Brain serotonin metabolism in isolated aggressive mice

GABABR1 in DRN mediated GABA to regulate 5-HT expression in multiple brain regions in male rats with high and low aggressive behavior

The identity of the mechanism that controls aggressive behavior in rodents is unclear. Serotonin (5-HT) and GABA are associated with aggressive behavior in rodents. However, the regulatory relationship between these chemicals in the different brain regions of rats has not been fully defined. This study aimed to clarify the role of GABABR1 in DRN-mediated GABA to regulate 5-HT expression in multiple brain regions in male rats with high and low aggressive behavior. Rat models of highly and less aggressive behavior were established through social isolation plus resident intruder. On this basis, GABA content in the DRN and 5-HT contents in the PFC, hypothalamus, hippocampus and DRN were detected using ELISA. Co-expression of 5-HT and GB1 in the DRN was detected by immunofluorescence and immunoelectron microscopy at the tissue and subcellular levels, respectively. GB1-specific agonist baclofen and GB1-specific inhibitor CGP35348 were injected into the DRN by stereotaxic injection. Changes in 5-HT levels in the PFC, hypothalamus and hippocampus were detected afterward. After modeling, rats with highly aggressive behavior exhibited higher aggressive behavior scores, shorter latencies of aggression, and higher total distances in the open field test than rats with less aggressive behavior. The contents of 5-HT in the PFC, hypothalamus and hippocampus of rats with high and low aggressive behavior (no difference between the two groups) were significantly decreased, but the change in GABA content in the DRN was the opposite. GB1 granules could be found on synaptic membranes containing 5-HT granules, which indicated that 5-HT neurons in the DRN co-expressed with GB1, which also occurred in double immunofluorescence results. At the same time, we found that the expression of GB1 in the DRN of rats with high and low aggressive behavior was significantly increased, and the expression of GB1 in the DRN of rats with low aggressive behavior was significantly higher than that in rats with high aggressive behavior. Nevertheless, the expression of 5-HT in DRN was opposite in these two groups. After microinjection of baclofen into the DRN, the 5-HT contents in the PFC, hypothalamus and hippocampus of rats in each group decreased significantly. In contrast, the 5-HT contents in the PFC, hypothalamus and hippocampus of rats in each group increased significantly after injection with CGP35348. The significant increase in GABA in the DRN combined with the significant increase in GB1 in the DRN further mediated the synaptic inhibition effect, which reduced the 5-HT level of 5-HT neurons in the DRN, resulting in a significant decrease in 5-HT levels in the PFC, hypothalamus and hippocampus. Therefore, GB1-mediated GABA regulation of 5-HT levels in the PFC, hypothalamus and hippocampus is one of the mechanisms of highly and less aggressive behavior originating in the DRN. The increased GB1 level in the DRN of LA-behavior rats exhibited a greater degree of change than in the HA-group rats, which indicated that differently decreased 5-HT levels in the DRN may be the internal mechanisms of high and low aggression behaviors.

Relationship between oxytocin and serotonin and the fearfulness, dominance, and trainability of horses

Oxytocin (OXT) and serotonin (5-HT) are essential neurotransmitters associated with the behavior of animals. Recently, we found that the plasma concentration of OXT is positively correlated with horse docility and friendliness toward humans. However, the relationships between the neurotransmitters and other temperaments such as fearfulness, dominance, and trainability are unknown. This study aimed to identify whether the plasma concentration of OXT or 5-HT is correlated with fearfulness, dominance, and trainability of horses. Blood samples of 34 horses were collected at the Horse Industry Complex Center of Jeonju Kijeon College. The concentration of OXT and 5-HT was measured in the plasma samples using enzyme-linked immunosorbent assays. The fearfulness, dominance, and trainability of horses were scored by three professors who were very familiar with the horses. One-way analysis of variance with the least significant difference post-hoc analysis was used to compare the scores for fearfulness and dominance among groups. The trainability of horses was compared using the student t-test. The 5-HT was negatively correlated with dominance, but it had no relation with fearfulness. The OXT appeared to be negatively correlated with fearfulness and dominance in horses. Furthermore, OXT was positively correlated with the trainability of horses. Additionally, 5-HT appeared to enhance trainability. In conclusion, the concentration of OXT or 5-HT in horse blood plasma can be used as a biomarker to monitor the fearfulness, dominance, or trainability of horses.

Brain serotonin deficiency affects female aggression

The neurotransmitter serotonin plays a key role in the control of aggressive behaviour. While so far most studies have investigated variation in serotonin levels, a recently created tryptophan hydroxylase 2 (Tph2) knockout mouse model allows studying effects of complete brain serotonin deficiency. First studies revealed increased aggressiveness in homozygous Tph2 knockout mice in the context of a resident-intruder paradigm. Focussing on females, this study aimed to elucidate effects of serotonin deficiency on aggressive and non-aggressive social behaviours not in a test situation but a natural setting. For this purpose, female Tph2 wildtype (n = 40) and homozygous knockout mice (n = 40) were housed with a same-sex conspecific of either the same or the other genotype in large terraria. The main findings were: knockout females displayed untypically high levels of aggressive behaviour even after several days of co-housing. Notably, in response to aggressive knockout partners, they showed increased levels of defensive behaviours. While most studies on aggression in rodents have focussed on males, this study suggests a significant involvement of serotonin also in the control of female aggression. Future research will show, whether the observed behavioural effects are directly caused by the lack of serotonin or by potential compensatory mechanisms.

Searching for neural and behavioral parameters that predict anti-aggressive effects of chronic SSRI treatment in rats

RATIONALE

Only a subset of impulsive aggressive patients benefits from selective serotonin reuptake inhibitor (SSRI) treatment, confirming contradictory results about the association between serotonin (5-hydroxytryptamine, 5-HT) and aggression. This shows the need to define behavioral characteristics within this subgroup to move towards individualized pharmacological treatment of impulsive aggression.

METHODS

Here we submitted an outbred strain of Long Evans rats to a crossover design treatment regimen with the SSRI citalopram, to test its anti-aggressive effect. Behavioral characteristics were baseline aggression, anxiety parameters as measured in the elevated plus maze and open field and cue responsivity as indicated by sign vs. goal tracking behavior. 5-HT_1A receptor densities as measured by ex vivo [¹⁸F]MPPF binding were determined in the dorsal raphe nucleus, dentate gyrus, orbitofrontal cortex, infralimbic cortex and prelimbic cortex, because of the receptors' involvement in the therapeutic delay of SSRIs and aggression.

RESULTS

We found statistically significant increased variance in aggressive behavior after citalopram treatment. However, none of the selected parameters predicted the citalopram treatment effect.

CONCLUSION

Since aggression after citalopram treatment decreased in a subgroup of animals and increased in the other, future research should focus on other possible predictors to support treatment strategies in aggressive patients.

Alcohol and violence: neuropeptidergic modulation of monoamine systems

Neurobiological processes underlying the epidemiologically established link between alcohol and several types of social, aggressive, and violent behavior remain poorly understood. Acute low doses of alcohol, as well as withdrawal from long-term alcohol use, may lead to escalated aggressive behavior in a subset of individuals. An urgent task will be to disentangle the host of interacting genetic and environmental risk factors in individuals who are predisposed to engage in escalated aggressive behavior. The modulation of 5-hydroxytryptamine impulse flow by gamma-aminobutyric acid (GABA) and glutamate, acting via distinct ionotropic and metabotropic receptor subtypes in the dorsal raphe nucleus during alcohol consumption, is of critical significance in the suppression and escalation of aggressive behavior. In anticipation and reaction to aggressive behavior, neuropeptides such as corticotropin-releasing factor, neuropeptide Y, opioid peptides, and vasopressin interact with monoamines, GABA, and glutamate to attenuate and amplify aggressive behavior in alcohol-consuming individuals. These neuromodulators represent novel molecular targets for intervention that await clinical validation. Intermittent episodes of brief social defeat during aggressive confrontations are sufficient to cause long-lasting neuroadaptations that can lead to the escalation of alcohol consumption.

Serotonin: a never-ending story

The neurotransmitter serotonin is an evolutionary ancient molecule that has remarkable modulatory effects in almost all central nervous system integrative functions, such as mood, anxiety, stress, aggression, feeding, cognition and sexual behavior. After giving a short outline of the serotonergic system (anatomy, receptors, transporter) the author's contributions over the last 40 years in the role of serotonin in depression, aggression, anxiety, stress and sexual behavior is outlined. Each area delineates the work performed on animal model development, drug discovery and development. Most of the research work described has started from an industrial perspective, aimed at developing animals models for psychiatric diseases and leading to putative new innovative psychotropic drugs, like in the cases of the SSRI fluvoxamine, the serenic eltoprazine and the anxiolytic flesinoxan. Later this research work mainly focused on developing translational animal models for psychiatric diseases and implicating them in the search for mechanisms involved in normal and diseased brains and finding new concepts for appropriate drugs.

Nitric oxide and serotonin interactions in aggression

Violence is a significant public health problem worldwide. Neurobiological research on violence and aggression attempts to elucidate the cellular and molecular pathways that increase the propensity toward this behavior. Research over the past 40 years has implicated several brain regions and neurotransmitters in aggression, mainly using rodent models. Perhaps the strongest association is the link between serotonin and aggression, which has compelling interactions with the nitric oxide system. Recently, new insights into these relationships have been added as modern techniques allow more sophisticated analyses. This chapter will discuss current developments implicating serotonin and nitric oxide in aggressive behavior. Recently developed high-resolution methods for examining the neurobiological basis of aggression will be considered, with emphasis on future directions for the field.

Infralimbic and dorsal raphé microinjection of the 5-HT(1B) receptor agonist CP-93,129: attenuation of aggressive behavior in CFW male mice

RATIONALE

Aggressive behavior and impaired impulse control have been associated with dysregulations in the serotonergic system and with impaired functioning of the prefrontal cortex. 5-HT(1B) receptors have been shown to specifically modulate several types of offensive aggression.

OBJECTIVE

This study aims to characterize the relative importance of two populations of 5-HT(1B) receptors in the dorsal raphé nucleus (DRN) and infralimbic cortex (ILC) in the modulation of aggressive behavior.

METHODS

Male CFW mice were conditioned on a fixed-ratio 5 schedule of reinforcement to self-administer a 6% (w/v) alcohol solution. Mice repeatedly engaged in 5-min aggressive confrontations until aggressive behavior stabilized. Next, a cannula was implanted into either the DRN or the ILC. After recovery, mice were tested for aggression after self-administration of either 1.0 g/kg alcohol or water prior to a microinjection of the 5-HT(1B) agonist, CP-93,129 (0-1.0 μg/infusion).

RESULTS

In both the DRN and ILC, CP-93,129 reduced aggressive behaviors after both water and alcohol self-administration. Intra-raphé CP-93,129 dose-dependently reduced both aggressive and locomotor behaviors. However, the anti-aggressive effects of intra-cortical CP-93,129 were behaviorally specific.

CONCLUSIONS

These findings highlight the importance of the serotonergic system in the modulation of aggression and suggest that the behaviorally specific effects of 5-HT(1B) receptor agonists are regionally selective. 5-HT(1B) receptors in a medial subregion of the prefrontal cortex, the ILC, appear to be critically involved in the attenuation of species-typical levels of aggression.

Pattern of forebrain activation in high novelty-seeking rats following aggressive encounter

We have previously demonstrated that selectively-bred High (bHR) and Low (bLR) novelty-seeking rats exhibit agonistic differences, with bHRs acting in a highly aggressive manner when facing homecage intrusion. In order to discover the specific neuronal pathways responsible for bHRs' high levels of aggression, the present study compared c-fos mRNA expression in several forebrain regions of bHR/bLR males following this experience. bHR/bLR males were housed with female rats for 2 weeks, and then the females were replaced with a male intruder for 10 min. bHR/bLR residents were subsequently sacrificed by rapid decapitation, and their brains were removed and processed for c-fos in situ hybridization. Intrusion elicited robust c-fos mRNA expression in both phenotypes throughout the forebrain, including the septum, amygdala, hippocampus, cingulate cortex, and the hypothalamus. However, bHRs and bLRs exhibited distinct activation patterns in select areas. Compared to bHR rats, bLRs expressed greater c-fos in the lateral septum and within multiple hypothalamic nuclei, while bHRs showed greater activation in the arcuate hypothalamic nucleus and in the hippocampus. No bHR/bLR differences in c-fos expression were detected in the amygdala, cortical regions, and striatum. We also found divergent 5-HT1A receptor mRNA expression within some of these same areas, with bLRs having greater 5-HT1A, but not 5-HT1B, receptor mRNA levels in the septum, hippocampus and cingulate cortex. These findings, together with our earlier work, suggest that bHRs exhibit altered serotonergic functioning within select circuits during an aggressive encounter.

Social instigation and aggression in postpartum female rats: role of 5-Ht1A and 5-Ht1B receptors in the dorsal raphé nucleus and prefrontal cortex

RATIONALE

5-HT(1A) and 5-HT(1B) receptor agonists effectively reduce aggressive behavior in males that has been escalated by social instigation. Important sites of action for these drugs are the receptors in dorsal raphé nuclei (DRN) and the ventral-orbital prefrontal cortex (VO PFC). DRN and VO PFC areas are particularly relevant in the inhibitory control of escalated aggressive and impulsive behavior.

OBJECTIVES

The objectives of this study are to assess the anti-aggressive effects of 5-HT(1A) (8-OH-DPAT) and 5-HT(1B) (CP-93,129) receptor agonists microinjected into DRN and VO PFC, respectively, and to study the aggressive behavior in postpartum female Wistar rats using the social instigation protocol to increase aggression.

METHODS AND RESULTS

8-OH-DPAT (0.56 μg) in the DRN increased aggressive behavior in postpartum female rats. By contrast, CP-93,129 (1.0 μg) microinjected into VO PFC decreased the number of attack bites and lateral threats. 5-HT(1A) and 5-HT(1B) receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming and the latter increasing these acts. When 8-OH-DPAT was microinjected into DRN and CP-93,129 was microinjected into VO PFC in female rats at the same time, maternal aggression decreased. Specific participation of 5-HT(1B) receptors was verified by reversal of the anti-aggressive effects using the selective antagonist SB-224,289 (1.0 μg).

CONCLUSIONS

The decrease in maternal aggressive behavior after microinjections of 5-HT(1B) receptor agonists into the VO PFC and DRN of female postpartum rats that were instigated socially supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner, due to activation of 5-HT(1B) receptors at the soma and terminals.

Variation in the gene encoding the serotonin transporter is associated with a measure of sociopathy in alcoholics

The present study examined the association between a measure of sociopathy and 5-HTTLPR genotype in a sample of individuals from Project MATCH, a multi-center alcohol treatment trial. 5-HTTLPR, an insertion-deletion polymorphism in SLC6A4, the gene encoding the serotonin transporter protein, results in functionally distinct long (L) and short (S) alleles. The S allele has been associated with a variety of psychiatric disorders and symptoms including alcohol dependence, but it is unknown whether 5-HTTLPR increases the risk for co-morbid sociopathy among those with alcohol dependence. Eight hundred sixty-two subjects diagnosed with alcohol dependence completed the California Psychological Inventory, a psychological assessment that includes a measure of socialization, which was used as a proxy measure of sociopathy. Subjects were genotyped for the insertion-deletion polymorphism, as well as a single nucleotide polymorphism (A→G) that is located in the inserted region. Regression analysis revealed that after controlling for age, which was negatively related to socialization score, 5-HTTLPR genotype interacted with sex to determine socialization score (P < 0.001). Males with the L'L' genotype (i.e. those homozygous for the L(A) allele) had lower socialization scores (i.e. greater sociopathy) than males who were carriers of the S' allele (P = 0.03). In contrast, women with the S'S' genotype had lower socialization scores than women with two L' alleles (P = 0.002) and tended to have lower Socialization Index of the California Psychological Inventory scores than women with one copy of the L' allele (P = 0.07). Among individuals with alcohol use disorders, the tri-allelic 5-HTTLPR polymorphism had opposite effects on socialization scores in men than women. The basis for this finding is unknown, but it may have implications for sub-typing alcoholics.

Differences in aggressive behavior and in the mesocorticolimbic DA system between A/J and BALB/cJ mice

The neurotransmitters DA and serotonin are known to be important modulators of aggression, but endogenous differences in these systems between aggressive and nonaggressive animals are poorly understood. To examine this issue, the mesocorticolimbic DA and serotonin systems of two mouse strains that differ in aggressive behavior, BALB/cJ and A/J, were analyzed using high performance liquid chromatography and quantitative receptor autoradiography. Significant differences in both serotonergic and dopaminergic systems were found between aggressive and nonaggressive mice. The nonaggressive A/J mice exhibited higher DA utilization in the nucleus accumbens and prefrontal cortex, higher D1 receptor expression in the rostral pole of the accumbens, and lower D2 receptor expression throughout the accumbens, as compared with aggressive BALB/cJ mice. Although correlative in nature, these data suggest that differences in mesocorticolimbic DA and serotonin systems may contribute to endogenous differences in aggression.

Escalated aggression after alcohol drinking in male mice: dorsal raphé and prefrontal cortex serotonin and 5-HT(1B) receptors

A significant minority of individuals engages in escalated levels of aggression after consuming moderate doses of alcohol (Alc). Neural modulation of escalated aggression involves altered levels of serotonin (5-HT) and the activity of 5-HT(1B) receptors. The aim of these studies was to determine whether 5-HT(1B) receptors in the dorsal raphé (DRN), orbitofrontal (OFC), and medial prefrontal (mPFC) cortex attenuate heightened aggression and regulate extracellular levels of 5-HT. Male mice were trained to self-administer Alc by performing an operant response that was reinforced with a delivery of 6% Alc. To identify Alc-heightened aggressors, each mouse was repeatedly tested for aggression after consuming either 1.0 g/kg Alc or H2O. Next, a cannula was implanted into either the DRN, OFC, or mPFC, and subsets of mice were tested for aggression after drinking either Alc or H(2)O prior to a microinjection of the 5-HT(1B) agonist, CP-94,253. Additional mice were implanted with a microdialysis probe into the mPFC, through which CP-94,253 was perfused and samples were collected for 5-HT measurement. Approximately 60% of the mice were more aggressive after drinking Alc, confirming the aggression-heightening effects of 1.0 g/kg Alc. Infusion of 1 microg CP-94,253 into the DRN reduced both aggressive and motor behaviors. However, infusion of 1 microg CP-94,253 into the mPFC, but not the OFC, after Alc drinking, increased aggressive behavior. In the mPFC, reverse microdialysis of CP-94,253 increased extracellular levels of 5-HT; levels decreased immediately after the perfusion. This 5-HT increase was attenuated in self-administering mice. These results suggest that 5-HT(1B) receptors in the mPFC may serve to selectively disinhibit aggressive behavior in mice with a history of Alc self-administration.

Serotonin and aggression

The neurotransmitter serotonin (5-HT) has been implicated in the modulation of aggression in animals and humans. A longstanding dogma that aggression and serotonergic activity are inversely related has to be abandoned in light of many new findings. Trait and state aggression are differentially regulated by the 5-HT system and different 5-HT receptors seem to be involved. Of the 14 different 5-HT receptors, the 5-HT(1B) receptor, particularly the postsynaptically located 5-HT(1B) heteroreceptor, plays a highly selective role in the modulation of offensive aggression. We are still far from understanding the complex role played by the serotonergic system in the modulation of a complex set of behaviors like aggression.

Corticotropin-releasing factor 1 and 2 receptors in the dorsal raphé differentially affect serotonin release in the nucleus accumbens

Corticotropin-releasing factor (CRF) is a neurohormone that mediates stress, anxiety, and affects serotonergic activity. Studies have shown that CRF has dose-dependent opposing effects on serotonergic activity. This effect has been hypothesized to be differentially mediated by CRF(1) and CRF(2) receptors in the dorsal raphé nucleus. We directly tested this hypothesis by using in vivo microdialysis to determine the effects of CRF and CRF receptor antagonists in the dorsal raphé nucleus on serotonin (5-HT) release in the nucleus accumbens, a brain region implicated in the neuropathology of stress-related psychiatric disorders. Male urethane-anesthetized rats were implanted with a microdialysis probe into the nucleus accumbens, and CRF (0, 100 or 500 ng) was infused into the dorsal raphé. Infusion of CRF into the dorsal raphé nucleus had dose-dependent opposite effects, with 100 ng of CRF significantly decreasing 5-HT levels in the nucleus accumbens and 500 ng CRF significantly increasing accumbal 5-HT levels. In subsequent experiments, the raphé was pre-treated with the CRF(1) receptor antagonist antalarmin (0.25 microg) or the CRF(2) receptor antagonist antisauvagine-30 (ASV-30; 2 microg) prior to CRF infusion. Antagonism of CRF(1) receptors in the dorsal raphé nucleus abolished the decrease in accumbal 5-HT levels elicited by 100 ng CRF, and CRF(2) receptor antagonism in the raphé blocked the increase in accumbal 5-HT levels elicited by 500 ng CRF. These results suggest that the opposing effects of dorsal raphé CRF on 5-HT release in the nucleus accumbens are dependent on differential activation of CRF(1) and CRF(2) receptors in the dorsal raphé nucleus.

Differential role of the 5-HT1A receptor in aggressive and non-aggressive mice: An across-strain comparison

Differential role of the 5-HT(1A) receptor in aggressive and non-aggressive mice: an across-strain comparison. PHYSIOL BEHAV 00(0) 000-000, 2006. According to the serotonin (5-HT)-deficiency hypothesis of aggression, highly aggressive individuals are characterized by low brain 5-HT neurotransmission. Key regulatory mechanisms acting on the serotonergic neuron involve the activation of the somatodendritic inhibitory 5-HT(1A) autoreceptor (short feedback loop) and/or the activation of postsynaptic 5-HT(1A) receptors expressed on neurons in cortico-limbic areas (long feedback loop). In this study, we examined whether low serotonin neurotransmission is associated with enhanced 5-HT(1A) (auto)receptor activity in highly aggressive animals. Male mice (SAL-LAL, TA-TNA, NC900-NC100) obtained through different artificial-selection breeding programs for aggression were observed in a resident-intruder test. The prefrontal cortex level of 5-HT and its metabolite 5-HIAA were determined by means of HPLC. The activity of the 5-HT(1A) receptors was assessed by means of the hypothermic response to the selective 5-HT(1A) agonists S-15535 (preferential autoreceptor agonist) and 8-OHDPAT (full pre- and postsynaptic receptor agonist). Highly aggressive mice had lower serotonin levels in the prefrontal cortex and two out of three aggressive strains had higher 5-HT(1A) (auto)receptor sensitivity. The results strengthen the validity of the serotonin-deficiency hypothesis of aggression and suggest that chronic exaggerated activity of the 5-HT(1A) receptor may be a causative link in the neural cascade of events leading to 5-HT hypofunction in aggressive individuals.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

Serotonin and aggressive behavior in rodents and nonhuman primates: predispositions and plasticity

This review analyzes psychosocial and genetic determinants of aggressive behavior in rodents and nonhuman primates and the role of the serotonin (5-HT) system on aggressive behaviors in order to trace possible evolutionary common origins between psychopathological and adaptive forms of aggression. Studies in primates suggest that deficit in serotonin activity, as indicated by the levels of the cerebrospinal fluid (CSF) serotonin major metabolite 5-hydroxyindoleacetic acid (5-HIAA) correlates with impulsive and aggressive behavior. It is possible that CSF 5-HIAA reflects the prevailing serotonergic tone and may be related to an aggressive trait. Superimposed on this tone are phasic serotonin changes that may be related to the inhibition of aggressive acts. Genetic factors determine aggressive behaviors as demonstrated by classic selection and strain comparison studies. Manipulations of genes targeting 5-HT receptors, transporters and enzymes can influence aggression. Some of these genes related to the serotonin transporter (5-HTT) and the monoamine oxidase A (MAO-A) show a polymorphism that may predispose, under specific environmental conditions, certain individuals to display pathological forms of aggression.

Severe irritability associated with statin cholesterol-lowering drugs

BACKGROUND

As use of a drug becomes widespread, the full spectrum of its effects becomes clearer. Although a link has been suggested between low or lowered cholesterol and irritability/aggression, less is known about possible links between irritability and statins.

AIM

To assess the possible connection of statin usage to severe irritability.

DESIGN

Case series.

METHODS

Six patients referred or self-referred with irritability and short temper on statin cholesterol-lowering drugs completed a survey providing information on character of behavioural effect, time-course of onset and recovery, and factors relevant to drug adverse effect causality.

RESULTS

In each case the personality disruption, once evident, was sustained until statin use was discontinued; and resolved promptly with drug cessation. In four patients, re-challenge with statins occurred, and led to recrudescence of the problem. All patients experienced other recognized statin adverse effects while on the drug. Manifestations of severe irritability included homicidal impulses, threats to others, road rage, generation of fear in family members, and damage to property.

DISCUSSION

Case series invariably raise more questions than they can answer. These case reports suggest that severe irritability may occur in some statin users. Although this adverse effect may be rare, potentially life-threatening adverse effects of drugs must be taken seriously.

Interaction of nitric oxide and serotonin in aggressive behavior

Nitric oxide (NO) modulates many behavioral and neuroendocrine responses. Genetic or pharmacological inhibition of the synthetic enzyme that produces NO in neurons evokes elevated and sustained aggression in male mice. Recently, the excessive aggressive and impulsive traits of neuronal NO synthase knockout (nNOS-/-) mice were shown to be caused by reductions in serotonin (5-HT) turnover and deficient 5-HT1A and 5-HT1B receptor function in brain regions regulating emotion. The consistently high levels of aggression observed in nNOS-/- mice could be reversed by 5-HT precursors and by treatment with specific 5-HT1A and 5-HT1B receptor agonists. The expression of the aggressive phenotype of nNOS-/- knockout mice requires isolated housing prior to testing. The effects of social factors such as housing condition and maternal care can affect 5-HT and aggression, but the interaction among extrinsic factors, 5-HT, NO, and aggression remains unspecified. Taken together, NO appears to play an important role in normal brain 5-HT function and may have significant implications for the treatment of psychiatric disorders characterized by aggressive and impulsive behaviors.

Aggressive behavior, increased accumbal dopamine, and decreased cortical serotonin in rats

Dopamine (DA) and serotonin have been implicated in the regulation of aggressive behavior, but it has remained challenging to assess the dynamic changes in these neurotransmitters while aggressive behavior is in progress. The objective of this study was to learn about ongoing monoamine activity in corticolimbic areas during aggressive confrontations in rats. Male Long-Evans rats were implanted with a microdialysis probe aimed at the nucleus accumbens (NAC) or medial prefrontal cortex (PFC); next, 10 min samples were collected before, during, and after a 10 min confrontation. Rats continued to display aggressive behavior while being sampled, and they performed two to six attack bites as well as 140 sec of aggressive acts and postures. Dopamine levels in NAC were significantly increased up to 60 min after the confrontation. Peak levels of 140% were achieved approximately 20-30 min after the confrontation. No concurrent changes in accumbal serotonin levels were seen during or after the confrontation. Dopamine and serotonin levels in PFC changed in the opposite direction, with a sustained decrease in serotonin to 80% of baseline levels during and after the confrontation and an increase in dopamine to 120% after the confrontation. The temporal pattern of monoamine changes, which followed rather than preceded the confrontation, points to a significant role of accumbal and cortical DA and 5-hydroxytryptamine in the consequences as opposed to the triggering of aggressive acts. The increase in accumbal DA in aggressive animals supports the hypothesis that this neural system is linked to the execution of biologically salient and demanding behavior.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

Psychopharmacologic treatment of pathologic aggression

Several drugs are apparently effective in treating pathologic anger and aggression. Because many of the studies on aggressive populations allowed the use of concomitant medications, it is unclear whether the efficacy of each drug in a particular population is dependent on the presence of other medications, such as antipsychotic agents. Finally, one needs to be circumspect in inferring efficacy of a particular drug in aggressive patients with neuropsychiatric conditions other than the ones in which some efficacy has been established. Lithium appears to be an effective treatment of aggression among nonepileptic prison inmates, mentally retarded and handicapped patients, and among conduct-disordered children with explosive behavior. Certainly, lithium would be the treatment of choice in bipolar patients with excessive irritability and anger outbursts, and it has been shown to be effective in this population. Anticonvulsant medications are the treatment of choice for patients with outbursts of rage and abnormal EEG findings. The efficacy of these drugs in patients without a seizure disorder, however, remains to be established, with the exception perhaps of valproate and carbamazepine. In fact, dyphenylhydantoin did not appear to be effective in treating aggressive behavior in children with temper tantrums and was found to be effective in only a prison population. There is some evidence for the efficacy of carbamazepine and valproate in treating pathologic aggression in patients with dementia, organic brain syndrome, psychosis, and personality disorders. As Yudofsky et al point out in their review of the literature, although traditional antipsychotic drugs have been used widely to treat aggression, there is little evidence for their effectiveness in treating aggression beyond their sedative effect in agitated patients or their antiaggressive effect among patients whose aggression is related to active psychosis. Antipsychotic agents appear to be effective in treating psychotic aggressive patients, conduct-disordered children, and mentally retarded patients, with only modest effects in the management of pathologic aggression in patients with dementia. Furthermore, at least in one study, these drugs were found to be associated with increased aggressiveness in mentally retarded subjects. On the other hand, atypical antipsychotic agents (i.e., clozapine, risperidone, and olanzapine) may be more effective than traditional antipsychotic drugs in aggressive and violent populations, as they have shown efficacy in patients with dementia, brain injury, mental retardation, and personality disorders. Similarly, benzodiazepines can reduce agitation and irritability in elderly and demented populations, but they also can induce behavioral disinhibition. Therefore, one should be careful in using this class of drugs in patients with pathologic aggression. Beta-blockers appear to be effective in many different neuropsychiatric conditions. These drugs seem effective in reducing violent and assaultive behavior in patients with dementia, brain injury, schizophrenia, mental retardation, and organic brain syndrome. As pointed out by Campbell et al in their review of the literature, however, systematic research is lacking, and little is known about the efficacy and safety of beta-blockers in children and adolescents with pathologic aggression. Although widely used in the management of pathologic aggression, the use of this class of drugs has been limited partially by marked hypotension and bradycardia, which are side effects common at the higher doses. The usefulness of the antihypertensive drug clonidine in the treatment of pathologic aggression has not been assessed adequately, and only marginal benefits were observed with this drug in irritable autistic and conduct disorder children. Psychostimulants seem to be effective in reducing aggressiveness in brain-injured patients as well as in violent adolescents with oppositional or conduct disorders, particu

Effect of isolation on behavioural models involving serotonergic 5-HT2 and 5-HT1A receptors

1. The effect of 7 days of isolation were observed in mice on behavioural models involving 5-HT2 and 5-HT1A receptors. 2. The sensitivity of 5-HT2 receptors as assessed through L-5-HTP or 5-MeODMT induced head-twitches was reduced. 3. The sensitivity of the 5-HT1A receptors implicated in the 8-OH-DPAT induced feeding was unchanged. 4. The sensitivity of the 5-HT1A receptors involved in the 8-OH-DPAT induced hypothermia was diminished. 5. On the whole, these results show that after 7 days of isolation, the responses to the stimulation of serotonergic receptors is unchanged or diminished according to both the receptor's subtype and the model used.

Aggression, anxiety and vocalizations in animals: GABAA and 5-HT anxiolytics

A continuing challenge for preclinical research on anxiolytic drugs is to capture the affective dimension that characterizes anxiety and aggression, either in their adaptive forms or when they become of clinical concern. Experimental protocols for the preclinical study of anxiolytic drugs typically involve the suppression of conditioned or unconditioned social and exploratory behavior (e.g., punished drinking or social interactions) and demonstrate the reversal of this behavioral suppression by drugs acting on the benzodiazepine-GABAA complex. Less frequently, aversive events engender increases in conditioned or unconditioned behavior that are reversed by anxiolytic drugs (e.g., fear-potentiated startle). More recently, putative anxiolytics which target 5-HT receptor subtypes produced effects in these traditional protocols that often are not systematic and robust. We propose ethological studies of vocal expressions in rodents and primates during social confrontations, separation from social companions, or exposure to aversive environmental events as promising sources of information on the affective features of behavior. This approach focuses on vocal and other display behavior with clear functional validity and homology. Drugs with anxiolytic effects that act on the benzodiazepine-GABAA receptor complex and on 5-HT1A receptors systematically and potently alter specific vocalizations in rodents and primates in a pharmacologically reversible manner; the specificity of these effects on vocalizations is evident due to the effectiveness of low doses that do not compromise other physiological and behavioral processes. Antagonists at the benzodiazepine receptor reverse the effects of full agonists on vocalizations, particularly when these occur in threatening, startling and distressing contexts. With the development of antagonists at 5-HT receptor subtypes, it can be anticipated that similar receptor-specificity can be established for the effects of 5-HT anxiolytics.

Increase in the isolation-induced social behavioural deficit by agonists at 5-HT1A receptors

The effect of 5-HT1A agonists was studied in the isolation-induced social behavioural deficit test. The drugs 8-OH-DPAT (0.125 mg/kg), buspirone (16 mg/kg) and ipsapirone (8 mg/kg) further increased the deficit. Unlike 8-OH-DPAT, the other two drugs may act non-specifically since they reduced spontaneous motor activity at 16 mg/kg, as measured in an activity meter. In addition, 8-OH-DPAT (0.25 mg/kg), buspirone (8 mg/kg) and ipsapirone (8 mg/kg) decreased exploratory activity in the open-field test. Since the smallest active doses were very close in the behavioural deficit and in the open-field tests, it is suggested that a common phenomenon, increased emotionality or reactivity, sustained both these reductions in activity. The increase in the behavioural deficit induced by 8-OH-DPAT, was likely to have resulted from stimulation of 5-HT1A receptors, since it was impaired by pretreatment with penbutolol, a beta-adrenergic-blocking drug, also known to bind to 5-HT1 receptors. Since it was previously shown that the behavioural deficit was reduced by agonists at 5-HT1B receptors, it is proposed that the behavioural inhibition, resulting from an isolation-induced increase in reactivity is bi-directionally modulated by serotonergic drugs, where 5-HT1A agonists increase and 5-HT1B agonists decrease this inhibition.

Paternal alcohol exposure: paradoxical effect in mice and rats

Male mice consumed liquid alcohol diets containing 25%, 10% or 0% ethanol-derived calories (EDC). Animals receiving the 10 and 0% EDC diets were pair fed to those consuming the 25% EDC diet. After 7 or 14 weeks of consumption, males were bred to non-treated females. Offspring were tested for swimming behavior at 75 days of age. Offspring sired by alcohol-consuming males were more immobile regardless of duration of paternal alcohol consumption or housing conditions (group or isolated). Imipramine (5, 15 mg/kg) reversed this effect such that after drug treatment, alcohol-sired offspring were less immobile than controls. Propranolol (1, 3 mg/kg) eliminated the group differences. Yohimbine (1, 10 mg/kg) significantly increased immobility in all groups but did not reverse the effects of paternal alcohol consumption. Metergoline (1 mg/kg) increased immobility in all groups but did not reverse the effects of paternal alcohol exposure. When rat offspring of alcohol-consuming fathers were tested, their swimming behavior was opposite to that of mice (i.e., rats were less immobile). However, as observed in mice, the effect was reversed by imipramine. These results extend the evidence for paternally mediated behavioral mutagenesis. They also indicate that the direction of induced changes are species dependent and that some of these effects, regardless of direction of change, are mediated by neurochemical changes sensitive to imipramine.

Interaction of dietary tryptophan and social isolation on territorial aggression, motor activity, and neurochemistry in mice

This study examined the interaction of dietary tryptophan (TRP) and differential housing on territorial-induced aggression, locomotor activity, and monoamine neurochemistry in mice. Groups of male CF-1 mice were singly-housed or group-housed and administered a semisynthetic basal diet supplemented with TRP (0.25-1.0%). Behavioral measures were taken at various intervals up to 2 weeks after dietary administration was instituted. Separate groups of mice were given the same experimental treatment and sacrificed for whole brain determination of the monoamines and their metabolites. Isolated mice were consistently more aggressive than grouped animals, suggesting that territorial-induced aggression is synergistic with intermale aggression based on social isolation. The combination of isolation and 0.50% TRP was particularly effective in producing increases in aggression that reached maximal levels after 10 days of diet administration. However, motor activity of singly-housed mice was unaffected by TRP, while that of grouped mice was decreased after 5 days of 0.50% TRP. By day 14 of administration behavioral changes tended to return to baseline levels. Neurochemical measures indicated increased DA and 5-HT turnover in isolated mice, with the 5-HT system most affected by dietary TRP. Because housing conditions were a prominent factor in the aggression and neurochemistry, the results suggest the involvement of both transmitter systems in this behavior. However, there were no changes in monoamine turnover that could account for the development of behavioral tolerance.

Isolation reduces midbrain tryptophan hydroxylase activity in mice

Male C57 mice were divided into two experimental groups: "Isolated" and "Grouped". Isolated mice were housed individually for 30 or 45 days; Grouped mice were housed five per cage. Midbrain tryptophan hydroxylase activity was determined at the end of the isolation period. Isolated mice showed 35% less tryptophan hydroxylase activity than grouped mice (P less than 0.001). Prolongation of the isolation (45 vs 30 days) did not further reduce the enzyme activity. The changes in tryptophan hydroxylase activity may be related to behavioral changes induced by isolation.

The interaction of alcohol and stress. A review

The literature on the interaction of ethanol and stress is reviewed. Stress has amethystic properties in both experimental animals and human subjects when analgesia or various behavioral parameters were evaluated. In addition studies have shown that ethanol counteracts some effects of stress in non-alcoholic subjects. Improvement in performance and in the effective state of humans and reversal of some behavioral and pathological concomitants of stress in experimental animals have been reported. Although there is indication that ethanol improves the affective state in humans, reduction of anxiety has not been a universal finding. Recent studies have pointed out a number of variables (the drinking environment, cognitive set, mood and personality of subjects, prior experience with ethanol, sex, dose and type of beverage) which can significantly alter the effects of alcohol in human subjects. These may account for the variability in results in the literature. Although some studies have shown that alcohol ingestion increases under stressful conditions, others have failed to do so in both experimental animals and in humans. In alcoholics ethanol ingestion, in general, does not appear to relieve anxiety. In fact anxiety usually increases with time during a drinking binge. Therefore more research needs to be done to assess the validity of the anxiety-reducing theory for alcohol abuse. Possible mechanisms for the interaction of stress and alcohol are discussed.

Postmortem changes of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in mouse brain and their prevention by pargyline and microwave irradiation

Postmortem (pm) manipulations of brain tissue of decapitated mice produced a maximum decline in 5-HT and a maximum rise in 5-HIAA of 20 and 40%, respectively. The pm treatments included freezing and thawing, mincing, and leaving over. Microwave irradiation or pretreatment of the animals with the enzymatic inhibitors NSD 1015 or pargyline suppressed the pm effects. The possible role of pm effects in the initial accumulation of 5-HT and decline of 5-HIAA in the brain following administration of pargyline was studied. Out data suggest that, when MAO inhibitors are used, 5-HT turnover might be overestimated by pm changes.

Isolation induced aggression and catecholamine variations in discrete brain areas of the mouse

Norepinephrine (NE) and dopamine (DA) levels and turnover were measured in 17 discrete brain regions of Swiss-Webster (NIH) mice made aggressive by prolonged isolation. The NE steady state level was significantly lower in olfactory tubercle and substantia nigra and significantly higher in the septal area of the aggressive mice when compared to the isolated non-fighter controls. NE turnover was only higher in the A-10 region of the aggressors. DA steady state level and turnover was lower in olfactory tubercle and higher in caudate putamen of the aggressors. The significance of these changes in isolation-induced aggression is discussed.

Effects of environmental complexity and deprivation on brain chemistry and physiology: a review

Within the last two decades it has become evident that the brain exhibits a wide range of chemical and physiological responses to its sensory environment. Comparison of the brains of animals reared in relatively complex as opposed to deprived sensory environments has yielded significant insight into the nature of these effects, which are reviewed in this paper. Greater complexity of the sensory environment results in increased total cholinesterase and acetylcholinesterase enzyme activity, while other neurotransmitter related substances, the catecholamines, show more variable responses. RNA concentration is slightly greater as is DNA transcriptional activity, while protein precursor uptake shows a variety of regional and temporal patterns. In general, responses for most substances tend to show regional and temporal specificity with the largest effects most often in the occipital cortex. Electrophysiological measures have revealed shorter visual cortex evoked potential latencies and greater amounts of sleep in the complexity-reared subjects. The wide range of environmentally responsive parameters is consistent with an adaptive functioning of brain chemistry and physiology and with recent models in the physical sciences which view the universe as composed of dynamic webs of relationships rather than isolated independent units.

Altered behavioral responses to intense foot shock in socially-isolated rats

Male Wistar rats were isolated immediately after weaning for 12 weeks and exposed to electric foot shock of various intensities. The shock-elicited jumping behavior was measured every ten minutes for one hr. The frequency of jumping in isolated rats was lower than that in grouped and the difference between two groups was the greatest with the most intense shock. In these experimental situations, there was no significant difference in monoamine turnover rate between the two groups while noradrenaline turnover markedly increased in both groups. Chlorpromazine suppressed the jumping in a dose-dependent manner in both groups with stronger suppression in isolated rats. Methamphetamine facilitated the jumping in grouped rats dose-dependently while the drug rather depressed it in the isolated. From these results and the behavioral similarity between the isolated and 6-hydroxydopamine treated rats under foot shock situation, it was suggested that the receptor supersensitivity of central catecholaminergic neurons was involved in the behavioral change in isolated rats.