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Nashville, TN, USA. A link between behavior and the activity of dopamine receptors in response to an aggressive event have been demonstrated in experiments at Vanderbilt University. According to researchers, the brain processes aggression as a reward — much like sex, food and drugs. The new findings, published in the journal Psychopharmacology, offers insights into our propensity to fight and our fascination with violent sports like boxing and football.
“Aggression occurs among virtually all vertebrates and is necessary to get and keep important resources such as mates, territory and food,” Craig Kennedy, professor of special education and pediatrics, said. “We have found that the ‘reward pathway’ in the brain becomes engaged in response to an aggressive event and that dopamine is involved.”
The rewarding effect of aggression is reduced by nucleus accumbens dopamine receptor antagonism in mice. Maria H. Couppis and Craig H. Kennedy. Psychopharmacology. ISSN 0033-3158 (Print); 1432-2072 (Online). doi 10.1007/s00213-007-1054-y.
Dopamine is a phenethylamine that occurs in both vertebrates and invertebrates. It activates receptors in the brain (neurotransmission) and is also a neurohormone released by the hypothalamus to inhibit prolactin release from the pituitary's anterior lobe. Rewarding stimuli, such as food, sex and drugs, can stimulate dopamine production. The question before the researchers was whether dopamine has a role in the reinforcement of aggression.
For the experiments, a pair of mice — one male, one female — was kept in one cage and five intruder” mice were kept in a separate cage. The female mouse was temporarily removed, and an intruder mouse was introduced in its place, triggering an aggressive response by the “home” male mouse. Aggressive behavior included tail rattle, an aggressive sideways stance, boxing and biting.
The home mouse was then trained to poke a target with its nose to get the intruder to return, at which point it again behaved aggressively toward it. The home mouse consistently poked the trigger, which was presented once a day, indicating it experienced the aggressive encounter with the intruder as a reward.
The same home mice were then treated with a drug that suppressed their dopamine receptors. After this treatment, they decreased the frequency with which they instigated the intruder’s entry.
In a separate experiment, the mice were treated with the dopamine receptor suppressors again and their movements in an open cage were observed. They showed no significant changes in overall movement compared to times when they had not received the drugs. This was done to demonstrate that their decreased aggression in the previous experiment was not caused by overall lethargy in response to the drug, a problem that had confounded previous experiments.
The research was supported by a Discovery Grant from Vanderbilt University.
“It is well known that dopamine is produced in response to rewarding stimuli such as food, sex and drugs of abuse,” Maria Couppis, who conducted the study as her doctoral thesis at Vanderbilt, said. “What we have now found is that it also serves as positive reinforcement for aggression.”
“We learned from these experiments that an individual will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it,” Kennedy said. “This shows for the first time that aggression, on its own, is motivating, and that the well-known positive reinforcer dopamine plays a critical role.”
The rewarding effect of aggression is reduced by nucleus accumbens dopamine receptor antagonism in mice. Maria H. Couppis and Craig H. Kennedy. Psychopharmacology. ISSN 0033-3158 (Print); 1432-2072 (Online). doi 10.1007/s00213-007-1054-y.
Abstract
Rationale. Dopamine (DA) receptors within the nucleus accumbens (NAc) are implicated in the rewarding properties of stimuli. Aggressive behavior can be reinforcing but the involvement of NAc DA in the reinforcing effects of aggression has yet to be demonstrated.
Objective. To microinject DA receptor antagonists into the NAc to dissociate their effects on reinforcement from their effects on aggressive behavior and general movement.
Materials and methods. Male Swiss Webster mice were implanted with guide cannulae aimed for the NAc and tested for aggressive behavior in a resident–intruder procedure. Aggressive mice were then conditioned on a variable-ratio 5 (VR-5) schedule with presentation of the intruder as the reinforcing event. The D1- and D2-like receptor antagonists SCH-23390 and sulpiride were microinfused (12–50 ng) before the mice responded on the VR-5 schedule and attacked the intruder. Open-field activity was also determined following the highest doses of these drugs.
Results. SCH-23390 and sulpiride dose-dependently reduced VR responding but did not affect open-field activity. The 50-ng SCH-23390 dose suppressed response rates by 40% and biting behaviors by 10%; other aggressive behaviors were not affected. The 25 and 50 ng sulpiride doses almost completely inhibited VR responding; the 50-ng dose suppressed biting by 50%.
Conclusions. These results suggest that both D1- and D2-like receptors in the ventral striatum are involved in the rewarding properties of aggression, but that D1-like receptors may be related to the motivation to earn reinforcement as opposed to aggressive behavior.
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For the experiments, a pair of mice — one male, one female — was kept in one cage and five intruder” mice were kept in a separate cage. The female mouse was temporarily removed, and an intruder mouse was introduced in its place, triggering an aggressive response by the “home” male mouse. Aggressive behavior included tail rattle, an aggressive sideways stance, boxing and biting. 
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