Combined neonatal stress and young-adult glucocorticoid stimulation in rats reduce BDNF expression in hippocampus: effects on learning and memory

Epidemiological studies suggest that multiple developmental disruptions are involved in the etiology of psychiatric illnesses including schizophrenia. In addition, altered expression of brain-derived neurotrophic factor (BDNF) has been implicated in these illnesses. In the present study, we examined the combined long-term effect of an early stress, in the form of maternal deprivation, and a later stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone, on BDNF expression in the hippocampus of rats. To assess whether there were behavioral effects, which may correlate with the BDNF changes, learning and memory was tested in the Y-maze test for short term spatial memory, the Morris water maze for long-term spatial memory, and the T-maze test for working memory. Four groups of rats received either no stress, maternal deprivation, corticosterone treatment, or both. Dorsal hippocampus sections obtained from parallel groups were used for BDNF mRNA in situ hybridization. Rats which had undergone both maternal deprivation and corticosterone treatment displayed a unique and significant 25-35% reduction of BDNF expression in the dentate gyrus (DG), and similar trends in the CA1 and CA3 regions of the hippocampus. These "two-hit" animals exhibited a learning delay in the Morris water maze test, a marked deficit in the Y-maze, but little change in the T-maze test. However, some aspects of cognition were also altered in rats with either maternal deprivation or corticosterone treatment. This study demonstrates a persistent effect of two developmental disruptions on BDNF expression in the hippocampus, with parallel, but not completely correlative changes in learning and memory.