Differences in intrinsic efficacy of benzodiazepines are reflected in their concentration-EEG effect relationship

From mechanisms to markers: novel noninvasive EEG proxy markers of the neural excitation and inhibition system in humans

Brain function is a product of the balance between excitatory and inhibitory (E/I) brain activity. Variation in the regulation of this activity is thought to give rise to normal variation in human traits, and disruptions are thought to potentially underlie a spectrum of neuropsychiatric conditions (e.g., Autism, Schizophrenia, Downs' Syndrome, intellectual disability). Hypotheses related to E/I dysfunction have the potential to provide cross-diagnostic explanations and to combine genetic and neurological evidence that exists within and between psychiatric conditions. However, the hypothesis has been difficult to test because: (1) it lacks specificity-an E/I dysfunction could pertain to any level in the neural system- neurotransmitters, single neurons/receptors, local networks of neurons, or global brain balance - most researchers do not define the level at which they are examining E/I function; (2) We lack validated methods for assessing E/I function at any of these neural levels in humans. As a result, it has not been possible to reliably or robustly test the E/I hypothesis of psychiatric disorders in a large cohort or longitudinal patient studies. Currently available, in vivo markers of E/I in humans either carry significant risks (e.g., deep brain electrode recordings or using Positron Emission Tomography (PET) with radioactive tracers) and/or are highly restrictive (e.g., limited spatial extent for Transcranial Magnetic Stimulation (TMS) and Magnetic Resonance Spectroscopy (MRS). More recently, a range of novel Electroencephalography (EEG) features has been described, which could serve as proxy markers for E/I at a given level of inference. Thus, in this perspective review, we survey the theories and experimental evidence underlying 6 novel EEG markers and their biological underpinnings at a specific neural level. These cheap-to-record and scalable proxy markers may offer clinical utility for identifying subgroups within and between diagnostic categories, thus directing more tailored sub-grouping and, therefore, treatment strategies. However, we argue that studies in clinical populations are premature. To maximize the potential of prospective EEG markers, we first need to understand the link between underlying E/I mechanisms and measurement techniques.

Intravenous and Intramuscular Allopregnanolone for Early Treatment of Status Epilepticus: Pharmacokinetics, Pharmacodynamics, and Safety in Dogs

Allopregnanolone (ALLO) is a neurosteroid that modulates synaptic and extrasynaptic GABAA receptors. We hypothesize that ALLO may be useful as first-line treatment of status epilepticus (SE). Our objectives were to (1) characterize ALLO pharmacokinetics-pharmacodynamics PK-PD after intravenous (IV) and intramuscular (IM) administration and (2) compare IV and IM ALLO safety and tolerability. Three healthy dogs and two with a history of epilepsy were used. Single ALLO IV doses ranging from 1-6 mg/kg were infused over 5 minutes or injected IM. Blood samples, vital signs, and sedation assessment were collected up to 8 hours postdose. Intracranial EEG (iEEG) was continuously recorded in one dog. IV ALLO exhibited dose-proportional increases in exposure, which were associated with an increase in absolute power spectral density in all iEEG frequency bands. This relationship was best described by an indirect link PK-PD model where concentration-response was described by a sigmoidal maximum response (Emax) equation. Adverse events included site injection pain with higher IM volumes and ataxia and sedation associated with higher doses. IM administration exhibited incomplete absorption and volume-dependent bioavailability. Robust iEEG changes after IM administration were not observed. Based on PK-PD simulations, a 2 mg/kg dose infused over 5 minutes is predicted to achieve plasma concentrations above the EC50, but below those associated with heavy sedation. This study demonstrates that ALLO is safe and well tolerated when administered at 1-4 mg/kg IV and up to 2 mg/kg IM. The rapid onset of effect after IV infusion suggests that ALLO may be useful in the early treatment of SE. SIGNIFICANCE STATEMENT: The characterization of the pharmacokinetics and pharmacodynamics of allopregnanolone is essential in order to design clinical studies evaluating its effectiveness as an early treatment for status epilepticus in dogs and people. This study has proposed a target dose/therapeutic range for a clinical trial in canine status epilepticus.

NS11821, a partial subtype-selective GABAA agonist, elicits selective effects on the central nervous system in randomized controlled trial with healthy subjects

NS11821 is a partial GABAA agonist with relatively dominant α2,3 and α5 subtype efficacy but negligible α1 agonism. This first-in-human study was performed in healthy male subjects using a single-dose, parallel, double blind, placebo-controlled, randomized, dose-escalation study design. In total six cohorts (N=48) were enrolled. The eight subjects of each cohort received NS11821 (10 mg, 30 mg, 75 mg, 150 mg, 300 mg or 600 mg) or placebo in a 6:2 ratio. At low dose levels, NS11821 had a relatively low exposure and a more-than-proportional increase of the area under the curve and maximum plasma concentrations, probably due to poor solubility. Saccadic peak velocity decreased in a dose-related manner while limited impairments were seen on body sway and the visual analogue scale for alertness. The most common adverse events were somnolence and dizziness, which were more prominent with the higher doses. Although no positive control was used in this study, the results were compared post hoc with a Centre for Human Drug Research dataset for lorazepam 2 mg. The maximum saccadic peak velocity effects seemed comparable to the typical effects of lorazepam, whereas the other central nervous system effects were smaller. These results support the pharmacological selectivity of NS11821 and show that pharmacodynamic effective doses of NS11821 were safe and well tolerated in healthy subjects.

Pharmacokinetic-pharmacodynamic modelling in anaesthesia

Anaesthesiologists adjust drug dosing, administration system and kind of drug to the characteristics of the patient. They then observe the expected response and adjust dosing to the specific requirements according to the difference between observed response, expected response and the context of the surgery and the patient. The approach above can be achieved because on one hand quantification technology has made significant advances allowing the anaesthesiologist to measure almost any effect by using noninvasive, continuous measuring systems. On the other the knowledge on the relations between dosing, concentration, biophase dynamics and effect as well as detection of variability sources has been achieved as being the benchmark specialty for pharmacokinetic-pharmacodynamic (PKPD) modelling. The aim of the review is to revisit the most common PKPD models applied in the field of anaesthesia (i.e. effect compartmental, turnover, drug-receptor binding and drug interaction models) through representative examples. The effect compartmental model has been widely used in this field and there are multiple applications and examples. The use of turnover models has been limited mainly to describe respiratory effects. Similarly, cases in which the dissociation process of the drug-receptor complex is slow compared with other processes relevant to the time course of the anaesthetic effect are not frequent in anaesthesia, where in addition to a rapid onset, a fast offset of the response is required. With respect to the characterization of PD drug interactions different response surface models are discussed. Relevant applications that have changed the way modern anaesthesia is practiced are also provided.

A PK-PD model of ketamine-induced high-frequency oscillations

OBJECTIVE

Ketamine is a widely used drug with clinical and research applications, and also known to be used as a recreational drug. Ketamine produces conspicuous changes in the electrocorticographic (ECoG) signals observed both in humans and rodents. In rodents, the intracranial ECoG displays a high-frequency oscillation (HFO) which power is modulated nonlinearly by ketamine dose. Despite the widespread use of ketamine there is no model description of the relationship between the pharmacokinetic-pharmacodynamics (PK-PDs) of ketamine and the observed HFO power.

APPROACH

In the present study, we developed a PK-PD model based on estimated ketamine concentration, its known pharmacological actions, and observed ECoG effects. The main pharmacological action of ketamine is antagonism of the NMDA receptor (NMDAR), which in rodents is accompanied by an HFO observed in the ECoG. At high doses, however, ketamine also acts at non-NMDAR sites, produces loss of consciousness, and the transient disappearance of the HFO. We propose a two-compartment PK model that represents the concentration of ketamine, and a PD model based in opposing effects of the NMDAR and non-NMDAR actions on the HFO power.

MAIN RESULTS

We recorded ECoG from the cortex of rats after two doses of ketamine, and extracted the HFO power from the ECoG spectrograms. We fit the PK-PD model to the time course of the HFO power, and showed that the model reproduces the dose-dependent profile of the HFO power. The model provides good fits even in the presence of high variability in HFO power across animals. As expected, the model does not provide good fits to the HFO power after dosing the pure NMDAR antagonist MK-801.

SIGNIFICANCE

Our study provides a simple model to relate the observed electrophysiological effects of ketamine to its actions at the molecular level at different concentrations. This will improve the study of ketamine and rodent models of schizophrenia to better understand the wide and divergent range of effects that ketamine has.

EEG-β/γ spectral power elevation in rat: a translatable biomarker elicited by GABA(Aα2/3)-positive allosteric modulators at nonsedating anxiolytic doses

Benzodiazepine drugs, through interaction with GABA(Aα1), GABA(Aα2,3), and GABA(Aα5) subunits, modulate cortical network oscillations, as reflected by a complex signature in the EEG power spectrum. Recent drug discovery efforts have developed GABA(Aα2,3)-subunit-selective partial modulators in an effort to dissociate the side effect liabilities from the efficacy imparted by benzodiazepines. Here, we evaluated rat EEG and behavioral end points during dosing of nine chemically distinct compounds that we confirmed statistically to selectively to enhance GABA(Aα2,3)-mediated vs. GABA(Aα1) or GABA(Aα5) currents in voltage clamped oocytes transfected with those GABA(A) subunits. These compounds were shown with in vivo receptor occupancy techniques to competitively displace [(3)H]flumazenil in multiple brain regions following peripheral administration at increasing doses. Over the same dose range, the compounds all produced dose-dependent EEG spectral power increases in the β- and and γ-bands. Finally, the dose range that increased γ-power coincided with that eliciting punished over unpunished responding in a behavioral conflict model of anxiety, indicative of anxiolysis without sedation. EEG γ-band power increases showed a significant positive correlation to in vitro GABA(Aα2,3) modulatory intrinsic activity across the compound set, further supporting a hypothesis that this EEG signature was linked specifically to pharmacological modulation of GABA(Aα2,3) signaling. These findings encourage further evaluation of this EEG signature as a noninvasive clinical translational biomarker that could ultimately facilitate development of GABA(Aα2,3)-subtype-selective drugs for anxiety and potentially other indications.

Electroencephalogram effects of armodafinil: comparison with behavioral alertness

Development of central nervous system-acting drugs would be enhanced by suitable biomarkers that reflect the targeted pathophysiologic brain state. The electroencephalogram (EEG) has several characteristics of an ideal biomarker and can be promptly adapted to pre-clinical and clinical testing. The aim of this study was to evaluate EEG as a measure of the wakefulness-promoting effect of armodafinil in sleep deprived healthy subjects. Armodafinil pharmacodynamics were simultaneously assessed by EEG- and behavioral-based measures including a well-established measure of alertness. Using two quantitative EEG-based measures-power spectral and event-related brain activity analyses-we observed that armodafinil mitigated the slowing of brain activity and the decrease of the event-related brain activity caused by sleep deprivation. Armodafinil-induced changes in EEG are in agreement and explain up to 73.1% of the armodafinil-induced changes in alertness. Our findings suggest that EEG can serve as a marker of the wakefulness-promoting drug effect.

Dynamics and Kinetics of a Modified-Release Formulation of Zolpidem: Comparison With Immediate-Release Standard Zolpidem and Placebo

Modified-release (MR) zolpidem was developed to maintain effective plasma concentrations during the 3- to 6-hour post-dosage interval, corresponding to the middle portion of the typical sleep interval. Modified-release zolpidem (12.5 mg), standard immediate-release (IR) zolpidem (10 mg), and placebo were compared in a double-blind, single-dose, 3-way crossover daytime study of healthy volunteers (n = 70 completers). Effect areas for electroencephalographic beta amplitude during 0 to 8 hours and 3 to 6 hours after dosage were greater for MR compared to IR (P < .001). The digit-symbol substitution test and sedation rating scales behaved similarly. MR and IR did not differ in effects at 8 hours post-dosage nor in halflife or clearance. Time of peak plasma concentration (tmax) was significantly longer for MR (2.4 vs 2.0 hours, P < .004), and dose-normalized peak plasma concentration (Cmax) was lower (12.2 vs 14.0 ng/mL/mg, P < .001). MR zolpidem also had greater area under the plasma concentration curve (AUC) during the 3- to 6-hour interval (P < .001). Thus, MR zolpidem produces sustained plasma levels compared to IR, with resulting enhancement of pharmacodynamic effects in the 3- to 6-hour post-dosage interval.

The hypnotic zolpidem increases the synchrony of BOLD signal fluctuations in widespread brain networks during a resting paradigm

Networks of brain regions having synchronized fluctuations of the blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) time-series at rest, or "resting state networks" (RSNs), are emerging as a basis for understanding intrinsic brain activity. RSNs are topographically consistent with activity-related networks subserving sensory, motor, and cognitive processes, and studying their spontaneous fluctuations following acute drug challenge may provide a way to understand better the neuroanatomical substrates of drug action. The present within-subject double-blind study used BOLD fMRI at 3T to investigate the functional networks influenced by the non-benzodiazepine hypnotic zolpidem (Ambien). Zolpidem is a positive modulator of γ-aminobutyric acid(A) (GABA(A)) receptors, and engenders sedative effects that may be explained in part by how it modulates intrinsic brain activity. Healthy participants (n=12) underwent fMRI scanning 45 min after acute oral administration of zolpidem (0, 5, 10, or 20mg), and changes in BOLD signal were measured while participants gazed at a static fixation point (i.e., at rest). Data were analyzed using group independent component analysis (ICA) with dual regression and results indicated that compared to placebo, the highest dose of zolpidem increased functional connectivity within a number of sensory, motor, and limbic networks. These results are consistent with previous studies showing an increase in functional connectivity at rest following administration of the positive GABA(A) receptor modulators midazolam and alcohol, and suggest that investigating how zolpidem modulates intrinsic brain activity may have implications for understanding the etiology of its powerful sedative effects.

The EEG signal: a window on the cortical brain activity

The accurate assessment of the depth of anesthesia, allowing a more accurate adaptation of the doses of hypnotics, is an important end point for the anesthesiologist. It is a particularly crucial issue in pediatric anesthesia, in the context of the recent controversies about the potential neurological consequences of the main anesthetic drugs on the developing brain. The electroencephalogram signal reflects the electrical activity of the neurons in the cerebral cortex. It is thus the key to assessment of the level of hypnosis. Beyond visual analysis, several monitoring devices allow an automated treatment of the electroencephalographic (EEG) signal, combining time and frequency domain analysis. Each of these monitors focuses on a specific combination of characteristics of the signal and provides the clinician with useful information that remains, however, partial. For a comprehensive approach of the EEG-derived indices, the main features of the normal EEG, in adults and children, will be presented in the awake state and during sleep. Age-related modifications accompanying cerebral maturation during infancy and childhood will be detailed. Then, this review will provide an update on how anesthetic drugs, particularly hypnotics, influence the EEG signal, and how the main available monitors analyze these drug-induced modifications. The relationships between pain, memory, and the EEG will be discussed. Finally, this review will focus on some specific EEG features such as the electrical epileptoid activity observed under sevoflurane anesthesia. The EEG signal is the best window we have on cortical brain activity and provides a fair pharmacodynamic feedback of the effects of hypnotics. However, the cortex is only one of several targets of anesthesia. Hypnotics and opiates, have also subcortical primary targets, and the EEG performances in the evaluation or prediction of nociception are poor. Monitoring subcortical structures in combination with the EEG might in the future allow a better evaluation and a more precise adaptation of balanced anesthesia.

Comparison of effects of propofol and midazolam at sedative concentrations on sympathetic tone generation in the isolated spinal cord of neonatal rats

BACKGROUND

Propofol and midazolam are common sedatives for critically ill patients. Little is known about the effects of propofol and midazolam on central sympathetic activity when drug concentrations in extracellular milieu are under precise control. Previous work using an in vitro neonatal rat splanchnic nerve-spinal cord preparation has demonstrated that tonic sympathetic activity is generated spontaneously in the thoracic spinal cord. The aim of this study was to investigate the concentration effects of propofol and midazolam on spinally generated sympathetic activity.

METHODS

Using an in vitro neonatal rat splanchnic nerve-spinal cord preparation that allows the precise control of drug concentrations, the central sympathetic effects elicited by the application of propofol (10-640 microM) and midazolam (10-640 microM) were compared.

RESULTS

There was a prompt decrease in sympathetic activity on application of propofol or midazolam in a concentration-dependent manner. A significant decrease in sympathetic activity was observed on application of propofol at 80-640 microM; however, the application of propofol at 10-40 microM caused only a slight alteration in activity. The sympathetic activity was not altered significantly by 10 microM of midazolam, but the application of midazolam at 20-640 microM caused a significant decrease in activity. Thus, in these experimental conditions, the minimum concentration of propofol causing a significant decrease in sympathetic activity was 80 microM and that of midazolam was 20 microM.

CONCLUSIONS

The current findings suggest that the administration of 9-19 microM of propofol or 0.7-0.9 microM of midazolam, the clinically relevant concentrations for sedation, does not alter central sympathetic outflow at the spinal cord level. However, propofol at a concentration of 86 microM, which could be achieved by a single-bolus loading dose to induce sedation, depresses central sympathetic activity.

Mechanism-Based Pharmacokinetic-Pharmacodynamic Modeling: Biophase Distribution, Receptor Theory, and Dynamical Systems Analysis

Mechanism-based PK-PD models differ from conventional PK-PD models in that they contain specific expressions to characterize, in a quantitative manner, processes on the causal path between drug administration and effect. This includes target site distribution, target binding and activation, pharmacodynamic interactions, transduction, and homeostatic feedback mechanisms. As the final step, the effects on disease processes and disease progression are considered. Particularly through the incorporation of concepts from receptor theory and dynamical systems analysis, important progress has been made in the field of mechanism-based PK-PD modeling. This has yielded models with much-improved properties for extrapolation and prediction. These models constitute a theoretical basis for rational drug discovery and development.

A recursive-partitioning model for blood–brain barrier permeation

A series of bagged recursive partitioning models for log(BB) is presented. Using a LGO-CV, three sets of physical property descriptors are evaluated and found to have Q2 values of 0.51 (CPSA), 0.53 (Ro5x) and 0.53 (MOE). Extrapolating these models to Pfizer chemical space is difficult due to P-glycoprotein (P-gp) mediated efflux. Low correlation coefficients for this test set are improved (R2 = 0.39) when compounds known to be P-gp substrates or statistical extrapolations are removed. The use of simple linear models for specific chemical series is also found to improve the correlation over a limited chemical space.

Relationship between exposure and nonspecific binding of thirty-three central nervous system drugs in mice

Unbound fractions in mouse brain and plasma were determined for 31 structurally diverse central nervous system (CNS) drugs and two active metabolites. Three comparisons were made between in vitro binding and in vivo exposure data, namely: 1) mouse brain-to-plasma exposure versus unbound plasma-to-unbound brain fraction ratio (fu(plasma)/fu(brain)), 2) cerebrospinal fluid-to-brain exposure versus unbound brain fraction (fu(brain)), and 3) cerebrospinal fluid-to-plasma exposure versus unbound plasma fraction (fu(plasma)). Unbound fraction data were within 3-fold of in vivo exposure ratios for the majority of the drugs examined (i.e., 22 of 33), indicating a predominately free equilibrium across the blood-brain and blood-CSF barriers. Some degree of distributional impairment at either the blood-CSF or the blood-brain barrier was indicated for 8 of the 11 remaining drugs (i.e., carbamazepine, midazolam, phenytoin, sulpiride, thiopental, risperidone, 9-hydroxyrisperidone, and zolpidem). In several cases, the indicated distributional impairment is consistent with other independent literature reports for these drugs. Through the use of this approach, it appears that most CNS-active agents freely equilibrate across the blood-brain and blood-CSF barriers such that unbound drug concentrations in brain approximate those in the plasma. However, these results also support the intuitive concept that distributional impairment does not necessarily preclude CNS activity.

Local and remote functional connectivity of neocortex under the inhibition influence

The current paper focuses on a relatively new and promising area of the study of EEG transformations during brain information processing based on the reduction of the signal to the discrete quasi-stationary segment sequences which may reflect individual brain microstates or discrete operations. In this framework, the complex brain functions require integration of several operations throughout the whole neocortex. However, the role of inhibitory brain systems in such processes is still unsettled. The effects of a single dose (30 microg/kg) of lorazepam on the operational activity of neuronal populations and on the temporal binding between them were examined in a double-blind randomized crossover placebo-controlled study with eight healthy volunteers. EEG measures at 20 channels were evaluated on two occasions: (1) eyes closed, (2) eyes open. In short, we conducted a two-by-two factorial study where one factor manipulated GABAergic neurotransmission (lorazepam vs. placebo), and the other factor was simply brain state (eyes closed vs. eyes opened). We were primarily interested in the main effect of lorazepam. In the present study, a connection between the mesoscopic level, described by the local functional processes (neuronal assemblies or populations) and the macroscopic level, described as a sequence of metastable brain states (remote functionally synchronized neuronal populations) was established. The role of inhibitory brain systems facilitated by lorazepam in the operational dynamics of neuronal populations and in the process of EEG structural synchrony (SS) (topological peculiarities) was addressed for the first time. It was shown that GABA signaling reorganized the dynamics of local neuronal populations and the remote functional connectivity between them.

Enhancement of GABA-related signalling is associated with increase of functional connectivity in human cortex

Structural or operational synchrony analysis with EEG was conducted in order to detect functional interaction between cortical areas during an enhanced inhibition induced by the GABAergic agonist lorazepam in a double-blind, randomized, placebo-controlled, cross-over study in eight healthy human subjects. Specifically, we investigated whether a neuronal inhibitory system in the brain mediates functional decoupling of cortical areas. Single-dose lorazepam administration resulted in a widespread increase in the inter-area functional connectivity and an increase in the strength of functional long-range and interhemispheric connections. These results suggest that inhibition can be an efficient mechanism for synchronization of large neuronal populations.

The interplay of lorazepam-induced brain oscillations: microstructural electromagnetic study

OBJECTIVE

The effects on cortical rhythms of a single-dose (30 microg/kg) administration of the GABAA agonist lorazepam were examined in a randomized, double-blind, cross-over, placebo-controlled study with 8 healthy volunteers using simultaneous electroencephalography (EEG) and magnetoencephalography (MEG).

METHODS

The oscillations were assessed by means of adaptive classification of short-term spectral patterns.

RESULTS

Lorazepam (a) decreased the percentage of EEG/MEG segments with fast-theta, delta-alpha, fast-theta-alpha and alpha activity and increased percentage of EEG/MEG segments with delta, delta-slow-theta, delta-beta, slow-theta and polyrhythmic activity; (b) decreased diversity of EEG/MEG signals (in terms of spectral patterns) and increased the general instability of the signal; (c) increased stabilization periods of the spectral patterns (reduced brain information processing); (d) maintained larger maximum periods of temporal stabilization for delta, slow-theta, delta-slow-theta, delta-beta and polyrhythmic activity (in terms of spectral patterns); (e) did not increase power in the independent beta rhythm.

CONCLUSIONS

Lorazepam caused significant reorganization of the EEG/MEG microstructure. These results suggest also that adaptive classification analysis of single short-term spectral patterns may provide additional information to conventional spectral analyses.

Behavioral effects of flumazenil in the social conflict test in mice

RATIONALE

Flumazenil, a competitive antagonist of benzodiazepine receptors (BZRs), has been used as a probe to detect effects of putative endogenous ligands for BZRs in anxiety. Flumazenil is renowned for its highly inconsistent behavioral effects.

OBJECTIVE

To ascertain effects of flumazenil in the social conflict test in mice, which provides complex measures for prediction of anxiolytic and anxiogenic activity of drugs in behaviorally different groups of animals.

METHODS

Singly housed male mice treated with flumazenil (5, 20 or 80 mg/kg i.p.) or vehicle were paired with untreated non-aggressive group-housed male mice in a novel cage. Behavior was analyzed from video tapes of the social interactions in three populations of mice: timid (n=21), aggressive (n=11), and sociable (n=7). Levels of gamma-aminobutyric acid (GABA) were measured in vivo in the prefrontal cortex.

RESULTS

Flumazenil reduced timid (defensive-escape) and increased locomotor activities in timid mice. The drug reduced aggressive and increased sociable (social investigation) activities in aggressive mice. These behavioral changes were produced at the lowest dose of flumazenil tested (5 mg/kg) and were not increased further by higher doses of the drug (20 mg/kg or 80 mg/kg). A tendency to increased timidity was found after flumazenil in sociable mice. Concentrations of GABA were markedly higher in the prefrontal cortex of sociable mice than in timid or aggressive mice.

CONCLUSIONS

Flumazenil produced moderate anxiolytic-like behavioural changes and a slight anxiogenic-like effect. The present data might be reflecting antagonism of corresponding endogenous BZR ligands. However, these putative ligands seem to exert only modest modulatory influence.

Quantified EEG analysis monitoring in a novel model of general anaesthesia in rats

The aim of this research was to evaluate the safety and reliability of an anaesthetic mixture (Equitensine: pentobarbital, chloral hydrate, dihydroxypropane, ethanol) which, unlike other 'classic' anaesthetics, such as ketamine [The Electroencephalogram in Anaesthesia, Springer, Berlin, 1984], has been demonstrated not to induce alterations in the extracellular concentrations of cerebral excitatory amino acids. Quantified EEG analysis monitoring and behavioural observation were used to quantify the degree and the time course of the changes in cerebral electrical activity, analgesia and sedation induced, in rats, by the compound under investigation. Equitensine (0.33 ml/100 g), administered intraperitoneally, induced analgesia (monitored by the tail flick method) for 60-70 min and a pattern of behavioural sedation (loss of the righting reflex) lasting, on average, 130-150 min. The EEG monitoring revealed a pattern typical of burst suppression which lasted 15-20 min, followed by another, lasting 270-300 min, characterized by slow waves of high amplitude. The quantified EEG analysis demonstrated that the changes in cerebral electrical activity lasted longer than behavioural observation suggested. The compound under examination was found to be safe, reliable and non-invasive to administer and sustain in all the animals, and quantified EEG analysis proved to be a very sensitive method for highlighting the functional changes in the central nervous system.

Pharmacodynamic analysis of the interaction between tiagabine and midazolam with an allosteric model that incorporates signal transduction

PURPOSE

The objective of this study was to characterize quantitatively the pharmacodynamic interaction between midazolam (MDL), an allosteric modulator of the gamma-aminobutyric acid subtype A (GABAA) receptor, and tiagabine (TGB), an inhibitor of synaptic GABA uptake.

METHODS

The in vivo concentration-response relation of TGB was determined through pharmacokinetic/pharmacodynamic (PK/PD) modeling. Rats received a single intravenous dose of 10 mg/kg TGB in the absence and the presence of a steady-state plasma concentration of MDL. The EEG response in the 11.5- to 30-Hz frequency band was used as the pharmacodynamic end point.

RESULTS

Infusion of MDL resulted in a mean steady-state plasma concentration of 66 +/- 3 ng/ml. A significant pharmacokinetic interaction with TGB was observed. MDL inhibited TGB clearance by 20 +/- 7 ml/min/kg from the original value of 89 +/- 6 ml/min/kg. However, no changes in plasma protein binding of both drugs were observed. The concentration-EEG relation of TGB was described by the sigmoid-Emax model. The pharmacodynamic parameter estimates of TGB were: Emax = 327 +/- 10 microV, EC50 = 392 +/- 20 ng/ml, and nH = 3.1 +/- 0.3. These values were not significantly different in the presence of MDL. Factors that may explain the lack of synergism were identified by a mechanism-based interaction model that separates the receptor activation from the signal-transduction process. High efficiency of signal transduction and the presence of a baseline response were shown to diminish the degree of synergism.

CONCLUSIONS

We conclude that the in vivo pharmacodynamic interaction between MDL and TGB is additive rather than synergistic. This strongly suggests that allosteric modulation of the antiseizure activity of a GAT-1 inhibitor by a benzodiazepine does not offer a therapeutic advantage.

Neuroactive steroids differ in potency but not in intrinsic efficacy at the GABA(A) receptor in vivo

The objective of the present investigation was to characterize the in vivo EEG effects of (synthetic) neuroactive steroids on the basis of a recently proposed mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) model. After intravenous administration, the time course of the EEG effect of pregnanolone, 2beta-3alpha-5alpha-3-hydroxy-2-(2,2-dimethylmorpholin-4-yl)-pregnan-11,20-dione (ORG 21465), 2beta-3alpha-5alpha-21-chloro-3-hydroxy-2-(4-morpholinyl)-pregnan-20-one (ORG 20599), and alphaxalone was determined in conjunction with plasma concentrations in rats. For each neuroactive steroid the PK/PD correlation was described on the basis of a two-compartment pharmacokinetic model with an effect compartment to account for hysteresis. The observed concentration EEG effect relationships were biphasic and characterized with a mechanism-based pharmacodynamic model, which is based on a separation between the receptor activation process and the stimulus-response relationship. A single unique biphasic stimulus-response relationship could be identified for all neuroactive steroids, which was successfully described by a parabolic function. The receptor activation process was described by a hyperbolic function. Estimates for the maximum activation (e(PD)) were similar for the different neuroactive steroids but values of the potency estimate (K(PD)) ranged from 157 +/- 16 ng. ml(-1) for pregnanolone, 221 +/- 83 ng. ml(-1) for ORG 20599, and 483 +/- 42 ng. ml(-1) for alphaxalone to 1619 +/- 208 ng. ml(-1) for ORG 21465. A statistically significant correlation was observed between the in vivo potency and the IC(50) in an in vitro [(35)S]t-butylbicyclophosphorothionate binding assay (r = 0.91). It is concluded that the new PK/PD model constitutes a new mechanism-based approach to the quantification of the effects of (synthetic) neuroactive steroids in vivo effects. The results show that the neuroactive steroids differ in potency but not in intrinsic efficacy at the GABA(A) receptor in vivo.

Mechanism-based pharmacodynamic modeling of the interaction of midazolam, bretazenil, and zolpidem with ethanol

The pharmacokinetic and pharmacodynamic interactions of ethanol with the full benzodiazepine agonist midazolam, the partial agonist bretazenil and the benzodiazepine BZ1 receptor subtype selective agonist zolpidem have been determined in the rat in vivo, using an integrated pharmacokinetic-pharmacodynamic approach. Ethanol was administered as a constant rate infusion resulting in constant plasma concentrations of 0.5 g/l. The pharmacokinetics and pharmacodynamics of midazolam, bretazenil, and zolpidem were determined following an intravenous infusion of 5.0, 2.5, and 18 mg/kg respectively. The amplitude in the 11.5-30 Hz frequency band of the EEG was used as measure of the pharmacological effect. For each of the benzodiazepines the concentration-EEG effect relationship could be described by the sigmoid Emax pharmacodynamic model. Significant differences in both EC50 and Emax were observed. The values of the EC50 were 76 +/- 11, 12 +/- 3, and 512 +/- 116 ng/ml for midazolam, bretazenil, and zolpidem respectively. The values of the Emax were 113 +/- 9, 44 +/- 3, and 175 +/- 10 microV/s. In the presence of ethanol the values of the EC50 of midazolam and zolpidem were reduced to approximately 50% of the original value. The values for Emax and Hill-factor were unchanged Due to a large interindividual variability no significant change in EC50 was observed for bretazenil. Analysis of the data on basis of a mechanism-based model showed only a decrease in the apparent affinity constant KPD for all three drugs, indicating that changes in EC50 can be explained entirely by a change in the apparent affinity constant KPD without concomitant changes in the efficacy parameter ePD and the stimulus-effect relationship. The findings of this study show that the pharmacodynamic interactions with a low dose of ethanol in vivo are qualitatively and quantitatively similar for benzodiazepine receptor full agonists, partial agonists, and benzodiazepine BZ1 receptor subtype selective agonists. This interaction can be explained entirely by a change in the affinity of the biological system for each benzodiazepine.

Effect of 7-day exposure to midazolam on electroencephalogram pharmacodynamics in rats: a model to study multiple pharmacokinetic-pharmacodynamic relationships in individual animals

The objective of this study was to determine the concentration-electroencephalogram (EEG) relationships for midazolam, a full-agonist benzodiazepine ligand, on multiple occasions in individual rats, and to examine the effect of chronic midazolam exposure on that relationship. Rats were chronically instrumented with venous and arterial cannulas, and cortical EEG electrodes. The rats received either: 7 days of midazolam 10 mg kg(-1) intravenously once a day (midazolam group); or midazolam on days 1 and 7 and vehicle on days 2-6 (vehicle group). Concentration-effect relationships were determined on days 1, 4 and 7 from multiple blood and EEG samples before and after the administration of the midazolam dose. The concentration-EEG effect relationships were consistent with a sigmoidal Emax (maximal effect) model. No differences in pharmacokinetic or pharmacodynamic parameters were found between day 1 and day 7 in either group. However, in the midazolam group, both the fraction unbound of midazolam in serum and the EC50 (concentration at half-maximal effect) for free midazolam increased from days 1-7 by 35 +/- 3% and 54 +/- 25%, respectively (means +/- s.d., P< 0.05). This may be related to decreased serum albumin levels in the midazolam group (-19+5%, P < 0.05) which, in turn, could be explained by the sedation associated with daily midazolam treatment. We concluded that concentration-EEG effect relationships can be studied on multiple occasions in individual animals, reducing animal use and variability. A modest degree of tolerance to midazolam was found with this paradigm, the effect only being evident after correction for the fraction unbound of midazolam.

Correlation and prediction of a large blood-brain distribution data set--an LFER study

We report linear free energy relation (LFER) models of the equilibrium distribution of molecules between blood and brain, as log BB values. This method relates log BB values to fundamental molecular properties, such as hydrogen bonding capability, polarity/polarisability and size. Our best model of this form covers 148 compounds, the largest set of log BB data yet used in such a model, resulting in R(2)=0.745 and e.s.d.=0.343 after inclusion of an indicator variable for carboxylic acids. This represents rather better accuracy than a number of previously reported models based on subsets of our data. The model also reveals the factors that affect log BB: molecular size and dispersion effects increase brain uptake, while polarity/polarisability and hydrogen-bond acidity and basicity decrease it. By splitting the full data set into several randomly selected training and test sets, we conclude that such a model can predict log BB values with an accuracy of less than 0.35 log units. The method is very rapid-log BB can be calculated from structure at a rate of 700 molecules per minute on a silicon graphics O(2).

Pharmacokinetic-pharmacodynamic modelling of tiagabine CNS effects upon chronic treatment in rats: lack of change in concentration-EEG effect relationship

The pharmacodynamics of the gamma-aminobutyric acid (GABA) uptake inhibitor (R)-N-(4,4-di-(methylthien-2-yl)but-3-enyl) nipecotic acid (tiagabine) was quantified in rats following chronic (14 days) administration by an integrated pharmacokinetic-pharmacodynamic (PK/PD) modelling approach. The increase in beta activity (11.5-30 Hz) of the EEG as derived by fast Fourier transformation analysis was used as pharmacodynamic endpoint. Two groups of male Wistar rats were treated for 14 days with either tiagabine at a steady-state concentration of 198+/-10 ng ml(-1) or placebo. Chronic treatment with tiagabine resulted in an increase of the EEG effect parameter by 38+/-2 microV. In the PK/PD experiment the time course of the EEG effect was determined in conjunction with the decline of drug concentrations after an i.v. bolus administration of 10 mg kg(-1). The pharmacokinetics of tiagabine was most adequately described by a bi-exponential function. No influence of chronic treatment on the pharmacokinetics was observed. Hysteresis between plasma concentration and EEG effect was accounted for by incorporation of an 'effect-compartment' in the model. The observed relationship between tiagabine concentrations and EEG effect was non-linear and described on the basis of the Hill equation. Between the treatment groups no differences in the pharmacodynamic parameters were observed. The population means for the different pharmacodynamic parameters were: maximum EEG effect 82 microV, EC(50) 486 ng ml(-1), Hill factor 2.0 and k(e0) 0.060 min(-1). In the in vitro [(3)H]GABA uptake assay no changes in affinity or functionality for the GABA uptake transporter were observed, consistent with the absence of adaptation. It is concluded that chronic treatment with tiagabine in an effective dose range for 14 days does not produce functional adaptation to tiagabine-induced GABA-ergic inhibition in vivo.

Differences in pharmacodynamics but not pharmacokinetics between subjects with panic disorder and healthy subjects after treatment with a single dose of alprazolam

The pharmacokinetics and pharmacodynamics of the benzodiazepine alprazolam (1 mg, administered orally) were compared between eight patients with panic disorder and eight age- and sex-matched healthy volunteers. Subjects received orally administered placebo and alprazolam in a randomized, double-blind, single-dose crossover study. The elimination half-life, time of maximum plasma concentration, maximum concentration, volume of distribution, and clearance of alprazolam were similar for both groups. For each cohort, alprazolam treatment (vs. placebo) produced significant changes in typical benzodiazepine agonist effects, such as increased sedation and impaired cognitive performance on the digit-symbol substitution test. For the panic disorder group only, there was a significant increase in the subjective rating of"contented" and a reduction in the rating of "easily irritated." For the healthy volunteer group, alprazolam produced increases in ratings of "fatigued" and "slowed thinking," but also increases in ratings of "relaxed." In each group, alprazolam significantly increased the electroencephalographic (EEG) measure of relative beta amplitude (range, 13-30 Hz) compared with placebo. Concentration-EEG response curves fit a sigmoid E(max) model, and there was greater sensitivity to EEG effects, as measured by a 28% reduction in the EC50 value, in the panic disorder group compared with healthy control subjects. After alprazolam treatment, there was increased sensitivity to EEG and mood effects and fewer aversive effects in the panic disorder group compared with healthy subjects. There were no differences in the pharmacodynamic measures of sedation and cognition or differences in pharmacokinetics between the two groups.

Mechanism-based modeling of functional adaptation upon chronic treatment with midazolam

PURPOSE

A mechanism-based model is applied to analyse adaptive changes in the pharmacodynamics of benzodiazepines upon chronic treatment in rats.

METHODS

The pharmacodynamics of midazolam was studied in rats which received a constant rate infusion of the drug for 14 days, resulting in a steady-state concentration of 102 +/- 8 ng x ml(-1). Vehicle treated rats were used as controls. Concentration-EEG effect data were analysed on basis of the operational model of agonism. The results were compared to data obtained in vitro in a brain synaptoneurosomal preparation.

RESULTS

The relationship between midazolam concentration and EEG effect was non-linear. In midazolam pre-treated rats the maximum EEG effect was reduced by 51 +/- 23 microV from the original value of 109 +/-15 microV in vehicle treated group. Analysis of this change on basis of the operational model of agonism showed that it can be explained by a change in the parameter tissue maximum (Em) rather than efficacy (tau). In the in vitro studies no changes in density, affinity or functionality of the benzodiazepine receptor were observed.

CONCLUSIONS

It is concluded that the observed changes in the concentration-EEG effect relationship of midazolam upon chronic treatment are unrelated to changes in benzodiazepine receptor function.

Substantia nigra: the involvement of central and peripheral benzodiazepine receptors in physical dependence on diazepam as evidenced by behavioral and EEG effects

Male rats chronically exposed to diazepam (DZ) slowly released from subcutaneously implanted silastic capsules along with empty capsule control rats were focally injected (1 microl) into the substantia nigra (SNR) with the central (CBR) and peripheral (PBR) benzodiazepine receptor antagonists, flumazenil [(FLU) 6.25, 12.5, 25 microg] and PK 11195 [(PK) 3.125, 6.25, 12.5, 25 microg], respectively (weekly intervals; Latin square design). Rats were observed for signs of withdrawal and the EEG was recorded simultaneously from the site of injection (SNR), caudate putamen, thalamus, hippocampus, and frontal cortex. In DZ-dependent rats the Precipitated Abstinence Score (PAS) was significantly related to dose of FLU. The PAS increased with increasing doses of PK (3.125-12.5 microg); however, the highest dose of PK (25 microg) showed less effect. The rapid onset of the PAS was accompanied by a rise in the total power (1-32 Hz) of the EEG (TP(EEG)) in the SNR and other brain areas. The PAS and TP(EEG) had similar time courses. Intranigrally injected FLU and PK did not evoke clonic and tonic-clonic convulsions; however, both antagonists induced dose-related twitches and jerks. Additionally, FLU precipitated a dose-related tachypnea and increases in turning and backing. Chronic DZ treatment altered the spectral content of the EEG, as indicated by a decrease and an increase of the slow and fast frequency bands, respectively. FLU and PK rapidly but transiently reversed the EEG. Data suggest that in the SNR the CBR mediate autonomic and motor signs of DZ withdrawal, while both the CBR and PBR are responsible for twitches and jerks and alteration of the EEG. It is possible that PK also acts on the site linked to a GABA(A)/CBR/ionophore.

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.

Adaptive changes in the pharmacodynamics of midazolam in different experimental models of epilepsy: kindling, cortical stimulation and genetic absence epilepsy

1. The objective of this investigation was to determine quantitatively whether experimental epilepsy is associated with a change in the pharmacodynamics of benzodiazepines in vivo. For that purpose the pharmacodynamics of midazolam were quantified by an integrated pharmacokinetic-pharmacodynamic approach in three different models of experimental epilepsy: amygdala kindling, cortical stimulation and genetic absence epilepsy. 2. The time course of the EEG effect was determined in conjunction with the decline of drug concentrations after intravenous administration of 10 mg kg(-1) midazolam. The pharmacokinetics of midazolam were most adequately described by a bi-exponential equation. No influence of epilepsy on the pharmacokinetics of midazolam was observed. 3. The increase in beta activity (11.5-30 Hz) of the EEG as derived by Fast Fourier Transformation analysis was used as pharmacodynamic endpoint. For each individual rat the increase in beta activity was directly related to the concentration in blood on the basis of the sigmoidal Emax pharmacodynamic model. In all three models a significant reduction in the maximal effect was observed, in amygdala kindling 28%, in the cortical stimulation model 49% and in genetic absence epilepsy 37%. No differences in the other pharmacodynamic parameters, E0 EC50,u and Hill factor, were observed. 4. It is inferred that in three different models of epilepsy there is a similar change in GABAergic functioning which is associated with a significant reduction in the intrinsic activity of midazolam in vivo. These models provide therefore a useful basis for further studies on the mechanism of epilepsy-induced changes in pharmacodynamics of anti-epileptic drugs.

Enhanced selective attention after low-dose administration of the benzodiazepine antagonist flumazenil

Although recognized for their sedative properties, benzodiazepines are also known to impair sustained and selective attention. Flumazenil at low doses may act to antagonize benzodiazepine-induced effects. This study examined whether low doses of flumazenil would improve event-related brain potential (ERP) indicators of selective attention and induce feelings of activation and anxiety in healthy men. Data from 11 men (24-30 years) who received intravenous flumazenil (0.2 mg, plus 0.3 mg 30 minutes later) and placebo were analyzed according to a double-blind crossover design. ERPs were recorded while subjects performed an auditory selective attention task. Mismatch negativity (MMN), processing negativity (PN), and the P3 component were extracted from the ERP as markers of preattentive mismatch processing, selective attention, and stimulus processing within working memory, respectively. Counting accuracy and performance on a letter cancellation test were used as behavioral indicators of attention. Mood was assessed by an adjective checklist and the State-Trait Anxiety Inventory. Flumazenil significantly increased PN over frontocortical areas, indicating improved selective attention (p < 0.05). Increases in the P3 amplitude and MMN after drug treatment remained nonsignificant. Subjects felt more activated and extraverted after flumazenil treatment than after placebo (p < 0.05). Anxiety was not increased. The findings of this study confirm the concept that flumazenil administered at a low dose in humans exerts effects opposite to those of benzodiazepines.

Neuromonitoring in the operating room: why, when, and how to monitor?

This review considers the main principles and indications of EEG and evoked potential (EP) neuromonitoring in the operating room. Neuromonitoring has a threefold purpose: to warn the surgeon that he has to adjust his strategy, to confirm his decision, and to help him improve subsequent procedures. The pathophysiology of intraoperative events liable to alter the EEG or the EPs is first considered. The usefulness of neuromonitoring in preventing neurological complication relies on its ability to detect neurological dysfunction at a reversible stage. This applies especially to ischemia and compressive damage. The anesthetic influences on EEG and EPs are then considered. Knowledge of them is essential to disentangle these neurophysiological alterations due to intraoperative events from those merely due to anesthesia and to use neurophysiological parameters to evaluate the depth of anesthesia. Third, the main indications and limitations of neuromonitoring are considered: prevention of ischemic brain or spinal cord damage, prevention of mechanical injuries of the brain, spinal cord or peripheral nerve, and localization of the motor cortex in cortical neurosurgery or of cranial nerves in posterior fossa surgery. Finally, the 3 levels of neuromonitoring (neurophysiological feature extraction, neurophysiological pattern recognition, clinical integration of the neurophysiological patterns) are discussed together with the rules that should guide the dialogue between the surgeon, the anesthesiologist, and the neurophysiologist.

Pharmacodynamics of temazepam in primary insomnia: assessment of the value of quantitative electroencephalography and saccadic eye movements in predicting improvement of sleep

BACKGROUND AND OBJECTIVE

Quantitative electroencephalographic parameters and saccadic eye movements are frequently used as pharmacodynamic measures of benzodiazepine effect. We investigated the relationship between these measures and the hypnotic effect.

METHODS

The correlation between the pharmacodynamic measures and sleep quality was determined in 21 patients with primary insomnia. The pharmacokinetic-pharmacodynamic relationships were characterized after administration of 20 mg oral temazepam. The hypnotic effect was determined on the basis of polysomnographic sleep recordings and a subjective sleep evaluation questionnaire. Correlations between pharmacodynamic measures and the improvement of sleep were investigated.

RESULTS

The pharmacokinetic-pharmacodynamic relationships for the parameters derived from electroencephalography and saccadic eye movements showed considerable interindividual variability. Administration of temazepam led to a significant improvement in the objective parameters sleep period efficiency, wake time after sleep onset, and sleep efficiency and in the subjective assessment of sleep quality. No significant correlations were observed between the pharmacokinetic-pharmacodynamic-derived parameters and the improvement in objective or subjective sleep parameters.

CONCLUSION

In subjects with primary insomnia the administration of 20 mg oral temazepam results in changes in both the pharmacodynamic measures and in quality of sleep. No individual correlations between the pharmacodynamic measures and quality of sleep were observed. We concluded that the investigated pharmacodynamic measures are of value in the first assessment of clinical efficacy and for the selection of the dose(s) to be investigated in subsequent trials that aim at showing clinical efficacy. However, the conclusive quantification of clinical efficacy should be performed only on the basis of the clinical end point itself.

Dorsal raphe and substantia nigra response to flumazenil in diazepam-dependent rats

Flumazenil (FLU; 25 micrograms) and DMSO-vehicle were focally injected (1 microliter) into the substantia nigra (SN) and the dorsal raphe nucleus (DR) in rats chronically implanted with silastic capsules containing diazepam (DZ; 540 mg/week). FLU precipitated an abstinence syndrome in the SN as indicated by a significant abstinence score, several abstinence signs and reduced total power of the fast frequency bands of the electroencephalogram (EEG) in the injections sites frontal cortex, (FC) and hippocampus (H). In contrast, FLU did not produce an abstinence syndrome in the DR, and its effect on the power of the EEG in DR, FC and H was not significantly different from that of the DMSO-vehicle. The data show regional heterogeneity in the response of the SN and the DR to chronic DZ treatment in terms of a focally precipitated abstinence syndrome.

Pharmacokinetic-pharmacodynamic modeling of mivacurium in rats

The pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of the neuromuscular blocking agent mivacurium were evaluated separately in two groups of rats receiving 0.6 mg kg-1 of mivacurium in a 2.5-min intravenous continuous (iv) infusion. The PK parameters for mivacurium were determined in the first group. A two-compartment model describes the kinetics of mivacurium in plasma. The estimates of the apparent volume of distribution at steady-state and plasma clearance [mean(SE)] were 650 (123) mL kg-1 and 9.9 (0.75) mL min-1 kg-1, respectively. In the second group, the evoked tibialis anterior muscle tension was monitored. The PK parameters derived from the first group were used to compute mivacurium plasma concentrations (C) at the times the PD measurements were recorded in the second group. The concentration-neuromuscular effect [% depression of initial twitch tension (E)] relationship was analyzed by two approaches. (1) The relationship of estimated effect site concentrations versus E; a sigmoidal Emax model described the effect compartment concentrations versus E relationship. The estimate [mean(SE)] of Cess50 (steady-state plasma concentration eliciting half of maximum E) was 0.65 (0.01) microgram mL-1. The value [mean-(SE)] of Keo (rate constant of equilibration between plasma and effect site) was estimated at 0.32 (0.03) min-1. (2) The relationship of descending limb C versus E; a sigmoidal Emax model described such relationship. The estimate [mean(SE)] of C50 (post-infusion C eliciting half of maximum E) was 0.57(0.03) microgram mL-1. The PD properties of mivacurium were also evaluated in another two groups of animals receiving either 5- or 10-min continuous iv infusion; PK and PD parameters obtained from the 2.5-min infusion experiments were used to predict the time course of E in the groups receiving 0.6 mg kg-1 of mivacurium in 5- and 10-min infusions; simulations using the estimated parameters adequately describe the time course of E in those groups. The effect of mivacurium on the mean arterial blood pressure (MAP) was also investigated; a 10% nonsignificant decrease (p > 0.05) in MAP was found in all groups.

Relationship between immobilised artificial membrane chromatographic retention and the brain penetration of structurally diverse drugs

Retention factors were determined for a set of 26 drugs, for which brain/blood concentration data are available, using immobilised artificial membrane (IAM) chromatography. The compound set represented acidic, basic and neutral drugs from various structural classes. The relationship between IAM retention and lipophilicity (n-octanol-water partition coefficient Koct), molecular size and acid/base character of the drugs and the relationship between brain distribution and IAM retention and the other parameters were analysed. IAM retention was increased with increases in lipophilicity and solute size, and decreased by the ionisation of acidic groups. Ionisation of basic groups had no significant effect. A three-parameter regression model with log Koct, molecular weight and an indicator parameter for the presence of carboxyl group explained 93% of the variation in log kIAM. The concentration ratio between brain and blood (log BB) was only weakly correlated with the IAM chromatographic retention or n-octanol-water partitioning. Three-parameter models taking ionisation and size into account, in addition to either log Koct or log kIAM, explained about 85% of the variation of log BB in the test set. Although IAM chromatography offers no advantage in these models, it seems to provide a better model than n-octanol-water partitioning for the membrane distribution of ionised compounds.

Determination of the steady state tissue distribution of midazolam in the rat

The aim of this study was to determine the steady state tissue:blood (Kb), tissue:plasma (Kp), and tissue:plasma water (Ku) partition coefficients of midazolam in all major organs and tissues in the rat, after development of a gas chromatographic assay. The Kp in 12 tissues ranged between 1.3 (in muscle) and 9.0 (in fat). Ku ranged correspondingly between 16 and 115.

Time course of discriminative stimulus effects of bretazenil and chlordiazepoxide in rats

The time course of the discriminative stimulus effects of a benzodiazepine partial agonist, bretazenil, and a benzodiazepine full agonist, chlordiazepoxide, were determined in rats after administration of two doses of either drug. As in man, bretazenil was considerably shorter-acting than chlordiazepoxide, with 0% drug-appropriate responding at the 2-h time point after the training dose of 7 mg/kg and < 20% drug-appropriate responding at 9 h after the 14 mg/kg dose. With chlordiazepoxide, responding on the water-appropriate lever did not occur in all rats until 7 h after administration of the training dose of 7 mg/kg, compared to 17 h after administration of 14 mg/kg. Although there was considerable individual variability with both drugs, it would appear that the drug discrimination procedure can be a valuable tool for studying the time course of the interoceptive effects of psychoactive drugs.

Pharmacokinetic-pharmacodynamic relationships for benzodiazepines

This article reviews the literature on the plasma concentration-effect relationships for benzodiazepines, in humans and in experimental animals. Only literature that explicitly links pharmacokinetics to pharmacodynamics is included. The following questions are evaluated. Can concentration-effect relationships be demonstrated? If so, are these relations stable? Are the influences of specific factors such as age and disease on these relationships established? It is clear that, when studies are conducted and interpreted appropriately, relations can be found for a wide range of benzodiazepine effects. These include objective measures such as electroencephalography, semisubjective measures such as psychomotor performance, and subjective measures such as mood/sedation scales. A generally applicable model of the relationship which will allow prediction of effect is, however, not yet established. The relationship appears to be dependent on route and rate of administration, because of factors such as distributional delay, formation of active metabolites and, probably, acute tolerance. Furthermore, intra- and interindividual variability is considerable, probably due to varying experimental conditions and intrinsic interindividual differences. The limited data available on factors influencing the plasma concentration-effect relationships for benzodiazepines demonstrate clear changes in the pharmacodynamics after multiple doses, suggesting the development of tolerance, and a subsensitivity in patients with panic disorder. The influence of factors such as age, disease and drug interactions on the pharmacokinetic-pharmacodynamic relationship remains less clear.

Pharmacodynamic modeling of the electroencephalographic effects of flumazenil in healthy volunteers sedated with midazolam

The purpose of this study was to model pharmacodynamically the reversal of midazolam sedation with flumazenil. Ten human volunteers underwent four different sessions. In session 1, individual midazolam pharmacokinetics and electroencephalographic pharmacodynamics were determined. In sessions 2 and 3, a computer-controlled infusion of midazolam with individual volunteer pharmacokinetic data was administered, targeting a plasma concentration corresponding to a light or deep level of sedation (20% or 80% of the maximal midazolam electroencephalographic effect) for a period of 210 minutes. After obtaining a stable electroencephalographic effect and constant midazolam plasma concentrations, a zero-order infusion of flumazenil was started until complete reversal of midazolam electroencephalographic effect was obtained. The flumazenil infusion was then stopped and the volunteer was allowed to resedate because of the constant midazolam drug effect. The electroencephalographic response was measured during a 180-minute period and analyzed by aperiodic analysis and fast-Fourier transforms. In session 4, a midazolam plasma concentration corresponding to a deep level of sedation was targeted for 210 minutes to examine for the possible development of acute tolerance. No flumazenil was given in session 4. For a light sedation level, with a mean midazolam plasma concentration of 160 +/- 64 ng/ml, the mean half-life of the equilibration rate constant of flumazenil reversal is 5.0 +/- 2.5 minutes, and the mean effect site concentration causing 50% of Emax is 13.7 +/- 5.8 ng/ml. For a deep level of sedation, with a mean midazolam plasma concentration of 551 +/- 196 ng/ml, the mean half-life of the equilibration rate constant is 3.9 +/- 1.5 minutes, and the mean effect site concentration causing 50% of Emax is 20.6 +/- 6.8 ng/ml. This study provides an estimate of the magnitude of the blood/central nervous system equilibration delay for flumazenil antagonism of midazolam sedation and further defines the usefulness of the electroencephalogram as a measure of midazolam pharmacodynamic effect.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Pharmacodynamic interaction between midazolam and a low dose of ethanol in vivo

The pharmacokinetic and pharmacodynamic interactions between midazolam and ethanol were studied in the rat in vivo. Ethanol was given as a constant rate intravenous infusion (1.85 mg/min). The pharmacokinetics and pharmacodynamics of midazolam were determined following an intravenous dose of 5 mg/kg in 15 minutes. Amplitudes in the 11.5-30 Hz (beta) frequency band of the EEG was used as a measure of the pharmacological effect. Ethanol infusion resulted in a constant plasma alcohol concentration of 0.44 +/- 0.04 g/l (Mean +/- SE) and had no effect on the baseline value of the EEG effect parameter. Also the pharmacokinetics of midazolam were unchanged. However, a significant parallel shift of the midazolam concentration-EEG effect relationship to lower concentrations was observed. These findings show that there is a pharmacodynamic interaction between midazolam and ethanol in vivo.

The prognostic significance of diazepam-induced EEG changes in epilepsy: a follow-up study

Diazepam (0.1-0.3 mg/kg) was injected intravenously, its effect on scalp EEG was evaluated visually and by computer in 84 cases of epilepsy, and long-term follow-up was carried out in an attempt to explore relationships between the EEG changes produced by diazepam and the prognosis in these patients. The average length of follow-up was 3 years (range 2-3.5) in 48 out of 84 cases (57%). The EEG visually responded to diazepam (abolition of abnormal activity with emergence of fast activity) in 33/48 cases. Subsequent follow-up showed that 29 (88%) of these had a good prognosis (seizure-free or a 50% or more reduction in seizures) and 4 others (12%) had a poor result (frequency of seizures increased, unchanged or decreased less than 50%). Fifteen patients had a negative EEG response to the drug, 4 (27%) of whom had a favorable outcome and 11 (73%) an unfavorable result. These results were statistically significant. The percentage of diazepam-induced EEG changes in beta activity (PDICB) was also significantly positively related to the percentage of reduction in seizure frequency in these patients (r = 0.55, p < 0.001). In 79% of patients with PDICB values more than 2, and 30% of those with values less than 2, a good outcome occurred whereas 21% and 70%, respectively, had a poor outcome (p < 0.001). These results showed that the patterns of EEG change induced by diazepam are intimately related to the outcome of epilepsy.