Differences in hippocampal CREB phosphorylation in trace fear conditioning of two inbred mouse strains

Abstract

The effects of genetic background on fear trace conditioning were evaluated in relation to phosphorylated levels of cAMP response element-binding protein (CREB) in the hippocampus using two different inbred strains of mice, C57BL/6 and DBA/2. The male mice received a trace fear conditioning protocol and unpaired control groups were included to assess nonassociative effects on test performance. Both C57BL/6 and DBA/2 mice with paired training displayed higher freezing responses during testing than those with unpaired training, respectively. The C57BL/6 mice with paired training also displayed higher freezing responses to the tone-CS during testing than the DBA/2 mice with paired training. Because much evidence implicates the hippocampus as an important neural substrate for trace fear conditioning, the engagement of the hippocampus was examined after testing by measuring levels of CREB and phosphorylated CREB (pCREB). The results revealed that hippocampal CREB levels in both strains of mice were not significantly altered according to the type of training (unpaired vs. paired). However, the hippocampal pCREB levels were significantly higher in the paired training group than the unpaired control group in C57BL/6 mice, but not in DBA/2 mice. These findings indicate that hippocampal pCREB is closely tied to this form of associative conditioning only in C57BL/6 mice and that different neural substrates may support trace conditioning in C57BL/6 and DBA/2 strains.

Delay and trace fear conditioning in C57BL/6 and DBA/2 mice: issues of measurement and performance

“…To date, only two studies have compared B6 and D2 CS responses using trace fear conditioning (Holmes et al 2002;Hwang et al 2010). Consistent with the present data (Fig.…”

“…However, these baseline measures are not reliably low across different reports. Altered context freezing, sometimes referred to as "generalized freezing," has been reported to differ in B6 and D2 mice (e.g., Young et al 2000;Balogh et al 2002;Balogh and Wehner 2003;Hwang et al 2010), but these differences are not consistently found (e.g., Lu and Wehner 1997;Nie and Abel 2001) and were absent in our data ( Fig. 2B,C; Supplemental Fig.…”

“…This is particularly true when one considers that the efficacy of a particular interval to act as a trace interval changes depending on the overall temporal context in which it is placed (e.g., Balsam 1984). Hwang et al (2010) also looked at TFC in B6 and D2 mice. Interestingly, their results for CS strain differences were variable.…”

“…B6 and D2 mice differ on a number of behavioral measures prior to training in any paradigm (Logue et al 1997;Podhorna and Brown 2002), which may or may not have an effect on the learned behavior. In three of the papers reviewed here (Valentinuzzi et al 1998;Bolivar et al 2001;Hwang et al 2010), adjusting the raw data to a baseline or control group changed the significance of the strain comparison. This leaves the question of which is truly representative of learning differences in these strains, the raw data or the adjusted measures?…”

“…However, this is challenging in studies of fear conditioning in which no more than a few US presentations are given. Although not ideal, the most commonly used method is to present "unpaired" sets of the CS and US in which the CS -US interval is very long or the US is given in the absence of the CS (Paylor et al 1994;Gerlai 1998;Hwang et al 2010). The duration of this interval is critical to these assumptions, given that trace fear conditioning between the CS and US can form across a wide range of inter-stimulus intervals (Misane et al 2005).…”