The role and importance of dorsal and ventral hippocampal catecholaminergic innervations in social memory of rats

This study aims to investigate the role and importance of the catecholaminergic innervations in the dorsal (DH) and ventral hippocampus (VH) in social memory of rats. Female Wistar rats were injected bilaterally with 6-OHDA in either DH or VH. Then, following a 10-day recovery period, sets of behavioral tests were conducted to assess both motor and cognitive functions. In the open field locomotor activity test, the depletion of catecholamines in the VH increased the locomotor activity compared to naive, whereas in the DH it did not affect the locomotor activity. Regarding olfaction, there is no difference in the latency to find pellets between all groups in the buried food-seeking test. In addition, impairment in social memory has been observed as both the DH and VH groups were unable to recognize a familiar from a novel juvenile rat, while the VH group demonstrated a deficit in sociability in the 3-chamber social memory test. In the 2- trial direct interaction, the DH group failed to recognize a previously encountered juvenile rat confirming the previous test outcome. However, the VH group showed a reduction in time spent interacting with both familiar and novel juvenile rats, which indicates low sociability. Moreover, no difference was observed between the groups in the Y maze, novel object recognition, sucrose preference, or elevated plus maze tests. Using demonstrative immunofluorescence staining, a reduction of TH+ terminal has been observed in dorsal CA1, and ventral CA1 regions of the hippocampus in DH and VH groups respectively. This shows that, the catecholaminergic innervations in the CA1 region of the DH and VH are important in modulating rat social memory. Furthermore, the catecholaminergic innervation of the VH may be more significant than DH in controlling sociability and locomotor activity. Additionally, a minor role of this innervation in spatial, recognition, olfaction, and anxiety processing could be proposed, which cannot be demonstrated with such a mild lesion made in this study. These findings could highlight the importance of catecholaminergic innervations in the VH in the behavioral deficits associated with neurodevelopmental and neuropsychiatric disorders.