However, the results obtained with PPDA (a NMDAR2C- and D-preferring antagonist), ifenprodil (a NMDAR2B antagonist) and NVP-AAM077 (a NMDAR2A antagonist; see Methods) did not reveal clear different pharmacological profile between synaptic and tonic currents (Fig. unaffected when inhibiting vesicular release of transmitters from MPEP HCl neurons but was increased upon inhibition of the enzyme converting glutamate in glutamine in glial cells. These observations indicate that ambient glutamate is mainly of glial origin. Finally, experiments with the use-dependent antagonist MK801 indicated that NMDARs mediating the tonic conductance are probably extra-synaptic NMDARs. Concentration of transmitters in the extracellular space of the central nervous system is determined by a balance between release, degradation and uptake mechanisms. During fast synaptic transmission, vesicular release of neurotransmitters such as glutamate and GABA leads to a rapid rise of neurotransmitter concentration which reaches the millimolar range within the synaptic cleft (Clements, 1996). Diffusion and efficient uptake by membrane-bound transporters ensure a rapid decay of the transmitter concentration in the cleft and a minimal spread of transmitter to neighbouring synapses (for review see Bergles 1999; Attwell & Gibb, 2005). It is thus generally assumed that between each episode of synaptic activation the concentration of transmitter within and outside the cleft is maintained at a very low level thereby preventing continuous activation or desensitization of receptors. However, microdialysis experiments suggest that the ambient concentration of amino acids such as glutamate, glycine and GABA reaches a low micromolar range, i.e. a concentration value sufficiently high to activate several types of glutamate and GABA receptors (Cavelier 2005). Accordingly, tonic currents mediated by the activation of GABAA receptors have been recorded in different cerebellar and cortical neurons (Semyanov 2004; Farrant & Nusser, 2005). This tonic conductance shows cell-type specific differences in magnitude and pharmacology, changes during postnatal development and is mediated by extrasynaptic receptors. but also GATA6 electrophysiological studies indicate that tonic GABAA receptor-mediated inhibition influences synaptic integration during sensory processing (Hamann 2002; Chadderton 2004). In comparison, the role of ambient glutamate has been much less studied. Yet, tonic activation of NMDA receptors (NMDARs) by ambient glutamate has been observed in pyramidal and granule cells of the MPEP HCl hippocampus (Sah 1989; Dalby & Mody, MPEP HCl 2003; Angulo 2004; Cavelier & Attwell, 2005). Moreover, blocking glutamate uptake in organotypic cultures (Jabaudon 1999) and in acute slices (Cavelier & Attwell, 2005) of the hippocampus unmask a similar excitatory tonic current which most likely results from a glial release of glutamate. Although the identification of the release mechanism awaits for the development of more specific pharmacological tools (Cavelier & Attwell, 2005), the presence of ambient glutamate raises several questions on its physiological or pathological roles. On the one hand, Sah and co-workers proposed that this tonic current modulates the input/output function of CA1 neurons (Sah 1989). On the other hand, if the receptors mediating this tonic excitation are extra-synaptic, as those mediating tonic inhibition (see above), they may play a crucial role in triggering cell death (Hardingham 2002). In the present study, we aimed at further characterizing the receptors responsible for the tonic excitatory current observed in CA1 pyramidal cells and the source of the ambient glutamate activating these receptors. Methods MPEP HCl Slice preparation All experiments followed European Union and institutional guidelines for the care and use of laboratory animals (Council directive 86/609EEC). Fourteen- to 29-day-old Wistar.
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