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Abstract
BACKGROUND: Δ9-Tetrahydrocannabinol (Δ9-THC) induces analgesic effects and alterations of alertness. It has been reported that propofol increases endocannabinoid levels in the brain, but the effects of Δ9-THC on propofol sedation remain unclear. Our aim was to characterize the interaction between Δ9-THC and propofol in terms of sedation and analgesia.
METHODS: Sedation was monitored by a rota-rod and analgesia by tail-flick latencies. Twenty mice received intraperitoneal injections of 50 mg/kg Δ9-THC with 50, 75 and 100 mg/kg propofol after baseline values were established for each drug. Control experiments were performed with Δ9-THC and thiopental or Intralipid.
RESULTS: Injection of 50 mg/kg propofol caused a rapid onset of sedation with a minimum of 24 s on the rota-rod. Fifty mg/kg Δ9-THC alone had no sedative effects. Administration of Δ9-THC significantly reduced the sedative effect of propofol to at least 60 s on the rota-rod (P < 0.001). After increasing the propofol dose to 100 mg/kg in the presence of Δ9-THC, sedation was re-established with 27 s on the rota-rod. Thiopental sedation was significantly reduced (P < 0.01) in the presence of Δ9-THC.
CONCLUSION: The results indicate a dose-dependent antagonistic interaction between Δ9-THC and propofol, and also between Δ9-THC and thiopental.
IMPLICATIONS: It has been reported that propofol increases cerebral endocannabinoid concentrations. We found that the sedative effect of a low propofol concentration was abolished by 50 mg/kg delta9-tetrahydrocannabinol (D9-THC) that produced no sedation when given alone. Our results indicate a dose-dependent antagonistic interaction between D9-THC and propofol.
The interaction of cannabinoids with anesthetics has aroused interest for more than three decades. Cannabinoids have been reported to have analgesic and sedative properties and to prolong sleeping times after administration of both IV, as well as volatile, anesthetics.1,2 Cannabinoids are often used recreationally in Western populations. It was shown that cannabinoids, acting via a mechanism involving the central nervous system (CNS), may be used therapeutically, especially for treating chronic pain. Delta-9-tretrahydrocannabinol (Δ9-THC) is the most abundant and major psychoactive ingredient of the cannabis plant Cannabis sativa. In animals, a variety of effects of Δ9-THC have been described. Δ9-THC can induce both excitation and depression of the CNS resulting in ataxia, sedation and catalepsy among other effects.3 Propofol is often used as an anesthetic for inducing and maintaining anesthesia. Although it has been reported that propofol may release endogenous cannabinoids, the interaction of exogenous cannabinoids with propofol remains unclear and may be different compared with other anesthetics.4 We hypothesize that the sedative effect of propofol, similar to that of other anesthetics,1,2 is increased in the presence of Δ9-THC in mice.
METHODS
Animals
With approval of the institutional animal care committee, two groups (A and B) each consisting of 10 male 129S2/SVHsd mice were used (Table 1). The experiments were not performed in the sequence shown in Table 1, but in a randomized order automatically created by a random number generator from 1 to 10 for group A and B. Mice were fed ad libitum weighing between 21 and 46 g and were maintained on a day and night rhythm of 12 h.
Drug Application
Mice were given a fixed dose of 50 mg/kg Δ9-THC (Delta 9 Pharma, Neumarkt, Germany) combined with administration of either 50, 75, 100 mg/kg propofol (Astra Zeneca, Wedel, Germany). Intralipid 20% (Baxter, Unterschleissheim, Germany), a soy bean emulsion similar to the solvent used in commercial propofol solutions, or 20 and 50 mg/kg thiopental (Byk Gulden, Konstanz, Germany) was used in the control groups. Table 1 depicts the specific drug combinations used and the number of mice submitted to each group. Drugs were administered intraperitoneally (i.p.) using a constant volume of 1 mL/100 g body weight. Additional sodium chloride solution (0.9%) was added as necessary to reach the constant volume, except in the Intralipid experiments where only Intralipid was added. All substances were administered by i.p. injection via 20 gauge needles. When THC was administered it always preceded the administration of the sedative by 15 min. The injection time of the sedative drug was defined as t = 0. After administration of the experimental solutions, analgesia was tested first, followed by determination of the sedative effect. Initial dose finding studies showed that the Δ9-THC dose chosen exerted a significant half-maximal analgesic effect measured by tail-flick testing. Propofol and thiopental doses used in this investigation showed significant sedation in the rota-rod tests and were comparable to those described in previous investigations.2,4 The data for derivation of the doses used is shown in the Appendix. The animals were allowed to recover after completion of a single experiment for at least 10 days until studied in a subsequent experiment. Baseline values were performed before the following experiment to eliminate any residual drug effects. A single mouse was used for a maximum of 10 experiments.
Measurement of Analgesia
Analgesia was measured using a tail-flick unit (Ugo Basile, Comerio, Italy) as described previously.5 Briefly, the mouse was restrained to prevent major movements apart from free tail movement. Heat from an infrared source was administered to the tail with a radiation intensity of 40 mW/cm2. Tail-flick latency was defined as the time of heat exposure until withdrawal of the tail, and was recorded by a single blinded observer at pre-set time points starting 1 min after drug administration. In order to avoid tissue damage a cut-off time of 10 s was defined as the maximum analgesic effect.
Measurement of Sedation
Mice were sequentially placed on the tail-flick unit immediately followed by the measurement of sedation on a treadmill. The treadmill consists of a rotating rod (16 rpm) with a diameter of 3 cm. Sedation using a rotating rod (Rota-rod, Ugo Basile, Comerio, Italy), was determined as described previously.6 The time a mouse was able to stay on the rota-rod was recorded by a blinded single observer at pre-set time intervals 1, 2.5, 5, 7.5, 10, 12.5, 15, 20, 30, 45, 60, 75, 90, 120, 150, 180, 210, and 240 min after drug administration. When a mouse was able to stay on the rota-rod for more than 60 s it was defined as not sedated. All mice were conditioned to the rota-rod during a 2-wk period before the start of the experiments.
Statistics
The basis of our sample size calculation was pilot data on the effect size of propofol sedation. Given a difference of ≥10 s ± 6—7 s for time on rota-rod, for a α = 0.05 and a β = 0.1 a minimum of 10 animals was needed for an adequate sample size. The power calculation was performed with commercially available statistics software (Statmate 2.0, Graph Pad, San Diego, CA), and the posttrial statistical analysis was performed with Prism (Prism 4.03, Graph Pad, San Diego, CA). Data were analyzed with two way analysis of variance factoring for time and drug followed by Bonferroni correction for multiple comparisons. Differences within groups were analyzed by repeated measures analysis of variance. A P < 0.05 was considered statistically significant. Actual P values given are values after Bonferroni correction for multiple comparisons. All results are presented as mean ± sem. A post hoc power calculation was performed for those studies in which no difference was found (Power and Precision, Version 2.0, Biostat, Englewood, NJ).
RESULTS
Sedation
Injection of 50 mg/kg Δ9-THC (n = 20) did not affect the ability of the mice to stay on the rota-rod. Administration of propofol induced dose-dependent sedation. A dose of 50 mg/kg propofol (n = 20) resulted in a maximum sedative effect on the rota-rod (24 ± 5.6 s) 2.5 min after injection (Fig. 1A). Subsequently, times on the rota-rod increased until no sedative effect (60 s) was observed 15 min after propofol administration.
After administration of both 50 mg/kg propofol and 50 mg/kg Δ9-THC (n = 20), no sedation was observed (Fig. 1A), nor did increasing the propofol dose to 75 mg/kg in the presence of 50 mg/kg Δ9-THC cause sedation (data not presented). At a propofol dose of 100 mg/kg in the presence of Δ9-THC (n = 20) the sedative effect was not different from the administration of 50 mg/kg propofol (n = 20) in the absence of Δ9-THC, showing a maximum effect 2.5 min after administration of propofol (27 s) lasting for 10 min (Fig. 1A).
After administration of 20 mg/kg thiopental (n = 10) a maximum sedative effect was observed 2.5 min after injection (30 ± 6.6 s) on the rota-rod. The effect of thiopental slowly diminished thereafter and was abolished after 15 min (Fig. 1B). Sedation was significantly reduced by co-administration of Δ9-THC (n = 10) after 2.5 and 5 min postinjection (Fig. 1B). Δ9-THC 50 mg/kg with 50 mg/kg thiopental re-established sedation with <10 s on the rota-rod starting from 5 to 10 min postinjection, followed by only steady decrease. Sedation was finally abolished after 180 min (data not shown in figure).
Analgesia
Injection of 50 mg/kg Δ9-THC (n = 20) significantly increased tail-flick latency from 3.4 s at baseline to a maximum of 6.7 ± 0.5 s after 20 min (P < 0.05 at 30 and 45 min; P < 0.01 at 12.5 and 60 min; P < 0.001 at 15 and 20 min; Fig. 2). For the following 9.5 h, tail-flick latencies remained elevated between 6.4 s at 210 min and 5.3 s at 240 min postinjection and did not return to baseline during this period of time. Injection of 50 mg/kg propofol (n = 20) alone did not affect tail-flick-latency.
The combination of 50 mg/kg Δ9-THC and 50 mg/kg propofol (n = 20) resulted in a significant reduction of tail-flick latencies (P < 0.001) 15 min postinjection (Fig. 2). An increase of the propofol dose was followed by a reduction in tail-flick latencies. After administration of 100 mg/kg propofol i.p. in the presence of Δ9-THC (n = 20) (Fig. 2) tail-flick latency was reduced to 3.4 and 4.1 s after 7.5 min and 12.5 min (P < 0.01 vs single drug Δ9-THC application), respectively. The reduction in tail-flick-latency was also significant 15 and 20 min after injection (P < 0.001). Thirty minutes after injection of both Δ9-THC and Δ9-THC with propofol, tail-flick-latencies did not differ significantly.
Thiopental did not induce an analgesic effect. The combination of 20 mg/kg thiopental and 50 mg/kg Δ9-THC showed a parallel onset of the analgesic effect compared with Δ9-THC alone (P > 0.05; n = 10) (Fig. 3). Thirty minutes postinjection there was a trend to longer tail-flick latencies with the combination of both drugs, however this difference did not reach statistical significance. The post hoc power analysis yielded a number of >30 animals necessary to reach statistical significance given the observed small difference between groups.
As a control group, 10 mice were studied combining 50 mg/kg Δ9-THC with Intralipid, a soy bean solution comparable to the carrier substance of propofol. The addition of Intralipid did not influence the analgesic effect of Δ9-THC. No side effects were observed throughout the study period.
DISCUSSION
The effects of Δ9-THC on propofol-induced sedation were studied in mice showing that the sedative effect of a low propofol concentration was abolished by analgesic doses of Δ9-THC, a dose which produced no measurable sedation when given alone. With increasing doses of propofol, sedation was re-established in the presence of Δ9-THC in a dose-dependent manner although for a shorter period of time compared with propofol administration alone. The combination of Δ9-THC and thiopental also reduced barbiturate-induced sedation but Δ9-THC-induced analgesia was not altered by thiopental. Combining Δ9-THC with the lipid carrier substance of propofol, Intralipid, had no effect on Δ9-THC-mediated analgesia.
It was demonstrated in the present study that the antagonist effects of Δ9-THC on propofol sedation is propofol dose-dependent and not limited to propofol. With increasing doses of propofol, the antagonistic effect of Δ9-THC was overcome. The attenuation of the sedative properties of propofol by Δ9-THC has not been reported with other anesthetics. In contrast to the results of the present study, the anesthetic effect of volatile anesthetics such as halothane or isoflurane has been reported to be prolonged when combined with a specific cannabinoid 1 receptor (CB1) agonist or Δ9-THC.1,7 To our knowledge, only one study reported an interaction between cannabinoids and propofol in mice. It was demonstrated that i.p. injection of propofol induced an increase in the whole-brain content of anandamide, an endocannabinoid.4 The interpretation of higher CNS anandamide concentrations during propofol hypnosis remains unclear, however, it can be speculated that compensatory mechanisms may be activated attenuating propofol-induced hypnosis. This effect may be specific to mice, because human anandamide blood levels are not altered during propofol anesthesia.8 In contrast to our study, it was demonstrated that the combination of propofol with the selective CB1 receptor agonist Win 55212-2 enhances the loss-of-righting reflex in mice.4
It has been demonstrated that Δ9-THC interacts with other receptors besides CB receptors. There is evidence that γ-aminobutyric acid type A (GABAA) receptors are a possible target site of Δ9-THC action. A central interaction between the neurotransmitter GABA and cannabinoids has been demonstrated by extinction of aversive memory by Marsicano et al. who demonstrated that endocannabinoids/CB1 receptors are, in part, responsible for the reduction of GABA-mediated inhibitory currents.9 If central GABAA receptors play a critical role for the Δ9-THC effects on propofol sedation, a similar effect should be expected in the presence of thiopental-because thiopental induced anesthesia is thought to be mainly mediated through GABAA receptors.10 In the present study, thiopental sedation was altered in the presence of Δ9-THC. Thus, we conclude that an interaction at GABAA receptors may be a possible mechanism responsible for the antagonism of propofol or thiopental to Δ9-THC.
An indirect interaction between Δ9-THC and propofol also seems to be possible. The synaptic GABAergic transmission of interneurons in the amygdala is in part modulated by presynaptic CB1 receptors, where CB1 agonists inhibit GABAA receptor-mediated inhibitory postsynaptic currents. In the amygdala, presynaptical axon terminals with CB1 receptors are in close proximity to GABAergic receptors.11 Due to the close anatomic relationship, a negative feedback mechanism to presynaptic CB1 receptors changes the arousal state. This system is rapidly modulated due to the short half-life of in vivo endocannabinoids, which are inactivated by fatty acid amide hydrolase.12,13 The cannabinoid-mediated retrograde signaling and subsequent down-regulation of GABA release also controls hippocampal oscillations by CB1 expressing interneurons,14 opening another field of possible interactions with propofol.15 Another possible indirect interaction between Δ9-THC and propofol sedation could be caused by central Δ9-THC sympatho-excitation, as demonstrated in rabbits.16 An unspecific arousal mediated by Δ9-THC should affect thiopental sedation similarly to propofol, as demonstrated. However, others have reported that thiopental sedation is increased by Δ9-THC with higher thiopental concentrations, possibly attenuating the effects from lower thiopental concentrations.17
To exclude the possibility that the interaction of propofol and Δ9-THC is due to uptake of the highly lipophilic Δ9-THC into the lipid contained in the propofol emulsion, thus decreasing free Δ9-THC concentrations, we administered the lipid carrier Intralipid together with Δ9-THC. The effect of Δ9-THC was unchanged by the lipid carrier, suggesting that this pharmacokinetic interaction does not play a critical role for the observed effect.
In conclusion, the exact interaction between cannabinoids and propofol remains unclear at present. Pertwee demonstrated that endocannabinoids play an important role in the physiologic control of pain processing.18 In humans, differential effects of sevoflurane and propofol on endocannabinoid levels have been observed. Whereas sevoflurane decreases anandamide levels, propofol leaves anandamide concentrations unchanged.8 Propofol has been demonstrated to decrease the activity of fatty acid amid hydrolase, the major anandamide degradation pathway, however only at high concentrations.4 The specific effects of propofol compared with other anesthetics may explain the findings of our study. However, whether the endocannabinoid pathway is indeed the origin of the effects of propofol, or if alterations of endocannabinoids merely represent an epi-phenomenon remains to be shown.
Source, Graphs and Figures: Propofol Sedation Is Reduced by
BACKGROUND: Δ9-Tetrahydrocannabinol (Δ9-THC) induces analgesic effects and alterations of alertness. It has been reported that propofol increases endocannabinoid levels in the brain, but the effects of Δ9-THC on propofol sedation remain unclear. Our aim was to characterize the interaction between Δ9-THC and propofol in terms of sedation and analgesia.
METHODS: Sedation was monitored by a rota-rod and analgesia by tail-flick latencies. Twenty mice received intraperitoneal injections of 50 mg/kg Δ9-THC with 50, 75 and 100 mg/kg propofol after baseline values were established for each drug. Control experiments were performed with Δ9-THC and thiopental or Intralipid.
RESULTS: Injection of 50 mg/kg propofol caused a rapid onset of sedation with a minimum of 24 s on the rota-rod. Fifty mg/kg Δ9-THC alone had no sedative effects. Administration of Δ9-THC significantly reduced the sedative effect of propofol to at least 60 s on the rota-rod (P < 0.001). After increasing the propofol dose to 100 mg/kg in the presence of Δ9-THC, sedation was re-established with 27 s on the rota-rod. Thiopental sedation was significantly reduced (P < 0.01) in the presence of Δ9-THC.
CONCLUSION: The results indicate a dose-dependent antagonistic interaction between Δ9-THC and propofol, and also between Δ9-THC and thiopental.
IMPLICATIONS: It has been reported that propofol increases cerebral endocannabinoid concentrations. We found that the sedative effect of a low propofol concentration was abolished by 50 mg/kg delta9-tetrahydrocannabinol (D9-THC) that produced no sedation when given alone. Our results indicate a dose-dependent antagonistic interaction between D9-THC and propofol.
The interaction of cannabinoids with anesthetics has aroused interest for more than three decades. Cannabinoids have been reported to have analgesic and sedative properties and to prolong sleeping times after administration of both IV, as well as volatile, anesthetics.1,2 Cannabinoids are often used recreationally in Western populations. It was shown that cannabinoids, acting via a mechanism involving the central nervous system (CNS), may be used therapeutically, especially for treating chronic pain. Delta-9-tretrahydrocannabinol (Δ9-THC) is the most abundant and major psychoactive ingredient of the cannabis plant Cannabis sativa. In animals, a variety of effects of Δ9-THC have been described. Δ9-THC can induce both excitation and depression of the CNS resulting in ataxia, sedation and catalepsy among other effects.3 Propofol is often used as an anesthetic for inducing and maintaining anesthesia. Although it has been reported that propofol may release endogenous cannabinoids, the interaction of exogenous cannabinoids with propofol remains unclear and may be different compared with other anesthetics.4 We hypothesize that the sedative effect of propofol, similar to that of other anesthetics,1,2 is increased in the presence of Δ9-THC in mice.
METHODS
Animals
With approval of the institutional animal care committee, two groups (A and B) each consisting of 10 male 129S2/SVHsd mice were used (Table 1). The experiments were not performed in the sequence shown in Table 1, but in a randomized order automatically created by a random number generator from 1 to 10 for group A and B. Mice were fed ad libitum weighing between 21 and 46 g and were maintained on a day and night rhythm of 12 h.
Drug Application
Mice were given a fixed dose of 50 mg/kg Δ9-THC (Delta 9 Pharma, Neumarkt, Germany) combined with administration of either 50, 75, 100 mg/kg propofol (Astra Zeneca, Wedel, Germany). Intralipid 20% (Baxter, Unterschleissheim, Germany), a soy bean emulsion similar to the solvent used in commercial propofol solutions, or 20 and 50 mg/kg thiopental (Byk Gulden, Konstanz, Germany) was used in the control groups. Table 1 depicts the specific drug combinations used and the number of mice submitted to each group. Drugs were administered intraperitoneally (i.p.) using a constant volume of 1 mL/100 g body weight. Additional sodium chloride solution (0.9%) was added as necessary to reach the constant volume, except in the Intralipid experiments where only Intralipid was added. All substances were administered by i.p. injection via 20 gauge needles. When THC was administered it always preceded the administration of the sedative by 15 min. The injection time of the sedative drug was defined as t = 0. After administration of the experimental solutions, analgesia was tested first, followed by determination of the sedative effect. Initial dose finding studies showed that the Δ9-THC dose chosen exerted a significant half-maximal analgesic effect measured by tail-flick testing. Propofol and thiopental doses used in this investigation showed significant sedation in the rota-rod tests and were comparable to those described in previous investigations.2,4 The data for derivation of the doses used is shown in the Appendix. The animals were allowed to recover after completion of a single experiment for at least 10 days until studied in a subsequent experiment. Baseline values were performed before the following experiment to eliminate any residual drug effects. A single mouse was used for a maximum of 10 experiments.
Measurement of Analgesia
Analgesia was measured using a tail-flick unit (Ugo Basile, Comerio, Italy) as described previously.5 Briefly, the mouse was restrained to prevent major movements apart from free tail movement. Heat from an infrared source was administered to the tail with a radiation intensity of 40 mW/cm2. Tail-flick latency was defined as the time of heat exposure until withdrawal of the tail, and was recorded by a single blinded observer at pre-set time points starting 1 min after drug administration. In order to avoid tissue damage a cut-off time of 10 s was defined as the maximum analgesic effect.
Measurement of Sedation
Mice were sequentially placed on the tail-flick unit immediately followed by the measurement of sedation on a treadmill. The treadmill consists of a rotating rod (16 rpm) with a diameter of 3 cm. Sedation using a rotating rod (Rota-rod, Ugo Basile, Comerio, Italy), was determined as described previously.6 The time a mouse was able to stay on the rota-rod was recorded by a blinded single observer at pre-set time intervals 1, 2.5, 5, 7.5, 10, 12.5, 15, 20, 30, 45, 60, 75, 90, 120, 150, 180, 210, and 240 min after drug administration. When a mouse was able to stay on the rota-rod for more than 60 s it was defined as not sedated. All mice were conditioned to the rota-rod during a 2-wk period before the start of the experiments.
Statistics
The basis of our sample size calculation was pilot data on the effect size of propofol sedation. Given a difference of ≥10 s ± 6—7 s for time on rota-rod, for a α = 0.05 and a β = 0.1 a minimum of 10 animals was needed for an adequate sample size. The power calculation was performed with commercially available statistics software (Statmate 2.0, Graph Pad, San Diego, CA), and the posttrial statistical analysis was performed with Prism (Prism 4.03, Graph Pad, San Diego, CA). Data were analyzed with two way analysis of variance factoring for time and drug followed by Bonferroni correction for multiple comparisons. Differences within groups were analyzed by repeated measures analysis of variance. A P < 0.05 was considered statistically significant. Actual P values given are values after Bonferroni correction for multiple comparisons. All results are presented as mean ± sem. A post hoc power calculation was performed for those studies in which no difference was found (Power and Precision, Version 2.0, Biostat, Englewood, NJ).
RESULTS
Sedation
Injection of 50 mg/kg Δ9-THC (n = 20) did not affect the ability of the mice to stay on the rota-rod. Administration of propofol induced dose-dependent sedation. A dose of 50 mg/kg propofol (n = 20) resulted in a maximum sedative effect on the rota-rod (24 ± 5.6 s) 2.5 min after injection (Fig. 1A). Subsequently, times on the rota-rod increased until no sedative effect (60 s) was observed 15 min after propofol administration.
After administration of both 50 mg/kg propofol and 50 mg/kg Δ9-THC (n = 20), no sedation was observed (Fig. 1A), nor did increasing the propofol dose to 75 mg/kg in the presence of 50 mg/kg Δ9-THC cause sedation (data not presented). At a propofol dose of 100 mg/kg in the presence of Δ9-THC (n = 20) the sedative effect was not different from the administration of 50 mg/kg propofol (n = 20) in the absence of Δ9-THC, showing a maximum effect 2.5 min after administration of propofol (27 s) lasting for 10 min (Fig. 1A).
After administration of 20 mg/kg thiopental (n = 10) a maximum sedative effect was observed 2.5 min after injection (30 ± 6.6 s) on the rota-rod. The effect of thiopental slowly diminished thereafter and was abolished after 15 min (Fig. 1B). Sedation was significantly reduced by co-administration of Δ9-THC (n = 10) after 2.5 and 5 min postinjection (Fig. 1B). Δ9-THC 50 mg/kg with 50 mg/kg thiopental re-established sedation with <10 s on the rota-rod starting from 5 to 10 min postinjection, followed by only steady decrease. Sedation was finally abolished after 180 min (data not shown in figure).
Analgesia
Injection of 50 mg/kg Δ9-THC (n = 20) significantly increased tail-flick latency from 3.4 s at baseline to a maximum of 6.7 ± 0.5 s after 20 min (P < 0.05 at 30 and 45 min; P < 0.01 at 12.5 and 60 min; P < 0.001 at 15 and 20 min; Fig. 2). For the following 9.5 h, tail-flick latencies remained elevated between 6.4 s at 210 min and 5.3 s at 240 min postinjection and did not return to baseline during this period of time. Injection of 50 mg/kg propofol (n = 20) alone did not affect tail-flick-latency.
The combination of 50 mg/kg Δ9-THC and 50 mg/kg propofol (n = 20) resulted in a significant reduction of tail-flick latencies (P < 0.001) 15 min postinjection (Fig. 2). An increase of the propofol dose was followed by a reduction in tail-flick latencies. After administration of 100 mg/kg propofol i.p. in the presence of Δ9-THC (n = 20) (Fig. 2) tail-flick latency was reduced to 3.4 and 4.1 s after 7.5 min and 12.5 min (P < 0.01 vs single drug Δ9-THC application), respectively. The reduction in tail-flick-latency was also significant 15 and 20 min after injection (P < 0.001). Thirty minutes after injection of both Δ9-THC and Δ9-THC with propofol, tail-flick-latencies did not differ significantly.
Thiopental did not induce an analgesic effect. The combination of 20 mg/kg thiopental and 50 mg/kg Δ9-THC showed a parallel onset of the analgesic effect compared with Δ9-THC alone (P > 0.05; n = 10) (Fig. 3). Thirty minutes postinjection there was a trend to longer tail-flick latencies with the combination of both drugs, however this difference did not reach statistical significance. The post hoc power analysis yielded a number of >30 animals necessary to reach statistical significance given the observed small difference between groups.
As a control group, 10 mice were studied combining 50 mg/kg Δ9-THC with Intralipid, a soy bean solution comparable to the carrier substance of propofol. The addition of Intralipid did not influence the analgesic effect of Δ9-THC. No side effects were observed throughout the study period.
DISCUSSION
The effects of Δ9-THC on propofol-induced sedation were studied in mice showing that the sedative effect of a low propofol concentration was abolished by analgesic doses of Δ9-THC, a dose which produced no measurable sedation when given alone. With increasing doses of propofol, sedation was re-established in the presence of Δ9-THC in a dose-dependent manner although for a shorter period of time compared with propofol administration alone. The combination of Δ9-THC and thiopental also reduced barbiturate-induced sedation but Δ9-THC-induced analgesia was not altered by thiopental. Combining Δ9-THC with the lipid carrier substance of propofol, Intralipid, had no effect on Δ9-THC-mediated analgesia.
It was demonstrated in the present study that the antagonist effects of Δ9-THC on propofol sedation is propofol dose-dependent and not limited to propofol. With increasing doses of propofol, the antagonistic effect of Δ9-THC was overcome. The attenuation of the sedative properties of propofol by Δ9-THC has not been reported with other anesthetics. In contrast to the results of the present study, the anesthetic effect of volatile anesthetics such as halothane or isoflurane has been reported to be prolonged when combined with a specific cannabinoid 1 receptor (CB1) agonist or Δ9-THC.1,7 To our knowledge, only one study reported an interaction between cannabinoids and propofol in mice. It was demonstrated that i.p. injection of propofol induced an increase in the whole-brain content of anandamide, an endocannabinoid.4 The interpretation of higher CNS anandamide concentrations during propofol hypnosis remains unclear, however, it can be speculated that compensatory mechanisms may be activated attenuating propofol-induced hypnosis. This effect may be specific to mice, because human anandamide blood levels are not altered during propofol anesthesia.8 In contrast to our study, it was demonstrated that the combination of propofol with the selective CB1 receptor agonist Win 55212-2 enhances the loss-of-righting reflex in mice.4
It has been demonstrated that Δ9-THC interacts with other receptors besides CB receptors. There is evidence that γ-aminobutyric acid type A (GABAA) receptors are a possible target site of Δ9-THC action. A central interaction between the neurotransmitter GABA and cannabinoids has been demonstrated by extinction of aversive memory by Marsicano et al. who demonstrated that endocannabinoids/CB1 receptors are, in part, responsible for the reduction of GABA-mediated inhibitory currents.9 If central GABAA receptors play a critical role for the Δ9-THC effects on propofol sedation, a similar effect should be expected in the presence of thiopental-because thiopental induced anesthesia is thought to be mainly mediated through GABAA receptors.10 In the present study, thiopental sedation was altered in the presence of Δ9-THC. Thus, we conclude that an interaction at GABAA receptors may be a possible mechanism responsible for the antagonism of propofol or thiopental to Δ9-THC.
An indirect interaction between Δ9-THC and propofol also seems to be possible. The synaptic GABAergic transmission of interneurons in the amygdala is in part modulated by presynaptic CB1 receptors, where CB1 agonists inhibit GABAA receptor-mediated inhibitory postsynaptic currents. In the amygdala, presynaptical axon terminals with CB1 receptors are in close proximity to GABAergic receptors.11 Due to the close anatomic relationship, a negative feedback mechanism to presynaptic CB1 receptors changes the arousal state. This system is rapidly modulated due to the short half-life of in vivo endocannabinoids, which are inactivated by fatty acid amide hydrolase.12,13 The cannabinoid-mediated retrograde signaling and subsequent down-regulation of GABA release also controls hippocampal oscillations by CB1 expressing interneurons,14 opening another field of possible interactions with propofol.15 Another possible indirect interaction between Δ9-THC and propofol sedation could be caused by central Δ9-THC sympatho-excitation, as demonstrated in rabbits.16 An unspecific arousal mediated by Δ9-THC should affect thiopental sedation similarly to propofol, as demonstrated. However, others have reported that thiopental sedation is increased by Δ9-THC with higher thiopental concentrations, possibly attenuating the effects from lower thiopental concentrations.17
To exclude the possibility that the interaction of propofol and Δ9-THC is due to uptake of the highly lipophilic Δ9-THC into the lipid contained in the propofol emulsion, thus decreasing free Δ9-THC concentrations, we administered the lipid carrier Intralipid together with Δ9-THC. The effect of Δ9-THC was unchanged by the lipid carrier, suggesting that this pharmacokinetic interaction does not play a critical role for the observed effect.
In conclusion, the exact interaction between cannabinoids and propofol remains unclear at present. Pertwee demonstrated that endocannabinoids play an important role in the physiologic control of pain processing.18 In humans, differential effects of sevoflurane and propofol on endocannabinoid levels have been observed. Whereas sevoflurane decreases anandamide levels, propofol leaves anandamide concentrations unchanged.8 Propofol has been demonstrated to decrease the activity of fatty acid amid hydrolase, the major anandamide degradation pathway, however only at high concentrations.4 The specific effects of propofol compared with other anesthetics may explain the findings of our study. However, whether the endocannabinoid pathway is indeed the origin of the effects of propofol, or if alterations of endocannabinoids merely represent an epi-phenomenon remains to be shown.
Source, Graphs and Figures: Propofol Sedation Is Reduced by
