Chemical Gradients withinBrain Extracellular SpaceMeasured using Low Flow Push–Pull Perfusion Sampling in Vivo

摘要

Although populations of neurons are known to vary on the micrometer scale, little is known about whether basal concentrations of neurotransmitters also vary on this scale. We used low-flow push–pull perfusion to test if such chemical gradients exist between several small brain nuclei. A miniaturized polyimide-encased push–pull probe was developed and used to measure basal neurotransmitter spatial gradients within brain of live animals with 0.004 mm³ resolution. We simultaneously measured dopamine (DA), norepinephrine, serotonin (5-HT), glutamate, γ-aminobutyric acid (GABA), aspartate (Asp), glycine (Gly), acetylcholine (ACh), and several neurotransmitter metabolites. Significant differences in basal concentrations between midbrain regions as little as 200 μm apart were observed. For example, dopamine in the ventral tegmental area (VTA) was 4.8 ± 1.5 nM but in the red nucleus was 0.5 ± 0.2 nM. Regions of high glutamate concentration and variability were found within the VTA of some individuals, suggesting hot spots of glutamatergic activity. Measurements were also made withinthe nucleus accumbens core and shell. Differences were not observedin dopamine and 5-HT in the core and shell; but their metaboliteshomovanillic acid (460 ± 60 nM and 130 ± 60 nM respectively)and 5-hydroxyindoleacetic acid (720 ± 200 nM and 220 ± 50nM respectively) did differ significantly, suggesting differencesin dopamine and 5-HT activity in these brain regions. Maintenanceof these gradients depends upon a variety of mechanisms. Such gradientslikely underlie highly localized effects of drugs and control of behaviorthat have been found using other techniques.