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Thursday, October 22, 2009

Absolutes, Relatives, Brain Imaging & Steroids

The functional brain imaging methods most commonly used in humans today are functional magnetic resonance imaging (fMRI) using blood oxygen-level dependent (BOLD) signals and the subtractive water method with positron emission tomography (PET). Both procedures record changes in local cerebral blood flow from a baseline. Local cerebral blood flow is associated with energy demands of activated nerve cells. The cells consume glucose sugar and oxygen to process information which cannot be stored in the brain and must be supplied on demand with the blood stream. Hence, blood flow increases with increased nerve cell activity. In the simplest conception of both procedures known as block design, measurements acquired over several minutes of mental activation are compared with measurements acquired during equivalent epochs of rest. The statistically significant difference is considered the result of cerebral activation.

In conventional fMRI, blood oxygen-level dependent signals are used to detected changes in blood flow (Ogawa and others, 1993). With the subtractive water method (Fox and others, 1984), water labeled with 15O, a radioactive positron-emitting isotope of oxygen, is used as tracer that freely diffuses into the brain tissue. The local concentration of the tracer is commensurate with local blood flow and can be imaged in a PET scanner. However, calibration for absolute blood flow is wrought with difficulty and has not found wide-spread application. Because the blood flow is not quantified, the differences between the compared mental states remain relative. That is, cerebral activation is usually expressed in percent difference from the state used as reference or in units of statistically significant difference (statistical parametric mapping).

By contrast, the cerebral glucose consumption is more directly related to nerve cell activity than cerebral blood flow. The deoxyglucose method of Sokoloff and others (1977) permits us to measure the local cerebral rate of glucose utilization. Deoxyglucose is an analogue to glucose that accumulates in the brain tissue commensurate with glucose consumption. Tagging deoxyglucose with 18F, a radioactive positron-emitting isotope of fluorine, the tracer's accumulation in the brain can be imaged with PET. The [18F]fluorodeoxyglucose method, therefore, provides a snapshot of the brain's energy consumption (Reivich and others, 1979). Although this snapshot needs 90 minutes to develop because of the tracer kinetics involved, the procedure constitutes an indispensable tool for the detection of long-term, pseudo-stationary changes in absolute cerebral metabolic activity as a consequence of disease or trauma. Below, I discuss one example.

After the collapse of the regime of Nicolae Ceauşescu at the end of 1989, U.S. parents began to adopt children from Romanian orphanages. The children had been kept in circumstances of great depravity, producing profound behavioral problems similar to autism (American RadioWorks report, 2006). Visiting scientists reported behavioral patterns resembling those the eminent American psychologist Harry Harlow had so aptly described in primates raised in isolation and with surrogates.


Although the adoptees were brought to the U.S. at very young age, some developed cognitive and behavioral differences, including impulsive reactions as well as attention and social deficits, in the years after their arrival.

Research at the orphanages provided evidence that the children had persistently augmented levels of cortisol n their blood stream as a result of the severe stress they endured (Carlson and Earls, 1997). Cortisol is a known steroid stress hormone produced in the adrenal glands and can fundamentally affect brain maturation. The hormone suppresses the activity of glia. A type of glia, astrocytes, helps regulate the extracellular glutamate concentration. Glutamate constitutes the most prevalent excitatory neurotransmitter in the brain, playing a major role in the stabilization of connections between nerve cells during brain maturation. Elevated concentrations of extracellular glutamate can trigger pre-programmed cell death known as apoptosis, otherwise occurring only during early stages of brain development. Presumably, the orphans' excessive stress-related exposure to cortisol led to modifications of nerve cell networks, underlying the children's behavioral differences. Imaging the brain's energy consumption provided a method to uncover whether and where nerve cell activity changed in cerebral cortex as a consequence of the children's stay in the orphanages. 

Using the fluorodeoxyglucose method, Chugani and others (2001) could show that the use of glucose was drastically reduced in the cerebral cortex of the orphans enrolled in the study, particularly in temporal and prefrontal cortical areas and in structures of the limbic system, notably the amygdala. The cortical regions are involved in executive functions and short-term memory crucial for social behavior and affect. The amygdala play an important role in fearful reactions. The observed reductions in energy consumption could not have been detected with the standard fMRI or PET procedures discussed above. The fluorodeoxyglucose method, hence, constitutes the procedure of choice when the fundamental metabolic state of the brain is in question.

Addendum
  • Take some time and listen to this show on National Public Radio's This American Life with the title "Unconditional Love". The first half of the show is about an orphaned Romanian boy adopted by an American couple at the age of eight. It demonstrates in great clarity the at times overwhelming difficulties the family faced to remedy important steps of personality development that were missed early in the boy's life. Finally, the challenges were overcome with passion and a professional attitude. It is reassuring to find out that success is possible (10/23/10).

References

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