Early emergence of altered 5-HT receptor-evoked behavior, neural activation and gene expression following maternal separation.

The early stress of Maternal Separation (MS) contributes to the establishment of adult psychopathology. The serotonergic (5-HT) system is implicated during this temporal window in mediating the development of mood-related behaviors. MS is reported to evoke altered 5-HT receptor function in adulthood. However, the ontogeny of altered 5-HT receptor responsivity following MS remains unknown. Here, we examined 5-HT receptor agonist, DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) (2mg/kg) evoked responses, namely stereotypical head-twitch behaviors in control and MS Sprague-Dawley rat pups at postnatal day 21 (P21). MS involved a separation of pups from the dam for 3h daily from postnatal day 2-14. MS pups at P21 exhibited significantly enhanced head-twitch behaviors compared to controls. Using c-Fos cell counting we examined neural activation in control and MS pups following DOI treatment. MS pups exhibited altered DOI-evoked c-Fos expression within all mPFC subdivisions, but not in the hippocampus, lateral septum and hypothalamus, suggesting differential prefrontal neural activation upon 5-HT receptor stimulation following early stress. Gene profiling of 5-HT receptor-regulated immediate early genes (IEGs) indicated a decline in the expression of Fos, Fra1 and Egr1 mRNA under baseline conditions in the mPFC of MS pups. MS pups also showed an altered pattern in the regulation of several 5-HT receptor-regulated IEGs (Fos, Fra1, Bdnf, Egr1, Egr3) following DOI treatment. Collectively, these results highlight an early emergence of altered 5-HT receptor-evoked behavioral responses and neural activation patterns in multiple brain regions in animals with a history of MS.