Sprouting of nervous fibers and upregulation of C-X-C chemokine receptor type 4 expression in hippocampal formation of rats with enhanced spatial learning and memory.

Hippocampal mossy fiber sprouting following training in the Morris water maze (MWM) is associated with spatial learning, memory and neural plasticity. The C-X-C chemokine receptor type 4 (CXCR4) is the main receptor for stromal cell-derived factor-1 (SDF-1), which is a chemokine that can regulate axonal elongation. This study aimed to investigate the relationship between the morphological plasticity of hippocampal formation and CXCR4 expression. A model of spatial learning and memory was established in rats by training using the MWM. Mossy fiber sprouting in the striatum oriens of the CA3 area of the hippocampus was found in trained rats by Neo-Timm's method. As shown by immunohistochemistry, the CXCR4 immunopositive neurons were distributed in all layers and areas of hippocampal formation. There were no differences among groups regarding the distribution or shape of the immunopositive neurons. However, the immunoreactive staining intensity was increased in trained rats as compared with the control rats. Both CXCR4 gene transcription and translation were significantly upregulated in the trained group as compared with the control group (P < 0.01). Morphological plasticity in the form of axonal sprouting in the hippocampal formation can be induced by enhanced spatial learning and memory activity, and CXCR4 mRNA and protein expression is upregulated, indicating a positive correlation between CXCR4 expression and axonal sprouting.