Benefit 2 Greater Ability to Measure Physiological Parameters

Animal studies afford unique opportunities to measure the physiological parameters that may underlie personality. This is because many of the techniques used to examine the biological events that lead to the expression of personality traits require decapitation and examination of brain areas, which are not possible in humans.

a. Measuring Hormone Receptor Density

Autoradiographic technology can be employed in animals to examine the density of hormone receptors in various parts of the brain. High densities in particular brain regions indicate important sites of action for the particular hormone under investigation. Using receptor binding density as an outcome variable, researchers can investigate how various hormones interact to influence personality.

In one study, Delville, Mansour and Ferris (1996b) castrated golden hamsters and implanted half of them with testosterone capsules. Vasopressin and testosterone had been previously linked to aggression, so the researchers decided to examine the effects of testosterone on vasopressin receptor binding. After sacrificing the animals, the researchers performed in vitro autoradi-ography. They found that castrated animals had a very low density of vasopressin receptor binding in the ventrolateral hypothalamus area of the brain. In addition, microinjections of vasopressin failed to increase aggression in castrated males. The authors concluded that testosterone may influence aggression by activating vasopressin receptors within the ventrolateral hypothalamus. Consistent with this result, DeLeon, Grimes, and Melloni (2002) found that anabolic-androgenic steroid treatment in adolescent male hamsters led to increased vasopressin receptor binding in the ventrolateral hypothalamus. In addition, these steroid-treated animals were more aggressive.

Without the ability to measure hormone receptor binding via autoradiography in animals, it would have been much more difficult to examine the testosterone-vasopressin receptor relationship in aggression. Furthermore, this research provides an excellent candidate model for how anabolic steroids may influence aggression in human adolescents through their effects on vasopressin receptors in key areas of the brain.

b. Measuring Gene Expression

The notion that genes and the environment exert independent effects on behavior is now considered simplistic and obsolete. Scientists now know that gene expression itself is influenced by both heredity and the environment (Hamer, 2002; Robinson, 2004). Variation in gene expression affects protein activity, brain processes, and ultimately behavior. Through the development of new genomic techniques using animal models, investigators can measure gene expression by quantifying the amount of messenger RNA (mRNA) produced by a particular gene. The ability to measure gene expression through mRNA allows researchers to consider complex, dynamic models of gene-behavior relationships. Not only can scientists investigate how environmental and hereditary factors interact to influence gene expression, but gene expression variation can also be examined as a predictor of subsequent brain processes and behavior (Gosling & Mollaghan, in press). Thanks to research conducted in animals, psychologists have begun to understand the interplay between hereditary and environmental influences on genetic activity and individual differences.

As an example, consider the animal research examining the effects of social defeat on aggression and the expression of serotonin genes. Social defeat and subordination have been associated with increased serotonin (Blanchard, Sakai, McEwen, Weiss, & Blanchard, 1993) and with decreased aggression (Huhman et al., 2003). Two substances, serotonin transporter (SERT) and MAO-A, are involved in the inactivation of serotonin (Filipenko, Beilana, Alekseyenko, Dolgov, & Kudryavtseva, 2002). Thus the effects of social defeat on SERT and MAO-A gene expression may provide clues about the genetic and biological processes that precede aggression. In one study, repeated exposure of adult male mice to social defeat resulted in greater expression of SERT and MAO-A mRNA than in either mice exposed to social victories or control mice (Filipenko et al., 2002). Thus it seems that social defeat induces the activation of the SERT and MAO-A genes. These findings suggest that the effects of social experiences (social defeats or victories) on aggression may be mediated by differential expression of various genes (SERT and MAO-A) within the serotonin system.

In other animal research, the effects of anabolic androgenic steroids on aggression have been linked to impaired functioning of the neurotransmitter gamma-aminobutyric acid (GABA) and its related genes (e.g., Miczek, Fish, & De Bold, 2003). When female mice underwent long-term testosterone therapy, they increased in aggression but decreased in mRNA expression for 5-alpha reductase type 1, a protein involved in GABA's functioning (Pinna, Costa, & Guidotti, 2005). This result suggests that testosterone treatment causes changes in gene expression, which in turn facilitate a disruption in the GABA neurotransmitter system and lead to increased aggression.

c. Other Opportunities for Physiological Measurement

In addition, animal studies offer several other opportunities for physiological measurement. For example, new imaging techniques allow animal researchers to measure neuronal activity in response to particular stimuli with a greater degree of precision than is possible in human functional magnetic resonance imaging (fMRI) studies (e.g., Whitlow, Freedland, & Porrino, 2002). Consequently, researchers can observe the specific brain areas and neurons involved in a particular biological process or behavior. Another advantage of animal studies is the ease with which neurotransmitter or hormone concentrations can be measured, because these data are normally collected through intrusive access to cerebrospinal fluid, blood, or specific brain areas.

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