Semi-quantitative estimation of changes in noradrenaline content and intraneuronal distribution in the rat vas...

A semi-quantitative histochemical assay for noradrenaline was developed, based on the assumption that the rate of reaction of noradrenaline with paraformaldehyde depends on transmitter concentration. Changes in organ noradrenaline content caused by drugs or cold-stress were associated with similar changes in fluorescence intensity of organ samples taken for microscopy. Differences in the fluorescence intensity of experimental and control tissues were also found when there was no change in total noradrenaline content, suggesting that fluorescence intensity is not a simple function of whole organ noradrenaline content. Changes in the relative fluorescence of experimental tissues with different paraformaldehyde exposures suggested that the intraneuronal distribution of noradrenaline may affect the rate of development of fluorescence. Analysis of the time course of the fluorescence reaction showed that this was best described by the sum of two first-order exponential components of different half-life. Further results suggested that the first, fast component represents vesicle-bound noradrenaline, while the slow component represents extrangranular transmitter.