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Anesth Analg 2005;101:722-725
© 2005 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000156951.83242.8D

ANESTHETIC PHARMACOLOGY

Subanesthetic Ketamine Does Not Affect 11C-Flumazenil Binding in Humans

Elina Salmi, MD, Jaakko W. Långsjö, MD, Sargo Aalto, MSc, Kjell Någren, PhD, Liisa Metsähonkala, MD, Kaike K. Kaisti, MD, Esa R. Korpi, MD, Jarmo Hietala, MD, and Harry Scheinin, MD

Turku PET Centre, Centre for Cognitive Neuroscience, and the Department of Pharmacology and Clinical Pharmacology, University of Turku, and the Departments of Anesthesiology and Intensive Care, Child Neurology, and Psychiatry, Turku University Hospital, Turku, Finland; Institute of Biomedicine, Pharmacology, University of Helsinki, Helsinki, Finland

Address correspondence to Elina Salmi, MD, Turku PET Centre, PO Box 52, FIN-20521 Turku, Finland. Address e-mail to anelsa{at}utu.fi.


   Abstract
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 Abstract
Introduction
 Methods
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 Discussion
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Positron emission tomography (PET) studies suggest that propofol and inhaled anesthetics increase 11C-flumazenil binding in the living human brain, thus supporting the involvement of {gamma}-aminobutyric acid type A (GABAA) receptors in the mechanism of action of these drugs. Ketamine produces its anesthetic effects primarily by N-methyl-d-aspartate receptor antagonism, but it may also have GABAA receptor agonistic properties. By using PET, we studied the cerebral 11C-flumazenil binding in 10 healthy subjects before and during a subanesthetic racemic ketamine infusion reaching a serum concentration of 350 ± 42 ng/mL. Ketamine did not affect 11C-flumazenil binding to GABAA receptor in the brain, indicating that this mechanism is of minor importance in the actions of subanesthetic ketamine.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Ketamine is a unique anesthetic with a rapid onset and a short duration of action. It is considered to be hemodynamically supportive, and it has moderate analgesic effects. Ketamine is believed to produce its anesthetic effects primarily by N-methyl-d-aspartate receptor antagonism (1), whereas various other IV and inhalational anesthetics act more on {gamma}-aminobutyric acid type A (GABAA) receptors in producing at least some supraspinal effects of anesthetics, such as sedation and amnesia (2,3). However, it has been proposed that ketamine also has GABAA receptor agonistic properties (4–7).

The effects of different anesthetics on GABAergic neurotransmission in the living human brain can be studied using positron emission tomography (PET) and 11C-flumazenil. Volatile anesthetics and propofol enhance 11C-flumazenil binding in both cortical and subcortical gray matter brain regions (8,9), supporting the involvement of GABAA receptors in their actions. The aim of the present study was to assess the effects of subanesthetic racemic ketamine on GABAergic neurotransmission in vivo in humans using PET and 11C-flumazenil.


   Methods
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 Introduction
 Methods
Results
 Discussion
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Ten healthy subjects were included in the study. All were right-handed, nonsmoking males with a mean age of 24 yr. Laboratory data and physical examination were normal. Subjects refrained from using alcohol, coffee, or any medication for 48 h before the study. The Hospital Ethics Committee approved the study protocol and all subjects gave written informed consent. Each subject underwent 2 dynamic 60-min 11C-flumazenil PET scans during the same day at least 2 h apart. The first scan was done before and the second scan during ketamine infusion.

Subjects received no premedication. Ketamine was administered as a continuous IV target-controlled infusion using a Harvard 22 pump (Harvard Apparatus, South Natick, MA) connected to a portable computer running STANPUMP software with pharmacokinetic variables from Domino et al. (10,11). The target ketamine serum concentration level was set to 300 ng/mL based on our previous study (12). After commencing the infusion, a 15-min stabilization period was allowed before the second scan. At the end of the scan, before the ketamine infusion was terminated, a 5-mL arterial blood sample was collected for determination of serum ketamine concentration (13). Hemodynamic variables and effects of ketamine on mood were monitored as previously described (12).

The preparation of 11C-flumazenil was done according to a published procedure (14). An IV dose of 370 MBq of 11C-flumazenil was administered manually in 60 s. The mean ± sd injected amount of 11C-flumazenil was 3.22 ± 1.41 µg. PET studies were performed as described previously (9,15,16).

Description of the realignment of dynamic 11C-flumazenil images and coregistration of magnetic resonance images to mean PET images is given in our previous article (9). Regions of interest were drawn using Imadeus software (version 1.15; Forima Inc, Turku, Finland) on the coregistered individual magnetic resonance images in several brain regions (Table 1). A two-compartment two-parameter model was used to describe 11C-flumazenil kinetics of regional and pixel tissue time activity curves. Distribution volumes (DV) were estimated directly without division (17). To visualize the effects of ketamine on DV, an independent explorative voxel-based statistical analysis was made using Statistical Parametric Mapping, Version 99 (Wellcome Department of Cognitive Neurology, University College London, England) and MatLab 6.1 for Windows (MathWorks, Natick, MA).


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  		Table 1. Summary of 11C-Flumazenil Binding in Different Brain Regions Before and During Ketamine Infusion

 

Quantitative DV values were analyzed using repeated-measures analysis of variance having the treatment (baseline, ketamine) and side (left, right) as within factors. Hemodynamic variables were analyzed with paired sample Student’s t-test. A two-sided value of P < 0.05 was considered statistically significant. Data are presented as mean ± sd unless otherwise stated. Statistical analyses were made using SPSS version 12.01 (SPSS Inc., Chicago, IL).


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
All PET scans were successful. The measured ketamine concentration was 350 ± 42 ng/mL (approximately 1.5 µM). Ketamine induced a significant increase in heart rate (33% ± 14%, P < 0.001) and mean arterial blood pressure (19% ± 9%, P < 0.001). End-tidal carbon dioxide, oxygen saturation, and respiratory rate remained stable, and there were no significant differences between the two scans (data not shown). Subjects reported significant perceptual distortion and reduced vigilance (P = 0.020). There were no significant treatment or side-by-treatment interaction effects in the DV values in any of the brain regions in the region of interest based analysis (Table 1). No significant changes were seen either in the Statistical Parametric Mapping analysis (data not shown).


   Discussion
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 Methods
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Based on in vitro and animal studies, ketamine appears to produce its anesthetic effects primarily by glutamate N-methyl-d-aspartate receptor antagonism (1). However, it has been demonstrated that ketamine increases GABA-induced chloride currents in recombinant GABAA receptors expressed in Xenopus laevis oocytes at the concentration of 365 µM (6) and in native neuronal receptors in olfactory cortical and hippocampal slices at the concentration of 500 µM (4,7). We now demonstrated that a small, subanesthetic, micromolar serum concentration of ketamine had no effect on GABAA receptor ligand binding in vivo in humans.

We decided to use subanesthetic ketamine, as even small doses (8 mg/kg) of ketamine have been shown to inhibit the convulsive effects of bicuculline, a GABAA receptor antagonist, in mice (5). Furthermore, we have previously shown in healthy volunteers that the same subanesthetic concentration of ketamine as used in the present study has significant effects on cerebral blood flow and glucose metabolism, which increased by 12%–38% (12) and 9%–15% (16), respectively, while the subjects reported perceptual distortion and changes in mood. Based on the present results these profound cerebral effects are not mediated by enhancement in GABAergic neurotransmission. It is possible, however, that ketamine affects only the GABAA receptor subtypes lacking the benzodiazepine binding sites, such as the flumazenil-insensitive {delta} subunit-containing receptors (18).

The mean ketamine serum concentration in the present experiment was approximately 1.5 µM (of which approximately 50% is protein bound) (19,20). The concentrations used in previous in vitro studies demonstrating GABAergic enhancement (4,6,7) have been are several orders of magnitude higher than in the present study. In vivo and in vitro concentrations cannot be directly compared, however. The tissue concentration of ketamine is unknown at 1.5 µM (pseudo) steady-state serum concentration, and as ketamine is a highly lipophilic drug, a substantial concentration gradient is presumable. Indeed, there appears to be a clear contradiction between the ketamine doses or concentrations acting on GABAA receptors in vitro and in vivo.

It has recently been shown that anesthesia at 1 minimum alveolar concentration of sevoflurane or effective plasma concentration 50 of propofol induces only an 8%–22% change in 11C-flumazenil DV (9). Furthermore, a considerable enhancement of 11C-flumazenil binding has been demonstrated when anesthetic exposure to isoflurane was increased from 1 to 1.5 minimum alveolar concentration (8). These findings indicate that even if the benzodiazepine antagonist flumazenil is the PET ligand of choice for in vivo quantification of GABAA receptors in the human brain (8,9), the binding of flumazenil at the GABAA-benzodiazepine receptor complex may not be very sensitive in detecting in vivo GABAergic enhancement by allosteric modulators. It should be noted, however, that there was not even a trend towards an increase in DV in any gray matter region in the present study; rather the opposite (Table 1). Nevertheless, we cannot exclude that larger, anesthetic doses of ketamine could affect brain 11C-flumazenil binding. This possibility remains to be studied.

In conclusion, subanesthetic ketamine did not enhance GABAA receptor ligand binding despite evident subjective effects and increases seen previously on cerebral blood flow and glucose metabolism at the same concentration level.

We thank Steven L. Shafer, MD (Professor, Department of Anesthesia, Stanford University, Stanford, CA) for the free use of his STANPUMP computer program.


   Footnotes
 
Supported, in part, by Turku University Hospital EVO-grant No. 13323, Turku, Finland and the Finnish-Norwegian Medical Foundation, Helsinki, Finland.

Accepted for publication January 5, 2005.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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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