The counterpart to the inhibitory control of the brain is provided by the excitatory mechanisms which use, among others, acetylcholine and and the catecholamines norepinephrine (NE) and dopamine (DA) as neurotransmitters.
Experiments injecting acetylcholine into the hippocampus, hypothalamus, and amygdala of cats and rats demonstrates that there is a strong association between aggressive behavior and the excitatory neurotransmitter acetylcholine (Glusman 1986.) After injection, the animals became extremely vicious and attacked other animals as well as the experimenter without provocation. There was also found to be a positive correlation between the amount of acetylcholine administered and the degree of aggression displayed (Allikmets 1974.) Acetylcholine is the neurotransmitter used in neuromuscular junctions, in the peripheral autonomic system, and is also found in brain circuits. It is produced in the nucleus basalis at the base of the brain and used to communicate with the hippocampus and cerebral cortex. If an excess of this neurotransmitter is released, it seems likely that the overactivation of the hippocampus would result, leading to aggressive behavior.
More implicated excitatory neurotransmitters are norepinephrine and dopamine. NE is used virtually everywhere in the brain - the limbic system, the cerebral cortex, and the hypothalamus are the areas of particular interest for this web page however. The levels of NE in the brain are related to how well an animal or human learns or remembers. For example, people tend to remember events that were associated with strong emotinons such as anger or fear. These emotional states typically involve increased levels of NE. In a study performed by Brown, the NE metabolite MHPG was measured in 26 men with life histories of aggressive, violent, and impulsive behaviors. There was a positive correlation between the amount of MHPG measured and aggressive behavior. Dopamine is used to communicate between the hypothalamus and the pituitary gland, in the control of movement, and in the communication between the limbic system and frontal cortex.
Evidence suggests that, for the most part, dopamine and norepinephrine do not exert specific action on aggression but that they exert a general excitatory effect on behavior.
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