Behavioral Aspects of Serotonin

The practices of modern psychiatry are based on the current understanding of the functioning of the human mind (Churchland, 1986). Psychotropic medications act on the system of neurotransmitters at the synapses of neurons. The neuron is the fundamental cellular communication unit of the nervous system. Within the neuron, the process of transmitting information is electrical in nature. There is a gap between the end of the axon of one neuron and the dendrites of the next neuron, this gap is called the synapse (Kruck, 1991). Neurons communicate by producing chemical substances called neurotransmitters that are sent across the synapse (Heston, 1992). The action of neurotransmitters is the basis for brain functioning. These chemicals have been linked to behaviors as diverse as learning and memory, motor activity, thirst, pain, thermo-regulation, pleasure, stress, emotions, mood and sexual receptivity (Moss, 1984).

Most theories assign a role of general inhibition to serotonin, either behavioral of emotional. Some assign a central role in the states of anxiety to serotonin (Zuckermann, 1991). One behavioral manifestation in humans and other animals that is believed to be related to levels of a neurotransmitter is aggression. Aggression is specifically thought to be related to levels of serotonin. Although not under the control of serotonin, aggression appears to be greatly influenced by it. In 1973 Luigi Valzelli studied aggressive behavior in mice. Valzelli isolated male mice and then examined the turnover rate of serotonin after four weeks. Turnover is analogous to usage of serotonin; it is the amount that is transmitted across the synapse and re-synthesized to replace the amount of serotonin used. Valzelli found that there was a decrease in serotonin turnover rate as a result of social isolation. The mice were also found to be more aggressive when placed with other males. When Valzelli compared the different genetic strains of mice, he found that the ones that fought the most had the lowest levels of serotonin turnover.

Valzelli also isolated male rats, measured their serotonin turnover rates and placed the rats with mice. The male rats had different reactions to social isolation. Some rats had a decrease in serotonin turnover level, other maintained a constant rate, and some even had increased rates of turnover. When placed with the mice, the rats with the low turnover rates attacked and killed the mice. Those rats with a constant serotonin turnover rate ignored the mice, and those with an increased rate became friendly towards the mice (Valzelli, 1973). In humans, low levels of serotonin are thought to be related to violent behavior, including impulsive homicide and suicide. The depletion of serotonin is believed to disinhibit aggressive behavior (Soubrie, 1986).

5-HIAA is the metabolite of serotonin (5-hydroxytryptamine) after the neurotransmitter is catalyzed by Monamine Oxidase. The level of 5-HIAA is thought to correspond to the amount of serotonin transmitted (Changeux, 1985). In groups of non-depressed people, levels of 5-HIAA were found to be positively correlated with socialization, assertiveness and irritability. 5-HIAA levels were negatively correlated with impulsivity (Schalling, 1984). A study by Brown et al. in 1979 shows that the manifestation of aggression can be predicted by the level of serotonin.

What are the implications of these studies? First, it may be that re-adjustment of serotonin turnover to a higher, normal level would decrease aggressive action, and a further re-adjustment to above normal levels may be even more beneficial. Fluoxetine has been shown to reduce murderous aggression in rats (Fuller, 1992). There are also social implications, such as the possibly counterproductive nature of solitary confinement in prisons. In Valzelli's studies on rats, the conditions of social isolation may have caused the low levels of serotonin turnover, which was correlated with aggression. The immediate cause of the aggressive behavior may have been the low serotonin turnover, however the root cause was an environmental condition. Also, different individuals reacted differently to the same situation. There may have been a predisposition to the aggressive condition that was triggered by the stimulus of being socially isolated.

In a normally functioning brain, serotonin may act as a mechanism through which the brain controls states of anxiety, aggression and temperament. Examining the levels of serotonin may reveal the conditions of the system and may reflect the temperament of the organism. If there is an abnormality in this system, it may be manifested in temperament and behavior.

The area of social interactions may be considered in relation to neurotransmitter levels. The level of social interaction initiated by the person on the medication is often influenced. Not only is the amount of social interaction influenced by serotonin levels, the actual social status of the person can be effected as well (Kramer, 1993). It has been demonstrated that low levels of serotonin turnover is correlated with aggressive behavior. Higher levels of serotonin are associated with social dominance. How are these two factors related?

An animal (including humans) that has social dominance may have attained status through conflict, however once the animal is at a high level of status it no longer has to initiate conflicts. The types of conflicts associated with social rank or territorial defense rarely lead to risky physical struggle. These interactions involve ritualized behaviors that signal social status (Ricklefs, 1993). Higher status individuals have less incentive to initiate social conflict. They also have more to lose if social status is called into question. Lower status individuals may exhibit more aggressive behaviors as a sort of pro-active self defense. Kramer (1993) believes that serotonin is related to feelings of security. Those with high levels of serotonin would feel more secure, giving them a cool yet not docile behavior. Those with a lesser sense of security may be more timid, and would also be more prone to striking out as a form of defense. The elevation of serotonin levels by fluoxetine may give the effects of a perceived higher social status. This may reduce violent tendencies based on anxiety in that organism.