reuptake function. Furthermore, nitric oxide, which promotes SNAP-25 binding to syntaxin 1A to form the SNARE complex, also potentiates the interaction of syntaxin 1A with GAT-1 and suppresses GABA reuptake by GAT-1. Thus our findings delineate a further molecular mechanism for the regulation of GABA reuptake by a target SNARE complex and suggest a direct coordination between GABA release and reuptake.

Endogenous nitric oxide modulates GABAergic transmission to granule cells in adult rat cerebellum

Nitric oxide (NO) is a gaseous neurotransmitter which plays an important role in neuronal signalling and plasticity throughout the brain. In the cerebellum, NO synthase (NOS) is expressed in parallel fibres and within the internal granule cell layer (IGL). During development there are changes in NOS concentration, distribution and activity within the IGL, suggesting NO may play a role in IGL function. Therefore, the actions of NO in the IGL were investigated. The similar actions of a range of NOS inhibitors and NO scavengers strongly suggested the presence of a tonic level of endogenous NO in the IGL. Both the neuronal and inducible forms of NOS appeared to be sources of this endogenous NO. The effects observed following a reduction in the concentration of endogenous NO were consistent with enhanced granule cell GABAA receptor activation. For example, a reduction in NO concentration led to an increase in the frequency of action potential-dependent phasic GABAergic inhibitory postsynaptic currents (IPSCs) and produced a TTX-insensitive GABAA receptor-mediated current. A direct action of NO on Golgi cell membrane potential and input resistance accounted for the increase in the frequency of phasic GABA release. The mechanism underlying the tonic GABA current is unclear but does not appear to be via the modulation of GABA uptake or the activation of nicotinic acetylcholine receptors. NO is a potentially novel mechanism for tuning GABAergic signalling to granule cells and therefore modulating the throughput of an important cerebellar circuit.

Pretreatment of rat brain synaptosomes with GABA increases subsequent GABA uptake via GABA(B) receptor activation

Pretreatment with 100 microM GABA of synaptosomes purified from rat brain results in an increased uptake of the labelled neurotransmitter in subsequent incubations. The effect is blocked by a GABA(B) receptor antagonist, 2-hydroxy-saclofen. The effect is mimicked by baclofen and the baclofen effect is blocked by saclofen too. Lower GABA concentrations (up to 50 microM) do not result in an increase of subsequent GABA uptake. Treatment of synaptosomes with 8-Br-cAMP results in a decreased GABA uptake. Since the uptake incubations were run with saturating concentrations of labelled GABA, the data indicates that GABA(B) receptor activation in brain synaptosomes up-regulates their GABA uptake capacity by an increase in Vmax. This mechanism appears of physiological relevance under conditions of sustained GABA release and substantial increase of its extracellular concentration.

Subfractionation of ficoll gradient purified synaptosomes: GABA uptake by the subfractions

The classical synaptosomes preparation purified on discontinuous Ficoll gradient has been sufractionated into three further subfractions by introducing more Ficoll density layers. Among the three subfractions, the lightest one was the one which was more difficult to obtain in reproducible amounts and was only partially characterized in terms of labelled GABA uptake. The heaviest one most probably is largely made up of partially damaged nerve endings. The central one was the most reproducible in terms of yield and labelled GABA uptake and actually was the one we studied more thoroughly in terms of morphology, labelled GABA uptake and its pharmacology. A comparison has been made with the classical "total" purified synaptosomes fraction. An interesting result of these experiments is a paradoxical effect of the glial uptake inhibitor beta-alanine. This substance appears to favor, in both "total" and "central" fraction synaptosomes, a redistribution of taken up GABA from contaminating glia to actual nerve endings.

GABA uptake by rabbit restiform body homogenates

Restiform body (inferior cerebellar peduncle) preparations were obtained from rabbit brain stem slices and homogenized. When challenged with labelled GABA, these homogenates took it up briskly. We have characterized pharmacologically this uptake which resulted almost equally neuronal and glial. The neuronal component of the GABA uptake might be due to the adjacent cochlear nuclei coming along in the preparations, whereas the glial component probably belongs to the restiform body proper. Another possibility is that actually both components are due to the myelinated fibers and glia which make up the restiform body.

Presenilins and Alzheimer's disease

Mutations in the genes encoding the presenilins cause the majority of early-onset cases of Alzheimer's disease (AD). The identification of the presenilin genes has provided new opportunities for elucidating the molecular mechanisms underlying the etiology and pathogenesis of AD. Recent progress has been made in attempts to understand the normal and pathological functions of the presenilins, emphasizing the effects of presenilin familial AD mutations on the amyloid beta-protein precursor, the presenilins themselves, and apoptotic cell death.