Escalated Aggressive Behavior: New Pharmacotherapeutic Approaches and Opportunities

Psychopharmacologic studies of aggressive behavior in animals under controlled laboratory conditions have been instrumental in developing and evaluating specific and effective novel drug treatments that reduce aggressive behavior. An initial contribution of this research is to create experimental conditions that enable the display of aggressive and defensive acts and postures in species that engage in either dominance or territorial or maternal aggression. Quantitative ethological analyses allow the precise delineation of the sequential organization of aggressive bursts, providing a benchmark for assessing excessive or pathological forms of aggressive behavior. A second contribution of preclinical research is the development of experimental models of escalated forms of aggressive behavior, such as focusing on genetic predispositions or social provocations and frustrative experiences. A critical role of preclinical research is in the pharmacological and neurochemical analysis of aggressive behavior; for example, a host of undesirable side effects prompted a shift from classic dopaminergic neuroleptic compounds to the more recently developed atypical neuroleptics with effective and more specific anti-aggressive effects. The long-established role of brain serotonin in impulsive and escalated forms of aggressive behavior continues to be a focus of preclinical studies. New evidence differentiates dynamic state changes in corticolimbic serotonergic neurons during the termination of aggressive behavior from the deficient-serotonin trait in violence-prone individuals. It can be anticipated that currently developed tools for targeting the genes that code for specific subtypes of serotonin receptors will offer new therapeutic options for reducing aggressive behavior, and the 5-HT(1B) receptor appears to be a promising target. The modulation of GABA and GABA(A) receptors by 5-HT in corticolimbic neurons promises to be particularly relevant for specific forms of escalated aggressive behavior such as alcohol-heightened aggression.

Development of an ethanol model using social insects: IV. Influence of ethanol on the aggression of Africanized honey bees (Apis mellifera L.)

Experiments were designed to determine whether ethanol influenced aggression in honey bees. Two experiments are reported. In Exp. 1, harnessed honey bees were fed a 1%, 5%, 10%, or 20% ethanol solution. Two control groups received either a sucrose solution only or no pretreatment, respectively. The dependent variable was the number of sting extensions over 10 min. Analysis showed that aggression in harnessed bees was not influenced by prior ethanol consumption. Because there was some suspicion that the extension of the sting apparatus may be hindered by harnessing, and the authors wanted to use a design that increased ecological validity, Exp. 2 was conducted with free-flying bees. Sucrose or 20% ethanol solutions were placed in front of beehives, and the number of stings on a leather patch dangled in front of the hive served as the dependent variable. The experiment was terminated after 5 hr. because bees exposed to ethanol became dangerously aggressive. A unique aspect of the study was that Africanized honey bees were used.

Gestational treatment with cocaine and fluoxetine alters oxytocin receptor number and binding affinity in lactating rat dams

Cocaine administered chronically throughout gestation has been correlated with deficits in maternal behavior, increased maternal aggressive behavior and decreased oxytocin levels in rats. In addition to its effects on oxytocin levels, cocaine is a potent serotonergic, dopaminergic and noradrenergic reuptake inhibitor. Alterations in the dopaminergic and serotonergic systems have been suggested as possibly having a role in cocaine-induced maternal aggression. This study was in part, an attempt to understand some of the mechanisms by which cocaine increases postpartum aggression, particularly as they relate to changes in the oxytocin system. Oxytocin receptor number and binding affinity in the medial preoptic area of the hypothalamus, ventral tegmental area, hippocampus and amygdala were determined for lactating rat dams on postpartum day 6 (PPD 6) that were gestationally treated with cocaine, fluoxetine, saline or an amfonelic acid/fluoxetine drug combination. Cocaine and fluoxetine treatment both resulted in a significant up-regulation of oxytocin receptor number and lower receptor affinity in the amygdala of lactating rat dams compared to saline controls and the amfonelic acid/fluoxetine combination treatment group. Cocaine treatment also resulted in a significant down-regulation of oxytocin receptors in the medial preoptic area and both cocaine and fluoxetine treated dams had the highest affinity for oxytocin receptors in this brain region. Results of the present study support previous data indicating that alterations in oxytocinergic and perhaps serotonergic system dynamics in the amygdala may play a role in cocaine-induced postpartum aggression.

Neuroendocrine responses in free-living female and male lizards after aggressive interactions

Although female aggression is found in many species and in a variety of contexts, little is known about its physiological bases. To compare neuroendocrine responses to aggression in females and males, we staged aggressive interactions between free-living territorial mountain spiny lizards and same-sex intruders and measured brain monoamines, plasma steroid hormone levels, and plasma glucose levels. Both females and males that had participated in a staged aggressive interaction had similar changes in serotonin (5-HT) activity in telencephalic tissue punches as indicated by a lowered ratio of forebrain:brainstem 5-HT concentrations. In addition, both females and males had elevated plasma corticosterone (B) after an aggressive interaction when compared to controls. The only difference detected between males and females was that females had a higher ratio of forebrain:brainstem norepinephrine (NE) concentrations throughout the brain compared to males. Together, these data indicate that acute neural and hormonal responses that accompany aggressive interactions in females are similar to those in males.

5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review

The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.

Differential effects of CGS 12066B and CP-94,253 on murine social and agonistic behaviour

Although it has been previously proposed that 5-HT1B agonism specifically attenuates rodent agonistic behaviour, more recent investigations have indicated that such influences may be ancillary to an anxiogenic effect. The present study examined the influences of two 5-HT1B agonists, CGS 12066B and CP-94,253, on murine agonistic behaviour. In a resident-intruder paradigm, CGS 12066B (0.5-5.0 mg/kg) decreased resident offensive aggression, social interest, and exploration while dose-dependently enhancing defensive behaviours across the dose range tested. CP-94,253 (2.5-10.0 mg/kg) also reduced elements of resident offensive behaviour whereas defensive behaviours were largely unchanged. Some elements of resident nonsocial and social behaviour were enhanced at 2.5 and 5.0 mg/kg but decreased at 10.0 mg/kg. The behavioural profile of CP-94,253, but not CGS 12066B, supports the proposal that 5-HT1B receptors inhibit agonistic behaviour without concomitant sedative or anxiogenic effects. Findings are discussed in relation to 5-HT1A/1B/2C receptors involved in agonistic behaviour and anxiety.

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.

Antianxiety and behavioral suppressant actions of the novel 5-HT1A receptor agonist, flesinoxan

Flesinoxan is a potent and selective 5-HT1A receptor agonist. In this study, the effects of this compound on behavior in the murine elevated plus-maze have been assessed using a recently developed ethological scoring method. Results show that, at low doses (0.1-0.5 mg/kg), flesinoxan inhibited risk assessment behaviors (stretched attend postures and closed arm returns) indicative of a reduction in anxiety. These effects were maintained at a higher dose of 1.0 mg/kg, which also increased percent open entries and time spent on the central platform and open arms. However, this more convincing anxiolytic profile was associated with significant reductions in total arm entries and rearing, suggesting a combination of anxiolysis and behavioral suppression at high doses. The plus-maze profile observed with flesinoxan is very similar to that previously reported for 8-OH-DPAT in the same test but, despite superficial similarities, can be distinguished from that seen with buspirone. Data are discussed in relation to behavioral similarities and differences between 5-HT1A receptor agonists, and the advantages of a more detailed approach to the analysis of plus-maze behavior.

Test-specific effects of FG-7142 on isolation-induced aggression in mice

Treatment with FG-7142 decreased isolation-induced attack, but not defense, by male mice when the residents' home cages contained only a sawdust substrate. When a small wooden nesting box was added to the cage (Experiment 2), however, FG-7142 somewhat increased levels of attack. Time spent in the nesting box was also increased, while overall levels of social interaction were decreased, by drug treatment in Experiment 2. The latter findings are consistent with FG-7142's well-documented anxiogenic properties and indicate that this drug is probably proaggressive in its actions. The antiaggressive effects of FG-7142 in Experiment 1 may have resulted from drug-induced fear behaviors that were incompatible with attack. Alternatively, Experiment 2 suggests the possibility that species-typical attack may be defensively motivated under some circumstances. Although the antiagressive properties of eltoprazine and yohimbine were unaffected by the addition of the nest box (Experiment 3), the provision of some sort of refuge within the testing apparatus may be an important methodological consideration for studies employing resident-intruder paradigms.

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.