Tonic activation of NMDA receptors by ambient glutamate of non‐synaptic origin in the rat hippocampus

Abstract

In several neuronal types of the CNS, glutamate and GABA receptors mediate a persistent current which reflects the presence of a low concentration of transmitters in the extracellular space. Here, we further characterize the tonic current mediated by ambient glutamate in rat hippocampal slices. A tonic current of small amplitude (53.99 ± 6.48 pA at +40 mV) with the voltage dependency and the pharmacology of NMDA receptors (NMDARs) was detected in virtually all pyramidal cells of the CA1 and subiculum areas. Manipulations aiming at increasing D-serine or glycine extracellular concentrations failed to modify this current indicating that the glycine binding sites of the NMDARs mediating the tonic current were saturated. In contrast, non-transportable inhibitors of glutamate transporters increased the amplitude of this tonic current, indicating that the extracellular concentration of glutamate primarily regulates its magnitude. Neither AMPA/kainate receptors nor metabotropic glutamate receptors contributed significantly to this tonic excitation of pyramidal neurons. In the presence of glutamate transporter inhibitors, however, a significant proportion of the tonic conductance was mediated by AMPA receptors. The tonic current was unaffected when inhibiting vesicular release of transmitters from 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.

Organization, control and function of extrasynaptic NMDA receptors

“…In line with this idea, it was observed that exogenous applications of co-agonist do not enhance the amplitude of NMDAR-mediated tonic current [58]. If true, it means there is little room for regulatory mechanisms and modulation of NMDAR activity through the co-agonist-binding site at extrasynaptic locations, but this has never been addressed for SICs or any other manifestation of extrasynaptic NMDAR activity.…”

“…Similar to what is known for GABAR-mediated tonic currents [56], NMDAR-mediated tonic current seems to be allowed by sufficient concentrations of ambient agonist [42,55] and generated by receptors located outside of synapses [55,57,58]. Indeed, NMDAR-mediated tonic current persists when synaptic activity is suppressed with tetrodotoxin, and conversely synaptic NMDAR activity remains unaffected after NMDARs mediating tonic current are blocked with MK801 [58], which establishes those receptors as distinct, and somehow distant, from synaptic receptors. However, the origin of the so-called ambient glutamate that allows such tonic activation is still unclear, and little evidence for the source exists, aside from the observation that it is of non-synaptic origin [58,59].…”

“…Indeed, NMDAR-mediated tonic current persists when synaptic activity is suppressed with tetrodotoxin, and conversely synaptic NMDAR activity remains unaffected after NMDARs mediating tonic current are blocked with MK801 [58], which establishes those receptors as distinct, and somehow distant, from synaptic receptors. However, the origin of the so-called ambient glutamate that allows such tonic activation is still unclear, and little evidence for the source exists, aside from the observation that it is of non-synaptic origin [58,59]. Interestingly, the amplitude of NMDAR-mediated tonic current is strongly enhanced either by blocking glutamate reuptake through the glutamate transporter 1 (GLT1) [58,60], which is predominantly present on glial cells and mediates 95% of glutamate transport, or when slices are challenged with glial toxins [60].…”

“…When combined with low-frequency stimulation of afferent fibres, this method allows the selective blockade of synaptic NMDARs and leaves intact most of their extrasynaptic counterparts [19,26,27,118,119]. Conversely, this approach also allows the blockade of extrasynaptic NMDARs mediating tonic currents, while leaving intact unstimulated synaptic NMDARs [58]. Either way, this method presents the advantage that it only relies on whether receptors are active or not during MK801 application, and therefore circumvents caveats associated with the use of subunit-selective reagents or immature animals.…”

“…However, the origin of the so-called ambient glutamate that allows such tonic activation is still unclear, and little evidence for the source exists, aside from the observation that it is of non-synaptic origin [58,59]. Interestingly, the amplitude of NMDAR-mediated tonic current is strongly enhanced either by blocking glutamate reuptake through the glutamate transporter 1 (GLT1) [58,60], which is predominantly present on glial cells and mediates 95% of glutamate transport, or when slices are challenged with glial toxins [60]. Additionally, a recent study showed that the extent of NMDAR-mediated tonic current depends on glial coverage in the supraoptic nucleus of the hypothalamus [60].…”