Vol 4 n° 2 - Cerebral aspects of reproductive endocrinology
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1 9 2 Posters & images in neuroscience Functional neuroimaging as a direct probe of the effects of gonadal steroid hormones on the brain Measurable parameters of brain function No single parameter completely, or even best, describes the functional status of the brain. Any measurement of brain “activity” subsumes a complex set of biochemical and physiological phenomena subserving diverse neu- ronal activities, such as cellular homeostasis, neuronal excitation and inhibition, maintenance of membrane potentials, and plastic change at the cellular or subcellu- lar level.The choice of which parameter to measure in a given study must be guided by the particular research question, and the use of multiple imaging methods to obtain information about several different parameters in the  same  patients  is  perhaps  the  most  informative approach (Figure 1). Measures of general neuronal activity The  idea  of  measuring  regional  cerebral  blood  flow (rCBF, with positron emission tomography [PET] and IV H215O or inhaled 15O2 or C15O) or blood oxygenation level   (with   functional   magnetic   resonace   imaging [fMRI]) to assess neural activity is well grounded in a firm theoretical base beginning with observations in the late 1800s that an augmented level of tissue function is sustained by increasing the rate of oxygen consumption and, therefore, the flow of oxygenated blood to the tis- sue (in this case, brain). Because these parameters can be measured in less than a minute and repeatedly, they are well suited to delineating the cerebral concomitants of transient mental phenomena such as cognition and emotion. The brain’s energy requirements, among the highest of any organ system, are normally provided by blood glu- cose.The PET [18F]fluorodeoxyglucose (FDG) technique for measuring local cerebral metabolic rate of glucose (CMRGlu) is based on the fact that deoxyglucose and glucose are transported across the blood–brain barrier by the same carrier, but in cerebral tissues they are phos- phorylated to deoxyglucose-6-phosphate and glucose- 6-phosphate, respectively, which have differing fates.The latter is metabolized to CO2 and water, while the former is “trapped” in neurons long enough to be imaged, if radiolabeled.  The  long  measurement  period  of  this method (a 20- to 30-min scan carried out 30 to 40 min postinjection, when CMRGlu is assumed to have reached a steady state) limits its temporal resolution and sensi- tivity to cognitive and acute pharmacological activations; it is, therefore, best suited to providing detailed, quanti- tative maps of trait-like brain functional characteristics, as opposed to mental states. Measurement of neurochemical systems Using PET to image and quantify the functional activity of various neurochemical system components (eg, neuro- There is considerable evidence from animal studies that gonadal steroid hormones modulate neuronal activity and affect behavior. In humans, however, the behavioral and cognitive evidence has not been conclusive, and, until recently, there have been few direct neurophysiological data. Functional brain imaging offers unique opportunities to characterize in humans the effects of gonadal steroid hormones on basic neurobiological parameters, such as neuronal metabolism and neurochemical systems, and to clarify the interactions between these hormones and cognition and mood regulation in health and disease. The most commonly used tools within the considerable armamentarium available for such research and the parameters of neural function that they can access are briefly reviewed here.