The activity of 5-hydroxytryptaminergic neurons has been estimated from 
measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the 
concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of 
accumulation of 5-hydroxytryptophan following the administration of an aromatic 
L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation 
of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid 
following the administration of a monoamine oxidase inhibitor (e.g., pargyline). 
The purpose of the present study was to compare these different methods under 
conditions of changing neuronal impulse traffic produced by electrical 
stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with 
chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation 
of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations 
of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in 
nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus 
were measured using HPLC coupled to an electrochemical detector. In each brain 
region, stimulation elicited an increase in the concentration of 
5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic 
acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an 
increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but 
did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic 
acid in pargyline-treated rats. The results o f this study indicate that 
although the first three methods serve as valid indices of 
5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques 
are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.