Effects of serotonin receptor agonists and antagonists on offensive aggression in mice

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.

Brain serotonin receptors and transporters: initiation vs. termination of escalated aggression

RATIONALE

Recent findings have shown a complexly regulated 5-HT system as it is linked to different kinds of aggression.

OBJECTIVE

We focus on (1) phasic and tonic changes of 5-HT and (2) state and trait of aggression, and emphasize the different receptor subtypes, their role in specific brain regions, feed-back regulation and modulation by other amines, acids and peptides.

RESULTS

New pharmacological tools differentiate the first three 5-HT receptor families and their modulation by GABA, glutamate and CRF. Activation of 5-HT(1A), 5-HT(1B) and 5-HT(2A/2C) receptors in mesocorticolimbic areas, reduce species-typical and other aggressive behaviors. In contrast, agonists at 5-HT(1A) and 5-HT(1B) receptors in the medial prefrontal cortex or septal area can increase aggressive behavior under specific conditions. Activation of serotonin transporters reduce mainly pathological aggression. Genetic analyses of aggressive individuals have identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT(1B), 5-HT transporter, Pet1, MAOA) or indirectly (e.g., Neuropeptide Y, αCaMKII, NOS, BDNF). Dysfunction in genes for MAOA escalates pathological aggression in rodents and humans, particularly in interaction with specific experiences.

CONCLUSIONS

Feedback to autoreceptors of the 5-HT(1) family and modulation via heteroreceptors are important in the expression of aggressive behavior. Tonic increase of the 5-HT(2) family expression may cause escalated aggression, whereas the phasic increase of 5-HT(2) receptors inhibits aggressive behaviors. Polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT modulate aggression, often requiring interaction with the rearing environment.

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Role of Alcohol Consumption in Escalation to Violence

No other drug has been associated with aggressive and violent behavior more than alcohol has. A major characteristic of the link between alcohol and social interactions is the very large variation in who becomes more aggressive while drinking and who does not. Tracing the origins of these individual differences has led to a focus on predispositions, such as the antisocial behavior of Type 2 alcoholics. Successful development of an experimental procedure to model heightened aggressive behavior after voluntary consumption of alcohol has facilitated the neurobiologic analysis of the link between alcohol and aggression. From a pharmacologic perspective, consumption of low to moderate doses of alcohol engenders heightened aggressive behavior in a significant minority of individuals before the circulation of appreciable amounts of the aldehyde metabolite. Ionophoric receptors such as NMDA, 5-HT(3) and GABA(A) have been identified in the brain as major sites of action for alcohol in the dose range that is relevant for engendering heightened aggression. Actions at the GABA(A) receptor complex that depend on particular GABA(A) subunits appear to be necessary for alcohol-heightened aggression. Genes that encode the synthesis of these alpha and gamma subunits are potentially significant markers for those individuals that are prone to engage in heightened aggressive behavior after the consumption of alcohol. Of particular importance are the reciprocal interactions between GABA and serotonin. Activating specific serotonin receptor subtypes such as 5-HT(1B) receptors reduces alcohol-heightened aggressive behavior. How these GABAergic and serotonergic corticolimbic mechanisms for alcohol-heightened aggression develop during the adolescent period remains an area of urgent study.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.

Antisuicide properties of psychotropic drugs: a critical review

The authors consider the extent to which psychotropic medications demonstrate benefits in the prevention of suicidal behavior in psychiatric patients. Results of a MedLine search are critically reviewed for lithium, divalproex and other anticonvulsants, conventional and atypical antipsychotics, and antidepressants. The existing literature is almost entirely limited to noncontrolled, often retrospective studies that do not control for potential biases in treatment selection, the use of multiple medications, the impact of medication nonadherence, and nonrandomized treatment discontinuations. Nevertheless, an extensive literature has arisen regarding observed reductions in suicidal behavior with lithium for mood disorders and, to a lesser extent, with clozapine for schizophrenia. A substantially smaller literature suggests more negative than positive data with divalproex or carbamazepine in bipolar disorder, while minimal information exists regarding suicidality with atypical antipsychotics other than clozapine. Studies of antidepressants have mostly been short-term and have focused more on whether they induce (rather than ameliorate) suicidal thoughts or behaviors. The sum of existing studies is generally inconclusive about whether antidepressants appreciably reduce risk for suicide completions. Relatively little is known about pharmacotherapy effects on suicidal ideation as distinct from behaviors. Possible mechanistic considerations for understanding antisuicide properties include a therapeutic impact on depression, impulsivity, or aggression, potentially mediated through serotonergic or other neuromodulatory systems. Recommendations are provided to guide future research as well as clinical practice.

Differential modulation of feline defensive rage behavior in the medial hypothalamus by 5-HT1A and 5-HT2 receptors

Previous studies have established that the expression of defensive rage behavior in the cat is mediated over reciprocal pathways that link the medial hypothalamus and the dorsolateral quadrant of the midbrain periaqueductal gray matter (PAG). The present study was designed to determine the roles played by 5-HT(1A) and 5-HT(2C) receptors in the medial hypothalamus on the expression of defensive rage behavior elicited from electrical stimulation of the PAG. Monopolar stimulating electrodes were placed in the midbrain PAG from which defensive rage behavior could be elicited by electrical stimulation. During the course of this study, defensive rage was determined by measuring the latency of the "hissing" component of this behavior. Cannula-electrodes were implanted into sites within the medial hypothalamus from which defensive rage behavior could also be elicited by electrical stimulation in order that serotonergic compounds could be microinjected into behaviorally identifiable regions of the hypothalamus at a later time. Microinjections of the 5-HT(1A) receptor agonist 8-OHDPAT (0.1, 1.0 and 3.0 nmol) into the medial hypothalamus suppressed PAG-elicited hissing in a dose-dependent manner. Administration of the 5-HT(1A) antagonist p-MPPI (3.0 nmol) blocked the suppressive effects of 8-OHDPAT upon hissing. The suppressive effects of 8-OHDPAT were specific to defensive rage behavior because this drug (3 nmol) facilitated quiet biting attack. Microinjections of the 5-HT(2C) receptor agonist (+/-)-DOI hydrochloride into the medial hypothalamus (0.5, 1.0, and 3.0 nmol) facilitated the occurrence of PAG-elicited hissing in a dose-dependent manner. In turn, these facilitating effects were blocked by pretreatment with the selective 5-HT(2) antagonist, LY-53,857, which was microinjected into the same medial hypothalamic site. The findings of this study provide evidence that activation of 5-HT(1A) and 5-HT(2) receptors within the medial hypothalamus exert differential modulatory effects upon defensive rage behavior elicited from the midbrain PAG of the cat.

Testosterone and its metabolites modulate 5HT1A and 5HT1B agonist effects on intermale aggression

Our understanding of the neurochemical and neuroendocrine systems' regulating the display of offensive intermale aggression has progressed substantially over the past twenty years. Pharmacological studies have shown that serotonin, via its action at 5HT1A and/or 5HT1B receptor sites, modulates the display of intermale aggressive behavior and that its effects serve to decrease behavioral expression. Neuroendocrine investigations, in turn, have demonstrated that male-typical aggression is testosterone-dependent and studies of genetic effects, metabolic function and steroid receptor binding have shown that facilitation of behavioral displays can occur via independent androgen-sensitive or estrogen-sensitive pathways. Remarkably, there have been virtually no studies that examined the interrelationship between these facilitative and inhibitory systems. As an initial step toward characterizing the interaction between the systems, studies were conducted that assessed hormonal modulation of serotonin function at 5HT1A and 5HT1B receptor sites. They demonstrated: (1) that the androgenic and estrogenic metabolites of testosterone differentially modulate the ability of systemically administered 8-OH-DPAT (a 5HT1A agonist) and CGS12066B (a 5HT1B agonist) to decrease offensive aggression; and (2) when microinjected into the lateral septum (LS) or medial preoptic area (MPO), the aggression-attenuating effects of 1A and 1B agonists differ regionally and vary with the steroidal milieu. In general, the results suggest that estrogens establish a restrictive environment for attenuation of T-dependent aggression by 8-OH-DPAT and CGS 12066B, while androgens either do not inhibit, or perhaps even facilitate, the ability of 5HT1A and 5HT1B agonists to reduce aggression. Potential mechanisms involved in the production of these steroidal effects are discussed and emerging issues that may impact on efforts to develop an integrative neurobiological model of offensive, intermale aggression are considered.

Androgens and estrogens modulate 5-HT1A and 5-HT1B agonist effects on aggression

Intermale offensive aggressive behavior is facilitated by gonadal steroids and inhibited by serotonin (5-HT), presumably through its effects at 5-HT1A and 5-HT1B receptor sites. To examine the interaction between these neuroendocrine and neurochemical regulatory systems, CF-1 male mice were gonadectomized and implanted with silastic capsules containing either diethylstilbestrol (DES, a synthetic estrogen), the nonaromatizable androgens methyltrienolone (R1881) or dihydrotestosterone (DHT), or testosterone (T). Two weeks later, they were given 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT1A agonist; 0.1 or 1.0 mg/kg), CGS12066B (a 5-HT1B agonist; 4.0 or 8.0 mg/kg), 0.1 or 1.0 mg/kg 8-OH-DPAT + 4.0 mg/kg CGS12066B, or vehicle, and tested for aggression. In the presence of DES, the higher 8-OH-DPAT dose given in combination with CGS attenuated aggression in comparison to vehicle controls. When given nonaromatizable androgen (R1881 or DHT), all drug treatments except 0.1 mg/kg 8-OH-DPAT significantly reduced offensive attack behavior. In the presence of T, which provides estrogenic and androgenic stimulation, aggression scores were significantly reduced when males were given the high dose of 8-OH-DPAT or CGS12066B, as well as in the 1.0 mg/kg 8-OH-DPAT + CGS12066B condition. Assessments of changes in motor behavior showed significant impairment when 8.0 mg/kg CGS12066B was administered across all hormonal conditions, indicating that reductions in offensive aggression in these treatment groups were nonspecific. The results demonstrate differential effects of the steroidal environment on the ability of 5-HT1A and 5-HT1B agonists to modulate aggression, with estrogens producing a more restrictive environment than androgens for serotonergic inhibition of male-typical aggressive behavior.

Effect of tryptophan treatment on self-biting and central nervous system serotonin metabolism in rhesus monkeys (Macaca mulatta)

Two studies were conducted to examine the effects of oral L-tryptophan (TRP) supplementation as a treatment for self-injurious behavior (SIB) and to investigate behavior and central serotonin turnover of male rhesus monkeys. In Study One, TRP was administered to seven individually housed rhesus monkeys with a recent history of spontaneous SIB. While the monkeys were on TRP treatment (100 mg/kg twice a day), cisternal cerebrospinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid increased markedly (p = .0013) above baseline (baseline mean = 207.6 pmol/ml +/- 39; TRP mean = 320.3 pmol/ml +/- 83.4), and the duration of self-biting behavior decreased below baseline (p = .03). In Study Two, 14 individually housed rhesus monkeys without a history of SIB were placed on three different doses of TRP in random order (50, 100, and 200 mg/kg twice a day). TRP had no effect on any behavioral or biochemical variables in the normal monkeys.

CONCLUSIONS

Supplemental tryptophan in well-tolerated doses reduced self-biting and increases serotonin turnover rate in male monkeys with a recent history of SIB. The same doses of TRP do not affect behavior or serotonin metabolism in male monkeys without a history of SIB.

Serotonin 5-HT1A and 5-HT2/1C receptors in the midbrain periaqueductal gray differentially modulate defensive rage behavior elicited from the medial hypothalamus of the cat

Recent studies have established that the expression of defensive rage behavior in the cat is mediated over a descending pathway from the medial hypothalamus to the dorsolateral quadrant of the midbrain periaqueductal gray matter (PAG). The present study was designed to determine the roles played by 5-HT1A and 5-HT2/1C receptors in this region of PAG in modulating defensive rage behavior elicited from the cat's medial hypothalamus. Monopolar stimulating electrodes were implanted into the medial hypothalamus from which defensive rage behavior could be elicited by electrical stimulation. During the course of the study, the 'hissing' component of the defensive rage response was used as a measure of defensive rage behavior. Cannula-electrodes were implanted into sites within the PAG from which defensive rage could also be elicited by electrical stimulation in order that 5-HT compounds could be microinjected into behaviorally identifiable regions of the PAG at a later time. Microinjections of the selective 5-HT1A agonist, (+)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OHDPAT) (50 pmol, 2.0 and 3.0 nmol), into the PAG suppressed the hissing response in a dose-dependent manner. Administration of the selective 5-HT1A antagonist, 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI) (1.5 and 3.0 nmol), blocked the suppressive effects of 8-OHDPAT upon hissing. In contrast, microinjections of the 5-HT2/1C receptor agonist (+)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride ((+)-DOI hydrochloride) (0.01, 1.0 and 1.5 nmol) facilitated the occurrence of hissing elicited from the medial hypothalamus in a dose-dependent manner. Immunohistochemical analysis revealed the presence of 5-HT axons and preterminals throughout the PAG, and in particular, in its dorsolateral aspect which receives major inputs from the medial hypothalamus in association with defensive rage behavior. The overall findings of the study provide evidence that activation of 5-HT1A and 5-HT2/1C receptors within the midbrain PAG differentially modulate the expression of defensive rage behavior elicited from the medial hypothalamus of the cat.

alpha-Melanocyte-stimulating hormone (alpha-MSH) and melanin-concentrating hormone (MCH) modify monoaminergic levels in the preoptic area of the rat

The effect of perfusion of melanin-concentrating hormone (MCH) or alpha-melanocyte-stimulating hormone (alpha-MSH) (100 ng/microliter) in the ventromedial nucleus (VMN) or medial preoptic area (MPOA) on monoaminergic levels of female rats was measured using microdialysis and HPLC-electrochemical detection. In the MPOA, alpha-MSH raised 5-HIAA concentration, whereas MCH reduced both 5-HT and 5-HIAA. Neither peptide had any effect in the VMN. The opposite effects of the peptides on the serotonergic system might be responsible for their antagonistic or opposite actions previously reported on several CNS functions. Dopamine may mediate the similar effects of the two peptides, because alpha-MSH inhibits dopaminergic release in the MPOA (but not VMN) and MCH tends to follow the same pattern.

The effect of tryptophan depletion and enhancement on subjective and behavioural aggression in normal male subjects

In order to investigate the link between aggression and 5-HT, we looked at effects of changes in plasma tryptophan on healthy male subjects. Twenty-four with high trait aggression (H) and 24 with low (L) drank an amino acid mixture with (T+) or without (T-) tryptophan. These caused plasma tryptophan enhancement and depletion, respectively, at 4.5 h. Group H subjects given T- became more angry, aggressive, annoyed, hostile and quarrelsome on subjective measures, whereas those given T+ responded in the opposite way. On a behavioural measure of aggression, group H subjects responded more aggressively after T- than T+. In contrast, there was no consistent effect on subjective or behavioural aggression in group L subjects. Feelings of well-being in group H were decreased by T- and increased by T+. In group L, T+ reduced feelings of well-being, possibly due to the sedative effect of tryptophan in this group, which correlated positively with plasma tryptophan concentration. Changes in plasma tryptophan are probably followed by changes in central 5-HT turnover. We conclude that, in those with pre-existing aggressive traits, acute falls in central 5-HT can cause increased subjective and objective aggression, while rises can have the opposite effect. The absence of changes in a low aggressive group suggests that the primary effect may be on impulsivity, possibly mediated by 5-HT1a receptors, expressing underlying aggressive traits. The findings on mood changes provide support for earlier reports of a lowering of mood with tryptophan depletion.

Time-dependent effects of PCPA on social aggression in chicks

We investigated the effects of para-chlorophenylalanine (PCPA), a serotonin (5-HT) antagonist, on social aggression and brain neurochemistry in young domestic chickens (Gallus domesticus). In Experiment 1, the effects of four different doses of PCPA (0, 100, 200, and 400 mg/kg) were examined for 3 days after injection. Immediately after PCPA injection, aggressive pecking was low and then increased over the 3-day test period. PCPA significantly decreased 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and 5-HT turnover. In addition, the frequency of aggression was negatively correlated with levels of 5-HIAA. In Experiment 2, the time-dependent effects of a single 400-mg/kg dose of PCPA were examined for 5 and 7 days after drug exposure. PCPA-treated chicks observed for 5 days after injection had significantly greater frequencies of aggression 4 days following drug exposure and significantly reduced 5-HT levels when measured on the next day. Similarly, chicks observed for 7 days exhibited significantly elevated aggression 5 days after injection, after which their pecking decreased to control levels on days 6 and 7. Coinciding with this behavioral pattern, 5-HT levels from these PCPA-treated chicks when assessed 7 days after drug exposure were the same as those for control birds. We concluded that PCPA increased social aggression in birds, an effect that diminished as brain 5-HT levels recovered over a 1-week period.

5-HT1A receptor influences on rodent social and agonistic behavior: a review and empirical study

Investigations examining the influence of 5-HT1A receptors in murine agonistic and social behavior have reported either specific or nonspecific attenuation of offensive behavior. To clarify this situation, the effects of three 5-HT1A agonists were examined on isolation-induced aggression and social behavior in male mice. 8-OH-DPAT (0.025-1.25 mg/kg) increased social behavior, rearing, and digging. Offensive behavior was reduced, without concomitant sedation. Ipsapirone (0.1-10.0 mg/kg) reduced naso-nasal behavior, whilst enhancing stretched-attend behavior, cage-exploration, and rearing. Offensive and defensive behaviors were attenuated, without reductions in activity. MDL 73005 EF (0.25-8.0 mg/kg) reduced social behaviors, cage-exploration and rearing while maintenance behavior was increased. Offensive and defensive behaviors showed attenuation. Current results corroborate previous findings with respect to 5-HT1A receptor involvement in murine agonistic behavior and anxiety. Data also connote that the behavioral specificity of 5-HT1A ligands should be interpreted in terms of response competition rather than solely concomitant sedation.

Cloning and functional characterization of the human 5-HT2B serotonin receptor

Recently, we have reported the cloning of the rat 5-HT2B receptor cDNA. This receptor is particularly interesting since it may be involved in diseases such as migraine. Here, we describe the isolation of a human 5-HT2B receptor clone from a cDNA library derived from SH-SY5Y cells. Although the receptor sequence was only 80% homologous to the rat sequence, the exon-intron distribution was conserved between the two species. In the human body, the receptor mRNA was detected in most peripheral organs. Only low expression levels were found in the brain. After expression in HEK 293 cells, activation of the receptor stimulated the production of phosphatidylinositol. The pharmacology of this functional response correlated well with that of the rodent receptor.

Neurobiological mechanisms controlling aggression: preclinical developments for pharmacotherapeutic interventions

Current pharmacotherapeutic approaches to the management of violent and aggressive behavior rely mostly on agents that act as receptor agonists or antagonists at subtypes of brain dopaminergic, GABAergic, and serotonergic receptors. Ethological experimental studies in animals have shown that drugs may modulate aggression by inhibiting motor activity, by distorting aggression-provoking or -inhibiting signals, by fragmenting behavioral sequences or temporal patterning, or by increasing the rate and intensity of aggressive acts. Evidence from animal studies points to large changes in selected brain dopamine, serotonin, and GABA systems during and following aggressive and defensive behavior. However, the specificity of drugs that are currently used to control aggressive behavior through their action as agonists or antagonists at subtypes of dopamine, serotonin or GABA receptors continues to be of concern. Similar to the effects of widely used traditional neuroleptics that nonselectively antagonize dopamine receptors, the range of behaviors which is suppressed by either D1 or D2 receptor antagonists is pervasive. At present, systemic administration of dopamine receptor antagonists in animal preparations does not target aggression-specific mechanisms. The GABAA/Benzodiazepine/Chloride ionophore receptor complex is implicated in the aggression-heightening effects of alcohol and benzodiazepines. Although early reports focused on the "taming" effects of benzodiazepine anxiolytics, low doses may enhance aggression in both animals and humans. Benzodiazepine antagonists block heightened aggression after low doses of alcohol or benzodiazepines. Agonists at certain 5-HT1 receptor subtypes such as eltoprazine are potently effective in reducing aggressive behavior of males and females of various animal species under conditions that promote charging offensive-type aggression, without adversely affecting nonaggressive components of the behavioral repertoire. However, recent reports indicate that eltoprazine and related compounds may potentiate anxiety reactions in rodents, and question the behavioral specificity of these substances. Opioid receptor antagonists modulate primarily physiological and behavioral responses of defense and submission. Defeated animals show tolerance to opiate analgesia and withdrawal responses upon challenge with opioid receptor antagonists. Defensive and submissive vocalizations are potently blocked by opioid peptides. Substances that target specific receptor subtypes at serotonergic, GABAergic and opioidergic synapses are most promising for the selective modification of aggressive, defensive and submissive behavior patterns.

Effects of pindobind 5-hydroxytryptamine1A (5-HT1A), a novel and potent 5-HT1A antagonist, on social and agonistic behaviour in male albino mice

In view of inconsistent results reported for 5-hydroxytryptamine1A (5-HT1A) receptor involvement in murine social conflict, this study examined the effects of N1-(bromoacetyl)-N8-[3-(4-indolyloxy)-2- hydroxypropyl]-(Z)-1,8-diamino-p-menthane (pindobind) 5-HT1A, a novel 5-HT1A antagonist, on agonistic and social behaviour in mice. Employing a resident-intruder paradigm, administration of pindobind 5-HT1A (0.5-10 mg/kg) to resident animals produced a reduction in offensive sideways and chasing behaviour. Defensive postures were unchanged except for evasion, which was reduced. Within social behaviour, nonspecific social behaviour and following behaviour were reduced while stretch/attend behaviour was enhanced. Nonsocial behavioural changes included an increase in resident cage exploration and rearing. Intruder data indicated no significant change in offensive behaviours, an attenuation of defensive sideways posturing and evasion, decreases in attend behaviour, and increases in cage exploration, rearing, and digging. Results are discussed in relation to the effects of 5-HT1A receptor (ant)agonism on murine offensive behaviour.

Fluoxetine and suicidal behaviour. Some clinical and theoretical aspects of a controversy

“In the practical decisions of life it will scarcely ever be possible to go through all the arguments in favour of or against one possible decision, and one will therefore always have to act on insufficient evidence” Werner Heisenberg (1962).

The development of selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRIs) has added a new dimension to the pharmacotherapy of depression. Their lack of sedative and anticholinergic effects together with relative safety in overdose, has led some authors to suggest that the older tricyclic antidepressants (TCAs) should not now be considered a first-line treatment (Montgomery, 1988). However, since a report by Teicher et al (1990) of suicidal preoccupation associated with fluoxetine treatment, there has been both intense media interest and animated correspondence in clinical and scientific journals (O'Donnell, 1991).

Rodent models of aggressive behavior and serotonergic drugs

Various models of rodent agonistic behaviour are described, which differentiate between offensive and defensive/flight models. Particular attention is given to one male and one female paradigm for offensive aggression, viz. resident-intruder or territorial (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 agonists buspirone, 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 non-specific anti-aggressive profile. Non-selective 5-HT1 agonists, such as RU 24969, eltoprazine (DU 28853), and TFMPP reduced aggression quite specific and did not decrease social interest or exploration, but sometimes even increased these behaviours. In RI and MA the behavioural effects of these drugs were roughly similar. In contrast, MA was more sensitive to the treatment with the 5-HT reuptake blocker fluvoxamine, which blocked RI aggression only non-specifically at the highest dose. DOI, a 5-HT2 and 5-HT1C agonist, decreased aggressive behaviour 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 non-specific 5-HT agonist (and 5-HT3 antagonist) quipazine also induced 'wet dog shaking' at doses which suppressed aggression, social interest and exploration but increased inactive behaviours (sitting and lying). The discussion attempts to delineate a role for 5-HT 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 stressing the consistent role of 5-HT in different forms of aggression.

An ethological study of the effects of buspirone and the 5-HT3 receptor antagonist, BRL 43694 (granisetron) on behaviour during social interactions in female and male mice

Buspirone (12.8 mg/l; 2.3-2.6 mg/kg daily) and the 5-HT3 receptor antagonist, BRL 43694 (granisetron) (40 micrograms/l; 10 micrograms/kg daily), were each given in drinking fluid to male and female DBA/2 mice for 5-10 days. Controls received tap water. Effects on behaviour were examined by ethological procedures during 5 min encounters with unfamiliar BKW partners. One group of DBA/2 males acted as intruders in a resident-intruder paradigm and another group encountered oestrous females in a neutral cage. The DBA/2 females each encountered a group-housed male in a neutral cage. Both buspirone and BRL 43694 decreased flight in females and increased the duration of their active social investigation. In females, BRL 43694 also reduced the occurrence of "scan" and prolonged the bout length of exploration. In male mice, buspirone increased social investigation, including the specific elements "sniff" and "follow" in encounters with female partners, but its only effect on behaviour during encounters with isolated resident males, was to decrease duration of the element, "attend". In males, BRL 43694 did not significantly affect behaviour in heterosexual encounters and had only a slight effect on behaviour during encounters with resident males, decreasing the occurrence of "eat". Overall, these results suggest that records of effects of drugs on flight responses of female mice, in encounters with male partners, may provide a sensitive index of the anxiolytic profile of novel compounds.

Postsynaptic 5-HT1 receptors and offensive aggression in rats: a combined behavioural and autoradiographic study with eltoprazine

The present study was designed to assess whether the antiaggressive effects of eltoprazine are mediated via presynaptic and/or postsynaptic 5-HT1 receptors. We describe the effects of central 5-HT depletion 1) on the behaviour of resident TMD-S3 rats in a territorial situation, 2) on the efficacy of eltoprazine to inhibit offensive aggression, and 3) on the 5-HT1A, 5-HT1B and 5-HT1C receptor binding in brains of rats previously used in behavioural studies. Male resident rats were given combined 5,7-dihydroxytryptamine (5,7-DHT) injections into the dorsal and median raphe nuclei. Two to four weeks after the lesions, rats were confronted with an intruder Wiser rat in their home cage for a 10-min period. The 5,7-DHT treatment resulted in a modest reduction of offensive behaviour, while having no effects on other social and nonsocial behaviours. Oral administration of eltoprazine (1 mg/kg) specifically reduced offensive aggression in both sham- and 5,7-DHT-lesioned animals, leaving social interest and exploration intact or even increasing it. A low dose (0.3 mg/kg) of eltoprazine did not affect the behavioural repertoire of sham-operated rats, whereas this dose significantly reduced offense behaviours in the 5,7-DHT-lesioned residents. Quantitative autoradiographic studies 5 weeks after 5,7-DHT treatment revealed a significant increase in radioligand binding to 5-HT1A, 5-HT1B and 5-HT1C sites in many brain regions studied, except for the raphe nuclei where [3H]8-OH-DPAT binding to 5-HT1A sites was markedly reduced. The concentrations of 5-HT and 5-HIAA in frontal cortex were reduced to approximately 10% of controls. The results indicate that serotonin has a stimulatory rather than an inhibitory influence on offensive aggressive behaviour. Central 5-HT depletion does not prevent the antiaggressive effects of eltoprazine, indicating a role for postsynaptic 5-HT1 receptors in the modulation of offensive aggression. The 5,7-DHT-induced overall upregulation of 5-HT1A, 5-HT1B and 5-HT1C binding sites suggests that these three receptor subtypes receive a tonic serotonergic influence. It is conceivable that this postsynaptic 5-HT1 receptor supersensitivity is reflected by the increased efficacy of eltoprazine to inhibit offensive aggression.

Eltoprazine, a drug which reduces aggressive behaviour, binds selectively to 5-HT1 receptor sites in the rat brain: an autoradiographic study

Eltoprazine, a phenylpiperazine derivative, selectively reduces offensive aggression in animal models. The present study was designed to localize and characterize the binding sites of [3H]eltoprazine in the rat brain and to compare the distribution of these sites with the distribution of [3H]5-HT binding sites. The binding of [3H]eltoprazine to whole tissue sections was saturable and revealed an apparent dissociation constant (Kd) of 11 nM. Autoradiographic studies demonstrated a widespread distribution of [3H]eltoprazine binding sites throughout the brain. Specific [3H]eltoprazine binding was completely displaced by 5-HT; conversely, unlabelled eltoprazine reduced [3H]5-HT binding to the levels of non-specific binding. The overall distribution of [3H]eltoprazine binding sites showed a strong resemblance to the location of 5-HT1 binding sites labelled with [3H]5-HT. Yet, regions enriched in 5-HT1A and 5-HT1C sites (e.g. dentate gyrus and choroid plexus, respectively) revealed relatively more [3H]5-HT binding as compared to [3H]eltoprazine binding, whereas [3H]eltoprazine binding was more pronounced in 5-HT1B receptor dense areas such as the dorsal subiculum, substantia nigra, ventral pallidum and globus pallidus. Displacement of [3H]eltoprazine with various selective serotonergic drugs demonstrated binding of [3H]eltoprazine to 5-HT1 receptor subtypes. The pharmacological and anatomical data indicate that eltoprazine binds to 5-HT1A, 5-HT1B and to a lesser extent to 5-HT1C binding sites in the rat brain. These results emphasize the important role of serotonin in the regulation of offensive aggression and suggest that eltoprazine may serve as a new tool to study the involvement of central 5-HT1 receptors in the expression of this behaviour.

Neurochemical profile of eltoprazine

In this paper we present the neurochemical profile of eltoprazine, a drug that specifically inhibits offensive aggression. Eltoprazine interacts selectively with serotonin (5-HT) receptor subtypes (Ki-values for 5-HT1A, 5-HT1B and 5-HT1C receptors are 40, 52 and 81 nM respectively). Affinity for other neurotransmitter receptors is much lower (Ki-values greater than 400 nM) than for 5-HT1 receptors. The selective interaction with 5-HT1 receptor subtypes is confirmed by in vitro autoradiographic studies using radiolabelled eltoprazine. The overall distribution of [3H]eltoprazine bears a strong resemblance to the localization of 5-HT1 binding sites labelled by [3H]5-HT, although some differences are observed. Eltoprazine (1 microM) inhibits the forskolin stimulated c-AMP production in hippocampus slices of the rat, indicating an agonistic action on the 5-HT1A receptor. The K+ stimulated release of 5-HT from rat cortex slices is inhibited by eltoprazine (pD2 = 7.8). The maximal response, however, was clearly less than that of the full agonist 5-HT, indicating partial agonistic activity on the 5-HT1B receptor (alpha = 0.5). Eltoprazine has a weak antagonistic action (IC50 = 7 microM) on the 5-HT1C receptor as revealed by inhibition of the 5-HT-induced accumulation of inositol phosphates in the choroid plexus of the pig. In vivo, eltoprazine reduces 5-HIAA levels in the striatum, without affecting the 5-HT levels. Eltoprazine also reduces the 5-HT synthesis rate as shown by 5-HTP accumulation after decarboxylase inhibition. These data indicate that eltoprazine acts as a 5-HT agonist in vivo in a dose range that affects aggressive behaviour (0.3-3 mg/kg p.o.). Taken together from a variety of neurochemical studies there is strong evidence both in vitro and in vivo that the pharmacological actions of eltoprazine can be attributed to an interaction with the 5-HT system, most probably via a (partial) agonistic action on 5-HT1A and 5-HT1B receptors.

Serotonergic modulation of social interactions in isolated male mice

Several serotonergic drugs were tested in isolation-induced aggressive behavior in male mice using ethological methodology. Eltoprazine, a mixed 5-HT1 agonist, reduced aggression but enhanced social interest and exploration. Several 5-HT1A agonists (8-OH-DPAT, ipsapirone, buspirone, 5-Me-ODMT) and a 5-HT uptake blocker (fluvoxamine) also reduced aggression. Although these drugs somewhat differentially affect aggressive behavior, the isolation-induced paradigm alone is not sensitive enough to successfully differentiate and screen the various serotonergic drugs with regard to their influence on social behavior in mice. It is argued that various animal paradigms in several species are necessary to describe specific effects of serotonergic drugs.

'Taming' of wild rats (Rattus rattus) by 5HT1A agonists buspirone and gepirone

A battery of tests designed to elicit reactions to a variety of nonpainful threat stimuli was used to study the effects of the 5HT1A agonists buspirone (5-20 mg/kg), and gepirone (5-20 mg/kg) on the defensive repertoire of wild Rattus rattus. These two compounds produced very similar patterns of results on the test battery, with gepirone generally more effective: Both compounds failed to interfere with either spontaneous motor activity or avoidance/flight to an approaching experimenter. However, both reduced defensive reactivity to proximal threat stimuli, increasing passive contacts with the experimenter in an inescapable situation and reducing "proximal" defensive reactions: jump/flinch reactions to dorsal contact, and, boxing, and biting to a number of threat stimuli. Defensive threat vocalizations and jump attacks were also reduced, but less consistently, as was the experimenter's rating of subject's defensiveness to being picked up. This pattern of results suggested specific "taming" effects of buspirone and, especially, gepirone on defensive reactions. In combination with findings indicating somewhat different (benzodiazepines) to very different (ethanol) profiles for other anxiolytics in the same test battery, these results suggest that the Defense Test Battery may be capable of providing behavioural differentiation among various classes of anxiolytics.