Functional brain imaging in humans has revealed task-specific increases in brain activity that are associated with various mental activities. In the same studies, mysterious, task-independent decreases have also frequently been encountered, especially when the tasks of interest have been compared with a passive state, such as simple fixation or eyes closed. These decreases have raised the possibility that there might be a baseline or resting state of brain function involving a specific set of mental operations. We explore this possibility, including the manner in which we might define a baseline and the implications of such a baseline for our understanding of brain function.

https://www.biac.duke.edu/education/courses/fall04/fmri/readings/week10/2001_NatRevNeuro_Gusnard.pdf

Searching for a baseline: Functional imaging and the resting human brain

The development of functional magnetic resonance imaging (fMRI) has brought together a broad community of scientists interested in measuring the neural basis of the human mind. Because fMRI signals are an indirect measure of neural activity, interpreting these signals to make deductions about the nervous system requires some understanding of the signaling mechanisms. We describe our current understanding of the causal relationships between neural activity and the blood-oxygen-level-dependent (BOLD) signal, and we review how these analyses have challenged some basic assumptions that have guided neuroscience. We conclude with a discussion of how to use the BOLD signal to make inferences about the neural signal.

Interpreting the BOLD signal.

From over 100 children studied with 2-deoxy-2[18F]fluoro-D-glucose and positron emission tomography we selected 29 children (aged 5 days to 15.1 years) who had suffered transient neurological events not significantly affecting normal neurodevelopment. These 29 children were reasonably representative of normal children and provided an otherwise unobtainable population in which to study developmental changes in local cerebral metabolic rates for glucose (lCMRGlc). In infants less than 5 weeks old lCMRGlc was highest in sensorimotor cortex, thalamus, brainstem, and cerebellar vermis. By 3 months, lCMRGlc had increased in parietal, temporal, and occipital cortices; basal ganglia; and cerebellar cortex. Frontal and dorsolateral occipital cortical regions displayed a maturational rise in lCMRGlc by approximately 6 to 8 months. Absolute values of lCMRGlc for various grey matter regions were low at birth (13 to 25 mumol/min/100 gm), and rapidly rose to reach adult values (19 to 33 mumol/min/100 gm) by 2 years. lCMRGlc continued to rise until, by 3 to 4 years, it reached values of 49 to 65 mumol/min/100 gm in most regions. These high rates were maintained until approximately 9 years, when they began to decline, and reached adult rates again by the latter part of the second decade. The highest increases of lCMRGlc over adult values occurred in cerebral cortical structures; lesser increases were seen in subcortical structures and in the cerebellum. This time course of lCMRGlc changes matches that describing the process of initial overproduction and subsequent elimination of excessive neurons, synapses, and dendritic spines known to occur in the developing brain. The determination of changing metabolic patterns accompanying normal brain development is a necessary prelude to the study of abnormal brain development with positron emission tomography.

Positron emission tomography study of human brain functional development.

Functional brain imaging with positron emission tomography and magnetic resonance imaging has been used extensively to map regional changes in brain activity. The signal used by both techniques is based on changes in local circulation and metabolism (brain work). Our understanding of the cell biology of these changes has progressed greatly in the past decade. New insights have emerged on the role of astrocytes in signal transduction as has an appreciation of the unique contribution of aerobic glycolysis to brain energy metabolism. Likewise our understanding of the neurophysiologic processes responsible for imaging signals has progressed from an assumption that spiking activity (output) of neurons is most relevant to one focused on their input. Finally, neuroimaging, with its unique metabolic perspective, has alerted us to the ongoing and costly intrinsic activity within brain systems that most likely represents the largest fraction of the brain's functional activity.

Brain Work and Brain Imaging

Normal development of brain circuits.

Sokoloff later noted, "This report provides evidence that the increase in glucose utilization evoked by functional activation
 in neural tissue is due to the opening of Na+ channels and activation of Na+, K+ -ATPase activity."

Activity-Dependent Energy Metabolism in Rat Posterior Pituitary Primarily Reflects Sodium Pump Activity

Extracellularly applied N-succinimidyl [2,3-3H]propionate was used in vivo to covalently label intra-axonal proteins in the rat sciatic nerve. This technique permitted a unique view of axonal transport of proteins independent of biosynthesis. The proteins detected in slow anterograde transport (1 to 2 millimeters per day) correspond to cytoskeletal proteins described in previous papers. The slowly retrogradely transported component (3 to 6 millimeters per day) was composed primarily of a single protein with a molecular weight of 68,000.

https://science.sciencemag.org/content/sci/208/4441/303.full.pdf

Retrograde axonal transport of endogenous proteins in sciatic nerve demonstrated by covalent labeling in vivo

The injection of [2,3-3H]N-succinimidyl propionate ([3H]N-SP) into the rat sciatic nerve was used to covalently label both intra- and extra-axonal proteins. While extra-axonal proteins (e.g., myelin proteins) remained in the injection site, the intra-axonal proteins were transported in both the anterograde and retrograde directions. The mobile labeled proteins appeared to move by normal axonal transport processes because: (a) autoradiographic studies showed that they were localized exclusively within the axon at considerable distances from the injection site, (b) specific and identifiable proteins (by SDS gel electrophoresis) moved at expected rates in the anterograde direction, and (c) an entirely different profile of proteins moved in the anterograde vs. retrograde direction. This novel experimental approach to axonal transport, which is independent of de novo protein synthesis, provided a unique view of slow anterograde transport, and particularly of retrograde transport of endogenous proteins. A large quantity of a 68,000 mol wt proteins, moving at approximately 3-6 mm/day, dominated the retograde transport profile. [3H]N-SP, therefore, represents a new and unique "vital stain" which may find many applications in cell biology.

/pdf/axonal-transport-of-proteins-a-new-view-using-in-vivo-26nd9jvob3.pdf

Axonal transport of proteins. A new view using in vivo covalent labeling.

The use of a labeled acylating probe for the study of fast axonal transport, in vivo.

Reverse transcriptase polymerase chain reaction (RT-PCR) can be used to amplify mRNA to determine expression of rare messages from small numbers of cells. Quantitative analysis of these messages can be achieved by a modification known as the competitive polymerase chain reaction (cPCR), ~l-s~ in which an internal standard is coamplified with the sample and the relative amount of each product is determined. Because the efficiency of amplification of the standard is identical to that of the sample template, cPCR circumvents the problems associated with tube-to-tube or sample-to-sample variability of the PCR reaction, 11~ thereby providing a means to estimate the level of mRNA accurately. Different types of internal standards may be created for cPCR; these include templates different in length from the target sequence, ~'6-8~ templates mutated to either create or delete a restriction site, Is'4~ and templates derived from other genes that fortuitously use the same primers but whose internal sequence remains unknown. ~2~ Although each of these techniques provides certain advantages, the steps of mutagenesis and cloning needed to construct these internal standards are often time-consuming. We describe here a rapid and simple method that uses PCR to generate deletion constructs that can be used as internal standards to measure the level of a specific mRNA by cPCR. The basis of this method is PCR amplification of part of the target gene sequence with the same 3' primer but a recombinant S' primer to produce a shortened template that can be amplified by the original primer pair and is then used as an internal standard for cPCR. We demonstrate the use of this method to produce a competitive internal standard for the ciliary neurotrophic factor (CNTF) gene that is 100 bp shorter than the native sequence, and the use of this standard to quantitate the amount of CNTF mRNA in rat sciatic nerve.

David J. Fink

Papers

Activity-Dependent Energy Metabolism in Rat Posterior Pituitary Primarily Reflects Sodium Pump Activity

Retrograde axonal transport of endogenous proteins in sciatic nerve demonstrated by covalent labeling in vivo

Axonal transport of proteins. A new view using in vivo covalent labeling.

The use of a labeled acylating probe for the study of fast axonal transport, in vivo.

Rapid Construction of