Comparison of effects of a range of 5-HT receptor modulators on consumption and preference for a sweetened ethanol solution in rats

Serotonin2C receptors in the nucleus accumbens are involved in enhanced alcohol-drinking behavior

Dopamine and serotonin (5-HT) in the nucleus accumbens (ACC) and ventral tegmental area of the mesoaccumbens reward pathways have been implicated in the mechanisms underlying development of alcohol dependence. We used a C57BL/6J mouse model with increased voluntary alcohol-drinking behavior by exposing the mice to alcohol vapor for 20 consecutive days. In the alcohol-exposed mice, the expression of 5-HT(2C) receptor mRNA increased in the ACC, caudate nucleus and putamen, dorsal raphe nucleus (DRN), hippocampus and lateral hypothalamus, while the protein level of 5-HT(2C) receptor significantly increased in the ACC. The expression of 5-HT(7) receptor mRNA increased in the ACC and DRN. Contents of 5-HT decreased in the ACC shell (ACC(S) ) and DRN of the alcohol-exposed mice. The basal extracellular releases of dopamine (DA) and 5-HT in the ACC(S) increased more in the alcohol-exposed mice than in alcohol-naïve mice. The magnitude of the alcohol-induced ACC(S) DA and 5-HT release in the alcohol-exposed mice was increased compared with the control mice. Intraperitoneal (i.p.) administration or local injection into ACC(S) of the 5-HT(2C) receptor antagonist, SB-242084, suppressed voluntary alcohol-drinking behavior in the alcohol-exposed mice. But the i.p. administration of the 5-HT(7) receptor antagonist, SB-258719, did not have significant effects on alcohol-drinking behavior in the alcohol-exposed mice. The effects of the 5-HT(2C) receptor antagonist were not observed in the air-exposed control mice. These results suggest that adaptations of the 5-HT system, especially the upregulation of 5-HT(2C) receptors in the ACC(S) , are involved in the development of enhanced voluntary alcohol-drinking behavior.

Effects of fenfluramine, 8-OH-DPAT, and tryptophan-enriched diet on the high-ethanol intake by rats bred for susceptibility to stress

The swim-test susceptible (SUS) line of rats has been bred in our laboratory for the characteristic of reduced motor activity in the swim test following exposure to an acute stressor. Testing of multiple generations of SUS rats has also revealed that they consume large amounts of ethanol voluntarily. As reported for lines of rats that show a propensity for high-ethanol intake, the SUS rats show evidence of low serotonergic function. Because serotonergic function has often been shown to be involved in the regulation of alcohol consumption, here we examined the effects of manipulations of serotonin transmission on intake of ethanol by SUS rats. Fenfluramine, a serotonin-releasing drug, was injected at various doses (0.625, 1.25, 2.5, and 5.0mg/kg) twice per day and ethanol intake was measured using a two-bottle free-choice method. The 8-OH-DPAT, a 5‑HT(1A) agonist, was injected at various doses (0.03125, 0.0625, 0.125, 0.25, 0.5, and 1.0mg/kg) before a 1-h session of exposure to ethanol (single-bottle test, water available the other 23h per day). A diet enriched with 3% tryptophan (TRP), the amino acid precursor for serotonin synthesis, was administered in a restricted feeding schedule (5h per day) with ethanol intake measured the last 4h. Fenfluramine decreased ethanol intake at all doses tested. The 8-OH-DPAT increased ethanol intake at lower doses, presumably acting at autoreceptors, which inhibit serotonergic neurons, and decreased intake at higher doses, presumably acting at postsynaptic 5-HT(1A) receptors. TRP-enriched diet also significantly decreased ethanol intake. Food and water intake were less or unaffected by these three manipulations. With all three manipulations, ethanol intake remained suppressed one or more days after the day of tests that decreased ethanol intake. These data suggest that SUS rats, like many other lines/strains of rodents that consume large amounts of alcohol, show an inverse relationship between serotonin transmission and voluntary intake of ethanol.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

5-HT(1B) receptor inhibition of alcohol-heightened aggression in mice: comparison to drinking and running

RATIONALE

Serotonin 5-HT(1B) receptors are promising targets for the management of several mood and impulse disorders.

OBJECTIVE

These experiments examine a 5-HT(1B) agonist, CP-94,253, and attempt to distinguish between its effects on seeking to perform three rewarding behaviors: aggression, drinking, and wheel running.

MATERIALS AND METHODS

Male CFW mice perform nose-poke responses that are maintained by a fixed interval schedules of 10-min (FI10) schedule to gain access to one of three rewarding activities. The first experiment studies mice reinforced by the opportunity to confront an intruder mouse after drinking water or alcohol; the second studies mice reinforced by the presentation of alcoholic or non-alcoholic solutions (i.e., 6% ethanol, 0.05% saccharin vs 0.05% saccharin); the third studies mice reinforced by access to a running wheel.

RESULTS

CP-94,253 (1.0-10 mg/kg i.p.) dose-dependently reduces aggression, drinking, and wheel running. Of these behaviors, alcohol-heightened aggression is the most sensitive to the 5-HT(1B) receptor agonist (ED50 = 4.8 mg/kg). Responding for the opportunity to drink or engage in alcohol-heightened aggression is suppressed by the highest dose of CP-94,253, whereas CP-94,253 does not affect responding that is reinforced by wheel running or species-typical aggression.

CONCLUSIONS

These results confirm the inhibitory effects of 5-HT(1B) receptor stimulation on aggressive performance and drinking. They also reveal an inhibition of voluntary wheel running, contrary to the stimulation of running in a novel, open arena. 5-HT(1B) receptor agonists may be particularly useful for the treatment of aggressive behavioral disorders, but their efficacy and potency appear to be sensitive to the intensity and context of the behavior.

Effects of Atypical Anxiolytic N-Phenyl-2-[1-[3-(2-Pyridinylethynyl)Benzoyl]-4-Piperidine]Acetamide (JNJ-5234801) on Alcohol Intake in Alcohol-Preferring P Rats

BACKGROUND

N-Phenyl-2-[1-[3-(2-pyridinylethynyl)benzoyl]-4-piperidine]acetamide (JNJ-5234801) is a structurally novel atypical anxiolytic with an overall in vivo profile in animals suggestive of the potential to show anxiolytic efficacy in humans at doses that will not cause CNS-related side effects. Furthermore, unlike the benzodiazepines, JNJ-5234801 does not have an adverse interaction with ethanol even at doses 20 to 40 times the minimal effective dose in the rat elevated plus maze (MED=1.0 mg/kg, p.o.).

METHODS

In the present study, JNJ-5234801 was evaluated for potential efficacy in reducing alcohol intake in alcohol-preferring rats. Alcohol-preferring P rats were allowed to drink water or alcohol (10%, v/v) in a 2-bottle choice procedure. Once stable baselines were established, the acute effects of JNJ-5234801 [(10-40 mg/kg, intraperitoneally (i.p.)] were assessed. In a separate study, chronic treatment with JNJ-5234801 (40 mg/kg once daily, i.p.) for 12 consecutive days was compared with naltrexone (20 mg/kg, twice daily, i.p.).

RESULTS

There was a selective dose-dependent reduction in alcohol intake in the alcohol-preferring (P) rats after acute administration of JNJ-5234801 (10-40 mg/kg, i.p.). There were no significant effects on food or water intake. When administered subchronically, both JNJ-5234801 (40 mg/kg once daily, i.p.) and naltrexone (20 mg/kg, twice daily, i.p.) considerably reduced alcohol intake, but tolerance to the alcohol-suppressing effects appeared to occur sooner in the naltrexone-treated group. While both compounds slightly but significantly reduced food intake at the beginning, only JNJ-5234801 increased water intake and decreased alcohol preference.

CONCLUSIONS

The novel atypical anxiolytic JNJ-5234801 has a favorable profile effects on alcohol intake and related measures compared with naltrexone, which is recommended for the treatment of alcoholism.

Manipulations of serotonin function in the nucleus accumbens core produce differential effects on ethanol and sucrose seeking and intake

OBJECTIVE

Behaviorally relevant stimuli, including alcohol, are processed through the nucleus accumbens/ventral tegmental area (VTA)/prefrontal cortex circuit. It is hypothesized that serotonin affects ethanol-directed behaviors by interacting with this system via projections from the dorsal raphe to the nucleus accumbens and VTA. The current studies utilized two different operant paradigms, one focusing on reinforcer seeking and one focusing on reinforcer self-administration (both with an ethanol and a sucrose solution as the reinforcer) to elucidate serotonin-specific regulation of these behaviors.

METHODS

The present experiments assessed the effects of microinjections of a serotonin1B agonist (CGS12066B) and a serotonin1A agonist (8-OH-DPAT) in the nucleus accumbens core on ethanol- and sucrose-reinforced seeking and intake. In four separate experiments, male Long-Evans rats were trained to complete a single response requirement that resulted in access to 10% ethanol or 2% sucrose for a 20-min drinking period.

RESULTS

Before microinjections, ethanol-reinforced subjects were consuming an average of 0.5-0.95 g/kg ethanol and making 50-100 responses during intermittent nonreinforced sham (no drug) sessions (sucrose groups had similar baseline response levels). In summary, findings from the four experiments showed the following: (1) manipulations of serotonin function that had effects on ethanol-reinforced responding had either no effect or less pronounced effects on sucrose-reinforced responding; (2) administration of the serotonin1B agonist decreased seeking behaviors to a greater degree than drinking behaviors; and (3) administration of the serotonin1A agonist decreased ethanol intake but not seeking with no impact at all on sucrose-reinforced behaviors.

CONCLUSIONS

Manipulations of serotonin activity in the nucleus accumbens core had little effect on sucrose-reinforced behaviors and differential effects on ethanol seeking versus intake, suggesting that this area may play a complex but selective role in the stimulus processing of external and internal alcohol-associated cues.

Effects of fluvoxamine on a multiple schedule of ethanol- and food-maintained behavior in two rat strains

RATIONALE

Previous studies show that selective serotonin reuptake inhibitors (SSRIs), including fluvoxamine, have a greater effect on ethanol-maintained responding compared with an alternative reinforcer. However, none of these studies matched baseline responding for reinforcers. Because behavioral effects of many drugs depend on the baseline response rate, the preferential effects of SSRIs may be due to different baseline response rates.

OBJECTIVES

Fluvoxamine effects on ethanol- and food-maintained responding were compared using a multiple schedule of behavior, providing matched baseline responding and allowing within-subject analysis in two strains of rats.

METHODS

The multiple schedule consisted of three consecutive 5-min, fixed-ratio five components (Food1, Ethanol, Food2). Fluvoxamine (3-30 mg/kg, i.p.) was administered 30 min presession. In Lewis rats, fluvoxamine effects were determined at several available ethanol concentrations [8, 16, 32, and 8% (w/v) redetermination]. In Sprague-Dawley rats, fluvoxamine effects were determined when the available ethanol concentration was 8% (w/v).

RESULTS

Baseline responding was stable and well matched under all conditions except 32% ethanol, when responding for ethanol was lower than for food. After the administration of 17.8 mg/kg fluvoxamine, ethanol-maintained responding was 15-33% lower than food-maintained responding in four of the five conditions tested. Breath ethanol assessments indicated that rats had blood ethanol levels of 33 mg/dl following responding for 8% ethanol.

CONCLUSIONS

These results are in agreement with previous findings that SSRIs preferentially reduce ethanol-maintained responding and suggest this is not likely due to different baseline levels of responding between the comparison conditions. Further, these results support the hypothesis that alteration of synaptic serotonin can modulate ethanol reinforcement.

Granisetron attenuates exercise-induced conditioned taste aversion in the rat

A conditioned taste aversion (CTA) paradigm was used in the present study to investigate whether CTA produced by exercise could be attenuated by the 5-HT(3) receptor antagonist granisetron. Male Sprague-Dawley rats were randomly allocated to one of four groups (Ns=6) and were exposed to salty (0.128 M sodium chloride) or sour (0.00138M citric acid) solutions. Subjects were injected with either saline solution (1.0 ml, 0.9%) or granisetron (0.5mg/kg, IP) and were exposed to 30 min of forced wheel running exercise (70 revolutions/30min) 10 min after injection. Exercise induced CTA to both the salty (3.7 ml intake) and sour-flaroured (3.1ml intake) solutions as compared with no exercise (intake 14.0 and 13.7 ml), and administration of granisetron significantly attenuated the exercise-induced CTA to the salty- and sour-flavoured solutions.

5-HT(3A) receptor subunit is required for 5-HT3 antagonist-induced reductions in alcohol drinking

The ionotropic serotonin subtype-3 (5-HT3) receptor has emerged as a potential therapeutic target in the treatment of alcohol abuse and alcoholism because selective pharmacological antagonists reduce alcohol consumption in preclinical and clinical models. 5-HT binds to the extracellular N-terminus of the 5-HT(3A) receptor subunit but receptor activation is also enhanced by distinct allosteric sites, which indicates the presence of other receptor subunits. It is not known if specific molecular subunits of the 5-HT3 receptor modulate alcohol drinking. To address this issue, we characterized acute locomotor response to alcohol and alcohol consumption in a two-bottle home-cage procedure by congenic C57BL/6J mice with a targeted deletion of the 5-HT(3A) receptor subunit gene. 5-HT(3A)-null mice did not differ from wild-type littermate controls on measures of spontaneous locomotor activity, habituation to a novel environment, or locomotor response to ethanol (0, 0.5, 1, or 2 g/kg). Moreover, null mice did not differ from controls on measures of ethanol (2-10%) intake and preference during or after a two-bottle home-cage sucrose fading procedure. Systemic administration of the 5-HT3 antagonist LY-278,584 (0-10 mg/kg) decreased intake of both sweetened (2% sucrose+10% ethanol) and unsweetened (10% ethanol) ethanol in wild-type mice only. These findings indicate that reduction of alcohol drinking produced by 5-HT3 antagonism is dependent on the presence of 5-HT(3A)-containing receptors.

The 5-HT3 antagonist Y-25130 blocks cocaine-induced lowering of ICSS reward thresholds in the rat

Serotonin-3 (5-HT(3)) receptor antagonists have been shown to attenuate drug-induced increases in mesolimbic dopamine (DA), locomotor activation, and drug self-administration. In the present study, we tested whether the selective 5-HT(3) antagonist Y-25130 would attenuate cocaine-induced lowering of intracranial self-stimulation (ICSS) reward thresholds. Rats (n=6) were surgically prepared with bipolar stimulation electrodes and trained to self-administer electrical stimulation delivered to the medial forebrain bundle-lateral hypothalamus (MFB-LH). A discrete-trial, rate-free threshold determination procedure was used to detect pharmacologically induced changes from baseline reward thresholds. Four doses of Y-25130 (0.0, 0.03, 0.3, and 3.0 mg/kg ip) were given alone and in combination with cocaine (4.0 mg/kg ip). Y-25130 did not significantly alter reward thresholds or response latencies when given alone as compared to baseline measures. While there were no significant effects at lower doses, the middle and highest doses of Y-25130 (0.3 and 3.0 mg/kg) did attenuate the threshold-lowering effect of cocaine. These findings suggest that the rewarding effects of cocaine are mediated through 5-HT(3) receptor activity.

Reappraisal of the serotonin 5-HT(1B) receptor gene in alcoholism: of mice and men

Because pharmacological and genetic data supported the idea that serotonin receptors of the 5-HT(1B) type can play a modulatory role in alcohol consumption in both human and rodents, the 5-HT(1B) receptor gene is considered as a candidate gene for alcohol dependence. However, contradictory results have been reported as a positive association between alcohol dependence, and either the 861C or the 861G allele of the G861C polymorphism of the 5-HT(1B) receptor gene can be found in the literature. Further investigations in a population of 136 male alcoholics compared with 72 male control subjects demonstrated that none of these alleles was actually associated with alcohol dependence. In addition, in contrast with previous results of the literature, ethanol intake under free choice conditions (i.e., ethanol solution vs. water) was found to be similar in 5-HT(1B)-/- knock mice and paired wild-type controls. The 5-HT(1B) receptor gene may thus not be a key component in the genetic background underlying alcohol dependence in human and alcohol preference in rodents, although these results should be considered as preliminary according to the small size of our sample.

A multiple schedule model of limited access drinking in the cynomolgus macaque

The present study reports on the development of a multiple schedule procedure of oral ethanol self-administration in cynomolgus macaques. Six adult cynomolgus macaques (four female, two male) were trained to self-administer ethanol and water under a 60 min, four-component multiple schedule of ethanol and water access with 1 g food pellets presented every 900 s (fixed-time 900 s). Water was available for the first and third 15 min components, ethanol in the second and fourth components. Total ethanol dose was stable at between 1-1.25 g/kg at ethanol concentration of 4%, 6% and 8%. Subsequently water was replaced with a sweetened drink (sugar-free Tang powder, General Foods). Ethanol and Tang were self-administered in similar volumes and both served as reinforcers compared with water. Acute pretreatment with 0.25 to 1.5 g/kg of intragastrically gavaged (i.g.) ethanol failed to alter ethanol or Tang self-administration significantly despite producing mean blood ethanol levels of up to 199 mg/dl when combined with self-administered ethanol. However, 1.0 g/kg i.g. ethanol administered for 15 consecutive days significantly decreased ethanol self-administration by 23%. The results suggest that ethanol self-administration under a multiple schedule is insensitive to alteration by acute ethanol pretreatment, but can be decreased by more prolonged chronic ethanol pretreatment.

Male and female 5-HT(1B) receptor knockout mice have higher body weights than wildtypes

5-HT(1B) receptors have a regulatory role in serotonergic activity and influence feeding behavior and body weight. Because the absence of 5-HT(1B) receptors may cause changes in this regulation, body weight was measured in male and female 5-HT(1B) receptor knockout (5-HT(1B) KO) and wildtype (WT) mice from weaning until the age of 30 weeks. In both genders, 5-HT(1B) KO mice had a higher body weight than WT mice (17% and 9%, respectively). Body weight was significantly higher for males over the entire period and for females from Week 18 onwards. Absolute food and water consumption were related to body weight. However, relative to body weight, males consumed more than females. 5-HT(1B) KO males drank strikingly more water. Housing mice singly reduced food and water intake in males, but not in females. Plasma leptin levels and most organ weights did not differ between genotypes, indicating that higher body weight in 5-HT(1B) KO mice is not related to obesity. Relative to body weight, brains and adrenals were larger in females, while heart and liver were smaller. Kidneys were smaller in females, but larger in 5-HT(1B) KO mice, while lungs showed opposite effects. Spleen and testes were smaller in 5-HT(1B) KO mice. Although 5-HT(1B) KO males are more aggressive, testosterone levels were not different from WT mice. Basal corticosterone levels were similar in all groups and increased in response to mild stress, particularly in females. Lifelong absence of 5-HT(1B) receptors in mice resulted in clear phenotypic differences in body weights and food and water intake. Lacking this receptor increases body growth, without signs of obesity. A potential genetic background effect influencing this phenotype is discussed.