Benzodiazepine Receptors In Brain Research Articles

A high-resolution in vivo atlas of the human brain's benzodiazepine binding site of GABAA receptors

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the human brain and plays a key role in several brain functions and neuropsychiatric disorders such as anxiety, epilepsy, and depression. For decades, several in vivo and ex vivo techniques have been used to highlight the mechanisms of the GABA system, however, no studies have currently combined the techniques to create a high-resolution multimodal view of the GABA system.Here, we present a quantitative high-resolution in vivo atlas of the human brain benzodiazepine receptor sites (BZR) located on postsynaptic ionotropic GABAA receptors (GABAARs), generated on the basis of in vivo [11C]flumazenil Positron Emission Tomography (PET) data. Next, based on ex vivo autoradiography data, we transform the PET-generated atlas from binding values into BZR protein density. Finally, we examine the brain regional association between BZR protein density and ex vivo mRNA expression for the 19 subunits in the GABAAR, including an estimation of the minimally required expression of mRNA levels for each subunit to translate into BZR protein.This represents the first publicly available quantitative high-resolution in vivo atlas of the spatial distribution of BZR densities in the healthy human brain. The atlas provides a unique neuroscientific tool as well as novel insights into the association between mRNA expression for individual subunits in the GABAAR and the BZR density at each location in the brain.

Influence of Maternal Alcoholism on the Brain Benzodiazepine Receptor Development in Human Embryo and Fetus during Ontogeny

Graphical Abstract: Influence of maternal alcoholism on the brain benzodiazepine receptor development in human’s embryo and fetus showed decreases their affinity and increases density as compensatory adaptation of the fetal nervous system to the effects of alcohol. Abstract: Signaling mechanisms that are required for proper neuronal development and how these processes are impaired by ethanol resulting in harmful consequences to brain development are very important for our understanding. The aim of the present work was to study the development of synaptic Benzodiazepine Receptors (BzDR) (functionally associated with the brain GABAergic system) in the brains of embryos and fetuses aged 8-15 weeks obtained from alcoholic female patients. Material and method: Abortive material (samples of brain tissue) was obtained from 33 women with grade II alcoholism (ICD-10 F10.201 and F10.202) and 30 control women were studied. The properties of BzDR were studied by radioreceptor binding with the selective ligand [3H]-flunitrazepam using a crude embryo brain synaptosomal fraction. Results: In contrast to controls, brain cells developing in conditions of prenatal alcoholization showed formation of synaptic benzodiazepine receptors and alterations in their properties: decreases in their affinity and increases in their density. A decrease in the ability of receptors to bind agonist ligands impairs the ligand: receptor protein ratio, leading to decreased binding of the major neurotransmitter GABA and impairment to synaptic transmission. Conclusion: These are interpreted as compensatory reactions promoting adaptation of the fetal nervous system to the effects of alcohol and functional deficiency of the GABAergic system.

Synaptogenesis and the Formation of Benzodiazepine Receptors in the Human Brain in Conditions of Prenatal Alcoholization

The aim of the present work was to study the relationship between the development of synaptic contacts and benzodiazepine receptors (functionally associated with the brain GABAergic system) in the brains of embryos and fetuses aged 8–15 weeks obtained from alcoholic female patients. Materials from 33 women with grade II alcoholism (ICD-10 F10.201 and F10.202) and 30 control women were studied. In contrast to controls, brain cells developing in conditions of prenatal alcoholization showed slowed formation of synaptic benzodiazepine receptors and increases in their density. These are interpreted as compensatory reactions promoting adaptation of the fetal nervous system to the effects of alcohol and functional deficiency of the GABAergic system.

The combined effects of midazolam and propofol sedation on muscle power

We performed a randomised, crossover study to investigate the effects of intravenous sedation on grip strength and bite force. Twenty male volunteers received a bolus intravenous injection of midazolam (0.02 mg.kg(-1)) together with a 30-min propofol infusion designed to achieve an effect-site concentration of 1.0 μg.ml(-1). Observed variables included bispectral index, observer's assessment of alertness/sedation, correct answer rate of Stroop colour-word test, grip strength and bite force. Grip strength decreased from a median (IQR [range]) of 483 (443-517 [380-586]) N to 358 (280-405 [108-580]) N (p < 0.001) during sedation and recovered following flumazenil administration, while bite force increased from 818 (593-1026 [405-1406]) N to 1377 (1243-1585 [836-2357]) N (p < 0.001) during sedation. Although bite force gradually returned to baseline following flumazenil administration, it remained increased throughout the experimental period. We conclude that bite force increased during intravenous sedation and that this may have clinical implications.

1614 – Prenatal alcoholization of human embryos and fetuses alters structure of synapses and properties of benzodiazepine receptors in their brain

Influence of prenatal alcoholization on development of embryo's human brain and process of synaptogenesis and neurotransmission has been studied inadequately. It has been noticed that acute impact of the alcohol reinforces GABAergic transmission and chronic alcoholization increases selectivity of reverse agonists of benzodiazepine receptor (BDR), that indicates presence of relationship between action of ethanol and functioning of GABA-benzodiazepine receptor complex. Objective of present investigation appeared to be a study of dynamic of formation and development of synaptic contacts and benzodiazepine receptors of synapses of the embryonic brain at weeks 7-15 of development obtained from healthy women and alcoholic ones. Investigations with electronic microscope in cells of embryonic brain obtained from women with alcohol addiction have shown slower formation of synaptic structures as compared with the norm. In addition, we revealed increasing of synaptic contacts number during brain development and alteration of properties of benzodiazepine receptors of synaptosomal membranes during embryo development stage that is expressed in decrease of affinity of receptors (decrease of affinity [ 3 H]flunitrazepam binding) and increase of their density with radioreceptors assay. In cells of the embryo brain obtained from women suffering from alcoholism slowing of formation of synaptic structures as compared with norm and transformation of properties of synaptosomal BDR has been found out, what may be a cause of alteration of neuromediator transmission. Provided data indicate presence of relationship between action of ethanol on formation of synaptic structures and functioning of benzodiazepine receptor in human embryo and fetus brain.

Pheromones: Fish Fear Factor

Fish, like many other animals, panic when another individual is injured. Now, the chemical nature of a substance that mediates this reaction has been uncovered.

Autoradiographic analysis of GABAA receptor binding in the neural anxiety network of postpartum and non-postpartum laboratory rats

Postpartum female rats exhibit a suppression of anxiety-related behaviors when compared to diestrous virgin females, pregnant females, and males. This blunted anxiety promotes optimal maternal care and involves elevated GABA neurotransmission, possibly including greater density of GABAA and benzodiazepine receptors in the postpartum brain. We here examined autoradiographic binding of [3H]muscimol to measure the total population of GABAA receptors and [3H]flunitrazepam to assess density of benzodiazepine sites in the medial prefrontal cortex, bed nucleus of the stria terminalis, amygdala, hippocampus, and periaqueductal gray of female rats sacrificed on day 7 postpartum, day 10 of pregnancy, or as diestrous virgins. A group of sexually naïve male rats was also included. We found that [3H]muscimol binding did not differ among groups in any site but that diestrous virgin females had greater [3H]flunitrazepam binding in the CA1 and dentate gyrus of the hippocampus compared to mid-pregnant females and males. Notably, postpartum and diestrous virgin females did not significantly differ in binding of either ligand in any site examined. This is the first study to evaluate the densities of GABAA and benzodiazepine binding sites simultaneously across three female reproductive states and sex with a focus on brain sites influencing anxiety-related behaviors. The results suggest that changes in other GABAA receptor characteristics such as subunit composition, or increased presynaptic GABA release during interactions with offspring, must instead play a greater role in the postpartum suppression of anxiety in laboratory rats.

Does ethyl-beta-carboline-3-carboxylate interact with mouse brain benzodiazepine receptors in vivo?

Does ethyl-beta-carboline-3-carboxylate interact with mouse brain benzodiazepine receptors in vivo?

Increased number of brain benzodiazepine receptors after in-vivo administration of estazolam to rats.

Estazolam significantly increased the Kd of [3H]flunitrazepam in-vitro, like other benzodiazepines (BDZs) acting competitively at the receptor site. At variance with other BDZs, estazolam significantly raised the Bmax for [3H]flunitrazepam, at concentrations lower than its Ki for BDZ receptors. This effect may be responsible for the observed increase in [3H]diazepam binding after in-vivo administration of estazolam to rats.

Thyroid hormones do not alter rat brain benzodiazepine receptor function in-vivo.

Journal Article Thyroid hormones do not alter rat brain benzodiazepine receptor function in-vivo Get access C K Atterwill, C K Atterwill M.R.C. Developmental Neurobiology Unit, 33 John’s Mews, London WCIN 2NS Correspondence. M.R.C. Developmental Neurobiology Unit, 33 John’s Mews, London WCIN 2NS and University Department of Psychiatry, Research Unit, Littlemore Hospital, Oxford, U.K. Search for other works by this author on: Oxford Academic Google Scholar D J Nutt D J Nutt University Department of Psychiatry, Research Unit, Littlemore Hospital, Oxford, U.K. Search for other works by this author on: Oxford Academic Google Scholar Journal of Pharmacy and Pharmacology, Volume 35, Issue 11, November 1983, Pages 767–768, https://doi.org/10.1111/j.2042-7158.1983.tb02893.x Published: 12 April 2011 Article history Received: 10 August 1983 Published: 12 April 2011

P01-106 - Neuromorphologic and neurochemical regularities of prenatal exposure of developing human brain to alcohol

ObjectiveTo identify neuromorphologic and neurochemical patterns of prenatal exposure to alcohol of developing brain of man as risk factor for development of mental pathology in offspring of mothers with alcoholism.MethodsWe studied brains of embryos and fetuses (7–12 weeks). There were obtained 53 embryos: 23 - from alcoholics and 30 - from healthy women. Age of patients was 26–39 years, disease duration - from 3 to 13 years. All cases were diagnosed with stage II alcoholism (F 10.201; F 10.202). To evaluate the properties of benzodiazepine receptors in brains of embryos sinaptoneurosomas we used method of radio receptor binding of selective ligand 3H-flyunitrazepame.ResultsDamage to cytoplasmic membrane and internal cell membrane systems, emergence and subsequent transformation of different types of spherical formations in perinuclear space, variety of options of mitochondria as a normal structure, and with signs of functional and structural pathology, increased development of Golgi complex, appearance of lipofuscin, multi-vesicular cells and myelin formation, delay of development of synaptic contacts of vesicular type, manifested in decreasing length of postsynaptic density of synaptic contacts, reducing area and perimeter of presynaptic terminals in main study group, changing nature of vascularization of brain tissue, what was expressed in reduction of average area of vessels, an increase in their number per unit of area and reducing perimeter of brain.ConclusionWe have found that alcoholism during pregnancy results in decrease of affinity of synaptosomal receptors of brains of embryos. This may affect overall neurotransmitter processes in brain.

Evaluation of the pharmacokinetic interaction of midazolam with ursodeoxycholic acid, ketoconazole and dexamethasone by brain benzodiazepine receptor occupancy

To clarify whether alterations in midazolam pharmacokinetics resulting from changes in cytochrome P450 3A (CYP3A) activity lead to changes in its pharmacodynamic effects, benzodiazepine receptor occupancy was measured in the brain of rats after oral administration of midazolam. Receptor occupancy was measured by radioligand binding assay in rats pretreated with ursodeoxycholic acid (UDCA), ketoconazole and dexamethasone, and the plasma concentration of midazolam was simultaneously determined. There was a significant increase in the apparent dissociation constant and decrease in the maximum number of binding sites for specific [(3) H]flunitrazepam binding after oral administration of midazolam at pharmacologically relevant doses, suggesting that midazolam binds significantly to brain benzodiazepine receptors. Pretreatment with UDCA significantly enhanced the binding. This correlated well with significant enhancement by UDCA of the plasma midazolam concentration. The brain benzodiazepine receptor binding of oral midazolam was significantly enhanced by pretreatment with ketoconazole, a potent inhibitor of CYP3A, whereas it was significantly reduced by treatment with dexamethasone, an inducer of this enzyme. These effects paralleled changes in the plasma concentration of midazolam. The results indicate that pharmacokinetic changes such as altered CYP3A activity significantly influence the pharmacodynamic effect of midazolam by affecting occupancy of benzodiazepine receptors in the brain. They also suggest in-vivo or ex-vivo time-dependent measurements of receptor occupancy by radioligand binding assay to be a tool for elucidating the pharmacokinetic interaction of benzodiazepines with other agents in pre-clinical and clinical evaluations.

Peripheral benzodiazepine receptors in patients with chronic schizophrenia: a PET study with [11C]DAA1106

Inflammatory/immunological process and glial contribution are suggested in the pathophysiology of schizophrenia. We investigated peripheral benzodiazepine receptors in brains of patients with chronic schizophrenia, which were reported to be located on mitochondria of glial cells, using [11C]DAA1106 with positron emission tomography. Fourteen patients and 14 age- and sex-matched normal controls participated in this study. PET data were analysed by two-tissue compartment model with metabolite-corrected plasma input. Clinical symptoms were assessed using the Positive and Negative Syndrome Scale. There was no significant difference between [11C]DAA1106 binding of the cortical regions of normal controls and patients with schizophrenia, whereas the patients showed a positive correlation between cortical [11C]DAA1106 binding and positive symptom scores. There was also a positive correlation between [11C]DAA1106 binding and duration of illness. Although the correlations need to be interpreted very cautiously, involvement of glial reaction process in the pathophysiology of positive symptoms or progressive change of schizophrenia might be suggested.

Quantitative Analysis of Peripheral Benzodiazepine Receptor in the Human Brain Using PET with 11C-AC-5216

Peripheral benzodiazepine receptor (PBR) is upregulated in activated glial cells and is therefore a useful biomarker for inflammation in the brain and neurodegenerative disorders. We developed a new PET radioligand, (11)C-AC-N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-pheyl-7,8-dihydro-9H-purin-9-yl)acetamide ((11)C-AC-5216), that allows the imaging and quantification of PBRs in monkey and mouse brains. The aim of this study was to evaluate a quantification method of (11)C-AC-5216 binding in the human brain. A 90-min dynamic PET scan was obtained for each of 12 healthy men after an intravenous injection of (11)C-AC-5216. Regions of interest were drawn on several brain regions. Binding potential, compared with nondisplaceable uptake (BP(ND)), was calculated by a nonlinear least-squares fitting (NLS) method with the 2-tissue-compartment model, and total volume of distribution (V(T)) was estimated by NLS and graphical analysis methods. BP(ND) was highest in the thalamus (4.6 +/- 1.0) and lowest in the striatum (3.5 +/- 0.7). V(T) obtained by NLS or graphical analysis showed regional distribution similar to BP(ND). However, there was no correlation between BP(ND) and V(T) because of the interindividual variation of K(1)/k(2). BP(ND) obtained with data from a scan time of 60 min was in good agreement with that from a scan time of 90 min (r = 0.87). Regional distribution of (11)C-AC-5216 was in good agreement with previous PET studies of PBRs in the human brain. BP(ND) is more appropriate for estimating (11)C-AC-5216 binding than is V(T) because of the interindividual variation of K(1)/k(2). (11)C-AC-5216 is a promising PET ligand for quantifying PBR in the human brain.

Toluene and TCE Decrease Binding to Mu‐Opioid Receptors, but Not to Benzodiazepine and NMDA Receptors in Mouse Brain

In vitro and in vivo studies have shown that abused solvents affect different neurotransmitter systems, including the GABAergic, glutamatergic, and opioidergic. The first purpose of this study was to determine in mice whether an acute exposure to 4,000 ppm toluene or 12,000 ppm 1,1,1-trichloroethane (TCE) modifies receptor binding levels to: (a) DAMGO, a mu-opioid receptor selective agonist; (b) MK-801, a noncompetitive selective NMDA-receptor antagonist; and (c) flunitrazepam, a benzodiazepine binding site selective agonist. In addition, in separate groups of animals, nociceptive effects of toluene alone or co-administered with morphine were evaluated in the hot-plate test. Mice were exposed to toluene or TCE in static exposure chambers for 30 min, and their brains were removed 24 h later for autoradiography. Acute toluene inhalation produced a significant decrease in mu-opioid receptor binding levels in cingulate and piriform cortices, caudate putamen, thalamus, amygdala, and periaqueductal gray, whereas TCE significantly decreased mu-opioid receptor levels, but only in thalamus and periaqueductal gray. Both toluene and TCE decreased benzodiazepine receptor binding levels in discrete brain areas, but had no effect on NMDA receptor levels. In the hot-plate test, a single toluene exposure counteracted morphine antinociceptive response when the solvent exposure time was immediately followed by morphine treatment, but not when morphine was administered 24, 48, 72, and 96 h later. However, co-administration of morphine and toluene 24, 48, 72, and 96 h after the single solvent exposure resulted in morphine-induced analgesia blockade. Present results suggest that mu-opioid receptors are an important molecular target for organic solvents, and that the inhalation of these compounds may affect the analgesic properties of opioids.

CNS 7056

A new benzodiazepine derivative, CNS 7056, has been developed to permit a superior sedative profile to current agents, i.e., more predictable fast onset, short duration of sedative action, and rapid recovery profile. This goal has been achieved by rendering the compound susceptible to metabolism via esterases. The authors now report on the profile of CNS 7056 in vitro and in vivo. The affinity of CNS 7056 and its carboxylic acid metabolite, CNS 7054, for benzodiazepine receptors and their selectivity profiles were evaluated using radioligand binding. The activity of CNS 7056 and midazolam at subtypes (alpha1beta2gamma2, alpha2beta2gamma2, alpha3beta2gamma2, alpha5beta2gamma2) of the gamma-aminobutyric acid type A (GABAA) receptor was evaluated using the whole cell patch clamp technique. The activity of CNS 7056 at brain benzodiazepine receptors in vivo was measured in rats using extracellular electrophysiology in the substantia nigra pars reticulata. The sedative profile was measured in rodents using the loss of righting reflex test. CNS 7056 bound to brain benzodiazepine sites with high affinity. The carboxylic acid metabolite, CNS 7054, showed around 300 times lower affinity. CNS 7056 and CNS 7054 (10 mum) showed no affinity for a range of other receptors. CNS 7056 enhanced GABA currents in cells stably transfected with subtypes of the GABAA receptor. CNS 7056, like midazolam and other classic benzodiazepines, did not show clear selectivity between subtypes of the GABAA receptor. CNS 7056 (intravenous) caused a dose-dependent inhibition of substantia nigra pars reticulata neuronal firing and recovery to baseline firing rates was reached rapidly. CNS 7056 (intravenous) induced loss of the righting reflex in rodents. The duration of loss of righting reflex was short (< 10 min) and was inhibited by pretreatment with flumazenil. CNS 7065 is a high-affinity and selective ligand for the benzodiazepine site on the GABAA receptor. CNS 7056 does not show selectivity between GABAA receptor subtypes. CNS 7056 is a potent sedative in rodents with a short duration of action. Inhibition of substantia nigra pars reticulata firing and the inhibition of the effects of CNS 7056 by flumazenil show that it acts at the brain benzodiazepine receptor.

Reduced γ-Aminobutyric AcidA–Benzodiazepine Binding Sites in Insular Cortex of Individuals With Panic Disorder

Benzodiazepine drugs are highly effective anxiolytic medications, but the role of the benzodiazepine-gamma-aminobutyric acid(A)-chloride ion channel macromolecular complex in the pathophysiologic mechanism of anxiety is not well understood. Previous human imaging studies have indicated involvement of specific regions of the brain in anxiety disorders, especially the frontal-prefrontal, temporal, and cingulate cortical and the limbic areas. To identify potential abnormalities of brain benzodiazepine receptor binding number and distribution in anxiety disorders. At the University of Michigan positron emission tomography facility, 11 individuals with DSM-IV-defined anxiety syndrome panic disorder were compared with 21 unaffected healthy control subjects. Design and Main Outcome Measure In a between-group comparison, we used positron emission tomography and the benzodiazepine receptor ligand flumazenil labeled with carbon 11 to assess the regional brain pattern of receptor binding. We observed decreased binding specifically in the insular cortex bilaterally. No binding abnormality was observed in any other brain region, and there was no evidence of abnormal cerebral blood flow anywhere in the brain. Individuals with panic disorder and comorbid depression, indicative of a more severe disorder, had the lowest binding. No significant correlations were observed for binding with age, sex, or duration of disorder. A previous smaller study with the same ligand reported a probable binding abnormality in the right insula. Because gamma-aminobutyric acid is a major inhibitory neurotransmitter in the brain and because benzodiazepines facilitate this effect of gamma-aminobutyric acid, decreased benzodiazepine binding is consistent with localized brain activation (ie, loss of inhibition). Because the insula is strongly involved in visceral-somatic afferent and efferent function, activation of the insula is consistent with the occurrence of the physical symptoms prominently associated with panic disorder.

Kinetic evaluation in nonhuman primates of two new PET ligands for peripheral benzodiazepine receptors in brain

Peripheral benzodiazepine receptors (PBRs) are upregulated on activated microglia and are, thereby, biomarkers of cellular inflammation in brain. We recently developed two PET ligands with an aryloxyanilide structure to image PBRs and now evaluate the kinetics of these radiotracers in monkey to determine whether they are suitable to explore in human. Baseline and receptor-blocking scans were performed with [(11)C]PBR01 and [(18)F]PBR06 in conjunction with serial measurements of the arterial plasma concentration of parent radiotracer separated from radiometabolite. We used brain and plasma data with compartmental modeling to calculate regional brain distribution volume, which is equal to the ratio at equilibrium of the concentration of radioligand in brain to that of plasma. The distribution volume of [(11)C]PBR01 was inaccurately estimated in the baseline scans, possibly because of the short half-life of (11)C or the presence of radiometabolite in brain. In contrast, the distribution volume of [(18)F]PBR06 was stably determined within 200 min of scanning, and nondisplaceable uptake was only approximately 10% of total brain uptake. [(18)F]PBR06 is promising for use in human because brain activity could be quantified with standard compartmental models and showed higher ratios ( approximately 10:1) of specific to nonspecific uptake. A critical factor for human use will be whether the tracer has adequately fast wash out from brain relative to the half-life of the radionuclide to obtain stable values of distribution volume.

Acute treatment with pentobarbital alters the kinetics of in vivo receptor binding in the mouse brain

The effect of pentobarbital, a sedative-hypnotic barbiturate, on the in vivo binding of benzodiazepine receptors in the mouse brain was investigated. Dose-related changes in the apparent binding of [3H]Ro15-1788 ([3H]flumazenil) in the cerebral cortex, cerebellum and pons-medulla were observed by pretreatment with pentobarbital. For quantification of the kinetic properties of the in vivo binding of [3H]Ro15-1788, time courses of radioactivity following its injection were examined, and kinetic analysis was performed using the compartment model. The time courses of radioactivity following injection of [3H]Ro15-1788 with 3 mg/kg Ro15-1788 were used as input function. In all regions studied, rate constants between input compartment and specific binding compartment were significantly decreased by pentobarbital. However, no significant alterations in the binding potential (BP=K3/K4) of benzodiazepine receptors by pentobarbital were observed in any of the regions. A saturation experiment indicated that the decrease in the input rate constant (K3), which includes both the association rate constant (k(on)) and the number of binding sites available (B(max)), was mainly due to decrease in k(on). These results suggest that apparent increases in binding at 20 min after tracer injection were due to the decrease in the association and dissociation rates of binding in vivo.

Development of tolerance to nitrous oxide (N2O)‐induced anxiolysis and cross‐tolerance to chlordiazepoxide (CP) in mice

Our laboratory has long studied the pharmacology of the anesthetic gas N2O, and we have implicated brain benzodiazepine receptors in N2O-induced anxiolytic effects (Li and Quock, Pharmacol. Biochem. Behav. 68:789-796, 2001). This study was conducted to determine the extent to which tolerance develops to the anxiolytic effect of N2O and whether there is cross-tolerance to the anxiolytic effects of the benzodiazepine CP. Male NIH Swiss mice, 18–30 g, were exposed to 70% N2O in oxygen (O2) for 1½ to 24 hr, removed to 100% O2 (to reduce diffusion hypoxia) and placed in a room air environment to complete the washout of N2O. Thirty minutes following removal from N2O, mice were individually tested in the light/dark exploration test to determine behavioral responsiveness to 70% N2O in O2. Results show that it required a continuous exposure of 9 hr or longer of 70% N2O to induce tolerance to the anxiolytic-like effects of a subsequent exposure to 70% N2O in the light/dark box paradigm. Preliminary results suggest that N2O-tolerant mice also exhibited reduced responsiveness to the anxiolytic-like effects of some doses of CP when compared to control mice that were exposed for an equal period of time to compressed air. These results provide evidence for development of tolerance of mice to the anxiolytic effects of N2O and possible cross-tolerance to the effects of benzodiazepines. (Supported in part by NIH Grant DA-10343.)