Effect of dopamine content in rat brain striatum on anesthetic requirement: an in vivo microdialysis study

Do dopamine receptors mediate part of MAC?

BACKGROUND

Depletion of central nervous system catecholamines, including dopamine, can decrease MAC (the minimum alveolar concentration of an inhaled anesthetic required to suppress movement in response to a noxious stimulus in 50% of test subjects); release of central nervous system catecholamines, including dopamine, can increase MAC; and increased free dopamine concentrations in the striatum can decrease MAC. Such findings suggest that dopamine receptors might mediate part of the capacity of inhaled anesthetics to provide immobility in the face of noxious stimulation.

METHODS

We measured the effect of blockade of D2 dopamine-mediated transmission with 0.3 mg/kg or 3.0 mg/kg droperidol on the MAC of cyclopropane, desflurane, halothane, isoflurane, or sevoflurane in rats, and the effect of 3.0 mg/kg droperidol on the dose or concentration of etomidate (an anesthetic known to act principally by enhancing the response of gamma-aminobutyric acid(A) receptors to gamma-aminobutyric acid) required to suppress movement in response to noxious stimulation.

RESULTS

Blockade of D2 dopamine-mediated transmission with droperidol does not decrease the MAC of cyclopropane, desflurane, halothane, isoflurane, or sevoflurane or its equivalent for etomidate in rats.

CONCLUSIONS

These data, plus data from studies by others about D1 dopamine receptors, indicate that dopamine receptors do not mediate the immobility produced by inhaled anesthetics.

Pentobarbital inhibits l-DOPA-induced dopamine increases in the rat striatum: An in vivo microdialysis study

Pentobarbital is reported to inhibit ketamine-induced dopamine (DA) release in the rat nucleus accumbens. The accumbens is a part of the limbic dopaminergic system in the brain, and the dopaminergic neural activity of other components may also be sensitive to pentobarbital. We investigated the effect of pentobarbital administration on DA release in the striatum known as DA-rich basal ganglia, and the interaction between pentobarbital and L-DOPA, using in vivo microdialysis techniques. Male SD rats were implanted microdialysis probe into the right striatum. The probe was perfused with modified Ringer's solution and dialysate was directly injected to an HPLC. Every group of rats was consisted of six to seven animals. In the first experiment, rats were given saline, 25 and 50 mg kg(-1) pentobarbital. The second, each rat was given a local administration of 2 and 5 microg ml(-1) of L-DOPA with perfusate. Finally, other sets of rats were given 5 microg ml(-1) of L-DOPA and 25, 50, or 100 mg kg(-1) pentobarbital. Pentobarbital anaesthesia decreased the extracellular concentration of DA, and local administration of L-DOPA significantly increased DA concentration. Pretreatment with pentobarbital diminished the L-DOPA-induced DA increase. The results of the present investigation demonstrate that administration of pentobarbital might inhibit dopaminergic neural activity not only in the nucleus accumbens but also in the rat striatum. Pentobarbital anaesthesia antagonizes DA increase induced by L-DOPA and suggests the inhibition of metabolism of L-DOPA. The results of some animal experiments on dopaminergic activity under pentobarbital anaesthesia should be reconsidered.

Isoflurane induces dopamine transporter trafficking into the cell cytoplasm

Previous investigations have shown that the binding of a selective hydrophilic positron emission tomography radiotracer for the dopamine transporter (DAT) (2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-18F-fluoroethyl)nortropane) decreased in monkey striatum during deep isoflurane anesthesia. Immunohistochemistry experiments suggested but did not prove that isoflurane induced a decrease in cell surface DAT. The present investigation was undertaken to demonstrate quantitatively the isoflurane-induced internalization of DAT using a rapid and relatively uncomplicated biochemical technique in human embryonic kidney (HEK-293) cells stably expressing the human DAT (h-DAT) protein. Biotinylation followed by Western blot analysis was used to determine the extent of change in cell surface expression of the DAT under control conditions and in the presence of a clinically relevant concentration of isoflurane. Isoflurane treatment for 30 min resulted in a highly significant decrease in the amount of h-DAT on the cell surface (21 +/- 15% of control; P < 0.01) (mean +/- SD; n = 4). These data are consistent with the hypothesis that isoflurane results in internalization of DAT from the cell membrane and further validate our qualitative results reported previously. In addition, the current results confirm the hypothesis that biotinylation can be used to quantitate the extent of disappearance of DAT from the cell surface making dose-response studies and comparisons of DAT internalization with other general anesthetics practical.

Halothane decreases impulse-dependent but not cytoplasmic release of dopamine from rat striatal slices

Using in vitro superfusion techniques and electrical field stimulation, a volatile anesthetic, halothane, decreased impulse-dependent vesicular release, but did not affect amphetamine-induced cytoplasmic release of dopamine (DA) from the rat striatal slice preparations loaded with [3H]-DA. Contrary to previous in vivo studies, halothane at concentrations applied (1% to 4%) did not enhance the release of DA from slice preparation in which the cell bodies were absent, and therefore, the possible site of action was located on the axon terminals. In this in vitro experiment, halothane decreased the fractional release of DA in a concentration-dependent manner and attenuated the increase of impulse-dependent DA release when amphetamine or nomifensine administration was combined with electrical stimulation. D2-receptor agonists (quinpirole and apomorphine) reduced the release, and antagonists (sulpiride and haloperidol) enhanced the release of DA. In the presence of halothane, D2-receptor antagonists had no effect on DA release. It is concluded that halothane may have some modulatory effect on D2-receptor mediated presynaptic control of DA release.