Brain amine measurement

As outlined previously, initial biological hypotheses of mood disorders were derived from the pharmacological actions of antidepressant drugs, which increase synaptic concentrations of noradrenaline (NA] and/or serotonin. Consequently, the biogenic amine hypothesis was formulated, but it lacks consistent proof that NA and/or serotonin release is indeed diminished. This failure is in part a result of the limited access to relevant brain areas. Measurements of NA and serotonin and their metabolites in blood, urine, and CSF... 

Catecholamines. The quantitative determination of dopamine and noradrenaline in tissue samples of 0.1-10 mg at levels in the order of 0.5 pmol has been described [84]. These methods are based on extraction, formation of the pentafluorpropionyl derivatives, and the use of the homologues, a-methyidopamine and a-methylnoradrenaline as internal standards in SIM. Higher sensitivity than obtainable with fluorimetric or enzymic assays is reported [462J. Applications have been to amine determination in specific regions of rat brain [84] and to measurement of heart ventricle concentrations [463]. A combination of assays of this type with the use of synthesis inhibitors or radioisotope labelled precursors allows direct estimation of brain amine turnover in animals. 

Fig. 5. Possible approaches to LC measurements of brain amines and metabolites.

Electrochemical techniques in vivo use the standard three electrode voltammetric system described earlier with the electrodes implanted in the brain of the animal subject. Measurements are made by acquiring some stable baseline signal and then stimulating release of the biogenic amine neurotransmitters. The change in signal is then a measure of the concentration of neurotransmitter in the extracellular fluid. 

Tryptamines that are unsubstituted on the terminal amine are good substrates for oxidative deamination by MAO. Furthermore, it has been demonstrated that tryptamine and 5-methoxytryptamine cross the blood-brain barrier with great difficulty administration of 50 mg/kg 5-methoxytryptamine to rats results in a low brain/plasma ratio when measured 15 min postadministration (242). 

Biochemical changes in animal central nervous sterns have been reported by Skillen et who noted a decrease in brain 5-hydroxytiypt-amine (serotonin) in rats exposed to ozone at 6 ppm for 4 h, and by Trams et who observed decreases in catecholamines and catechol-O-methyl-transferase in dogs chronically exposed to ozone at 1,2, or 3 ppm. Electro-encephalographic (eeg) measurements in the same dogs were recently presented by Johnson et who noted alterations in eeg patterns at 9 months of ozone exposure, but not after 18 months of exposure. Previously, Xintaras et o/. had observed alterations in the visual evoked electric response in rats acutely exposed to 0.5-1.0 ppm. As pointed out by Johnson et it is not clear whether these findings indicate a direct neurotoxic action of ozone or are secondary to damage in other organs. 

Because it was known by 1960 that the pontine brain stem was crucial to REM sleep generation, it was natural to assume that the newly discovered modulatory elements played an important role in its generation. It was even reasonable, on an a priori basis, that each of the elements had responsibility for one state, viz, dopamine controls waking, serotonin controls slow wave sleep, and norepinephrine controls REM. Early lesion and parenteral pharmacological studies—some even armed with measures of amine concentrations in the brain—gave initial support to this concept. For example, Michel Jouvet produced insomnia in cats by blocking the enzyme that is essential to convert tryptophane into serotonin. He interpreted this result to mean that serotonin was a sleep mediator. 

As scientists improved their techniques for detecting low levels of amines, they began to measure the amine concentrations in postmortem human brains. Several researchers measured levels of serotonin and norepinephrine in the brains of people who had committed suicide as a result of depression and compared the levels to individuals of the same age who had been killed in accidents. The suicides brains had lower levels of serotonin. Subsequent studies also revealed that certain depressed patients had lower levels of serotonin metabolites in their cerebrospinal fluid. These findings accelerated the effort to develop drugs that would be effective on serotonergic neurons. 

The kinetic parameters of Tc-ECD in the human brain have been compared with regional cerebral blood flow measurements with Xe SPECT (Devous et al. 1993) and with positron emission tomography (PET) (Tsuchida et al. 1992). The response of Tc-ECD, Tc-examethylpropylene amine oxime HMPAO and I-IMP (AT-isopro-pyl-p-[ I]-iodoamphetamine) to changes in cerebral blood flow was studied in primates (Dormehl et al. 1997). 

The amine uptake inhibiting properties of potential antidepressant compounds can also be measured biochemically by means of isolated synaptosomes, artificially tom-off nerve endings which retain their transmitter uptake system [18]. Synaptosomes can be isolated from whole brain homogenates or brain areas. By incubation of these synaptosomes with radioactively labelled NA or 5-HT after preincubation with various concentrations of inhibiting compounds, the uptake-inhibiting properties of these compounds can be measured. 

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