The "mouse-killing" rat: ethological discussion on an experimental model of agression

Behavioural pharmacology of the serenic, eltoprazine

In this paper the effects of serenics (eltoprazine and fluprazine) are described in several animal models for offensive agonistic, defensive agonistic and predatory behaviour. They are compared with the effects of a number of other putative anti-aggressive compounds or drugs used clinically in order to ameliorate aggressive behaviour of psychiatric patients. In isolation-induced offensive aggression in mice, eltoprazine has a marked and potent anti-aggressive activity, although numerous other psychoactive drugs also exert anti-aggressive effects. The behavioural specificity of this anti-aggressive profile was investigated using an ethologically derived animal model, social interaction in male mice. In this model, eltoprazine has a very specific anti-aggressive (serenic) profile, inhibiting aggression while social interaction and exploration are not decreased but even enhanced; inactivity, a measure for sedation, is not affected. Such a profile contrasts sharply with that of neuroleptics (chlorpromazine, haloperidol), psychostimulants (d-amphetamine) or benzodiazepines (chlordiazepoxide), which exert severe sedation (neuroleptics) or even aggression-enhancing effects (BDZ). After subchronic treatment no tolerance for the anti-aggressive effects of eltoprazine occurred. The specific anti-aggressive effects of eltoprazine were also found in rat models of offensive agonistic behaviour. In one such model - resident-intruder aggression - eltoprazine reduced offensive behaviour specifically, leaving social interactions and exploration intact, and did not induce sedation or other unwanted side-effects. The neuroleptic haloperidol was very sedative in this model, as was the 5-HT1A-agonist buspirone. Benzodiazepines (chlordiazepoxide) have a biphasic effect in this paradigm, enhancing offence at low doses and decreasing it at higher doses, due to muscle relaxation. In another offensive model, colony-aggression, in which a dominant and subordinate male in a colony are confronted with a male intruder, eltoprazine reduced offensive behaviour of both the dominant and the subordinate against the intruder. In contrast, chlordiazepoxide enhanced aggression, at least at lower doses, whereas alcohol had, up to very high doses, no effect on the offensive behaviour. In a brain-stimulation induced offensive model--hypothalamically-induced aggression in rats--eltoprazine specifically reduces offence. Locomotion, a measure for sedation, was either unaffected or even somewhat enhanced, indicating the absence of any sedatory activity of this serenic compound. In contrast, haloperidol heavily sedated animals, making them incapable of aggression.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of mouse killing behavior by serotonin-mimetic drugs: effects of partial alterations of serotonin neurotransmission

Rats which do not kill mice and which acquire mouse killing behavior after partial lesion of the serotonin neurotransmission, either by p-chlorophenylalanine treatment or by electrolytical lesions of dorsal and median raphe nucleus, were treated by IP injection of serotonin-mimetics. The following drugs were used: 5-methoxy-N-N-dimethyl-tryptamine and 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide, serotonin-agonists, fluoxetine and citalopram, inhibitors of serotonin uptake. All these serotonin-mimetics inhibit mouse killing behavior without apparent secondary effects. When these compounds were tested on killer rats, a stronger antimuricidal effect was observed in rats having altered serotonin neurotransmission. These results support a role for the serotoninergic supersensitivity in a model of aggressive behavior.

Modulatory actions of benzodiazepine receptor ligands on agonistic behaviour

Several experiments were conducted to establish the role of benzodiazepine (BDZ) receptor ligands in aggressive behaviour in male and female rats. In particular, the pro-aggressive effects of BDZ agonists was subject of investigation. Predatory aggression (mouse killing) was facilitated by chlordiazepoxide (CDP) when tested in naive female rats, but CDP was unable to induce muricide in non-killing rats with extensive experience with mice. In experiments on maternal aggression in rats a post-hoc analysis revealed that the pro-aggressive action of CDP on maternal aggression was base line dependent: the increase in aggression in spontaneously low aggressive females was significantly higher than in females with a higher base line level. A further study aimed at unravelling the underlying factors contributing to this pro-aggressive action by determining the role of opponent size on the effects induced by CDP. Normally large opponents evoke less aggression from lactating females than smaller opponents and CDP exerted its pro-aggressive effect particularly strongly in the 'large opponent' situation. An ethological analysis was made of lateral display--an ambivalent posture frequently occurring in agonistic behaviour--to establish whether CDP indirectly increases aggression by reducing fear by means of its anxiolytic properties. The data partly support this hypothesis. These findings stress the importance of environmental and experiential factors in the possible outcome of CDP effects on aggression. Moreover, they point to possible explanations of seemingly contradictory data. In two final experiments an inverse benzodiazepine receptor agonist (beta-CCE) was tested in maternal aggression. beta-CCE reduced aggression, although not in a completely specific way. A neutral BDZ-receptor antagonist (Ro 15-1788) was tried in an attempt to antagonize the pro-aggressive effects of CDP in maternal aggression. Ro 15-1788 did not counteract the pro-aggressive action of CDP, but antagonized CDP effects on exploration. The modulatory role of the benzodiazepine receptor complex in aggression remains an intriguing area of research in which many subtleties in testing conditions play a role and in which more BDZ agonists, inverse agonists and antagonists have to be tested.

A comparison of the effects of acute and subacute administration of beta-phenylethylamine and d-amphetamine on mouse killing behavior of rats

beta-Phenylethylamine (PEA) is an endogenous amine that in some instances acts biochemically and behaviorally like amphetamine. In the present experiments, the effects of PEA on mouse killing by rats were compared and constrasted with the effects of d-amphetamine on this behavior. When given acutely to experienced mouse killing rats, PEA (16 and 32 mg/kg) inhibited killing in a direct dose dependent manner. This is similar to the dose dependent inhibition of killing by amphetamine reported previously. However, d-amphetamine but not PEA showed physiologic tolerance following 8 days of twice daily administration. Cross tolerance between the two drugs only occurred when d-amphetamine was administered subacutely. It was concluded that PEA and d-amphetamine have sililar acute effects but differed when given subacutely since PEA did not show tolerance and there was not bidirectional cross-tolerance. These data suggest that these drugs have different pharmacologic actions when given repeatedly. One possible difference may be the duration of action.