Partial antagonism of propofol anaesthesia by physostigmine in rats is associated with potentiation of fast (80-200 Hz) oscillations in the thalamus.

BACKGROUND: Positron emission tomography studies in human subjects show that propofol-induced unconsciousness in humans is associated with a reduction in thalamic blood flow, suggesting that anaesthesia is associated with impairment of thalamic function. A recent study showed that antagonism of propofol-induced unconsciousness by the anticholinesterase physostigmine is associated with a marked increase in thalamic blood flow, supporting the implication of the thalamus. The aim of the present study was to assess the role of the thalamus in the antagonistic effects of physostigmine during propofol anaesthesia using electrophysiological recordings in a rat model. METHODS: Local field potentials were recorded from the barrel cortex and ventroposteromedial thalamic nucleus in 10 chronically instrumented rats to measure spectral power in the gamma/high-gamma range (50-200 Hz). Propofol was given i.v. by target-controlled infusion at the lowest concentration required to abolish righting attempts. Physostigmine was given during anaesthesia to produce behavioural arousal without changing anaesthetic concentration. RESULTS: Compared with baseline, gamma/high-gamma power during anaesthesia was reduced by 31% in the cortex (P=0.006) and by 65% in the thalamus (P=0.006). Physostigmine given during anaesthesia increased gamma/high-gamma power in the thalamus by 60% (P=0.048) and caused behavioural arousal that correlated (P=0.0087) with the increase in power. Physostigmine caused no significant power change in the cortex. CONCLUSIONS: We conclude that partial antagonism of propofol anaesthesia by physostigmine is associated with an increase in thalamic activity reflected in gamma/high-gamma (50-200 Hz) power. These findings are consistent with the view that anaesthetic-induced unconsciousness is associated with impairment of thalamic function.

Selective 5-HT(1A)-R-agonist repinotan prevents remifentanil-induced ventilatory depression and prolongs antinociception.

BACKGROUND: 5-HT(1A)-R-agonist repinotan was shown to counteract a morphine-induced ventilatory depression but had pronociceptive effects at small doses (0.2 µg/kg). It remained to be clarified (1) whether a moderate dose of repinotan, sufficient to stimulate spontaneous breathing, impairs antinociception if plasma concentration decreases over time, and if (2) moderate doses prevent ventilatory depression if given before the opioid. METHODS: A dose-response curve of the repinotan effects on spontaneous minute ventilation during continuous remifentanil infusion in anesthetized rats was established to identify moderate doses: (1) tail-flick reflex latencies to assess nociception were recorded until 60 min after cessation of a continuous remifentanil infusion with or without a concomitant moderate repinotan dose (10 µg/kg), and (2) remifentanil boluses (2.5 µg/kg) were given after repinotan (10 and 20 µg/kg). RESULTS: (1) Remifentanil-induced antinociception lasted only 5 min after infusion was stopped (tail-flick reflex latencies; median [interquartile range], 97 [54-100]% of maximum possible effect; P = 0.034), but was extended by repinotan (10 µg/kg) to 30 min (tail-flick reflex latencies, 100 [75-100]% of maximum possible effect; P = 0.031). Repinotan (10 µg/kg) alone did not have any significant antinociceptive effect. (2) The ventilatory depression by remifentanil boluses (2.5 µg/kg; minute ventilation, -65 [-81 to -56]%; P = 0.031, n = 5) was blunted by repinotan (20 µg/kg; minute ventilation, -24 [-53 to 13]%; P = 0.313, compared with the pretreatment level). CONCLUSIONS: Repinotan prevented remifentanil-induced ventilatory depression in spontaneously breathing, anesthetized rats. Although repinotan did not depress nociception itself, it prolonged the profound antinociception after discontinuation of remifentanil infusion.

Magnesium sulfate prevents remifentanil-induced postoperative hyperalgesia in patients undergoing thyroidectomy.

BACKGROUND: In a randomized, double-blind, prospective study, we investigated whether an intraoperative high versus low dose of remifentanil increased postoperative hyperalgesia and whether magnesium can prevent remifentanil-induced hyperalgesia. METHODS: Ninety patients undergoing thyroidectomy were randomly assigned to 1 of 3 groups. Remifentanil was intraoperatively infused at 0.05 µg/kg/min (group LO) or 0.2 µg/kg/min (groups HI and HM). Patients in group HM received MgSO(4) 30 mg/kg at induction followed by an intraoperative infusion of 10 mg/kg/h. Mechanical pain thresholds on the forearm and periincisional area were assessed by von Frey filament the evening before surgery and postoperatively at 24 and 48 hours. Pain measured on a verbal numerical rating scale (VNRS) (0-10) and additional analgesics were recorded in the postanesthesia care unit postoperatively at 6, 24, and 48 hours. RESULTS: There was a significantly greater decrease in pain threshold on the periincisional area at 24 and 48 hours postoperatively in group HI, as compared with the other 2 groups. The 95% confidence intervals for the mean difference in pain thresholds on the periincisional area at 24 and 48 hours postoperatively were 0.31 to 1.11 and 0.36 to 1.14 for group HI versus group LO, 0.45 to 1.26 and 0.54 to 1.32 for group HI versus group HM (values are log(10) of force in milligrams). The change in pain threshold on the forearm was similar among the groups. Group HI had significantly higher VNRS scores (median [interquartile range], 3 [2-4]) than group LO (2 [1-3] and group HM (2 [1-3]) at 48 hours postoperatively. The 95% confidence intervals for median difference in VNRS score at 48 hours postoperatively were 1 to 2 for group HI versus group LO and 0 to 2 for group HI versus group HM. There were no significant differences in the number of patients who requested rescue analgesics in the postoperative anesthesia care unit and general ward during 48 hours postoperatively among the 3 groups. CONCLUSIONS: A relatively high dose of intraoperative remifentanil enhances periincisional hyperalgesia. Intraoperative MgSO(4) prevents remifentanil-induced hyperalgesia. However, hyperalgesia did not reach clinical relevance in terms of postoperative pain or analgesic consumption in patients undergoing thyroidectomy.

Critical involvement of the thalamus and precuneus during restoration of consciousness with physostigmine in humans during propofol anaesthesia: a positron emission tomography study.

BACKGROUND: Functional brain imaging offers a way to investigate how general anaesthetics impair consciousness. However, functional imaging changes may result from drug effects unrelated to hypnosis. Establishing a causal link with loss of consciousness is thus difficult. METHODS: To identify changes of neuronal activity functionally linked to the level of consciousness, physostigmine was used to restore consciousness without changing the anaesthetic concentration in 11 subjects anaesthetized with propofol. Eight subjects (responders) regained consciousness after physostigmine and three did not (non-responders). Positron emission tomography was used to measure regional cerebral blood flow (rCBF); during baseline (awake), after anaesthesia-induced loss of consciousness, after physostigmine administration, and recovery. In addition to subtraction analyses, we used conjunction analysis in the responders to identify changes common to the baseline-anaesthesia and physostigmine-anaesthesia contrasts. RESULTS: Complete data were available for seven subjects (four responders and three non-responders). The analyses revealed that unconsciousness was associated with rCBF decreases in the thalamus and precuneus. Restoration of consciousness by physostigmine was associated with rCBF increases in these same structures, with the strongest effect in the thalamus. CONCLUSIONS: The results provide strong evidence that reductions in rCBF in the thalamus and precuneus are functionally related to propofol-induced unconsciousness independently of any non-specific effects of propofol. These observations confirm that the thalamus and precuneus are key elements to understand how general anaesthetics cause unconsciousness and how patients wake up from anaesthesia. Furthermore, they are consistent with the notion that anaesthetic-induced unconsciousness is associated with reduced cholinergic activation.

The use of flumazenil after midazolam-induced conscious sedation.

OBJECTIVE: To investigate the use of flumazenil after midazolam-induced conscious sedation. DESIGN AND SETTING: A prospective audit was carried out in the Department of Sedation and Special Care Dentistry at Guy's Hospital, King's College, London, 2009.Subjects Patients sedated with midazolam for dental treatment. METHOD: All clinical staff completed the data capture proforma when flumazenil was administered to a patient after sedation with midazolam. RESULTS: Four hundred and fifty-three patients were sedated with midazolam. Flumazenil was used in 32 cases. No cases required flumazenil for the emergency treatment of respiratory depression. CONCLUSIONS: The results of the audit confirmed the safe and appropriate use of midazolam for conscious sedation within the Department of Sedation and Special Care Dentistry at Guy's Hospital and demonstrated that flumazenil use was low and in accordance with current best practice. The audit has highlighted distinct indications for the post-operative use of flumazenil in specifically selected cases. Each case should be individually considered, justified and documented within the patient's clinical record.

Propofol and aminophylline antagonize each other during the mobilization of intracellular calcium in human umbilical vein endothelial cells.

This study examined whether propofol and aminophylline affect the mobilization of intracellular calcium in human umbilical vein endothelial cells. Intracellular calcium was measured using laser scanning confocal microscopy. Cultured and serum-starved cells on round coverslips were incubated with propofol or aminophylline for 30 min, and then stimulated with lysophosphatidic acid, propofol and aminophylline. The results were expressed as relative fluorescence intensity and fold stimulation. Propofol decreased the concentration of intracellular calcium, whereas aminophylline caused increased mobilization of intracellular calcium in a concentration-dependent manner. Propofol suppressed the lysophosphatidic acid-induced mobilization of intracellular calcium in a concentration-dependent manner. Propofol further prevented the aminophylline-induced increase of intracellular calcium at clinically relevant concentrations. However, aminophylline reversed the inhibitory effect of propofol on the elevation of intracellular calcium by lysophosphatidic acid. Our results suggest that propofol and aminophylline antagonize each other on the mobilization of intracellular calcium in human umbilical vein endothelial cells at clinically relevant concentrations. Serious consideration should be given to how this interaction affects mobilization of intracellular calcium when these two drugs are used together.

Propofol causes neurite retraction in neurones.

BACKGROUND: The mechanism by which anaesthetic agents produce general anaesthesia is not yet fully understood. Retraction of neurites is an important function of individual neurones and neural plexuses during normal and pathological conditions, and it has been shown that such a retraction pathway exists in developing and mature neurones. We hypothesized that propofol decreases neuronal activity by causing retraction of neuronal neurites. METHODS: Primary cultures of rat cortical neurones were exposed in concentration- and time-response experiments to 0.02, 0.2, 2, and 20 microM propofol or lipid vehicle. Neurones were pretreated with the GABA(A) receptor (GABA(A)R) antagonist, bicuculline, the myosin II ATPase activity inhibitor, blebbistatin, and the F-actin stabilizing agent, phalloidin, followed by administration of propofol (20 microM). Changes in neurite retraction were evaluated using time-lapse light microscopy. RESULTS: Propofol caused a concentration- and time-dependent reversible retraction of cultured cortical neurone neurites. Bicuculline, blebbistatin, and phalloidin completely inhibited propofol-induced neurite retraction. Images of retracted neurites were characterized by a retraction bulb and a thin trailing membrane remnant. CONCLUSIONS: Cultured cortical rat neurones retract their neurites after exposure to propofol in a concentration- and time-dependent manner. This retraction is GABA(A)R mediated, reversible, and dependent on actin and myosin II. Furthermore, the concentrations and times to full retraction and recovery correspond to those observed during propofol anaesthesia.

Clinical assessment of propofol-induced yawning with heart rate variability: a pilot study.

STUDY OBJECTIVES: To investigate the proportion of propofol-induced yawning and sympathovagal balance during propofol-induced yawning. DESIGN: Prospective, observational, clinical study. SETTING: University hospital and 2400-bed tertiary medical center. PATIENTS: 546 ASA physical status I and II patients undergoing elective surgery with general anesthesia. INTERVENTIONS: Standard induction of anesthesia was performed with intravenous (IV) propofol two to four mg/kg (group P), or pretreatment with atropine 0.1 mg/kg (group AP) or with fentanyl 1 to 3 microg/kg (group FP) before propofol. Continuous standard electrocardiogram for heart rate variability (HRV) was performed in another 20 patients to investigate sympathovagal balance during propofol-induced yawning. MEASUREMENTS AND MAIN RESULTS: The proportions of yawning were 53.5% (207/386), 61.1% (55/90), and 0% (0/50) in the P, AP, and FP groups, respectively. Propofol-induced yawning could be dramatically decreased by pretreatment with IV fentanyl (P < 0.001, chi2 test). Significant increased ratio of low-frequency/high-frequency power was detected during HRV monitoring in 9 patients with yawning in comparison with 11 patients without yawning (P < 0.05, Wilcoxon signed-rank test). CONCLUSIONS: Pretreatment with fentanyl may inhibit propofol-induced yawning. Fluctuations in autonomic function have been noted during propofol-induced yawning.

Aminophylline reversal of prolonged postoperative sedation induced by propofol.

Propofol is frequently used for intravenous sedation or anesthesia in ambulatory and office-based anesthesia. Although awakening is usually rapid, there are instances of delayed recovery from propofol anesthesia. It has been reported that aminophylline antagonizes the sedative effects of several anesthetic and analgesic drugs. The case reports presented here demonstrate that intravenous aminophylline effectively reversed prolonged propofol-induced sedation/anesthesia in the postoperative period. There were no side effects or delayed re-sedation after the administration of aminophylline. Our study suggests that aminophylline could be a clinically useful propofol antagonist.

The effects of aminophylline on bispectral index during inhalational and total intravenous anaesthesia.

Aminophylline is usually used during anaesthesia to treat bronchospasm but recent findings suggest that it can also be used to shorten recovery time after general anaesthesia. However, it is unclear whether aminophylline shows similar properties during a steady-state phase of deep surgical anaesthesia. We therefore wanted to test the hypothesis that the administration of aminophylline leads to an increase in bispectral index as a surrogate parameter suggesting a lighter plane of anaesthesia. The study was designed as a double-blind, randomised, controlled trial with two main groups (aminophylline and placebo) and two subgroups (sevoflurane and propofol). We studied 60 patients. The injection of aminophylline 3 mg x kg(-1) was associated with significant increases in bispectral index up to 10 min after its injection, while heart rate and blood pressure did not change. It appears that aminophylline has the ability to partially antagonise the sedative effects of general anaesthetics.

A comparison of midazolam with remifentanil for the prevention of myoclonic movements following etomidate injection.

Etomidate is a popular anaesthetic induction agent, but it frequently causes myoclonic movements. Although both benzodiazepines and opioids reduce myoclonus, there has been no comparative study between these agents. Thus, we conducted a prospective, randomized study to compare midazolam and remifentanil as pre-treatment agents for reducing etomidate-induced myoclonus in 90 adults undergoing surgery. Patients were pre-treated before the etomidate injection, either with saline (Group C), midazolam 0.5 mg/kg (Group M) or remifentanil 1 microg/kg (Group R). Both Groups M and R showed a significantly lower incidence of myoclonus compared with Group C (17%, 17% and 77%, respectively). The incidence of myoclonus was not significantly different between Groups M and R, but 10% (n = 10) of the patients in Group R experienced remifentanil-related side-effects. We conclude that midazolam is probably a better choice than remifentanil for reducing etomidate-induced myoclonus during anaesthesia induction.

Antagonistic effects of atipamezole, flumazenil and 4-aminopyridine against anaesthesia with medetomidine, midazolam and ketamine combination in cats.

Antagonistic effects of atipamezole (ATI), flumazenil (FLU) and 4-aminopyridine (4AP) alone and in various combinations after administration of medetomidine-midazolam-ketamine (MED-MID-KET) were evaluated in cats. Animals were anaesthetised with MED (50 microg/kg), MID (0.5 mg/kg) and KET (10 mg/kg) given intramuscularly. Twenty minutes later, physiological saline, ATI (200 microg/kg), FLU (0.1 mg/kg), 4AP (0.5 mg/kg), ATI-FLU, FLU-4AP, ATI-4AP or ATI-FLU-4AP was administered intravenously. FLU, 4AP alone, or FLU-4AP did not effectively antagonise the anaesthesia, hypothermia, bradycardia, and bradypnoea induced by MED-MID-KET. ATI alone was effective. ATI-FLU, ATI-4AP and ATI-FLU-4AP combinations produced an immediate and effective recovery from anaesthesia. The combination of ATI-FLU-4AP was the most effective in antagonising the anaesthetic effects, but was associated with tachycardia, tachypnoea, excitement, and muscle tremors. Combinations with ATI are more effective for antagonising anaesthesia, but ATI-FLU-4AP is not suitable.

Characteristics of propofol-evoked vascular pain in anaesthetized rats.

BACKGROUND: In this study we have assessed vascular pain caused by the i.v. anaesthetic agent, propofol, using the flexor reflex response and compared this with that of capsaicin in anaesthetized intact rats. METHODS: Experiments were performed on 133 male Sprague-Dawley rats weighing 280-340 g. The animals were anaesthetized with urethane (1.3 g kg(-1), i.p.), and an arterial cannula was inserted to the level of the bifurcation of the femoral artery. The magnitude of the flexor reflex was examined by recording the electromyogram from the posterior biceps femoris/semitendinosus muscles. RESULTS: Our data show that the flexor reflexes evoked by intra-arterial (i.a.) injection of propofol (1%, 25-100 microl) and capsaicin (0.05-0.2 microg) were dose dependent. An initial i.a. injection of procaine (2%, 200 microl) blocked both responses. Furthermore, the flexor reflex induced by these chemical stimuli were inhibited by morphine (5 mg kg(-1), s.c.) and restored with naloxone (1.5 mg kg(-1), s.c.). Pre-treatment with capsazepine (20 microg, i.a.), a selective VR1 antagonist, inhibited the capsaicin-evoked response, but not that of propofol. Indomethacin (10 mg kg(-1), i.p.), a non-selective cyclo-oxygenase inhibitor, inhibited only the propofol-evoked response and this recovered with arterial PGE2 (5 microg). CONCLUSIONS: Collectively our data suggest that propofol-evoked vascular pain is mainly initiated by prostanoids.

Pharmacological properties of GABAA receptors in rat hypothalamic neurons expressing the epsilon-subunit.

The pharmacological properties and functional role of native GABA(A) receptors (GABA(A)Rs) were investigated in rat hypothalamic neurons expressing the epsilon-subunit with the help of whole-cell patch-clamp recording and single-cell reverse transcription-PCR. Two cell groups were identified: histaminergic tuberomamillary and orexinergic/hypocretinergic neurons. Approximately 25% of histaminergic and 70% of orexinergic neurons contained mRNA encoding for the epsilon-subunit. Double-immunofluorescence staining revealed a somatic localization of this protein in these two neuronal groups. Constitutive activity, diazepam modulation, fast desensitization of maximal currents, and activation by propofol (6-98 microm) of GABA(A)Rs did not correlate with epsilon-subunit expression. Propofol at 3-12 microm potentiated GABA-mediated currents similarly in all neurons. However, noise variance analysis of GABA-mediated currents enhanced by propofol revealed a significant difference between epsilon-positive and epsilon-negative neurons. The former displayed no difference between control and potentiated responses, and, in the latter, noise was decreased in the presence of propofol. Spontaneous IPSCs recorded in cultured hypothalamic neurons were prolonged in the presence of propofol in all epsilon-negative neurons, whereas propofol-resistant IPSCs were recorded in epsilon-positive cells. The infrequent expression of the epsilon-subunit may be a key factor in the recently discovered central role of the tuberomamillary nucleus in anesthesia.

Mechanisms underlying the sympathomimetic cardiovascular responses elicited by gamma-hydroxybutyrate.

Gamma-hydroxybutyrate (GHB) is generally thought to be a central nervous system depressant; however, GHB also has sympathomimetic cardiovascular actions. Radio telemetry was used to record the cardiovascular responses elicited by GHB (180-1000 mg/kg IV) in conscious rats. GHB elicited increases in mean arterial pressure (MAP) (24 +/- 3 to 60 +/- 5 mm Hg) lasting from 28 +/- 8 to 227 +/- 37 minutes. GHB (560 and 1000 mg/kg IV) also elicited a prolonged tachycardic response (85 +/- 23 and 95 +/- 22 bpm). The hypertension and tachycardia elicited by GHB (560 mg/kg) were reversed by the intravenous and intracerebroventricular administration of the GABAb receptor antagonist CGP 35348. CGP 35348 also reversed GHB-mediated increases in renal sympathetic nerve activity (RSNA). Administration of the purported GHB receptor antagonist NCS-382 reversed the increase in heart rate but not the pressor response elicited by GHB in telemetered rats. These data indicate that the intravenous administration of GHB markedly increases MAP, heart rate, and RSNA in conscious rats via activation of central GABAb receptors. In addition, GHB receptors appear to selectively mediate the increase in heart rate elicited by large doses of GHB.

Protection by the GABAB receptor antagonist, SCH 50911, of gamma-hydroxybutyric acid-induced mortality in mice.

Different effects of moderate to high doses of gamma-hydroxybutyric acid, including sedation/hypnosis, have been found to be blocked by gamma-aminobutyric acidB (GABAB) receptor antagonists. The present study investigated whether the protective effect of GABAB receptor antagonists extends also to gamma-hydroxybutyric acid-induced mortality. To this aim, the present study investigated the effect of the GABAB receptor antagonist, (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911; 100 mg/kg, ip), on mortality induced by gamma-hydroxybutyric acid (1-6 g/kg, ip) in DBA mice. Pretreatment with SCH 50911 resulted in a significant shift to the right of the dose-response curve of gamma-hydroxybutyric acid-induced mortality. Accordingly, the LD50 in SCH 50911-pretreated mice was significantly higher than that obtained in water-pretreated mice. The results of the present study support the hypothesis that (a) the GABAB receptor is a relevant site of action of gamma-hydroxybutyric acid, and (b) GABAB receptor antagonists may constitute potentially effective therapeutic interventions for gamma-hydroxybutyric acid intoxication.

The anticonvulsant action of propofol on epileptiform activity in rat hippocampal slices.

We used extracellular electrophysiological recordings from the CA1 region in rat hippocampal slices to investigate the effects of propofol on the field excitatory postsynaptic potential (fEPSP), population spike, and epileptiform activity induced by a Mg(2+)-free condition. Propofol depressed the population spike, fEPSP, and epileptiform activity. Both aminophylline, a nonselective adenosine receptor antagonist, and 8-cyclopentyl-1,3-dipropylxanthine, an A(1) receptor antagonist, significantly reduced the effect of propofol on fEPSP amplitude. However, 3,7-dimethyl-1-propagylxanthine, an A(2) receptor antagonist, did not alter the effect of propofol on fEPSP amplitude. Picrotoxin, a specific chloride channel blocker, partly reduced the effect of propofol on epileptiform activity, but bicuculline, a competitive gamma-aminobutyric acid(A) receptor antagonist, failed to antagonize it. Aminophylline significantly reduced the action of propofol on the epileptiform activity. The anticonvulsant action of propofol was partly reduced by 8-cyclopentyl-1,3-dipropylxanthine, whereas 3,7-dimethyl-1-propagylxanthine failed to affect it. Adenosine depressed the amplitude of fEPSPs in a dose-dependent manner, and propofol enhanced this inhibition. The results demonstrated that, in rat hippocampal slices, propofol inhibits epileptiform activity. In addition, adenosine neuromodulation through the A(1) receptor may contribute to the anticonvulsant action of propofol.