Alterations in endogenous circadian rhythm of core temperature in senescent Fischer 344 rats

We assessed whether alterations in endogenous circadian rhythm of core temp erature (CRT) in aging rats are associated with chronological time or with a biological marker of senescence, i.e., spontaneous rapid body weight loss . CRT was measured in male Fischer 344 (F344) rats beginning at age 689 day s and then continuously until death. Young rats were also monitored. The ra ts were housed under constant dim red light at 24-26 degrees C, and core te mperature was recorded every 10 min via biotelemetry. The CRT amplitude of the body weight-stable (presenescent) old rats was significantly less than that of young rats at all analysis periods. At the onset of spontaneous rap id weight loss (senescence), all measures of endogenous CRT differed signif icantly from those in the presenescent period. The suprachiasmatic nucleus (a circadian pacemaker) of the senescent rats maintained its light responsi veness as determined by an increase in c-fos expression after a brief light exposure. These data demonstrate that some characteristics of the CRT are altered slowly with chronological aging, whereas others occur rapidly with the onset of senescence.