Effects of post-ethanol administration of NMDA and non-NMDA receptor antagonists on the development of ethanol tolerance in C57B1 mice

Integrative Pharmacology in the Treatment of Substance Use Disorders

The detrimental physical, mental, and socioeconomic effects of substance use disorders (SUDs) have been apparent to the medical community for decades. However, it has become increasingly urgent in recent years to develop novel pharmacotherapies to treat SUDs. Currently, practitioners typically rely on monotherapy. Monotherapy has been shown to be superior to no treatment at all for most substance classes. However, many randomized controlled trials (RCTs) have revealed that monotherapy leads to poorer outcomes when compared with combination treatment in all specialties of medicine. The results of RCTs suggest that monotherapy frequently fails since multiple dysregulated pathways, enzymes, neurotransmitters, and receptors are involved in the pathophysiology of SUDs. As such, research is urgently needed to determine how various neurobiological mechanisms can be targeted by novel combination treatments to create increasingly specific yet exceedingly comprehensive approaches to SUD treatment. This article aims to review the neurobiology that integrates many pathophysiologic mechanisms and discuss integrative pharmacology developments that may ultimately improve clinical outcomes for patients with SUDs. Many neurobiological mechanisms are known to be involved in SUDs including dopaminergic, nicotinic, N-methyl-D-aspartate (NMDA), and kynurenic acid (KYNA) mechanisms. Emerging evidence indicates that KYNA, a tryptophan metabolite, modulates all these major pathophysiologic mechanisms. Therefore, achieving KYNA homeostasis by harmonizing integrative pathophysiology and pharmacology could prove to be a better therapeutic approach for SUDs. We propose KYNA-NMDA-α7nAChRcentric pathophysiology, the "conductor of the orchestra," as a novel approach to treat many SUDs concurrently. KYNA-NMDA-α7nAChR pathophysiology may be the "command center" of neuropsychiatry. To date, extant RCTs have shown equivocal findings across comparison conditions, possibly because investigators targeted single pathophysiologic mechanisms, hit wrong targets in underlying pathophysiologic mechanisms, and tested inadequate monotherapy treatment. We provide examples of potential combination treatments that simultaneously target multiple pathophysiologic mechanisms in addition to KYNA. Kynurenine pathway metabolism demonstrates the greatest potential as a target for neuropsychiatric diseases. The investigational medications with the most evidence include memantine, galantamine, and N-acetylcysteine. Future RCTs are warranted with novel combination treatments for SUDs. Multicenter RCTs with integrative pharmacology offer a promising, potentially fruitful avenue to develop novel therapeutics for the treatment of SUDs.

Effects of AMPA receptor antagonist, NBQX, and extrasynaptic GABAA agonist, THIP, on social behavior of adolescent and adult rats

Adolescence is characterized by high significance of social interactions, along with a propensity to exhibit social facilitating effects of ethanol while being less sensitive than adults to the inhibition of social behavior that emerges at higher doses of ethanol. Among the neural characteristics of adolescence are generally enhanced levels of glutamatergic (especially NMDA receptor) activity relative to adults, whereas the GABA system is still developmentally immature. Activation of NMDA receptors likely plays a role in modulation of social behavior in adolescent animals as well as in socially facilitating and suppressing effects of ethanol. For instance, adolescent and adult rats differ in their sensitivities to the effects of NMDA antagonists and ethanol on social behavior, with adolescents but not adults demonstrating social facilitation at lower doses of both drugs and adults being more sensitive to the socially suppressing effects evident at higher doses of each. The roles of AMPA and extrasynaptic GABA_A receptors in modulation of social behavior during adolescence and in adulthood are still unknown. The present study was designed to assess whether pharmacological blockade of AMPA receptors and/or activation of extrasynaptic GABA_A receptors results in age-dependent alterations of social behavior. Adolescent and adult male and female Sprague-Dawley rats were injected with an assigned dose of either a selective AMPA antagonist, NBQX (Experiment 1) or extrasynaptic GABA_A agonist, THIP (Experiment 2) and placed into a modified social interaction chamber for a 30-min habituation period prior to a 10-min social interaction test with a novel age- and sex-matched partner. Behaviors such as social investigation, contact behavior and play behavior were scored from video recordings of the interaction tests. In Experiment 1, NBQX produced similar social inhibition at higher doses in both age groups. In Experiment 2, THIP induced inhibition in adolescents, but not adults. No social facilitation was evident following low doses of either drug. Therefore, AMPA and extrasynaptic GABA_A receptors appear to play little role if any in modulation of peer-directed social behavior in adolescence and adulthood and not likely to contribute to previously observed age differences in the social effects of acute ethanol.

Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Glutamate transporter 1: target for the treatment of alcohol dependence

Emerging evidence indicates that many aspects of alcohol and drug dependence involve changes in glutamate transmission. A number of studies have reported that drugs of abuse, including alcohol and cocaine, alter glutamate transport. Extracellular glutamate is regulated by a number of glutamate transporters in various brain regions. Of these transporters, glutamate transporter (GLT1) is a key player in the removal of most of the extracellular glutamate. Similar to neurodegenerative disease models, in which there is dysfunction of the glutamatergic excitatory system, the role of GLT1 has been tested in drug dependence models that show dysfunction of glutamate transmission. We and others have recently found that ceftriaxone, an FDA-approved drug known to elevate GLT1 expression, attenuates cue-induced cocaine relapse. Moreover, we recently found that alcohol-preferring rats treated with ceftriaxone showed a significant dosedependent reduction in alcohol consumption. We also demonstrated that ceftriaxone-induced upregulation of GLT1 expression was associated with increases in glutamate uptake in Huntington's disease mouse model. Importantly, ceftriaxone is currently in clinical trials for the treatment of amyotrophic lateral sclerosis. This review provides information about the potential therapeutic role of GLT1 for the treatment of alcohol abuse and dependence.

Timing-dependent reduction in ethanol sedation and drinking preference by NMDA receptor co-agonist d-serine

NMDA receptors become a major contributor to acute ethanol intoxication effects at high concentrations as ethanol binds to a unique site on the receptor and inhibits glutamatergic activity in multiple brain areas. Although a convincing body of literature exists on the ability of NMDA receptor antagonists to mimic and worsen cellular and behavioral ethanol effects, receptor agonists have been less well-studied. In addition to a primary agonist site for glutamate, the NMDA receptor contains a separate co-agonist site that responds to endogenous amino acids glycine and d-serine. d-serine is both selective for this co-agonist site and potent in boosting NMDA dependent activity even after systemic administration. In this study, we hypothesized that exogenous d-serine might ameliorate some acute ethanol behaviors by opposing NMDA receptor inhibition. We injected adult male C57 mice with a high concentration of d-serine at various time windows relative to ethanol administration and monitored sedation, motor coordination and voluntary ethanol drinking. d-serine (2.7 g/kg, ip) prolonged latency to a loss of righting reflex (LoRR) and shortened LoRR duration when given 15 min before ethanol (3 g/kg) but not when it was injected with or shortly after ethanol. Blood samples taken at sedative recovery and at fixed time intervals revealed no effect of d-serine on ethanol concentration but an ethanol-induced decrease in l-serine and glycine content was prevented by acute d-serine pre-administration. d-serine had no effect on ethanol-induced (2 g/kg) rotarod deficits in young adult animals but independently and interactively degraded motor performance in a subset of older mice. Finally, a week-long series of daily ip injections resulted in a 50% decrease in free choice ethanol preference for d-serine treated animals compared to saline-injected controls in a two-bottle choice experiment.

Role of major NMDA or AMPA receptor subunits in MK-801 potentiation of ethanol intoxication

BACKGROUND

The glutamate system plays a major role in mediating EtOH's effects on brain and behavior, and is implicated in the pathophysiology of alcohol-related disorders. N-methyl-D-aspartate receptor (NMDAR) antagonists such as MK-801 (dizocilpine) interact with EtOH at the behavioral level, but the molecular basis of this interaction is unclear.

METHODS

We first characterized the effects of MK-801 treatment on responses to the ataxic (accelerating rotarod), hypothermic and sedative/hypnotic effects of acute EtOH administration in C57BL/6J and 129/SvImJ inbred mice. Effects of another NMDAR antagonist, phencyclidine, on EtOH-induced sedation/hypnosis were also assessed. Gene knockout of the NMDAR subunit NR2A or l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate GluR1 or pharmacological antagonism of the NMDAR subunit NR2B (via Ro 25-6981) was employed to examine whether inactivating any one of these glutamate signaling molecules modified MK-801's effect on EtOH-related behaviors.

RESULTS

MK-801 markedly potentiated the ataxic effects of 1.75 g/kg EtOH and the sedative/hypnotic effects of 3.0 g/kg EtOH, but not the hypothermic effects of 3.0 g/kg EtOH, in C57BL/6J and 129/SvImJ mice. Phencyclidine potentiated EtOH-induced sedation/hypnosis in both inbred strains. Neither NR2A nor GluR1 KO significantly altered basal EtOH-induced ataxia, hypothermia, or sedation/hypnosis. Ro 25-6981 modestly increased EtOH-induced sedation/hypnosis. The ability of MK-801 to potentiate EtOH-induced ataxia and sedation/hypnosis was unaffected by GluR1 KO or NR2B antagonism. NR2A KO partially reduced MK-801 + EtOH-induced sedation/hypnosis, but not ataxia or hypothermia.

CONCLUSIONS

Data confirm a robust and response-specific potentiating effect of MK-801 on sensitivity to EtOH's intoxicating effects. Inactivation of three major components of glutamate signaling had no or only partial impact on the ability of MK-801 to potentiate behavioral sensitivity to EtOH. Further work to elucidate the mechanisms underlying NMDAR x EtOH interactions could ultimately provide novel insight into the role of NMDARs in alcoholism and its treatment.

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.

Ethanol-induced hypnotic tolerance is absent in N-methyl-D-aspartate receptor epsilon 1 subunit knockout mice

Recent pharmacological studies suggest that N-methyl-D-aspartate (NMDA) receptors play an important role in neuroadaptive processes in the development of tolerance to addictive drugs, such as opioids, amphetamine, and cocaine. In the present study, we investigated the contribution of the NMDA receptor to ethanol-induced hypnotic tolerance using NMDA receptor epsilon1 subunit knockout mice. Hypnotic sensitivity to a single injection of 3, 3.5, and 4 g/kg ethanol was not significantly different between wild-type mice and NMDA receptor epsilon1 subunit knockout mice. In contrast, although wild-type mice displayed hypnotic tolerance after repeated administration of 4 g/kg ethanol for 4 consecutive days, no change in the duration of hypnosis was observed in knockout mice. No significant differences were observed in blood ethanol clearance between wild-type and knockout mice on day 4. Our results indicate epsilon1 subunit containing the NMDA receptor might be involved in the development of ethanol-induced hypnotic tolerance.

The Effects of NMDA and GABAA Pharmacological Manipulations on Acute and Rapid Tolerance to Ethanol During Ontogeny

BACKGROUND

Sensitivity to several ethanol effects increases during ontogeny, perhaps in part because of a notable decline in acute tolerance. In contrast, rapid tolerance to ethanol-induced sedation emerges slowly during ontogeny. This study tested the hypothesis that ontogenetic differences in glutamate and/or gamma-aminobutyric acid systems influence tolerance expression.

METHODS

Sprague-Dawley rats at postnatal day (P)26 or P70 received (+)MK-801, muscimol, or saline before ethanol (3.5 or 4.5 g/kg) or saline on day 1 and ethanol only on day 2. Loss of and time to regain the righting reflex and blood alcohol levels at recovery were recorded. The presence of acute tolerance was indicated as a positive slope of the linear regression of blood alcohol levels at recovery versus ethanol dose. Rapid tolerance was estimated on day 2 by comparing animals given ethanol only on day 2 with those given ethanol on both days.

RESULTS

Acute tolerance on day 1 only was observed at P26; this was disrupted by (+)MK-801 but not muscimol. Evidence for acute tolerance also emerged in adults on day 2. Whereas both drugs increased ethanol sedation at both ages, they did not facilitate ontogenetic expression of rapid tolerance: rapid tolerance was not evident at P26 regardless of pretreatment when indexed in terms of recovery time.

CONCLUSIONS

These data provide further evidence for an ontogenetic dissociation in the expression of acute and rapid tolerance to ethanol-induced sedation. Pharmacological attenuation of the expression of acute tolerance was sufficient but not necessary to delay recovery of righting after ethanol. The greater propensity of young animals to develop acute tolerance, seemingly modulated in part by NMDA receptors, may contribute to their relative resistance to ethanol, although other factors, including pharmacokinetic factors, also contribute to their more rapid recovery from ethanol sedation.

Metabotropic glutamate receptors mediate lipopolysaccharide-induced fever and sickness behavior

Several mechanisms have been proposed for neuroimmune communication supporting the sickness syndrome (fever, anorexia, inactivity, and cachexia) following infection. We examined the role of glutamate as a neurochemical intermediary of sickness behavior induced by intraperitoneal lipopolysaccharide (LPS). Mice implanted with biotelemetry devices capable of detecting body temperature (Tb) were administered LPS (50 or 500 microg/kg i.p., serotype 0111:B4) with or without i.p. pretreatment with vehicle or broad-spectrum antagonists selective for N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic (AMPA)/kainite, or metabotropic glutamate (mGlu) receptors. While NMDA and AMPA/kainate receptor antagonism failed to attenuate LPS-induced sickness behavior, antagonism of metabotropic receptors with l(+)-AP3 reduced the febrile (0-11h: control: 37.32+/-0.16 degrees C, l(+)-AP3: 36.66+/-0.27), anorexic (control: -87+/-5%, l(+)-AP3: 48+/-12% scotophase food intake), and cachexic (control: -8.9+/-0.4%, l(+)-AP3: -6.1+/-1.3% body weight) effects of 500 microg/kg LPS, and produced a biphasic Tb effect in response to 50 microg/kg LPS (1h: -0.90+/-0.26; 6h: 1.78+/-0.35 degrees C relative to baseline). At this dose the Tb of l(+)-AP3-treated mice was 1.18 degrees C lower than controls 2h post-injection, and 0.68 degrees C greater that controls 8h post-injection. These results suggest a role for mGlu receptors in mediating fever, anorexia, and cachexia possibly via activation of extra-vagal pathways, since the attenuating effect of l(+)-AP3 increased with increasing dosages of LPS. Given the critical role ascribed to mGlu receptors in neurotransmitter release and astrocytic processes, it is possible that these observations reflect an l(+)-AP3-induced attenuation of these systems.

Effect of natural and synthetic polyamines on ethanol intake in UChB drinker rats

Because of the important glutamatergic mediation of the behavioral effects of ethanol, glutamatergic agents have attracted attention for the treatment of ethanol abuse and dependence in preclinical and clinical studies. In the present study, we investigated the effect of pharmacological doses of the natural polyamines putrescine, spermine, and spermidine and the synthetic polyamine N,N'-bis-(3-aminopropyl)cyclohexane-1,4-diamine (DCD) on alcohol consumption in a free-choice paradigm carried out in genetically high-ethanol-consumer UChB rats. Short 3-day treatment with either polyamine, administered p.o., significantly reduced ethanol intake without modifying water and food intakes. Neither polyamine was able to increase markedly blood acetaldehyde in rats submitted to a standard challenge dose of ethanol, to rule out a disulfiram-like effect. Besides, blood ethanol disappearance after a test dose of ethanol was not affected by the synthetic polyamine DCD. Long-term treatment with DCD dose-dependently reduced ethanol intake in UChB rats without producing any observable effect on overt behavior, food consumption, and total fluid intake. The present results indicate that pharmacological doses of polyamines can reduce ethanol consumption in genetically drinking rats without producing significant side effects, suggesting that modulation of brain N-methyl-d-aspartate receptors by polyamines could represent a suitable strategy to reduce appetite for ethanol. However, caution must be exercised in interpreting the results because polyamines can also affect neuronal excitability by acting at other receptor targets, such as AMPA and kainate receptors, as well as at some voltage-dependent ion channels.

Neurobehavioral effects of alcohol in AMPA receptor subunit (GluR1) deficient mice

Of the ionotropic glutamatergic receptors, the NMDA receptor is clearly implicated in the acute and chronic effects of ethanol; however, the role of the AMPA receptor in mediating the effects of ethanol in vivo is as yet unclear. Using mice deficient in the AMPA receptor subunit GluR1 (GluR1-/- mice), we investigated whether the AMPA receptor had a significant role in mediating the effects of ethanol. GluR1-/- mice showed greater locomotor activity in a novel environment, but by the fifth day of repeated testing their activity was the same as that of wild-type mice. In contrast to their enhanced locomotor activity, on an accelerating rotarod GluR1-/- mice performed consistently worse than wild-types. With regard to the effects of ethanol on motor responses, GluR1-/- mice did not differ significantly from wild-type mice in ethanol's sedative or incoordinating effects. However, the GluR1-/- mice were insensitive to the hypothermic effects of a hypnotic dose of ethanol in contrast to wild-types; this effect was dissociable from the hypnotic effects of ethanol. Further, tolerance to ethanol developed equally for GluR1-/- mice versus wild-type mice. In terms of alcohol drinking behavior, compared to wild-types, GluR1-/- mice differed neither in the acquisition of voluntary ethanol consumption nor in stress-induced ethanol drinking, nor in the expression of an alcohol deprivation effect (ADE) which is used as a model of relapse-like drinking behavior. In summary, although the loss of a hypothermic effect of ethanol in GluR1-/- mice indicates a critical role for the AMPA receptors in this effect, the GluR1 subunit of the AMPA receptor does not seem to play a critical role in the etiology of alcohol dependence. However, changes observed in activity patterns may be related to the putative role of AMPA receptors in attention deficit hyperactivity disorder.

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.

Effect of NMDA antagonists, an NMDA agonist, and serotonin depletion on acute tolerance to ethanol

The effect of N-methyl-D-aspartate (NMDA) antagonists [dizocilpine, (+)MK-801, and ketamine], an NMDA agonist (D-cycloserine) and of brain serotonin (5-HT) depletion with p-chlorophenylalanine (p-CPA) on acute tolerance to ethanol was examined, using the tolerance model proposed by Radlow [Psychopharmacology 114 (1994) 1-8] and Martin and Moss [Alcohol Clin Exp Res 17 (1993) 211-216]. This model is based on the concept of a linear increase of acute tolerance with time; the rate of acute tolerance development is the slope of the output function that relates blood alcohol concentrations (BACs) and intoxication. Pretreatment with NMDA antagonists inhibited the development of acute tolerance to ethanol, whereas pretreatment with D-cycloserine enhanced it. Depletion of 5-HT by p-CPA also blocked acute tolerance to ethanol. These results on acute tolerance are similar to those previously found on rapid and chronic tolerance to ethanol.

Differential effects of a dihydropyridine calcium channel antagonist on the components of ethanol tolerance

The dihydropyridine calcium channel antagonist, nimodipine, was found to decrease the extent of tolerance that developed to the ataxic action of ethanol in experimental designs in which the tolerance was not context-specific, when ethanol was given by liquid diet. When ethanol was given by injection, so that cues were present for the effects of ethanol during the chronic treatment, tolerance to the ataxic actions of ethanol was unaffected. Nimodipine, however, decreased the tolerance to the hypothermic actions of ethanol, when the ethanol was given by injection. When the rats were given practice sessions on the motor task while under the influence of the ethanol, during the chronic treatment, nimodipine did not affect tolerance to the ataxic actions of ethanol. When nimodipine was given before the motor task learning and ethanol after the practice sessions, the tolerance to the ataxic effect of ethanol was increased. A similar schedule of drug treatment with the NMDA antagonist CGP37849 given before the practice sessions, and ethanol afterwards, resulted in decreased tolerance to ethanol. It is suggested that these changes in ethanol tolerance may be explained by dual actions of nimodipine in, firstly, decreasing the form of tolerance to ethanol that is not dependent on contextual cues and, secondarily, in increasing the learning of a motor task.

In search of a new pharmacological treatment for drug and alcohol addiction: N-methyl-D-aspartate (NMDA) antagonists

The most challenging aspect of treating alcohol and drug addiction is the relapsing course of these disorders. Although substitution therapies for nicotine and opioid dependence have proven to be relatively effective, there is a need for new pharmacotherapies designed to decrease the frequency and severity of relapse. The aim of this paper is to provide an overview of the potential utility of N-methyl-D-aspartate (NMDA) receptor antagonists as treatments for substance abuse as shown in preclinical models and preliminary clinical trials. It is hypothesized that NMDA receptors mediate the common adaptive processes that are involved the development, maintenance, and expression of drug and alcohol addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists offers a novel treatment approach. It is proposed that NMDA antagonists may have multiple functions in treating addictions, including an attenuation of withdrawal effects, normalization of the affective changes following initiation of abstinence which arise from neurochemical changes resulting from chronic addiction, and an attenuation of conditioned responses arising from drug-related stimuli.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Pharmacological treatment of alcoholism

1. Pharmacological treatments are effective as part of a treatment plan that includes substantial education, psychological therapy and social support. This paper reviews recent literature on animal models of and treatment for alcohol abuse under seven categories: agents to block craving or reduce alcohol intake, agents to induce aversion to alcohol, agents to treat acute alcohol withdrawal, agents to treat protracted alcohol withdrawal, agents to diminish drinking by treating associated psychiatric pathology, agents to decrease drinking by treating associated drug abuse, and agents to induce sobriety in intoxicated individuals. 2. The benzodiazepines provide safe and effective treatment for detoxification, although current research focuses on finding drugs with a smaller likelihood of dependence. As yet, there are no drugs that effectively reverse the intoxicating effects of alcohol. 3. Currently, only two major groups of drugs that are relatively safe have shown any effect at reducing alcohol consumption: aversives such as disulfiram, and opioid antagonists such as naltrexone. 4. Finally, it is important to customize therapy for each patient rather than putting everyone through a standard treatment plan, especially in regards to the use of antidepressant or antipsychotic medications. Tailoring the program to the patient's needs dramatically improves the outcome of therapy and reduces the risk of adverse effects.

More vasopressin mRNA in the paraventricular hypothalamic nucleus of alcohol-preferring rats and high alcohol-drinking rats selectively bred for high alcohol preference

Both the selectively bred alcohol-preferring (P) and high alcohol-drinking (HAD) rats exhibit alcohol preference, and develop tolerance to alcohol more quickly than their counterparts, the alcohol-nonpreferring (NP) and low alcohol-drinking (LAD) rats, respectively. It has been shown that the P rats retain developed tolerance longer than do NP rats, and alcohol drinking increases concurrently with the development of tolerance. Although alcohol preference and tolerance are fundamental elements of alcoholism, the exact mechanisms underlying these two phenotypes in P and HAD rats are not well understood. Recent studies have suggested that arginine vasopressin (AVP) may be involved in modulation of alcohol tolerance. Accordingly, this study was designed to examine whether the AVP mRNA level in the hypothalamus differs in rats that have been selectively bred for alcohol preference and nonpreference. A 35S-AVP antisense oligodeoxynucleotide probe was used for in situ hybridization to localize AVP mRNA in the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus (SON), two major sites for AVP synthesis in the hypothalamus. Quantitative autoradiography demonstrated that P rats had higher levels of AVP mRNA in the PVN than NP rats. Similarly, higher levels of AVP mRNA were also found in the PVN of HAD rats, compared with LAD rats. The AVP mRNA levels in the SON were similar in the alcohol-preferring and alcohol-nonpreferring rat lines. Basal plasma AVP levels were higher in NP rats than in P rats as determined by radioimmunoassay, whereas plasma AVP levels were not significantly different between HAD and LAD rats. The results suggest that increased AVP gene expression in the PVN may contribute to alcohol preference and the development of alcohol tolerance.

NMDA-receptor antagonists block the development of rapid tolerance to ethanol in mice

Several studies have emphasized the role of learning in the development of rapid and chronic tolerances. Recently, it was shown that the NMDA antagonists MK-801(dizocilpine) and ketamine block the development of tolerance to ethanol in rats submitted to tilt-plane apparatus. The present study examines the generality of this inhibition using mice submitted to the rota-rod test. Mice were tested in the rota-rod apparatus at 5, 10 and 15 minutes after intraperitoneal ethanol injections. The first experiment evaluated the time course of acute effects of different doses of ethanol (1.0-2.25 g/kg) in the rota-rod test. In the second experiment, the most effective dose of ethanol to produce rapid tolerance (RT) was determined. Mice were injected on day 1 with ethanol or saline and tested on the rota-rod. After 24 hours, all groups were injected with the same doses of ethanol and tested. The third experiment investigated whether ketamine (1.0-5.0 mg/kg) injected before ethanol on day 1 influenced the development of RT to ethanol. The last experiment compared the actions of the (+) and (-)MK-801 isomers (0.015-0.060 mg/kg) on RT to ethanol. Maximum motor impairment was obtained 5 minutes after ethanol injections. Pretreatment of animals with ketamine (2.5 and 5 mg/kg) or with (+)MK-801 (0.030 and 0.060 mg/kg) significantly blocked the development of RT. The (-)MK-801 isomer did not affect RT, suggesting that the blockade by MK-801 is stereospecific. These results confirm and extend previous studies showing that NMDA receptor antagonists block RT to the motor impairment produced by ethanol in other animals tested in different models.

Effect of NMDA antagonists on development of rapid tolerance to various barbiturates

We recently reported that the noncompetitive antagonists, (+)-MK-801 and ketamine, block the development of rapid tolerance to ethanol. We now show that pretreatment with these NMDA antagonists also blocks rapid tolerance to the various barbiturates (pentobarbital, barbital, and phenobarbital) examined. Tolerance to pentobarbital occurred under three difference conditions: (a) in groups of rats that were tested at repeated times on day 1 (intoxicated practice or testing group), (b) in groups of rats that were not tested on the apparatus but handled at the same times on day 1 (dummy testing or associative learning group), and (c) in groups of rats that were not subjected to testing at all on day 1 (nontesting). However, NMDA antagonists blocked intoxicated practice and associative tolerance, but not tolerance produced in the nontesting group. In the last experiment NMDA antagonist failed to block tolerance (unlearned) when animals were treated in the animal quarters and tested in a different room (i.e., in the laboratory). These findings suggest that NMDA antagonists affect barbiturate tolerance in a manner similar to their effect on ethanol tolerance.

Effect of NMDA antagonists on rapid tolerance to benzodiazepines

We have reexamined the effect of NMDA antagonists [(+)MK-801 and ketamine] on rapid tolerance to chlordiazepoxide. (+)MK-801 and ketamine blocked the development of rapid tolerance to chlordiazepoxide, but this effect was dependent on the dose ratio of the NMDA antagonist to that of the benzodiazepine used to produce rapid tolerance. Furthermore, NMDA antagonists blocked both learned and unlearned tolerance to chlordiazepoxide. It appears that in addition to impairment of memory and learning, NMDA antagonists may also influence some other mechanism involved in the production of drug-tolerance.

N-Methyl-D-aspartate-induced enhancement of activator protein-1 DNA binding activity is blocked by ethanol in cerebellar granule cells

The effects of ethanol on N-methyl-D-aspartate (NMDA) and non-NMDA receptor agonist-stimulated activator protein-1 (AP-1) DNA binding activity in primary cultures of rat cerebellar granule cells were investigated. The application of intoxicating concentrations of ethanol produced a concentration-dependent inhibition of NMDA-enhanced AP-1 binding with a significant reduction obtained at 50 mM ethanol. The inhibitory actions of ethanol on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-stimulated AP-1 binding were considerably weaker as compared to the effects seen following administration of NMDA. The AMPA-induced enhancement of AP-1 DNA binding activity was demonstrated both in the absence and presence of cyclothiazide, a drug, which is known to block the desensitization of AMPA receptors. Our data suggest that moderate concentrations of ethanol modulate glutamate-induced alterations of gene expression in brain neurons.

Effects of decahydroisoquinoline-3-carboxylic acid monohydrate, a novel AMPA receptor antagonist, on glutamate-induced CA2+ responses and neurotoxicity in rat cortical and cerebellar granule neurons

In this study, we examined the effects of a novel water-soluble, putative AMPA receptor antagonist, (-)(3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]-1,2,3, 4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid monohydrate (LY326325), on glutamate-, N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, and kainic acid (KA)-induced elevations of intracellular Ca2+ concentrations ([Ca2+]i) and 45Ca2+ uptake, as well as glutamate agonist-induced neurotoxicity in primary cultures of intact rat cortical and cerebellar granule neurons. In some experiments, the actions of LY326325 were tested in the presence of cyclothiazide, a compound that is known to block glutamate-induced desensitization of AMPA-preferring subtypes of glutamate receptors, thereby largely potentiating the functional effects of AMPA. LY326325 fully blocked the elevations of [Ca2+]i induced by NMDA and non-NMDA glutamate receptor agonists in both cortical and cerebellar granule neurons. The application of increasing concentrations of cyclothiazide was not able to reverse the LY326325-induced blockade of glutamate receptors in cortical neurons. In contrast, the same cyclothiazide treatment fully reversed the blockade produced by the noncompetitive AMPA/KA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine HCl (GYKI 52466). In 45Ca2+ uptake studies. LY325325 inhibited the NMDA-, AMPA-, and KA-induced enhancement of 45Ca2+ uptake in a concentration-dependent fashion in both cortical and cerebellar granule cells. In analogy to the results obtained with [Ca2+]i recordings, cyclothiazide failed to counteract the LY326325-induced blockade of KA-stimulated 45Ca2+ uptake in cerebellar granule neurons, whereas the blockade induced by the noncompetitive AMPA/KA receptor blocking agent GYKI 52466 was fully reversed by cyclothiazide. Because a similar, although not identical pattern of actions was seen following the application of the competitive AMPA/KA receptor antagonist 6-nitro-7-sulphamoyl-benzo(f)quinoxaline-2-3-dione (NBQX), it is suggested that the inhibitory actions of LY326325 are similar to those produced by NBQX but clearly differ from those caused by the noncompetitive AMPA/KA receptor antagonist GYKI 52466. Finally, when the neuroprotective actions of LY326325 on glutamate agonist-induced neurotoxicity were examined in cerebellar granule neurons, we found that LY326325 almost completely blocked the neurotoxic actions of NMDA, AMPA, and KA, respectively, whereas it produced only a partial blockade of glutamate-induced neurotoxicity. Taken together, our current results suggest that although LY326325 blocked both nonNMDA and NMDA-induced Ca2+ responses, it still displayed a preferential affinity of nonNMDA receptors as compared to NMDA receptors. However, LY326325 appears to be a less selective AMPA/KA receptor antagonist than NBQX and GYKI52466, respectively.