Effect of naltrexone on acute tolerance to ethanol in UChB rats

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

Selective mu- and kappa-opioid receptor antagonists administered into the nucleus accumbens interfere with rapid tolerance to ethanol in rats

BACKGROUND

Previous findings have shown that intra-accumbens injection of naltrexone, a non-selective opioid antagonist, blocks the acquisition of rapid tolerance to ethanol in rats. This study investigates the effects of intra-accumbens injection of the selective mu-, delta-, and kappa-opioid antagonists, respectively, naloxonazine, naltrindole, and nor-binaltorphimine, on rapid tolerance to ethanol.

METHODS

Male Wistar rats with guide cannulae directed to the shell or the core portions of the nucleus accumbens received a microinjection of naloxonazine (2-4 microg), naltrindole (2-4 microg), nor-binaltorphimine (2.5-5 microg), or vehicle. After 5 min, each group was divided in two groups that received ethanol (2.7 g/kg i.p.) or saline. Rats were then tested for motor coordination on the tilting plane apparatus. Twenty four hours later, all rats received a challenge dose of ethanol (2.7 g/kg i.p.) and were tested on the tilt plane again.

RESULTS

Repeated injections of ethanol caused a reduction in motor impairment suggesting the development of tolerance. However, rats injected with 4 microg naloxonazine into either core or shell portions of the nucleus accumbens did not exhibit tolerance when challenged with ethanol on day 2. Rats treated with 5 microg nor-binaltorphimine into accumbens core plus intraperitoneal saline on day 1 showed reduced motor impairment when challenged with ethanol on day 2, suggesting cross-tolerance to ethanol.

CONCLUSIONS

Taken together, our results suggests that mu-opioid receptors in both shell and core portions of the nucleus accumbens, and possibly kappa-opioid in the core, participate in the modulation of rapid tolerance to ethanol.

Baclofen reduces ethanol intake in high-alcohol-drinking University of Chile bibulous rats

ABSTRACT Treatment with gamma-aminobutiric acid (GABA(B)) receptor agonist, +/-baclofen, has been shown to reduce ethanol intake in selectively bred Sardinian alcohol-preferring rats. The general goal of the present study was to characterize the high ethanol consumption high-alcohol-drinking University of Chile bibulous (UChB) rats with regard to the anti-alcohol effect of GABA(B) receptor stimulation. UChB rats were treated with the more active enantiomer of baclofen [R(+)-baclofen; at a dose of 1.0, 2.0 or 3.0 mg/kg] administered intraperitoneally once daily for four consecutive days or a single dose. When comparing ethanol and saccharin consumption in a free-choice regimen with unlimited access 24 hours/day, the dose of baclofen required to attenuate ethanol consumption significantly was 1.0 mg/kg administered once a day for three consecutive days while the dose that was sufficient to affect saccharin consumption significantly was 2.0 mg/kg, indicating that baclofen was more potent in reducing ethanol intake by UChB rats than reducing saccharin consumption. The reduction of ethanol or saccharin intake can not be attributed to baclofen-induced motor impairment, since baclofen (1.0, 2.0 or 3.0 mg/kg) did not alter spontaneous locomotor activity in UChB rats. Baclofen dose-dependently suppressed the motor activity stimulated by ethanol administration, a phenomenon mediated by activation of the mesolimbic dopamine system. In conclusion, these results showed that the activation of GABA(B) receptor by R(+)-baclofen reduced ethanol and saccharin consumption, as well as ethanol-induced motor stimulation, implicating the GABA(B) receptor in the neural substrates mediating effects that sustain voluntary ethanol in take in UChB rats.

The UChA and UChB rat lines: metabolic and genetic differences influencing ethanol intake

Ethanol non-drinker (UChA) and drinker (UChB) rat lines derived from an original Wistar colony have been selectively bred at the University of Chile for over 70 generations. Two main differences between these lines are clear. (1) Drinker rats display a markedly faster acute tolerance than non-drinker rats. In F2 UChA x UChB rats (in which all genes are 'shuffled'), a high acute tolerance of the offspring predicts higher drinking than a low acute tolerance. It is further shown that high-drinker animals 'learn' to drink, starting from consumption levels that are one half of the maximum consumptions reached after 1 month of unrestricted access to 10% ethanol and water. It is likely that acquired tolerance is at the basis of the increases in ethanol consumption over time. (2) Non-drinker rats carry a previously unreported allele of aldehyde dehydrogenase-2 (Aldh2) that encodes an enzyme with a low affinity for Nicotinamide-adenine-dinuclectide (NAD+) (Aldh2(2)), while drinker rats present two Aldh2 alleles (Aldh2(1) and Aldh2(3)) with four- to fivefold higher affinities for NAD+. Further, the ALDH2 encoded by Aldh2(1) also shows a 33% higher Vmax than those encoded by Aldh2(2) and Aldh2(3). Maximal voluntary ethanol intakes are the following: UChA Aldh2(2)/Aldh2(2) = 0.3-0.6 g/kg/day; UChB Aldh2(3)/Aldh2(3) = 4.5-5.0 g/kg/day; UChB Aldh2(1)/Aldh2(1) = 7.0-7.5 g/kg/day. In F2 offspring of UChA x UChB, the Aldh2(2)/Aldh2(2) genotype predicts a 40-60% of the alcohol consumption. Studies also show that the low alcohol consumption phenotype of Aldh2(2)/Aldh2(2) animals depends on the existence of a maternally derived low-activity mitochondrial reduced form of nicotinamide-adenine-dinucleotide (NADH)-ubiquinone complex I. The latter does not influence ethanol consumption of animals exhibiting an ALDH2 with a higher affinity for NAD+. An illuminating finding is the existence of an 'acetaldehyde burst' in animals with a low capacity to oxidize acetaldehyde, being fivefold higher in UChA than in UChB animals. We propose that such a burst results from a great generation of acetaldehyde by alcohol dehydrogenase in pre-steady-state conditions that is not met by the high rate of acetaldehyde oxidation in mitochondria. The acetaldehyde burst is seen despite the lack of differences between UChA and UChB rats in acetaldehyde levels or rates of alcohol metabolism in steady state. Inferences are drawn as to how these studies might explain the protection against alcoholism seen in humans that carry the high-activity alcohol dehydrogenase but metabolize ethanol at about normal rates.

Can experimental paradigms and animal models be used to discover clinically effective medications for alcoholism?

Evaluating medications in animal laboratory paradigms can reveal whether the compound is effective in an established alcoholism model, at clinically relevant doses and exposure conditions, when administered orally (or transdermally) and without serious limiting side effects. Positive outcomes constitute a possible discovery for relevance to alcoholism and, under favorable marketing conditions, encourage further development. Medication testing using animal models of alcoholism might also guide clinical testing by discriminating clinically effective from clinically ineffective compounds. This ability rests on whether there are tests or, more reasonably, batteries of tests having this discriminative ability. The present paper examines this possibility. Effects of naltrexone and acamprosate in animal paradigms which model behavioral aspects of alcoholism are reviewed and compared with the effects of compounds which have limited effects in alcoholics. It is not clear at present whether any single paradigm or combination of paradigms differentiates clinically effective from clinically limited compounds. Steps are suggested to improve the use of preclinical laboratory tests to predict which compounds are likely to be effective medications for reducing drinking and sustaining abstinence in human alcoholics.

Systemic and intra-accumbens microinjections of naltrexone interfere with tolerance to ethanol in rats

RATIONALE

Evidence suggests a role for the opioid system in the control of ethanol reinforcement and drinking. Previous findings have shown that naltrexone, an opioid antagonist that decreases ethanol consumption in humans and experimental animals, reduces the acquisition of acute ethanol tolerance in rats. However, there are few data regarding the role of the opioid system in the acquisition of ethanol tolerance, particularly in brain areas involved in the rewarding actions of ethanol.

OBJECTIVES

This study investigates the effects of systemic and of intra-accumbens injections of naltrexone on the development of rapid tolerance to ethanol.

METHODS

Wistar rats received intraperitoneal injections of naltrexone (0.1-3.0 mg/kg) or microinjections into the core or shell portions of the nucleus accumbens (5-20 microg) before ethanol (2.7 g/kg i.p.). The animals were tested for motor coordination on the tilting plane apparatus. Tolerance was assessed 24 h later by administering the same dose of ethanol to all animals and retesting them on the tilting plane.

RESULTS

The second injection of ethanol resulted in less motor incoordination on Day 2, suggesting the development of rapid tolerance. Pretreatment with naltrexone, either i.p. (0.3 and 0.6 mg/kg) or intra-accumbens (5-20 microg), on Day 1, blocked the development of rapid tolerance to the motor-incoordinating effects of ethanol on Day 2 without affecting the motor performance of the animals on Day 1.

CONCLUSIONS

The results suggest that the opioid system may be involved in the development of ethanol tolerance, and that the nucleus accumbens may play a role in this phenomenon.

Influence of drugs acting on nitric oxide-dependent pathways on ethanol tolerance in rats

RATIONALE

Our previous studies have shown that the inhibition of nitric oxide (NO) synthesis with drugs administered either by systemic or ICV routes blocks the development of tolerance to some of the effects of ethanol.

OBJECTIVES

The aim of this study was to further investigate the role of NO-dependent pathways in tolerance to the incoordinating effect of ethanol through ICV administration of drugs that activate or interfere with NO-dependent pathways.

METHODS

Male Wistar rats were pretreated with IP ethanol (2.7 g/ kg) or saline before receiving ICV injections of the soluble guanylyl cyclase (sGC) inhibitors methylene blue (30 nmol), 6(phenylamino)-5,8-quinolinedione (LY83583, 10 nmol), 1H-(1,2,4)-oxodiazolo (4,3-a)quinoxalin-1-one (ODQ, 1 nmol), and 4H-8-bromo-1,2,4-oxadiazolo (3,4-d)benz(b)(1,4)oxazin-1-one (NS2028, 10 nmol), or the respective control solutions. The animals were tested on the tilt plane apparatus. Tolerance was assessed 24 h after the first ethanol injection, by administering ethanol to all animals and re-testing them on the tilt plane. The effects of the cyclic guanylate 3',5'-monophosphate (cGMP) analogue, 8-bromo-cGMP (40 nmol or 80 nmol) and of the NO donors S-nitroso-N-acetylpenicillamine (SNAP, 40 or 80 nmol) and sodium nitroprusside (SNP, 40 or 80 nmol) were also studied.

RESULTS

All sGC inhibitors significantly blocked rapid tolerance, whereas SNP (40 nmol) and 8-bromo-cGMP (80 nmol) increased the magnitude of ethanol tolerance (ANOVA+Tukey's test).

CONCLUSIONS

The present results suggest that activation or inhibition of NO-dependent pathways increases or blocks rapid tolerance, respectively. These results give additional support to the hypothesis that brain NO plays a role in the development of tolerance to ethanol, but it remains to be confirmed if the same basic cellular mechanisms are also applicable to tolerance to other behavioural and/or physiological effects of this drug.

Beta-phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids and compounds derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2001 to June 2002 is reviewed, with 581 references cited.

Effects of intracerebroventricular administration of 7-nitroindazole on tolerance to ethanol

Previous studies have shown that nitric oxide (NO) is involved in the development of rapid tolerance to the motor incoordination produced by ethanol. In order to further investigate this involvement, three experiments were undertaken using the tilt-plane and the hypothermia tests. The first demonstrated that 7-nitroindazole (7-NI), a preferential neuronal NO synthase (nNOS) inhibitor, injected by intracerebroventricular (i.c.v.) route, blocked the development of rapid tolerance to ethanol-induced motor incoordination. This effect was prevented by i.c.v. injection of L-arginine. The second experiment showed that D-arginine did not influence the blockade of tolerance produced by 7-NI. The third experiment revealed that i.c.v. injection of 7-NI also blocked the development of tolerance to the hypothermic effect of ethanol. These results support the hypothesis that nNOS-derived NO participates in the development of rapid tolerance to ethanol.