Carbohydrate and tryptophan induced increase in brain serotonin: biochemical and behavioral correlates

Behavioral and biochemical correlates of the carbohydrate and tryptophan induced increase in brain serotonin were investigated in a series of 4 experiments. Experiment 1 was conducted to establish the nadir of brain tryptophan during the dark phase of the light cycle, Following a 16 hour fast, brains were removed, at 1600, 1800, 2000, 2.400, and 0400, hrs for tryptophan determination. Analyses indicated no differences in brain tryptophan throughout the dark period. The time course and peak concentrations of the carbohydrate and tryptophan induced increase in brain tryptophan and serotonin were determined in Experiment 2. Rats were fasted from 0030 to 1730 and then offered a control diet and injected with saline or 50 mg/kg tryptophan, or offered a high carbohydrate, protein-free meal and injected with saline. One hour after treatment and hourly for the next 3 hours, brains were obtained for analysis of tryptophan and serotonin. Tryptophan injected rats exhibited a peak in brain tryptophan at 1 hour post injection and a fall in tryptophan to control levels by 2 hours. Carbohydrate fed animals exhibited an increase in brain tryptophan at all times observed. Elevated brain serotonin was found in both tryptophan and carbohydrate treated animals. Experiment 3 was conducted to establish a behavioral correlate of brain serotonin. Behaviors investigated included: latency to step-down and explore a novel chamber and acquisition and extinction of a passive avoidance response. Animals were fed ad libitum, and 1 hour (1700) prior to behavioral testing injected with saline or 50 mg/kg tryptophan. Animals did not differ on measures of passive avoidance acquisition or extinction. However, tryptophan injected animals were found to exhibit a longer latency to step-down and explore a novel chamber than controls. In Experiment 4, plasma corticosterone, latency to step-down, rearing, urination, and defecation in a novel chamber were assessed. Animals were fasted from 2400 to 1700 and injected and fed as in Experiment 2. One and 2 hours following treatment, behaviors were observed. Thereafter, brains were removed for determination of tryptophan and serotonin and blood obtained for plasma corticosterone analysis. In tryptophan administered rats, brain tryptophan was observed to peak at 1 hour post injection and to remain higher than controls at 2 hours post injection. Carbohydrate fed rats were found to exhibit higher levels of brain tryptophan than control animals at both times assayed. Brain serotonin was found to peak in tryptophan treated rats at 1 hour post injection and to remain elevated at 2 hours. No changes in brain serotonin were revealed in carbohydrate fed animals. No group differences were observed for any of the behavioral measures taken. However, increased plasma corticosterone was found in rats fed the high carbohydrate meal. These data revealed that injection of tryptophan resulted in an increased latency to step-down and explore a novel chamber when animals were fed ab libitum, whereas carbohydrate ingestion resulted in an increase in plasma corticosterone with no effect on behavior. Confirmation that serotonin mediated these biochemical and behavioral changes awaits further research.Land and Food Systems, Faculty ofGraduat