Serotonin2C receptors and serotonin2C receptor-mediated phosphoinositide hydrolysis in the brain of alcohol-preferring and alcohol-nonpreferring rats

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

Ethanol deprivation and central 5-HT deficiency differentially affect the mRNA editing of the 5-HT2C receptor in the mouse brain

BACKGROUND

Serotonin (5-HT) 5-HT2C receptor mRNA editing (at five sites, A-E), implicated in neuropsychiatric disorders, including clinical depression, remains unexplored during alcohol abstinence-often accompanied by depressive symptoms.

METHODS

We used deep sequencing to investigate 5-HT2C receptor editing in mice during early ethanol deprivation following prolonged alcohol exposure and mice lacking tryptophan hydroxylase (TPH)2, a key enzyme in central 5-HT production. We also examined Tph2 expression in ethanol-deprived animals using quantitative real-time PCR (qPCR).

RESULTS

Cessation from chronic 10% ethanol exposure in a two-bottle choice paradigm enhanced immobility time and decreased latency in the forced swim test (FST), indicating a depression-like phenotype. In the hippocampus, ethanol-deprived "high ethanol-drinking" mice displayed reduced Tph2 expression, elevated 5-HT2C receptor editing efficiency, and decreased frequency of the D mRNA variant, encoding the less-edited INV protein isoform. Tph2-/- mice showed attenuated receptor editing in the hippocampus and elevated frequency of non-edited None and D variants. In the prefrontal cortex, Tph2 deficiency increased receptor mRNA editing at site D and reduced the frequency of AB transcript, predicting a reduction in the corresponding partially edited VNI isoform.

CONCLUSIONS

Our findings reveal differential effects of 5-HT depletion and ethanol cessation on 5-HT2C receptor editing. Central 5-HT depletion attenuated editing in the prefrontal cortex and the hippocampus, whereas ethanol deprivation, coinciding with reduced Tph2 expression in the hippocampus, enhanced receptor editing efficiency specifically in this brain region. This study highlights the interplay between 5-HT synthesis, ethanol cessation, and 5-HT2C receptor editing, providing potential mechanism underlying increased ethanol consumption and deprivation.

Alcohol Dependence and Withdrawal Impair Serotonergic Regulation of GABA Transmission in the Rat Central Nucleus of the Amygdala

Excessive serotonin (5-HT) signaling plays a critical role in the etiology of alcohol use disorder. The central nucleus of the amygdala (CeA) is a key player in alcohol-dependence associated behaviors. The CeA receives dense innervation from the dorsal raphe nucleus, the major source of 5-HT, and expresses 5-HT receptor subtypes (e.g., 5-HT2C and 5-HT1A) critically linked to alcohol use disorder. Notably, the role of 5-HT regulating rat CeA activity in alcohol dependence is poorly investigated. Here, we examined neuroadaptations of CeA 5-HT signaling in adult, male Sprague Dawley rats using an established model of alcohol dependence (chronic intermittent alcohol vapor exposure), ex vivo slice electrophysiology and ISH. 5-HT increased frequency of sIPSCs without affecting postsynaptic measures, suggesting increased CeA GABA release in naive rats. In dependent rats, this 5-HT-induced increase of GABA release was attenuated, suggesting blunted CeA 5-HT sensitivity, which partially recovered in protracted withdrawal (2 weeks). 5-HT increased vesicular GABA release in naive and dependent rats but had split effects (increase and decrease) after protracted withdrawal indicative of neuroadaptations of presynaptic 5-HT receptors. Accordingly, 5-HT abolished spontaneous neuronal firing in naive and dependent rats but had bidirectional effects in withdrawn. Alcohol dependence and protracted withdrawal did not alter either 5-HT1A-mediated decrease of CeA GABA release or Htr1a expression but disrupted 5-HT2C-signaling without affecting Htr2c expression. Collectively, our study provides detailed insights into modulation of CeA activity by the 5-HT system and unravels the vulnerability of the CeA 5-HT system to chronic alcohol and protracted withdrawal.SIGNIFICANCE STATEMENT Elevated GABA signaling in the central nucleus of the amygdala (CeA) underlies key behaviors associated with alcohol dependence. The CeA is reciprocally connected with the dorsal raphe nucleus, the main source of serotonin (5-HT) in the mammalian brain, and excessive 5-HT signaling is critically implicated in the etiology of alcohol use disorder. Our study, using a well-established rat model of alcohol dependence, ex vivo electrophysiology and ISH, provides mechanistic insights into how both chronic alcohol exposure and protracted withdrawal dysregulate 5-HT signaling in the CeA. Thus, our study further expands our understanding of CeA cellular mechanisms involved in the pathophysiology of alcohol dependence and withdrawal.

Anxiety during alcohol withdrawal involves 5-HT2C receptors and M-channels in the lateral habenula

Anxiety disorders often co-occur with alcohol use disorders, but the mechanisms underlying this comorbidity remain elusive. Previously, we reported that rats withdrawn from chronic alcohol consumption (Post-EtOH rats) exhibited robust anxiety-like behaviors (AB), which were accompanied by neuronal hyperexcitability, and the downregulation of M-type potassium channels (M-channels) in the lateral habenula (LHb); and that serotonin (5-HT) stimulated LHb neurons via type 2C receptors (5-HT_2CRs). Also, 5-HT_2CR activation is known to inhibit M-current in mouse hypothalamic neurons. The present study investigated whether LHb 5-HT_2CRs and M-channels contribute to AB in adult male Long-Evans rats. We used the intermittent-access to 20% ethanol two-bottle free-choice drinking paradigm to induce dependence. We measured AB with the elevated plus-maze, open-field, and marble-burying tests at 24 h withdrawal. We found that intra-LHb infusion of SB242084, a selective 5-HT_2CR antagonist alleviated AB and reduced the elevated c-Fos expression in the LHb of Post-EtOH rats. By contrast, intra-LHb infusion of the selective 5-HT_2CR agonist WAY161503 induced AB and increased c-Fos expression in the LHb in alcohol-naive but not Post-EtOH rats. Also, intra-LHb SB242084 significantly reduced self-administration of alcohol intake in the operant chambers. Furthermore, both 5-HT_2CR protein levels and 5-HIAA/5-HT ratio was increased in the LHb of Post-EtOH rats. Finally, intra-LHb SB242084 increased LHb KCNQ2/3 membrane protein expression in Post-EtOH rats. Collectively, these results suggest that enhanced LHb 5-HT_2CR signaling that interacted with M-channels triggers AB in Post-EtOH rats and that 5-HT_2CRs may be a promising target for treating comorbid anxiety disorders in alcoholics.

Expression Analysis of Serotonin Receptors, Serotonin Transporter and l-Amino Acid Decarboxylase in the Mouse Sphenopalatine Ganglion by RT-PCR, Northern Blot Analysis and In Situ Hybridization

The sphenopalatine ganglion (SPG) is a gathering of the cell bodies of parasympathetic fibers that dominate the nasal gland, lacrimal gland and cerebral blood vessels. The SPG controls nasal secretions, tears, and the dilation of cerebral blood vessels. However, it is unclear how serotonin regulates SPG functions. In this study, we investigated the expression of genes involved in the serotonergic system in the mouse SPG. We examined the mRNA expression levels of 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1F, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT_3A, 5-HT_3B, 5-HT₄, 5-HT_5A, 5-HT_5B, 5-HT₆ and 5-HT₇ receptors, as well as serotonin transporter, tryptophan hydroxylases 1 and 2, and L-amino acid decarboxylase (AADC) by RT-PCR. It revealed that the 5-HT_3A and 5-HT_3B ionotropic receptors and AADC were likely to be highly expressed in the SPG, as measured by RT-PCR. We next performed in situ hybridization on the SPG to examine the expression of these three genes at the cellular level after validating the specificity of each cRNA probe by northern blotting. The 5-HT_3A receptor, 5-HT_3B receptor, and AADC were expressed in 96.5% ± 1.0%, 29.7% ± 10.7%, and 57.4% ± 2.9% of neuronal cell bodies in the SPG, respectively, indicating that the 5-HT_3A receptor was virtually expressed in all SPG neurons. Our results on the expression of these critical serotonin system genes in the parasympathetic SPG provide insight into the pathogenetics of rhinitis, conjunctivitis and headache. Furthermore, our findings suggest that targeting the 5-HT_3A receptor might have therapeutic potential in the treatment of these ailments.

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.

A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

Application of palladium-mediated (18)F-fluorination to PET radiotracer development: overcoming hurdles to translation

New chemistry methods for the synthesis of radiolabeled small molecules have the potential to impact clinical positron emission tomography (PET) imaging, if they can be successfully translated. However, progression of modern reactions from the stage of synthetic chemistry development to the preparation of radiotracer doses ready for use in human PET imaging is challenging and rare. Here we describe the process of and the successful translation of a modern palladium-mediated fluorination reaction to non-human primate (NHP) baboon PET imaging–an important milestone on the path to human PET imaging. The method, which transforms [¹⁸F]fluoride into an electrophilic fluorination reagent, provides access to aryl–¹⁸F bonds that would be challenging to synthesize via conventional radiochemistry methods.

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.

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.

HTR2C promoter polymorphisms are associated with risperidone efficacy in Chinese female patients

AIMS

A number of studies demonstrate that the polymorphisms in the 5 region of HTR2C play a pivotal role in antipsychotic drug efficacy. Since risperidone is an antagonist of HTR2C, polymorphic variations in HTR2C may explain variability in response to risperidone treatment. We analyzed HTR2C polymorphisms for association with efficacy of risperidone monotherapy.

MATERIALS & METHODS

We genotyped five SNPs distributed throughout the HTR2C gene and examined them for association using the Brief Psychiatric Rating Scale score in 130 Chinese schizophrenic patients following an 8-week period of risperidone monotherapy. All the patients were receiving the atypical antipsychotic drug treatment for the first time and had a 4-week medication-free period before research began.

RESULTS

We found rs518147, rs1023574 and rs9698290 were significantly associated with risperidone treatment in female patients (F = 4.75, degrees of freedom = 2 and p = 0.011; F = 4.329, degrees of freedom = 2 and p = 0.016; F = 4.188, degrees of freedom = 2 and p = 0.019, respectively) and they were also found to be in one linkage disequilibrium block.

CONCLUSION

Our results indicate that variants in the HTR2C promoter region are likely to affect the risperidone therapeutic effect in female mainland patients. It may be helpful to investigate a combination of other clinical factors to predict atypical antipsychotic efficacy.

Innate differences in the expression of brain-derived neurotrophic factor in the regions within the extended amygdala between alcohol preferring and nonpreferring rats

BACKGROUND

Animal lines such as alcohol-preferring (P) and nonpreferring (NP) rats appear to be suitable animal models to investigate the biological basis of alcohol-drinking behaviors. The extended amygdala serves as a neuroanatomical substrate for alcohol-drinking behaviors. Brain-derived neurotrophic factor (BDNF) in the amygdala has been implicated in alcohol-drinking behaviors; however, its expression in the extended amygdala of P and NP rats is unknown. Therefore, we examined the basal expression of BDNF in the extended amygdala of alcohol naive P and NP rats.

METHODS

We determined the basal mRNA and protein levels of BDNF by in situ RT-PCR and immuno-histochemical procedure, respectively, in the amygdaloid [central nucleus of amygdala (CeA), medial nucleus of amygdala (MeA), and basolateral amygdala (BLA)], nucleus accumbal (NAc shell and core), and bed nucleus of stria terminalis (BNST) [lateral BNST (lBNST), medial BNST (mBNST), and ventral BNST (vBNST)] brain structures of P and NP rats. In addition, we examined the localization of BDNF in neurons using double-immunofluorescence labeling of BDNF with neuron-specific nuclear protein (NeuN) and also determined the number of NeuN-positive neurons in the amygdaloid structures of P and NP rats.

RESULTS

The mRNA and protein levels of BDNF were found to be significantly lower in both the CeA and MeA, but not in the BLA, of P compared with NP rats. We also found that BDNF was expressed in neurons in the amygdaloid structures of P and NP rats. In addition, we found that the number of NeuN-positive neurons was similar in the amygdaloid structures of P and NP rats. Interestingly, the mRNA and protein levels of BDNF were also significantly lower in the lBNST, mBNST, and vBNST of P compared with NP rats. On the other hand, mRNA and protein levels of BDNF were similar in the NAc shell and core structures of P and NP rats.

CONCLUSIONS

P and NP rats are selectively bred for higher and lower alcohol preference, respectively; therefore it is possible that lower BDNF levels in the amygdaloid and BNST structures may be associated with the excessive alcohol-drinking behaviors of P rats.

Porcine maternal infanticide as a model for puerperal psychosis

Childbirth is a period of substantial rapid biological and psychological change and a wide range of psychotic disorders can occur ranging from mild 'baby blues' to severe episodes of psychotic illnesses. Puerperal psychosis is the most extreme form of postnatal psychosis, occurring in 1 in 1,000 births. In this study, we have used the pig as an animal model for human postnatal psychiatric illness. Our aim was to identify quantitative trait loci (QTL) associated with maternal (infanticide) sow aggression. This is defined by sows attacking and killing their own newborn offspring, within 24 hr of birth. An affected sib pair whole genome linkage analysis was carried out with 80 microsatellite markers covering the 18 porcine autosomes and the X chromosome, with the aim of identifying chromosomal regions responsible for this abnormal behavior. Analysis was carried out using the non-parametric linkage test of Whittemore and Halpern, as implemented in the Merlin software. The results identified 4 QTL mapping on Sus scrofa chromosomes 2 (SSC2), 10 (SSC10), and X (SSCX). The peak regions of these QTL are syntenic to HSA 5q14.3-15, 1q32, Xpter-Xp2.1, and Xq2.4-Xqter, respectively. Several potential candidate genes lie in these regions in addition to relevant abnormal behavioral QTL, found in humans and rodents.

Regulation of 5-HT2A/C receptors and DOI-induced behaviors by protein kinase Cgamma

Protein kinase Cgamma (PKCgamma) null mutant mice demonstrate increased behavioral impulsivity and ethanol consumption. Pharmacological studies have shown that 5-HT(2A/C) receptors modulate impulsivity and ethanol consumption in rodents and that PKC can regulate 5-HT(2A/C) receptors. To determine whether PKCgamma plays a selective role in 5-HT(2A/C) receptor regulation, biochemical and behavioral experiments were performed in PKCgamma mutant and wild-type mice. DOI-stimulated phosphoinositol hydrolysis and [(125)I]-DOI saturation binding in the PFC, and quantitative autoradiography of [(125)I]-DOI binding sites in 15 brain regions were analyzed. DOI-induced head twitch responses (HTR) were measured in naive mice after an acute 2.5 mg/kg injection of DOI. Results indicated that DOI-induced HTR was significantly greater in mutant mice compared to wild-type mice. Results of the phosphoinositol hydrolysis, membrane binding, and autoradiography experiments indicated that in mutant mice, increased HTR was associated with increased 5-HT(2A/C) receptor function in the PFC, but not increased receptor number or affinity suggesting that PKCgamma regulates receptor function but not receptor number. These data support a role for 5-HT(2A/C) receptors in the PFC in mediating some of the behavioral differences observed between PKCgamma mutant and wild-type mice.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Pharmacological modulation of repeated ethanol withdrawal-induced anxiety-like behavior differs in alcohol-preferring P and Sprague-Dawley rats

Previous work with Sprague-Dawley (SD) rats indicated that subjecting these rats to multiple episodes of ethanol diet could provoke anxiety-like responses. Because alcohol-preferring P rats have been reported to have neurochemical alterations in many systems shown to modulate anxiety-like responses, P rats were compared to SD rats. Rats were subjected to one or three cycles of 5 days' exposure to 4.5% or 7% ethanol diet to assess anxiety-like behavior. The social interaction test was conducted 5 h after ethanol was removed. Other groups of P and SD rats were injected with flumazenil (5 mg/kg), a benzodiazepine (BZD) receptor antagonist, CP-154,526 (10 mg/kg), CRF1 receptor antagonist, SB243,213, a 5-HT2C receptor inverse agonist, or vehicle during the 1st and 2nd withdrawals but not the third. After a single 5-day cycle of ethanol exposure, SD rats did not exhibit a change in social interaction, but P rats exhibited a decrease after exposure to the 7% ethanol. Both strains of rats exhibited anxiety-like behavior following three cycles of exposure to ethanol and the concentration of ethanol in the diet did not influence the response. It was confirmed that flumazenil, CP-154,523, and SB243,213 had prophylactic effects on anxiety-like behavior in the SD rats. Neither flumazenil nor SB243,213 was as effective in the P rats, while the CRF1 receptor antagonist completely counteracted the reduced social interaction in repeatedly withdrawn P rats. A small study showed that buspirone, a 5-HT1A agonist, also had prophylactic effects in P rats. These findings show that alcohol-preferring P rats exhibit anxiety-like behavior more readily following exposure to ethanol-containing diets and that this behavior is counteracted more readily by pretreatment with a CRF1 receptor antagonist than with BZD or 5-HT2C receptor antagonists.

Analysis of 5-hydroxytryptamine 2c receptor gene promoter variants as alcohol-dependence risk factors

AIMS

To examine whether polymorphic variants of the HTR2C gene are associated with diagnosis of alcohol dependence.

METHODS

We compared allele frequencies of five HTR2C promoter polymorphisms in a Nordic population of alcohol dependent individuals (Males: n = 309; Females: n = 127) and ethnically matched controls (Males: n = 83; Females: n = 190) in whom alcohol dependence was established, or any diagnosis of substance disorder was excluded, respectively. Patients were further subtyped into Type I (late onset) and Type II (early onset) alcoholics.

RESULTS

None of the individual polymorphisms indicated significant association with alcohol dependence. A common promoter haplotype (GAGG) exhibited different distribution frequencies between males and females (Type I), however on Bonferroni's multiple-testing correction, this observation proved to be insignificant.

CONCLUSIONS

Although we report a lack of association between alcohol dependence and five common promoter polymorphisms, and the constituted haplotypes, the analysis tends to indicate gender and sub-type differences. We suggest that a follow up study with larger sample numbers should be performed to improve the power to detect the genetic influences of HTR2C in alcohol dependence.

The status of serotonin-selective pharmacotherapy in the treatment of alcohol dependence

Research performed during the past 20 years has shown that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission is related to alcohol dependence. Both theoretical and empirical research have supported the idea that alcohol dependence is a chronic disease and that, in addition, biological vulnerabilities contribute to the pathogenesis of alcohol dependence. Preclinical studies have consistently demonstrated that there is a relationship between 5-HT function and alcohol consumption. Furthermore, there is evidence building that lends support for the existence of distinct alcoholic subtypes that may be differentiated by the type or complexity of their 5-HT dysfunction. Beyond excessive drinking, behaviors that are indicators of 5-HT dysregulation are depression, anxiety, impulsiveness, and early-onset problem drinking. This chapter will discuss the usefulness of 5-HT-selective pharmacotherapy in treating alcohol dependence and will provide both historical and current perspectives on its use.

Regional CNS densities of serotonin 1A and dopamine D2 receptors in periadolescent alcohol-preferring P and alcohol-nonpreferring NP rat pups

The objective of the present study was to use quantitative autoradiography to determine binding densities of serotonin(1A) (5-HT(1A)) and dopamine (DA) D(2) receptors in alcohol-naive periadolescent P and NP rat pups. P (n=8) and NP (n=7) rat pups, 25 days of age, from different litters were used. Coronal brain sections were incubated with 2 nM [3H]8-OH-DPAT or 20 nM [3H]sulpiride for 5-HT(1A) or D(2) binding, respectively. Approximately 15-40% higher densities of [3H]8-OH-DPAT binding were observed in the anterior cortical regions of the periadolescent P rat compared with NP rat pups. Similar differences were also observed in posterior cortical regions with P rats having 25-40% higher [3H]8-OH-DPAT binding than NP rats. [3H]8-OH-DPAT binding was approximately 10-20% higher in posterior hippocampal regions of the P rat pups compared with the NP line. [3H]sulpiride binding was significantly different only in the ventral tegmental area (VTA), where binding was approximately 20% lower in the periadolescent P rats compared with the NP rat pups. Overall, these results are very similar to findings observed in adult alcohol-naive P and NP rats, and suggest that the innate differences in the neural systems implicated in high alcohol drinking behaviors may already be established in the periadolescent animal.

MK-801- and ethanol-induced activity in inbred long-sleep and short-sleep mice: dopamine and serotonin systems

Low doses of (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801; dizocilpine) or ethanol induce less locomotor activation in inbred long-sleep (ILS) than short-sleep (ISS) mice. These differences may involve altered dopamine and/or 5-hydroxytryptamine (serotonin; 5-HT) neurotransmission. To address this possibility, the dopaminergic and serotonergic mechanisms underlying the locomotor-stimulant effects of MK-801 and ethanol in ILS and ISS mice were studied. Dopamine D1, D2 and 5-HT(2A) receptor antagonists reduced MK-801-stimulated activity in ILS mice without having any effect in ISS mice. The 5-HT reuptake inhibitor fluoxetine potentiated MK-801-stimulated activity selectively in ILS mice. Strain differences in 5-HT transporters do not explain this selective effect of fluoxetine in ILS mice since [3H]citalopram binding and [3H]5-HT uptake studies found no differences in the affinity, number or function of 5-HT transporters between ILS and ISS mice. Ethanol-induced activity in ISS mice was depressed by dopamine D2 and 5-HT(2C) receptor antagonists and enhanced by a 5-HT(1A) receptor antagonist. These results suggest that in ILS mice the locomotor-stimulant effects of MK-801 require increased dopamine and/or 5-HT neurotransmission. Conversely, in ISS mice, the effects of MK-801 appear to be monoamine-independent. Thus, even though both MK-801 and ethanol inhibit N-methyl-D-aspartate receptors, their stimulant effects appear to involve different neuronal systems.

Pharmacogenetics and the serotonin system: initial studies and future directions

Serotonin (5-hydroxytryptamine, 5-HT) appears to play a role in the pathophysiology of a range of neuropsychiatric disorders, and serotonergic agents are of central importance in neuropharmacology. Genes encoding various components of the 5-HT system are being studied as risk factors in depression, schizophrenia, obsessive-compulsive disorder, aggression, alcoholism, and autism. Recently, pharmacogenetic research has begun to examine possible genetic influences on therapeutic response to drugs affecting the serotonin system. Genes regulating the synthesis (TPH), storage (VMAT2), membrane uptake (HTT), and metabolism (MAOA) of 5-HT, as well as a number of 5-HT receptors (HTR1A, HTR1B, HTR2A, HTR2C, and HTR5A), have been studied and this initial research is reviewed here. After a brief introduction to serotonin neurobiology and a general discussion of appropriate genetic methodology, each of the major 5-HT-related genes and their encoded proteins are reviewed in turn. For each gene, relevant polymorphisms and research on functional variants are discussed; following brief reviews of the disorder or trait association and linkage studies, pharmacogenetic studies performed to date are covered. The critical and manifold roles of the serotonin system, the great abundance of targets within the system, the wide range of serotonergic agents-available and in development-and the promising preliminary results suggest that the serotonin system offers a particularly rich area for pharmacogenetic research.

Effect of alcohols on G-protein coupling of serotonin(1A) receptors from bovine hippocampus

The serotonin(1A) (5-hydroxytryptamine [5-HT](1A)) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. Serotonergic signalling has been shown to play an important role in alcohol intake, preference and dependence. G-protein coupling of the 5-HT(1A) receptor serves as an important determinant for serotonergic signalling. We have studied the effect of alcohols on G-protein coupling of bovine hippocampal 5-HT(1A) receptors in native membranes. This was done by monitoring the modulation of ligand (agonist and antagonist) binding in presence of alcohols by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), a non-hydrolyzable analogue of GTP. Our results show that alcohols inhibit the specific binding of the agonist 8-hydroxy-2-(di-N-propylamino)tetralin (except in case of ethanol) and the antagonist 4-(2'-methoxy)-phenyl-1-[2'-(N-2"-pyridinyl)-p-fluorobenzamido]eth yl- piperazine to 5-HT(1A) receptors in a concentration-dependent manner. Further, we show here that the action of alcohols on the bovine hippocampal 5-HT(1A) receptors could be modulated by guanine nucleotides and that the mode of action of ethanol on the 5-HT(1A) receptor may be quite different than that of other alcohols. The effect of GTP-gamma-S on the agonist and the antagonist binding is found to be markedly different. Our results could be significant in the overall context of the role of G-protein coupling in serotonergic neurotransmission and its role in alcohol tolerance and dependence.

Molecular serotonergic mechanisms appear to mediate genetic sensitivity to cocaine-induced convulsions

Cocaine-induced convulsions appear to be mediated by serotonin (5-HT) neurotransmission, acting primarily at 5-HT(2) receptors. However, this effect of cocaine is attenuated by cocaine binding at sigma and muscarinic M(1) and M(2) sites. This study examined whether the aforementioned neural sites mediate the nearly two-fold difference in sensitivity to cocaine-induced convulsions across C57BL/6J (6J) and C57BL/6ByJ (6ByJ) mice. Experiment 1 compared 5-HT transporter densities across several brain regions of 6J and 6ByJ mice and cocaine-induced convulsions following pretreatment with the 5-HT reuptake inhibitor fluoxetine. Experiment 2 compared 5-HT(2) receptor densities across these mice and cocaine-induced convulsions following pretreatment with the 5-HT(2) antagonist cinanserin. There were no differences in 5-HT transporter densities, however, fluoxetine produced a greater facilitation of cocaine-induced convulsions in 6ByJ relative to 6J mice, suggesting that sensitivity to convulsions is mediated postsynaptically. Indeed, 5-HT(2) density was higher in 6ByJ relative to 6J mice in the amygdaloid ridge, hypothalamus, and midbrain. In addition, cinanserin attenuated convulsions more potently in 6J relative to 6ByJ mice. There were no differences in the densities or affinities of 5-HT(1), muscarinic, or sigma receptors across these strains, suggesting that density of these latter sites does not mediate genetic sensitivity to cocaine-induced convulsions. Since 6ByJ mice are less sensitive to convulsions despite the fact that they have more 5-HT(2) receptors, we hypothesized that these mice may exhibit a weaker linkage of 5-HT(2) sites to their second-messenger system relative to 6J mice. However, in experiment 3 we demonstrated that 5-HT(2)-receptor mediated phosphoinositide hydrolysis was higher in 6ByJ relative to 6J mice in the same regions also displaying higher 5-HT(2) densities. This study suggests that 5-HT(2) receptors mediate genetic sensitivity to cocaine-induced convulsions, further supporting the role of these sites in mediating this toxic effect of cocaine.

Serotonin-2C and serotonin-1A receptor genes are not associated with psychotic symptomatology of mood disorders

The serotonergic system is involved in both pathophysiology and treatment of mood disorders. In the present study we investigated the possible influence of the polymorphisms of the serotonin-1A and 2C receptor genes on the symptomatology of mood disorders. Eighty-four inpatients affected by mood disorders (72 bipolar and 12 major depressive disorder) were assessed by the Operational Criteria Checklist for Psychotic Illness to score their lifetime psychotic symptomatology. The subjects were also typed for 5HT1A and 5HT2C variants using polymerase chain reaction techniques. No association was found between 5HT2C and psychopathology as defined by the four symptomatologic factors used as phenotype definition (mania, depression, delusion, and disorganization) even when bipolar subjects were analyzed separately. Only one subject with the 5HT1A variant was observed. Genetic variation at the 5HT1A and 5HT2C receptor genes does not, therefore, play a major role in the pathogenesis of mood disorders symptomatology. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:161-166, 2000.

Acoustic startle and fear-potentiated startle in alcohol-preferring (P) and -nonpreferring (NP) lines of rats

The objective of the present study was to determine whether alcohol-preferring P and -nonpreferring NP rats differ in their acoustic startle response and in fear-potentiated startle. In Experiment 1, male P and NP rats were tested on the startle response to acoustic stimuli ranging from 90-115 dB. Experiments 2 and 3 examined fear-potentiated startle and extinction of the response. In Experiment 2, rats received two light foot shock training sessions separated by 3-4 h. Testing consisted of ten acoustic startle (115 dB) and fear-potentiated startle (light preceding the acoustic startle) presentations administered every 24 h for 9 consecutive days. To test potentiated startle learning under reduced training conditions, a single training session was administered in Experiment 3, and a single within-session extinction test of 50 startle and 50 potentiated startle trials occurred the following day. Results of Experiment 1 indicated that P and NP rats did not differ in startle at any of the acoustic intensities tested. Following fear-potentiated startle conditioning in Experiment 2, however, both acoustic startle and potentiated startle responding were consistently greater in P than NP rats over most of the first 6 test days with P rats having approximately a 100% greater acoustic startle and 50-100% greater potentiated startle response. Moreover, following a single training session in Experiment 3, only P rats showed significant fear-conditioned startle. Additionally, P rats exhibited a 50-100% elevated acoustic startle response over that observed in NP rats. Taken together, the data indicate that, although experimentally naive male P and NP rats show similar acoustic startle responses, P rats become more responsive to both startle-alone and potentiated startle stimuli following fear conditioning. The change in general startle reactivity of the P rat following aversive conditioning, along with facilitated light foot shock learning, suggests that stress exposure may be an important variable in examining associations between anxiety and alcohol drinking behavior.

Serotonin receptor 2A, 2C, 1A genes and response to lithium prophylaxis in mood disorders

The aim of this study was to investigate the influence of serotonin receptors 2A, 2C and 1A gene variants on lithium prophylactic efficacy in mood disorders. One hundred and twenty-four subjects affected by bipolar (n=102) and major depressive (n=22) disorder were followed prospectively for an average of 52 months and were typed for 5-HT2A (T102C: n=111, HTP: n=104), 5-HT2C (n=110) and 5-HT1A (n=61) variants. Both 5-HT2A and 5-HT2C variants were not associated with lithium outcome. Consideration of possible stratification effects like gender, polarity, family history, age at onset and duration of lithium treatment did not influence results. No 5-HT1A gene variant was identified. 5-HT2A and 2C variants are not, therefore, associated with lithium prophylactic efficacy in mood disorders.

Role of serotonin and serotonin-selective pharmacotherapy in alcohol dependence

The majority of studies that have examined the usefulness of pharmacotherapies selective for serotonin (5-hydroxytryptamine; 5-HT) as a treatment for alcohol dependence have been standard, double-blind clinical trials that include patients with a variety of clinical presentations. Almost all of the early studies evaluated heavy social drinkers and found only a modest advantage for 5-HT pharmacotherapies in reducing the number of drinks per day. Also, the advantage of these pharmacotherapies was observed primarily when these agents were given at higher daily dosages than suggested prescribing practices for use as an antidepressant. The few studies that evaluated treatment-seeking patients found that 5-HT pharmacotherapies were not instrumental in reducing drinking rates compared with placebo. These results led to a dampening of enthusiasm for use of these agents in treating alcohol dependence. However, more recent investigations have begun to target subgroups with potential abnormalities in 5-HT neurotransmission. The thinking is that these medications should be most useful in alcohol-dependent individuals who have more clearly delineated suggestive signs of 5-HT dysfunction, such as concomitant depression or anxiety. Although few results are available to date, there is growing evidence to suggest that alcohol-dependent subgroups are differentially responsive to 5-HT pharmacotherapies with respect to drinking-related outcomes. This may explain the modest and variable 5-HT pharmacotherapeutic effects that were reported in the earlier studies, which included large heterogeneous patient groups. Further investigations are needed to confirm these initial optimistic results.

Biological phenotypes associated with individuals at high risk for developing alcohol-related disorders: Part 1

This article reviews the results of studies concerning particular classes of biological phenotypes that may have relevance for alcohol dependence. Broadly defined, these classes include brain neurotransmitter systems and neuroelectric potentials. Evidence is presented concerning genotypic variation in alcoholics and high-risk relatives suggesting that the etiology of alcoholism and other addictive diseases is mediated in part through suboptimal neurotransmitter functioning. Research opportunities are offered with respect to specific candidate genes that have been cloned from these neurotransmitter systems that could be most fully utilized in family-based genetic analyses. Additional evidence is offered, suggesting that characteristics of particular neuroelectric potentials (e.g. the amplitude of the P300 component of the event-related potential) may provide another dimension of potential markers that could be used to identify children at risk. Finally, methodological considerations specific to high risk studies are discussed. Among these are the need to include a plan for studying more severe cases of alcohol dependence that are relatively uncomplicated by other major psychiatric disorders. Plans for long-term follow-up of children at highest risk for developing the disorder should also be included. Multiple domains of inquiry should not be viewed as "unfocused" but rather as an economical means for utilizing highly characterized samples of individuals meeting rigorous research criteria.