Regulation of adult hippocampal neurogenesis and affective behaviors by tactile stimulation in rats

Abstract

Tactile stimulation (TS) is a commonly employed treatment strategy for different clinical conditions, including developmental and psychiatric conditions. It was shown to have potential neurogenic effects by promoting cell proliferation in the spinal cord, however, the theoretical basis and underlying biological mechanisms of tactile stimulation in the brain remain unknown. Brushing, a specific type of tactile stimulation, has been employed as a treatment to decrease sensory over-responsiveness and enhance the recovery of cognitive function and emotions in human. For further understanding of the mechanisms, this study aims to investigate the effect of single and repeated tactile stimulation on affective behaviors, adult hippocampal neurogenesis and dendritic growth in the rat brain. The present study was divided into two experiments. First, young male adult Sprague-Dawley (SD) rats were treated with TS for 1 ,14 and 28 days to assess the effect of novel and repeated TS on hippocampal neurogenesis, depression- and anxiety-like behaviors. TS was applied by a brush treatment over the back, forelimbs, hind limbs and whisker pads of rats. The results reveal that both novel and repeated TS may enhance dendritic growth but not neurogenesis in the hippocampus. Behavioral tests indicated that TS induced stress in rats. In experiment two, young male adult SD rats were treated with high-dose corticosterone to induce suppression of neurogenesis, depression-like and anxiety-like behaviors. Repeated brush treatment (14 days) reduced the depression-like and anxiety-like behaviors in corticosterone (cort)-treated rats. In addition, the rate of neuronal cell differentiation in the hippocampus was enhanced by brush treatment. Moreover, the suppression of dendritic complexity in the hippocampal DG of cort-treated rats was reversed by repeated TS. Affective behaviors were also improved by TS in cort-treated rats. The findings indicate that repeated tactile stimulation has anxiolytic and antidepressant-like effects and promoted neurogenesis in hypercortisolemic animal model. In conclusion, novel and repeated TS enhanced hippocampal dendritic growth but induced stress in healthy rats. In addition, the habituation effect was found, and affective behaviors were improved after rats receiving 28-days TS. In hypercortisolemic rat model, repeated TS reversed the neurogenic impairment in the hippocampus and improved affect behaviors. The result not only suggests that neurogenesis may be the underlying mechanism of TS, but also provides evidence to support the clinical practice of treatments incorporating TS.