Biomedical subjects

Arnaud Ruiz

Publications and source records attributed to Arnaud Ruiz.

6 recordsLinked to original sources

PubMed2005

Distinct subunits in heteromeric kainate receptors mediate ionotropic and metabotropic function at hippocampal mossy fiber synapses.

Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2.

Animals↗

PubMed2005

Presynaptic, extrasynaptic and axonal GABAA receptors in the CNS: where and why?

Although GABA(A) receptors are widely distributed at inhibitory synapses on dendrites and cell bodies of neurons, they also occur in other places, in particular at synapses made on axons and in extrasynaptic membranes. This review summarises some of the evidence that presynaptic receptors modulate transmission not only at primary afferents in the spinal cord, but also at a variety of sites in the brain, including hippocampal mossy fibres. These receptors modulate transmitter release via several different mechanisms. Another form of unconventional GABA(A) receptor-mediated signalling is the mediation of a tonic conductance, seen in granule cells of the cerebellum and dentate gyrus and also in hippocampal interneurons. Tonic signalling appears to be mediated by extrasynaptic receptors. The adaptive significance of this form of signalling remains poorly understood.

Adaptation, Physiological↗

PubMed2003

Endogenous zinc inhibits GABA(A) receptors in a hippocampal pathway.

Depending on their subunit composition, GABA(A) receptors can be highly sensitive to Zn(2+). Although a pathological role for Zn(2+)-mediated inhibition of GABA(A) receptors has been postulated, no direct evidence exists that endogenous Zn(2+) can modulate GABAergic signaling in the brain. A possible explanation is that Zn(2+) is mainly localized to a subset of glutamatergic synapses. Hippocampal mossy fibers are unusual in that they are glutamatergic but have also been reported to contain GABA and Zn(2+). Here, we show, using combined Timm's method and post-embedding immunogold, that the same mossy fiber varicosities can contain both GABA and Zn(2+). Chelating Zn(2+) with either calcium-saturated EDTA or N,N,N',N'-tetrakis (2-pyridylmethyl)ethylenediamine had no effect on stratum-radiatum-evoked inhibitory postsynaptic currents (IPSCs), but enhanced IPSCs evoked by stimuli designed to recruit dentate granule cells. We also show that IPSCs recorded in CA3 pyramidal neurons in acute hippocampal slices are depressed by exogenous Zn(2+). This depression was of similar amplitude whether the IPSCs were evoked by stimulation in s. radiatum (to recruit local interneurons) or in the s. granulosum of the dentate gyrus (to recruit mossy fibers). These results show for the first time that GABAergic IPSCs can be modulated by endogenous Zn(2+) and are consistent with GABA release at Zn(2+)-containing mossy fiber synapses.

Animals↗

PubMed2003

GABAA receptors at hippocampal mossy fibers.

Presynaptic GABAA receptors modulate synaptic transmission in several areas of the CNS but are not known to have this action in the cerebral cortex. We report that GABAA receptor activation reduces hippocampal mossy fibers excitability but has the opposite effect when intracellular Cl- is experimentally elevated. Synaptically released GABA mimics the effect of exogenous agonists. GABAA receptors modulating axonal excitability are tonically active in the absence of evoked GABA release or exogenous agonist application. Presynaptic action potential-dependent Ca2+ transients in individual mossy fiber varicosities exhibit a biphasic dependence on membrane potential and are altered by GABAA receptors. Antibodies against the alpha2 subunit of GABAA receptors stain mossy fibers. Axonal GABAA receptors thus play a potentially important role in tonic and activity-dependent heterosynaptic modulation of information flow to the hippocampus.

Animals↗

PubMed2003

GABA and GABAA receptors at hippocampal mossy fibre synapses.

Anatomical and electrophysiological evidence has raised the possibility that corelease of GABA and glutamate occurs at hippocampal mossy fibre synapses which, however, lack the vesicular GABA transporter VGAT. Here, we apply immunogold cytochemistry to show that GABA, like glutamate, has a close spatial relation to synaptic vesicles in rat mossy fibre terminals, implying that a mechanism exists to package GABA in synaptic vesicles. We also show that GABAA and AMPA receptors are colocalized at mossy fibre synapses. The expression of GABA and GABAA receptors is, however, weaker than in inhibitory synapses. Electrical stimuli that recruit mossy fibres evoke monosynaptic GABAA receptor-mediated signals in post-synaptic targets that show marked frequency-dependent facilitation and sensitivity to group II metabotropic receptors, two features that are characteristic of mossy fibre transmission. These results provide further evidence for GABA and glutamate cotransmission at mossy fibre synapses, although paired pre- and post-synaptic recordings will be required to determine the role of GABA at this unusual synapse.

Animals↗

PubMed2002

Do mossy fibers release GABA?

PURPOSE: Mossy fibers are the sole excitatory projection from dentate gyrus granule cells to the hippocampus, forming part of the trisynaptic hippocampal circuit. They undergo significant plasticity during epileptogenesis and have been implicated in seizure generation. Mossy fibers are a highly unusual projection in the mammalian brain; in addition to glutamate, they release adenosine, dynorphin, zinc, and possibly other peptides. Mossy fiber terminals also show intense immunoreactivity for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and immunoreactivity for GAD67. The purpose of this review is to present physiologic evidence of GABA release by mossy fibers and its modulation by epileptic activity. METHODS: We used hippocampal slices from 3- to 5-week-old guinea pigs and made whole-cell voltage clamp recordings from CA3 pyramidal cells. We placed stimulating electrodes in stratum granulosum and adjusted their position in order to recruit mossy fiber to CA3 projections. RESULTS: We have shown that electrical stimuli that recruit dentate granule cells elicit monosynaptic GABAA receptor-mediated synaptic signals in CA3 pyramidal neurons. These inhibitory signals satisfy the criteria that distinguish mossy fiber-CA3 synapses: high sensitivity to metabotropic glutamate-receptor agonists, facilitation during repetitive stimulation, and N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation. CONCLUSIONS: We have thus provided compelling evidence that there is a mossy fiber GABAergic signal. The physiologic role of this mossy fiber GABAergic signal is uncertain, but may be of developmental importance. Other evidence suggests that this GABAergic signal is transiently upregulated after seizures. This could have an inhibitory or disinhibitory effect, and further work is needed to elucidate its actual role.

Animals↗