Midazolam In Rats Research Articles

Dizocilpine enhances the discriminative stimulus effects of midazolam in rats discriminating pregnanolone

The discriminative stimulus effects of the neuroactive steroid pregnanolone are thought to be mediated by multiple neurotransmitter systems including positive modulation at GABAA receptors and antagonism of NMDA receptors. This study investigated the contribution of different receptor mechanisms by establishing pregnanolone as a discriminative stimulus and studying the NMDA receptor antagonist dizocilpine alone and in combination with the positive GABAA modulator midazolam. Male Sprague‐Dawley rats (n=10) discriminated pregnanolone (7.5 mg/kg, i.p.) and responded under a fixed ratio 10 schedule for food. Pregnanolone and midazolam produced >80% pregnanolone‐lever responding. In contrast, dizocilpine produced >80% pregnanolone‐lever responding in two rats and <20% pregnanolone‐lever responding in the remaining eight rats, while the mu opioid receptor agonist morphine produced predominantly vehicle‐lever responding. When administered 30 minutes before sessions, dizocilpine shifted the midazolam dose‐effect curve leftward 2‐ to 6‐fold; however, morphine did not alter the midazolam dose‐effect curve. Although dizocilpine did not substitute for pregnanolone, enhancement of the effects of midazolam by dizocilpine suggests that the pregnanolone discriminative stimulus has both GABAA and NMDA components.

Separate evaluation of intestinal and hepatic metabolism of three benzodiazepines in rats with cannulated portal and jugular veins: comparison with the profile in non-cannulated mice

Pharmacokinetic analyses of three kinds of benzodiazepines—midazolam (MDZ), triazolam (TRZ) and alprezolam (APZ)—were performed in rats with cannulated portal and jugular veins. Each drug was administered to the double-cannulated rats, and pharmacokinetic data for the parent drugs and their 1′- and 4-hydroxylated metabolites were compared with those obtained in non-cannulated mice.In bioavailability, the drugs ranked APZ >> TRZ = MDZ in rats, and APZ > TRZ >> MDZ in mice, with the values for MDZ remarkably different between rats and mice (19% in rats versus 2.3% in mice). In contrast, hepatic availability (Fh) was similar (APZ > TRZ > MDZ) in both species. Highly significant relationships were found between the ratio of the area under the plasma concentration–time curve (AUC) for the parent drugs in portal blood (AUCpor) to that in systemic blood (AUCsys) and Fh in rats and mice.The double-cannulated rat is useful for estimating the hepatic availability of drug candidates by determining the AUC values for the parent drugs in portal and systemic blood samples.

Differential effect of Shenmai injection, a herbal preparation, on the cytochrome P450 3A-mediated 1′-hydroxylation and 4-hydroxylation of midazolam

Shenmai injection ( SMI), one of the most popular herbal preparations, is widely used for the treatment of coronary atherosclerotic cardiopathy and viral myocarditis. The purpose of this study was to investigate the effect of Shenmai injection ( SMI) on the CYP3A-mediated metabolism of midazolam (MDZ). The present study demonstrated that SMI could significantly inhibit MDZ 4-hydroxylation but activate its 1′-hydroxylation in human liver microsomes (HLMs), rat liver microsomes (RLM) and recombinant human CYP3A4 and CYP3A5. The opposing effect of SMI was characterized by the kinetic change of increasing V max/ K m for MDZ 1′-hydroxylation and decreasing V max/ K m for MDZ 4-hydroxylation in HLM and RLM. The presence of SMI enhanced the inhibition of ketoconazole on MDZ 4-hydroxylation but weakened or reversed its inhibition on MDZ 1′-hydroxylation in HLM. After single or multiple pretreatment with SMI, the ratios of AUC 4-OH MDZ/AUC MDZ in rats were significantly decreased, while the ratios of AUC 1′-OH MDZ/AUC MDZ were increased. Among the major components in SMI, total ginsenoside (TG), ophiopogon total saponins (OTS), ophiopogon total flavone (OTF), ginsenoside Rd, ophiopogonin D and ophiopogonone A exhibited significant inhibition on both 4-hydroxylation and 1′-hydroxylation of MDZ in HLM and RLM, while no activation on MDZ metabolism was observed in the presence of these major constituents alone or together. To further explore the responsible components, 3 mL of SMI was loaded on a solid phase extraction (SPE) C 18 cartridge and then separated by different concentrations of methanol. The fractions eluted with 60% and 90% methanol both showed significant activation on MDZ 1′-hydroxylation in HLM, but the fraction eluted with 30% methanol had no such effect. The results indicated that the activation of SMI on MDZ 1′-hydroxylation might be mainly resulted from the lipid-soluble components in SMI.

Effects of Midazolam on Brain Injury After Transient Focal Cerebral Ischemia in Rats*

The benzodiazepine, midazolam, is commonly used for sedation and anesthesia in the operating room and the intensive care unit where there is a risk of cerebral ischemia. We therefore examined its ability to reduce damage subsequent to cerebral ischemia. Male Wistar rats were randomly assigned to a high-dose midazolam group or a matched vehicle group and a lower dose of midazolam group or a matched vehicle group. The rats underwent 90 minutes of middle cerebral artery occlusion. In the midazolam groups, the first dose of midazolam (10 or 25 mg/kg) was given by a 10-minute intravenous infusion before ischemia; a second dose of (1/2) the initial dose (5 or 12.5 mg/kg) was given 1 hour after the onset of ischemia. In the vehicle groups, a similar volume of vehicle was given at the same time intervals. Infarct size, NeuN immunopositive cells in the ischemic penumbral and core regions, and neurologic outcome were determined 7 days after ischemia. Compared with vehicle-treated rats, the higher-dose midazolam (25 mg/kg)-treated rats had a smaller infarct size (93.9+/-63.5 mm vs. 152.0+/-53.7 mm, P<0.05), more NeuN immunopositive cells in the ischemic core region (206.7+/-211.3/mm vs. 40.0+/-66.3/mm, P<0.01), and better neurologic outcome (P<0.05). Midazolam at a lower dose (10 mg/kg) had no significant effects. Although midazolam generated dose-dependent hemolysis, this hemolysis was transient. Midazolam (25 mg/kg) caused a loss of the righting reflex in rats that lasted until 19.9+/-1.3 min after the injection, the anesthetic dose of midazolam in rats is approximately 100x greater than the anesthetic dose for humans. An anesthetic dose of midazolam in rats reduced neuronal damage and improved neurologic outcome 7 days after focal cerebral ischemia, however, it also caused transient hemolysis.

Nonionic surfactants are strong inhibitors of cytochrome P450 3A biotransformation activity in vitro and in vivo

Nonionic surfactants are commonly used as excipients in pharmaceutical formulation, but recent studies demonstrate that the ingredients affect the pharmacokinetics of the active drugs. However, the mechanisms are largely unknown. Here, we examined the effects of four common nonionic surfactants polysorbate 20, polyoxyl 35 castor oil, polyoxyl 40 stearate and poloxamer 188, on cytochrome P450 3A in vitro and in vivo using midazolam as a probe. We first examined the effects of these surfactants on the 1′-hydroxylation of midazolam in isolated rat liver and intestinal microsomes. All the surfactants tested inhibited midazolam 1′-hydroxylation in a concentration-dependent manner and presented a mixed competitive inhibitory model with decreased V max and increased K m values in vitro. Among the tested nonionic surfactants, polysorbate 20 was the most potent inhibitor of midazolam 1′-hydroxylation with an IC 50 of 2.06 and 0.39 mg ml −1 in the liver and intestinal microsomes, respectively. These surfactants were also tested in vivo as we investigated their effects on the pharmacokinetics of midazolam in rats. These four surfactants displayed different inhibitory patterns in terms of the AUC of midazolam and 1′-hydroxymidazolam. Polysorbate 20 significantly increased both AUC 0–4 h and AUC 0– ∞ of midazolam and decreased the AUC 0–4 h of 1′-hydroxymidazolam to about 40% ( p < 0.05) in both single- and multiple-treated rats, along with a significant decrease of the metabolic ratio of 1′-hydroxymidazolam/midazolam to 25%. Polyoxyl 35 castor oil, polyoxyl 40 stearate and poloxamer 188 displayed complicated inhibition on the 1′-hydroxylation of midazolam dependent on the administration formula. These results confirmed that effects of these surfactants would have potential inhibitory effects on cytochrome P450 3A and altered midazolam bioavailability. Therefore, caution is needed when selecting nonionic surfactants in drug formulation.

Inhibitory effect of docosahexaenoic acid (DHA) on the intestinal metabolism of midazolam: In vitro and in vivo studies in rats

The aim of this study was to evaluate the effects of docosahexaenoic acid (DHA) on the intestinal cytochrome P450 isoenzyme (CYP3A) and P-glycoprotein (P-gp) functions using midazolam and rhodamine-123 as specific substrates of CYP3A and P-gp, respectively. Perfused everted intestinal segments from rats were employed to determine the effects of DHA on midazolam metabolism and rhodamine-123 transport. In addition, the effects of DHA on in vitro midazolam metabolism in rat intestinal microsomes and on midazolam bioavailability in rats were examined. The intestinal extraction ratio (ER G) of midazolam was determined to be 0.43 and decreased significantly to 0.12, 0.07, and 0.06 in the presence of 50, 100, and 200 μM DHA, respectively, in a concentration-dependent manner. The results from an in vitro study using rat intestinal microsomes demonstrated that DHA competitively inhibited the intestinal CYP3A activity with K i of 15.7 and 27.1 μM for the formations of 1′-OH midazolam and 4-OH midazolam, respectively. Moreover, the oral administration of DHA (100 mg/kg) increased the AUC ∞, C max, and oral bioavailability ( F) of midazolam by about 50% in rats, without affecting the T 1/2, V dss/ F, or CL tot/ F. In contrast, DHA did not change the serosal-to-mucosal transport of rhodamine-123 in the perfused everted intestine and oral administration of DHA (100 mg/kg) had no influence on the pharmacokinetics of intravenously administered midazolam in rats, thus suggesting that DHA has little effect on the intestinal P-gp activity and hepatic clearance of midazolam. This study provided the first direct evidence to show that DHA has an inhibitory effect on the intestinal pre-systemic metabolism of a CYP3A substrate and that DHA has little, if any, effect on the P-gp activity in the gut.

Impact of Curcumin-Induced Changes in P-Glycoprotein and CYP3A Expression on the Pharmacokinetics of Peroral Celiprolol and Midazolam in Rats

The aim of this study was to evaluate whether curcumin could modulate P-glycoprotein (P-gp) and CYP3A expression, and in turn modify the pharmacokinetic profiles of P-gp and CYP3A substrates in male Sprague-Dawley rats. Intragastric gavage of the rats with 60 mg/kg curcumin for 4 consecutive days led to a down-regulation of the intestinal P-gp level. There was a concomitant upregulation of hepatic P-gp level, but the renal P-gp level was unaffected. Curcumin also attenuated the CYP3A level in the small intestine but induced CYP3A expression in the liver and kidney. Regular curcumin consumption also caused the C(max) and area under the concentration-time curve (AUC(0-8) and total AUC) of peroral celiprolol (a P-gp substrate with negligible cytochrome P450 metabolism) at 30 mg/kg to increase, but the apparent oral clearance (CL(oral)) of the drug was reduced. Similarly, rats treated with curcumin for 4 consecutive days showed higher AUC (AUC(0-4) and total AUC) and lower CL(oral) for peroral midazolam (a CYP3A substrate that does not interact with the P-gp) at 20 mg/kg in comparison with vehicle-treated rats. In contrast, curcumin administered 30 min before the respective drug treatments did not significantly modify the pharmacokinetic parameters of the drugs. Analysis of the data suggests that the changes in the pharmacokinetic profiles of peroral celiprolol and midazolam in the rat model were contributed mainly by the curcumin-mediated down-regulation of intestinal P-gp and CYP3A protein levels, respectively.

Effect of methamphetamine on the pharmacokinetics of dextromethorphan and midazolam in rats

Methamphetamine is the fourth most frequently reported compound associated with drug abuse on admission of patients to treatment centres after cocaine, heroin and marijuana. It is metabolized in the organism with a reaction that is catalyzed by cytochrome P450, mainly by the CYP2D and CYP3A subfamily, 4-hydroxyamphetamine and amphetamine being dominant metabolites. The present pharmacokinetic study was undertaken to investigate the possible influence of methamphetamine (10 mg/kg, i.p., once daily for six days) on the pharmacokinetics of dextromethorphane as a model substrate for rat cytochrome P-4502D2 and midazolam as a model substrate for CYP3A1/2. Animals received a single injection of dextromethorphane (10 mg/kg) or midazolam (5 mg/kg) in the tail vein 24 h after the last dose of methamphetamine or administration of placebo. The results of pharmacokinetic analysis showed a significantly increased rate of dextrorphane and 3-hydroxymorphinan formation, and a marked stimulatory effect of methamphetamine on CYP2D2 metabolic activity. Similarly, the kinetics of midazolam's metabolic conversion to hydroxy derivates of midazolam indicated a significant increase in CYP3A1/2 activity. The results showed that the administration of methamphetamine significantly stimulated the metabolic activity of CYP2D2 as well as that of CYP3A1/2. With regard to the high level of homology between human and rat CYP isoforms studied, the results may have a clinical impact on future pharmacotherapy for methamphetamine abuse.

Cerebral and plasma kinetics of a high dose of midazolam and correlations with its respiratory effects in rats

Benzodiazepine poisoning causes coma and respiratory depression. Our objective was to determine whether, and to what extent, arterial blood gas disturbances correlated with blood or cerebral kinetics of midazolam. A 160 mg kg −1 single dose of midazolam was infused intravenously over 20 min in catheterized male Sprague–Dawley rats. Midazolam kinetics was simultaneously determined in plasma and brain using striatal microdialysis. Midazolam concentrations were measured using a high-performance liquid chromatographic assay with ultraviolet detection. Midazolam (160 mg kg −1) reproducibly induced deep coma with respiratory acidosis. Plasma midazolam kinetics was well described by a bi-exponential model, with an elimination half-life of 6.4 ± 1.8 h. The striatal dialysate concentration peaked at 50.0 ± 8.9 min after the end of infusion, with a significant delay to peak concentration compared to plasma. Respiratory depression, assessed by the elevation in PaCO 2, was more closely correlated with midazolam striatal dialysate rather than plasma kinetics. These results suggest a central mechanism for midazolam respiratory effects at toxic doses in rats. In conclusion, our study showed a delayed onset in peak PaCO 2 and pH effects after the slow infusion of a toxic dose of midazolam in rats. The effects on arterial blood gases were better correlated with midazolam striatal concentrations than with plasma concentrations. This study may contribute to better understanding of benzodiazepine-induced respiratory depression in poisonings.

Mechanism-based modeling of the pharmacodynamic interaction of alphaxalone and midazolam in rats.

The objective of the present investigation was to characterize the pharmacodynamic interaction between the synthetic neuroactive steroid alphaxalone and the benzodiazepine midazolam. The time course of the electroencephalographic (EEG) effect (11.5-30 Hz) was determined in rats in conjunction with plasma concentrations. Alphaxalone was administered as a continuous intravenous infusion of 0, 1.2, 2.2, or 5.2 mg over 360 min. Midazolam was administered as a 5-min intravenous bolus infusion of 4 mg.kg-1. The pharmacokinetic profiles of both drugs were described by a two-compartment model. No pharmacokinetic interaction was observed. The EEG effect versus time profiles of midazolam and alphaxalone, when administered separately and in combination, were modeled on the basis of the recently proposed mechanism-based pharmacokinetic/pharmacodynamic model for GABAA receptor modulators, which contains separate expressions to describe the drug-receptor interaction and the stimulus-response relationship. The pharmacodynamic interaction between alphaxalone and midazolam was best characterized using an independent drug-drug interaction model without an expression for allosteric modulation of the effect of midazolam by alphaxalone. The final model contained an exponential expression to account for acute functional adaptation to the EEG effect upon continuous infusion of alphaxalone. The mechanism-based analysis showed that this functional adaptation is best explained by a change in the system-specific stimulus-response relationship, rather than the drug-receptor activation process. It is concluded that the pharmacodynamic interaction between alphaxalone and midazolam in vivo is best described using an independent interaction model without allosteric modulation.

In vitro, pharmacokinetic, and pharmacodynamic interactions of ketoconazole and midazolam in the rat.

Interactions of midazolam and ketoconazole were studied in vivo and in vitro in rats. Ketoconazole (total dose of 15 mg/kg intraperitoneally) reduced clearance of intravenous midazolam (5 mg/kg) from 79 to 55 ml/min/kg (p < 0.05) and clearance of intragastric midazolam (15 mg/kg) from 1051 to 237 ml/min/kg (p < 0.05), increasing absolute bioavailability from 0.11 to 0.36 (p < 0.05). Presystemic extraction occurred mainly across the liver as opposed to the gastrointestinal tract mucosa. Midazolam increased electroencephalographic (EEG) amplitude in the beta-frequency range. Ketoconazole shifted the concentration-EEG effect relationship rightward (increase in EC(50)), probably because ketoconazole is a neutral benzodiazepine receptor ligand. Ketoconazole competitively inhibited midazolam hydroxylation by rat liver and intestinal microsomes in vitro, with nanomolar K(i) values. At a total serum ketoconazole of 2 microg/ml (3.76 microM) in vivo, the predicted reduction in clearance of intragastric midazolam by ketoconazole (to 6% of control) was slightly greater than the observed reduction in vivo (to 15% of control). However, unbound serum ketoconazole greatly underpredicted the observed clearance reduction. Although the in vitro and in vivo characteristics of midazolam in rats incompletely parallel those in humans, the experimental model can be used to assess aspects of drug interactions having potential clinical importance.

Effects of Single and Repeated Treatment with Itraconazole on the Pharmacokinetics of Midazolam in Rats

To estimate the influence of repeated administration of drug metabolism inhibitors on the extent of drug interaction, we investigated the effects of single intravenous or repeated oral administration of itraconazole on the pharmacokinetics of midazolam in rats. In the single administration study, the plasma concentration of itraconazole was maintained by intravenous infusion, and midazolam was administered into the portal vein to investigate its kinetics. In the repeated administration study, the kinetics of midazolam was investigated after seven-day oral treatment with itraconazole. The in vitro metabolism of midazolam and the contents of cytochrome P450 were investigated using liver microsomes from the itraconazole-treated rats. The area under the curve (AUC) of midazolam was increased by 1.45- or 1.44-fold after single or repeated itraconazole treatment, respectively. Meanwhile, the liver concentrations of itraconazole after single administration and repeated administration were 38.2 and 20.3 (nmol/g), respectively. In vitro maximum metabolic reaction velocity (V(max)) and Michaelis-Menten constant (K(m)) of midazolam were increased from 2.26 to 3.84 (nmol/min/mg protein) and from 8.28 to 13.0 (microM) by single itraconazole treatment, respectively, and decreased from 2.23 to 1.17 (nmol/min/mg protein) and from 7.86 to 4.47 (microM) by repeated treatment, respectively. Correspondingly, the content of CYP3A2 was significantly altered by single or repeated itraconazole administration. The increases in AUC could be predicted only when the changes in V(max) and K(m) were taken into consideration, in addition to the hepatic unbound concentration of itraconazole. In conclusion, changes in enzyme kinetics should be taken into account to predict the extent of drug interaction after repeated treatment with inhibitors.

In-vivo kinetics of the interaction between midazolam and erythromycin in rats, taking account of metabolic intermediate complex formation.

To predict, quantitatively, the extent of drug interaction during repeated administration of a metabolic inhibitor, we analysed the effects of erythromycin treatment under several regimens on the area under the concentration curve (AUC) of midazolam in rats. Midazolam was administered into the portal vein 12 h after erythromycin treatment for 1, 2 or 3 days, or 12, 24, 36, 48, 72 and 96 h after erythromycin treatment for 4 days, and the plasma-concentration profiles of midazolam were analysed to assess the AUC. Moreover, the contents of total cytochrome P450 and inactive metabolic intermediate (MI) complex were simultaneously quantitated. While the AUC value of midazolam was not affected by the administration of erythromycin for 1 day, repeated administration of erythromycin evoked an increase in AUC ratio (AUC in erythromycin-treated rats/AUC in vehicle-treated rats), which reached a maximum value of 1.99 at 12 h after 4 days' treatment with erythromycin. The total content of cytochrome P450 in liver microsomes was unaffected by erythromycin treatment. Although the MI complex was undetectable after 1 day's treatment with erythromycin, its content increased with duration of erythromycin treatment, and the complex disappeared after the end of erythromycin treatment with a half-life of 12.3 h. In conclusion, the interaction between erythromycin and midazolam could be well predicted when the formation of MI complex in the liver was taken into account.

Rate of change of blood concentrations is a major determinant of the pharmacodynamics of midazolam in rats.

The objective of this investigation was to characterize quantitatively the influence of the rate of increase in blood concentrations on the pharmacodynamics of midazolam in rats. The pharmacodynamics of midazolam were quantified by an integrated pharmacokinetic-pharmacodynamic modelling approach. Using a computer controlled infusion technique, a linear increase in blood concentrations up to 80 ng ml(-1) was obtained over different time intervals of 16 h, resulting in rates of rise of the blood concentrations of respectively, 1.25, 1.00, 0.87, 0.46, 0.34 and 0.20 ng ml(-1) min(-1). In one group of rats the midazolam concentration was immediately brought to 80 ng ml(-1) and maintained at that level for 4 h. Immediately after the pretreatment an intravenous bolus dose was given to determine the time course of the EEG effect in conjunction with the decline of midazolam concentrations. The increase in beta activity (11.5-30 Hz) of the EEG was used as pharmacodynamic endpoint. For each individual animal the relationship between blood concentration and the EEG effect could be described by the sigmoidal Emax model. After placebo, the values of the pharmacodynamic parameter estimates were Emax = 82+/-5 microV, EC50,u = 6.4+/-0.8 ng ml(-1) and Hill factor = 1.4+/-0.1. A bell-shaped relationship between the rate of change of midazolam concentration and the value of EC50,u was observed with a maximum of 21+/-5.0 ng ml(-1) at a rate of change of 0.46 ng ml(-1) min(-1); lower values of EC50,u were observed at both higher and lower rates. The findings of this study show that the rate of change in plasma concentrations is an important determinant of the pharmacodynamics of midazolam in rats.

FG 7142- and Restraint-Induced Alterations in the Ataxic Effects of Alcohol and Midazolam in Rats Are Time Dependent

The purpose of this study was to examine whether acute stress exposure would alter the ataxic properties of midazolam or ethanol in rats. Rats were administered either vehicle or FG 7142 (10 mg/kg) and placed back in their home cages, or placed in restraining tubes for 90 min. Three and one-half or 24 h following injection all subjects were then administered an ataxic dose of either ethanol or midazolam and after 10 min, motoric impairment was assessed by rotarod performance. Neither FG 7142 administration nor restraint had an impact on rotarod performance 3-1/2 h later for ethanol nor 24 h later in response to midazolam. However, midazolam-induced ataxia was significantly modified 3-1/2 h following both restraint and FG 7142 exposure. Similarly, at the 24-h time point, both manipulations had a significant effect on ethanol-induced motor incoordination. Importantly, prior exposure to FG 7142 and restraint was without effect on rotarod performance in saline-treated subjects. Functional alterations in behavioral reactivity to low doses of two classes of CNS depressants by the acute stress of restraint and/or FG 7142 administration suggest the anxiogenic nature of these stressors may be the critical factor.

Dose-dependent Intestinal and Hepatic First-pass Metabolism of Midazolam, a Cytochrome P450 3A Substrate with Differently Modulated Enzyme Activity in Rats

The dose-dependent first-pass metabolism of midazolam, a cytochrome P450 (CYP) 3A substrate, was separately estimated in the intestine and liver after administration into a jejunal loop of rats with differently modulated enzyme activity. Modulation of CYP3A enzyme activity of Sprague-Dawley rats was performed by pretreating the rats with inducers such as dexamethasone or by co-administering ketoconazole (an inhibitor) with midazolam. Bioavailabilities of midazolam administered into the jejunal loop at a dose of 10 micromol were 12% in untreated (control) rats, and 2% in dexamethasone-pretreated rats. Co-administered ketoconazole (2 micromol) significantly increased the bioavailability to 53% and 7%, respectively, in these rats. The intestinal first-pass metabolism of midazolam administered into the jejunal loop at a dose of 50 nmol in untreated and dexamethasone-pretreated rats, estimated by the mesenteric blood-collecting method in-situ, was 25% and 49% of absorbed amount, respectively. The intestinal first-pass metabolism of midazolam was reduced when ketoconazole (0.5 micromol) was co-administered or when the dose of midazolam was increased to 0.5 micrommol in these rats. Assuming that the contribution of intestinal first-pass metabolism could be negligible when midazolam was administered at a much higher dose of 10 micromol, the estimated hepatic first-pass metabolism of midazolam at a dose of 10 micromol in untreated rats, dexamethasone-pretreated rats, untreated rats given ketoconazole, and dexamethasone-pretreated rats given ketoconazole was, respectively, 86, 97, 46, and 92% of the amounts absorbed. In conclusion, the dose-dependent intestinal first-pass metabolism and the hepatic first-pass metabolism of midazolam in rats with differently modulated CYP3A activities was quantitatively estimated by in-vivo and in-situ absorption studies.

Effects of glucocorticoids on pharmacokinetics and pharmacodynamics of midazolam in rats

We investigated the effect of dexamethasone (80 mg kg per day for 2 days) and prednisolone (600 mg kg per day for 2 days, equivalent to dexamethasone for glucocorticoid (GC) potency) on both pharmacokinetics and pharmacodynamics of midazolam (MDZ), a substrate for cytochrome P450 (CYP) 3A, in 8-week-old male Sprague-Dawley rats. Animals received a single injection of MDZ (pharmacokinetic study, 10 mg kg ; pharmacodynamic study, 55.5 mg kg ) in the tail vein 24 h after the last dose of GC or placebo. The elimination half-life (t 1 2 ) and the area under the concentration-time curve of MDZ were significantly reduced by pretreatment with dexamethasone to 58.9% and 44.7% of the control value, respectively, and the clearance of MDZ was significantly increased by dexamethasone. Similar changes observed by prednisolone pretreatment did not reach significance. The t 1 2 of the dexamethasone pretreatment group (14.4 ± 0.7 min) was significantly shorter than that of the prednisolone group (20.9 ± 1.5 min). The amount of CYP3A2 protein and the activity of erythromycin N-demethylase were significantly increased by dexamethasone and prednisolone pretreatments, but dexamethasone showed a greater effect than prednisolone. Sleeping time was significantly shortened by dexamethasone and prednisolone pretreatment to 38.7% and 57.1% of control value, respectively. The current study demonstrates that the anesthetic effect of MDZ would be reduced in patients treated with dexamethasone or prednisolone, and that the CYP3A induction was greater by dexamethasone than by prednisolone, implying that the potency of CYP3A induction may differ among GCs even when GC activity is the same.

The use of etorphine/methotrimeprazine and midazolam as an anaesthetic technique in laboratory rats and mice.

The results of a preliminary evaluation of etorphine/methotrimeprazine ('Small Animal Immobilon') and midazolam in rats and mice are reported, and this regimen is compared to fentanyl/fluanisone/midazolam in mice. In rats, a surgical plane of anaesthesia with good muscle relaxation was produced, but blood gas analysis showed the presence of severe hypoxia, hypercapnia and acidosis. In mice etorphine/methotrimeprazine/midazolam and fentanyl/fluanisone/midazolam produced adequate anaesthesia, but blood gas analysis showed severe respiratory depression with both regimens. Since etorphine/methotrimeprazine/midazolam produced severe respiratory depression in rats and mice it is suggested that this regimen is used with caution. Administration of supplemental oxygen would seem advisable when using either etorphine/methotrimeprazine/midazolam or fentanyl/fluanisone/midazolam in rats and mice.

Discriminative stimulus effects of presynaptic GABA agonists in pentobarbital-trained rats

The discriminative stimulus effects of indirect-acting GABAergic drugs were compared to those of pentobarbital (PB) and midazolam in rats trained to discriminate 5 mg/kg PB from saline under a two-lever fixed-ratio 32 schedule of food reinforcement. PB and midazolam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Valproic acid, an antiepileptic drug and GABA transaminase inhibitor, substituted for PB but not only at a dose that produced response rate suppression. Vigabatrin, an irreversible GABA transaminase inhibitor, failed to substitute for PB, but did produce a dose-dependent decrease in response rates. The GABA uptake inhibitors, 1-[2-[bis[4-(trifluoromethyl)phenyl]- methoxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid (CI-966) and ( R(−)- N-[4,4- bis(3-methylthien-2-yl) but-3-enyl] nipecotic acid HCl (tiagabine), produced no greater than 40% PB-lever responding. Aminoxyacetic acid (AOAA), which is described as a nonselective presynaptic GABA agonist, yielded a maximum of 43% PB-lever responding. These results indicate that the discriminative stimulus effects of the indirect GABA A agonists, PB and midazolam, although similar to one another, differ from those of presynaptic GABAergic drugs. Differences in the discriminative stimulus properties of GABA transaminase inhibitors and uptake inhibitors also exist, indicating that not all presynaptic GABA agonists have similar behavioral profiles. These results contribute to a further understanding of the similarities and differences in the behavioral effects of drugs that enhance GABAergic neurotransmission.

Antinociceptive and motor effects of intrathecal morphine combined with intrathecal clonidine, noradrenaline, carbachol or midazolam in rats

This study investigated antinociceptive effects of intrathecal morphine combined with intrathecal clonidine, noradrenaline, carbachol or midazolam in rats. Each animal received intrathecally, on 3 separate occasions (i) 2 μg morphine (M), (ii) a dose (D) of one of the non-opioid drugs, and (iii) a combination, 1 2 (M + D), consisting of 1 μg morphine plus half the dose of the non-opioid drug. Antinociceptive effects were assessed by the hot-plate and tail-flick tests over the duration of drug action. All non-opioid drugs studied led to dose-related antinociceptive effects when given alone. Addition of morphine caused a left shift in the dose-response curves of all the non-opioid drugs, indicating at least some degree of additive effects. Effects were considered supra-additive when the effect of the combination, 1 2 (M + D), was significantly greater than both the effect of 2 μg morphine and the dose of non-opioid. Evidence of supra-additive antinociceptive effects was obtained only with the clonidine-morphine combination.