The 5-HT4 receptor antagonist, GR113808, reduces ethanol intake in alcohol-preferring rats

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

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.

Enhanced 5-HT(2A) receptor status in the hypothalamus and corpus striatum of ethanol-treated rats

AIM

Brain is the major target for the actions of ethanol and it can affect the brain in a variety of ways. In the present study we have investigated the changes in 5-HT level and the 5-HT(2A) receptors in the ethanol-treated rats.

METHODS

Wistar adult male rats of 180-200 g body weight were given free access to 15% (v/v) (approx.7.5 g/Kg body wt./day) ethanol for 15 days. Controls were given free access to water for 15 days. Brain 5-HT and its metabolites were assayed by high performance liquid chromatography (HPLC) integrated with an electrochemical detector (ECD) fitted with C-18-CLS-ODS reverse phase column. 5-HT(2A) receptor binding assay was done with different concentrations of [3H] MDL 100907.

RESULTS

The hypothalamic 5-HT content significantly increased (P < 0.001) with a decreased (P < 0.001) 5-HIAA/5-HT turnover in the ethanol-treated rats when compared to control. The corpus striatum 5-HT content significantly decreased (P < 0.01) with increased (P < 0.01) 5-HIAA/5- HT turnovers in the ethanol-treated rats when compared to control. Scatchard analysis of [(3)H] MDL 100907 against ketanserin in hypothalamus showed a significant increase (P < 0.001) in B(max )with a decreased affinity (P < 0.001) in ethanol-treated rats when compared to control. The competition curve for [3H] MDL 100907 against ketanserin fitted one-site model in all the groups with unity as Hill slope value. An increased K(i) and log (EC(50)) value were also observed in ethanol-treated rats when compared to control. Scatchard analysis of [3H] MDL 100907 against ketanserin in the corpus striatum of ethanol-treated rats showed a significant increase (P < 0.001) in B(max) and in affinity (P < 0.01) when compared to control. The change in affinity of the receptor protein in both corpus striatum and hypothalamus shows an altered receptor. The competition curve for [(3)H] MDL 100907 against ketanserin fitted one-site model in all the groups with unity as Hill slope value. There was no significant change in K(i) and log (EC (50)) value in ethanol-treated rats when compared to control.

CONCLUSION

The present study demonstrated the enhanced 5-HT(2A) receptor status in hypothalamus and corpus striatum. The ethanol-induced enhanced 5-HT(2A) receptors in the hypothalamus and corpus striatum has clinical significance in the better management of ethanol addiction. This will have therapeutic application.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.

Genetically selected Marchigian Sardinian alcohol-preferring (msP) rats: an animal model to study the neurobiology of alcoholism

The present article provides an up-to-date review summarizing almost 18 years of research in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. The results of this work demonstrate that msP rats have natural preference for ethanol characterized by a spontaneous binge-type of drinking that leads to pharmacologically significant blood ethanol levels. This rat line is highly vulnerable to relapse and presentation of stimuli predictive of alcohol availability or foot-shock stress can reinstate extinguished drug-seeking up to 8 months from the last alcohol experience. The msP rat is highly sensitive to stress, shows an anxious phenotype and has depressive-like symptoms that recover following ethanol drinking. Interestingly, these animals have an up-regulated corticotrophin releasing factor (CRF) receptor 1 system. Clinical studies have shown that alcoholic patients often drink ethanol in the attempt to self-medicate from negative affective states and to search for anxiety relief. We propose that msP rats represent an animal model that largely mimics the human alcoholic population that due to poor ability to engage in stress-coping strategies drink ethanol as a tension relief strategy and for self-medication purposes.

Role of the serotonergic system in the neurobiology of alcoholism: implications for treatment

Preclinical studies have contributed greatly to our understanding of the neurochemical pathways associated with the development and maintenance of alcohol-seeking behaviour. These studies have demonstrated the important role of serotonin pathways, particularly as they relate to dopaminergic function, which mediates alcohol-induced reward associated with its abuse liability. Naturally, this has led to the study of serotonergic agents as treatments for alcoholism.SSRIs do not appear to be effective treatment for a heterogeneous alcoholic group. However, they may be useful as treatment for late-onset alcoholics, or alcoholism complicated by comorbid major depression. Buspirone, a serotonin 5-HT1A partial agonist, does not appear to be an effective treatment for alcoholics without comorbid disease. Buspirone may, however, have some utility for treating alcoholics with comorbid anxiety disorder. The 5-HT2 antagonist ritanserin, at pharmacologically relevant clinical doses, does not appear to be an effective treatment for alcoholism. Ondansetron, a 5-HT3 antagonist, is an efficacious and promising medication for the treatment of early-onset alcoholism. Preliminary evidence suggests that combining the mu antagonist naltrexone with the 5-HT3 antagonist ondansetron promises to be more effective for treating alcoholism than either alone. The differential treatment effect of SSRIs and ondansetron among various subtypes of alcoholic is intriguing. Future research is needed to understand more clearly the molecular genetic differences and the interactions of such differences with the environment that typify a particular alcoholic subtype. Such an understanding could enable us to make comfortable predictions as to which alcoholic subtype might respond best to a particular serotonergic agent, which could then be provided.

5-HT3 antagonist ICS 205–930 enhances naltrexone's effects on ethanol intake

Opioid receptor antagonist naltrexone has shown some efficacy in decreasing ethanol consumption in humans. However, naltrexone treatment is not always efficacious and produces several aversive effects such as nausea, anxiety and weight loss. Serotonin-3 (5-HT3) receptor antagonists also modulate some of the behavioral effects of alcohol and may decrease alcohol consumption. We examined the effects of the combination of 5-HT3 receptor antagonist ICS 205-930 ((3-tropanyl-indole-1-carboxylate, tropisetron) and naltrexone on ethanol and food intake in Sprague-Dawley rats. Both naltrexone (0.56-10 mg/kg) and ICS 205-930 (5.6 mg/kg), when administered intraperitoneally 30 min before the scheduled 3-h access to ethanol, significantly suppressed ethanol intake. Naltrexone (1 mg/kg) when given in combination with ICS 205-930 (5.6 mg/kg) was significantly more efficacious in suppressing ethanol intake in comparison with naltrexone (1 mg/kg) administered alone. The drug combination did not affect the food intake. These data suggest that 5-HT3 receptor antagonist ICS 205-930 may be used as an effective adjunct for pharmacotherapy of alcoholism.

Immunocytochemical study of the forebrain serotonergic innervation in Sardinian alcohol-preferring rats

RATIONALE

The anxiolytic effect of ethanol is generally considered to be causally related to the development of alcohol dependence, and serotonin (5-HT) has been involved in both alcohol abuse and anxiety disorders. Several lines of evidence suggest an inverse relationship between alcohol abuse and central serotonergic neurotransmission.

OBJECTIVES

When tested in the elevated plus-maze, selectively bred Sardinian alcohol-preferring (sP) rats display a higher degree of anxiety than Sardinian alcohol-non-preferring rats (sNP); this behavior is reversed by voluntary ethanol intake. The present study examined whether sP rats differed with respect to the 5-HT innervation in different forebrain areas.

METHODS

We performed an immunohistochemistry study using an antibody raised against serotonin transporter (SERT), a marker for 5-HT fibers, coupled with an unbiased stereology, the method used to count the number of 5-HT neurons in the raphe nuclei.

RESULTS

The SERT-positive innervation density was found to be significantly lower in the medial-prefrontal cortex and in the shell of the nucleus accumbens of the ethanol-naive sP rats (sP-N) when compared with the sNP and unselected Wistar rats. No differences were found in the caudate putamen and hippocampus. The stereological analysis showed a significant difference in the number of 5-HT neurons in the dorsal but not in the median raphe of sP-N rats, compared with sNP and Wistar rats. Analysis of the cell body cross-sectional area revealed no differences among the three lines of rats either in the dorsal or in the median raphe. In sP rats that had voluntarily drunk ethanol for 14 consecutive days (sP-exp), no differences were found in the 5-HT innervation relative to sP-N animals.

CONCLUSIONS

These results indicate a selective reduction of innervation in the medial portion of the mesocorticolimbic 5-HT system in sP rats, suggesting that this genetically determined difference may be involved in the contrasting alcohol preference and consumption of sP and sNP animals.

5-HT4 receptors in the hippocampus modulate rat locomotor activity

In the present study, the ability of 5-hydroxytryptamine-4 (5-HT4) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion via microdialysis probes. The local infusion of 5-HT bilaterally into the striatum did not alter rat motor activity. The local infusion of 1.0 mM 5-HT into the bilateral hippocampus, but not lower doses, significantly increased motor activity as compared with the baseline values or the control rats. During the day hours (0700-1900, light on), the local infusion of either 5-HT4 agonist, 5-MeOT (100 microM) or mosapride (10 microM), but not in their lower concentrations, into the bilateral hippocampus significantly increased motor activity as compared with the baseline values or the control rats. Almost all increased motor activity was normal forward locomotion. This 5-MeOT-induced hyperlocomotion was completely reversed by the combined infusion of a 5-HT4 antagonist, either GR125487D (100 microM), SB204070 (100 microM) or RS23597-190 (100 microM). During the night hours (1900-0700, light off), the local infusion of either SB204070 (100 microM) or RS23597-190 (100 microM), but not in their lower concentrations, into the bilateral hippocampus significantly decreased rat motor activity and inhibited rat nocturnal hyperactivity. These hypoactivities during the night hours induced by 5-HT4 antagonist were reversed by the combined infusion of a 5-HT4 agonist, 5-MeOT (100 microM). The present study demonstrates that the serotonergic neurons projecting to the hippocampus, but not to the striatum, modulate rat locomotor activity by stimulating 5-HT4 receptors in the hippocampus.

Central 5-HT(4) receptors and dopamine-dependent motor behaviors: searching for a functional role

In this study, we evaluated the role of central 5-HT(4) receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT(4) receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT(4) antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 microl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT(4) antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT(4) receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission.

5-HT2A and 5-HT2C/5-HT1B receptors are differentially involved in alcohol preference and consummatory behavior in cAA rats

The present study aimed to investigate the role of serotonin 5-HT2A and 5-HT2C receptors in the control of alcohol preference and consummatory behavior in alcohol-preferring cAA rats. Effects of the 5-HT(2A/2C) receptor agonist, DOI, the 5-HT(2C/1B) receptor agonist, mCPP, the 5-HT(2A/2C) receptor antagonist, ritanserin, and the 5-HT2A receptor antagonist, MDL 100,907, on ethanol (EtOH, 10% v/v) preference and intake, as well as total fluid and food intake were evaluated in a 12-h limited-access two-bottle paradigm. DOI (0.3-3 mg/kg, i.p.) reduced EtOH intake and preference, but not total fluid or food intake; whereas mCPP (1-10 mg/kg, s.c.) reduced EtOH, total fluid, and food intake. Therefore, it is concluded that DOI induces a specific and selective antialcohol effect, whereas mCPP rather induces a general suppressive effect on consummatory behavior. Ritanserin (1-10 mg/kg, i.p.) did not affect EtOH intake and preference, nor total fluid and food consumption. MDL 100,907 (0.1-1 mg/kg, i.p.) induced only a small reduction of food intake at the highest dose tested. Pretreatment with ritanserin (3 mg/kg, i.p.) and MDL 100,907 (0.3 mg/kg, i.p.) blocked the effects of DOI (3 mg/kg, i.p.), but not those of mCPP (10 mg/kg, i.p.). It is suggested that activation of 5-HT2C and/or 5-HT1B receptors results in a general decrease of consummatory behavior, whereas activation of 5-HT2A receptors selectively decreases EtOH intake and preference, as assessed in the cAA rat model of alcoholism.

Serotonin-4 receptor antagonists reverse cocaine-induced cardiac arrhythmia

The effect of the 5-HT4 antagonists GR113808A and GR125487D on cocaine-induced cardiac arrhythmia was examined in the rat. Cocaine alone, given i.v. at a rate of 2 mg/kg every 5 min, produced an initial increase in blood pressure followed by a severe drop in pressure and bradycardia. Sustained ventricular fibrillation was noted after 6-12 mg/kg cocaine and quickly progressed to asystole. Pretreatment with both GR113808A and GR125487D antagonized these effects in a dose-dependent manner. When given after the onset of arrhythmia, both drugs reversed the cocaine-induced arrhythmia's. Thus, the 5-HT4 antagonists may be useful in the treatment of cocaine toxicity.

5-HT4 receptor antagonism does not affect motor and reward mechanisms in the rat

5-HT4 receptors are concentrated in areas of the brain which are rich in dopamine neuronal markers, which may suggest that they influence motor and reward processes. We tested this hypothesis by examining the effects of a 5-HT4 receptor antagonist, 8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1,4-dioxan-5-car boxylate hydrochloride (SB-204070-A) on amphetamine- and nicotine-induced locomotor stimulation in intact rats. In rats with unilateral 6-hydroxydopamine-induced lesions of the ascending nigrostriatal dopaminergic projection, SB-204070-A was tested for its effects on amphetamine-induced rotation. SB-204070-A was also tested for its effects on rewarded behaviour maintained by intracranial self-stimulation. SB-204070-A did not alter behaviour under any of these conditions, suggesting a lack of involvement of the 5-HT4 receptor in motor and reward processes.

Central 5-HT3 receptors in P and in AA alcohol-preferring rats: An autoradiographic study

Considerable evidence exists for an involvement of serotonergic mechanisms in the control of alcohol consumption. In the present study, an extensive 5-hydroxytryptamine (5-HT3) receptor autoradiographical investigation was performed using two genetically selected rat strains, alcohol preferring (P) and Alko alcohol (AA) alcohol-preferring rats, as well as the corresponding alcohol nonpreferring (NP) and Alko nonalcohol (ANA) alcohol-nonpreferring rats. The aim was to determine if there are any differences in 5-HT3 binding levels that may illuminate mechanisms of alcohol preference in these animals. For quantitating 5-HT3 binding sites, [3H]S(-)zacopride (0.5 nM) was used. Non-specific binding was measured in the presence of granisetron 10(-6) M. The [3H]S(-)zacopride binding density was measured in two subregions of the amygdaloid nucleus, frontal cortex, piriform cortex, cingulate laminae, parietal anterior cortex, parietal medial cortex, hippocampus CA1, hippocampus CA3, and entorhinal cortex. In all the brain areas investigated, the results showed no differences between AA and ANA rats. In P rats, compared to NP controls, there was a 30% lower 5-HT3 binding level in the lateral nucleus and the posteromedial cortical nucleus of the amygdala. These findings suggest that the expression of high alcohol preference in genetically selected P and AA rats is not associated with a general alteration of central 5-HT3 receptors, although a lower 5-HT3 receptor level in the amygdala of P rats may contribute to the phenotype of this strain of animals.

Acute and chronic actions of ethanol on CA1 hippocampal responses to serotonin

The effects of acute or chronic ethanol on serotonin (5-HT)-induced membrane hyperpolarization and inhibition of the slow Ca2(+)-dependent after hyperpolarization (sAHP) were recorded in rat CA1 pyramidal neurons in hippocampal slices using sharp intracellular electrodes. 5-HT (1-100 microM) caused concentration-dependent hyperpolarization of the membrane that was not altered by simultaneous 30 mM ethanol treatment, but blunted by 10 microM buspirone, a weak 5-HT1A agonist. 5-HT (1-30 microM) also partially inhibited (approximately 40%) the sAHP following a burst of five or more action potentials. Initially ethanol (30 mM) alone did not alter the sAHP, but over a period of 38 min, a slow increase in amplitude (approximately 40%) was observed. 5-HT-mediated inhibition of the sAHP was significantly greater with ethanol present, regardless of the length of exposure. Pyramidal neurons in hippocampal slices prepared from ethanol-dependent animals showed no obvious signs of withdrawal related hyperexcitability and neither concentration-dependent membrane hyperpolarization nor sAHP inhibition caused by 5-HT were significantly changed from responses in controls. These results suggest that hyperpolarizing responses to 5-HT in hippocampal CA1 pyramidal neurons are functionally resistant to acute or chronic ethanol treatment. 5-HT-mediated inhibition of the sAHP is enhanced by ethanol acutely, but does not show an adaptive change as a result of ethanol dependence.

Pharmacotherapies for alcohol problems: a review of research with focus on developments since 1991

Research on medications to treat alcohol problems has flourished in the last 5 years. Whereas before this time most projects focused on withdrawal agents, at least equal interest has now extended to drugs that may directly reduce urge to drink. The most promising medications in this regard are the opiate antagonists and acamprosate. Considerable attention has also been devoted to serotonergic agents. As aids to detoxification, pharmacologic agents that affect the multiple neural systems disrupted by acute alcohol withdrawal remain under active investigation. Significant progress is also being made in identifying medications to assist alcoholics suffering collateral psychopathology, especially depression and anxiety based disorders. Unfortunately, fewer gains have been realized in the development of medications to assist patients simultaneously dependent on both alcohol and illicit drugs. Also, research to develop amethystic agents remains in its very early stages.