Push-pull cannula experiment

Fig. 4. Schematic diagram of the push-pull cannula experiment.

Also in the intact brain push pull cannula and microdialysis experiments have demonstrated K+-evoked Ca +-dependent Asp release from the striatum (Girault et al., 1986 Paulsen and Fonnum, 1989 but see Zuiderwijk et al., 1996). Recently Lada et al. (1998) demonstrated Ca -dependent release of Asp from the striatum after electrical stimulation of the prefrontal cortex. Experiments interfering with the exocytotic machinery have also been performed in the intact striatum. Analysis of microdialysates (Herrera-Marschitz et al., 1996) has shown that extracellular Asp levels were decreased during K+-induced depolarization after treatment with alpha-latrotoxin, which triggers sustained exocytosis (Henkel and Sankaranarayanan, 1999). This is presumably because of depletion of the vesicular content of Asp before the stimulated release. However, care should be taken in interpreting this result, because alpha-latrotoxin may release both vesicular and cytoplasmic pools of amino acids (McMahon et al., 1990). Nonetheless, most experiments in the intact brain, using both direct chemical depolarization of the tissue and stimulation of pathways, show Asp release consistent with exocytosis. 

Absolute qualitative identification can be assured only if samples are removed and analyzed. Two examples of such a procedure have been reported. The first was an attempt to determine if direct electrical stimulation of the caudate nucleus resulted in the release of dopamine as well as ascorbic acid from that tissue. Micro voltammetric and stimulating electrodes were micromanipulated into excised caudate tissue which was flushed with warmed, oxygenated buffer. Reference and auxiliary electrodes were nearby. Quantitative information was taken, stored, manipulated, and displayed by a minicomputer. Simultaneously a push-pull cannula device sampled the caudate and delivered the perfusate to an iced vial. Changes in the electrochemical signal that followed stimulation were correlated with changes in the dopamine and ascorbic acid content of the perfusate as determined via HPLC with electrochemical detection. It was found that little if any ascorbic acid was released as a result of electrical stimulation in these experiments. Although there is some question concerning the stability of ascorbate in an iced vial, the above example does illustrate this coincident analytical technique. 

Neuroscientists have been interested for many years in being able to make measurements of the concentration of neurotransmitters in extracellular fluid. Two non-electrochemical methods have seen widespread use for these measurements. One is the use of the push-pull cannula. In this experiment, two concentric tubes are placed in the brain region of interest. Fluid is pumped into the brain through the inner tube and removed by the outer tube (Figure 4). This sampling technique, coupled with the use... 

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