Inhibition of 5-HT3 receptors by propofol: equilibrium and kinetic measurements

Anti-emetic effects of thalidomide: Evidence, mechanism of action, and future directions

The rationale for using thalidomide (THD) as a treatment for nausea and vomiting during pregnancy in the late 1950s appears to have been based on its sedative or hypnotic properties. In contrast to contemporaneous studies on the anti-emetic activity of phenothiazines, we were unable to identify publications reporting preclinical or clinical evaluation of THD as an anti-emetic. Our survey of the literature revealed a clinical study in 1965 showing THD reduced vomiting in cancer chemotherapy which was substantiated by similar studies from 2000, particularly showing efficacy in the delayed phase of chemotherapy-induced nausea and vomiting. To identify the mechanism(s) potentially involved in thalidomide's anti-emetic activity we reviewed its pharmacology in the light of nausea and vomiting mechanisms and their pharmacology with a particular emphasis on chemotherapy and pregnancy. The process identified the following potential mechanisms: reduced secretion of Growth Differentiation Factor 15, suppression of inflammation/prostaglandin production, downregulation of cytotoxic drug induced upregulation of iNOS, and modulation of BK (KCa1.1) channels and GABAA/glutamate transmission at critical points in the emetic pathways (nucleus tractus solitarius, area postrema). We propose ways to investigate these hypothesized mechanisms and discuss the associated challenges (e.g., objective quantification of nausea) in addition to some of the more general aspects of developing novel drugs to treat nausea and vomiting.

Natural Negative Allosteric Modulators of 5-HT₃ Receptors

Chemotherapy-induced nausea and vomiting (CINV) remain the most common and devastating side-effects associated with cancer chemotherapy. In recent decades, several lines of research emphasize the importance of 5-hydroxytryptamine3 (5-HT3; serotonin) receptors in the pathogenesis and treatment of CINV. 5-HT₃ receptors are members of ligand-gated ion channels that mediate the rapid and transient membrane-depolarizing effect of 5-HT in the central and peripheral nervous system. These receptors play important roles in nausea and vomiting, as well as regulation of peristalsis and pain transmission. The development of antagonists for 5-HT₃ receptor dramatically improved the treatment of CINV in cancer patients. In fact, the most common use of 5-HT₃ receptor antagonists to date is the treatment of nausea and vomiting. In recent years, there has been an increasing tendency to use natural plant products as important therapeutic entities in the treatment of various diseases. In this article, we examined the results of earlier studies on the actions of natural compounds on the functional properties of 5-HT₃ receptors. It is likely that these natural modulators of 5-HT₃ receptors can be employed as lead structures for the synthesis of therapeutic agents for treating CINV in future clinical studies.

Noncompetitive Inhibition of 5-HT3 Receptors by Citral, Linalool, and Eucalyptol Revealed by Nonlinear Mixed-Effects Modeling

Citral, eucalyptol, and linalool are widely used as flavorings, fragrances, and cosmetics. Here, we examined their effects on electrophysiological and binding properties of human 5-HT₃ receptors expressed in Xenopus oocytes and human embryonic kidney 293 cells, respectively. Data were analyzed using nonlinear mixed-effects modeling to account for random variance in the peak current response between oocytes. The oils caused an insurmountable inhibition of 5‐HT–evoked currents (citral IC₅₀ = 120 µM; eucalyptol = 258 µM; linalool = 141 µM) and did not compete with fluorescently labeled granisetron, suggesting a noncompetitive mechanism of action. Inhibition was not use‐dependent but required a 30-second preapplication. Compound washout caused a slow (∼180 seconds) but complete recovery. Coapplication of the oils with bilobalide or diltiazem indicated they did not bind at the same locations as these channel blockers. Homology modeling and ligand docking predicted binding to a transmembrane cavity at the interface of adjacent subunits. Liquid chromatography coupled to mass spectrometry showed that an essential oil extracted from Lippia alba contained 75.9% citral. This inhibited expressed 5‐HT₃ receptors (IC₅₀ = 45 µg ml⁻¹) and smooth muscle contractions in rat trachea (IC₅₀ = 200 µg ml⁻¹) and guinea pig ileum (IC₅₀ = 20 µg ml⁻¹), providing a possible mechanistic explanation for why this oil has been used to treat gastrointestinal and respiratory ailments. These results demonstrate that citral, eucalyptol, and linalool inhibit 5-HT₃ receptors, and their binding to a conserved cavity suggests a valuable target for novel allosteric modulators.

Identification of Glycyrrhiza as the rikkunshito constituent with the highest antagonistic potential on heterologously expressed 5-HT3A receptors due to the action of flavonoids

The traditional Japanese phytomedicine rikkunshito is traditionally used for the treatment of gastrointestinal motility disorders, cachexia and nausea. These effects indicate 5-HT₃ receptor antagonism, due to the involvement of these receptors in such pathophysiological processes. E.g., setrons, specific 5-HT₃ receptor antagonists are the strongest antiemetics, developed so far. Therefore, the antagonistic effects of the eight rikkunshito constituents at heterologously expressed 5-HT_3Areceptors were analyzed using the two-electrode voltage-clamp technique. The results indicate that tinctures from Aurantii, Ginseng, Zingiberis, Atractylodis and Glycyrrhiza inhibited the 5-HT_3A receptor response, whereas the tinctures of Poria cocos, Jujubae and Pinellia exhibited no effect. Surprisingly, the strongest antagonism was found for Glycyrrhiza, whereas the Zingiberis tincture, which is considered to be primarily responsible for the effect of rikkunshito, exhibited the weakest antagonism of 5-HT_3A receptors. Rikkunshito contains various vanilloids, ginsenosides and flavonoids, a portion of which show an antagonistic effect on 5-HT₃ receptors. A screening of the established ingredients of the active rikkunshito constituents and related substances lead to the identification of new antagonists within the class of flavonoids. The flavonoids (-)-liquiritigenin, glabridin and licochalcone A from Glycyrrhiza species were found to be the most effective inhibitors of the 5-HT-induced currents in the screening. The flavonoids (-)-liquiritigenin and hesperetin from Aurantii inhibited the receptor response in a non-competitive manner, whereas glabridin and licochalcone A exhibited a potential competitive antagonism. Furthermore, licochalcone A acts as a partial antagonist of 5-HT_3A receptors. Thus, this study reveals new 5-HT_3A receptor antagonists with the aid of increasing the comprehension of the complex effects of rikkunshito.

Fast and slow interactions of n-alkanols with human 5-HT3A receptors: Implications for anesthetic mechanisms

This is part of a continuing patch-clamp study exploring molecular actions of anesthetics and systematically varied related substances on 5-HT3A receptors as prototypes of ligand-gated ion channels. Specifically, n-alkanols, related to but simpler in structure than propofol, were studied to explore the complex actions of this leading intravenous anesthetic. Outside-out patches excised from HEK 293 cells heterologously expressing human 5-HT3A receptors were superfused with even-numbered n-alkanols (ethanol through n-tetradecanol) of different concentrations. Fast solution exchange for varying durations allowed separation of drug actions by their kinetics. Compared with propofol the electrophysiological responses to n-alkanols were not much simpler. n-Alkanols produced fast and slow inhibition or potentiation of current amplitudes, and acceleration of current rise and decay time constants, depending on exposure time, concentration, and chain-length of the drug. Inhibition dominated, characterized by fast and slow processes with time constants separated by two orders of magnitude which were similar for different n-alkanols and for propofol. Absolute interaction energies for ethanol to n-dodecanol (relative to xenon) ranged from -10.8 to -37.3kJmol(-1). No two n-alkanols act completely alike. Potency increases with chain length (until cutoff) mainly because of methylene groups interacting with protein sites rather than because of their tendency to escape from the aqueous phase. Similar wash-in time constants for n-alkanols and propofol suggest similar mechanisms, dominated by the kinetics of conformational state changes rather than by binding reactions.

Activation of human 5-hydroxytryptamine type 3 receptors via an allosteric transmembrane site

In common with other members of the Cys-loop family of pentameric ligand-gated ion channels, 5-hydroxytryptamine type 3 receptors (5-HT3Rs) are activated by the binding of a neurotransmitter to an extracellular orthosteric site, located at the interface of two adjacent receptor subunits. In addition, a variety of compounds have been identified that modulate agonist-evoked responses of 5-HT3Rs, and other Cys-loop receptors, by binding to distinct allosteric sites. In this study, we examined the pharmacological effects of a group of monoterpene compounds on recombinant 5-HT3Rs expressed in Xenopus oocytes. Two phenolic monoterpenes (carvacrol and thymol) display allosteric agonist activity on human homomeric 5-HT3ARs (64 ± 7% and 80 ± 4% of the maximum response evoked by the endogenous orthosteric agonist 5-HT, respectively). In addition, at lower concentrations, where agonist effects are less apparent, carvacrol and thymol act as potentiators of responses evoked by submaximal concentrations of 5-HT. By contrast, carvacrol and thymol have no agonist or potentiating activity on the closely related mouse 5-HT3ARs. Using subunit chimeras containing regions of the human and mouse 5-HT3A subunits, and by use of site-directed mutagenesis, we have identified transmembrane amino acids that either abolish the agonist activity of carvacrol and thymol on human 5-HT3ARs or are able to confer this property on mouse 5-HT3ARs. By contrast, these mutations have no significant effect on orthosteric activation of 5-HT3ARs by 5-HT. We conclude that 5-HT3ARs can be activated by the binding of ligands to an allosteric transmembrane site, a conclusion that is supported by computer docking studies.

Molecular interactions between general anesthetics and the 5HT2B receptor

BACKGROUND

Serotonin modulates many processes through a family of seven serotonin receptors. However, no studies have screened for interactions between general anesthetics currently in clinical use and serotonergic G-protein-coupled receptors (GPCRs). Given that both intravenous and inhalational anesthetics have been shown to target other classes of GPCRs, we hypothesized that general anesthetics might interact directly with some serotonin receptors and thus modify their function.

METHODS

Radioligand binding assays were performed to screen serotonin receptors for interactions with propofol and isoflurane as well as for affinity determinations. Docking calculations using the crystal structure of 5-HT2B were performed to computationally confirm the binding assay results and locate anesthetic binding sites.

RESULTS

The 5-HT2B class of receptors interacted significantly with both propofol and isoflurane in the primary screen. The affinities for isoflurane and propofol were determined to be 7.78 and .95 μM, respectively, which were at or below the clinical concentrations for both anesthetics. The estimated free energy derived from docking calculations for propofol (-6.70 kcal/mol) and isoflurane (-5.10 kcal/mol) correlated with affinities from the binding assay. The anesthetics were predicted to dock at a pharmacologically relevant binding site of 5HT2B.

CONCLUSIONS

The molecular interactions between propofol and isoflurane with the 5-HT2B class of receptors were discovered and characterized. This finding implicates the serotonergic GPCRs as potential anesthetic targets.

Which agonist properties are important for the activation of 5-HT3A receptors?

PURPOSE

Why do anesthetics not activate excitatory ligand-gated ion channels such as 5-HT3 receptors in contrast to inhibitory ligand-gated ion channels? This study examines the actions of structural closely-related 5-HT derivatives and 5-HT constituent parts on 5-HT3A receptors with the aim of finding simpler if not minimal agonists and thus determining requirements for successful agonist action.

EXPERIMENTAL APPROACH

Responses to 5-HT derivatives of human 5-HT3A receptors stably expressed in HEK 293 cells have been examined with the patch-clamp technique in the outside-out configuration combined with a fast solution exchange system.

RESULTS

Phenol, pyrrole and alkyl amines, constituents of 5-HT, even at high concentrations, cannot activate 5-HT3A receptors but they can inhibit them. To date, tyramines are the smallest known agonists. However, an aromatic ring is not required for activation as acetylcholine is also an agonist of similar strength.

CONCLUSION

Simultaneous interactions of adequate strength at two separate subsites within the 5-HT binding domain appear to be essential for successful agonist function. Anesthetics either fail to achieve this or the activation they produce is so weak that it is masked by a comparatively very strong inhibition.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

[What do we know about anesthetic mechanisms?: hypnosis, unresponsiveness to surgical incision and amnesia]

Despite the increase of molecular knowledge in anesthesia research over the past decades there is still ongoing discussion about the mechanisms of anesthesia. This article focuses on presenting anesthetic sensitive ligand and voltage gated ion channels. The impact on anesthetic modulated ion channels is summarized for clinically commonly used anesthetics isoflurane, propofol and ketamine. Furthermore, the anesthetic features hypnosis, unresponsiveness to surgical incision and amnesia and their putative relevant anatomical sites in the central nervous system are briefly introduced.

Microfluidics: a new cosset for neurobiology

Recently, microfluidic systems have shown great potential in the study of molecular and cellular biology. With its excellent properties, such as miniaturization, integration and automation, to name just a few, microfluidics creates new opportunities for the spatial and temporal control of cell growth and environmental stimuli in vitro. In the field of neuroscience, microfluidic devices offer precise control of the microenvironment surrounding individual cells, and the delivery of biochemical or physical cues to neural networks or single neurons. The intent of this review is to outline recent advances in microfluidic-based applications in neurobiology, with emphasis on neuron culture, neuron manipulation, neural stem cell differentiation, neuropharmacology, neuroelectrophysiology, and neuron biosensors. It also aims to stimulate development of microfluidic-based applications in neurobiology by involving scientists from various disciplines, especially neurobiology and microtechnology.

Effects of midazolam on acquisition and extinction of conditioned taste aversion memory in rats

Some intravenous anesthetic agents such as midazolam are known to induce anterograde and retrograde amnesia. We analyzed the effect of midazolam by the conditioned taste aversion (CTA) acquisition and retention. After the rats were offered 0.1% sodium saccharin (Sac) as conditioned stimulus (CS), an intraperitoneal (i.p.) injection of several concentrations (5-30mg/kg) of midazolam was followed by an i.p. injection of 0.15M LiCl (2% of body weight) as unconditioned stimulus (US). The rats, which acquired CTA by every CS-US paradigm, strongly avoided Sac on the 1st test day after conditioning and maintained the avoidance for 3 days. We have already reported that Sac intake abruptly increased on the 2nd test day and the almost complete extinction occurred on the 3rd test day after conditioning by injection of subhypnotic dose of propofol before LiCl-injection. In contrast, we found that subhypnotic dose of midazolam suppressed not only CTA acquisition, but also CTA retention. On the other hand, an alpha2-adrenergic blocker, yohimbin (1mg/kg) suppressed only the CTA retention. These results suggest that the subhypnotic doses of midazolam firstly affect the acquisition mechanism of the CTA memory (CTAM), resulting the suppression of the retention of CTAM.

Antimalarial drugs inhibit human 5-HT(3) and GABA(A) but not GABA(C) receptors

BACKGROUND AND PURPOSE

Antimalarial compounds have been previously shown to inhibit rodent nicotinic acetylcholine (nACh) and 5-HT(3) receptors. Here, we extend these studies to include human 5-HT(3A), 5-HT(3AB), GABA(A) alpha1beta2, GABA(A) alpha1beta2gamma2 and GABA(C) rho1 receptors.

EXPERIMENTAL APPROACH

We examined the effects of quinine, chloroquine and mefloquine on the electrophysiological properties of receptors expressed in Xenopus oocytes.

KEY RESULTS

5-HT(3A) receptor responses were inhibited by mefloquine, quinine and chloroquine with IC(50) values of 0.66, 1.06 and 24.3 microM. At 5-HT(3AB) receptors, the potencies of mefloquine (IC(50)=2.7 microM) and quinine (15.8 microM), but not chloroquine (23.6 microM), were reduced. Mefloquine, quinine and chloroquine had higher IC(50) values at GABA(A) alpha1beta2 (98.7, 0.40 and 0.46 mM, respectively) and GABA(A) alpha1beta2gamma2 receptors (0.38, 1.69 and 0.67 mM, respectively). No effect was observed at GABA(C) rho1 receptors. At all 5-HT(3) and GABA(A) receptors, chloroquine displayed competitive behaviour and mefloquine was non-competitive. Quinine was competitive at 5-HT(3A) and GABA(A) receptors, but non-competitive at 5-HT(3AB) receptors. Homology modelling in combination with automated docking suggested orientations of quinine and chloroquine at the GABA(A) receptor binding site.

CONCLUSIONS AND IMPLICATIONS

The effects of mefloquine, quinine and chloroquine are distinct at GABA(A) and GABA(C) receptors, whereas their effects on 5-HT(3AB) receptors are broadly similar to those at 5-HT(3A) receptors. IC(50) values for chloroquine and mefloquine at 5-HT(3) receptors are close to therapeutic blood concentrations required for malarial treatment, suggesting that their therapeutic use could be extended to include the treatment of 5-HT(3) receptor-related disorders.

Inhibition of human 5-HT(3A) and 5-HT(3AB) receptors by etomidate, propofol and pentobarbital

The actions of intravenous anaesthetics on 5-HT(3AB) receptors have not been studied. Using oocyte electrophysiology, the effects of etomidate, propofol, and pentobarbital on human 5-HT(3A) and 5-HT(3AB) receptors were studied and compared. Inhibition of peak currents by all three compounds in both receptor subtypes was anaesthetic concentration-dependant and non-competitive. Because the half-maximal inhibitory concentrations for etomidate, propofol and pentobarbital in 5-HT(3A) and 5-HT(3AB) receptors were all above their respective anaesthetic concentrations, the results of our study suggest that neither 5-HT(3) receptor subtype contributes to the anaesthetic actions of etomidate, propofol or pentobarbital.

Interactions of metoclopramide and ergotamine with human 5-HT(3A) receptors and human 5-HT reuptake carriers

The actions of metoclopramide and ergotamine, drugs which are used as a combined migraine medication, on human (h)5-HT3A receptors and 5-HT reuptake carriers, stably expressed in HEK-293 cells, were studied with patch-clamp- and ([3H]5-HT)-uptake techniques. At clinical concentrations, metoclopramide inhibited peak and integrated currents through h5-HT3A receptors concentration-dependently (IC50 = 0.064 and 0.076 microM, respectively) when it was applied in equilibrium (60 s before and during 5-HT (30 microM) exposure). The onset and offset time constants of metoclopramide action were 1.3 and 2.1 s, respectively. The potency of metoclopramide when exclusively applied during the agonist pulse decreased more than 200-fold (IC50 = 19.0 microM, peak current suppression). Metoclopramide (0.10 microM) did not alter the EC50 of 5-HT-induced peak currents. In contrast to the lack of competitive interaction between metoclopramide and 5-HT in this functional assay, metoclopramide inhibited specific [3H]GR65630 binding to human h5-HT3A receptors in a surmountable manner. This seeming discrepancy between functional studies and radioligand binding experiments may be accounted for by (1) the slow kinetics of inhibition of peak currents by metoclopramide compared with the fast onset and offset kinetics of 5-HT-induced currents and (2) the low efficacy of metoclopramide in inhibiting radioligand binding (e.g. only 20% binding inhibition compared to 79% peak current suppression by 200 nM metoclopramide). At low concentrations (1-10 nM), ergotamine had no effect on 5-HT (30 microM)-induced peak currents. Above clinical concentrations, ergotamine (>3 microM) inhibited them. When both drugs were applied together (0.10 microM metoclopramide +0.001 to 0.01 microM ergotamine), an inhibition of both, peak and integrated current responses was observed. Neither metoclopramide (< or =30 microM) nor ergotamine (< or =30 microM) had an effect on the 5-HT reuptake carrier as they did not alter the citalopram-sensitive [3H]5-HT uptake.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Influence of sodium substitutes on 5-HT-mediated effects at mouse 5-HT3 receptors

1 The influence of sodium ion substitutes on the 5-hydroxytryptamine (5-HT)-induced flux of the organic cation [14C]guanidinium through the ion channel of the mouse 5-HT3 receptor and on the competition of 5-HT with the selective 5-HT3 receptor antagonist [3H]GR 65630 was studied, unless stated otherwise, in mouse neuroblastoma N1E-115 cells. 2 Under physiological conditions (135 mm sodium), 5-HT induced a concentration-dependent [14C]guanidinium influx with an EC50 (1.3 microm) similar to that in electrophysiological studies. 3 The stepwise replacement of sodium by increasing concentrations of the organic cation hydroxyethyl trimethylammonium (choline) concentration dependently caused both a rightward shift of the 5-HT concentration-response curve and an increase in the maximum effect of 5-HT. Complete replacement of sodium resulted in a 34-fold lower potency of 5-HT and an almost two times higher maximal response. A low potency of 5-HT in choline buffer was also observed in other 5-HT3 receptor-expressing rodent cell lines (NG 108-15 or NCB 20). 4 Replacement of Na+ by Li+ left the potency and maximal effects of 5-HT almost unchanged. Replacement by tris (hydroxymethyl) methylamine (Tris), tetramethylammonium (TMA) or N-methyl-d-glucamine (NMDG) caused an increase in maximal response to 5-HT similar to that caused by choline. The potency of 5-HT was only slightly reduced by Tris, to a high degree decreased by TMA (comparable to the decrease by choline), but not influenced by NMDG. 5 The potency of 5-HT in inhibiting [3H]GR65630 binding to intact cells was 35-fold lower when sodium was completely replaced by choline, but remained unchanged after replacement by NMDG. 6 The results are compatible with the suggestion that choline competes with 5-HT for the 5-HT3 receptor; the increase in maximal response may be partly due to a choline-mediated delay of the 5-HT-induced desensitization. For studies of 5-HT-evoked [14C]guanidinium flux through 5-HT3 receptor channels, NMDG appears to be an 'ideal' sodium substituent since it increases the signal-to-noise ratio without interfering with 5-HT binding.

Mechanism of block by fluoxetine of 5-hydroxytryptamine3 (5-HT3)-mediated currents in NCB-20 neuroblastoma cells

The effect of fluoxetine (Prozac) on 5-hydroxytryptamine(3) (5-HT(3))-mediated currents in NCB-20 neuroblastoma cells was examined using the whole-cell patch-clamp technique. Fluoxetine produced a significant reduction of peak amplitude without altering the activation time course of 5-HT(3)-mediated currents. These effects were concentration-dependent, with an IC(50) value of 4.15 microM. No voltage dependence was evident in fluoxetine's block of 5-HT(3)-mediated currents over the entire voltage range tested. The extent of block by pre-application of fluoxetine was significantly greater than that by co-application. Fluoxetine also increased the apparent rate of current desensitization to 5-HT application. Using a first-order kinetics analysis, the open-channel blocking rate constants were 0.06 microM(-1)s(-1) (k(+1), association rate constant) and 0.05 s(-1) (k(-1), dissociation rate constant), with an apparent K(d) (=k(-1)/k(+1)) of 0.83 microM. This value is close to an IC(50) of 1.11 microM obtained from the reduction in tau, the time constant of desensitization. Intracellular application of fluoxetine for long durations had no effect on the amplitude or kinetics of 5-HT(3)-mediated currents. Similarly, norfluoxetine, the major metabolite of fluoxetine, reduced the peak current, and enhanced the rate of current desensitization in a concentration-dependent manner with an IC(50) of 2.66 microM, indicating that norfluoxetine is more potent than fluoxetine in blocking 5-HT(3)-mediated currents. These results indicate that, at clinically relevant concentrations, fluoxetine and its metabolite, norfluoxetine, block 5-HT(3)-mediated currents in NCB-20 neuroblastoma cells.

Subhypnotic doses of propofol accelerate extinction of conditioned taste aversion

Subhypnotic doses of propofol accelerate extinction of conditioned taste aversion. Some intravenous anesthetic agents including propofol is known to induce anterograde and retrograde amnesia. We evaluated whether propofol affect the long-term memory formed by the conditioned taste aversion (CTA) paradigm. Rats were allowed a 4h access to water through the experiments. After preconditioning water intake, the rats were offered 0.1% sodium saccharin (Sac) as conditioned stimulus (CS) for 20 min. An intraperitoneal (i.p.) injection of several concentrations (0.5-100 mg/kg) of propofol 10 min after Sac exposure was followed by an i.p. injection of 0.15M LiCl (2% of body weight) as unconditioned stimulus (US) 30 min after CS-exposure. The volumes of intake of Sac for 20 min were measured on the successive 4 days. The rats, which acquired CTA by every CS-US paradigm, strongly avoided Sac on the first test day after conditioning and maintained the avoidance for 3 days. However, when subhypnotic dose of propofol was injected before LiCl-injection, Sac intake abruptly increased on the second test day and the almost complete extinction occurred on the third test day after conditioning. The extinction process of CTA was barely affected by hypnotic dose of propofol. These results suggest that propofol affects the retention mechanism of the CTA memory in a dose-dependent manner. Subhypnotic dose of propofol may affect the sub-cellular process of the memory consolidation in CTA.

An electrophysiological study of the effects of propofol on native neuronal ligand-gated ion channels

1. Pharmacological evidence suggests that some of the clinical actions of propofol may be mediated, at least in part, by positive modulation of the GABA(A) receptor chloride channel. The effect of propofol at other native neuronal ligand-gated ion channels is unclear. 2. To gain some insight into the effects of propofol at a range of native neuronal receptors, the present study has used an extracellular recording technique and determined its effects at GABA(A), 5-HT3, P2X and nicotinic acetylcholine (nACh) receptors of the rat isolated vagus nerve and the GABA(A) and strychnine-sensitive glycine receptor of the rat isolated optic nerve. In addition, we have used patch-clamp recording techniques to further investigate the effects of propofol at the GABA(A) and strychnine-sensitive glycine receptors in rat cultured hippocampal neurons. 3. Propofol (0.3-100 micromol/L) concentration-dependently potentiated submaximal GABA-evoked responses in the vagus nerve and shifted the GABA concentration-response curve to the left. In contrast, propofol at concentrations ranging from 1 to 10 micromol/L had little or no effect on 5-HT3, P2X or nACh receptor-mediated responses in the vagus nerve but, at 100 micromol/L, propofol inhibited these responses to approximately 50% of control. In the optic nerve, EC20 GABA-evoked responses were also potentiated by propofol (10 micromol/L), while EC20 glycine-evoked responses were minimally enhanced. 4. Further investigations using cultured hippocampal neurons showed that submaximal (10 micromol/L) GABA-evoked currents were potentiated by propofol (1-10 micromol/L), in a non-voltage-dependent manner, whereas submaximal (100 micromol/L) glycine-evoked currents were unaffected. 5. These data suggest that propofol, at therapeutic concentrations, exerts its principle pharmacological actions at GABA(A) receptors with relatively little effect at other neuronal ligand-gated ion channels.