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Anesth Analg 2002;95:1274-1281
© 2002 International Anesthesia Research Society

ANESTHETIC PHARMACOLOGY

The Effects of General Anesthetics on Norepinephrine Release from Isolated Rat Cortical Nerve Terminals

Victor N. Pashkov, PhD, and Hugh C. Hemmings, Jr., MD PhD

Departments of Anesthesiology and Pharmacology, Weill Medical College of Cornell University, New York, New York

Address correspondence to Dr. H. C. Hemmings Jr., Box 50, LC-203A, Weill Medical College of Cornell University, 525 East 68th St., New York, NY 10021. Address e-mail to hchemmi{at}med.cornell.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Intravenous and volatile general anesthetics inhibit norepinephrine (NE) release from sympathetic neurons and other neurosecretory cells. However, the actions of general anesthetics on NE release from central nervous system (CNS) neurons are unclear. We investigated the effects of representative IV and volatile anesthetics on [3H]NE release from isolated rat cortical nerve terminals (synaptosomes). Purified synaptosomes prepared from rat cerebral cortex were preloaded with [3H]NE and superfused with buffer containing pargyline (a monoamine oxidase inhibitor) and ascorbic acid (an antioxidant). Basal (spontaneous) and stimulus-evoked [3H]NE release was evaluated in the superfusate in the absence or presence of various anesthetics. Depolarization with increased concentrations of KCl (15–20 mM) or 4-aminopyridine (0.5–1.0 mM) evoked concentration- and Ca2+-dependent increases in [3H]NE release from rat cortical synaptosomes. The IV anesthetics etomidate (5–40 µM), ketamine (5–30 µM), or pentobarbital (25–100 µM) did not affect basal or stimulus-evoked [3H]NE release. Propofol (5–40 µM) increased basal [3H]NE release and, at larger concentrations, reduced stimulus-evoked release. The volatile anesthetic halothane (0.15–0.70 mM) increased basal [3H]NE release, but did not affect stimulus-evoked release. These findings demonstrate drug-specific stimulation of basal NE release. Noradrenergic transmission may represent a presynaptic target for selected general anesthetics in the CNS. Given the contrasting effects of general anesthetics on the release of other CNS transmitters, the presynaptic actions of general anesthetics are both drug- and transmitter-specific.

IMPLICATIONS. General anesthetics affect synaptic transmission both by altering neurotransmitter release and by modulating postsynaptic responses to transmitter. Anesthetics exert both drug-specific and transmitter-specific effects on transmitter release: therapeutic concentrations of some anesthetics stimulate basal, but not evoked, norepinephrine release, in contrast to evoked glutamate release, which is inhibited.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
General anesthetics have potent effects on synaptic transmission, both on presynaptic terminals, by altering neurotransmitter release, and on postsynaptic neurons, by modulating their responses to neurotransmitters (1). The anesthetic effects on GABAergic and glutamatergic neurotransmission occur at clinically relevant concentrations in the central nervous system (CNS), consistent with substantial involvement of these systems in the mechanisms of anesthesia (1–4). In contrast, the contribution of central catecholaminergic transmission to the actions of general anesthetics is less fully explored.

The catecholamines (dopamine, norepinephrine, epinephrine) act as neurotransmitters or hormones in both the peripheral and CNS. Catecholamine secretion has been implicated as a target for general anesthetic effects on the peripheral sympathetic nervous system in several studies involving peripheral neurosecretory cells. Thus, both volatile and IV anesthetics inhibit catecholamine release from isolated bovine adrenal chromaffin cells (1,5,6) (see Discussion). In the CNS, noradrenergic neurons modulate many functions including learning and memory, the sleep-wake cycle, anxiety, nociception, and behavioral vigilance (7), all of which are potential targets for anesthetic action. However, the effects of general anesthetics on central norepinephrine (NE) release have not been defined clearly (8–13), and only a single study has directly addressed presynaptic anesthetic actions (8). The present study is an initial characterization of the effects of representative general anesthetics on basal and evoked [3H]NE release from isolated rat cortical nerve terminals. Our findings suggest that presynaptic anesthetic actions on central noradrenergic transmission may have a role in the central effects of some (e.g., propofol, halothane) but not other anesthetics.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
These studies were approved by Weill Medical College of Cornell University Institutional Animal Care and Use Committee.

L-[2,5,6-3H)]-NE (54.7 Ci/mmol) was obtained from NEN Life Science Products (Boston, MA); pargyline, L-ascorbic acid, dimethyl sulfoxide (DMSO), HEPES, 4-aminopyridine, veratridine, pentobarbital, and ketamine from Sigma Chemical Co. (St. Louis, MO); Percoll from Pharmacia AB (Uppsala, Sweden); bovine serum albumin (fraction 5) from J. T. Baker (Phillipsburg, NJ); and BAPTA-AM (1,2-bis[2-aminophenoxy]ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester) from RBI (Natick, MA). Halothane (thymol free) was obtained from Halocarbon Products (River Edge, NJ); propofol was purchased from Aldrich Chemicals (Milwaukee, WI) or was a gift from AstraZeneca Pharmaceuticals (Macclesfield, UK); and etomidate was a gift from Janssen Research Products (Flanders, NJ).

Synaptosomes provide an enriched source of sealed nerve endings, and contain the essential machinery for neurotransmitter release and uptake. This preparation allows the study of presynaptic mechanisms that regulate transmitter release while minimizing interference by intact neuronal circuits and glia, which complicate more intact preparations such as brain slices (14). Synaptosomes from rat cerebral cortex were prepared by the Percoll gradient method as described (15). Synaptosomes were washed from Percoll with HEPES-buffered medium (HBM) composed of (in mM): NaCl 140, KCl 5, NaHCO3 5, MgCl2 1, Na2HPO4 1.2, D-glucose 10 and HEPES 20, titrated to pH 7.4 with NaOH. Protein content of the synaptosomes was measured with the Bio-Rad Protein Assay Kit (Hercules, CA) using bovine serum albumin as a standard. Synaptosomes were stored as a pellet on ice and used within 5 h of preparation.

For each experiment, synaptosomes from a single preparation were resuspended to 1.5 mg protein/mL in oxygenated HBM containing 1.3 mM CaCl2, 0.1 mM pargyline, and 0.2 mM ascorbic acid, equilibrated at 37°C for 5 min, and labeled with [3H]NE (final concentration 100 nM) for 20 min at 37°C. After washing by centrifugation, synaptosomes were resuspended in HBM and loaded into 12 chambers (0.1 mg of protein each) of a Brandel SF-12 superfusion system (Brandel Scientific; Gaithersburg, MD) equipped with a circulating water bath set at 37°C. Synaptosomes were embedded in glass fiber filters (Whatman GF/F; Maidstone, UK), washed for 30 min, and superfused at 0.5 mL/min with HBM containing various additives. The system pump was located between the chamber outlets and the fraction collector to avoid pressurization of the chamber. Fractions (1.5 min, except as noted) of superfusate were collected into scintillation vials. A single synaptosome preparation was used for each experiment, and each chamber received a single treatment. Typically, each treatment was performed in triplicate, such that a single experiment contained two to three determinations of each treatment, and each experiment was performed at least three times. The data for baseline release were obtained by averaging release over three consecutive time points (fractions). Evoked release was stimulated with either increased KCl (15–20 mM) or 4-aminopyridine (4AP), which differ in their mechanisms of action. Increased KCl induces Ca2+-dependent transmitter release that is sensitive to Ca2+ channel blockade but resistant to Na+ channel blockade, whereas 4AP induces release that is sensitive to both Na+ and Ca2+ channel blockers by blocking K+ channels (15).

Radioactivity in samples and filters was determined by liquid scintillation spectrometry after the addition of 5 mL of Bio-Safe 2 scintillation cocktail (Research Products International, Mount Prospect, IL). Synaptosomes in filters were solubilized in 0.5 mL of 0.25% (v/v) Triton X-100 for 30 min before the addition of scintillation cocktail. [3H]NE release was expressed as fractional release: the radioactivity in each fraction divided by the total amount of radioactivity present in the synaptosomes at the time collection of that fraction began. Fractional release data were normalized to the baseline release at the beginning of each experiment to facilitate comparisons between experiments.

Synaptosomes (1.5 mg) for BAPTA experiments were labeled with [3H]NE, washed by centrifugation, resuspended in 3.2 mL of HBM without Ca2+, divided in 2 equal parts, and incubated with 0.25 mM BAPTA-AM/0.2% (v/v) DMSO or 0.2% (v/v) DMSO alone (control) at 37°C for 20 min to allow loading and hydrolysis. Treated synaptosomes were loaded into the chambers of the superfusion system for determination of [3H]NE release as above.

Stock solutions of pentobarbital and ketamine were prepared in water. Stock solutions of propofol and etomidate were prepared in DMSO, and appropriate dilutions were made into HBM immediately before use. DMSO used as a vehicle at a final concentration up to 0.2% (v/v) did not affect [3H]NE release (data not shown). Propofol concentrations in the superfusate were analyzed by using high-performance liquid chromatography to correct for systematic losses (16). Samples containing propofol were collected from chambers with a glass syringe during superfusion. Measured free propofol concentrations (referred to in text and figures) were approximately 50% of the nominal superfusate concentrations. The free aqueous anesthetic concentrations reported to produce general anesthesia are 2.2 µM for propofol (17), 10 µM for etomidate (18), 20 µM for ketamine (19), and 25 µM for pentobarbital (2). Intravenous anesthetic effects were screened at 1–4 times these concentrations; larger concentrations were also examined if effects were observed at these clinical concentrations.

Halothane was prepared as saturated solutions in HBM at room temperature (10–12 mM). Required volumes were diluted into superfusate reservoirs containing appropriate buffers, which were quickly sealed and placed in the system water bath at 37°C. Actual halothane concentrations in the chambers during superfusion were determined by gas chromatography (20) by withdrawal of 0.2-mL aliquots of the superfusate using a gas-tight Hamilton glass microsyringe, and extraction into 0.1 mL of n-heptane. An aliquot (5 µL) of heptane was injected into a gas chromatograph (GC-8A; Shimadzu Corp., Kyoto, Japan) equipped with a thermal conductivity detector. Separation was achieved on a 1.8 m/6 mm inner diameter glass column packed with Porapack Q (Supelco, Bellefonte, PA). Column temperature was 210°C, injector temperature was 230°C, and carrier gas (He) flow was 40 mL/min. Halothane concentrations measured at the chambers (referred to in text and figures) were 50%–70% of the nominal concentration in the superfusate reservoir; concentrations in the text and figures refer to the actual measured concentrations. The aqueous halothane concentration corresponding to 1 minimum alveolar anesthetic concentration (MAC) in rat is 0.35 mM (21). Halothane effects were screened up to 2 MAC, which was the largest concentration achievable with this system at 37°C.

The results were expressed as mean ± SD. Statistical significance was assessed by using analysis of variance with Newman-Keuls multiple comparison test or Student’s two-tailed t-test using GraphPad Prism, version 2.01 (GraphPad Software, Inc., San Diego, CA).


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Increased concentrations of extracellular KCl (final concentration 15–20 mM with compensatory reductions in NaCl) or 4AP (0.5–1.0 mM) stimulated concentration- and Ca2+-dependent [3H]NE release. The peak effect of evoked release for both secretogogues was 300%–600% of control (basal) release; the magnitude of the peak effect varied with synaptosome preparation. Both KCl- and 4AP-evoked release of [3H]NE was >90% Ca2+-dependent (Fig. 1).



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  		Figure 1. Ca2+ dependence of KCl-evoked and 4-aminopyridine (4AP)-evoked [3H]norepinephrine release. [3H]norepinephrine release (mean ± SD) observed for superfusion of synaptosomes in the presence of 1.3 mM Ca2+ ({blacksquare} = KCl, • = 4AP) and in the absence of Ca2+ but with 0.1 mM EGTA ({square} = KCl, {circ} = 4AP). Basal release in the presence of Ca2+ ({blacktriangleup}). Solid aclinic lines indicate times of KCl and 4AP exposure. Data from a single representative experiment are shown (n = 3; fraction time = 1 min).

 
Etomidate (5–40 µM) or ketamine (5–30 µM) did not significantly affect basal, KCl-evoked, or 4AP-evoked [3H]NE release (Fig. 2, A and B, and data not shown). Pentobarbital (25–100 µM) reduced both KCl-evoked (Fig. 2C) and 4AP-evoked release (data not shown) by 10%–20% without reaching statistical significance (P > 0.05), and did not affect basal release.



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  		Figure 2. Intravenous anesthetic effects on [3H]norepinephrine release. A, Effects of etomidate on basal and KCl-evoked [3H]norepinephrine release. B, Effects of ketamine on basal and 4-aminopyridine (4AP)-evoked [3H]norepinephrine release (fraction time = 1 min). C, Effects of pentobarbital on basal and KCl-evoked [3H]norepinephrine release. A representative time course observed in each series of experiments (n = 3) is shown. Each point represents the mean ± SD of three determinations within each experiment. Solid aclinic lines indicate times of drug exposure.

 
Propofol facilitated basal [3H]NE release in a concentration-dependent manner (Fig. 3). Small but significant effects were observed at >5 µM; saturation was not observed over the concentration range tested (Fig. 3, inset). Superfusion of synaptosomes in Ca2+-free HBM (plus 0.1 mM EGTA) reduced basal release of [3H]NE (Fig. 4, bar 1 versus 3). Propofol stimulated [3H]NE efflux both in the absence (1.96-fold) or presence (1.74-fold) of Ca2+. The inorganic Ca2+ channel antagonist LaCl3 [100 µM; (22)] or the Na+ channel antagonist tetrodotoxin [1 µM; (23)], drugs which inhibit KCl-evoked or 4AP-evoked release, respectively (data not shown), did not affect the ability of propofol to enhance basal [3H]NE release in the presence of Ca2+ (Fig. 4). This effect of propofol thus appeared independent of extracellular Ca2+ entry through Ca2+ channels. Preloading synaptosomes with the membrane permeable Ca2+ chelator BAPTA-AM did not significantly reduce stimulation of [3H]NE release by propofol (1.64-fold), suggesting that the effect is independent of changes in intrasynaptosomal [Ca2+] as well (Fig. 4).



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  		Figure 3. Effects of propofol on basal [3H]norepinephrine release. Data from a single representative concentration-effect experiment are shown. Curves show fractional [3H]norepinephrine release (mean ± SD) observed for superfusion of synaptosomes without propofol ({square}), or with 5 µM ({blacksquare}), 12.5 µM ({circ}), 25 µM (•), and 40 µM ({blacktriangleup}) propofol. Each point represents the mean ± SD of three determinations. A solid aclinic line indicates time of propofol exposure. The inset shows the concentration-effect relationship for facilitation of [3H]norepinephrine release by propofol determined at the plateau of the effect by taking the mean value of fractions 11–13 (n = 3–7). *P < 0.05 by analysis of variance.

 


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  		Figure 4. Propofol-evoked basal [3H]norepinephrine release is Ca2+-independent. Open bars, no propofol; filled bars, +25 µM propofol. The bars shown are the mean ± SD values derived from the mean of 3 consecutive fractions (11–13) at the plateau of the effect, each assayed 2–3 times per experiment (n = 3). Additions included 1.3 mM CaCl2, 100 µM LaCl3 (an inorganic Ca2+ channel blocker), 1 µM tetrodotoxin (TTX, a selective Na+ channel blocker), or preexposure to 0.25 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (BAPTA-AM) (a membrane permeant Ca2+ chelator). *P < 0.05 versus respective assay without propofol; #P < 0.05 versus +Ca2+ assay, by Student’s t-test.

 
Superfusion with increased KCl (Fig. 5A) or 4AP (Fig. 5B) in the presence of propofol (introduced 10–12 min before secretogogues) did not produce an additive effect on propofol-stimulated release. Peak 4AP-evoked [3H]NE release was reduced 28% by 25 µM propofol (548% ± 26% to 392% ± 12% control fractional release, *P < 0.05), whereas peak KCl-evoked release was unaffected (325% ± 30% to 305% ± 27% control fractional release; n = 3). The effects of propofol (up to 50 µM) on basal and secretogogue-evoked release were reversible. After a 15-min washing step, a second pulse of KCl or 4AP resulted in the same rates of basal and evoked [3H]NE release in propofol-treated and untreated synaptosomes, although the amplitude of the second response (Fig. 5; fractions 32–40) was reduced compared with the initial response.



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  		Figure 5. Effects of propofol on KCl-evoked (A) and 4-aminopyridine (4AP)-evoked (B) [3H]norepinephrine release. The data are from a single representative experiment (n = 3). Each point represents the mean ± SD of three determinations. Solid aclinic lines indicate times of drug exposure. A second stimulus (fractions 36–38) was applied after a 15-min washing step. *P < 0.05 by analysis of variance.

 
Halothane (0.15–0.70 mM) enhanced basal release of [3H]NE in the absence or presence of extracellular Ca2+ (Fig. 6). Halothane up to 0.70 mM (2 MAC) had no effect on KCl-evoked (Fig. 6) or 4AP-evoked release (n = 3; data not shown). Application of halothane did not affect subsequent KCl-evoked release after a 15-min washing step (data not shown).



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  		Figure 6. Effect of halothane on KCl-evoked [3H]norepinephrine release. The data are from a single representative experiment (n = 3). Each point represents the mean ± SD of three determinations. Solid aclinic lines indicate times of drug exposure. The inset demonstrates the Ca2+-independence of halothane-evoked (0.35 mM) [3H]norepinephrine release determined at the plateau of the effect by taking the mean value of fractions 9–11 (n = 3).

 

   Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Etomidate, ketamine, pentobarbital, propofol, and halothane did not significantly affect evoked NE release from isolated rat cerebrocortical nerve terminals at clinically relevant concentrations. Large concentrations of propofol selectively inhibited 4AP-evoked, but not KCl-evoked, release. Halothane and propofol enhanced basal (spontaneous) efflux of [3H]NE in a Ca2+-independent manner. These actions differ from the marked inhibitory effects of propofol (15) and of volatile anesthetics (24,25) on evoked release of the excitatory amino acid transmitter glutamate from isolated rat cerebrocortical nerve terminals. This is consistent with the distinct pharmacologic profiles for specific anesthetics (3), and with differences in the neurochemical mechanisms underlying the release of various transmitters (26).

A role for noradrenergic pathways in general anesthesia was suggested by the ability of drugs that modulate CNS NE levels to inversely modulate anesthetic potency (9,27–30). Subsequent studies showed that IV and volatile anesthetics inhibited NE release from intact adrenal medulla (31,32) and sympathetic nerves (33,34). Pentobarbital competitively inhibited catecholamine secretion evoked by activation of nicotinic acetylcholine receptors with threefold greater potency than release evoked by increased KCl or activation of histamine receptors (29). Halothane was even more selective for nicotinic receptor-mediated catecholamine secretion (31). These studies indicate that inhibition by barbiturates and volatile anesthetics of evoked catecholamine secretion from intact adrenal glands is primarily a postsynaptic effect mediated by blockade of nicotinic acetylcholine receptors, rather than a presynaptic effect mediated by inhibition of Ca2+ entry through voltage-gated Ca2+ channels and direct presynaptic inhibition of transmitter release. Our findings indicate that KCl-evoked NE release from isolated CNS nerve terminals is similarly insensitive to these drugs.

In isolated chromaffin cells, nicotinic receptor-mediated secretion of catecholamines was about twice as sensitive to inhibition by pentobarbital as KCl-evoked secretion (5), and five- to eightfold more sensitive to volatile anesthetics (6). In electropermeabilized cells, pentobarbital and volatile anesthetics had no effect on catecholamine secretion evoked by varying Ca2+ concentrations, suggesting that exocytotic mechanisms were unaffected. In rat PC12 pheochromocytoma cells, KCl depolarization-evoked and nicotinic receptor-evoked catecholamine release were both sensitive to volatile anesthetics in the same range (35). However, in human SH-SY5Y neuroblastoma cells, halothane (36), propofol, and thiopental (37) inhibited KCl-evoked [3H]NE release and associated increases in intracellular Ca2+, but only thiopental also inhibited carbachol-evoked release. Thus, barbiturates and volatile anesthetics inhibit nicotinic receptor-mediated release in native neurosecretory (chromaffin) cells more potently than KCl-evoked release, but in neurosecretory cell lines, the differences between nicotinic receptor- and KCl-evoked release may be diminished for volatile anesthetics. These results suggest that the Ca2+ channels coupled to catecholamine release are relatively insensitive to general anesthetics, and that differences in release-modulating receptors and cell-signaling mechanisms expressed in different cell lines may selectively affect sensitivities to specific drugs. These factors complicate extrapolation of results obtained in isolated peripheral neurosecretory cells and cell lines to CNS neurons, and illustrate the necessity for direct analysis of anesthetic effects on native CNS neuronal preparations.

Before this study, the effects of general anesthetics on CNS noradrenergic transmission have not been as fully characterized as their effects on peripheral sympathetic transmission. Anesthetic-induced changes in central noradrenergic transmission were suggested by the findings that halothane and cyclopropane produced drug-specific changes in tissue levels of NE and dopamine in discrete regions of rat brain (38). Pentobarbital reduced NE efflux from the rat medial preoptic area by in vivo microdialysis (39). In slices of rat cerebral cortex, clinical concentrations of halothane, enflurane, and methoxyflurane inhibited [3H]NE efflux, but not [3H]acetylcholine efflux, evoked by 16–28 mM KCl (11). In rat striatal slices, clinical concentrations of anesthetic barbiturates, and large concentrations of propofol and alphaxalone, but not ketamine, inhibited NE and dopamine efflux evoked by 40 mM KCl; effects on basal efflux were not reported (12,13). But these studies with brain slices, which preserve intact neuronal microcircuits, do not distinguish direct presynaptic effects from indirect postsynaptic effects on release (14). A large concentration of pentobarbital (200 µM) inhibited [3H]NE and [14C]{gamma}-aminobutyric acid release from isolated mouse cortical nerve terminals evoked by increased KCl, but not by the Ca2+ ionophore A23187 (8). This latter finding supports the results from electropermeabilized chromaffin cells (6) which indicated no effect on intracellular Ca2+-release coupling. However, our results indicate that more appropriate concentrations of pentobarbital (<100 µM) have minimal effects on NE release evoked by KCl or 4AP.

Stimulation of Ca2+-independent basal release by anesthetics has been reported previously for another catecholamine, dopamine. Halothane, enflurane, and isoflurane, but not thiopental or ketamine, increased basal [3H]dopamine release from rat striatal synaptosomes (40). All 5 anesthetics also produced slight reductions in [3H]dopamine release evoked by 15 mM KCl, but did not affect release by 50 mM KCl (40,41). In another study, halothane or isoflurane had no effect on basal [3H]dopamine release from whole brain synaptosomes, although both anesthetics inhibited dopamine uptake (42). Further studies will be required to reconcile these disparate results on dopamine release. Taken together with our findings, these results suggest that clinical concentrations of volatile anesthetics enhance the spontaneous release of the catecholamines dopamine and NE [(40,41), and this report]. The mechanisms underlying Ca2+-independent release of neurotransmitters are not fully understood (43), but they seem to be distinct from those involved in evoked Ca2+-dependent exocytosis (44). Rat noradrenergic synaptosomes contain a pool of vesicles that can undergo protracted Ca2+-independent, but tetanus toxin-sensitive, spontaneous exocytosis (45). Stimulation of this process may explain the enhanced Ca2+-independent basal transmitter efflux produced by propofol, halothane, and isoflurane.

The effects of general anesthetics on NE and dopamine release differ from those on glutamate release, suggesting distinct presynaptic anesthetic mechanisms. This may be attributable to fundamental differences in Ca2+ channel-release coupling, release regulatory mechanisms, etc., specific to each transmitter. Glutamate release involves the fusion of small synaptic vesicles at the active zone, where they are closely coupled to voltage-gated Ca2+ channels. In contrast, catecholaminergic neurons contain two types of vesicles, small dense core vesicles, which seem to release their contents at active zones, and large dense core vesicles, which are dispersed throughout the terminal and undergo exocytosis at ectopic sites (46). The relative importance of these two vesicle types in catecholamine release is unclear. A variety of general anesthetics inhibit the release of neurotransmitters such as glutamate from small synaptic vesicles (24,25,47,48), possibly via inhibition of presynaptic Na+ channels (15,24,25,49), whereas evoked release of neurotransmitters such as NE (this study) and cholecystokinin (50) from large dense core vesicles is insensitive to anesthetics. These findings suggest that differences in the mechanisms involved in the release of small versus large synaptic vesicles affect anesthetic sensitivity. Considerable evidence indicates that there are complex transmitter-, brain region-, secretogogue-, and drug-specific presynaptic actions of general anesthetics on neurotransmitter release. Differences in presynaptic receptors, regulation by intracellular signaling pathways, coupling to various voltage-gated Ca2+ and Na+ channel subtypes, exocytotic mechanisms, and/or Ca2+-release coupling may contribute to these differences.


   Acknowledgments
 
Supported by National Institutes of Health Grant GM58055, and by departmental funds.


   Footnotes
 
Presented in part at the annual meeting of the Society for Neuroscience, Miami, FL, October, 1999.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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Accepted for publication June 4, 2002.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press