On the Origin of Extracellular Glutamate Levels Monitored in the Basal Ganglia of the Rat by In Vivo Microdialysis

Abstract

Several putative neurotransmitters and metabolites were monitored simultaneously in the extracellular space of neostriatum, substantia nigra, and cortex and in subcutaneous tissue of the rat by in vivo microdialysis. Glutamate (Glu) and aspartate (Asp) were at submicromolar and y-aminobutyric acid (GABA) was at nanomolar concentrations in all brain regions. The highest concentration of dopamine (DA) was in the neostriatum. Dynorphin B (Dyn B) was in the picomolar range in all brain regions. Although no GABA, DA, or Dyn B could be detected in subcutaneous tissue, Glu and Asp levels werẽ 5 and coO.4 fiM, respectively. Lactate and pyruvate concentrations were cru200 and rmlO 1iM in all regions. The following criteria were applied to ascertain the neuronal origin of substances quantified by microdialysis: sensitivity to (a) Kdepolarization, (b) Na~channel blockade, (c) removal of extracellular Ca 2~,and (d) depletion of presynaptic vesicles by local administration of c~-latrotoxin. DA, Dyn B, and GABA largely satisfied all these criteria. In contrast, Glu and Asp levels were not greatly affected by Kdepolarization and were increased by perfusing with tetrodotoxin or with Ca2~-free medium, arguing against a neuronal origin. However, Glu and Asp, as well as DA and GABA, levels were decreased under both basal and K~-depolarizingconditions by ce-latrotoxin. Because the effect of Kdepolarization on Glu and Asp could be masked by reuptake into nerve terminals and glial cells, the reuptake blocker dihydrokainic acid (DHKA) or L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was included in the microdialysis perfusion medium. The effect of K~depolarization on Glu and Asp levels was increased by DHKA, but GABA levels were also affected. In contrast, PDC increased only Glu levels. It is concluded that there is a pool of releasable Glu and Asp in the rat brain. However, extracellular levels of amino acids monitored by in vivo microdialysis reflect the balance between neuronal release and reuptake into surrounding nerve terminals and glial elements.

Chemical Gradients within Brain Extracellular Space Measured using Low Flow Push–Pull Perfusion Sampling in Vivo

“…10,48 Another possible reason for the variable Glu in the VTA is tissue damage caused by the probe, for example, disruption of neurons could cause release of intracellular Glu. This explanation is discounted because we have previously observed that Glu concentration in perfusate is elevated immediately after push−pull probe insertion into brain tissue and that it decreases and to a stable level within 0.5 h. 22,23 Similar findings on microdialysis have been reported 49 and is attributed to the disruption of neurons and subsequent dissipation of the neurotransmitters. Thus, sampling in these experiments has occurred after Glu released by cell disruption has been removed.…”

Dopamine Receptor Activation Is Required for Corticostriatal Spike-Timing-Dependent Plasticity

“…In the striatum, dopamine is present in low nanomolar concentrations in vivo (Herrera-Marschitz et al, 1996) and in vitro (Kawagoe et al, 1992;Zhou et al, 2001;Partridge et al, 2002). These concentrations are thought to tonically activate only the dopamine D 2 receptors attributable to their enhanced dopamine affinity compared with dopamine D 1 receptors (Richfield et al, 1989).…”

RepeatedN-Acetylcysteine Administration Alters Plasticity-Dependent Effects of Cocaine

“…Aside from synaptic glutamate maintained by vesicular release, extrasynaptic glutamate is sustained primarily by nonvesicular release (Herrera-Marschitz et al, 1996;Timmerman and Westerink, 1997). In support, basal extrasynaptic glutamate sampled using microdialysis are mostly independent of vesicular glutamate (Westerink, 1995).…”