Inhibitory Effect Of Midazolam Research Articles

Anesthetic drug midazolam inhibits cardiac human ether-à-go-go-related gene channels: mode of action.

Midazolam is a short-acting benzodiazepine that is in wide clinical use as an anxiolytic, sedative, hypnotic, and anticonvulsant. Midazolam has been shown to inhibit ion channels, including calcium and potassium channels. So far, the effects of midazolam on cardiac human ether-à-go-go-related gene (hERG) channels have not been analyzed. The inhibitory effects of midazolam on heterologously expressed hERG channels were analyzed in Xenopus oocytes using the double-electrode voltage clamp technique. We found that midazolam inhibits hERG channels in a concentration-dependent manner, yielding an IC50 of 170 μM in Xenopus oocytes. When analyzed in a HEK 293 cell line using the patch-clamp technique, the IC50 was 13.6 μM. Midazolam resulted in a small negative shift of the activation curve of hERG channels. However, steady-state inactivation was not significantly affected. We further show that inhibition is state-dependent, occurring within the open and inactivated but not in the closed state. There was no frequency dependence of block. Using the hERG pore mutants F656A and Y652A we provide evidence that midazolam uses a classical binding site within the channel pore. Analyzing the subacute effects of midazolam on hERG channel trafficking, we further found that midazolam does not affect channel surface expression. Taken together, we show that the anesthetic midazolam is a low-affinity inhibitor of cardiac hERG channels without additional effects on channel surface expression. These data add to the current understanding of the pharmacological profile of the anesthetic midazolam.

Inhibitory effect of midazolam on MMP-9, MMP-1 and MMP-13 expression in PMA-stimulated human chondrocytes via recovery of NF-κB signaling.

IntroductionMidazolam, a benzodiazepine, has a hypnotic effect and is widely used as an intravenous sedative. Past studies have clearly established that midazolam has beneficial effects in attenuating ischemia-reperfusion injury more than other currently used sedative drugs. However, the role of midazolam on chondroprotection via inhibition of matrix metalloproteinases (MMPs) is warrant investigation. The aim of this study was to examine the mechanisms of action of midazolam on MMP expression via nuclear factor κB (NF-κB) signaling in activated chondrosarcoma cells maintained in vitro.Material and methodsChondrocytes, SW1353 cells, were stimulated with phorbol 12-myristate 13-acetate (PMA) in the absence or presence of various concentrations of midazolam (5-20 µM). Release of MMP-9 into the culture media was determined by gelatin zymography. The expressions of MMP-1, MMP-9 and MMP-13, phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinases and degradation of IκB-α were determined by western blotting assay.ResultsMidazolam significantly down-regulated PMA-induced MMP-9 protein expression at concentrations of 5, 10 and 20 µM, the values were 1.95 ±0.09 (p < 0.01), 1.71 ±0.12 (p < 0.01) and 1.35 ±0.20 (p < 0.001), respectively. At concentrations of 5, 10 and 20 µM, it was significantly inhibited the PMA-induced expressions of MMP-1 (2.27 ±0.10, 1.98 ±0.11 and 1.56 ±0.15; p < 0.001) and MMP-13 (0.89 ±0.04, 0.81 ±0.07, and 0.74 ±0.09; p < 0.001), respectively. Midazolam at concentrations of 10 and 20 µM for 15 min significantly reversed the rate of degradation (0.895 ±0.051; p < 0.05 and 0.926 ±0.060; p < 0.01, respectively) of IκB-α in PMA-chondrocyte cells. In addition, this sedative drug inhibited PMA-induced levels of phos-ERK (1.243 ±0.12, 1.108 ±0.16 and 0.903 ±0.19, respectively) and phos-p38 (1.146 ±0.10, 1.063 ±0.13 and 0.946 ±0.18, at concentrations of (5, 10 and 20 µM), respectively.ConclusionsThese results are important for understanding the mechanism of midazolam in inhibiting PMA-induced MMP expression through the signaling pathways of either NF-κB or ERK/p38 MAPKs down-regulation.

Antiaggregatory effect of midazolam on human platelets during monitored anesthesia care for trans-vaginal oocyte retrieval

BackgroundMidazolam plays a major role in sedation and large doses might be used under certain circumstances. Trans-vaginal oocyte retrieval is one of the most painful procedures in which monitored anesthesia care (MAC) is needed. The mechanism by which midazolam exerts anti-aggregation of human platelets is still unclear.This work aimed to investigate the in vivo suppressing effect of midazolam on human platelet aggregation.MethodsSixty adult females ASA I–II, scheduled for trans-vaginal ultrasound oocyte retrieval by combined intravenous midazolam sedation with paracervical local nerve block were included. At the end of the procedure, it was found that the range of total midazolam consumption was 4.9–9 mg. Thereby, patients were divided into three groups according to the total consumption of midazolam: Group I (4.9–6.2 mg), Group II (6.3–7.6 mg), Group III (7.7–9 mg). Patients who needed other drugs for sedation were excluded. Pre- and post-operative Bleeding Time (BT) were recorded. Two blood samples were collected from each patient to test the inhibitory effect of midazolam on platelet aggregation. Platelet aggregation was tested using platelet-aggregometer. ELISA was used to measure thromboxane B2 formation. Flowcytometry was used to evaluate whether glycoprotein IIb–IIIa complex is the receptor site for the antiplatelet action of midazolam.ResultsOnly 51 patients completed the study: Group I (n = 16), Group II (n = 24), and Group III (n = 11). Bleeding Time (BT) showed significant prolongation in group III compared to basal levels. Midazolam suppressed platelet aggregation in a dose-dependent manner as detected by aggregometer. Glycoprotein IIb–IIIa complex is not its site of action as shown by flowcytometric analysis. Lastly, thromboxane B2 was significantly inhibited by midazolam.ConclusionThis study revealed that midazolam dose-dependently inhibits platelet aggregation by a mechanism not involving the binding of glycoprotein IIb–IIIa complex (fibrinogen receptor). It was also found that midazolam inhibits thromboxane A2 formation.

Direct assessment of the antioxidant properties of midazolam by electron spin resonance spectroscopy

Some antioxidant anesthetics directly inhibit lipid peroxidation mediated via the generation of reactive oxygen species (ROS). To date, the scavenging effects of midazolam on ROS have not been directly assessed. We investigated the inhibitory effect of midazolam on ROS [hydroxyl radical (HO(·)) and superoxide (O (2) (·-) )] by in vitro X-band electron spin resonance with the spin-trapping agent 5,5-dimethyl-1-pyrroline-N-oxide. Our results indicated that HO(·) and O (2) (·-) were not affected by midazolam at clinically relevant concentrations, but were directly scavenged by midazolam at high concentrations (i.e., >4.6 and >1.5mM, respectively).

The Effects of Benzodiazepines on Urotensin II-Stimulated Norepinephrine Release from Rat Cerebrocortical Slices

Urotensin II (UII) and its receptor (UT) are implicated in mood disorders, such as stress and anxiety, and this may result, at least in part, from increased norepinephrine release from the cerebral cortex. Benzodiazepines have been widely used as hypnotics and anxiolytics, producing a decrease in cerebrocortical norepinephrine release. We hypothesized that there was some interaction between benzodiazepines and the UII system in the cerebral cortex. In the present study, we have examined the effects of benzodiazepines on UII-increased norepinephrine release from rat cerebrocortical slices and intracellular Ca(2+) concentrations ([Ca(2+)]i) in HEK293 cells expressing rat UT receptor (HEK293-rUT cells). Midazolam, diazepam and flunitrazepam concentration-dependently inhibited UII-evoked norepinephrine release but did not affect [Ca(2+)]i. The IC(50) of midazolam for inhibition of UII-evoked norepinephrine release (0.32 microM, P < 0.01) was significantly lower than that of diazepam (187 microM) or flunitrazepam (40 microM). The inhibitory effects of midazolam on UII-evoked norepinephrine release were significantly attenuated by flumazenil, a benzodiazepine site antagonist. The present study suggests that midazolam, at clinically relevant concentration, significantly inhibited UII-evoked norepinephrine release. This inhibitory effect may be partially mediated via central benzodiazepine receptors.

Effects of Midazolam on Organ Dysfunction in Rats with Endotoxemia Induced by Lipopolysaccharide

Sepsis is associated with high mortality and septic patients sometimes need sedation to attenuate anxiety and agitation. Recently, it has been suggested that midazolam, a benzodiazepine, modulates the responses to cytokines released from macrophages in vitro. The purpose of the present study was to evaluate the effect of midazolam on hemodynamics and organic function in rats with septic syndromes induced by intravenous endotoxin. Wistar rats were randomly allocated to three groups and treated intravenously as follows: (1) control group, in which the rats were given saline vehicle (1 mL/kg) at time 0; (2) LPS group, in which the rats were given Escherichia coli lipopolysaccharide (10 mg/kg) in an infusion over 10 minutes; (3) LPS+Midazolam group, in which the rats were given E. coli LPS infusion as in the LPS group followed immediately by infusion of midazolam, given at a rate of 1 microg/kg per hour. All hemodynamic and biochemical parameters were measured during the 4-hour observation period. The injection of LPS led to hypotension, tachycardia, and vascular hyporeactivity to vasoconstrictors. In addition, initial hyperglycemia, delayed hypoglycemia, hypoalbuminemia, elevated serum indicators of hepatic and renal injury and high mortality were also observed in rats treated with LPS. However, midazolam given in the designated dosage did not offer any modulatory effects on hemodynamic responses, multiple organic dysfunction and survival rate in rats with endotoxemia. No deleterious or beneficial effects on hemodynamics and organ dysfunction were observed in the endotoxemic rats treated with midazolam at the designated dosage. However, previous reports which showed substantial benefits from the inhibitory effects of midazolam on proinflammatory factors in vitro should be subjected to further elucidation.

Midazolam attenuates the antinociception induced by d-serine or morphine at the supraspinal level in rats

Our recent study has shown that the intracerebroventricular administration of d-serine, an endogenous and selective agonist for the glycine site of the N-methyl- d-aspartate receptor, alone or in combination with morphine, leads to the potentiation of antinociception on the tail-flick response. Although there is a variety of information concerning the effects of benzodiazepines on opioid-induced antinociception, little is known about the effect of benzodiazepines on the N-methyl- d-aspartate receptor agonist-induced antinociception. To clarify the analgesic interactions among the benzodiazepine/GABA A, N-methyl- d-aspartate and opioid receptors at the supraspinal level, we investigated the effects of intracerebroventricular administration of midazolam, a benzodiazepine receptor agonist, on the antinociception evoked by the intracerebroventricular application of d-serine or morphine. The intracerebroventricular administration of midazolam alone produced hyperalgesia on the tail-flick response in a benzodiazepine receptor antagonist, flumazenil-reversible manner. The antinociception induced by the intracerebroventricular application of d-serine or morphine was attenuated by the intracerebroventricular administration of midazolam. In addition, this inhibitory effect of midazolam on the antinociception of d-serine or morphine was antagonized by the intracerebroventricular administration of flumazenil. Together with the facts that d-serine and midazolam act as selective agonists for the glycine site of the N-methyl- d-aspartate receptor and benzodiazepine/GABA A receptor, respectively, these observations suggest a functional interaction between the NMDA and benzodiazepine/GABA A receptors in the regulation of antinociception at the supraspinal level.

The Effect of Midazolam on Outward K+Channel Currents in Rabbit Cerebral Arterial Smooth Muscle Cells

Background: Midazolam has a direct relaxing effect on vascular smooth muscle, but the mechanisms that this agent produces muscle relaxation are not fully understood. The current study was performed to identify the effects of the midazolam on K+ channels current in rabbit cerebral arterial smooth muscle cells. Methods: Whole cell patch-clamp recording technique was used to evaluate the effects of midazolam (0.1 to 100μM) on outward K+ channel currents in dispersed rabbit cerebral arterial smooth muscle cells. Results: Outward K+ currents of rabbit cerebral artery smooth muscle cells were voltage-dependent. Midazolam (10, 100μM) tested significantly inhibited outward K+ currents in a dose-dependent manner and half-blocking concentration (IC50) was 15.94μM at 60 mV. Conclusions: Midazolam inhibit outward K+ currents of rabbit cerebral arterial smooth muscle cells. Further study will be needed to determine the effect of midazolam on calcium channel current because it is unclear if the inhibitory effect of midazolam on outward K+current induces vasoconstriction. (Korean J Anesthesiol 2007; 52: 67～71)

Intrathecal midazolam regulates spinal AMPA receptor expression and function after nerve injury in rats

Spinal γ-aminobutyric acid (GABA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been implicated in the mechanisms of neuropathic pain after nerve injury; however, how these two receptors interact at the spinal level remains unclear. Here we show that intrathecal midazolam through activation of spinal GABAA receptors attenuated the expression and function of spinal AMPA receptors in rats following peripheral nerve injury. Thermal hyperalgesia and mechanical allodynia induced by chronic constriction nerve injury (CCI) in rats were attenuated by the short-acting benzodiazepine midazolam (20 = 10 > 5 μg > vehicle) administered intrathecally once daily for 7 postoperative days. CCI-induced upregulation of AMPA receptors within the spinal cord dorsal horn was also significantly reduced by the intrathecal midazolam (10, 20 μg) treatment. The inhibitory effects of midazolam (10, 20 μg) on neuropathic pain behaviors and AMPA receptor expression were prevented by co-administration of midazolam with the GABAA receptor antagonist bicuculline (3 μg), whereas intrathecal treatment with bicuculline (1 or 3 μg) alone in naive rats induced the upregulation of spinal AMPA receptor expression and nociceptive responses, indicating a tonic regulatory effect from endogenous GABAergic activity on the AMPA receptor expression and spinal nociceptive processing. These results indicate that modulation of spinal AMPA receptor expression and function by the GABAergic activity may serve as a mechanism contributory to the spinal nociceptive processing.

The spinal nitric oxide involved in the inhibitory effect of midazolam on morphine-induced analgesia tolerance

Previous studies had shown that pretreatment with midazolam inhibited morphine-induced tolerance and dependence. The present study was to investigate the role of spinal nitric oxide (NO) in the inhibitory effect of midazolam on the development of morphine-induced analgesia tolerance. Subcutaneous injection of 100 mg/kg morphine to mice caused an acute morphine-induced analgesia tolerance model. To develop chronic morphine tolerance in mice, morphine was injected for three consecutive days (10, 20, 50 mg/kg sc on Day 1, 2, 3, respectively). In order to develop chronic tolerance model in rats, 10 mg/kg of morphine was given twice daily at 12 h intervals for 10 days. Midazolam was intraperitoneally injected 30 min prior to administration of morphine. Tail-flick test, hot-plate and formalin test were conducted to assess the nociceptive response. Immunocytochemistry, histochemistry and western blot were performed to determine the effect of midazolam on formalin-induced expression of Fos protein, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and nitric oxide synthase (NOS) in chronic morphine-tolerant rats, respectively. The results showed that pretreatment with midazolam significantly inhibited the development of acute and chronic morphine tolerance in mice, which could be partially reversed by intrathecal injection of NO precursor L-arginine (L-Arg). In chronic morphine-tolerant rats, pretreatment with midazolam significantly decreased the formalin-induced expression of Fos and Fos/NADPH-d double-labeled neurons in the contralateral spinal cord and NADPH-d positive neurons in the bilateral spinal cord. Both inducible NOS (iNOS) and neuronal NOS (nNOS) protein levels in the spinal cord were significantly increased after injection of formalin, which could be inhibited by pretreatment with midazolam. The above results suggested that the decrease of the activity and expression of NOS contributed to the inhibitory effect of midazolam on the development of morphine tolerance.

Assessment of the recovery of dynamic balance after intravenous sedation with midazolam

To assess street fitness after sedation, computerized dynamic posturography (CDP) involving movement of the center of gravity may be more accurate than the conventional computerized static posturography (CSP). The purpose of this study was to evaluate the recovery of dynamic balance function after intravenous sedation by CDP in comparison with CSP, and to find a simple dynamic balance test that is well correlated with CDP. The subjects were 20 male volunteers aged 20-27 years. After intravenous injection of midazolam (0.07 mg . kg(-1)), four balance tests were performed. The recovery time and the percentage of subjects showing recovery (difference from the baseline value < or =10%) were compared. As CDP, a test in which unexpected perturbation stimuli are given using an unstable platform was performed. As CSP, standing sway tests were performed. Maximum speed walking (MSW) and usual speed walking (USW) tests were performed as simple balance tests. The recovery time in CDP (80 min) was longer than that in CSP (40-60 min). The percentage of subjects showing recovery in CDP (20%) was significantly lower than that in CSP (55%-70%) 60 min after the administration of midazolam. There was a significant positive correlation between the CDP test and the MSW test (r = 0.67). CDP with perturbation stimuli detects the balance inhibitory effects of midazolam with greater sensitivity than CSP. The MSW test is well correlated with CDP with perturbation stimuli.

The Inhibitory Effects of Midazolam and Diazepam on Smooth Muscle Tension in Rat Trachea

Background: It has been demonstrated that a group of minor tranquilizers, benzodiazepines, are able to relax airway smooth muscle, but the mechanism by which these agents produce muscle relaxation are not fully understood. This study was undertaken to determine the effects and mechanism of diazepam and midazolam on Ca²+and K+ channel in isolated rat trachea muscles by measuring isometric tension. Methods: Our experiment was performed to evaluate the effects of midazolam and diazepam by cumulative administration from 10-6 M to 3×10-5 M to tracheal smooth muscle contraction which was induced by contractile agonists such as ACh 10-5 M, carbachol 3×10-7 M, and KC1 40 mM. The effects of midazolam and diazepam were evaluated on Ca²+ and K+ channels by inhibition of contraction using a nonspecfic K+ channel blocker such as tetraethyl ammonium (TEA) elicited by a 2 mM Ca²+ space addition to Ca²+ free on high K' depolarizing rat tracheal muscle. Also, to elucidate any mechanism involved, the effects of flumazenil (a specific central antagonist of benzodiazepines), propranolol (a beta adrenergic antagonist), and atropine (a muscarinic antagonist) and tracheal epithelium removal were examined. Results: In a concentration-dependent way, both midazolam and diazepam relaxed airway smooth muscle directly and had inhibitory effects on voltage-dependent Ca²+ (VDCC) and K channels. Conclusions: These results suggest that benzodiazepines relax airway smooth muscle, not via a neural pathway or benzodiazepine receptor but through a direct action on Ca²+ and K+ channels. Benzodiazepine enhanced K+ conductance, leading to a decrease in VDCC opening, thus reducing Ca²+ thtough the voltage-dependent Ca²+ channel, in addition to inhibiting of intracellular Ca²+ release.

Effect of Midazolam on Development of Acute Tolerance to Alfentanil

This study in rats was performed to explore whether the inhibitory effect of midazolam on the development of acute tolerance to the analgesic effect of alfentanil is due to pharmacokinetic mechanisms.Analgesia was determined with tail-compression and hot-plate tests. Alfentanil and midazolam concentrations in plasma and the brain were measured using a radioimmunoassay and chromatographic technique, respectively. After the 4-h period of alfentanil administration (155 micro g[centered dot]kg-1[centered dot]h-1 after a 50-micro g/kg bolus), when acute tolerance had already developed, midazolam (2-mg/kg bolus) enhanced the alfentanil-induced analgesia by 120% (P<0.001) with the tail-compression test and 76% (P<0.01) with the hot-plate test. At the height of midazolam-induced enhancement of the analgesic effect of alfentanil, the measurements of the alfentanil and midazolam plasma and brain concentrations did not demonstrate any significant changes in the drugs' concentrations. The results confirm that midazolam attenuates the development of acute tolerance to the analgesic effect of alfentanil and indicate that this interaction is not pharmacokinetic. (Anesth Analg 1997;85:182-7)

Effect of Midazolam on Development of Acute Tolerance to Alfentanil

This study in rats was performed to explore whether the inhibitory effect of midazolam on the development of acute tolerance to the analgesic effect of alfentanil is due to pharmacokinetic mechanisms. Analgesia was determined with tail-compression and hot-plate tests. Alfentanil and midazolam concentrations in plasma and the brain were measured using a radioimmunoassay and chromatographic technique, respectively. After the 4-h period of alfentanil administration (155 microg x kg(-1) x h(-1) after a 50-microg/kg bolus), when acute tolerance had already developed, midazolam (2-mg/kg bolus) enhanced the alfentanil-induced analgesia by 120% (P < 0.001) with the tail-compression test and 76% (P < 0.01) with the hot-plate test. At the height of midazolam-induced enhancement of the analgesic effect of alfentanil, the measurements of the alfentanil and midazolam plasma and brain concentrations did not demonstrate any significant changes in the drugs' concentrations. The results confirm that midazolam attenuates the development of acute tolerance to the analgesic effect of alfentanil and indicate that this interaction is not pharmacokinetic.

Effect of the NMDA‐antagonist, MK 801, on benzodiazepine‐opioid interactions at the spinal and supraspinal level in rats

1. Benzodiazepines potentiate morphine antinociception at the spinal level via GABAergic mechanisms. At the supraspinal level, the inhibitory effect of midazolam on morphine antinociception cannot be easily explained by GABAA receptor activation. Since excitatory amino acids play a role in central transmission, we investigated the effect of dizocilpine (MK 801) on this interaction in spinal cord and brain. 2. In rats with an intrathecal or intracerebroventricular catheter, the mechanisms of the antinociceptive effect of benzodiazepine-morphine combinations were tested during thermal nociceptive tests. 3. The principal findings of this study were that at the spinal level, midazolam potentiation of morphine antinociception can be antagonized by the NMDA antagonist, MK 801 (10 micrograms), as assessed by hot-plate and tail-flick tests. When drugs were administered supraspinally, midazolam inhibited morphine antinociception only in the hot-plate test, an effect also inhibited by MK 801. In the tail-flick assay, midazolam failed to influence the morphine response. 4. The NMDA antagonist significantly affected midazolam antinociception at the spinal level, but was not effective following i.c.v. administration of the drugs. MK 801 had no effect on morphine antinociception after i.t. and i.c.v. administration of the drugs. 5. The paradoxical effect of midazolam on morphine antinociception and its reversal by MK 801 might be due to modulation at various levels of the neuraxis and/or modulation of different pathways mediated via both GABAA and NMDA receptor mechanisms.

Differential foetal development of the O‐ and N‐demethylation of codeine and dextromethorphan in man.

1. Codeine and dextromethorphan were N-demethylated in human foetal liver microsomes at high rates which were close to the activities in adult livers. In contrast, foetal liver microsomes did not catalyze the O-demethylation of these drugs at mid-gestation. 2. The metabolic data were in accordance with the absence of P450IID6 and the presence of P450 IIIA as determined by Western blotting with anti-human P450 IID6 (MAb 114/2) and anti-rat P450 IIIA (PCN 2-13-1/C2) monoclonal antibodies, respectively. 3. The inhibitory effects of midazolam and dehydroepiandrosterone support the contention that the N-demethylase is a human foetal form of the cytochrome P450 IIIA family. Consistent with this we found that blotting with the MAb PCN 2-13-1/C2, which recognizes an epitope specific for the P450 III family, correlated well with the N-demethylase activities.

Benzodiazepine inhibition of the stimulation-evoked catecholamine release from bovine adrenomedullary cells in culture

Effect of benzodiazepines on evoked catecholamine (CA) release from a primary culture of bovine adrenal medullary cells was investigated. Midazolam at high doses (> 10 ? M) inhibited CA release evoked by acetylcholine (ACh), excess K(+) and veratridine but not by A23187 or caffeine in Ca(2+) -free media. Other benzodiazepines, diazepam, clonazepam, nitrazepam and R05-4864, as well as 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195) and ethyl-?-carboline-3-carboxylate (?CCE) also inhibited ACh-evoked CA release but only at high concentrations. The inhibitory effect of midazolam on ACh-evoked CA release was not affected by R015-1788, a central-type benzodiazepine receptor antagonist which itself had no effect on basal and ACh-evoked CA release. Facilitatory action of Bay K 8644 on CA release evoked by 20 mM K(+) was reduced by midazolam, PK11195 and R05-4864. Further, ACh-evoked (45)Ca uptake was markedly reduced by midazolam and R05-4864 in association with the inhibition of CA release. These results suggest that benzodiazepines at high doses, inhibit the evoked CA release from adrenal chromaffin cells possibly through the blockade of Ca(2+) influx. Possible involvement of receptor subtypes of benzodiazepines in regulating CA secretion is discussed.

Effects of the benzodiazepine receptor ligands midazolam, Ro 15-1788, and Ro 5-4864, alone and in combinations, on platelet serotonin uptake.

The benzodiazepine (BZ) agonist midazolam, the BZ antagonist Ro 15-1788, and the peripheral BZ binding site ligand Ro 5-4864 have been assessed, separately and in combinations, as to their effect on the active uptake of serotonin (5HT) in human blood platelets in vitro, in an artificial, protein-free medium. Midazolam had a moderate noncompetitive (or mixed competitive/noncompetitive) inhibitory effect on the uptake. This effect was not influenced by Ro 15-1788, which in itself had only a very weak inhibitory effect. Ro 4-5864 showed in several independent experiments (but not always) a biphasic inhibitory effect, with a moderate but significant inhibition in the concentration range of about 10(-8) to 10(-6) M, and a stronger, noncompetitive inhibitory effect above 10(-6) M. When Ro 5-4864 was tested in the presence of a fixed concentration of midazolam (4 X 10(-6) M), the inhibitory effect of the former was markedly reduced (at higher concentrations), eliminated (at intermediate concentrations), or even reversed into a relative stimulatory effect (at low concentrations). Thus, low concentrations of Ro 5-4864 reduced the inhibitory effect of midazolam. A possible explanation of this interaction is that low concentrations of Ro 5-4864 have both an inhibitory and a stimulatory effect on platelet 5HT uptake (often seen to vary in relative strength between experiments), and that the stimulatory component is unmasked in the presence of a drug that already has an inhibitory effect, probably mediated by the same site.(ABSTRACT TRUNCATED AT 250 WORDS)