Neurohypophyseal peptides maintain tolerance to the incoordinating effects of ethanol

Role of Oxytocin and Vasopressin in Neuropsychiatric Disorders: Therapeutic Potential of Agonists and Antagonists

Oxytocin (OT) and vasopressin (AVP) are hypothalamic neuropeptides classically associated with their regulatory role in reproduction, water homeostasis, and social behaviors. Interestingly, this role has expanded in recent years and has positioned these neuropeptides as therapeutic targets for various neuropsychiatric diseases such as autism, addiction, schizophrenia, depression, and anxiety disorders. Due to the chemical-physical characteristics of these neuropeptides including short half-life, poor blood-brain barrier penetration, promiscuity for AVP and OT receptors (AVP-R, OT-R), novel ligands have been developed in recent decades. This review summarizes the role of OT and AVP in neuropsychiatric conditions, as well as the findings of different OT-R and AVP-R agonists and antagonists, used both at the preclinical and clinical level. Furthermore, we discuss their possible therapeutic potential for central nervous system (CNS) disorders.

Vasopressin and alcohol: a multifaceted relationship

BACKGROUND

Arginine vasopressin (VP) has been implicated in a number of neuropsychiatric disorders with an emphasis on situations where stress increased the severity of the disorder. Based on this hypothesized role for VP in neuropsychiatric disorders, much research is currently being undertaken in humans and animals to test VP as a target for treatment of a number of these disorders including alcohol abuse.

OBJECTIVES

To provide a summary of the literature regarding the role of VP in alcohol- and stress-related behaviors including the use of drugs that target VP in clinical trials.

RESULTS

Changes in various components of the VP system occur with alcohol and stress. Manipulating VP or its receptors can alter alcohol- and stress-related behaviors including tolerance to alcohol, alcohol drinking, and anxiety-like behavior. Finally, the hypothalamic-pituitary-adrenal axis response to alcohol is also altered by manipulating the VP system. However, clinical trials of VP antagonists have had mixed results.

CONCLUSIONS

A review of VP's involvement in alcohol's actions demonstrates that there is much to be learned about brain regions involved in VP-mediated effects on behavior. Thus, future work should focus on elucidating relevant brain regions. By using previous knowledge of the actions of VP and determining the brain regions and/or systems involved in its different behavioral effects, it may be possible to identify a specific receptor subtype target, drug treatment combination, or specific clinical contexts that may point toward a more successful treatment.

Binge drinking in alcohol-preferring sP rats at the end of the nocturnal period

Sardinian alcohol-preferring (sP) rats have been selectively bred for high alcohol preference and consumption using the standard 2-bottle "alcohol (10%, v/v) vs. water" choice regimen with unlimited access; under this regimen, sP rats daily consume 6-7 g/kg alcohol. The present study assessed a new paradigm of alcohol intake in which sP rats were exposed to the 4-bottle "alcohol (10%, 20%, and 30%, v/v) vs. water" choice regimen during one of the 12 h of the dark phase of the daily light/dark cycle; the time of alcohol exposure was changed daily in a semi-random order and was unpredictable to rats. Alcohol intake was highly positively correlated with the time of the drinking session and averaged approximately 2 g/kg when the drinking session occurred during the 12th hour of the dark phase. Alcohol drinking during the 12th hour of the dark phase resulted in (a) blood alcohol levels averaging approximately 100 mg% and (b) severe signs of alcohol intoxication (e.g., impaired performance at a Rota-Rod task). The results of a series of additional experiments indicate that (a) both singular aspects of this paradigm (i.e., unpredictability of alcohol exposure and concurrent availability of multiple alcohol concentrations) contributed to this high alcohol intake, (b) alcohol intake followed a circadian rhythm, as it decreased progressively over the first 3 h of the light phase and then maintained constant levels until the beginning of the dark phase, and (c) sensitivity to time schedule was specific to alcohol, as it did not generalize to a highly palatable chocolate-flavored beverage. These results demonstrate that unpredictable, limited access to multiple alcohol concentrations may result in exceptionally high intakes of alcohol in sP rats, modeling - to some extent - human binge drinking. A progressively increasing emotional "distress" associated to rats' expectation of alcohol might be the neurobehavioral basis of this drinking behavior.

Increase in alcohol intake, reduced flexibility of alcohol drinking, and evidence of signs of alcohol intoxication in Sardinian alcohol-preferring rats exposed to intermittent access to 20% alcohol

BACKGROUND

This study assessed in Sardinian alcohol-preferring (sP) rats a procedure known to promote alcohol drinking and based on the intermittent (once every other day) access to 2 bottles containing alcohol (20%, v/v) and water, respectively (Wise, 1973).

METHODS

To this end, sP rats were exposed - under the 2-bottle choice regimen - to: (i) 10% (v/v) alcohol with continuous access (CA10%; i.e., the procedure under which sP rats had been selectively bred); (ii) 10% (v/v) alcohol with intermittent access (IA10%); (iii) 20% (v/v) alcohol with continuous access (CA20%); (iv) 20% (v/v) alcohol with intermittent access (IA20%; the "Wise" condition) (Experiment 1). Additional experiments assessed the influence of (i) adulteration with quinine of the alcohol solution (Experiment 2) and (ii) concurrent presentation of a saccharin solution (Experiment 3) on alcohol drinking under the CA10% and IA20% conditions. Finally, it was assessed whether alcohol drinking under the CA10% and IA20% conditions resulted in motor incoordination at the Rota-Rod task, as a possible sign of alcohol intoxication (Experiment 4).

RESULTS

Daily alcohol intake markedly escalated in rats exposed to the IA20% condition, averaging 9.0 g/kg (in comparison with the average intake of 6.5 g/kg in the CA10% rat group). CA20% and IA10% rats displayed intermediate values of daily alcohol intake between those of CA10% and IA20% rats. Alcohol intake was virtually abolished by addition of quinine or by concurrent presentation of the saccharin solution in CA10% rats; conversely, alcohol intake in IA20% rats was only partially affected by gustatory aversion or concurrent presentation of an alternative reinforcer. Finally, alcohol intake in IA20%, but not in CA10%, rats resulted in clear motor-incoordinating effects.

CONCLUSIONS

These data suggest that the "Wise" procedure is effective in inducing marked increases in alcohol intake in sP rats. These increases are associated with a reduced flexibility of alcohol drinking (suggesting the development of "behavioral" dependence) and produce signs of alcohol intoxication that are not detected when sP rats are exposed to the more conventional CA10% condition.

Ethanol-induced elevation of 3 alpha-hydroxy-5 alpha-pregnan-20-one does not modulate motor incoordination in rats

BACKGROUND

Ethanol administration elevates the levels of GABAergic neuroactive steroids in brain and contributes to some of its behavioral actions. In the present study, we investigated whether such elevation of GABAergic neuroactive steroids contributes to the motor incoordinating effects of ethanol.

METHODS

Sprague-Dawley rats were administered ethanol (2 g/kg intraperitoneally) or saline, and the level of 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) was measured across time in cerebral cortex and in various brain regions at the peak time by radioimmunoassay. To study whether increases in GABAergic neuroactive steroids are responsible for the motor incoordinating actions of ethanol, rats were subjected to chemical (5alpha-reductase inhibitor, finasteride) and surgical (adrenalectomy) manipulations before receiving ethanol (2 g/kg intraperitoneally) injections. The rats were then subjected to different paradigms to evaluate motor impairment including the Majchrowicz motor intoxication rating scale, Rotarod test, and aerial righting reflex task at different time points.

RESULTS

The radioimmunoassay of 3alpha,5alpha-THP in different brain regions showed that ethanol increases 3alpha,5alpha-THP levels by 3- and 9-fold in cerebral cortex and hippocampus, respectively. There was no change in 3alpha,5alpha-THP levels in cerebellum and midbrain. The time course of 3alpha,5alpha-THP elevations in the cerebral cortex showed significant increases 20-min after ethanol injection with a peak at 60 min. In contrast, motor toxicity peaked between 5 and 10 min after ethanol injections and gradually decreased over time. Furthermore, adrenalectomy or pretreatment with finasteride (2 x 50 mg/kg, subcutaneously) did not reduce motor incoordinating effects of ethanol as assessed by the Majchrowicz intoxication rating score, Rotarod test, or aerial righting reflex task.

CONCLUSIONS

Ethanol increases GABAergic neuroactive steroids in a time- and brain region-selective manner. The role of neuroactive steroids in alcohol action is specific for certain behaviors. Alcohol-induced deficits in motor coordination are not mediated by elevated neuroactive steroid biosynthesis.

Different sensitivity to the motor incoordinating effects of γ-hydroxybutyric acid (GHB) and baclofen in GHB-sensitive and GHB-resistant rats

The present study investigated whether the differential sensitivity of selectively bred gamma-hydroxybutyric acid (GHB)-sensitive (GHB-S) and GHB-resistant (GHB-R) rats to GHB- and baclofen-induced sedation/hypnosis generalized to the motor incoordinating effect of the two drugs. To this aim, GHB-S and GHB-R rats were tested on a Rota-Rod after the acute administration of GHB (100-500 mg/kg, i.p.) and baclofen (1.25-5 mg/kg, i.p.). Significant line differences were observed in the dose-response curves for both GHB and baclofen, with GHB-S rats displaying a greater sensitivity to the motor incoordinating effects of both drugs than GHB-R rats. No line difference was observed in diazepam (1.25-5 mg/kg, i.p.), pentobarbital (5-15 mg/kg, i.p.), and ethanol (1-1.5 g/kg, i.p.) dose-response curves. These results suggest that the differential sensitivity of GHB-S and GHB-R rats to GHB and baclofen extends to the effects produced by doses of the two drugs which are 5-10 times lower than those for which rats have been selectively bred. These results are discussed in terms of the GABA(B) receptor being the likely neural substrate on which the differential sensitivity of GHB-S and GHB-R rats resides.

Cross-tolerance to ethanol and gamma-hydroxybutyric acid

In the present study, the development of tolerance to the motor impairing effects of gamma-hydroxybutyric acid (GHBA) and ethanol was compared (Experiment 1). Rats were required to perform a motor coordination task daily shortly after ethanol (3.5 g/kg) and GHBA (1.0 g/kg) administration for 9 consecutive days. Tolerance to the motor impairing effects of ethanol and GHBA developed to a similar extent but with different patterns. On the tenth day, the presence of cross-tolerance to the motor impairing effects of GHBA and ethanol was assessed (Experiment 2). Administration of 1.0 g/kg GHBA produced a significantly lower impairment in ethanol-tolerant rats than in ethanol-naive rats. Similarly, administration of 3.5 g/kg ethanol induced a significantly lower impairment in GHBA-tolerant rats than in GHBA-naive rats. The presence of cross-tolerance between GHBA and ethanol is discussed in terms of common pathways of neuroadaptation to chronic GHBA and ethanol.

Neuropeptides and alcohol addiction in monkeys

Neuropeptides have been implicated in experimental drug addiction. Desglycinamide (Arg8) vasopressin (DGAVP) attenuates heroin and cocaine intake during initiation of drug self-administration in rats. beta-Endorphin is self-administered in rats and a role of endogenous opioids in cocaine reward has been proposed. The present studies deal with voluntary alcohol consumption in monkeys under free choice conditions. Monkeys initiated alcohol drinking within a few days and after a stable drinking pattern was acquired increased their ethanol consumption during a short period following interruption of the alcohol supply (relapse). The alcohol drinking behavior seems under the control of reinforcement principles. DGAVP reduced the acquisition of alcohol drinking in the majority of treated monkeys. Initiation of alcohol drinking induced modifications in neuroendocrine homeostasis e.g. an increased plasma beta-endorphin. Both the opioid antagonist naltrexone and the opioid agonist morphine dose-dependently decreased alcohol intake during continuous supply and after imposed abstinence. The monkeys were more sensitive to both drugs after imposed abstinence. The effects are interpreted in the context of the endorphin compensation hypothesis of addictive behavior. It is suggested that endorphins may be particularly implicated in craving for addictive drugs and in relapse of addictive behavior.

The NMDA receptor antagonist dizocilpine differentially affects environment-dependent and environment-independent ethanol tolerance

Antagonists of the N-methyl-D-aspartate subtype of glutamate receptor have been reported to block the development of tolerance to various effects of ethanol and opiates, using paradigms in which tolerance is believed to be governed by learning. There is considerable evidence to implicate the N-methyl-D-aspartate receptor in learning processes, and therefore the ability of the antagonists to block tolerance has been attributed to their effects on learning. To evaluate this hypothesis, we compared, in C57BL/6 mice, the effect of the uncompetitive N-methyl-D-aspartate receptor antagonist, dizocilpine, on environment-dependent (associative) tolerance to ethanol, which is governed by learning, and on environment-independent (nonassociative) ethanol tolerance, in which learning plays a minimal role. Environment-dependent tolerance was induced by repeated ethanol injections, and dizocilpine blocked the development of this type of tolerance to the hypothermic and incoordinating effects of ethanol. In contrast, when environment-independent ethanol tolerance was induced by feeding the mice an ethanol-containing liquid diet, dizocilpine treatment had no effect on the development of tolerance to the hypothermic, incoordinating or hypnotic effects of ethanol. The results support the hypothesis that the effect of N-methyl-D-aspartate receptor antagonists on ethanol tolerance reflects the more general role of this receptor in processes involving learning and memory.

The effect of desglycinamide-(Arg8)-vasopressin (DGAVP) on the acquisition of free-choice alcohol drinking in rhesus monkeys

The vasopressin analog desglycinamide-(Arg8)-vasopressin (DGAVP) has been reported to reduce the acquisition of heroin and cocaine self-injection behavior in rats. This led to the hypothesis that DGAVP can reduce the self-administration of psycho-active drugs (including ethanol) by attenuating central reinforcement processes. Under forced ingestion conditions, DGAVP has been reported, however, to enhance alcohol drinking in rats. We studied the effect of DGAVP on the acquisition of voluntary, free-choice alcohol drinking in naive rhesus monkeys, that had concurrent access to either 1% and 2% (n = 12) or to 4% and 8% (n = 8) ethanol/water solutions in addition to drinking water. Half of the monkeys were injected twice per day with 50 micrograms.kg-1 of DGAVP for 14 successive days, the other half received placebo. Subsequently, all subjects had access to the same solutions for another 14 days without treatment. DGAVP did not significantly affect concentration preference behavior. With regard to net ethanol ingestion in animals drinking 1% and 2% solutions, DGAVP decreased net ethanol intakes, having a time-dependent and long lasting effect; placebo-treated animals gradually increased net ethanol intakes over time. The placebo-treated animals in the 4% and 8% group, showed a different acquisition pattern; DGAVP reduced net ethanol intake in two animals in a similar way as above. Two animals behaved differently. It is concluded that in a free-choice condition DGAVP did not enhance the acquisition of alcohol drinking in monkeys, but rather inhibited ethanol self-administration in the majority of the subjects.

The role of arginine vasopressin in the development of tolerance to ethanol in normal and Brattleboro rats

Administration of AVP and related peptide fragments following ethanol (EtOH) administration has been shown to enhance retention of tolerance to ethanol. The present studies were designed specifically to: (1) examine the influence of AVP given concurrently with EtOH on the development of tolerance to the ataxic and hypothermic effects of EtOH in Long-Evans rats, and (2) to determine if tolerance to these effects develops in Brattleboro rats which are deficient in AVP. In Experiment 1, EtOH (2.5 g/kg, 15% v/v) was administered IP to 2 groups of rats in combination with a SC injection of either AVP (6 micrograms/kg) or an equal volume of saline. Two additional control groups received IP saline injections in combination with either saline or AVP. After 13 days, EtOH-treated rats were significantly more tolerant than saline-treated animals. AVP significantly increased the hypothermic and ataxic effects of EtOH and failed to enhance tolerance development. AVP delayed the extinction of tolerance to the hypothermic (but not the ataxic) effects of ethanol when administered during the extinction phase to rats previously treated with EtOH. In Experiment 2, Brattleboro rats were injected with EtOH or an equivalent volume of saline and tested for ataxia and hypothermia. Rats receiving EtOH failed to demonstrate significant tolerance to either effect of ethanol after 12 treatment days.

Effect of an imidazobenzodiazepine, Ro15-4513, on the incoordination and hypothermia produced by ethanol and pentobarbital

The imidazobenzodiazepine, Ro15-4513, which is a partial inverse agonist at brain benzodiazepine receptors, reversed the incoordinating effect of ethanol in mice, as measured on an accelerating Rotarod. This effect was blocked by benzodiazepine receptor antagonists. In contrast, Ro15-4513 had no effect on ethanol-induced hypothermia in mice. However, Ro15-4513 reversed the hypothermic effect of pentobarbital, and, at a higher dose, also reversed the incoordinating effect of pentobarbital in mice. The data support the hypothesis that certain of the pharmacological effects of ethanol are mediated by actions at the GABA-benzodiazepine receptor-coupled chloride channel.

Intraventricular arginine vasopressin maintains ethanol tolerance

Tolerance to the hypnotic effect of ethanol in mice is prolonged by once daily intraventricular injections of arginine vasopressin. This action is similar to that reported previously when vasopressin was administered subcutaneously. The results indicate that maintenance of ethanol tolerance by vasopressin is a centrally mediated action of the peptide, and is not due to possible aversive properties of peripherally administered vasopressin.