1 Effect of brain - derived neurotrophic factor haploinsufficiency on stress - induced remodeling of hippocampal neurons

Chronic restraint stress (CRS) induces the remodeling (i.e. retraction and simplification) of the apical dendrites of hippocampal CA3 pyramidal neurons in rats, suggesting that intrahippocampal connectivity can be affected by a prolonged stressful challenge. Since the structural maintenance of neuronal dendritic arborizations and synaptic connectivity requires neurotrophic support, we investigated the potential role of brain derived neurotrophic factor (BDNF), a neurotrophin enriched in the hippocampus and released from neurons in an activity-dependent manner, as a mediator of the stress-induced dendritic remodeling. The analysis of Golgi-impregnated hippocampal sections revealed that wild type (WT) C57BL/6 male mice showed a similar CA3 apical dendritic remodeling in response to 3 weeks of CRS to that previously described for rats. Haploinsufficient BDNF mice (BDNF +/-) did not show such remodeling, but, even without CRS, they presented shorter and simplified CA3 apical dendritic arbors, like those observed in stressed WT mice. Furthermore, unstressed BDNF +/-mice showed a significant decrease in total hippocampal volume. The dendritic arborization of CA1 pyramidal neurons was not affected by CRS or genotype. However, only in WT mice, CRS induced changes in the density of dendritic spine shape subtypes in both CA1 and CA3 apical dendrites. These results suggest a complex role of BDNF in maintaining the dendritic and spine morphology of hippocampal neurons and the associated volume of the hippocampal formation.