Dendrites of the dorsal and ventral hippocampal CA1 pyramidal neurons of singly housed female rats exhibit lamina-specific growths and retractions during adolescence that are responsive to pair housing.

Adolescence is accompanied by increased vulnerability to psychiatric illnesses, including anxiety, depression, schizophrenia, and eating disorders. The hippocampus is important for regulating emotional state through its ventral compartment and spatial cognition through its dorsal compartment. Previous animal studies have examined hippocampal development at stages before, after or at single time points during adolescence. However, only one study has investigated morphological changes at multiple time points during adolescence, and no study has yet compared developmental changes of dorsal versus ventral hippocampi. We analyzed the dorsal and ventral hippocampi of rats to determine the developmental trajectory of Golgi-stained hippocampal CA1 neurons by sampling at five time points, ranging from postnatal day (P) 35 (puberty) to 55 (end of adolescence). We show that the dorsal hippocampus undergoes transient dendritic retractions in stratum radiatum (SR), while the ventral hippocampus undergoes transient dendritic growths in SR. During adulthood, stress and hormonal fluctuations have been shown to alter the physiology and morphology of hippocampal neurons, but studies of the impact of these factors upon adolescent hippocampi are scarce. In addition, we show that female-female pair housing from P 36-44 significantly increases branching in the dorsal SR and reduces branching in the ventral SR. Taken together with data on spine density, these results indicate that pyramidal cells in the dorsal and ventral CA1 of female adolescents are remodeled differently following single housing. Social housing during adolescence elicits pathway-specific changes in the hippocampus that may underlie behavioral benefits, including stability of emotion regulation and superior cognition.