Quantification of enzyme activities in brain sections by microphotometry

A quantitative histochemistry technique for measuring regional distribution of acetylcholinesterase in the brain using digital scanning densitometry

Studies of brain acetylcholinesterase (AChE) are traditionally based on biochemical assays, immunoreactivity, and histochemistry. Conventional histochemistry yields rich morphological data from tissue sections but yields quantitative results only with great difficulty. Several histochemical methods developed in recent years, including microdensitometry, microphotometry, and video-based histochemistry, are effective in quantitative and detailed study of AChE in tissue sections. However, they are usually time-consuming. As we report here, we adapted digital scanning densitometry to quantitate AChE histochemical staining in brain sections. The AChE and butyrylcholinesterase (BuChE), as measured by the method, were heterogeneously distributed throughout the brain, results that are consistent with those obtained by biochemical methods. The staining intensity is dependent on section thickness, substrate concentration, and reaction time. The cholinesterase inhibitor methyl paraoxon significantly decreased AChE staining intensity. Furthermore, data acquired from densitometry are similar to those obtained by video-based microscopy or by spectrophotometry. The advantage of the densitometric measurements compared to other quantitative histochemical methods is that it is very rapid while collecting data that are equivalent in quality. Because the digital scanning densitometers provide high quality and sensitive imaging, wide dynamic ranges, and convenient image analysis software, they are very useful tools in quantitative histochemistry.

Mitochondrial enzymes related to glutamate and GABA metabolism in the hippocampus of young and aged rats: a quantitative histochemical study

Quantitative histochemistry (scanning microphotometry) was used to determine the activities of the mitochondrial enzymes NAD-linked isocitrate dehydrogenase (EC 1.1.1.41), L-glutamate dehydrogenase (EC 1.4.1.3) and GABA transaminase (EC 2.6.1.19) in various layers of the hippocampus (middle one third) of young (3-4 months old) and memory-impaired aged rats (28-30 months old). For comparison, determinations of cytochrome c oxidase (EC 1.9.3.1) as a marker for mitochondria and energy metabolism were also performed. The study showed that there was a layered reaction pattern in the hippocampus and that the cellular distribution and the levels of enzyme activity were different. However, the activities of the different enzymes (excepting GABA transaminase and cytochrome c oxidase) were significantly correlated in the hippocampus in both age groups. Age-dependent changes were only observed for NAD-linked isocitrate dehydrogenase and GABA transaminase (significant increases of activities in some layers of the hippocampus, preferentially in the terminal field of the perforant path). From the present study it is concluded that, 1. the enzymatic complement of mitochondria in neurons and glia depends upon layer specific metabolic processes of the hippocampus (also with respect to glutamatergic and GABAergic terminal fields) indicating a layer specific interaction of the enzymes studied to produce or catabolize glutamate and GABA, and 2. the age dependent changes of the studied enzymes are very restricted.

Microphotometric determination of enzymes in brain sections. IV. Isocitrate dehydrogenases

A polyvinyl alcohol-(PVA) containing incubation medium was adapted for the microphotometric determination (kinetic and end-point measurements) of the activities of NAD- and NADP-linked isocitrate dehydrogenases (ICDHs) in cryostat sections of the rat hippocampus. The following incubation medium is recommended for the quantification of NAD- and NADP- (differences in brackets) ICDHs: 100 mM DL-isocitrate, 10 mM sodium azide, 5 mM (4 mM) nitroblue tetrazolium (NBT), 7 mM NAD (4 mM NADP), 10 mM magnesium chloride, 0.25 mM phenazine methosulfate (PMS), with or without 5 mM ADP (without ADP), 23% PVA in 0.05 M Hepes buffer; the final pH was 7.5. With these incubation media a linear response of the reactions lasted at least 20 min. In kinetic and end-point measurements the same level of activities was demonstrable. The use of NaN3 (as a blocker of the respiratory chain) and PMS (as artificial electron carrier) was indispensible for the transfer of all reduction equivalents in the dehydrogenase reactions to the tetrazolium salt NBT. Furthermore, the activation by magnesium ions and the need of PVA to avoid diffusion artefacts of the loosely bound ICDHs were clearly shown. It is concluded that the quantification of ICDHs in situ could be a valuable tool for neurochemical investigations because ICDHs play a role not only in the substrate flux through the tricarboxylic acid cycle but also in providing alpha-ketoglutarate for the formation of glutamate which is an important amino acid in the brain.