Adult hippocampal neurogenesis in depression

The development of new treatments for depression is predicated upon identification of neural substrates and mechanisms that underlie its etiology and pathophysiology. The heterogeneity of depression indicates that its origin may lie in dysfunction of multiple brain regions. Here we evaluate adult hippocampal neurogenesis as a candidate mechanism for the etiology of depression and as a substrate for antidepressant action. Current evidence indicates that adult hippocampal neurogenesis may not be a major contributor to the development of depression, but may be required for some of the behavioral effects of antidepressants. We next revisit the functional differentiation of the hippocampus along the septo-temporal axis within the context of adult hippocampal neurogenesis and suggest that neurogenesis in the ventral dentate gyrus may be preferentially involved in regulation of emotion. Finally, we speculate on how increased adult hippocampal neurogenesis may modulate dentate gyrus function to confer the behavioral effects of antidepressants.

Reversal of neuropathology and motor dysfunction in a conditional model of Huntington's disease

Neurodegenerative disorders like Huntington's disease (HD) are characterized by progressive and putative irreversible clinical and neuropathological symptoms, including neuronal protein aggregates. Conditional transgenic models of neurodegenerative diseases therefore could be a powerful means to explore the relationship between mutant protein expression and progression of the disease. We have created a conditional model of HD by using the tet-regulatable system. Mice expressing a mutated huntingtin fragment demonstrate neuronal inclusions, characteristic neuropathology, and progressive motor dysfunction. Blockade of expression in symptomatic mice leads to a disappearance of inclusions and an amelioration of the behavioral phenotype. We thus demonstrate that a continuous influx of the mutant protein is required to maintain inclusions and symptoms, raising the possibility that HD may be reversible.

The SV40 72 base repair repeat has a striking effect on gene expression both in SV40 and other chimeric recombinants

By introduction of recombinant plasmids into monkey CV1 cells, we have unambiguously demonstrated that sequences entirely within the 72 bp repeat, which is located upstream of the SV40 early region, are crucial for T-antigen expression in vivo. We have also shown that a DNA fragment containing the 72 bp repeat, inserted directly before chicken conalbumin or adenovirus-2 major late promoter sequences in chimeric plasmids where these promoters replace that of the SV40 early genes, caused a dramatic increase in the expression of T-antigen in vivo. This effect was independent of the orientation of the 72 bp repeat, but was sensitive to its location within the plasmid, when the 72 bp repeat was separated from the promoter sequences, T-antigen expression was reduced. Insertion of the 72 bp repeat into equivalent plasmids containing no known eukaryotic promoter sequences (plasmids which were not detectably expressed in vivo) gave rise to a measurable, but smaller level of expression. The stimulation of expression by the 72 bp repeat is cis-acting : it required covalent linkage to the recombinant. We discuss the possibility that the 72 bp repeat region in SV40 may act as a bi-directional entry site for RNA polymerase B such that promoter sequences linked to the repeat are more efficiently utilised.

Lrp5 controls bone formation by inhibiting serotonin synthesis in the duodenum

Loss- and gain-of-function mutations in the broadly expressed gene Lrp5 affect bone formation, causing osteoporosis and high bone mass, respectively. Although Lrp5 is viewed as a Wnt coreceptor, osteoblast-specific disruption of beta-Catenin does not affect bone formation. Instead, we show here that Lrp5 inhibits expression of Tph1, the rate-limiting biosynthetic enzyme for serotonin in enterochromaffin cells of the duodenum. Accordingly, decreasing serotonin blood levels normalizes bone formation and bone mass in Lrp5-deficient mice, and gut- but not osteoblast-specific Lrp5 inactivation decreases bone formation in a beta-Catenin-independent manner. Moreover, gut-specific activation of Lrp5, or inactivation of Tph1, increases bone mass and prevents ovariectomy-induced bone loss. Serotonin acts on osteoblasts through the Htr1b receptor and CREB to inhibit their proliferation. By identifying duodenum-derived serotonin as a hormone inhibiting bone formation in an Lrp5-dependent manner, this study broadens our understanding of bone remodeling and suggests potential therapies to increase bone mass.

Serotonin receptor 1A knockout: an animal model of anxiety-related disorder

To investigate the contribution of individual serotonin (5-hydroxytryptamine; 5-HT) receptors to mood control, we have used homologous recombination to generate mice lacking specific serotonergic receptor subtypes. In the present report, we demonstrate that mice without 5-HT1A receptors display decreased exploratory activity and increased fear of aversive environments (open or elevated spaces). 5-HT1A knockout mice also exhibited a decreased immobility in the forced swim test, an effect commonly associated with antidepressant treatment. Although 5-HT1A receptors are involved in controlling the activity of serotonergic neurons, 5-HT1A knockout mice had normal levels of 5-HT and 5-hydroxyindoleacetic acid, possibly because of an up-regulation of 5-HT1B autoreceptors. Heterozygote 5-HT1A mutants expressed approximately one-half of wild-type receptor density and displayed intermediate phenotypes in most behavioral tests. These results demonstrate that 5-HT1A receptors are involved in the modulation of exploratory and fear-related behaviors and suggest that reductions in 5-HT1A receptor density due to genetic defects or environmental stressors might result in heightened anxiety.

Human Adult Neurogenesis: Evidence and Remaining Questions

Renewed discussion about whether or not adult neurogenesis exists in the human hippocampus, and the nature and strength of the supporting evidence, has been reignited by two prominently published reports with opposite conclusions. Here, we summarize the state of the field and argue that there is currently no reason to abandon the idea that adult-generated neurons make important functional contributions to neural plasticity and cognition across the human lifespan.

Enhanced aggressive behavior in mice lacking 5-HT1B receptor

The neuromodulator serotonin (5-hydroxytryptamine, 5-HT) has been associated with mood disorders such as depression, anxiety, and impulsive violence. To define the contribution of 5-HT receptor subtypes to behavior, mutant mice lacking the 5-HT1B receptor were generated by homologous recombination. These mice did not exhibit any obvious developmental or behavioral defects. However, the hyperlocomotor effect of the 5-HT1A/1B agonist RU24969 was absent in mutant mice, indicating that this effect is mediated by 5-HT1B receptors. Moreover, when confronted with an intruder, mutant mice attacked the intruder faster and more intensely than did wild-type mice, suggesting the participation of 5-HT1B receptors in aggressive behavior.

Effects of chronic fluoxetine in animal models of anxiety and depression

The onset of the therapeutic response to antidepressant treatment exhibits a characteristic delay. Animal models sensitive to chronic, but not acute, antidepressant treatment are greatly needed for studying antidepressant mechanisms. We initially assessed four inbred mouse strains for their behavioral response to chronic treatment with the selective-serotonin reuptake inhibitor fluoxetine (0, 5, 10 mg/kg/day in drinking water), which is used for the treatment of mood and anxiety disorders. Only the highly anxious BALB/c strain exhibited sensitivity to fluoxetine in the forced swim test. Additionally, fluoxetine reduced locomotion in C57BL/6 and 129SvEv, but not BALB/c and DBA/2, strains. We then evaluated the effects of subchronic (approximately 4 days) and chronic (approximately 24 days) fluoxetine treatment (0, 10, 18, 25 mg/kg/day) on measures of anxiety and depression in BALB/c mice. Anxiety measures were obtained using the open field and novelty-induced hypophagia tests. Antidepressant effects were evaluated using the forced swim test. We found 18 mg/kg/day of chronic fluoxetine to be active in all three paradigms; subchronic treatment had no effect. Anxiety-related measures were reduced by 18 mg/kg/day. In the forced swim test, 10 and 18 mg/kg/day increased swimming and reduced immobility. Here we present several novel effects of chronic, but not subchronic, antidepressant treatment.

Resistance of alpha -synuclein null mice to the parkinsonian neurotoxin MPTP

Parkinson's disease (PD) is most commonly a sporadic illness, and is characterized by degeneration of substantia nigra dopamine (DA) neurons and abnormal cytoplasmic aggregates of alpha-synuclein. Rarely, PD may be caused by missense mutations in alpha-synuclein. MPTP, a neurotoxin that inhibits mitochondrial complex I, is a prototype for an environmental cause of PD because it produces a pattern of DA neurodegeneration that closely resembles the neuropathology of PD. Here we show that alpha-synuclein null mice display striking resistance to MPTP-induced degeneration of DA neurons and DA release, and this resistance appears to result from an inability of the toxin to inhibit complex I. Contrary to predictions from in vitro data, this resistance is not due to abnormalities of the DA transporter, which appears to function normally in alpha-synuclein null mice. Our results suggest that some genetic and environmental factors that increase susceptibility to PD may interact with a common molecular pathway, and represent the first demonstration that normal alpha-synuclein function may be important to DA neuron viability.

Adenovirus-2 E1A products repress enhancer-induced stimulation of transcription

The adenovirus-2 early region 1A (E1A) products repress activation of transcription induced by the simian virus 40, polyoma virus and adenovirus-2 E1A enhancers. The repression probably involves an interaction between the enhancer elements and a trans-acting factor(s), possibly the E1A products themselves.

Efficient and accurate extraction of in vivo calcium signals from microendoscopic video data

In vivo calcium imaging through microendoscopic lenses enables imaging of previously inaccessible neuronal populations deep within the brains of freely moving animals. However, it is computationally challenging to extract single-neuronal activity from microendoscopic data, because of the very large background fluctuations and high spatial overlaps intrinsic to this recording modality. Here, we describe a new constrained matrix factorization approach to accurately separate the background and then demix and denoise the neuronal signals of interest. We compared the proposed method against previous independent components analysis and constrained nonnegative matrix factorization approaches. On both simulated and experimental data recorded from mice, our method substantially improved the quality of extracted cellular signals and detected more well-isolated neural signals, especially in noisy data regimes. These advances can in turn significantly enhance the statistical power of downstream analyses, and ultimately improve scientific conclusions derived from microendoscopic data.

Hyperactivity and impaired response habituation in hyperdopaminergic mice

Abnormal dopaminergic transmission is implicated in schizophrenia, attention deficit hyperactivity disorder, and drug addiction. In an attempt to model aspects of these disorders, we have generated hyperdopaminergic mutant mice by reducing expression of the dopamine transporter (DAT) to 10% of wild-type levels (DAT knockdown). Fast-scan cyclic voltammetry and in vivo microdialysis revealed that released dopamine was cleared at a slow rate in knockdown mice, which resulted in a higher extracellular dopamine concentration. Unlike the DAT knockout mice, the DAT knockdown mice do not display a growth retardation phenotype. They have normal home cage activity but display hyperactivity and impaired response habituation in novel environments. In addition, we show that both the indirect dopamine receptor agonist amphetamine and the direct agonists apomorphine and quinpirole inhibit locomotor activity in the DAT knockdown mice, leading to the hypothesis that a shift in the balance between dopamine auto and heteroreceptor function may contribute to the therapeutic effect of psychostimulants in attention deficit hyperactivity disorder.

Antidepressants recruit new neurons to improve stress response regulation

Recent research suggests an involvement of hippocampal neurogenesis in behavioral effects of antidepressants. However, the precise mechanisms through which newborn granule neurons might influence the antidepressant response remain elusive. Here, we demonstrate that unpredictable chronic mild stress in mice not only reduces hippocampal neurogenesis, but also dampens the relationship between hippocampus and the main stress hormone system, the hypothalamo-pituitary-adrenal (HPA) axis. Moreover, this relationship is restored by treatment with the antidepressant fluoxetine, in a neurogenesis-dependent manner. Specifically, chronic stress severely impairs HPA axis activity, the ability of hippocampus to modulate downstream brain areas involved in the stress response, the sensitivity of the hippocampal granule cell network to novelty/glucocorticoid effects and the hippocampus-dependent negative feedback of the HPA axis. Remarkably, we revealed that, although ablation of hippocampal neurogenesis alone does not impair HPA axis activity, the ability of fluoxetine to restore hippocampal regulation of the HPA axis under chronic stress conditions, occurs only in the presence of an intact neurogenic niche. These findings provide a mechanistic framework for understanding how adult-generated new neurons influence the response to antidepressants. We suggest that newly generated neurons may facilitate stress integration and that, during chronic stress or depression, enhancing neurogenesis enables a dysfunctional hippocampus to restore the central control on stress response systems, then allowing recovery.

Antidepressant-induced neurogenesis in the hippocampus of adult nonhuman primates

New neurons are generated in the adult hippocampus of many species including rodents, monkeys, and humans. Conditions associated with major depression, such as social stress, suppress hippocampal neurogenesis in rodents and primates. In contrast, all classes of antidepressants stimulate neuronal generation, and the behavioral effects of these medications are abolished when neurogenesis is blocked. These findings generated the hypothesis that induction of neurogenesis is a necessary component in the mechanism of action of antidepressant treatments. To date, the effects of antidepressants on newborn neurons have been reported only in rodents and tree shrews. This study examines whether neurogenesis is increased in nonhuman primates after antidepressant treatment. Adult monkeys received repeated electroconvulsive shock (ECS), which is the animal analog of electroconvulsive therapy (ECT), the most effective short-term antidepressant. Compared with control conditions, ECS robustly increased precursor cell proliferation in the subgranular zone (SGZ) of the dentate gyrus in the monkey hippocampus. A majority of these precursors differentiated into neurons or endothelial cells, while a few matured into glial cells. The ECS-mediated induction of cell proliferation and neurogenesis was accompanied by increased immunoreactivity for the neuroprotective gene product BCL2 (B cell chronic lymphocytic lymphoma 2) in the SGZ. The ECS interventions were not accompanied by increased hippocampal cell death or injury. This study demonstrates that ECS is capable of inducing neurogenesis in the nonhuman primate hippocampus and supports the possibility that antidepressant interventions produce similar alterations in the human brain.

Recent advances in animal models of chronic antidepressant effects: the novelty-induced hypophagia test

Animal models exhibiting sensitivity to chronic, but not acute, antidepressant treatment are greatly needed for studying the neural mechanisms of the antidepressant response. Although several models of acute antidepressant effects provide excellent tools for antidepressant discovery, they do not permit investigation into their therapeutic effects, which require several weeks of treatment to emerge. The inhibition of feeding produced by novelty, termed 'hyponeophagia', provides an anxiety-related measure that is sensitive to the effects of chronic, but not acute or subchronic, antidepressant treatment. This review evaluates the value of hyponeophagia-based tests as tools for investigating the neurobiology of the therapeutic response to antidepressant treatment. Criteria for the development and validation of animal models used to study neurobiological mechanisms of the antidepressant response are presented. Methodological considerations affecting the reliability, specificity, and ease of use of hyponeophagia-based models are also discussed. Lastly, we present a newly revised hyponeophagia paradigm, called the novelty-induced hypophagia (NIH) test, which attempts to maximize the predictive validity and practicality of the test. The NIH paradigm provides a promising new model for investigations into the neurobiology underlying the antidepressant response.

Treatment resistant depression: A multi-scale, systems biology approach

An estimated 50% of depressed patients are inadequately treated by available interventions. Even with an eventual recovery, many patients require a trial and error approach, as there are no reliable guidelines to match patients to optimal treatments and many patients develop treatment resistance over time. This situation derives from the heterogeneity of depression and the lack of biomarkers for stratification by distinct depression subtypes. There is thus a dire need for novel therapies. To address these known challenges, we propose a multi-scale framework for fundamental research on depression, aimed at identifying the brain circuits that are dysfunctional in several animal models of depression as well the changes in gene expression that are associated with these models. When combined with human genetic and imaging studies, our preclinical studies are starting to identify candidate circuits and molecules that are altered both in models of disease and in patient populations. Targeting these circuits and mechanisms can lead to novel generations of antidepressants tailored to specific patient populations with distinctive types of molecular and circuit dysfunction.

The mouse 5-hydroxytryptamine1B receptor is localized predominantly on axon terminals

The 5-hydroxytryptamine1B receptor is a serotonin receptor subtype which is expressed predominantly in the basal ganglia. It has been suggested to play a role in movement and appetite control as well as in certain pathological states such as migraine. The recent cloning of the 5-hydroxytryptamine1B gene as well as the discovery of a radioligand that labels in rodents 5-hydroxytryptamine1B and possibly 5-hydroxytryptamine1D alpha receptors (S-CM-G[125I]TNH2) allowed us to compare the distribution of the messenger RNA and of the protein in mouse brain sections. A high 5-hydroxytryptamine1B messenger RNA level is found in the caudate-putamen in medium spiny neurons that project to the globus pallidus and the substantia nigra. In contrast, no messenger RNA is expressed in the globus pallidus and substantia nigra although these structures reveal the highest level of 5-hydroxytryptamine1B binding sites. In the hippocampus, 5-hydroxytryptamine1B messenger RNA is localized in the cell bodies of pyramidal cells of the CA1 field while the protein is found predominantly in the dorsal subiculum, a projection zone for the CA1 pyramidal neurons. In the cerebellum, 5-hydroxytryptamine1B messenger RNA is expressed in the Purkinje cells, which display no receptor binding sites. Conversely, moderate binding is found in the deep nuclei of the cerebellum, the main projection zone of the Purkinje cells. 5-Hydroxytryptamine1B sites are also detected in the superficial gray layer of the superior colliculus and the lateral geniculate nucleus, brain regions containing the terminals of retinal ganglion cells. The soma of these ganglion cells express high levels of 5-hydroxytryptamine1B messenger RNA while no 5-hydroxytryptamine1B binding sites were found in the retina. This study demonstrates that the main brain regions, expressing 5-hydroxytrypamine1B messenger RNA contain low densities of 5-hydroxytryptamine1B binding sites. Conversely, the major projection areas of these anatomical structures do not express detectable levels of 5-hydroxytryptamine1B messenger RNA, but present a high density of binding sites. In addition, our data suggest that the distribution of the 5-hydroxytryptamine1D alpha binding sites is different from that of the 5-hydroxytryptamine1D alpha messenger RNA. These results together with previous lesion studies, indicate that the 5-hydroxytryptamine1B and possibly the 5-hydroxytryptamine1D alpha receptors are localized predominantly on axon terminals, while their expression is low or absent at the somatodendritic level. The 5-hydroxytryptamine1D alpha proteins might therefore contain an addressing signal allowing their transport toward nerve endings.(ABSTRACT TRUNCATED AT 400 WORDS)

Elevated alcohol consumption in null mutant mice lacking 5-HT1B serotonin receptors

Substantial evidence links alcohol drinking and serotonin (5-HT) functioning in animals. Lowered central 5-HT neurotransmission has been found in a subgroup of alcoholics, possibly those with more aggressive, assaultive tendencies. Several rodent studies have also suggested that intact 5-HT systems are important determinants of sensitivity and/or tolerance to ethanol-induced ataxia and hypothermia. Null mutant mice lacking the 5-HT1B receptor gene (5-HT1B-/-) have been developed that display enhanced aggression and altered 5-HT release in slice preparations from some, but not all, brain areas. We characterized these mice for sensitivity to several effects of ethanol. Mutant mice drank twice as much ethanol as wild-type mice, and voluntarily ingested solutions containing up to 20% ethanol in water. Their intake of food and water, and of sucrose, saccharin and quinine solutions, was normal. Mutants were less sensitive than wild-types on a test of ethanol-induced ataxia and, with repeated drug administration, tended to develop tolerance more slowly. In tests of ethanol withdrawal and metabolism, mutants and wild-type mice showed equivalent responses. Our results suggest that the 5-HT1B receptor participates in the regulation of ethanol drinking, and demonstrate that serotonergic manipulations lead to reduced responsiveness to certain ataxic effects of ethanol without affecting dependence.

Shared Synaptic Pathophysiology in Syndromic and Nonsyndromic Rodent Models of Autism

The genetic heterogeneity of autism poses a major challenge for identifying mechanism-based treatments. A number of rare mutations are associated with autism, and it is unclear whether these result in common neuronal alterations. Monogenic syndromes, such as fragile X, include autism as one of their multifaceted symptoms and have revealed specific defects in synaptic plasticity. We discovered an unexpected convergence of synaptic pathophysiology in a nonsyndromic form of autism with those in fragile X syndrome. Neuroligin-3 knockout mice (a model for nonsyndromic autism) exhibited disrupted heterosynaptic competition and perturbed metabotropic glutamate receptor–dependent synaptic plasticity, a hallmark of fragile X. These phenotypes could be rescued by reexpression of neuroligin-3 in juvenile mice, highlighting the possibility of reverting neuronal circuit alterations in autism after the completion of development.

Increased vulnerability to cocaine in mice lacking the serotonin-1B receptor

There is increasing evidence that genetic factors can influence individual differences in vulnerability to drugs of abuse. Serotonin (5-hydroxytryptamine, 5-HT), acting through many receptors can modulate the activity of neural reward pathways and thus the effects of various drugs of abuse. Here we examine the effects of cocaine in mice lacking one of the serotonin-receptor subtypes, the 5-HT1B receptor. We show that mice lacking 5-HT1B display increased locomotor responses to cocaine and that they are more motivated to self-administer cocaine. We propose that even drug-naive 5-HT1B-knockout mice are in a behavioural and biochemical state that resembles that of wild-type mice sensitized to cocaine by repeated exposure to the drug. This altered state might be responsible for their increased vulnerability to cocaine.

Dysregulation of adult hippocampal neuroplasticity in major depression: pathogenesis and therapeutic implications

Major depressive disorder (MDD) was previously hypothesized to be a disease of monoamine deficiency in which low levels of monoamines in the synaptic cleft were believed to underlie depressive symptoms. More recently, however, there has been a paradigm shift toward a neuroplasticity hypothesis of depression in which downstream effects of antidepressants, such as increased neurogenesis, contribute to improvements in cognition and mood. This review takes a top-down approach to assess how changes in behavior and hippocampal-dependent circuits may be attributed to abnormalities at the molecular, structural, and synaptic level. We conclude with a discussion of how antidepressant treatments share a common effect in modulating neuroplasticity and consider outstanding questions and future perspectives.

Dissecting the role of the serotonin system in neuropsychiatric disorders using knockout mice

RATIONALE

The serotonin system has an important role in the modulation of several processes relevant to psychiatry such as anxiety, affect, aggression, and drug abuse. This review summarizes the recent progress in elucidating the function of the serotonergic system using knockout mice. This review while not exhaustive, highlights recent findings of relevance to psychopharmacology.

OBJECTIVES

To familiarize the reader with the technique and the findings from serotonergic knockout mice.

METHODS

Information included in this review was drawn from our own experience in this field and relevant publications from other investigators.

RESULTS

We have focused on three main themes that have emerged from studies with mice bearing single-gene mutations of serotonergic genes: anxiety, aggression, and drug abuse. Mice lacking the 5-HT1A have been found to be more anxious in several behavioral paradigms. Elevated levels of aggression have been reported in mice lacking the monoamine oxidase A and the 5-HT1B receptor genes. The mice lacking the 5-HT1B receptor have also been reported to exhibit an increased vulnerability to cocaine. The molecular basis of this enhanced vulnerability has been linked to compensatory changes in the nucleus accumbens. These results and their correlation with pharmacological studies will be discussed.

CONCLUSION

Mice lacking key components of the serotonin system have provided us with important animal models of genetic vulnerability to conditions such as anxiety disorders, aggression, and drug abuse. Ongoing research with these mice may help elucidate the mechanistic functioning of this complex system.

Repression of the immunoglobulin heavy chain enhancer by the adenovirus-2 E1A products

The products of the adenovirus-2 (Ad2) immortalizing oncogene E1A repress the activity of the SV40, polyoma virus and E1A enhancers. Evidence is presented that Ad2 infection of MPC11 plasmocytoma cells results in an inhibition of transcription of both the gamma 2b heavy chain (IgH) and the kappa light chain immunoglobulin genes. This inhibition is caused by the Ad2 E1A products. Furthermore, the Ad2 E1A products repress transcription activated by the immunoglobulin heavy chain enhancer in chimeric recombinants, which are either stably integrated in the genome of lymphoid cells or are present as episomes. The implications of negative regulation of cellular enhancers are discussed.

Serotonin receptors and animal models of aggressive behavior

Various models of rodent agonistic behavior are described, which differentiate between offensive and defensive/flight models. Particular attention is given to one male and one female paradigm for offensive aggression, i.e. resident/intruder or territorial aggression (RI) and maternal aggression (MA). After an overview of the serotonin (5-HT) system in the CNS, a description is given of the ligands available. Subsequently, the effects of various drugs affecting serotonergic transmission in the RI- and MA-paradigms are described. The 5-HT1A receptor agonists busipirone, ipsapirone, and 8-OH-DPAT decreased aggression in RI and MA, but simultaneously led to a marked decrease in social interest and activity, indicative of a nonspecific antiaggressive profile. Nonselective 5-HT1 receptor agonists, such as RU24969, eltoprazine, and TFMPP reduced aggression quite specifically, did not decrease social interest or exploration, and sometimes even increased these behaviors. In RI and MA, the behavioral effects of these drugs were roughly similar. In contrast, MA was more sensitive to treatment with the 5-HT reuptake blocker fluvoxamine, which blocked RI aggression nonspecifically at the highest dose only. DOI, a 5-HT2A/2C# receptor agonist, decreased aggressive behavior and increased inactivity, without affecting social interest and exploration in RI as well as MA. This was, however, accompanied by "wet dog shaking" characteristic of 5-HT2 receptor stimulation. The nonspecific 5-HT receptor agonist (and 5-HT2 receptor antagonist) quipazine also induced "wet dog shaking" at doses which suppressed aggression, social interest, and exploration but increased inactive behaviors (sitting and lying). The discussion delineates a specific role for 5-HT1B receptor-subtype involvement in the modulation of aggression, with the restrictions we clearly face with regard to the lack of specific serotonergic agonists and antagonists for certain receptor subtypes. By and large, male and female rats react similarly to treatment with serotonergic drugs, and this fact underlines the consistent role of 5-HT in different forms of aggression.