Biology 202
2004 Second Web Paper
On Serendip

Aside from the strict science and reporting in genetic based cases, one of the major points that have been stressed in all studies of genes for behaviorism is the minimal affect they have compared to any environmental factors. All scientists agree on the idea that behavior stems from the nervous system, but the real question has been the degree that the nurturing environment plays in initiating certain behaviors. The easiest breakdown is comparing violence genes to those for alcoholism. Since it is assumed that alcoholism is genetic, one must understand that the genes cannot take any affect until someone is exposed to alcohol. In the same way, violence must be initiated by a case of abuse before the cycle can be perpetuated. It is quite obvious that no matter what chemicals or genes are found to be related to violence, all cases start from the impact of a patient's surroundings.

Although all violence was traditionally thought to be in the realm of sociology, or psychology, we are now finding increasing evidence of its biological initiation. Many recent studies support the notion of a genetic "deficiency" causing aggressive behavior. These genes code for certain enzymes that are responsible for the metabolism or synthesis of neurotransmitters. This genetic analysis will show the genes coding for the Monoamine oxidase (MAOA) and Tryptophan hydroxylase (TPH) enzymes (catalyzing proteins) have been linked to specific cases of violent behavior.

Each of these enzymes works on neurotransmitters to control activity in the brain. A neurotransmitter is essentially a type of chemical that carries a signal across a synapse between neurons. The primary neurotransmitters that have been associated with the onset of aggression or violence are Serotonin (5-HT), Norepinephrine, and Dopamine. These neurotransmitters are three of the most common chemicals found in the brain. Serotonin is responsible for different moods, appetite, sexual activity, homeostasis, and sleep. Norepinephrine is affected by stress and moods in the brain; it is also involved in the sympathetic nervous system. (2) Dopamine is used to regulate emotion, the "pleasure center" of the brain, and motivation. (3)

In order to comprehend the function and relation of neurotransmitters better, one must understand the way in which neurons communicate via synapses. A common nerve cell holds a partially negative charge, relative to its outside environment. There are various channels that allow the flow of positive sodium ions into the nerve cell. These sodium channels are activated when the channel prior to it has moved in ions. This essential "domino effect" of allowing positive ions into the cell creates what is known as an action potential. When the action potential reaches the end of the axon it will allow for the intake of calcium ions, and the release of synaptic vessels which contain the neurotransmitter chemicals. When these vessels reach the axon's cell membrane they release their neurotransmitters into the synaptic cleft. All of these neurotransmitters are taken up by the dendrites of the next cell's membrane.

The MAOA enzyme operates on the molecules leftover in the axon. Monoamine Oxidase is an enzyme used to metabolize the neurotransmitters Serotonin, Norepinephrine, and Dopamine. The purpose of the MAOA enzyme is essential to inhibit the reactivity of the neurotransmitters. (4) Any leftover neurotransmitters will be broken down by the MAOA enzymes. Since this enzyme is translated from a gene that is located on the X-chromosome, of which women have 2 copies and males only one, males have a greater probability of having a deficiency of the enzyme.

Another interesting aspect of the study conducted on MAOA stated that the link between violence and genetic mutations in which no genes for MAOA existed, proved inconclusive for an entire population. (4) The reason that these results are not conclusive on the entire population is in relation to the entire nature versus nurture battle. On a whole, the majority of the population has not experienced abusive situations. After narrowing the search criteria, the research did eventually find links between the MAOA enzyme and aggression. Such results further the notion that genetic backgrounds are not utilized without a behavioral initiator.

The most cohesive link was found between the MAOA enzyme
activity and adolescent conduct disorder in 'maltreated' males. (4) The conclusions drawn from these studies show that although there are instances of the MAOA enzyme being completely deficient, these cases are rare. There is, however, a large portion of the population which has a low MAOA enzyme activity. (4) Whenever neurotransmitters are released, from fear etc., they will remain in the synaptic cleft and cause more aggressive behavior. In previously abused children, this activity bolsters violent behavior by stopping Serotonin activity. (4)

The other enzyme that has been equally promoted as a cause of violent is TPH, an enzyme which is concerned with limiting the rate of synthesis of the neurotransmitter Serotonin. (5) The biology behind the TPH enzymes makes scientists aware of the fact that it is the only catalyst in the reaction producing Serotonin and therefore can limit its production.(1) Many studies have shown that altered Serotonergic activity exists in many males with suicidal and aggression issues. (6) Any deficiency in the amount of TPH produced creates a dearth of Serotonin in areas of the brain which use it to hinder impulsive behavior. Many published experiments show that in order to understand the prevalence of cases with TPH deficiency better, one must look at the genetics basis of the enzyme's production.

The TPH allele is associated with the gene A218C. One of the studies conducted with TPH enzymes showed that people with a single nucleotide substitution on the TPH gene, creating an A779C single, had more issues with aggression.(5) The presence of the A779C is what leads to a deficiency in the amount of TPH present in the brain.(1) The lack of this TPH will consequently cause a lower than normal level of Serotonin production. The low Serotonin level will lead to difficulties in inhibiting impulsive behaviors.

As seen in the MAOA enzymes, the lack of the TPH enzyme is also not something that is found in a majority of the population. When examining the various scientific studies, one cannot help but understand that genetics is not the sole factor in violent behavior. The scarcity of cases of violent behavior with relation to deficient enzymes shows that not all violent behavior can be accounted for through genetics. This is not to say that there is no genetic basis for behavior in genes, but one can safely maintain the notion that outside influence plays a larger role in the behavior of a person.

References

1) Dysfunction in the neural circuitry of Emotion regulation- a possible prelude to violence

2)Definition of Norepinephrine

3)Dopamine definition

4) Role of Genotype in the Cycle of Violence in Maltreated Children. Science Magazine Auguest 2, 2002. Vol. 297

5)TPH synthesis

6)Biology of Violence presentation

7) , Serotonin description


| Course Home Page | Course Forum | Brain and Behavior | Serendip Home |