Comparative effects of single dose and repeated oral tryptophan administration on indoleamine synthesis and memory functions in rats.

Brain functions can be affected by the availability of dietary precursors of neurotransmitters. The diet induced increase in tryptophan (TRP) availability has been shown to increase brain serotonin (5-HT, 5-hydroxytryptamine) synthesis and various related behaviors. A prominent role of serotonin in memory functions is widely acknowledged. Increased brain 5-HT concentration is shown to enhance cognitive function whereas decreased 5-HT metabolism in brain has been shown to impair memory. This study was designed to investigate the effects of single dose and repeated TRP administration on brain TRP, 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) levels and on memory functions in rats. TRP at a dose of 100 mg/kg body weight were orally administered to rats. Assessment of memory in rats was done using the water maze test (WM). Brain TRP, 5-HT and 5-HIAA levels were comparable to control after single TRP administration. Repeated administration of TRP for 6 weeks significantly increased brain TRP (P<0.05), 5-HT (P<0.01) and 5-HIAA (P<0.01) levels with respect to controls. Memory enhancement effect of TRP was not seen after single oral administration whereas repeated TRP intake significantly (P<0.01) enhanced memory functions of rats as evidenced by the decreased latency time to reach the hidden platform in WM. Our results indicate that repeated but not single oral TRP administration is involved in the enhanced memory functions in rats.