Effects of repeated tickling on conditioned fear and hormonal responses in socially isolated rats. 
Positive emotional states have been reported to modify human resilience to fear and anxiety, but few animal models are available to elucidate underlying mechanisms.  In the current study, we examined whether 2 weeks of tickling, which is considered to evoke positive emotions, alters conditioned fear and hormonal reactions in Fischer rats.  We conditioned rats to fear an auditory tone which was initially paired with a mild foot-shock (0.2mA), and retention test was conducted 48h and 96h after conditioning.  During these tests, we found that prior tickling treatment significantly diminished fear-induced freezing.  To examine the effects of tickling on sympatho-adrenal and hypothalamic-pituitary-adrenal responses associated with conditioned fear, we measured plasma catecholamine and corticosterone levels in the retention test 96h after conditioning.  The plasma catecholamine concentration of non-tickled rats was higher than basal levels, whereas tickled rats showed significantly reduced concentrations of both plasma adrenaline and noradrenaline.  No significant differences in plasma corticosterone levels were observed between tickled and non-tickled rats.  These results suggest that repeated exposure to tickling can modulate fear-related behavior and sympatho-adrenal stress responses.