Learning strategy selection in the water maze and hippocampal CREB phosphorylation differ in two inbred strains of mice

Abstract

Learning strategy selection was assessed in two different inbred strains of mice, C57BL/6 and DBA/2, which are used for developing genetically modified mouse models. Male mice received a training protocol in a water maze using alternating blocks of visible and hidden platform trials, during which mice escaped to a single location. After training, mice were required to choose between the spatial location where the platform had been during training (a place strategy) and a visible platform presented in a new location (a cued/response strategy). Both strains of mice had similar escape performance on the visible and hidden platform trials during training. However, in the strategy preference test, C57BL/6 mice selected a place strategy significantly more often than DBA/2 mice. Because much evidence implicates the hippocampus and striatum as important neural substrates for spatial/place and cued/response learning, respectively, the engagement of the hippocampus was then assessed after either place or cue training by determining levels of cAMP response element-binding protein (CREB) and phosphorylated CREB (pCREB) in these two mouse strains. Results revealed that hippocampal CREB levels in both strains of mice were significantly increased after place in comparison to cued training. However, the relation of hippocampal pCREB levels to training was strain dependent; pCREB was significantly higher in C57BL/6 mice than in DBA/2 mice after place training, while hippocampal pCREB levels did not differ between strains after cued training. These findings indicate that pCREB, specifically associated with place/spatial training, is closely tied to differences in spatial/place strategy preference between C57BL/6 and DBA/2 mice.

Figure 1.

Mean escape latency on each day for C57BL/6 (n = 9) and DBA/2 (n = 9) mice. Although the platform was stationary throughout training, it was visible on days 1, 2, 5, 6, 9, 10 and hidden on days 3, 4, 7, 8, 11, 12; there were no reliable differences between C57BL/6 mice and DBA/2 mice on either visible or hidden platform training days. See text for statistical analysis.

Figure 2.

Swim paths from a representative mouse choosing a “place strategy” (A) and a representative mouse using a “cued/response strategy” (B) on the competition test (day 13). Note, in A, that the mouse crossed the annulus where the escape platform had been during the previous 12 d of training, whereas in B, the mouse swam directly to the visible platform in its new location.

Figure 3.

Place (A) and cued (B) training performance of C57BL/6 and DBA/2 mice. No strain differences were observed on either or cued performance. See text for statistical analysis.

Figure 4.

Representative immunoblots of hippocampal CREB and pCREB from C57BL/6 and DBA/2 mice 30 min after place (A) or cued (B) training.

Figure 5.

Quantification of hippocampal CREB (A) and pCREB (B) levels (mean ± S.E.M) from C57BL/6 and DBA/2 mice 30 min after place or cue training. Data are expressed as the ratio of CREB/actin and pCREB/actin. Asterisk (*) indicates significantly greater hippocampal CREB following place in comparison to cue training in both mouse strains. Double asterisk (**) indicates hippocampal pCREB in C57BL/6 mice after place training was significantly greater than all other conditions. See text for statistical analysis.