Release of 5-hydroxytryptamine from caudate nucleus and septum.

1. The anterior horn of one lateral ventricle was perfused in anaesthetized cats treated with inhibitors of monoamine oxidase, and the effluent was tested for 5-hydroxytryptamine (5-HT).2. The basal release of 5-HT varied from 0.25 to 4 ng in 25 min, and was usually about 1 ng.3. The basal release rose and fell with body temperature.4. Electrical stimulation for 15 min of the nucleus linearis intermedius or of the nucleus linearis rostralis caused a release of 5-HT which rarely outlasted the collection period of 25 min.5. Low frequencies (0.5/sec) were, per stimulus, more effective than high frequencies in releasing 5-HT. Over a 15 min period of stimulation, however, the highest total yield was at 20/sec; it fell abruptly at still greater frequencies.6. No release was obtained if the stimulating electrode was positioned in a variety of brain structures outside the two linear nuclei.7. The experiments indicate that 5-HT acts as a transmitter of impulses in neurones originating in the linear nuclei and terminating in caudate nucleus and septum.