Abstract
Hippocampal synaptic plasticity is believed to comprise the cellular basis for spatial learning. Strain-dependent differences in synaptic plasticity in the CA1 region have been reported. However, it is not known whether these differences extend to other synapses within the trisynaptic circuit, although there is evidence for morphological variations within that path. We investigated whether Wistar and Hooded Lister (HL) rat strains express differences in synaptic plasticity in the dentate gyrus in vivo. We also explored whether they exhibit differences in the ability to engage in spatial learning in an eight-arm radial maze. Basal synaptic transmission was stable over a 24-h period in both rat strains, and the input–output relationship of both strains was not significantly different. Paired-pulse analysis revealed significantly less paired-pulse facilitation in the HL strain when pulses were given 40–100 ms apart. Low frequency stimulation at 1 Hz evoked long-term depression (>24 h) in Wistar and short-term depression (<2 h) in HL rats; 200 Hz stimulation induced long-term potentiation (>24 h) in Wistar, and a transient, significantly smaller potentiation (<1 h) in HL rats, suggesting that HL rats have higher thresholds for expression of persistent synaptic plasticity. Training for 10 days in an eight-arm radial maze revealed that HL rats master the working memory task faster than Wistar rats, although both strains show an equivalent performance by the end of the trial period. HL rats also perform more efficiently in a double working and reference memory task. On the other hand, Wistar rats show better reference memory performance on the final (8–10) days of training. Wistar rats were less active and more anxious than HL rats. These data suggest that strain-dependent variations in hippocampal synaptic plasticity occur in different hippocampal synapses. A clear correlation with differences in spatial learning is not evident however.
Highlights
Long-term potentiation (LTP) and long-term depression (LTD) comprise forms of synaptic plasticity that occur in the hippocampus in vivo
Responses in Hooded Lister (HL) animals (n = 8) were significantly lower in the 40- to 100-ms range compared to Wistar rats (n = 8), suggesting that paired-pulse facilitation is less in the HL rat strain (Figure 1B, ANOVA, p < 0.05)
We have shown that LTP and LTD are associated with different precise components of a spatial representation (ManahanVaughan and Braunewell, 1999; Kemp and Manahan-Vaughan, 2004, 2007) and that hippocampal LTD in the CA1 region and dentate gyrus, for example, is facilitated by very different aspects of a spatial context (Kemp and Manahan-Vaughan, 2008)
Summary
Long-term potentiation (LTP) and long-term depression (LTD) comprise forms of synaptic plasticity that occur in the hippocampus in vivo Both are input-specific, associative, and protein synthesis-dependent (Frey et al, 1988, 1996; Otani et al, 1989; Bliss and Collingridge, 1993; Nguyen et al, 1994; Bear and Abraham, 1996; Manahan-Vaughan et al, 2000) and fulfill the criteria for cellular mechanisms for information storage. Rodents readily express different forms of hippocampal synaptic plasticity, differences in the ability of various strains to respond with persistent LTP or LTD following afferent stimulation of Schaffer collateral afferents to the CA1 region have been reported (Manahan-Vaughan and Braunewell, 1999, 2005; ManahanVaughan, 2000a) It is not clear if similar differences exist at other hippocampal synapses. Despite these intriguing reports as to differences in synaptic plasticity in the dentate gyrus (in comparison to the much better studied CA1 region) little is known about the role of synaptic plasticity in the dentate gyrus, in hippocampus-dependent learning phenomena
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