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Although many animals that perform sexual reproduction exhibit sexual dimorphism, individuals with intersex traits between the traits of males and females appear in some species, depending on environmental factors.

γ-Aminobutyric acid (GABA) is an amino acid found in unpolished rice, chocolate, tea, and other foods. It is an important inhibitory neurotransmitter. However, the influence of GABA on object recognition and working memory is still unknown. In this study, the effects of GABA on novel object recognition (NOR) memory and working memory were examined. The proper retention interval and delay time were also investigated for the NOR test and T-maze test, respectively. Male 3-wk-old Wistar rats were allowed free access to food and water containing 0.5% GABA or 1% GABA for a month. After that, the rats performed the NOR test at a 48 h retention interval and T-maze test at a 900 s delay time to estimate the effects of GABA on learning behavior. The results showed that the object information in the NOR test was stored as long-term memory and the recognition index (RI) was significantly increased after GABA administration. The accuracy rate also significantly increased after GABA administration. These indicate that GABA may be involved in long-term object recognition memory and working memory.

Theanine, γ-glutamylethylamide, is one of the major amino acid components in green tea. In this study, cognitive function and the related mechanism were examined in theanine-administered young rats. Newborn rats were fed theanine through dams, which were fed water containing 0.3% theanine, and then fed water containing 0.3% theanine after weaning. Theanine level in the brain was under the detectable limit 6 weeks after the start of theanine administration. Theanine administration did not influence locomotor activity in the open-field test. However, rearing behavior was significantly increased in theanine-administered rats, suggesting that exploratory activity is increased by theanine intake. Furthermore, object recognition memory was enhanced in theanine-administered rats. The increase in exploratory activity in the open-field test seems to be associated with the enhanced object recognition memory after theanine administration. On the other hand, long-term potentiation (LTP) induction at the perforant path-granule cell synapse was not changed by theanine administration. To check hippocampal neurogenesis, BrdU was injected into rats 3 weeks after the start of theanine administration, and brain-derived neurotropic factor (BDNF) level was significantly increased at this time. Theanine intake significantly increased the number of BrdU-, Ki67-, and DCX-labeled cells in the granule cell layer 6 weeks after the start of theanine administration. This study indicates that 0.3% theanine administration facilitates neurogenesis in the developing hippocampus followed by enhanced recognition memory. Theanine intake may be of benefit to the postnatal development of hippocampal function.