Diazepam dependence prevented by glutamate antagonists

Inhibitory and excitatory synaptic neuroadaptations in the diazepam tolerant brain

Benzodiazepine (BZ) drugs treat seizures, anxiety, insomnia, and alcohol withdrawal by potentiating γ2 subunit containing GABA type A receptors (GABAARs). BZ clinical use is hampered by tolerance and withdrawal symptoms including heightened seizure susceptibility, panic, and sleep disturbances. Here, we investigated inhibitory GABAergic and excitatory glutamatergic plasticity in mice tolerant to benzodiazepine sedation. Repeated diazepam (DZP) treatment diminished sedative effects and decreased DZP potentiation of GABAAR synaptic currents without impacting overall synaptic inhibition. While DZP did not alter γ2-GABAAR subunit composition, there was a redistribution of extrasynaptic GABAARs to synapses, resulting in higher levels of synaptic BZ-insensitive α4-containing GABAARs and a concomitant reduction in tonic inhibition. Conversely, excitatory glutamatergic synaptic transmission was increased, and NMDAR subunits were upregulated at synaptic and total protein levels. Quantitative proteomics further revealed cortex neuroadaptations of key pro-excitatory mediators and synaptic plasticity pathways highlighted by Ca2+/calmodulin-dependent protein kinase II (CAMKII), MAPK, and PKC signaling. Thus, reduced inhibitory GABAergic tone and elevated glutamatergic neurotransmission contribute to disrupted excitation/inhibition balance and reduced BZ therapeutic power with benzodiazepine tolerance.

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.

Ketamine: A Potential Adjunct for Severe Benzodiazepine Withdrawal

Following the abrupt cessation of benzodiazepine therapy, patients can present with acute life-threatening withdrawal. Medical management of benzodiazepine withdrawal is typically undertaken with benzodiazepines either through loading dose with gradual taper or symptom triggered treatment, though adjuvant anxiolytics and anticonvulsants are often used. Ketamine, increasingly utilized as an adjunct in the treatment of alcohol withdrawal, may represent an effective medication in the treatment of benzodiazepine withdrawal. In this case report, a 27-year-old male with a history of benzodiazepine and opioid abuse presented to our emergency department with a chief complaint of drug withdrawal. Despite standard treatment with large amounts of benzodiazepine, barbiturate, opioid, and adjunctive medications, the patient remained with severe withdrawal syndrome until an infusion of ketamine (0.5mg/kg in 30 minutes) was administered resulting in significant improvement of the patient symptoms. This case demonstrates the potential role of ketamine as an adjunct medication in the treatment of benzodiazepine withdrawal.

Short- and medium-term exposures of diazepam induce metabolomic alterations associated with the serotonergic, dopaminergic, adrenergic and aspartic acid neurotransmitter systems in zebrafish (Danio rerio) embryos/larvae

INTRODUCTION

Diazepam is a well-known psychoactive drug widely used worldwide for the treatment of anxiety, seizures, alcohol withdrawal syndrome, muscle spasms, sleeplessness, agitation, and pre/post-operative sedation. It is part of the benzodiazepine family, substances known to primarily act by binding and enhancing gamma-aminobutyric acid (GABA_A) receptors. The objective of the present work was to investigate the influence of short and medium-term diazepam exposures on neurotransmitters measured through targeted metabolomics using a zebrafish embryo model.

METHODS

Short-term (2.5 h) and medium-term (96 h) exposures to diazepam were performed at drug concentrations of 0.8, 1.6, 16, and 160 μg/L. Intervention groups were compared with a vehicle control group. Each group consisted of 20 zebrafish eggs/larvae. Metabolites related with neurotransmission were determined by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS).

RESULTS

Thirty-six compounds were quantified. Significantly increased tryptophan and serotonin concentrations were found in the intervention groups receiving higher doses of diazepam in 2.5 h exposure (p < 0.05 control versus intervention groups). Tyrosine concentrations were higher (p < 0.05) at higher concentrations in 2.5 h exposure, but lower (p < 0.05) at higher concentrations in 96 h exposure. Both phenylalanine and aspartic acid concentrations were higher (p < 0.05) at higher doses in 2.5 h and 96 h exposure.

CONCLUSIONS

Short- and medium-term exposures to diazepam induce dose- and time-dependent metabolomic alterations associated with the serotonergic, dopaminergic/adrenergic, and aspartic acid neurotransmitter systems in zebrafish.

How chronic administration of benzodiazepines leads to unexplained chronic illnesses: A hypothesis

It is thought that an ill defined biochemical cascade may lead to protracted withdrawal symptoms subsequent to discontinuance of routine use of benzodiazepine class drugs and establish chronic illness in some patients. In this review, published findings are presented that support the novel concept that withdrawal from benzodiazepine class drugs can trigger elevated and sustained levels of a potent oxidant called peroxynitrite via potentiation of the L-type voltage-gated calcium channels, and in the later stages of withdrawal, via excessive N-methyl-D-aspartate receptor activity, as well. Potentiation of L-type voltage-gated calcium channels and excessive N-methyl-D-aspartate receptor activity both result in calcium influx into the cell that triggers nitric oxide synthesis. In pathophysiological conditions, such increased nitric oxide synthesis leads to peroxynitrite formation. The downstream effects of peroxynitrite formation that may occur during withdrawal ultimately lead to further peroxynitrite production in a system of overlapping vicious cycles collectively referred to as the NO/ONOO(-) cycle. Once triggered, the elements of the NO/ONOO(-) cycle perpetuate pathophysiology, perhaps including reduced GABA_A receptor functioning, that may explain protracted withdrawal associated symptoms while the vicious cycle nature of the NO/ONOO(-) cycle may explain how withdrawal becomes a chronic state. Suboptimal levels of tetrahydrobiopterin may be one risk factor for the development of the protracted withdrawal syndrome as this will lead to partial nitric oxide uncoupling and resultant peroxynitrite formation. Nitric oxide uncoupling results in superoxide production as calcium-dependent nitric oxide synthases attempt to produce nitric oxide in response to L-type voltage-gated calcium channel-mediated calcium influx that is known to occur during withdrawal. The combination of nitric oxide and superoxide produced, as when partial uncoupling occurs, react together in a very rapid, diffusion limited reaction to form peroxynitrite and thereby trigger the NO/ONOO(-) cycle. The NO/ONOO(-) cycle may explain the nature of the protracted withdrawal syndrome and the related constellation of symptoms that are also common in other illnesses characterized as NO/ONOO(-) disorders such as myalgic encephalomyelitis/chronic fatigue syndrome and fibromyalgia.

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.

The Effect of Chronic Alprazolam Intake on Memory, Attention, and Psychomotor Performance in Healthy Human Male Volunteers

Alprazolam is used as an anxiolytic drug for generalized anxiety disorder and it has been reported to produce sedation and anterograde amnesia. In the current study, we randomly divided 26 healthy male volunteers into two groups: one group taking alprazolam 0.5 mg and the other taking placebo daily for two weeks. We utilized the Cambridge Neuropsychological Test Automated Battery (CANTAB) software to assess the chronic effect of alprazolam. We selected Paired Associates Learning (PAL) and Delayed Matching to Sample (DMS) tests for memory, Rapid Visual Information Processing (RVP) for attention, and Choice Reaction Time (CRT) for psychomotor performance twice: before starting the treatment and after the completion of the treatment. We found statistically significant impairment of visual memory in one parameter of PAL and three parameters of DMS in alprazolam group. The PAL mean trial to success and total correct matching in 0-second delay, 4-second delay, and all delay situation of DMS were impaired in alprazolam group. RVP total hits after two weeks of alprazolam treatment were improved in alprazolam group. But such differences were not observed in placebo group. In our study, we found that chronic administration of alprazolam affects memory but attentive and psychomotor performance remained unaffected.

GABA withdrawal syndrome: GABAA receptor, synapse, neurobiological implications and analogies with other abstinences

The sudden interruption of the increase of the concentration of the gamma-aminobutyric acid (GABA), determines an increase in neuronal activity. GABA withdrawal (GW) is a heuristic analogy, with withdrawal symptoms developed by other GABA receptor-agonists such as alcohol, benzodiazepines, and neurosteroids. GW comprises a model of neuronal excitability validated by electroencephalogram (EEG) in which high-frequency and high-amplitude spike-wave complexes appear. In brain slices, GW was identified by increased firing synchronization of pyramidal neurons and by changes in the active properties of the neuronal membrane. GW induces pre- and postsynaptic changes: a decrease in GABA synthesis/release, and the decrease in the expression and composition of GABAA receptors associated with increased calcium entry into the cell. GW is an excellent bioassay for studying partial epilepsy, epilepsy refractory to drug treatment, and a model to reverse or prevent the generation of abstinences from different drugs.

The effects of repeated zolpidem treatment on tolerance, withdrawal-like symptoms, and GABAA receptor mRNAs profile expression in mice: comparison with diazepam

RATIONALE

Zolpidem is a short-acting, non-benzodiazepine hypnotic that acts as a full agonist at α1-containing GABAA receptors. Overall, zolpidem purportedly has fewer instances of abuse and dependence than traditionally used benzodiazepines. However, several studies have shown that zolpidem may be more similar to benzodiazepines in terms of behavioral tolerance and withdrawal symptoms.

OBJECTIVES

In the current study, we examined whether subchronic zolpidem or diazepam administration produced deficits in zolpidem's locomotor-impairing effects, anxiety-like behaviors, and changes in GABAAR subunit messenger RNA (mRNA).

METHODS

Mice were given subchronic injections of either zolpidem (10 mg/kg), diazepam (20 mg/kg), or vehicle twice daily for 7 days. On day 8, mice were given a challenge dose of zolpidem (2 mg/kg) or vehicle before open field testing. Another set of mice underwent the same injection regimen but were sacrificed on day 8 for qRT-PCR analysis.

RESULTS

We found that subchronic zolpidem and diazepam administration produced deficits in the acute locomotor-impairing effects of zolpidem and increased anxiety-like behaviors 1 day after drug termination. In addition, we found that subchronic treatment of zolpidem and diazepam induced distinct but overlapping GABAAR subunit mRNA changes in the cortex but few changes in the hippocampus, amygdala, or prefrontal cortex. Levels of mRNA measured in separate mice after a single injection of either zolpidem or diazepam revealed no mRNA changes.

CONCLUSIONS

In mice, subchronic treatment of zolpidem and diazepam can produce deficits in the locomotor-impairing effects of zolpidem, anxiety-like withdrawal symptoms, and subunit-specific mRNA changes.

Involvement of the arachidonic acid cascade in the hypersusceptibility to pentylenetetrazole-induced seizure during diazepam withdrawal

The present study was designed to clarify whether the arachidonic acid cascade contributes to the decreased threshold for pentylenetetrazole-induced seizure under benzodiazepine withdrawal in mice. The seizure threshold for pentylenetetrazole was significantly decreased by the discontinuation of chronic treatment with diazepam. The decrease in the seizure threshold for pentylenetetrazole during diazepam withdrawal was significantly suppressed by intracerebroventricular (i.c.v.) pretreatment with the phospholipase A(2) inhibitor quinacrine (30, 100 nmol) and the lipoxygenase inhibitor nordihydroguaiaretic acid (10, 30 nmol). In contrast, the decreased seizure threshold in the diazepam-withdrawal group was intensified by pretreatment with the cyclooxygenase inhibitor diclofenac (56 nmol). These compounds did not alter the threshold for seizure in a control group. These findings suggest that enhancement of the arachidonic acid cascade may contribute to the hypersusceptibility to pentylenetetrazole-induced seizure during diazepam withdrawal.

Reduction of group II metabotropic glutamate receptors during development of benzodiazepine dependence

Prolonged use of benzodiazepines often leads to dependence and withdrawal syndrome. However, the cellular mechanisms underlying benzodiazepine dependence have not been fully clarified. Several investigators have shown an involvement of metabotropic glutamate receptors (mGluRs) in the pathophysiology of dependence or withdrawal. This study was performed to elucidate the role of mGluRs in benzodiazepine dependence. Withdrawal signs were precipitated in mice by flumazenil injection (25 mg/kg) after continuous subcutaneous infusion of benzodiazepines for 7 days, and the effects of several Gi-coupled receptor ligands on forskolin-stimulated cyclic AMP accumulation were examined in the cerebral cortex of mice. The mRNA expression for mGluRs was determined by RT-PCR. A single injection of flumazenil precipitated typical withdrawal signs such as tail elevation and tremor in mice treated with diazepam or alprazolam, but not quazepam. The inhibitory effect of nonselective mGluR ligands on adenylate cyclase activity was diminished in mice that showed signs of benzodiazepine withdrawal. The mRNA expression levels of mGluR2 and mGluR3 were lowered in the cerebral cortex of mice pretreated with diazepam or alprazolam. Our findings suggest that the reduction in the expression of group II mGluRs subunits may be involved in the development of benzodiazepine dependence.

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.

Effect of nitric oxide synthase inhibitors on benzodiazepine withdrawal in mice and rats

This study was undertaken to evaluate the effect of nitric oxide (NO) synthase inhibitors on benzodiazepine withdrawal syndrome in mice and rats. Diazepam withdrawal in mice was read out as intensification of the seizures induced by a subthreshold dose of pentetrazole. In rats, the withdrawal syndrome resulting from chronic administration of diazepam, chlordiazepoxide, clonazepam and temazepam was characterized by audiogenic seizures, hypermotility and weight loss. Administration of the non-selective NO synthase inhibitors N(G)-nitro-L-arginine (L-NOARG) and N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) significantly attenuated the withdrawal syndrome (i.e., pentetrazole-induced seizures) in diazepam-dependent mice. L-NOARG significantly suppressed hypermotility in clonazepam-dependent rats and inhibited the decrease in body weight observed after 12 h of withdrawal in chlordiazepoxide- and clonazepam-dependent rats. Moreover, a clear propensity of L-NOARG to protect benzodiazepine-dependent rats against audiogenic seizures was observed. These findings suggest that the cGMP/NO system may participate in causing the signs of benzodiazepine withdrawal.

Specificity of action of epileptogenic agents in diazepam-tolerant rats

The development of tolerance in rats by repeated intraperitoneal injection of 0.5 mg/kg diazepam increased the sensitivity of the frontal cortex to the epileptogenic effect of NMDA and kainic acid. The median effective dose of NMDA was shown to decrease under these conditions. Intracerebroventricular injection of NMDA in these doses induced clonic seizures and tonic extension of the forelimbs.

Increased AMPA receptor GluR1 subunit incorporation in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Prolonged benzodiazepine treatment leads to tolerance and increases the risk of dependence. Flurazepam (FZP) withdrawal is associated with increased anxiety correlated with increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor (AMPAR)-mediated synaptic function and AMPAR binding in CA1 pyramidal neurons. Enhanced AMPAR synaptic strength is also associated with a shift toward inward rectification of synaptic currents and increased expression of GluR1, but not GluR2, subunits, suggesting augmented membrane incorporation of GluR1-containing, GluR2-lacking AMPARs. To test this hypothesis, the postsynaptic incorporation of GluR1 and GluR2 subunits in CA1 neurons after FZP withdrawal was examined by postembedding immunogold quantitative electron microscopy. The percentage of GluR1 positively labeled stratum radiatum (SR) synapses was significantly increased in FZP-withdrawn rats (88.2% +/- 2.2%) compared with controls (74.4% +/- 1.9%). In addition, GluR1 immunogold density was significantly increased by 30% in SR synapses in CA1 neurons from FZP-withdrawn rats compared with control rats (FZP: 14.1 +/- 0.3 gold particles/mum; CON: 10.8 +/- 0.4 gold particles/mum). In contrast, GluR2 immunogold density was not significantly different between groups. Taken together with recent functional data from our laboratory, the current study suggests that the enhanced glutamatergic strength at CA1 neuron synapses during benzodiazepine withdrawal is mediated by increased incorporation of GluR1-containing AMPARs. Mechanisms underlying synaptic plasticity in this model of drug dependence are therefore fundamentally similar to those that operate during activity-dependent plasticity.

Long-lasting modulation of glutamatergic transmission in VTA dopamine neurons after a single dose of benzodiazepine agonists

Initial effects of drugs of abuse seem to converge on the mesolimbic dopamine pathway originating from the ventral tegmental area (VTA). Even after a single dose, many drugs of abuse are able to modulate the glutamatergic transmission activating the VTA dopamine neurons, which may represent a critical early stage in the development of addiction. Ligands acting on the benzodiazepine site of the inhibitory gamma-aminobutyric acid type A (GABA(A)) receptors are known to be rewarding in animal models and have abuse liability in humans, but notably little evidence exists on the involvement of the mesolimbic dopamine system in their effects. Here we report that single in vivo doses of benzodiazepine-site agonists, similar to morphine and ethanol, induce a modulation in the glutamatergic transmission of VTA dopamine neurons. This is seen 24 h later as an increase in the ratio between alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory currents using whole-cell patch-clamp configuration in mouse VTA slices. The effect was due to increased frequency of spontaneous miniature AMPA receptor-mediated currents. It lasted at least 3 days after the injection of diazepam, and was prevented by coadministration of the benzodiazepine-site antagonist flumazenil or the NMDA receptor antagonist dizocilpine. A single injection of the GABA(A) receptor alpha1 subunit-preferring benzodiazepine-site ligand zolpidem also produced an increase in the AMPA/NMDA ratio in VTA dopamine neurons. These findings suggest a role for the mesolimbic dopamine system in the initial actions of and on neuronal adaptation to benzodiazepines.

The combination of huperzine A and imidazenil is an effective strategy to prevent diisopropyl fluorophosphate toxicity in mice

Diisopropyl fluorophosphate (DFP) causes neurotoxicity related to an irreversible inhibition of acetylcholinesterase (AChE). Management of this intoxication includes: (i) pretreatment with reversible blockers of AChE, (ii) blockade of muscarinic receptors with atropine, and (iii) facilitation of GABA(A) receptor signal transduction by benzodiazepines. The major disadvantage associated with this treatment combination is that it must to be repeated frequently and, in some cases, protractedly. Also, the use of diazepam (DZP) and congeners includes unwanted side effects, including sedation, amnesia, cardiorespiratory depression, and anticonvulsive tolerance. To avoid these treatment complications but safely protect against DFP-induced seizures and other CNS toxicity, we adopted the strategy of administering mice with (i) small doses of huperzine A (HUP), a reversible and long-lasting (half-life approximately 5 h) inhibitor of AChE, and (ii) imidazenil (IMI), a potent positive allosteric modulator of GABA action selective for alpha(5)-containing GABA(A) receptors. Coadministration of HUP (50 microg/kg s.c., 15 min before DFP) with IMI (2 mg/kg s.c., 30 min before DFP) prevents DFP-induced convulsions and the associated neuronal damage and mortality, allowing complete recovery within 18-24 h. In HUP-pretreated mice, the ED(50) of IMI to block DFP-induced mortality is approximately 10 times lower than that of DZP and is devoid of sedation. Our data show that a combination of HUP with IMI is a prophylactic, potent, and safe therapeutic strategy to overcome DFP toxicity.

The role of NMDA receptor antagonists in nicotine tolerance, sensitization, and physical dependence: a preclinical review

Nicotine, the primary psychoactive component of tobacco products, produces diverse neurophysiological, motivational, and behavioral effects through several brain regions and neurochemical pathways. Various neurotransmitter systems have been explored to understand the mechanisms behind nicotine tolerance, dependence, and withdrawal. Recent evidence suggests that glutamate neurotransmission has an important role in this phenomenon. The aim of the present review is to discuss preclinical findings concerning the role of N-methyl-D-aspartate (NMDA) receptor neurotransmission in mediating the behavioral effects of nicotine, tolerance, sensitization, dependence, and withdrawal. Based on preclinical findings, it is hypothesized that NMDA receptors mediate the common adaptive processes that are involved in the development, maintenance, and expression of nicotine addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists may prove to be useful in alleviating the symptoms of nicotine abstinence and facilitate tobacco-smoking cessation.

Abuse and dependence liability of benzodiazepine-type drugs: GABA(A) receptor modulation and beyond

Over the past several decades, benzodiazepines and the newer non-benzodiazepines have become the anxiolytic/hypnotics of choice over the more readily abused barbiturates. While all drugs from this class act at the GABA(A) receptor, benzodiazepine-type drugs offer the clear advantage of being safer and better tolerated. However, there is still potential for these drugs to be abused, and significant evidence exists to suggest that this is a growing problem. This review examines the behavioral determinants of the abuse and dependence liability of benzodiazepine-type drugs. Moreover, the pharmacological and putative biochemical basis of the abuse-related behavior is discussed.

Prostate small cell carcinoma and skin metastases: a rare entity

OBJECTIVES

To report a rare case of small cell carcinoma of the prostate with unusual skin metastasis.

CLINICAL PRESENTATION AND INTERVENTIONS

A 60-year-old was evaluated for difficulty in urinating. Abdominal computed tomography scans revealed a prostatic mass invading the surrounding tissues and multiple perirectal, periprostatic, para-aortic and pericaval lymph nodes. Needle biopsy specimens showed both small cell carcinoma and adenocarcinoma. He was treated with combination chemotherapy: cisplatin and etoposide and bilateral orchiectomy. After six cycles of the chemotherapy, disease progressed and the patient did not respond to salvage therapy; hence, palliative care was instituted. During the follow-up, papillary lesions were observed in the scrotal skin; biopsy showed metastatic small cell carcinoma.

CONCLUSION

Small cell carcinoma of the prostate is an aggressive disease with a highly metastatic potential; but skin metastases are very uncommon. It has poor prognosis despite therapy. Management resembles that of small cell carcinoma of the lung.

Benzodiazepine-induced hippocampal CA1 neuron α-amino-3-hydroxy-5-methylisoxasole-4-propionic acid (AMPA) receptor plasticity linked to severity of withdrawal anxiety: differential role of voltage-gated calcium channels and N-methyl-D-aspartic acid receptors

Withdrawal from 1-week oral administration of the benzodiazepine, flurazepam (FZP) is associated with increased alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic acid (AMPA) receptor (AMPAR) miniature excitatory postsynaptic currents (mEPSCs) but reduction of N-methyl-D-aspartic acid (NMDA) receptor (NMDAR)-evoked (e)EPSCs in hippocampal CA1 neurons. A positive correlation was observed between increased AMPAR-mediated mEPSC amplitude and anxiety-like behavior in 1-day FZP-withdrawn rats. These effects were disrupted by systemic AMPAR antagonist administration (GYKI-52466, 0.5 mg/kg, intraperitoneal) at withdrawal onset, strengthening the hypothesis that CA1 neuron AMPAR-mediated hyperexcitability is a central component of a functional anatomic circuit associated with the expression of withdrawal anxiety. Abolition of AMPAR current upregulation in 2-day FZP withdrawn rats by GYKI-52466 injection also reversed the reduction in NMDAR-mediated eEPSC amplitude in CA1 neurons from the same rats, suggesting that downregulation of NMDAR function may serve a protective, negative-feedback role to prevent AMPAR-mediated neuronal overexcitation. NMDAR antagonist administration (MK-801, 0.25 mg/kg intraperitoneally) had no effect on modifying increased glutamatergic strength or on withdrawal anxiety, whereas injection of an L-type voltage-gated calcium channel antagonist, nimodipine (10 mg/kg, intraperitoneally) averted AMPAR current enhancement and anxiety-like behavior, suggesting that these manifestations may be initiated by a voltage-gated calcium channel-dependent signal transduction pathway. An evidence-based model of likely cellular mechanisms in the hippocampus contributing to benzodiazepine withdrawal anxiety was proposed implicating regulation of multiple CA1 neuron ion channels.

The blockade of AMPA-kainate and NMDA receptors in the dorsal periaqueductal gray reduces the effects of diazepam withdrawal in rats

It is well established that the most persistent sign of withdrawal from chronic benzodiazepine use in humans is anxiety. In contrast to other types of drugs of abuse, the emergence of this anxiety does not seem to be linked directly to alterations in the levels of dopamine in the mesolimbic system. Some studies have proposed that fear-like behaviors elicited by benzodiazepine withdrawal could be the result either of alterations in the sensitivity of GABAA receptors or in the neuronal hyperexcitability that results from neuroadaptative responses to chronic treatment, probably mediated by glutamate. The increased fear-like behaviors induced by benzodiazepine withdrawal are similar to the defense reaction displayed by animals exposed to dangerous situations or submitted to electrical or chemical stimulation of the dorsal periaqueductal gray (dPAG), a key structure of the brain aversive system. However, the involvement of the dPAG in drug abuse has been investigated only in the context of the physical effects of drug dependence. Thus, in this study we investigated the effects of injections into the dPAG of the glutamic acid diethyl ester (GDEE) and 2-amino-7-phosphonoheptanoate (AP-7) (AMPA-kainate and NMDA receptors antagonists, respectively) on fear-like behaviors promoted by benzodiazepine withdrawal in rats submitted to aversive events (foot-shocks) immediately before chronic diazepam administration in a conditioning place-preference paradigm, using a light-dark box. Our results showed that inhibition of the glutamatergic neurotransmission in the dPAG reduces the consequence of the diazepam withdrawal in rats, implicating the excitatory amino acids of the dPAG in the modulation of the aversive state induced by benzodiazepine drugs withdrawal.

An NMDA antagonist (LY 235959) attenuates abstinence-induced withdrawal of planarians following acute exposure to a cannabinoid agonist (WIN 55212-2)

The mechanisms that facilitate the development and expression of cannabinoid physical dependence in humans and other mammals are poorly understood. The present experiments used a planarian model to provide evidence that pharmacological antagonism of NMDA receptors significantly attenuates the development of cannabinoid physical dependence. Abstinence-induced withdrawal from the cannabinoid agonist WIN 55212-2 (10 microM) was manifested as a significant (P<0.05) decrease in the rate of planarian spontaneous locomotor velocity (pLMV) when WIN 55212-2 (10 microM)-exposed planarians were placed into drug-free water. No change in pLMV occurred when WIN 55212-2 (10 microM)-exposed planarians were placed into water containing WIN 55212-2 (10 microM). WIN 55212-2 (10 microM)-exposed planarians placed into water containing LY 235959 (1 or 10 microM) did not display withdrawal (no significant difference, P>0.05, in pLMV). In addition, withdrawal was not observed (no significant difference, P>0.05, in pLMV) in planarians that were co-exposed to a solution containing WIN 55212-2 (10 microM) and LY 235959 (10 microM). The present results reveal that NMDA receptor activation mediates the development of cannabinoid physical dependence and the expression of cannabinoid withdrawal in planarians.

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.

Chlordiazepoxide interactions with scopolamine and dizocilpine: novel cooperative and antagonistic effects on spatial learning

The authors investigated the effects on spatial behavior of coadministrations of a benzodiazepine, chlordiazepoxide (CDP), with a noncompetitive N-methyl-d-aspartate receptor antagonist (NMDAR), dizocilpine (DZP), and a muscarinic cholinergic receptor antagonist, scopolamine (SCP). Rats solved the Morris swim task in 2 settings; 1 in which a hidden escape platform was always in the same location (performance) and a 2nd in which the platform had been moved to a different location (acquisition) for repeated daily sessions. CDP (3.0 mg/kg) administered alone did not disrupt escape latencies or swim path accuracies. SCP and DZP each impaired acquisition and performance in a dose-dependent manner. CDP coadministered with 0.3 mg/kg SCP impaired escape only in the acquisition setting and when coadministered with 1.0 mg/kg SCP selectively exacerbated the escape impairment in the acquisition setting. CDP ameliorated deleterious effects of DZP in both settings.

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.

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor autoradiography in mouse brain after single and repeated withdrawal from diazepam

Withdrawal from chronic treatment with benzodiazepines is associated with increased neuronal excitability leading to anxiety, aversive effects and increased seizure sensitivity. After repeated withdrawal experiences, seizure sensitivity increases while withdrawal-induced anxiety and aversion decrease. We used autoradiographical methods employing [(3)H]Ro48 8587, a selective ligand for glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, to study withdrawal-induced changes in AMPA receptor binding in areas of the mouse brain postulated to be involved in these responses. Mice were given 21 days treatment with diazepam (15 mg/kg, s.c. in sesame oil) followed by withdrawal (single withdrawal) or three blocks of 7 days treatment interspersed with 3-day periods to allow washout of drug (repeated withdrawal). In keeping with heightened excitability in withdrawal from chronic diazepam treatment, the single withdrawal group showed, 72 h after their final dose of diazepam, increased [(3)H]Ro48 8587 binding in several brain areas associated with emotional responses or seizure activity, including hippocampal subfields, amygdalar and thalamic nuclei and motor cortex. In contrast, the repeated withdrawal group showed no changes in [(3)H]Ro48 8587 binding in any brain area studied. These observations are consistent with up-regulation of AMPA receptor-mediated transmission being important in withdrawal-induced anxiety and aversion but not in increased seizure sensitivity associated with repeated withdrawal. As changes in AMPA receptor subunit expression alter the functionality of the receptor, future studies will address this possibility.

Late-onset motoneuron disease caused by a functionally modified AMPA receptor subunit

Amyotrophic lateral sclerosis (ALS) is a devastating disorder of the central nervous system in middle and old age that leads to progressive loss of spinal motoneurons. Transgenic mice overexpressing mutated human Cu(2+)/Zn(2+) superoxide dismutase 1 (SOD1) reproduce clinical features of the familial form of ALS. However, changes in SOD1 activity do not correlate with severity of motor decline in sporadic cases, indicating that targets unrelated to superoxide metabolism contribute to the pathogenesis of the disease. We show here that transgenic expression in mice of GluR-B(N)-containing L-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) receptors with increased Ca(2+) permeability leads to late-onset degeneration of neurons in the spinal cord and decline of motor functions. Neuronal death progresses over the entire lifespan but manifests clinically in late adulthood, resembling the course of a slow neurodegenerative disorder. Additional transgenic expression of mutated human SOD1 accelerates disease progression, aggravates the severity of motor decline, and decreases survival. These observations link persistently elevated Ca(2+) influx through AMPA channels with progressive motor decline and late-onset degeneration of spinal motoneurons, indicating that functionally altered AMPA channels may be causally related to pathogenesis of sporadic ALS in humans.

Effects of deletion of gria1 or gria2 genes encoding glutamatergic AMPA-receptor subunits on place preference conditioning in mice

RATIONALE

The conditioned place preference (CPP) paradigm has been used as a measure of the rewarding effects of a number of stimuli. Critically, this classical conditioning procedure requires the formation of associations between a rewarding stimulus and environmental cues, and the ability of these cues to direct subsequent behaviour.

OBJECTIVES

The purpose of the current experiments was to examine the role of glutamatergic transmission via subunit-specific populations of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the formation of stimulus--reward associations involving contextual stimuli.

METHODS

We investigated the ability of cocaine and food to induce a CPP in mice lacking either the GluR1 or GluR2 subunits of the AMPA receptor [gria1 or gria2 knockout (KO) mice]. In separate experiments, food pellets or cocaine (5--20 mg/kg IP) were paired with one compartment of the CPP apparatus, while no-food or vehicle was paired with an alternative compartment.

RESULTS

Following conditioning, gria1 KOs displayed a significant preference for the food or cocaine-paired compartment, and did not differ from wild-type (WT) controls. However, gria2 knockouts displayed a preference for a cocaine-paired compartment, but not a food-paired compartment, indicating a specific deficit in place preference conditioning to food.

CONCLUSIONS

These results obtained using knockout mice indicate that GluR2-containing AMPA receptors may be critical for learning about contextual stimuli relevant to food rewards, but not drug rewards. When the results are considered in relation to our previous findings with gria1 and gria2 knockout mice, they also raise questions about the CPP paradigm representing a model of conditioned reward over a conditioned approach interpretation. However, it would be important to confirm these findings with alternative approaches, should selective ligands become available.

Requirement of alpha5-GABAA receptors for the development of tolerance to the sedative action of diazepam in mice

Despite its pharmacological relevance, the mechanism of the development of tolerance to the action of benzodiazepines is essentially unknown. The acute sedative action of diazepam is mediated via alpha1-GABA(A) receptors. Therefore, we tested whether chronic activation of these receptors by diazepam is sufficient to induce tolerance to its sedative action. Knock-in mice, in which thealpha1-,alpha2-,alpha3-, oralpha(5)-GABA(A) receptors had been rendered insensitive to diazepam by histidine-arginine point mutation, were chronically treated with diazepam (8 d; 15 mg x kg(-1) x d(-1)) and tested for motor activity. Wild-type, alpha2(H101R), and alpha3(H126R) mice showed a robust diminution of the motor-depressant drug action. In contrast, alpha5(H105R) mice failed to display any sedative tolerance. alpha1(H101R) mice showed no alteration of motor activity with chronic diazepam treatment. Autoradiography with [3H]flumazenil revealed no change in benzodiazepine binding sites. However, a decrease in alpha5-subunit radioligand binding was detected selectively in the dentate gyrus with specific ligands. This alteration was observed only in diazepam-tolerant animals, indicating that the manifestation of tolerance to the sedative action of diazepam is associated with a downregulation of alpha5-GABA(A) receptors in the dentate gyrus. Thus, the chronic activation of alpha(5)-GABA(A) receptors is crucial for the normal development of sedative tolerance to diazepam, which manifests itself in conjunction with alpha1-GABA(A) receptors.

Participation of dorsal raphe nucleus in the behavioral alteration observed after discontinuation of chronic diazepam administration: Possible neural circuitry involved

Previous findings from our laboratory have demonstrated a positive correlation between the development of tolerance to diazepam (DZ) 5 mg/kg/day over 4 days, and increased hippocampal synaptic plasticity. It seems likely that a similar plastic phenomenon may occur on hippocampal formation after chronic (18 days) DZ administration. We postulate hippocampal long-term potentiation (LTP) underlying substrate to the behavioral alteration observed after chronic DZ administration. In the present study, we investigated the involvement of the serotonergic (5-HT) system in the possible neural circuitry recruited during DZ withdrawal and in the increased hippocampal synaptic plasticity associated with the discontinuation of chronic DZ administration. The results of the current research demonstrate an increased neuronal activity in the dorsal raphe nucleus (DRN) during withdrawal. Previous MK-801 administration impairs the development of anxiety signs observed during withdrawal and the concomitant increased electrical activity on 5-HT neurons on DRN. These results are discussed in terms of the participation of 5-HT system in the modulation of hippocampal plasticity developed on DZ withdrawal.

Transient plasticity of hippocampal CA1 neuron glutamate receptors contributes to benzodiazepine withdrawal-anxiety

Withdrawal from 1-week oral administration of the benzodiazepine (BZ), flurazepam (FZP) is associated with enhanced AMPA receptor (AMPAR)-mediated and reduced NMDA receptor (NMDAR)-mediated excitation in CA1 pyramidal neurons 2-days after cessation of FZP administration. The present study examined temporal regulation of glutamate receptor-mediated whole-cell currents in CA1 neurons from hippocampal slices prepared from 0-, 1-, 2-, and 4-day FZP-withdrawn rats in relation to expression of anxiety-like behavior during BZ withdrawal. AMPAR-mediated miniature excitatory postsynaptic current (mEPSC) amplitude was significantly increased in CA1 neurons from 1- and 2-day FZP-withdrawn rats, while evoked NMDAR EPSC amplitude was reduced only in neurons from 2-day FZP-withdrawn rats. Withdrawal-anxiety, measured in the elevated plus-maze, was observed 1 day, but not 0, 2, or 4 days, after FZP treatment with 1-day withdrawn rats spending significantly reduced time in open arms compared to controls. CA1 neuron hyperexcitability was evident from the significant increase in the frequency of extracellular, 4-AP-induced spike discharges in slices from 1-day FZP-withdrawn rats. Systemic injection of the NMDAR antagonist MK-801 (0.25 mg/kg) on day 1 of withdrawal prevented reduced NMDAR-mediated currents in CA1 neurons from 2-day FZP-withdrawn rats, whereas AMPAR-mediated currents remained upregulated. Furthermore, MK-801 'unmasked' withdrawal-anxiety in the same 2-day FZP-withdrawn rats. Systemic injection of the AMPAR antagonist GYKI-52466 (0.5 mg/kg) at the onset of withdrawal blocked increased AMPAR-mediated currents and withdrawal-anxiety in 1-day FZP-withdrawn rats. These findings suggest that increased CA1 neuron AMPAR-mediated excitation may contribute to hippocampal hyperexcitability and expression of withdrawal-anxiety after prolonged BZ exposure via NMDAR-mediated neural circuits.

Continuous exposure to nitric oxide enhances diazepam binding inhibitor mRNA expression in mouse cerebral cortical neurons

Effects of sustained exposure to nitric oxide (NO) formed by long-term activation of N-methyl-D-aspartate (NMDA) receptors and liberated from a long-lasting NO generator, DETA NONOate, on diazepam binding inhibitor (DBI) and its mRNA expressions were examined using mouse cerebral cortical neurons. Long-term exposure to NMDA increased DBI mRNA expression, and NO synthase inhibitors dose-dependently inhibited this increase. DETA NONOate dose-dependently increased DBI mRNA expression when exposing the neurons to this agent for 3 days and a maximal enhancement of the expression was found at 100 microM of the NO generator. In addition, a significant increase in DBI mRNA expression was observed 1 day after the exposure to 100 microM DETA NONOate, and the maximal expression was observed 2 days after the exposure, whereas transient exposure for less than 3 h to 100 microM DETA NONOate produced no changes in the expression. DETA NONOate (100 microM)-induced increase in DBI mRNA expression was completely abolished by concomitant exposure to hemoglobin. DBI content was also dose-dependently increased by DETA NONOate after the exposure for 3 days. The inhibition of cGMP formation by 1H-[1,2,4] oxadiazolo [4,3-alpha]quinoxalin-1-one (ODQ) showed no affects on the DETA NONOate-induced expression, suggesting that the increased expression of DBI mRNA is mediated via processes independent of cGMP. These results indicate that continuous exposure of the neurons to NO is an essential factor for increasing DBI mRNA expression in the neurons.

MK-801 prevents the increased NMDA-NR1 and NR2B subunits mRNA expression observed in the hippocampus of rats tolerant to diazepam

The chronic diazepam administration in rats has been show from our previous results, to produce an increased synaptic plasticity. Furthermore, this occurs with a concomitant over expression of the mRNA NR1 and NR2B N-methyl-D-aspartate receptor subunits. MK-801, a non-competitive antagonist of N-methyl-D-aspartate receptor, impairs both the development of conditioned tolerance to diazepam and the hippocampal long-term potentiation generation. In the present study, we have further investigated the hippocampal glutamatergic transmission in the development of tolerance to diazepam. Our results demonstrate that the development of tolerance to the hypolocomotive effect of diazepam, along with the increased hippocampal synaptic plasticity and the associated over expression of the mRNA NR1 and NR2B N-methyl-D-aspartate receptor subunits, were blocked by previous MK-801 administration. We suggest that the participation of hippocampal glutamatergic transmission is relevant to increased hippocampal synaptic plasticity, the latter being a neurobiological mechanism behind the development of the conditioned tolerance to diazepam.

NMDA-NR1 and -NR2B subunits mRNA expression in the hippocampus of rats tolerant to Diazepam

The development of tolerance to the hypolocomotor effects of Diazepam (DZ) is thought to be a contingent or learning phenomenon. In previous reports, we demonstrated a positive correlation between the development of tolerance to the sedative effects of DZ and hippocampal synaptic plasticity. Furthermore, previous exposure to the drug administration context blocks both the tolerance to sedative effects of DZ and the increased hippocampal plasticity. The results of the present investigation show that the development of tolerance to hypolocomotor action of DZ (5 mg/kg/day) for 4 days results in a significant increase in the hybridization signals for mRNA for N-methyl-D-aspartate (NMDA) glutamatergic receptor NR1 and NR2B subunits in the hippocampal dentate gyrus. Furthermore, we have observed more benzodiazepine binding sites in the hippocampus of non-tolerant animals. We conclude that the increased hippocampal synaptic efficacy in DZ tolerant rats, may be NMDA receptor dependent due to an increased recombinant NR1-NR2B complex observed in the hippocampal formation of tolerant rats.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

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.

GABAA receptors and benzodiazepines: a role for dendritic resident subunit mRNAs

This review is designed to describe the evolution of the seminal observation made simultaneously in 1975 by Dr. W. Haefely's laboratory (Hoffman La Roche, Basel, Switzerland) and in the Laboratory of Preclinical Pharmacology (NIH, St. Elizabeths Hospital, Washington DC), that benzodiazepine action was mediated by a modulation of GABA action at GABA(A) receptors. In fact, our suggestion was that the benzodiazepine receptor was "a receptor on a receptor" and that this receptor was GABA(A). Needless to say, this suggestion created opposition, but we did not abandon the original idea, in fact, as shown in this review, there is now universal agreement with our hypothesis on the mode of action of benzodiazepines. Hence, this review deals with the allosteric modulation of GABA(A) receptors by benzodiazepines, the role of GABA(A) receptors and benzodiazepine structure diversities in this modulation, and describes the results of our attempts to establish a benzodiazepine (imidazenil) devoid of tolerance, withdrawal symptoms, and changes in the expression of GABA(A) receptor subunits during tolerance. It also deals with the idea that the synthesis of GABA(A) receptor subunits triggered by tolerance resides in dendrites and spines where mRNAs and the apparatus for this translation is located. New analytic procedures may foster progress in the understanding of tolerance to and withdrawal from benzodiazepines.

Environmental changes modify the expression of Diazepam withdrawal

Early results from our laboratory have demonstrated a positive correlation between increased hippocampal synaptic plasticity and development of tolerance to hypolocomotive effect of Diazepam (DZ). We have found recently, that pre-exposure to DZ administration context impairs increase of hippocampal synaptic plasticity in conjunction with tolerance to DZ. These findings have suggested, that the tolerance to DZ is context specific. Furthermore, the hippocampus can be critically involved in the behavioral expression of conditioned tolerance to DZ. The results of the present investigation show that animals chronically treated with DZ for 18 days exhibit withdrawal signs, evaluated as an increased anxiety in an elevated plus maze. These animals also show, a facilitation in the threshold to induce long-term potentiation in the hippocampal formation. These phenomena have a strong dependency on the drug administration context, since both are reversed after the introduction of some changes in the drug administration environment. Furthermore, the alteration of some environmental cues increased the locomotive activity in animals that did not show anxiety as a withdrawal signs. We conclude that a common neural system could underlie the behavioral expression of the conditioned tolerance and dependence on DZ.

Chronic benzodiazepine administration alters hippocampal CA1 neuron excitability: NMDA receptor function and expression(1)

Rats are tolerant to benzodiazepine (BZ) anticonvulsant actions two days after ending one-week administration of the BZ, flurazepam (FZP). Concurrently, GABA(A) receptor-mediated inhibition is reduced and AMPA receptor-mediated excitation is selectively enhanced in CA1 pyramidal neurons in hippocampal slices. In the present study, the effects of chronic FZP exposure on NMDA receptor (NMDAR) currents were examined in CA1 pyramidal neurons in hippocampal slices and following acute dissociation. In CA1 neurons from chronic FZP-treated rats, evoked NMDAR EPSC amplitude was significantly decreased (52%) in slices, and the maximal current amplitude of NMDA-induced currents in dissociated neurons was also significantly reduced (58%). Evoked NMDAR EPSCs were not altered following acute desalkyl-FZP treatment. Using in situ hybridization and immunohistochemical techniques, a selective reduction in NR2B subunit mRNA and protein expression was detected in the CA1 and CA2 regions following FZP treatment. However, total hippocampal NMDAR number, as assessed by autoradiography with the NMDAR antagonist, [(3)H]MK-801, was unchanged by FZP treatment. These findings suggest that reduced NMDAR-mediated currents associated with chronic BZ treatment may be related to reduced NR2B subunit-containing NMDARs in the CA1 and CA2 regions. Altered NMDAR function and expression after chronic BZ exposure may contribute to BZ anticonvulsant tolerance or dependence.

Selective enhancement of AMPA receptor-mediated function in hippocampal CA1 neurons from chronic benzodiazepine-treated rats

Two days following one-week administration of the benzodiazepine, flurazepam (FZP), rats exhibit anticonvulsant tolerance in vivo, while reduced GABA(A) receptor-mediated inhibition and enhanced EPSP amplitude are present in CA1 pyramidal neurons in vitro. AMPA receptor (AMPAR)-mediated synaptic transmission in FZP-treated rats was examined using electrophysiological techniques in in vitro hippocampal slices. In CA1 pyramidal neurons from FZP-treated rats, the miniature excitatory postsynaptic current (mEPSC) amplitude was significantly increased (33%) without change in frequency, rise time or decay time. Moreover, mEPSC amplitude was not elevated in dentate granule neurons following 1-week FZP treatment or in CA1 pyramidal neurons following acute desalkyl-FZP treatment. Regulation of AMPAR number was assessed by quantitative autoradiography with the AMPAR antagonist, [(3)H]Ro48-8587. Specific binding was significantly increased in stratum pyramidale of hippocampal areas CA1 and CA2 and in proximal dendritic fields of CA1 pyramidal neurons. Regulation of AMPAR subunit proteins was examined using immunological techniques. Neither abundance nor distribution of GluR1-3 subunit proteins was different in the CA1 region following FZP treatment. These findings suggest that enhanced AMPAR currents, mediated at least in part by increased AMPAR number, may contribute to BZ anticonvulsant tolerance. Furthermore, these studies suggest an interaction between GABAergic and glutamatergic systems in the CA1 region which may provide novel therapeutic strategies for restoring BZ effectiveness.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

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.