Experimental Effects of Free Serotonin on the Brain and Its Relation to Brain Injury

S EROTONIN, a naturally occurring amine, can be found throughout the mammalian brain although the concentrations in the brain stem, limbic lobe, and hypothalmus are 20 to 40 times greater than those in the cortex, white matter, or cerebellumY 3 The material occupies an intracellular neuronal site, as demonstrated by fluorescent 1%17 and radioisotope 25,46 studies, where it accumulates in synaptic terminals in high concentration. Within the neurons, serotonin is found in morphologically distinct vesicles2 G,46 Apparently the vesicles hold serotonin safe f rom enzymatic destruction by monoamine oxidaseY v Despite extensive investigations, the neurophysiologic function of serotonin remains unknown. The best-known central nervous system effects are mood depression with low cerebral serotonin levels and mood elevation with increased concentrations. 47 Behavior can be experimentally altered in the cat by intraventricular serotonin injection. Sedation, diminished motor activity, lethargy, tremor, and convulsions have all been reported? 9,~9,as The body temperature response to third ventricular injection of serotonin is dose dependent; small doses cause hyperthermia, and larger doses, hypothermiaY