During the past decade, a substantial body of evidence has been accumulated which suggests that serotonin, 5-hydroxytryptamine (5-HT), may have an important role in brain function and that changes in the serotonin levels of the brain can lead to gross changes in the behavior of men and animals (WooLEu 1962). In most cases, these behavioral effects have been qualitatively described. There have been only a few behavioral studies which have attempted to alter levels of brain 5-HT and to correlate these changes with changes in behavior measured objectively.
One of the first of these studies was carried out by API~ISON and FERSTE~ (1961). They examined the effect of an increase in brain serotonln on the behavior of pigeons trained to peek a disc for intermittent food reinforcement. The elevation of brain serotonin was produced by injecting the birds with the precursor to serotonin, 5-hydroxytryptophan (5-HTP). This amino acid rapidly penetrates the "blood-brain" barrier where it is deearboxylated to form serotonin (UDENFRIE~D et al., 1957 b). The results showed that the drug lowered the rate of responding in proportion to the amount of 5-HTP injected.
In an experiment using rats trained on a discrete avoidance schedule, Jou and HuI~WlTZ (1964) found that injections of 5-HTP effected both reaction time and the number of shocks received. JOYCE and Hu!aWITZ interpreted the behavioral change as being primarily depressant. HINGTGEN and APalSON (1963) investigated the effect of decreased levels of brain serotonin on the behavior of pigeons trained on a reinforcement schedule similar to that used by API~ISON and FEI~STEI~ (1961). The birds were given c~-methyl-m-tyrosine (AIpha-MMT), a compound that in large amounts differentially depletes serotonin and norepinephrine in brain as well as in peripheral tissues (HEss et al., 1961 ; SoIn~KES et al.,