Effect of juvenile pretraining on adolescent structural hippocampal attributes as a substrate for enhanced spatial performance.

Research has demonstrated that Long-Evans rats (LER) display superior mnemonic function over Wistar rats (WR). These differences are correlated with endogenous and input-dependent properties of the hippocampus. The present work sought to determine if juvenile pretraining might enhance hippocampal structural markers and if this would be associated with spatial processing improvements. Male and female WR and LER were either handled or trained on a water maze task from postnatal day 16 (p16) to p26 (pretraining). All animals were then trained on the task from p40 to p44 followed by immunohistochemical assessment of synaptophysin (to mark presynaptic terminals), MAP-2 (to mark dendrites), and the phosphorylated (activated) form of the extracellular signal-regulated kinase-1 (pERK1) in the hippocampus. From p19 to p20, LER (both male and female) showed a dramatic improvement in locating the hidden platform compared to their WR counterparts. On the first day of training at p40, all pretrained groups showed shorter latencies to locate the platform compared to groups without pretraining. Over the next 4 d, only pretrained male LER showed enhanced memory. Immunohistochemical analysis revealed fewer pERK1-labeled neurons in the CA3 hippocampal region in all pretrained groups and fewer pERK1-labeled neurons in the CA1 region of pretrained male LER. Pretrained male LER also showed more MAP-2 staining in CA1 and dentate gyrus regions. Synaptophysin staining revealed a pattern of axonal redistribution in the CA3 region in the pretrained groups. Results suggest a pattern of structural hippocampal alterations that may help to identify network malleability following pretraining protocols.