The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat

Membrane cholesterol dependence of cannabinoid modulation of glycine receptor

Cannabinoids exert therapeutic effects on several diseases such as chronic pain and startle disease by targeting glycine receptors (GlyRs). Our previous studies have shown that cannabinoids target a serine residue at position 296 in the third transmembrane helix of the α1/α3 GlyR. This site is located on the outside of the ion channel protein at the lipid interface where the cholesterol concentrates. However, whether membrane cholesterol regulates cannabinoid-GlyR interaction remains unknown. Here, we show that GlyRs are closely associated with cholesterol/caveolin-rich domains at subcellular levels. Membrane cholesterol reduction significantly inhibits cannabinoid potentiation of glycine-activated currents in cultured spinal neurons and in HEK 293T cells expressing α1/α3 GlyRs. Such inhibition is fully rescued by cholesterol replenishment in a concentration-dependent manner. Molecular docking calculations further reveal that cholesterol regulates cannabinoid enhancement of GlyR function through both direct and indirect mechanisms. Taken together, these findings suggest that cholesterol is critical for the cannabinoid-GlyR interaction in the cell membrane.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Propofol Inhibits Cerebellar Parallel Fiber-Purkinje Cell Synaptic Transmission via Activation of Presynaptic GABAB Receptors in vitro in Mice

Propofol is a widely used intravenous sedative-hypnotic agent, which causes rapid and reliable loss of consciousness via activation of γ -aminobutyric acid A (GABA_A) receptors. We previously found that propofol inhibited cerebellar Purkinje cells (PC) activity via both GABA_A and glycine receptors in vivo in mice. We here examined the effect of propofol on the cerebellar parallel fiber (PF)-PC synaptic transmission in mouse cerebellar slices by whole-cell recording technique and pharmacological methods. We found that following blockade of GABA_A and glycine receptors activity, propofol reversely decreased the amplitude of PF-PC excitatory postsynaptic currents (PF-PC EPSCs), and significantly increased paired-pulse ratio (PPR). The propofol-induced decrease in amplitude of PF-PC EPSCs was concentration-dependent. The half-inhibitory concentration (IC₅₀) of propofol for inhibiting PF-PC EPSCs was 4.7 μM. Notably, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by GABA_B receptor antagonist, saclofen (10 μM), but not blocked by N-methyl-D-aspartate receptor (NMDA) receptor antagonist, D-APV (50 μM). Application of the GABA_B receptor agonist baclofen induced a decrease in amplitude and an increase in PPR of PF-PC EPSCs, as well masked the propofol-induced changes in PF-PC EPSCs. Moreover, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by a specific protein kinase A (PKA) inhibitor, KT5720. These results indicate that application of propofol facilitates presynaptic GABA_B receptors, resulting in a depression of PF-PC synaptic transmission via PKA signaling pathway in mouse cerebellar cortex. The results suggest that the interaction with GABA_B receptors may contribute to the general anesthetic action of propofol.

Gastrointestinal delivery of propofol from fospropofol: its bioavailability and activity in rodents and human volunteers

BACKGROUND

Propofol is a safe and widely used intravenous anesthetic agent, for which additional clinical uses including treatment of migraine, nausea, pain and anxiety have been proposed (Vasileiou et al. Eur J Pharmacol 605:1-8, 2009). However, propofol suffers from several disadvantages as a therapeutic outside anesthesia including its limited aqueous solubility and negligible oral bioavailability. The purpose of the studies described here was to evaluate, in both animals and human volunteers, whether fospropofol (a water soluble phosphate ester prodrug of propofol) would provide higher propofol bioavailability through non-intravenous routes.

METHODS

Fospropofol was administered via intravenous, oral and intraduodenal routes to rats. Pharmacokinetic and pharmacodynamic parameters were then evaluated. Based on the promising animal data we subsequently conducted an oral and intraduodenal pharmacokinetic/pharmacodynamic study in human volunteers.

RESULTS

In rats, bioavailability of propofol from fospropofol delivered orally was found to be appreciable, in the order of around 20-70%, depending on dose. Availability was especially marked following fospropofol administration via the intraduodenal route, where bioavailability approximated 100%. Fospropofol itself was not appreciably bioavailable when administered by any route except for intravenous. Pharmacologic effect following oral fospropofol was confirmed by observation of sedation and alleviation of thermal hyperalgesia in the rat chronic constrictive injury model of neuropathic pain. The human data also showed systemic availability of propofol from fospropofol administration via oral routes, a hereto novel finding. Assessment of sedation in human volunteers was correlated with pharmacokinetic measurements.

CONCLUSIONS

These data suggest potential utility of oral administration of fospropofol for various therapeutic indications previously considered for propofol.

Central depressant effects and toxicity of propofol in chicks

Propofol is an ultra-short acting anesthetic agent. The information on the pharmacological and toxicological effects of propofol in the chicken is rather limited. This study examines the toxicity and pharmaco-behavioral effects of propofol given intraperitoneally in 7–10 day-old chicks. The median effective doses of propofol for the induction of sedation, analgesia to electric stimulation and sleep in the chicks were 1.82, 2.21 and 5.71 mg/kg, respectively. The 24-h median lethal dose of propofol in chicks was 57.22 mg/kg. The therapeutic indices of propofol for sedation, analgesia and sleep were 31.4, 25.9 and 10, respectively. Propofol at 0.5 and 1 mg/kg reduced the locomotor activity and increased the duration of tonic immobility in chicks. Propofol at 2 and 4 mg/kg caused analgesia to electric stimulation as well as analgesia and anti-inflammatory responses against formalin test in chicks. Propofol at 5, 10 and 20 mg/kg induced sleep in chicks for 8.4 to 25 min. Physostigmine shortened the sleep duration of propofol. Data suggest that propofol induces anti-inflammatory action and central nervous system depression in chicks resulting in sedation, analgesia and anesthesia with wide safety margin. These effects could form the basis of further pharmacological and toxicological studies on propofol in the young chick model, and the drug could be safely applied clinically in the chicken.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Potentiating action of propofol at GABAA receptors of retinal bipolar cells

PURPOSE

Propofol (2,6-diisopropyl phenol), a widely used systemic anesthetic, is known to potentiate GABA(A) receptor activity in a number of CNS neurons and to produce changes in electroretinographically recorded responses of the retina. However, little is known about propofol's effects on specific retinal neurons. The authors investigated the action of propofol on GABA-elicited membrane current responses of retinal bipolar cells, which have both GABA(A) and GABA(C) receptors.

METHODS

Single, enzymatically dissociated bipolar cells obtained from rat retina were treated with propofol delivered by brief application in combination with GABA or other pharmacologic agents or as a component of the superfusing medium.

RESULTS

When applied with GABA at subsaturating concentrations and with TPMPA (a known GABA(C) antagonist), propofol markedly increased the peak amplitude and altered the kinetics of the response. Propofol increased the response elicited by THIP (a GABA(A)-selective agonist), and the response was reduced by bicuculline (a GABA(A) antagonist). The response to 5-methyl I4AA, a GABA(C)-selective agonist, was not enhanced by propofol. Serial brief applications of (GABA + TPMPA + propofol) led to a progressive increase in peak response amplitude and, at higher propofol concentrations, additional changes that included a prolonged time course of response recovery. Pre-exposure of the cell to perfusing propofol typically enhanced the rate of development of potentiation produced by (GABA + TPMPA + propofol) applications.

CONCLUSIONS

Propofol exerts a marked and selective potentiation on GABA(A) receptors of retinal bipolar cells. The data encourage the use of propofol in future studies of bipolar cell function.

Comparative molecular field analysis and synthetic validation of a hydroxyamide-propofol binding and functional block of neuronal voltage-dependent sodium channels

Voltage gated sodium channels represent an important therapeutic target for a number of neurological disorders including epilepsy. Unfortunately, medicinal chemistry strategies for discovering new classes of antagonist for trans-membrane ion channels have been limited to mostly broad screening compound arrays. We have developed new sodium channel antagonist based on a propofol scaffold using the ligand based strategy of comparative molecular field analysis (CoMFA). The resulting CoMFA model was correlated and validated to provide insights into the design of new antagonists and to prioritize synthesis of these new structural analogs (compounds 4 and 5) that satisfied the steric and electrostatic model. Compounds 4 and 5 were evaluated for [(3)H]-batrachotoxinin-A-20-alpha-benzoate ([(3)H]-BTX-B) displacement yielding IC(50)'s of 22 and 5.7 microM, respectively. We further examined the potency of these two compounds to inhibit neuronal sodium currents recorded from cultured hippocampal neurons. At a concentration of 50 microM, compounds 4 and 5 tonically inhibited sodium channels currents by 59+/-7.8% (n=5) and 70+/-7.5% (n=7), respectively. This clearly demonstrates that these compounds functionally antagonize native neuronal sodium channel currents. In summary, we have shown that CoMFA can be effectively used to discover new classes of sodium channel antagonists.

Increased phosphorylation of extracellular signal-regulated kinase in trigeminal nociceptive neurons following propofol administration in rats

Although propofol (PRO) is widely used in clinic as a hypnotic agent, the underlying mechanisms of its action on pain pathways is still unknown. Sprague-Dawley rats were assigned to receive PRO or pentobarbital (PEN) and were divided into 2 groups as LIGHT and DEEP hypnotic levels based on the EEG analysis. Rats in each hypnotic level received capsaicin injection into the face and phosphorylated extracellular signal-regulated kinase (pERK) immunohistochemistry was performed in subnucleus caudalis (Vc) and upper cervical spinal cord. In the rats with PEN or PRO administration, a large number of pERK-like immunoreactive (LI) cells was observed in the trigeminal spinal subnuclei interpolaris and caudalis transition zone (Vi/Vc), middle Vc, and transition zone between Vc and upper cervical spinal cord (Vc/C2) following capsaicin injection into the whisker-pad region. The number of pERK-LI cells in Vi/Vc, middle Vc, and Vc/C2 was significantly larger in rats with PRO infusion than those with PEN infusion. The number of pERK-LI cells was increased following an increase in the dose of PRO but not in PEN. The pERK-LI cells were mainly distributed in the Vi/Vc, middle Vc, and Vc/C2 after the bolus infusion of PRO. The expression of pERK-LI cells was depressed after the intravenous lidocaine application before bolus PRO infusion. The present findings suggest that PRO induced an enhancement of the activity of trigeminal nociceptive pathways through nociceptors innervating the venous structure, as indicated by a lidocaine-sensitive increase in pERK. This may explain deep pain around the injection regions during intravenous bolus infusion of PRO.

PERSPECTIVE

The effect of propofol administration on ERK phosphorylation in the subregions of the spinal trigeminal complex and upper cervical spinal cord neurons were precisely analyzed in rats with PRO infusion. A large number of pERK-LI cells was observed following intravenous PRO administration, suggesting an enhancement of trigeminal nociceptive activity and that PRO may produce pain through nociceptors innervating the venous structures during infusion.

Glycine receptors contribute to hypnosis induced by ethanol

BACKGROUND

Glycine is a major inhibitory neurotransmitter in the adult central nervous system (CNS), and its receptors (GlyRs) are well known for their effects in the spinal cord and the lower brainstem. Accumulating evidence indicates that GlyRs are more widely distributed in the CNS, including many supraspinal regions. Previous in vitro studies have demonstrated that ethanol potentiates the function of these brain GlyRs, yet the behavioral role of the brain GlyRs has not been well explored.

METHODS

Experiments were conducted in rats. The loss of righting reflex (LORR) was used as a marker of the hypnotic state. We compared the LORR induced by systematic administration of ethanol and of ketamine in the absence and presence of the selective glycine receptor antagonist strychnine. Ketamine is a general anesthetic that does not affect GlyRs.

RESULTS

Systemically administered (by intraperitoneal injection) ethanol and ketamine dose-dependently induced LORR in rats. Furthermore, systemically administered (by subcutaneous injection) strychnine dose-dependently reduced the percentage of rats exhibiting LORR induced by ethanol, increased the onset time, and decreased the duration of LORR. Strychnine had no effect, however, on the LORR induced by ketamine.

CONCLUSIONS

Given that hypnosis is caused by neuronal depression in upper brain areas, we therefore conclude that brain GlyRs contribute at least in part to the hypnosis induced by ethanol.

Behavior and cellular evidence for propofol-induced hypnosis involving brain glycine receptors

BACKGROUND

It is well documented that several general anesthetics, including propofol, potentiate glycine receptor function. Furthermore, glycine receptors exist throughout the central nervous system, including areas of the brain thought to be involved in sleep. However, the role of glycine receptors in anesthetic-induced hypnosis has not been determined.

METHODS

Experiments were conducted in rats where the loss of righting reflex (LORR) was used as a marker of the hypnotic state. Propofol-induced LORR was examined in the presence and absence of strychnine (a glycine receptor antagonist), GABAzine (a gamma-aminobutyric acid A receptor antagonist), as well as ketamine (an antagonist of N-methyl-D-aspartic acid subtype of glutamate receptors). Furthermore, the effects of propofol on the currents elicited by glycine and gamma-aminobutyric acid were analyzed in neurons isolated from the posterior hypothalamus of rats. The effects of strychnine and GABAzine on propofol-induced currents were also evaluated.

RESULTS

Strychnine and GABAzine dose-dependently reduced the percentage of rats exhibiting LORR induced by propofol. Furthermore, strychnine significantly increased the onset time and reduced the duration of LORR induced by propofol. In contrast, strychnine did not affect the LORR induced by ketamine. In addition, propofol markedly increased the currents elicited by glycine and GABA of hypothalamic neurons. Conversely, strychnine and GABAzine both profoundly attenuated the current induced by propofol.

CONCLUSION

Strychnine, the glycine receptor antagonist, dose-dependently reduced propofol-induced LORR in rats and propofol-induced current of rat hypothalamic neurons. These results suggest that neuronal glycine receptors partially contribute to propofol-induced hypnosis.

Effects of general anesthetics on visceral pain transmission in the spinal cord

Current evidence suggests an analgesic role for the spinal cord action of general anesthetics; however, the cellular population and intracellular mechanisms underlying anti-visceral pain by general anesthetics still remain unclear. It is known that visceral nociceptive signals are transmited via post-synaptic dorsal column (PSDC) and spinothalamic tract (STT) neuronal pathways and that the PSDC pathway plays a major role in visceral nociception. Animal studies report that persistent changes including nociception-associated molecular expression (e.g. neurokinin-1 (NK-1) receptors) and activation of signal transduction cascades (such as the protein kinase A [PKA]-c-AMP-responsive element binding [CREB] cascade)-in spinal PSDC neurons are observed following visceral pain stimulation. The clinical practice of interruption of the spinal PSDC pathway in patients with cancer pain further supports a role of this group of neurons in the development and maintenance of visceral pain. We propose the hypothesis that general anesthetics might affect critical molecular targets such as NK-1 and glutamate receptors, as well as intracellular signaling by CaM kinase II, protein kinase C (PKC), PKA, and MAP kinase cascades in PSDC neurons, which contribute to the neurotransmission of visceral pain signaling. This would help elucidate the mechanism of antivisceral nociception by general anesthetics at the cellular and molecular levels and aid in development of novel therapeutic strategies to improve clinical management of visceral pain.

Modulation of neuronal activity in CNS pain pathways following propofol administration in rats: Fos and EEG analysis

We studied Fos expression in the central nociceptive pathways at different sedative levels in order to clarify the central mechanism of propofol's nociceptive action. Sprague-Dawley rats received propofol (PRO) or pentobarbital (PEN) and were divided into two groups with different doses of drug administration (light and deep sedative levels) based on the electroencephalogram analysis. Rats at each sedative level received heat stimulation to their face and Fos immunohistochemistry was performed at various brain sites. We also infused lidocaine into the jugular vein to test whether PRO directly activated nociceptors distributed in the vein. Fos expression in two major ascending pain pathways (lateral and medial systems) and descending modulatory system were precisely analyzed following intravenous (i.v.) administration of PRO or PEN. Many Fos protein-like immunoreactive (Fos protein-LI) cells were expressed in the trigeminal spinal nucleus caudalis (Vc), parabrachial nucleus, parafascicular nucleus, a wide area of the primary somatosensory cortex, anterior cingulate cortex, amygdala, periaqueductal gray, solitary tract nucleus, and lateral hypothalamus following heating of the face during PRO or PEN infusion. The number of Fos protein-LI cells was significantly greater in many Central nervous system regions during PRO infusion compared with PEN. Fos expression was significantly greater in the Vc and Periaqueductal gray following greater amount of PRO infusions compared, whereas they were significantly smaller in the Vc in the rats with PEN infusion. The Fos expression was significantly depressed following i.v. infusion of lidocaine before PRO administration. The present findings suggest that PRO is involved in the enhancement of Vc activity through direct activation of the primary afferent fibers innervating veins, resulting in pain induction during infusion.

Caudal 0.2% ropivacaine is less effective during surgery than 0.2% levobupivacaine and 0.2% bupivacaine: a double-blind, randomized, controlled trial

BACKGROUND

We hypothesized that without the analgesic effects of volatile anesthetics, caudal 0.20% ropivacaine would be less effective during surgical stimulation than 0.20% bupivacaine or 0.20% levobupivacaine. This trial was designed to examine whether the combination of a caudal block with 0.20% ropivacaine and i.v. anesthesia resulted in reduced analgesic efficacy during surgery compared with caudal 0.20% levobupivacaine or 0.20% bupivacaine in children.

METHODS

Ninety ASA I-II children between 1 and 7 years old, scheduled for inguinal hernia repair or orchidopexy under propofol anesthesia were randomized to receive a caudal block with 1 ml x kg(-1) of 0.2% bupivacaine, 0.2% ropivacaine or 0.2% levobupivacaine. The primary outcome measure of the study was the clinical efficacy of the caudal block during surgery. Secondary outcome measures were postoperative pain relief and residual motor blockade.

RESULTS

Four children were excluded and 86 were analyzed. The proportion of children with effective caudal block during surgery was significantly higher in children receiving levobupivacaine (26/28) or bupivacaine (27/29) compared with patients receiving ropivacaine (21/29) (P = 0.03). There were no significant differences among groups in the analgesic onset time (P = 0.1), incidence of residual motor blockade (P = 0.4), number of patients requiring analgesia after operation or in the time from caudal injection to the first administration of analgesic medication (P = 0.3).

CONCLUSIONS

Combined with propofol anesthesia, 0.2% levobupivacaine and 0.2% bupivacaine are more effective than 0.2% ropivacaine for caudal use in children undergoing inguinal hernia repair or orchidopexy.

Structural features of phenol derivatives determining potency for activation of chloride currents via alpha(1) homomeric and alpha(1)beta heteromeric glycine receptors

Phenol derivatives constitute a family of neuroactive compounds. The aim of our study was to identify structural features that determine their modulatory effects at glycine receptors. We investigated the effects of four methylated phenol derivatives and two halogenated analogues on chloride inward currents via rat alpha(1) and alpha(1)beta glycine receptors, heterologously expressed in HEK 293. All compounds potentiated the effect of a submaximal glycine concentration in both alpha(1) homomeric and alpha(1)beta glycine receptors. While the degree of maximum potentiation of the glycine 10 microM effect in alpha(1)beta receptors was not different between the compounds, the halogenated compounds achieved half-maximum potentiating effects in the low microM range -- at more than 20-fold lower concentrations compared with their nonhalogenated analogues (P<0.0001). The coactivating effect was over-ridden by inhibitory effects at concentrations >300 microM in the halogenated compounds. Neither the number nor the position of the methyl groups significantly affected the EC(50) for coactivation. Only the bimethylated compounds 2,6 and 3,5 dimethylphenol (at concentrations >1000 microM) directly activated both alpha(1) and alpha(1)beta receptors up to 30% of the maximum response evoked by 1000 microM glycine. These results show that halogenation in the para position is a crucial structural feature for the potency of a phenolic compound to positively modulate glycine receptor function, while direct activation is only seen with high concentrations of compounds that carry at least two methyl groups. The presence of the beta subunit is not required for both effects.

2,6 di-tert-butylphenol, a nonanesthetic propofol analog, modulates alpha1beta glycine receptor function in a manner distinct from propofol

The anesthetic propofol (2,6 diisopropylphenol) mediates some of its effects by activating inhibitory chloride currents in the lower brainstem and spinal cord. The effects comprise direct activation of gamma-aminobutyric acid-A and glycine receptors in the absence of the natural agonist, as well as potentiation of the effect of submaximal agonist concentrations. Replacement of propofol's isopropyl groups by di-tert-butyl groups yields a compound without in vivo anesthetic effects. We have studied the effects of propofol and 2,6 di-tert-butylphenol on chloride inward currents via rat alpha1beta glycine receptors heterologously expressed in human embryonic kidney cells. Propofol, but not 2,6 di-tert-butylphenol, directly activated glycine receptors; half-maximal current activation was observed with propofol 114 +/- 27 microM. Both compounds potentiated the effect of a submaximal glycine concentration (10 microM) to a maximum value of 136% +/- 71% (propofol) and 279% +/- 109% (2,6 di-tert-butylphenol) of the response to glycine 10 microM. The 50% effective concentration for this effect was 12.5 +/- 6.4 microM and 9.4 +/- 10.2 microM for propofol and 2,6 di-tert-butylphenol, respectively. Propofol and its nonanesthetic structural analog do not differ in their ability to coactivate the glycine receptor but differ in their ability to directly activate the receptor in the absence of the natural agonist.

Intrathecal Baclofen for the Treatment of Tetanus

Tetanus remains a serious problem in public health, particularly in developing countries, despite efficient prevention programs. A retrospective study was conducted at an infectious diseases intensive care unit during 1998-2003 involving patients admitted with grade III tetanus. The aim of the study was to evaluate the efficacy and safety of intrathecal baclofen for the treatment of tetanus. Lumbar puncture was performed, and a subarachnoid catheter was inserted for drug administration. An intrathecal bolus of baclofen was followed by a continuous infusion of 20 microg/h, until a maximum daily dose of 2 mg was provided. Twenty-two patients were treated overall. Control of the symptoms was achieved in all patients but one. Seven patients had colonization of the catheter, and 1 patient developed meningitis. All patients except one recovered. In our study, this means of treatment was efficacious and well tolerated.

Propofol Suppresses the Cortical Somatosensory Evoked Potential in Rats

The dose-response curve for the effect of volatile anesthetics on the somatosensory evoked potential (SEP) is well described, but for propofol, the large dose segment of the curve is undefined. We describe the effect of increasing plasma concentrations of propofol on cortical SEPs in 18 rats. After surgical preparation under ketamine anesthesia, a remifentanil infusion was begun at 2.5, 5, or 10 microg x kg(-1) x min(-1). After 20 min, the propofol infusion was initiated at 20 mg x kg(-1) x h(-1) and was increased to 40, 60, and 80 mg x kg(-1) x h(-1) at 20-min intervals. SEP was recorded before remifentanil infusion, before propofol infusion rate changes, and 30 min after discontinuing propofol infusion. In six additional rats, the plasma concentrations of propofol after each 20-min infusion were measured using gas chromatography. Remifentanil did not have a significant effect, but propofol significantly depressed the SEP amplitude and prolonged the latency at infusion rates of 40 mg x kg(-1) x h(-1) and more. Propofol's effect was dose-dependent, but even at 80 mg x kg(-1) x h(-1) with an estimated plasma concentration of 31.6 +/- 3.4 microg/mL (10.8 50% effective concentration), a measurable response was present in 44.5% of rats. These results suggest that even at large doses, propofol and remifentanil provide adequate conditions for SEP monitoring.

IMPLICATIONS

Rats demonstrate dose-dependent somatosensory evoked potential (SEP) suppression with propofol but not with remifentanil. However, SEP suppression by 50% occurred only at large (1.5 EC(50)) concentrations of propofol, and a measurable SEP was present in 8 of 18 rats, even at 10.8 EC(50).