Pharmacodynamics and pharmacokinetics of ethanol, diazepam and pentobarbital in young and aged rats

Extrasynaptic GABAA receptors in central medial thalamus mediate anesthesia in rats

Neuronal depression in the thalamus underlies anesthetic-induced loss of consciousness, while the precise sub-thalamus nuclei and molecular targets involved remain to be elucidated. The present study investigated the role of extrasynaptic GABAA receptors in the central medial thalamic nucleus (CM) in anesthesia induced by gaboxadol (THIP) and diazepam (DZP) in rats. Local lesion of the CM led to a decrease in the duration of loss of righting reflex induced by THIP and DZP. CM microinjection of THIP but not DZP induced anesthesia. The absence of righting reflex in THIP-treated rats was consistent with the increase of low frequency oscillations in the delta band in the medial prefrontal cortex. CM microinjection of GABAA receptor antagonist SR95531 significantly attenuated the anesthesia induced by systemically-administered THIP, but not DZP. Moreover, the rats with declined expression of GABAA receptor δ-subunit in the CM were less responsive to THIP or DZP. These findings explained a novel mechanism of THIP-induced loss of consciousness and highlighted the role of CM extrasynaptic GABAA receptors in mediating anesthesia.

Zolpidem is a potent anticonvulsant in adult and aged mice

Zolpidem is a widely used hypnotic drug acting via benzodiazepine binding sites on GABA(A) receptors. Several studies suggested that zolpidem might have better anticonvulsant potency than previously thought. To compare the sedative and anticonvulsant potency of this drug, we studied in male mice the influence of zolpidem (0.1-10 mg/kg i.p.) on ambulatory locomotor activity (a measure of sedation) and on the threshold for myoclonus, clonic and tonic seizures, as well as death, in response to i.v. infusion of pentylenetetrazole (PTZ, 4.4 mg/min). Because older people take zolpidem more often than young people and have a higher incidence of epilepsy, we also compared the sedative and anticonvulsant properties of low doses of this drug (0.1 and 1 mg/kg i.p.) between adult (3 months) and aged (13 months) mice. Zolpidem in doses of 0.3-10 mg/kg decreased locomotion, as quantified by recording interruptions of infrared beams during 10 min, and in doses of 1-10 mg/kg increased the threshold for PTZ-induced seizures and death. The effect of zolpidem against tonic seizures was greater than against two other seizure components and death. In control aged mice the threshold for PTZ-induced myoclonus, clonic seizures and death was lower than in adult mice, while the locomotor activity was not different. In adult and in aged mice zolpidem in a dose of 1 mg/kg decreased locomotion and elevated the threshold for PTZ-induced seizures and death. Neither of these effects was age-dependent. The results suggest that in addition to strong sedative activity zolpidem has potent anticonvulsant properties.

Acute and rebound effects of lorazepam on orolingual motor function in young versus aged Fischer 344/Brown Norway rats

The purpose of this study was to measure the acute effects of lorazepam [a short-acting benzodiazepine (BZ) with no active metabolites] on orolingual motor function in young (6 months) versus aged (24 months) Fischer 344/Brown Norway hybrid (F344/BN) rats. Rats licked water from an isometric force-sensing operandum so that the number of licks per session, licking rhythm (licks/second), and lick force could be measured during daily sessions. Acute doses (1.0 and 2.0 mg/kg) of lorazepam were administered 30 min before the testing sessions, 4 days apart. Whereas aged rats produced more licks per session, lorazepam increased this measure primarily in the young group. On the days after each lorazepam dose, rats licked less than they did before receiving the drug. This effect was shown by both groups. Aged rats showed significantly slower licking rhythm than young rats. Lorazepam slowed this measure in both groups. Peak tongue forces were significantly increased by lorazepam. These findings suggest that BZs such as lorazepam can affect tongue force output and exacerbate age-related tongue motility deficits. They also suggest that although BZs can directly influence motivation to engage in water-reinforced tasks, opposite 'rebound' effects may occur, even after acute dosing.

The effect of chronic lorazepam administration in aging mice

To assess benzodiazepine tolerance in aged animals, lorazepam or vehicle was administered chronically to male Crl: CD-1(ICR)BR mice. Pharmacodynamic and neurochemical endpoints were examined on days 1 and 14 of drug administration. There was no age-related significant difference in plasma lorazepam levels. Young and middle-aged animals demonstrated behavioral tolerance to lorazepam, while the aged animals showed a similar trend which failed to reach significance. In addition, aged animals also showed a trend toward tolerance to the anticonvulsant effects of lorazepam. There were no changes in alpha1 mRNA levels in cortex or hippocampus following administration of lorazepam when compared to vehicle-treated animals in any age group. Aged animals, however, had an initial increase in alpha1 mRNA expression in cortex and hippocampus on day 1 of vehicle treatment followed by decreased expression on day 14. These age-related changes were abolished by lorazepam administration. In summary, age-related sensitivity to the effects of lorazepam was not demonstrated in the present study. However, comparison of these data to other studies indicates that the effect of chronic benzodiazepine treatment may be specific to the benzodiazepine administered, the technique used to quantify mRNA expression changes, the subunits of the GABA(A) receptor investigated and the brain region analyzed. The phenomenon of benzodiazepine sensitivity in the elderly is an area of research which remains controversial and may well be compound specific. Determining benzodiazepines that do not produce pharmacodynamic sensitivity, such as lorazepam, may allow more careful prescribing and dosing of these drugs, and perhaps even the development of specific agents which could avoid this sensitivity.

Impairment state of cognitive performance and the affecting factors in outpatients following gastrointestinal endoscopy after single-dose diazepam

Diazepam is commonly used as premedicant for endoscopic procedures. Wide interindividual differences have been observed in the residual cognitive effects of the drug after gastrointestinal endoscopy. Our aim was to clarify the major factors, including pharmacokinetic factors, contributing to this wide variation in residual cognitive effect after gastrointestinal endoscopy in the study. Sixty-one outpatients undergoing gastrointestinal endoscopy participated in the study. Cognitive effects were evaluated in the diazepam group (n=52) by the digit symbol substitution test (DSST) twice before and 30 min after an intravenous administration of 5 mg diazepam; in the intervening time gastrointestinal endoscopy was performed. Plasma concentrations of diazepam were determined by HPLC. The control group (n=9) was tested by DSST in the same manner. The cognitive effects according to the change in DSST score was significantly decline in the diazepam group compared with the control group (by 0.2 versus -4.6; P=0.014). This prospective study confirmed that cognition was significantly impaired after gastrointestinal endoscopy by premedication to subjects with 5 mg diazepam. There were very wide variations in change in DSST score. However we could not identify the independent variables that best predicted DSST score difference in a multiple regression analysis for age, plasma albumin level, and plasma diazepam concentration 30 min after intravenous administration. We should pay attention to patients' individual states in cognitive performance following gastrointestinal endoscopy after single-dose diazepam.

Rotarod studies in the rat of the GABAA receptor agonist gaboxadol: lack of ethanol potentiation and benzodiazepine cross-tolerance

All benzodiazepines and benzodiazepine site agonists impair motor performance dose-dependently and potentiate the effects of ethanol. In order to evaluate the risk of benzodiazepine and ethanol interaction with the direct acting GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo (5,4-c) pyridin-3-ol (gaboxadol), we studied impairment of motor coordination for combinations of gaboxadol, ethanol and a series of benzodiazepines (flunitrazepam, zolpidem and indiplon) in a rat rotarod model. All compounds produced a dose-dependent motor impairment and, in agreement with earlier data, a supra-additive effect of the benzodiazepine ligands and ethanol 1 g/kg was seen. In contrast, no significant potentiation of the effects of gaboxadol by ethanol was detected, and furthermore, no synergistic interaction between gaboxadol and any of the benzodiazepines was seen. A 30-day tolerance study was conducted with daily injections of gaboxadol (7.9 mg/kg) and zolpidem (1.25 mg/kg), respectively. A time-dependent tolerance developed to the motor impairment produced by both compounds. On day 31, cross-tolerance studies between zolpidem/gaboxadol and gaboxadol/zolpidem were conducted. No cross-tolerance was observed, indicating that the motor coordination effects observed with gaboxadol and zolpidem may arise from interaction with different receptor populations.

Naloxone potentiates anxiolytic-like actions of diazepam, pentobarbital and meprobamate but not those of Ro19-8022 in the rat

The elevated plus-maze test was used to determine if the opiate antagonist naloxone could potentiate the anxiolytic-like effects of the benzodiazepine diazepam, the barbiturate pentobarbital, the propanediol carbamate meprobamate and the partial benzodiazepine receptor agonist [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-methanol (Ro19-8022) in the rat. A subeffective dose of each of these compounds was combined with naloxone, 10 mg/kg. Naloxone had no effect by itself, but potentiated all drugs except Ro19-8022. The proportion of entries on the open arm increased while the total number of arms entries was not modified. These results coincide with and extend data previously obtained in the mouse. One possible explanation for naloxone's effect could be that it blocks opioid inhibition of GABAergic (gamma-aminobutyric acid) neurons thereby enhancing the effects of benzodiazepines. Another possibility is that naloxone blocks opioid effects on adenosinergic systems.

Effects of sodium pentobarbital on the components of electroretinogram in the isolated rat retina

Photovoltages, the fast P3(t) component of electroretinogram (ERG), were registered between two microelectrodes across the rod outer segments. The P2(t) component, obtained by subtracting the ERGs measured before the application of 50 microM APB from those measured after the application of 50 microM APB, was used as an indicator of depolarizing bipolar cell activity. Measurements of the scotopic threshold response (STR) and the oscillatory potentials (OPs) were used as indicators of third order neuron activity. The slow P3*(t) component, obtained by subtracting the photovoltages from the transretinal recording in the APB-treated retina was used as an indicator of Müller cell activity. The components of the ERG obtained in normal superfusate medium were compared with those obtained in the presence of 100 microM sodium pentobarbital. We found that sodium pentobarbital slowed the kinetics of the P2(t) component and increased its latency. The fast P3(t) component was not affected by pentobarbital. The slow P3*(t) component was slightly reduced in the presence of pentobarbital. The minor components of the ERG, the STR and the OPs, were strongly suppressed by pentobarbital. These results suggest that in rat retina pentobarbital does not affect photoreceptors, but it does affect bipolar cells and Müller cells, and it suppresses activity of third order neurons.

No effect of age on the dose requirement of thiopental in the rat

With increasing human age (20-80 years), the electroencephalogram (EEG) dose requirement for the intravenous anesthetic thiopental decreases approximately 10% per decade of life. The goal of this study was to compare the dose required to attain isoelectric EEG in young (4-5 month) vs. aged (24-25-month) Fischer 344 rats. One second isoelectricity was found to be an endpoint where minimal cardiorespiratory depression occurred. The effects of age, infusion rate, and repeated administration were examined in nine young and nine old rodents. Thiopental dose requirement increased with increasing infusion rates. Repeated administration at two-day intervals did not demonstrate tolerance to thiopental. No difference in thiopental dose requirement was detected in the young vs. elderly rats. In a separate group of five young and five old rats, thiopental plasma, brain, heart, and CSF concentrations were measured when five seconds of EEG isoelectricity was achieved: no consistent differences were noted. The rat may not be an appropriate model to investigate acute age-related anesthetic effects in humans, because cardiovascular changes with age are dissimilar between species.

Adaptive changes of dopamine-D2 receptors in rat brain following ethanol withdrawal: a quantitative autoradiographic investigation

The effect of subchronic treatment with two doses of ethanol (5 and 10 vol% drinking fluid) on the density of dopamine-D2 receptors was investigated at two different phases of withdrawal, namely 24 h and 5 days after the cessation of the ethanol application. The number of dopamine-D2 receptors was affected in regions receiving projections from both the substantia nigra as well as the ventral tegmentum. Twenty-four hours after the replacement of the ethanol solution by water, a dose-dependent decrease of D2 receptors was found in all regions (N. caudatus dorsalis, medialis and ventralis, N. accumbens lateralis and medialis, tuberculum olfactorium) and most of the analyzed planes [interaural 7.7-10.2 according to the atlas of Paxinos and Watson (35)]. At day 5 of withdrawal, the number of dopamine-D2 receptors of the animals treated with 5 vol% ethanol reached the level of water controls in most planes. In contrast, two- to three-fold higher numbers were detected in animals treated with the higher dose. Only in the most caudal parts of the investigated regions, was the number of receptors decreased with the higher dose. The mesocorticolimbic system seems to be less sensitive to the effects of ethanol than the nigrostriatal neurones. The findings of the present study suggest an increased activity of dopaminergic neurons with an adaptive reduction of dopamine-D2 receptors during the subchronic treatment with ethanol during the first day(s) of withdrawal. This phase is followed by a reduced turnover rate for up to 7 days (21). The reduced activity of dopaminergic neurones induces a compensatory increase of the number of receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenesis of peripheral benzodiazepine receptors: demonstration of selective up-regulation in rat testis as a function of maturation

Peripheral benzodiazepine receptors (PBR) have been localized to the outer mitochondrial membrane in a variety of organs, where they apparently play a role in steroidogenesis, oxidative processes, and/or growth and development. Previous studies have demonstrated ontogenetic changes in heart and lung PBR, with maximal PBR density at 31 days, as opposed to negligible changes in brain PBR during the prenatal through postnatal periods. The present study was designed to examine the influence of maturation and aging upon PBR binding characteristics. Rats aged 1, 2, 12, 18, and 24 months were sacrificed, and the following organs were removed according to standard protocol: heart, lungs, kidneys, adrenal gland, and testes. Binding studies were performed using [3H]PK 11195 as a radioligand. A 3-fold increase in PBR density was demonstrated in testis during maturation, with maximal values appearing at 18 months, followed by a decline at 24 months. None of the other organs examined showed significant changes in PBR density. No alterations were observed in affinity values for the various organs and ages. Since testicular PBR are putatively involved in testosterone production, these results might reflect critical interactions between PBR and gonadal hormone activity during development.

Clinical pharmacokinetics of anxiolytics and hypnotics in the elderly. Therapeutic considerations (Part II)

Part I of this article, which appeared in the previous issue of the Journal, discussed the scope of and scientific basis for special pharmacokinetic studies of anxiolytic and hypnotic drugs in the elderly, and examined the methodology and results of such studies and the prediction of pharmacokinetic changes. In Part II the authors continue their review, focusing on age-related pharmacodynamic changes in the effects of these drugs, the attempts to correlate pharmacokinetic with pharmacodynamic findings, and the clinical applications of these data.

Sensitivity to triazolam in the elderly

BACKGROUND

Elderly persons frequently appear to be sensitive to the effects of many drugs that depress the central nervous system. We studied the effect of age on the pharmacokinetics and pharmacodynamics of the benzodiazepine hypnotic agent triazolam, now the most frequently prescribed hypnotic drug in the United States.

METHODS

Twenty-six healthy young subjects (mean age, 30 years) and 21 healthy elderly subjects (mean age, 69 years) participated in a four-way crossover study. After a single-blind adaptation trial with placebo, each subject received, in random order and in double-blind fashion, single doses of placebo, 0.125 mg of triazolam, and 0.25 mg of triazolam. For 24 hours after the administration of each of the three study medications, plasma triazolam levels were determined and psychomotor performance, memory, and degree of sedation were assessed.

RESULTS

Plasma triazolam concentrations increased in proportion to the dose, but the elderly subjects had higher plasma concentrations due to reduced clearance of the drug. The degree of sedation as rated by an observer and the reduction in the subjects' performance on the digit-symbol substitution test were both greater in the elderly than in the young subjects after they were given the same doses. The relation of the plasma triazolam concentration to the degree of impairment was similar for the two groups. As part of the study, information was presented 1 1/2 hours after the administration of the drugs; the subjects' ability to recall the information 24 hours later was impaired by both doses of triazolam, and the percent decrease was similar in the young and elderly groups.

CONCLUSIONS

Triazolam caused a greater degree of sedation and greater impairment of psychomotor performance in healthy elderly persons than in young persons who received the same dose. These effects resulted from reduced clearance and higher plasma concentrations of triazolam rather than from an increased intrinsic sensitivity to the drug. On the basis of these results, the dosage of triazolam for elderly persons should be reduced on average by 50 percent.