Analysis of the 5-HT1C receptor and the serotonin uptake site in fawn-hooded rat brain

Serotonergic Neuroplasticity in Alcohol Addiction

Alcohol addiction is a debilitating disorder producing maladaptive changes in the brain, leading drinkers to become more sensitive to stress and anxiety. These changes are key factors contributing to alcohol craving and maintaining a persistent vulnerability to relapse.

Serotonin (5-Hydroxytryptamine, 5-HT) is a monoamine neurotransmitter widely expressed in the central nervous system where it plays an important role in the regulation of mood. The serotonin system has been extensively implicated in the regulation of stress and anxiety, as well as the reinforcing properties of all of the major classes of drugs of abuse, including alcohol. Dysregulation within the 5-HT system has been postulated to underlie the negative mood states associated with alcohol use disorders.

This review will describe the serotonergic (5-HTergic) neuroplastic changes observed in animal models throughout the alcohol addiction cycle, from prenatal to adulthood exposure. The first section will focus on alcohol-induced 5-HTergic neuroadaptations in offspring prenatally exposed to alcohol and the consequences on the regulation of stress/anxiety. The second section will compare alterations in 5-HT signalling induced by acute or chronic alcohol exposure during adulthood and following alcohol withdrawal, highlighting the impact on the regulation of stress/anxiety signalling pathways. The third section will outline 5-HTergic neuroadaptations observed in various genetically-selected ethanol preferring rat lines. Finally, we will discuss the pharmacological manipulation of the 5-HTergic system on ethanol- and anxiety/stress-related behaviours demonstrated by clinical trials, with an emphasis on current and potential treatments.

Depressive-like behavior and high alcohol drinking co-occur in the FH/WJD rat but appear to be under independent genetic control

This review will consider the evidence supporting the view that a specific substrain of Fawn-Hooded rat (FH/Wjd) exhibits co-occurring depressive-like behavior and high alcohol intake independently. First, the FH/Wjd rat is compared with other Fawn-Hooded substrains (FH/Har, FHH/Eur, FHL/Eur) and it is concluded that only the FH/Wjd rat is both highly immobile in the forced swim test and drinks substantial amounts of 5-10% alcohol voluntarily. Next it is demonstrated that the FH/Wjd rat fulfils many of the criteria proposed for an animal model of alcoholism (becomes tolerant, becomes dependent and expresses withdrawal symptoms, bar-presses for alcohol). Other literature in addition to the high swim test immobility suggests that the FH/Wjd rat may also be an animal model of depression (high basal corticosterone levels, blunted hormonal responses to serotonergic agonists). To study the phenotypes more closely an inbred strain (ACI/N) of rat that drank little alcohol voluntarily and exhibited considerable swimming in the forced swim test (i.e., low immobility) was obtained. A systematic intercrossing of the parental strains and the resulting F1 progeny was carried out to generate more than 800 F2s. Swim test immobility, alcohol intake and preference and saccharin intake are four of the 7 variables assessed in each of these rats. Using classical quantitative genetics methods, it was determined that these four phenotypes exhibited modest heritability and were influenced by multiple genes. Correlation coefficients between immobility and the other measures were near zero, whereas alcohol intake and preference were highly correlated (r=0.9) and alcohol and saccharin intakes were modestly correlated (r=0.3). A final study showed that chronic fluoxetine treatment counteracted the high immobility but did not affect alcohol intake, similar to human studies. These findings suggest that although depressive-like behavior and high alcohol intake co-occur in the FH/Wjd rat, they are independently regulated.

Modulation of high alcohol drinking in the inbred Fawn-Hooded (FH/Wjd) rat strain: implications for treatment

The Fawn-Hooded rat (FH/Wjd) is an inbred alcohol-preferring rat strain, unlike most of the other strains that were selectively bred for high alcohol intake and preference. It was chosen for study some 16 years ago because of a reported mutation that disrupted platelet serotonin function. Although the FH/Wjd rat has high alcohol intake (>5 g/kg/day) and preference (>65%), interbreeding with an alcohol-non-preferring inbred strain suggested that these measures are unrelated to the serotonin abnormality. Similarly, the exaggerated immobility of the FH/Wjd rats in the forced swim test did not correlate with the high alcohol intake. Many compounds have been tested in the FH/Wjd rats after both acute and chronic treatment and a substantial number of them have proved effective. However, as the case with opiate antagonists, tolerance to the effects of the drug can develop. An up-regulation of opioid receptors accompanied the chronic treatment and this mechanism may account for the development of tolerance. Tolerance to opiate antagonists has also been demonstrated in two of the selectively bred alcohol-preferring rat lines, but it is unknown whether this process may contribute to the relapses seen in individuals being treated with naltrexone. Other drugs that reliably decrease alcohol intake in the FH/Wjd rats include the 5-hydroxytryptamine-2A receptor antagonist, amperozide, the mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and herbal derivatives such as ibogaine, St. John's wort and kudzu extract. Thus, studies in the FH/Wjd rat have led to the discovery of a wide variety of targets for the development of novel agents to treat alcoholism. The fact that several of these drugs were shown to reduce alcohol intake in some of the selectively bred alcohol-preferring rat lines and/or alcohol-preferring vervet monkeys increases our confidence that they are good candidates for further development.

Cytokines, stressors, and clinical depression: augmented adaptation responses underlie depression pathogenesis

By influencing the central nervous system, cytokines, which regulate immune function innately and adaptively, may play a key role in mediating depression-like neuro-behavioral changes. However, the similarity between cytokine and stressor-effects in animal models raises a question about the degree to which behavioral and neurochemical outcomes of cytokine challenge represent depressive disorder per se. The present review attempts to illustrate the degree of overlap between cytokines and stressors with respect to their effects on neurochemistry and behavior in animal models. The review also shows how short-term effects of cytokine exposure in typical animals may be discerned from characteristics that might otherwise be described as depression-like. By comparing outcomes of immune challenge in typical rodent strains (e.g., Sprague-Dawley [SD], Wistar) and an accepted animal model of depression (e.g., Fawn Hooded [FH] rodent strain), differences between short-term effects of cytokines and depression-like characteristics in rodents are demonstrated. Additionally, because it is known that preexisting vulnerability to depression may affect outcomes of immune challenge, we further compare immunological, biochemical and behavioral effects of cytokines between SD and FH rodent strains. Interestingly, the acute neurochemical and behavioral effects of the cytokine interleukin 1alpha (IL-1alpha) reveal stressor-like responses during behavioral habituation in both strains, though this appears to a stronger degree in FH animals. Further, the subacute response to IL-1alpha vastly differed between strains, indicating differences in adaptive mechanisms. Thus, stressor-like effects of immune challenge, particularly in FH animals, provide validation for recent "cross-sensitization" models of depression pathogenesis that incorporate immune factors.

Functional interactions between 5-hydroxytryptamine receptors and the serotonin transporter in pulmonary arteries

Pulmonary arterial 5-hydroxytryptamine (serotonin) (5-HT) transporter (SERT)-, 5-HT receptor expression, and 5-HT-induced vasoconstriction can be increased in pulmonary hypertension. These variables were studied in normoxic and hypoxic Fawn-Hooded (FH) and Sprague-Dawley (SD) rats. Furthermore, we compared the functional effects of SERT inhibitors and 5-HT receptor antagonists against 5-HT-induced vasoconstriction of pulmonary arteries. SERT and 5-HT(1B) expression was greater in FH rat lungs than in SD rats, as was 5-HT-mediated vasoconstriction. The 5-HT(2A) receptor antagonist ketanserin and the 5-HT(1B) receptor antagonist SB224289 (1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro-spiro-[furo] 2, 3-f]indole-3,4'-piperidine]) inhibited responses to 5-HT in all vessels. The combined 5-HT(1B) receptor/SERT antagonist LY393558 (1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-1(2H)-pyridinyl]ethyl]-3-isopropyl-6-(methylsulfonyl)-3,4-dihydro-1H-2,1,3-benzothiadiazine-2,2-dioxide) was the most potent inhibitor of constriction in all vessels. SERT inhibitors citalopram and fluoxetine inhibited responses to 5-HT in SD vessels. However, these inhibitors potentiated responses to 5-HT in FH vessels. After exposure of rats to 2 weeks of hypoxia, there was increased 5-HT-mediated vasoconstriction and a profound decrease in SERT expression in both the FH and SD rat lung. Accordingly, citalopram had no effect on 5-HT-induced constriction in SD rat vessels and markedly less effect in FH rat vessels. Ketanserin, SB224289, and LY393558 inhibited responses to 5-HT in all hypoxic rat vessels. LY393558 was the most potent antagonist, and there was synergy between the effects of fluoxetine and SB224289 when given simultaneously. The results suggest that, in FH rats, SERT inhibitors may increase pulmonary vasoconstriction, but this can be inhibited by simultaneous 5-HT(1B) receptor antagonism. There is synergy between the inhibitory effects of 5-HT(1B) receptor antagonists and SERT inhibitors on 5-HT-induced pulmonary vasoconstriction.

Sex-dependent modulation of ethanol consumption in vesicular monoamine transporter 2 (VMAT2) and dopamine transporter (DAT) knockout mice

Several lines of evidence suggest that monoaminergic systems, especially dopaminergic and serotoninergic systems, modulate ethanol consumption. Humans display significant differences in expression of the vesicular and plasma membrane monoamine transporters important for monoaminergic functions, including the vesicular monoamine transporter (VMAT2, SLC18A2) and dopamine transporter (DAT, SLC6A3). In addition, many ethanol effects differ by sex in both humans and animal models. Therefore, ethanol consumption and preference were compared in male and female wild-type mice, and knockout (KO) mice with deletions of genes for DAT and VMAT2. Voluntary ethanol (2-32% v/v) and water consumption were compared in two-bottle preference tests in wild-type (+/+) vs heterozygous VMAT2 KO mice (+/-) and in wild-type (+/+) vs heterozygous (+/-) or homozygous (-/-) DAT KO mice. Deletions of either the DAT or VMAT2 genes increased ethanol consumption in male KO mice, although these effects were highly dependent on ethanol concentration, while female DAT KO mice had higher ethanol preferences. Thus, lifetime reductions in the expression of either DAT or VMAT2 increase ethanol consumption, dependent on sex.

Neuropeptide Y (NPY) and depression: from animal studies to the human condition

Neuropeptide Y (NPY) is widely distributed throughout the central nervous system (CNS) and is one of the most conserved peptides in evolution, suggesting an important role in the regulation of basic physiological functions. In addition, both pre-clinical and clinical evidence have suggested that NPY, together with its receptors, may have a direct implication in several psychiatric disorders, including depression and related illnesses. NPY-like immunoreactivity and NPY receptors are expressed throughout the brain, with varying concentrations being found throughout the limbic system. Such brain structures have been repeatedly implicated in the modulation of emotional processing, as well as in the pathogenesis of depressive disorders. This review will concentrate on the distribution of NPY, its receptors, and the putative role played by this peptide in depressive illness based on both pre-clinical and clinical evidence.

The Fawn-Hooded (FH/Wjd) rat: a genetic animal model of comorbid depression and alcoholism

The Fawn-Hooded (FH/Wjd) rat is an inbred strain of rat that has been reported to exhibit both high immobility in the forced swim test and high voluntary ethanol intake, measures that have been periodically linked with depression and alcoholism in humans. The present paper will first present a survey of the literature and previously unpublished findings that bear on the question of whether FH/Wjd rats should be considered genetic animal models of depression and alcoholism. Subsequently, behavioral studies of the FH/Wjd rats, the non-drinking ACI/N strain, and their F1 and F2 intercrosses will be described. Under free choice conditions, the FH/Wjd rat drinks up to 6 g/kg 10% ethanol per day. This intake was sufficient to render the rats tolerant to the hypothermic effects of injected ethanol (2.5 g/kg). Rats that had been voluntarily drinking for at least 6 weeks also exhibited withdrawal-induced anxiety in the social interaction, elevated plus maze, and ultrasonic vocalization tasks. The FH/Wjd rat exhibits a 25-30% increase in alcohol intake when the alcohol is returned after a 24-h period of deprivation. It responds to drugs that are effective in humans with a reduction in alcohol intake. Therefore, the FH/Wjd rat meets most of the criteria for an animal model of alcoholism. Chronic antidepressant treatments correct several of the abnormalities exhibited by the FH/Wjd rats, including the exaggerated immobility in the forced swim test. Therefore, the FH/Wjd rats also fulfill some of the criteria for an animal model of depression. On the contrary, inbred ACI/N rats do not drink much alcohol voluntarily and are quite active in the forced swim test. The FH/Wjd and ACI/N rats were intercrossed to obtain the F1 and F2 progenies, which were then tested for alcohol intake and immobility. Alcohol intake and immobility were distributed in different patterns in the F1 and F2 progenies. Alcohol intake was intermediate in the F1 progeny, while immobility was closer to the FH/Wjd parents. In the F2 progeny, chi-square analyses indicated that the distributions were significantly different. In addition, there were no significant litter effects, indicating that maternal effects did not appear to occur. There were also no significant differences among rats with different coat colors, suggesting that the Fawn-Hooded phenotype can be separated from the measures of alcohol intake and immobility. We conclude that the FH/Wjd rat is a genetic animal model of depression and alcoholism, but that the two measures reflective of these states are under separate genetic controls.

A comparison of N,N-dimethyltryptamine, harmaline, and selected congeners in rats trained with LSD as a discriminative stimulus

1. A series of N-substituted tryptamines was compared with a series of beta-carbolines in rats trained to discriminate LSD (0.1 mg/kg) from saline. 2. Intermediate levels of substitution were elicited by MDMT (76.4%), DMT (77.9%), and DET (48.7%). 6-F-DET produced 41.3% LSD-appropriate responding at a dose of 6.0 mg/kg but only 4 of 8 subjects completed the test session thus precluding statistical analysis. Bufotenine (25.8%) also failed to substitute. Although none of the tryptamines substituted completely for LSD, the pattern of substitution is consonant with what is known of their activity in humans. MDMT, DMT, and DET are well established in the literature as hallucinogens but the same cannot be said for 6-F-DET and bufotenine. 3. Of the beta-carbolines tested, none substituted for LSD completely and only harmane elicited intermediate substitution (49.5%). No significant generalization of the LSD stimulus to 6-methoxyharmalan, harmaline, or THBC was observed. Thus, in contrast to the tryptamines, scant ability to substitute for LSD was observed in the beta-carbolines tested. 4. Taken together, the present data indicate that the representative tryptamines employed in the present study exhibit greater similarity to the LSD stimulus than do representative beta-carbolines. The receptor interactions responsible for these differences remain to be determined.

Neuropeptide Y, neurokinin A and neurotensin in brain regions of Fawn Hooded "depressed", Wistar, and Sprague Dawley rats. Effects of electroconvulsive stimuli

1. Concentrations of neuropeptide Y (NPY)-, neurokinin A (NKA)- and neurotensin (NT)-like immunoreactivity (-LI) were measured in brain tissues of Fawn Hooded (FH) (a model of depression), Wistar (W) (control for depression) and Sprague Dawley (SD) rats (control for strain) with the aim to explore possible associations between neuropeptides and models of depression. 2. In addition, peptides were determined after six electroconvulsive stimuli (ECS) or six sham ECS ("baseline") in order to investigate ECS mechanisms of action. 3. Baseline NPY-LI concentrations were markedly lower in the hippocampus of the "depressed" FH compared to the W and SD animals. 4. Baseline NKA-LI concentrations were higher in the occipital cortex and NT-LI concentrations in the occipital cortex, frontal cortex, and hypothalamus of the FH and W compared to the SD rats. 5. ECS increased NPY-LI in the hippocampus, frontal cortex and occipital cortex of all three strains. In the hippocampus, the increase was significantly larger in the FH compared to the W and SD rats. ECS also increased NKA-LI in the hippocampus. 6. In contrast, ECS decreased NT-LI in the occipital cortex of the FH and W animals. 7. The results indicate that NPY may play a role in depression and that changes in NPY and NKA probably constitute one of the mechanisms of ECT action. More speculatively, NT may also be involved in depression.

Possible confounding influence of strain, age and gender on cognitive performance in rats

There are substantial differences in the performance of various rat strains in tasks of learning, memory and attention. Strain, age and sex differences are not consistent over procedures: poor performance in one paradigm does not predict poor performance in a different paradigm. Some strain differences are not readily apparent until a direct comparison is made between one or more strains. Moreover, large differences in nominally the same strain but obtained from different suppliers have been observed in behavioural, pharmacological and physiological parameters and can have important consequences for interpretation of drug effects. Longevity, and the effects of ageing can differ dramatically from one strain to another; drug effects can alter radically with increasing age and show strain (and individual) differences in their action. Sex can further complicate interpretation of results. Thus, non-cognitive factors may exert a major effect on results in cognitive testing, and strain-dependent effects may account for many conflicting results in the literature concerning mnemonic performance. Strain differences in particular must be identified and used to help identify fundamental effects on memory, rather than continue to be ignored and allowed to obscure interpretation of drug effects on cognitive processes.

Evidence that m-chlorophenylpiperazine-induced hyperthermia in rats is mediated by stimulation of 5-HT2C receptors

Intraperitoneal administration of m-chlorophenylpiperazine (m-CPP) to Wistar rats produced hyperthermia with a peak effect at 30 min. Pretreatment with low doses of metergoline (5-HT1/5-HT2 antagonist), mesulergine and mianserin (5-HT2C/5-HT2A antagonists) blocked m-CPP-induced hyperthermia. Pretreatment with propranolol (beta-adrenergic receptor antagonist that also has binding affinity for 5-HT1A, 5-HT1B and 5-HT2B sites), yohimbine (alpha 2-noradrenergic antagonist that also has binding affinity for 5-HT2B sites), MDL-72222 or ondansetron (5-HT3 antagonists) did not attenuate m-CPP-induced hyperthermia. Only high doses of ketanserin, LY-53857 and ritanserin (5-HT2A/5-HT2C antagonists) as well as spiperone (5-HT1A/5-HT2A/D2 antagonist) attenuated m-CPP-induced hyperthermia. Daily administration of m-CPP produced complete tolerance to its hyperthermic effect by day 5. However, there was no cross-tolerance to 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, a 5-HT2A agonist that also has high affinity for 5-HT2C receptors)-induced hyperthermia. m-CPP-induced increases in temperature were found to be significantly less in the Fawn-Hooded (FH) rat strain as compared to the Wistar rat strain; in prior studies, FH rats have been found to be subsensitive to other 5-HT2C-mediated pharmacologic responses. Altogether, these findings suggest that m-CPP-induced hyperthermia in rats is mediated by selective stimulation of 5-HT2C receptors.

Rat strain differences in the vulnerability of serotonergic nerve endings to neurotoxic damage by p-chloroamphetamine

Substituted amphetamines are known to selectively destroy serotonin (5-HT) nerve endings in distant projection fields of the dorsal raphe nuclei and the systemic administration of these drugs is widely used in investigations of the role of the central 5-HT system and of the mechanisms involved in their toxicity. Until now Sprague-Dawley rats were almost exclusively used for this purpose and the findings were thought to apply to other strains as well. We compared the long-term effects of the administration of different doses of para-chloroamphetamine (PCA) on three specific markers of the density of 5-HT presynapses, [3H]-paroxetine binding to 5-HT-transporters, tryptophan hydroxylase apoenzyme contents, and 5-HT levels in the frontal cortex of Sprague-Dawley and Wistar rats. PCA-treatment caused a dose dependent decline of all three parameters which was much more pronounced in Sprague-Dawley compared to Wistar rats. An i.p. dose of 4 mg PCA/kg body weight, which caused a severe, about 90% reduction of all three parameters of 5-HT innervation in Sprague-Dawley rats was almost ineffective in Wistar rats. The dose of 8 mg/kg which was required to eliminate about 80% of cortical 5-HT presynapses in Wistar rats was already lethal to Sprague-Dawley rats. The reasons of this different susceptibility of the 5-HT system in the two rat strains are unknown. Their elucidation will contribute to a better understanding of inherited differences in individual vulnerability to the neurotoxic effects of substituted amphetamines. The combined measurements of transporter density, of tryptophan hydroxylase apoenzyme contents, and of 5-HT levels is a powerful tool for the assessment of experimentally induced changes in the density of 5-HT innervation in distant projection fields of the raphe nuclei.

Long-term antidepressant treatments alter 5-HT2A and 5-HT2C receptor-mediated hyperthermia in Fawn-Hooded rats

We have recently demonstrated that hyperthermia induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and m-chlorophenylpiperazine (m-CPP) are separately mediated by selective stimulation of 5-HT2A and 5-HT2C receptors, respectively in Wistar rats. Furthermore, hyperthermia induced by either DOI or m-CPP was found to be significantly less in Fawn-Hooded rats (a rat strain suggested to represent a genetic model of depression) relative to Wistar rats. In the present study, we studied the effects of long-term antidepressant treatments on DOI (2.5 mg/kg)-induced and m-CPP (2.5 mg/kg)-induced hyperthermia in male Fawn-Hooded rats. Long-term (21 days) treatment with the tricyclic antidepressants, imipramine or clomipramine (each 5 mg/kg/day), attenuated DOI-induced hyperthermia, while m-CPP-induced hyperthermia was accentuated. On the other hand, long-term (21 days) treatment with the monoamine oxidase type-A inhibiting antidepressant, clorgyline (1 mg/kg/day), did not modify m-CPP-induced hyperthermia, but significantly attenuated DOI-induced hyperthermia. These findings demonstrate that long-term antidepressant treatments alter 5-HT2A and 5-HT2C receptor-mediated hyperthermia in a genetic animal model of depression.

Volitional consumption of ethanol by fawn-hooded rats: effects of alternative solutions and drug treatments

Behavioral and neurochemical measures of brain 5-hydroxytryptamine (5-HT) function in the Fawn-Hooded rat are abnormal relative to outbred strains of rats. Fawn-Hooded rats freely drink large amounts of 10% ethanol in the presence of water and have been proposed to be an animal model for studies related to alcoholism. In this study, Fawn-Hooded rats were given solutions of ethanol increasing in concentration from 3% to 30% (w/v in tap water) over 10 days with tap water in a second drinking tube and a third tube left empty. The solutions of ethanol that produced maximal drinking with a preference (ml ethanol/ml total fluid) near 50% ranged from 5% to 13%, which became the fixed individual concentrations for each rat. After a 5-day baseline period the rats were offered a solution in the third drinking tube of either 0.5% aspartame or chocolate Ultra SlimFast (diluted with water 2:1). The chocolate drink, but not aspartame, significantly reduced the consumption of alcohol by 73%. For the drug experiments, the rats were given successive 4-day periods of: baseline drinking; drug or saline injections b.i.d.; and a posttreatment period. Neither ipsapirone, a 5-HT1a partial agonist, nor naltrexone injected inhibited the intakes of ethanol solutions. Treatment with 2.5 mg/kg of amperozide, a 5-HT2 antagonist, decreased the consumption of ethanol by 38%, but also caused a decrease in consumption of food. These results show a pattern of drinking of increasing concentrations of ethanol different than other strains of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

To drink or not to drink: tests of anxiety and immobility in alcohol-preferring and alcohol-nonpreferring rat strains

Previous reports have provided mixed results about emotional states in rats that voluntarily drink substantial amounts of alcohol. The purpose of the present study was to compare several strains of alcohol-preferring rats (P, AA, FH) with several strains of alcohol-nonpreferring rats (NP, ANA, FRL), and the Maudsley strains on tests reflecting anxiety and immobility. At about 70 days of age the rats were placed in the elevated plus maze for a 5-min test; a forced swim test of 10 min was given 4 days later and this test was followed 4 days later by a modified forced swim test (the capsule), in which there were four false escape alleys. The FRL rats spent more time in the open arms of the elevated plus maze than any other strain, but there was no consistent relationship between elevated plus maze scores and alcohol intake. The alcohol-preferring P rats were the most active in the standard forced swim test and the alcohol-nonpreferring Maudsley Reactive rats were the least active, but there was no consistent relationship between immobility and alcohol intake overall. All rats were much more active in the capsule and there were no significant strain differences. However, the alcohol-preferring P and FH rats attempted to escape more than the other strains, resulting in an overall significant correlation between escape attempts and alcohol intake. These findings do not provide any support for the hypothesis that alcohol-preferring rats are drinking alcohol to reduce high anxiety states.

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of m-chlorophenylpiperazine

m-Chlorophenylpiperazine (mCPP), a major metabolite of the atypical antidepressant trazadone, has been observed to produce marked physiological and behavioral effects in both humans and animals. These effects have been attributed to the interaction of mCPP with serotonergic receptors. The present study was designed to characterize those interactions of mCPP with central serotonergic receptors which mediate mCPP-induced stimulus control. A series of serotonergic antagonists (mesulergine, pizotyline, ketanserin, spiperone, risperidone, ritanserin, metergoline, pirenpirone, and LY53857) was tested for the ability to block the mCPP stimulus. The affinity of these antagonists for 5-HT2A and 5-HT2C receptors was then correlated with maximal percent inhibition of the mCPP stimulus. Kd at the 5-HT2C receptor was inversely proportional (r = -0.75, P < 0.05), and Kd at the 5-HT2A receptor directly proportional (r = +0.67, P < 0.05) to the maximal percent inhibition of the mCPP stimulus. The 5-HT2C selectivity ratio [Kd(5-HT2A)/Kd(5-HT2C)] of the antagonists was directly proportional (r = +0.86, P < 0.01) to maximal percent inhibition of the mCPP stimulus. A multiple regressions analysis indicated that 81% of the variance in the ability of a given antagonist to block the mCPP stimulus could be predicted on the basis of its affinity for 5-HT2A and 5-HT2C receptors. It is concluded that the stimulus effects of mCPP are mediated predominantly by a combination of agonist activity at 5-HT2C receptors and antagonist activity at 5-HT2A receptors.

To drink or not to drink: open field behavior in alcohol-preferring and-nonpreferring rat strains

High open field activity has been associated with high alcohol intake in inbred mouse strains. The present study sought to determine if a similar relationship might exist in rats. Strains which voluntarily drink large amounts of alcohol (alcohol-preferring [P], alcohol-accepting [AA], Fawn-Hooded [FH]) or little or no alcohol (alcohol-nonpreferring [NP], alcohol-nonaccepting [ANA], Flinders Resistant Line [FRL]) were compared with the Maudsley strains of rats selectively bred for differences in open field defecation and activity. There were highly significant strain differences in open field activity, with the alcohol-preferring P rats exhibiting the highest activity and the alcohol-nonpreferring Maudsley Reactive rats exhibiting the lowest. However, the NP rats were almost as active as the P rats and the AA and ANA rats exhibited intermediate levels of activity which did not differ from each other. Thus, there was no consistent relationship between open field activity and high voluntary alcohol intake. Defecation was highest in the Maudsley Reactive rats, and there was a consistent negative relationship with alcohol intake (r = -0.455 across all strains). In a population of 57 FHxFRL F2 hybrids, there were no significant correlations between alcohol intake and open field activity (r = -0.01) or defecation (r = +0.12). We conclude, therefore, that there was no consistent relationship between voluntary alcohol intake and open field behavior across strains of rats.

Functional and biochemical evidence for altered serotonergic function in the fawn-hooded rat strain

Administration of various doses of DOI (a 5-HT2A/5-HT2C agonist) produced hyperthermia that was significantly less in the FH rat strain relative to the Wistar rat strain. Similarly, administration of various doses of ipsapirone (a 5-HT1A agonist) produced hypothermia that was significantly less in the FH rat strain relative to the Wistar rat strain. Furthermore, m-CPP (a 5-HT agonist)-induced increases in growth hormone levels were also significantly less in the FH rat strain relative to the Wistar rat strain. There was no significant difference in the levels of either 5-HT or 5-HIAA between the two rat strains in the frontal cortex, hippocampus, hypothalamus, and striatum. In the brain stem, however, both 5-HT and 5-HIAA levels were significantly lower in the FH rat strain relative to the Wistar rat strain. On the other hand, 5-HT turnover rate was significantly higher in the hypothalamus and striatum and significantly lower in the hippocampus in the FH rat strain relative to the Wistar rat strain. These findings provide further evidence for altered serotonergic function in the FH rat strain and, in addition, suggest that the FH rat strain may prove to be a useful genetic model for some neuropsychiatric disorders with possible abnormalities in serotonergic function such as depression, obsessive-compulsive disorder, and the eating disorders.

Enhanced anorexic responses to m-chlorophenylpiperazine during lithium administration to fawn-hooded rats

In the present study, we investigated whether functional adaptational changes in the serotonergic neurotransmitter mechanisms regulating food intake following long-term lithium treatment in Fawn-Hooded rats (a rat strain suggested to represent a genetic model of depression) were different or similar to those previously observed in Wistar rats. Long-term (21-25 days) lithium treatment accentuated m-chlorophenylpiperazine (m-CPP, a 5-HT agonist) induced decreases in food intake. There was no significant difference in either brain m-CPP concentrations or hypothalamic norepinephrine, dopamine and 5-hydroxyindoleacetic acid concentrations between control and long-term lithium-treated rats following m-CPP. However, hypothalamic serotonin concentrations were significantly higher in long-term lithium-treated compared to saline-treated animals. This finding contrasts with our previous report demonstrating attenuation of m-CPP-induced anorexia in Wistar rats following similar long-term lithium treatment, and therefore suggests a differential adaptation in the serotonergic neurotransmitter mechanisms regulating food intake in a genetic animal model of depression.