Extracellular Glutamate in the Nucleus Accumbens Is Nanomolar in Both Synaptic and Non-synaptic Compartments

Abstract

Summary In the CNS, glutamate is both phasically and tonically released into the extracellular space, and must be removed by excitatory amino acid transporters (EAATs) to prevent excitotoxic accumulation. There remains uncertainty, however, regarding the functional steady-state concentration, with estimates ranging from tens of nanomolar to tens of micromolar. Efforts to reconcile these disparate values have led to a hypothesis that the extracellular space is comprised of distinct compartments in which basal glutamate concentrations are maintained independently. We used electrophysiology and two-photon Ca2+ imaging to test this hypothesis in the nucleus accumbens (NAc), where it has been proposed that micromolar extracellular glutamate is necessary for normal function. We found that the average concentration of synaptic glutamate is nanomolar, in agreement with previous electrophysiological estimates. Furthermore, this held true when glutamate uptake was inhibited, indicating that extracellular glutamate is not compartmentalized by EAATs.

Neuronal glutamate transporters control dopaminergic signaling and compulsive behaviors

“…4L ). Second , we calculated the mGluRI open probability at the experimentally-measured extracellular glutamate concentration in the striatum (∼25 nM; (Chiu and Jahr, 2017)) and the estimated concentration of glutamate transporters (∼140 μM; (Lehre and Danbolt, 1998)). Third , we used steady-state equations to determine the effect of a progressive reduction in the glutamate transporter concentration on the extracellular glutamate concentration and the mGluRI open probability.…”

“…(L) We used a kinetic model of mGluRI (Marcaggi et al, 2009) to determine the open probability of these receptors at a range of extracellular glutamate concentrations (pink). The striatum maintains the extracellular glutamate concentration at ∼25 nM (Chiu and Jahr, 2017) in the presence of ∼140 μM glutamate transporters (Lehre and Danbolt, 1998). Reducing the concentration of glutamate transporters leads to increased extracellular glutamate concentrations (blue).…”

“…Briefly, under steady-state conditions, the relationship between glutamate transporter and extracellular glutamate concentration can be described by a modified version of the Michaelis-Menten equation (Sun et al, 2014), as follows: In this equation, K M =27 μM (Sun et al, 2014). The proportionality constant A can be calculated by setting [Glutamate]=25 nM (the experimentally measured ambient glutamate concentration in the striatum (Chiu and Jahr, 2017)) and [Transporter]=140 μM (the estimated concentration of glutamate transporters in the brain (Lehre and Danbolt, 1998)). The data shown in Fig.…”

A novel method using ambient glutamate for the electrophysiological quantification of extrasynaptic NMDA receptor function in acute brain slices

“…The presence of a tonic NMDA receptor-mediated current and a preferential MK-801 block of agonist responses over synaptic responses supports the hypothesis that ambient glutamate is present and largely excluded from the synapse (Lozovaya et al 2004;Le Meur et al 2007;Wu et al 2012) and is a prerequisite for our method. Since the compartmentalization of synapses by astrocytes is brain region dependent, this approach is not applicable in other brain regions without compartmentalization such as the nucleus accumbens (Chiu & Jahr, 2017). When applying our methodology to another cell type or brain region, functional assessment of the protective cap phenomenon according to Lozovaya et al (2004) might be performed to verify its presence as well as the presence of ambient glutamate.…”

Single synapse indicators of impaired glutamate clearance derived from fast iGluu imaging of cortical afferents in the striatum of normal and Huntington (Q175) mice

“…NMDARs are sensitive indicators of [Glu] and therefore well suited to detect a potentially existing Glu clearance deficits in the environment of active synapses, provided that the analysed responses are derived from one or few synapses only (Chiu and Jahr, 2017). CEF is known to stimulate the transcription of SLC1A2, i.e.…”