Adversity in early adolescence alters female rats' anxiety-related behaviours and serotonergic innervation profiles in adulthood

Stress during early development produces lasting effects on psychopathological outcomes. The impact of prior intermittent, physical stress (IPS) during early-adolescence (PD 22-33) on anxiety-related behaviour of female rats was analyzed in adulthood. After behavioural testing, serotonergic innervation was evaluated using immunohistochemistry for the serotonin transporter (SERT) in the medial prefrontal cortex (mPFC) and ventral hippocampus. Administration of IPS (i.e., water immersion, elevated platform, foot shock) in early adolescence increased rats’ anxiety-like behaviour in the elevated plus-maze but had no effects in the shock-probe burying test. In the social interaction test, IPS decreased social interaction, and this effect was driven by selective decreases in the duration of playfighting with no evident changes in contact or investigative behaviour. Selective stress-induced increases in SERT-immunoreactive axon density were found in the infralimbic (IL) subregion of the mPFC, but not in the cingulate or prelimbic (PL) subregions. IPS in early adolescence did not affect serotonergic innervation profiles in any sub-fields of the ventral hippocampus. The findings confirm and extend on earlier evidence that stress during early adolescence promotes the emergence of an anxious phenotype, and provide novel evidence that these effects may be mediated, at least in part, by increased serotonergic innervation of the IL mPFC.