Effects of aging and inflammatory molecules on the suprachiasmatic circadian clock

Abstract

The suprachiasmatic nucleus (SCN) is considered the master circadian pacemaker in the mammalian brain and is vital for generating physiological and behavioral circadian rhythms. During aging and inflammatory conditions the circadian system is frequently affected, but relatively little is known about the mechanisms underlying such effects.

We compared electrophysiological properties of the young and aged SCN in slice preparations in vitro and found that aged mice had an overall reduced GABAergic synaptic transmission and an increased proportion of silent cells, i.e. cells that do not fire spontaneous action potentials. The in vivo response of SCN neurons to pro-inflammatory cytokines was also compared between young and aged mice by measuring the neuronal induction of the protein encoded by the immediate early gene Fos.

Firstly, the response to a mixture of the pro-inflammatory cytokines tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma showed a clear day/night variation in both age groups with higher induction of Fos protein during the subjective night. This variation was paralleled by variations in the mRNA expression of suppressor of cytokine signaling (SOCS) molecules that are important negative regulators of cytokine signaling pathways; when SOCS molecule expression was low, TNF-alpha/IFN-gamma had the strongest effect, indicating that SOCS molecules may regulate the responses to cytokines in the SCN. Secondly, the Fos response during early subjective night was lower in aged as compared to young animals. There was no difference between the age groups in the mRNA expression of SOCS or the TNF-alpha receptor, whereas IFN-gamma receptor mRNA expression was lower in aged mice during subjective night, which may underlie the blunted responses in aged mice at this time point.

Neurophysiological effects of TNF-alpha on the SCN were also investigated. Spontaneous firing as well as synaptic transmission, was affected by the cytokine and the observed effects mostly persisted throughout the recording period. The effect on spontaneous firing could be blocked by the nitric oxide synthase (NOS) inhibitor L-NAME. TNF-alpha may therefore affect the SCN neuronal activity via induction of NO, to alter circadian output signals from the SCN.

The functional role of SOCS molecules in neurons of the central nervous system was verified in primary hippocampal neuronal cultures. In these cultures IFN-gamma induced SOCS1 and SOCS3 mRNA expression. Using neurons from socs1-/- mice, SOCS1 was found to negatively regulate the response to IFN-gamma in terms of transcriptional responses and antigen presentation.

In summary, this thesis shows that during aging the SCN synaptic network and spontaneous neuronal activity is compromised. TNF-alpha, the levels of which often are increased in aged individuals, affects SCN neuronal firing and synaptic activity and may thereby contribute to circadian rhythm disturbances during aging. SCN neurons in aged mice also responded differently to IFN-gamma/TNF-alpha exposure as compared to young mice. Furthermore, SOCS molecules, which are expressed in the SCN with a day/night rhythm, can regulate the responses of brain neurons to cytokine exposure, and may therefore play a role to regulate the sensitivity of the SCN to pro-inflammatory cytokines.