A modification of the Falck-Hillarp technique for 5-HT fluorescence employing hypertonic formaldehyde perfusion.

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Abstract
The addition of a perfusion step coupled with pharmacologic pretreatment markedly improves the Falck-Hillarp fluorescence technique for 5-HT [5-hydroxytryptamine]. Even with the present improvements the axonal and terminal fluorescence of 5-HT is still less intense and less stable than the CA [catecholamine] fluorescence produced by the glyoxylic acid modification. Most approaches up to this time have attempted to improve on the 5-HT fluorescence by increasing the amount of 5-HT available for conversion to the active fluorophore.