Tickling stimulation causes the up-regulation of the kallikrein family in the submandibular gland of the rat.

We recently showed that tactile stimulation (tickling) accompanied by positive emotion altered the expression of many genes in the rat hypothalamus (Hori et al., 2009 [15]). In this study, the effect of repeated tickling on gene expressions of the rat salivary gland was examined. After 4-week stimulation, several genes of the kallikrein (Klk) family were remarkably up-regulated and the alpha-amylase (amylase) gene was down-regulated in DNA microarray analysis. In quantitative analysis using real-time PCR of the submandibular gland of the rats tickled for 2 weeks, mRNAs of Klk1, Klk2 (Klk1c2, Tonin), Klk7 (Klk1l), Klk1b3 (Nerve growth factor, gamma), Klk1c10, Klks3 (Klk1c9) and GK11 were significantly 2-5-fold increased among 18 members of the Klk gene family examined and the submandibular amylase was decreased compared with the lightly touched and untouched control rats. In immunoblot analysis the increase in Klk7 protein was observed in the whole cell lysate fraction of the submandibular gland. In immunohistochemical analysis with anti-Klk7 polyclonal antibody, the immunostain was increased in duct cells of the submandibular gland of the tickled rat when compared with the lightly touched and untouched control rats. These results suggest that tactile sensory processing in the central nervous system affects the gene expression in the peripheral tissue probably via hormonal and/or autonomic neural activities. Submandibular Klks may be biochemical markers indicating positive emotional states.

Neurogenesis in the dentate gyrus of the rat hippocampus enhanced by tickling stimulation with positive emotion.

Hippocampal neurogenesis is influenced by many factors. In this study, we examined the effect of tactile stimulation (tickling), which induced positive emotion, on neurogenesis in the dentate gyrus (DG) of the hippocampus. Four week-old rats were tickled for 5 min/day on 5 consecutive days and received 5-bromo-2'-deoxyuridine (BrdU) administration for 4 days from the second tickling day. Then they were allowed to survive for 18 h or 3 weeks after the end of BrdU treatment. Neurogenesis in the DG was compared between the tickled and untickled rats by using immunohistochemistry with anti-BrdU antibody. The result showed that the number of BrdU- and NeuN (neural cell marker)-double positive neurons on 18h as well as 3 weeks of the survival periods was significantly increased in the tickled group as compared with the untickled group. The expression of mRNA of brain-derived neurotrophic factor (BDNF) in the hippocampus of the tickled rats was not altered when compared with the control rats. In conclusion, tickling stimulation which induces positive emotion may affect the generation and survival of new neurons of the DG through the BDNF-independent pathway.