Is the expression of positive emotions during rat tickling reduced by glucocorticoids?

Rat tickling is a type of heterospecific play involving the hand of an experimenter interacting with a rat. This activity is generally considered to be pleasurable to the individuals involved as this leads to the production of 50kHz ultrasonic vocalisations (USV), with a degree of individual variability (low or high USV rats). Neuropharmacological experiments have linked USV production to activation of the dopaminergic reward pathways. Tickling enjoyment has been shown to be reduced in anxiogenic conditions, and we asked whether stress hormones might be involved. Here, we tested the hypothesis that enjoyment of tickling might be reduced by injection of the synthetic glucocorticoid dexamethasone, with a view to explore the biology of positive emotions. Rats (N=24, 12 per treatment) were tickled over 3 sessions, which raised USV production, and the experiment was carried out on the fourth. Animals were injected with dexamethasone (2mg/kg, ip) or solvent control, tickled 6 hours later, and brains were removed for histochemical analysis 90 min later. Dexamethasone led to a mild reduction in USV production in the low USV individuals only (control=156.5±43.2 USV/min and DEX=76.8±17.1 USV/min). We are currently analysing cFos through immunohistochemistry, to investigate the effect of dexamethasone on tickling-induced neurone activation patterns. We will focus on characterising individual variability in USV emission. This work will improve our understanding of the neuroendocrine basis for positive emotions, and by extension will contribute towards improving animal welfare and human health.