The NMDA receptor competitive antagonist CPP modulates benzodiazepine tolerance and discontinuation

Application of gabapentinoids and novel compounds for the treatment of benzodiazepine dependence: the glutamatergic model

BACKGROUND

Current approaches for managing benzodiazepine (BZD) withdrawal symptoms are daunting for clinicians and patients, warranting novel treatment and management strategies. This review discusses the pharmacodynamic properties of BZDs, gabapentinoids (GBPs), endozepines, and novel GABAergic compounds associated with potential clinical benefits for BZD-dependent patients. The objective of this study was to review the complex neuromolecular changes occurring within the GABAergic and glutamatergic systems during the BZD tolerance and withdrawal periods while also examining the mechanism by which GBPs and alternative pharmacological therapies may attenuate withdrawal symptoms.

METHODS AND RESULTS

An elaborative literature review was conducted using multiple platforms, including the National Center for Biotechnology (NCBI), AccessMedicine, ScienceDirect, pharmacology textbooks, clinical trial data, case reports, and PubChem. Our literature analysis revealed that many distinctive neuroadaptive mechanisms are involved in the GABAergic and glutamatergic systems during BZD tolerance and withdrawal. Based on this data, we hypothesize that GBPs may attenuate the overactive glutamatergic system during the withdrawal phase by an indirect presynaptic glutamatergic mechanism dependent on the α₂δ₁ subunit expression.

CONCLUSIONS

GBPs may benefit individuals undergoing BZD withdrawal, given that the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor current significantly increases during abrupt BZD withdrawal in animal studies. This may be a conceivable explanation for the effectiveness of GBPs in treating both alcohol withdrawal symptoms and BZD withdrawal symptoms in some recent studies. Finally, natural and synthetic GABAergic compounds with unique pharmacodynamic properties were found to exert potential clinical benefits as BZD substitutes in animal studies, though human studies are lacking.

GABAA receptor subtypes and benzodiazepine use, misuse, and abuse

Benzodiazepines have been in use for over half a century. While they remain highly prescribed, their unfavorable side-effect profile and abuse liability motivated a search for alternatives. Most of these efforts focused on the development of benzodiazepine-like drugs that are selective for specific GABA_A receptor subtypes. While there is ample evidence that subtype-selective GABA_A receptor ligands have great potential for providing symptom relief without typical benzodiazepine side-effects, it is less clear whether subtype-selective targeting strategies can also reduce misuse and abuse potential. This review focuses on the three benzodiazepine properties that are relevant to the DSM-5-TR criteria for Sedative, Hypnotic, or Anxiolytic Use Disorder, namely, reinforcing properties of benzodiazepines, maladaptive behaviors related to benzodiazepine use, and benzodiazepine tolerance and dependence. We review existing evidence regarding the involvement of different GABA_A receptor subtypes in each of these areas. The reviewed studies suggest that α1-containing GABA_A receptors play an integral role in benzodiazepine-induced plasticity in reward-related brain areas and might be involved in the development of tolerance and dependence to benzodiazepines. However, a systematic comparison of the contributions of all benzodiazepine-sensitive GABA_A receptors to these processes, a mechanistic understanding of how the positive modulation of each receptor subtype might contribute to the brain mechanisms underlying each of these processes, and a definitive answer to the question of whether specific chronic modulation of any given subtype would result in some or all of the benzodiazepine effects are currently lacking from the literature. Moreover, how non-selective benzodiazepines might lead to the maladaptive behaviors listed in DSM and how different GABA_A receptor subtypes might be involved in the development of these behaviors remains unexplored. Considering the increasing burden of benzodiazepine abuse, the common practice of benzodiazepine misuse that leads to severe dependence, and the current efforts to generate side-effect free benzodiazepine alternatives, there is an urgent need for systematic, mechanistic research that provides a better understanding of the brain mechanisms of benzodiazepine misuse and abuse, including the involvement of specific GABA_A receptor subtypes in these processes, to establish an informed foundation for preclinical and clinical efforts.

Molecular and Regulatory Mechanisms of Desensitization and Resensitization of GABAA Receptors with a Special Reference to Propofol/Barbiturate

It is known that desensitization of GABA_A receptor (GABA_AR)-mediated currents is paradoxically correlated with the slowdown of their deactivation, i.e., resensitization. It has been shown that an upregulation of calcineurin enhances the desensitization of GABA_AR-mediated currents but paradoxically prolongs the decay phase of inhibitory postsynaptic currents/potentials without appreciable diminution of their amplitudes. The paradoxical correlation between desensitization and resensitization of GABA_AR-mediated currents can be more clearly seen in response to a prolonged application of GABA to allow more desensitization, instead of brief pulse used in previous studies. Indeed, hump-like GABA_AR currents were produced after a strong desensitization at the offset of a prolonged puff application of GABA in pyramidal cells of the barrel cortex, in which calcineurin activity was enhanced by deleting phospholipase C-related catalytically inactive proteins to enhance the desensitization/resensitization of GABA_AR-mediated currents. Hump-like GABA_AR currents were also evoked at the offset of propofol or barbiturate applications in hippocampal or sensory neurons, but not GABA applications. Propofol and barbiturate are useful to treat benzodiazepine/alcohol withdrawal syndrome, suggesting that regulatory mechanisms of desensitization/resensitization of GABA_AR-mediated currents are important in understanding benzodiazepine/alcohol withdrawal syndrome. In this review, we will discuss the molecular and regulatory mechanisms underlying the desensitization and resensitization of GABA_AR-mediated currents and their functional significances.

Valium without dependence? Individual GABAA receptor subtype contribution toward benzodiazepine addiction, tolerance, and therapeutic effects

Benzodiazepines are one of the most prescribed medications as first-line treatment of anxiety, insomnia, and epilepsy around the world. Over the past two decades, advances in the neuropharmacological understanding of gamma aminobutyric acid (GABA)_A receptors revealed distinct contributions from each subtype and produced effects. Recent findings have highlighted the importance of α₁ containing GABA_A receptors in the mechanisms of addiction and tolerance in benzodiazepine treatments. This has shown promise in the development of tranquilizers with minimal side effects such as cognitive impairment, dependence, and tolerance. A valium-like drug without its side effects, as repeatedly demonstrated in animals, is achievable.

NMDA Receptors and NO:cGMP Signaling Pathway Mediate the Diazepam-Induced Sensitization to Withdrawal Signs in Mice

The goal of the present study was to examine the effects of N-methyl-aspartate (NMDA) receptor antagonists-memantine and ketamine and the drugs modifying the NO:cGMP pathway-NG-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), the endogenous precursor of NO-L-arginine, and the guanylyl cyclase inhibitor-methylene blue (MB) on the development of sensitization to withdrawal signs precipitated after chronic, interrupted treatment with diazepam, a benzodiazepine receptor agonist, in mice. To develop the sensitization, the mice were divided into groups: continuously and sporadically (with two diazepam-free periods) treated with diazepam (15 mg/kg, sc). To precipitate the withdrawal syndrome (clonic and tonic seizures, and death), pentylenetetrazole (55 mg/kg, sc) with the benzodiazepine receptor antagonist, flumazenil (5.0 mg/kg, ip), were administered after the last injection of diazepam or saline. Memantine (2.5, 5.0 mg/kg), and ketamine (2.5, 5.0 mg/kg), L-NAME (100, 200 mg/kg) and 7-NI (20 and 40 mg/kg), L-arginine (250, 500 mg/kg) and MB (5 and 10 mg/kg) were administered ip in sporadically diazepam-treated mice during the diazepam-free periods. Our results indicated that both NMDA receptor antagonists and drugs that inhibit the NO:cGMP pathway, except L-arginine (the endogenous donor of NO), attenuated the diazepam-induced sensitization to withdrawal signs in mice. Thus, NMDA receptors and the NO:cGMP pathway are involved in the mechanisms of sensitization to benzodiazepine withdrawal.

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Mechanisms Underlying Tolerance after Long-Term Benzodiazepine Use: A Future for Subtype-Selective GABA(A) Receptor Modulators?

Despite decades of basic and clinical research, our understanding of how benzodiazepines tend to lose their efficacy over time (tolerance) is at least incomplete. In appears that tolerance develops relatively quickly for the sedative and anticonvulsant actions of benzodiazepines, whereas tolerance to anxiolytic and amnesic effects probably does not develop at all. In light of this evidence, we review the current evidence for the neuroadaptive mechanisms underlying benzodiazepine tolerance, including changes of (i) the GABA(A) receptor (subunit expression and receptor coupling), (ii) intracellular changes stemming from transcriptional and neurotrophic factors, (iii) ionotropic glutamate receptors, (iv) other neurotransmitters (serotonin, dopamine, and acetylcholine systems), and (v) the neurosteroid system. From the large variance in the studies, it appears that either different (simultaneous) tolerance mechanisms occur depending on the benzodiazepine effect, or that the tolerance-inducing mechanism depends on the activated GABA(A) receptor subtypes. Importantly, there is no convincing evidence that tolerance occurs with α subunit subtype-selective compounds acting at the benzodiazepine site.

Calcium/calmodulin-dependent protein kinase II mediates hippocampal glutamatergic plasticity during benzodiazepine withdrawal

Benzodiazepine withdrawal anxiety is associated with potentiation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) currents in hippocampal CA1 pyramidal neurons attributable to increased synaptic incorporation of GluA1-containing AMPARs. The contribution of calcium/calmodulin-dependent protein kinase II (CaMKII) to enhanced glutamatergic synaptic strength during withdrawal from 1-week oral flurazepam (FZP) administration was further examined in hippocampal slices. As earlier reported, AMPAR-mediated miniature excitatory postsynaptic current (mEPSC) amplitude increased in CA1 neurons from 1- and 2-day FZP-withdrawn rats, along with increased single-channel conductance in neurons from 2-day rats, estimated by non-stationary noise analysis. Input-output curve slope was increased without a change in paired-pulse facilitation, suggesting increased AMPAR postsynaptic efficacy rather than altered glutamate release. The increased mEPSC amplitude and AMPAR conductance were related to CaMKII activity, as intracellular inclusion of CaMKIINtide or autocamtide-2-related inhibitory peptide, but not scrambled peptide, prevented both AMPAR amplitude and conductance changes. mEPSC inhibition by 1-naphthyl acetyl spermine and the negative shift in rectification index at both withdrawal time points were consistent with functional incorporation of GluA2-lacking AMPARs. GluA1 but not GluA2 or GluA3 levels were increased in immunoblots of postsynaptic density (PSD)-enriched subcellular fractions of CA1 minislices from 1-day FZP-withdrawn rats, when mEPSC amplitude, but not conductance, was increased. Both GluA1 expression levels and CaMKII alpha-mediated GluA1 Ser(831) phosphorylation were increased in PSD-subfractions from 2-day FZP-withdrawn rats. As phospho-Thr(286)CaMKII alpha was unchanged, CaMKII alpha may be activated through an alternative signaling pathway. Synaptic insertion and subsequent CaMKII alpha-mediated Ser(831) phosphorylation of GluA1 homomers contribute to benzodiazepine withdrawal-induced AMPAR potentiation and may represent an important hippocampal pathway mediating both drug-induced and activity-dependent plasticity.

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.

Differential effects of two chronic diazepam treatment regimes on withdrawal anxiety and AMPA receptor characteristics

Withdrawal from chronic benzodiazepines is associated with increased anxiety and seizure susceptibility. Neuroadaptive changes in neural activity occur in limbo-cortical structures although changes at the level of the GABA(A) receptor do not provide an adequate explanation for these functional changes. We have employed two diazepam treatment regimes known to produce differing effects on withdrawal aversion in the rat and examined whether withdrawal-induced anxiety was accompanied by changes in AMPA receptor characteristics. Rats were given 28 days treatment with diazepam by the intraperitoneal (i.p.) route (5 mg/kg) and the subcutaneous (s.c.) route (15 mg/kg). Withdrawal anxiety in the elevated plus maze was evident in the group withdrawn from chronic s.c. diazepam (relatively more stable plasma levels) but not from the chronic i.p. group (fluctuating daily plasma levels). In the brains of these rats, withdrawal anxiety was accompanied by increased [3H]Ro48 8587 binding in the hippocampus and thalamus, and decreased GluR1 and GluR2 subunit mRNA expression in the amygdala (GluR1 and GluR2) and cortex (GluR1). The pattern of changes was different in the chronic i.p. group where in contrast to the chronic s.c. group, there was reduced [3H]Ro48 8587 binding in the hippocampus and no alterations in GluR1 and GluR2 subunit expression in the amygdala. While both groups showed reduced GluR1 mRNA subunit expression in the cortex overall, only the agranular insular cortex exhibited marked reductions following chronic i.p. diazepam. Striatal GluR2 mRNA expression was increased in the i.p. group but not the s.c. group. Taken together, these data are consistent with differential neuroadaptive processes in AMPA receptor plasticity being important in withdrawal from chronic benzodiazepines. Moreover, these processes may differ both at a regional and receptor function level according to the behavioral manifestations of withdrawal.

Termination of pseudopregnancy in the rat alters the response to progesterone, chlordiazepoxide, and MK-801 in the elevated plus-maze

RATIONALE

Allopregnanolone, a neurosteroid-reduced metabolite of progesterone, is a well-documented positive modulator of the gamma-aminobutyric type A (GABA(A)) receptor. As has been reported for other positive modulators of the GABA(A) receptor, chronic exposure to neurosteroids is hypothesized to decrease GABA(A) receptor function. Drawing from the literature on chronic exposure to benzodiazepines or alcohol, putative changes in N-methyl-D-aspartate (NMDA) receptor function are also expected after chronic neurosteroid exposure.

OBJECTIVES

To assess the sensitivity of the GABA(A) and NMDA receptors after chronic elevation of neurosteroid produced by termination of pseudopregnancy in behavioral tests of anxiety and sensorimotor coordination.

METHODS

Female rats ovariectomized on day 10 of pseudopregnancy were tested in the elevated plus-maze and on the rotor rod after an acute injection of progesterone (4 mg/0.2 ml, s.c.), chlordiazepoxide (5 or 15 mg/kg, i.p.), or MK-801 (0.025, 0.05, or 0.1 mg/kg, i.p.).

RESULTS

Pseudopregnancy termination produced an anxiogenic-like response in the plus-maze; an acute injection of progesterone restored baseline levels of behavior in this test. Pseudopregnancy termination eliminated the anxiolytic-like, sedative, and ataxic effects of chlordiazepoxide. In contrast, pseudopregnancy termination produced an increased sensitivity to the anxiolytic-like and ataxic effects of MK-801.

CONCLUSIONS

The effects of pseudopregnancy termination on the behavioral response to positive modulators of the GABA(A) receptor are consistent with results from studies in which chronic exposure to neurosteroids decreases the response to acute neurosteroid and benzodiazepine administration. However, unlike the enhanced glutamatergic tone resulting from discontinuation of chronic benzodiazepine or alcohol exposure, the termination of pseudopregnancy apparently decreases NMDA receptor function.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Seizures in the adult intensive care unit

Seizures are a common occurrence in the intensive care unit (ICU). The presentation of seizures is usually as focal or generalized motor convulsions, but other seizure types may occur. Etiologies of the seizures are typically secondary either to primary neurologic pathology or a consequence of critical illness and clinical management. Particularly important as precipitants of seizures are hypoxia/ischemia, drug toxicity, and metabolic abnormalities. It is important to properly diagnose the seizure type and its cause to ensure appropriate therapy. Most seizures occur singly, and recurrence is usually prevented with initiation of anticonvulsant therapy. However, status epilepticus may develop, which requires emergent treatment before irreversible brain injury occurs. Treatment with anticonvulsants is not without untoward risks, however, and primary toxicities of these agents is reviewed. After traumatic head injury, brain surgery, or cerebrovascular accidents, many patients are at risk for seizures. Current data on the benefits of prophylactic therapy for such patients is also reviewed.

Glutamic acid decarboxylase and glutamate receptor changes during tolerance and dependence to benzodiazepines

Protracted administration of diazepam elicits tolerance, whereas discontinuation of treatment results in signs of dependence. Tolerance to the anticonvulsant action of diazepam is present in an early phase (6, 24, and 36 h) but disappears in a late phase (72-96 h) of withdrawal. In contrast, signs of dependence such as decrease in open-arm entries on an elevated plus-maze and increased susceptibility to pentylenetetrazol-induced seizures were apparent 96 h (but not 12, 24, or 48 h) after diazepam withdrawal. During the first 72 h of withdrawal, tolerance is associated with changes in the expression of GABA(A) (gamma-aminobutyric acid type A) receptor subunits (decrease in gamma(2) and alpha(1); increase in alpha(5)) and with an increase of mRNA expression of the most abundant form of glutamic acid decarboxylase (GAD), GAD(67). In contrast, dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor GluR1 subunit mRNA and cognate protein, which are normal during the early phase of diazepam withdrawal, increase by approximately 30% in cortex and hippocampus in association with the appearance of signs of dependence 96 h after diazepam withdrawal. Immunohistochemical studies of GluR1 subunit expression with gold-immunolabeling technique reveal that the increase of GluR1 subunit protein is localized to layer V pyramidal neurons and their apical dendrites in the cortex, and to pyramidal neurons and in their dendritic fields in hippocampus. The results suggest an involvement of GABA-mediated processes in the development and maintenance of tolerance to diazepam, whereas excitatory amino acid-related processes (presumably via AMPA receptors) may be involved in the expression of signs of dependence after withdrawal.

Therapeutic potential of NMDA receptor antagonists in the treatment of alcohol and substance use disorders

Despite the fact that the use of alcohol, nicotine and other drugs is the major external factor contributing to mortality in industrialised countries, there are few medications available to treat alcohol and substance use disorders. In recent years, major advances have been made in the understanding of the neurobiological basis for these disorders and these advances should lead to the development of new pharmacotherapeutics. A substantial amount of the research suggests that N-methyl-D-aspartate (NMDA) receptor neurotransmission contributes to mediating the behavioural effects of alcohol and other drugs of abuse. This research supports the therapeutic potential of NMDA receptor antagonists in alcohol and substance use disorders. In this paper the authors present their opinion on the goals and stages of pharmacological treatment of these complex psychiatric disorders. Available preclinical research using designs that model aspects of alcohol and substance use disorders is summarised, with an emphasis on research published in the last two years. In animal models, NMDA antagonists inhibit physical dependence and the reinforcing effects of a variety of abused substances. The ability of NMDA antagonists to inhibit tolerance to drug effects and contribute possible antidepressant and anxiolytic effects are also important from the perspective of drug development. This review summarises the relevant clinical laboratory and treatment data. Finally, it presents the status of the current development of NMDA receptor antagonists and discusses candidates with the greatest potential for clinical development.

Low-dose subcutaneous ketamine infusion and morphine tolerance

The NMDA receptor has been reported to be involved in opioid tolerance. Adjuvant subcutaneous infusion treatment with (very) low-dose ketamine, a NMDA receptor antagonist, improves analgesia and at the same time appears to reduce morphine tolerance. Three cases are presented.

Lorazepam attenuates the behavioral effects of dizocilpine

To characterize the potential interaction between the excitatory and inhibitory neurotransmitter systems, the effects of dizocilpine, CPP, and lorazepam on open-field behavior and pentylenetetrazol-induced seizures were evaluated in mice. Dizocilpine (0.01-0.1 mg/kg), CPP (1-10 mg/kg), or vehicle was administered intraperitoneally 15 min prior to lorazepam (0.2-2 mg/kg) or vehicle. Behavioral monitoring began 25 min after the lorazepam injection. Upon completion of testing, unrestrained mice were infused intravenously with pentylenetetrazole until the onset of a full tonic-clonic seizure. The highest dose of dizocilpine by itself significantly increased the average distance traveled, the number of rears, and the number of stereotypies during the test period. Lorazepam alone dose dependently decreased activity on all behavioral parameters. Lorazepam also completely antagonized the hyperactivity produced by dizocilpine when the two compounds were coadministered. This antagonism is most likely due to an interaction in the regulation of dopaminergic tone which underlies motor activity. Lorazepam exerted a dose-dependent anticonvulsant effect. Dizocilpine alone had no effect on seizure induction and did not potentiate the anticonvulsive effect of lorazepam when coadministered with lorazepam. CPP reduced the number of rears and the number of stereotypies during the test period. CPP did not alter the pentylenetetrazol-induced seizure threshold and did not influence the anticonvulsant effect of lorazepam.

Does the noncompetitive NMDA receptor antagonist dizocilpine (MK801) really block behavioural sensitization associated with repeated drug administration?

The term 'behavioural sensitization' refers to the progressively augmented behavioural response that is produced by many drugs of abuse upon their repeated administration. From most of the available data, it has been concluded that co-administration of an N-methyl-D-aspartate (NMDA) receptor antagonist [such as dizocilpine (MK801)] together with the sensitizing drug can block the development of behavioural sensitization. However, the picture might not be that simple. Recent experimental evidence suggests that instead of blocking sensitization, co-administration of dizocilpine enhances the effect of the the sensitizing drug or has more complex effects on the development of sensitization. In this article, Thomas Tzschentke and Werner Schmidt present these two different views and emphasize that the conclusions that can be drawn from sensitization experiments about the effects of dizocilpine and related drugs on behavioural plasticity crucially depend on how, when and under what conditions a test for sensitization is conducted.