Antagonistic effects of ondansetron and tramadol? A randomized placebo and active drug controlled study.

UNLABELLED: Opposing effects of ondansetron and tramadol on the serotonin pathway have been suggested which possibly increase tramadol consumption and emesis when co-administered. In a randomized, double-blinded study, 179 patients received intravenous ondansetron, metoclopramide, or placebo for emesis prophylaxis. Analgesic regimen consisted of tramadol intraoperative loading and subsequent patient-controlled analgesia. Tramadol consumption and response to antiemetic treatment were compared. Additionally, plasma concentrations of ondansetron and (+)O-demethyltramadol and CYP2D6 genetic variants were analyzed as possible confounders influencing analgesic and antiemetic efficacy. Tramadol consumption did not differ between the groups. Response rate to antiemetic prophylaxis was superior in patients receiving ondansetron (85.0%) compared with placebo (66.7%, P = .046), with no difference to metoclopramide (69.5%). Less vomiting was reported in the immediate postoperative hours in the verum groups (ondansetron 5.0%, metoclopramide 5.1%) compared with placebo (18.6%; P = .01). Whereas plasma concentrations of (+)O-demethyltramadol were significantly correlated to CYP2D6 genotype, no influence was detected for ondansetron. Co-administration of ondansetron neither increased tramadol consumption nor frequency of PONV in this postoperative setting. PERSPECTIVE: Controversial findings were reported for efficacy of tramadol and ondansetron when co-administered due to their opposing serotonergic effects. Co-medication of these drugs neither increased postoperative analgesic consumption nor frequency of emesis in this study enrolling patients recovering from major surgery.

Characterization of the naturally occurring Arg344His variant of the human 5-HT 3A receptor.

The present study aimed at examining the function and pharmacological properties of the naturally occurring Arg344His variant of the human 5-HT(3A) receptor, identified in a schizophrenic patient. In intact human embryonic kidney (HEK) 293 cells expressing the wild-type (WT) or the variant receptor, the function was analyzed by indirect measurement of agonist-induced Ca(2+) current through the 5-HT(3A) receptor channel by an aequorin luminescence-based Ca(2+) assay. In cell membrane patches cation currents were determined electrophysiologically including technically demanding single channel analyses. The pharmacological properties were analyzed by [(3)H]GR65630 binding to cell membrane fragments. The density of [(3)H]GR65630 binding sites in cells expressing the variant receptor was reduced to 55% of that in cells expressing the WT receptor, which, however, was not accompanied by an analogous decrease in 5-HT-induced Ca(2+) influx through the receptor channel. However, the single channel analysis suggests an increase in single receptor channel mean open time (which is known to be subject of many variables) but not in unitary current amplitude. Radioligand competition experiments revealed that the affinity of five 5-HT(3) receptor agonists and four antagonists for the variant receptor did not differ from that for the WT receptor. In conclusion, the variant receptor resembles the WT receptor in that it forms functional homopentameric 5-HT(3A) receptors with identical pharmacological properties. In view of the lack of reduction in Ca(2+) flux through the variant receptor channels in spite of the decrease in its density on the cell membrane, the increase in single receptor channel mean open time appears to compensate for the reduction in variant receptor density.

The effects of fentanyl-like opioids and hydromorphone on human 5-HT3A receptors.

BACKGROUND: 5-HT(3) receptors are involved in various physiologic functions, including the modulation of emesis. 5-HT(3) antagonists are clinically widely used as potent antiemetics. Emesis is also a side effect of opioid analgesics. Intriguingly, the natural opioid morphine shows specific interactions with human 5-HT(3) receptors at clinically relevant concentrations. In the present study, we investigated whether this is a general effect of opioids, even when they are structurally diverse. Therefore, another morphine (phenanthrene-type) derivative, hydromorphone, and fentanyl including its (4-anilinopiperidine-type) derivatives were tested. METHODS: Whole-cell patches from human embryonic kidney-293 cells, stably transfected with the human 5-HT(3A) receptor cDNA, were used to determine the opioid effects on the 5-HT (3 microM)-induced currents using the patch clamp technique (voltage-clamp). RESULTS: None of the fentanyl derivatives affected currents through the 5-HT(3A) receptor (3 microM 5-HT) significantly in the clinically relevant nanomolar concentration range (IC(50) values >30 microM). In contrast, hydromorphone was considerably more potent (IC(50) = 5.3 microM), slowing the current activation- and desensitization-kinetics significantly (at 3 microM by a factor of 1.9 and 2.4, respectively), similar to morphine. At concentrations much higher than clinically relevant, but within the range predicted from Meyer-Overton correlations for nonspecific interactions, the fentanyl derivatives all showed at least a tendency to suppress current amplitudes, but they had diverse effects on the activation- and desensitization-kinetics of 5-HT(3A) receptors. CONCLUSIONS: Only morphine and hydromorphone, but not the fentanyl derivatives, reduced 5-HT-induced current amplitudes and slowed current kinetics near clinically relevant concentrations. The high potencies of morphine and hydromorphone, when compared to their lipophilicities, suggest a specific interaction with 5-HT(3A) receptors. In contrast, the effects of fentanyl-type opioids appear to be of unspecific nature. Because the rank order of opioid potencies for human 5-HT(3A) receptors is opposite of that for opioid receptors, the site involved is structurally different from opioid receptor binding sites. In agreement with recent data on different phenols, a phenolic OH-group (which morphine and hydromorphone possess) may contribute to specific interactions of morphine and hydromorphone with the 5-HT(3A) receptor. Future clinical studies could test whether corresponding differences in emetogenicity between different classes of opioids will be found.

Molecular actions of propofol on human 5-HT3A receptors: enhancement as well as inhibition by closely related phenol derivatives.

BACKGROUND: 5-Hydroxytryptamine type 3 (5-HT3) receptors are excitatory ligand-gated ion channels which are involved in postoperative nausea and vomiting. They are depressed by the anesthetic propofol, which, in contrast, enhances the activity of inhibitory ligand-gated ion channels such as gamma-aminobutyric acid type A receptors and glycine receptors. To investigate the molecular mechanisms responsible for these contrasting actions, we examined the kinetics of the action of propofol and its lesser hydrophobic derivatives 2-isopropylphenol and phenol on human 5-HT3A receptors. METHODS: Human embryonic kidney 293 cells containing stably transfected cDNA of the human 5-HT3A receptor subunit were patch clamped (excised outside-out patches). Drugs were applied with a fast solution exchange system (within 2 ms) and their concentrations were determined by high performance liquid chromatography. RESULTS: When applied in equilibrium (60 s before and during the 5-HT pulse), propofol inhibited human 5-HT3A receptors (IC50 = 18 +/- 1.0 microM). In equilibrium, the less hydrophobic 2-isopropylphenol was surprisingly a similarly potent inhibitor of human 5-HT3A receptors (IC50 = 17 +/- 3.2 microM), whereas phenol was considerably less potent (IC50 = 1.6 +/- 0.2 mM). Varying the duration of drug application before currents were elicited, and then applying 5-HT still in the presence of the drug revealed that fast and slow processes contributed to the (equilibrium) effects of propofol (tau(IN-1) = 35 ms and tau(IN-2) = 4.8 s), 2-isopropylphenol (tau(IN-1) = 64 ms and tau(IN-2) = 6.6 s), and phenol (tau(IN-1) < 10 ms, tau(IN-2) = 20.4 s). When applied transiently together with 5-HT (open channel application), propofol depressed currents and accelerated the 5-HT-induced desensitization significantly, whereas, in contrast, 2-isopropylphenol and phenol increased currents and slowed desensitization. Slowed desensitization was also observed for 5-hydroxyindole (1 mM), a 5-HT derivative, but not for benzene. The fast effects of phenol, 2-isopropylphenol, and propofol were more pronounced when the 5-HT concentration was decreased from 30 to 3 microM, whereas the slow effects were not sensitive to 5-HT. CONCLUSIONS: At least two separate inhibitory actions on 5-HT3A receptors could be identified for propofol, whereas the enhancing action seen for the two related smaller phenol derivatives could no longer be detected. 5-HT-dependent and 5-HT-independent interactions could be distinguished for all three drugs. Propofol was less potent than expected from its hydrophobic properties. Underlying mechanisms appear to involve the phenolic hydroxyl group, hydrophobic interactions, and steric restrictions.

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'.

Effects of tramadol and O-demethyl-tramadol on human 5-HT reuptake carriers and human 5-HT3A receptors: a possible mechanism for tramadol-induced early emesis.

([3H]5-HT)-uptake and patch-clamp techniques were used to study the actions of (+) and (-) tramadol and the active metabolites of tramadol, (+) and (-) O-demethyl-tramadol on the human serotonin (5-HT) transporter and the human 5-HT3A receptor, stably expressed in HEK-293 cells. The (+) and (-) enantiomers of tramadol suppressed the human 5-HT transporter concentration-dependently (IC50=1.0 and 0.8 microM, respectively), resulting in 97% and 87% transport inhibition at their respective initial plasma concentrations (9.5 microM). The (+) and (-) enantiomers of the active tramadol metabolite were less potent than tramadol in inhibiting the human 5-HT transporter (IC50=15 and 44 microM, respectively), resulting in 19.2% and 4.8% transport inhibition at their highest plasma concentrations (2.5 microM). In contrast to their potent suppression of the 5-HT transporter, both, (+) and (-) tramadol inhibited 5-HT (30 microM)-induced currents only at substantially higher concentrations (IC50=199 and 251 microM, respectively), resulting in only 6% and 4% inhibition at the initial maximum plasma concentration. A similar low potent inhibition of human 5-HT(3A) receptors was found for (+) and (-) O-demethyl-tramadol (IC50=158 and 63 microM, respectively). In conclusion, at clinical plasma concentrations tramadol potently suppresses the human 5-HT transporter, whereas it has only a slight effect on the human 5-HT3A receptor. The results are compatible with a possible mechanism for tramadol-induced early emesis involving the serotonergic system.

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.

Pharmacological and electrophysiological properties of the naturally occurring Pro391Arg variant of the human 5-HT3A receptor.

The 5-HT3A receptor, a ligand-gated ion channel, is involved in pain pathways, nausea and emesis, and irritable bowel syndrome, and may play a role in the pathogenesis of psychiatric diseases such as schizophrenia and depression. Recently, a naturally occurring variation (ProArg) in the second intracellular loop of the human (h) 5-HT3A receptor was identified in a schizophrenic patient. Because the substitution of proline, an alpha-imino acid, by arginine may affect the conformation of the whole receptor, the aim of the present study was to determine the pharmacological and functional properties of this variant compared to the wild-type receptor in stably transfected HEK293 cells. Studies of binding of [H]GR65630, a 5-HT3 receptor antagonist, to membranes (saturation and competition experiments with 5-HT3 receptor ligands) and patch-clamp studies of agonist-induced currents in outside-out patches were carried out. In comparison to the wild-type, the variant receptor exhibited no changes in the receptor density and the affinities for nine representative ligands (five agonists and four antagonists). The potencies and efficacies of three 5-HT3 receptor agonists in inducing currents through the ion channel and the potencies of two 5-HT3 receptor antagonists in blocking 5-HT-evoked currents did not differ between wild-type and variant receptors. In addition, there were no differences in the desensitization kinetics of both receptor isoforms. In conclusion, the ArgPro variation of the h5-HT3A receptor does not change ligand binding to the h5-HT3A receptor, nor does it modify current through the receptor channel.