Circadian clock resetting in the mouse changes with age

Abstract

The most widely recognised consequence of normal age-related changes in biological timing is the sleep disruption that appears in old age and diminishes the quality of life. These sleep disorders are part of the normal ageing process and consist primarily of increased amounts of wakefulness and reduced amounts of deep sleep. Changes in the amplitude and timing of the sleep-wake cycle appear to represent, at least in part, a loss of effective circadian regulation of sleep. Understanding alterations in the characteristics of stimuli that help to consolidate internal rhythms will lead to recommendations to improve synchronisation in old age. Converging evidence from both human and animal studies indicate that senescence is associated with alterations in the neural structure thought to be primarily responsible for the generation of the circadian oscillation, the suprachiasmatic nuclei (SCN). Work has shown that there are changes in the anatomy, physiology and ability of the clock to reset in response to stimuli with age. Therefore it is possible that at least some of the observed age-related changes in sleep and circadian timing could be mediated at the level of the SCN. The SCN contain a circadian clock whose activity can be recorded in vitro for several days. We have tested the response of the circadian clock to a number of neurochemicals that reset the clock in a manner similar to light, including glutamate, N-methyl-D-aspartate (NMDA), gastrin-releasing peptide (GRP) and histamine (HA). In addition, we have also tested agents which phase shift in a pattern similar to behavioural 'non-photic' signals, including neuropeptide Y (NPY), serotonin (5HT) and gamma-aminobutyric acid (GABA). These were tested on the circadian clock in young and older mice (approximately 4 and 15 months old). We found deficits in the response to specific neurochemicals but not to others in our older mice. These results indicate that some changes seen in the responsiveness of the circadian clock to light with age may be mediated at the level of the SCN. Further, the responsiveness of the circadian clock with age is attenuated to some, but not all stimuli. This suggests that not all clock stimuli lose their effectiveness with age, and that it may be possible to compensate for deficits in clock performance by enhancing the strength of those stimulus pathways which are intact.

Fig. 1

Frequency of SCN cells firing rates represented by a 1-h running mean with a 15-min lag over time for two individual slices recorded from a young (5 months) and older (15 months) mouse. ZT 12 is defined as the time of lights off in the animal’s previous light:dark cycle, so the above trace shows data obtained from recordings over 32 h in vitro, and allows visualisation of two peak times in both slices. As evident from the representative traces shown here, there was no difference between the mean peak times for untreated slices from young and older mice, although there was a significant difference in amplitude such that slices recorded from older mice showed attenuated amplitude

Fig. 2

Phase shifts of the pattern of SCN firing rates in young (5 months) and older (15 months) mice to at ZT 14 to glutamate (glut), NMDA, GRP and HA. These neurochemicals all phase shift the firing rate of the suprachiasmatic nucleus in a photic pattern, and show phase delays when applied early in the night at ZT 15. Mean phase delays ± the standard error of the mean. Asterisks indicate significant differences between phase shifts seen in young and older mice

Fig. 3

Phase shifts of the pattern of suprachiasmatic nucleus firing rates in young (5 months) and older (15 months) mice to at ZT 6 to neuropeptide Y (NPY), 8-OH-DPAT (serotonin agonist) and muscimol (GABA agonist). These neurochemicals all phase shift the firing rate of the suprachiasmatic nucleus in a non-photic pattern, showing phase advances when applied in the middle of the day at ZT 6. Mean phase delays ± the standard error of the mean. Asterisks indicate significant differences between phase shifts seen in young and older mice