Fading: a new cytofluorimetric measure quantifying serotonin in the presence of catecholamines at the cellular level in brain.

A new histopharmacological approach to the study of serotonergic neurons is described. Using a computerized microspectrofluorimeter, we demonstrated that a measure of fluorescence fading reliably detects changes in serotonin even in the presence of catecholamines. This "fading measure" was validated with model droplets containing serotonin, norepinephrine or mixtures of the two, and with in vivo studies using pargyline to increase brain serotonin. After various doses of pargyline, a correlation of 0.927 was found between our intraperikaryal fading measure and a standard florimetric measure of serotonin in dissected samples of the raphe. Further experiments were designed to test the feasibility of the cytofluorimetric fading measure for quantifying differential changes in serotonin in intraperikaryal and extraperikaryal regions. Both LSD (75 and 150 microgram/kg) and a low dose of the monoamine oxidase inhibitor pargyline (25 mg/kg) increased intraperikaryal serotonin without affecting extraperikaryal fluorescence. Conversely, the serotonin reuptake inhibitor fluoxetine produced the opposite effect of selectively increasing extraperikaryal serotonin. Another inhibitor of serotonin reuptake, chlorimipramine, which also blocks reuptake of norepinephrine in vivo, markedly increased both intraperikaryal and extraperikaryal serotonin. These results confirm the utility of cytofluorimetric measures of serotonin within raphe cell bodies from untreated rats, and indicate that changes in the intracellular and extracellular concentrations of serotonin can be differentiated.