Rats housed on corncob bedding show less slow-wave sleep

Abstract

Despite the reported advantages of corncob bedding, questions have emerged about how comfortable animals find this type of bedding as a resting surface. In this study, encephalography (EEG) was used to compare the effects of corncob and aspen-chip bedding on rat slow-wave sleep (SWS). According to a facility-wide initiative, rats that were weaned on aspen-chip bedding were switched to corncob bedding in home cages and EEG recording chambers. Spontaneous EEG recordings obtained for 5 wk after the switch to corncob bedding demonstrated that rats spent significantly less time in SWS as compared with levels measured on aspen chips just prior to the bedding switch. SWS remained low even after a 5-wk acclimation period to the corncob bedding. We then acutely switched back to aspen-chip bedding in EEG recording chambers. Acute reinstatement of aspen-chip bedding during EEG recording was associated with an average 22% increase in time spent in SWS, with overall levels of SWS comparable to the levels measured on aspen chips prior to the change to corncob bedding. Aspen-chip bedding subsequently was reinstated in both home cages and EEG recording chambers, and SWS baseline levels were restored. These data raise important concerns about the effects of corncob bedding on rodents used in research.

Figure 1.

Time spent in slow-wave sleep (SWS) was significantly decreased for 5 wk after the switch to corncob bedding, compared with baseline recordings obtained on aspen-chip bedding. Data are expressed as number of minutes (mean ± SEM) spent in slow-wave sleep during each hour of the 5-h EEG recording session during the light phase. Solid circles represent rats recorded on aspen chips prior to the bedding switch. Open symbols represent the number of weeks that rats were housed on corncob bedding, with the dotted line indicating that the bedding was novel. Significant (*, P < 0.05; †, P < 0.01; Newman–Keuls posthoc analysis) differences compared with aspen chip baseline value are indicated; ns, not significant.

Figure 2.

Example of light-cycle EEG histogram for the same rat recorded on (A) aspen-chip bedding (week 0) and (B) corncob bedding (week 3). Sleep EEG is characterized by a prevalence of high-amplitude 1- to 4-Hz EEG activity, compared with the low 1- to 4-Hz amplitude data seen during the waking state. This representative example shows that when on aspen-chip bedding (panel A), the rat exhibited a relatively normal sleep pattern, with high-amplitude sleep bouts throughout the entire 5 h of recording. When on corncob bedding (panel B), the rat showed high-amplitude SWS bouts during early hours of the recording, whereas low-amplitude awake-EEG patterns dominated during later portions of the recording session.

Figure 3.

(A) Decreases in slow-wave sleep (SWS) during weeks 1 through 5 on corncob bedding were (B) significantly ameliorated during week 6 by acutely reinstating aspen-chip bedding in the EEG recording chamber. In contrast, significant decreases in SWS remained when rats were kept on corncob bedding during recording sessions during week 6. (C) Permanent reinstatement of aspen-chip bedding in EEG recording chambers and home cages during week 7 also improved SWS time. Data are expressed as the total amount of time (min; mean ± SEM) spent in SWS during the entire 5-h EEG light-cycle recording session. Filled bars represent rats recorded on aspen-chip bedding, whereas open bars represent the same animals recorded on corncob bedding. Significant (†, P < 0.01; *, P < 0.05) difference compared with data from week 0 (aspen-chip bedding). Significant (×, P < 0.01; +, P < 0.05) compared with recordings from rats on aspen-chip bedding during week 6. Significant (•, P < 0.01; ○, P < 0.05) compared with recordings from rats on aspen-chip bedding during week 7; ns, not significant.