Circadian rhythm dysfunction in the suprachiasmatic nucleus : effects of Trypanosoma brucei brucei infection and inflammatory cytokines

Abstract

Alterations in the mammalian circadian pacemaker, the hypothalamic suprachiasmatic nuclei (SCN), were studied in an experimental rat model of African trypanosomiasis, or sleeping sickness, caused by infections with subspecies of Trypanosoma brucei (T b). Characteristic signs of the disease are marked disturbances in circadian rhythms, such as a fragmentation of the sleep-wake cycle. Circadian rhythms are mastered from neurons in the SCN, which have an endogenous rhythm of spontaneous firing that is increased during the subjective day. The SCN rhythm can be recorded as spontaneous single unit activity in slice preparations containing the SCN. This activity was recorded in slices from controls and rats infected with T. b. brucei. The rhythm in spontaneous neuronal activity was markedly altered in trypanosome-infectec rats, with a lower average frequency and a phase advance of the peak. No structural differences in the retinal afferents were detected.

The spontaneous postsynaptic activity was analyzed in slices by whole cell patch clamp recordings of S CN neurons. The inhibitory and excitatory postsynaptic events were identified as primarily gamma- ammobutyric acid (GABA) A and alpha-amino-hydroxy-5-mothyhsoxazole-4propionic acid (AMPA) receptor- dependent. The inhibitory and excitatory activity was compared between the subjective day and night in control rats. No significant difference was detected in amplitude or frequency of inhibitory synaptic events, but die frequency of excitatory events was significantly increased during the subjective day. In slices from rats infected with T. b. brucei, the frequency of excitatory events was significantly lower during the subjective day as compared with control rats. In addition, the protein expressions of AMPA glutamate receptor subunit 2 & 3, and N-methyl-D-aspartate receptor channel subunit zeta1 (NMDAR1), were decreased in trypanosome-infected rats.

Invasion of trypanosomes causes a substantial release of several cytokines as an immune response to the infection, for instance tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma, which act in synergy. TNF-alpha and IFN-gamma in combination with bacterial lipopolysaccharide were added to SCN slices and the spontaneous firing was examined. The cytokines altered the rhythm in firing frequency; i. c. caused an abolishment or shift of the peak.

The transcript and protein of the IFN-gamma receptor (IFN-gammaR) was detected in the SCN. The molecular identity of the receptor transcript was determined, confirming that the receptor molecule was identical to the IFN-gammaR in the immune system. The expression of the receptor, protein showed daily variations with a peak of expression during the early subjective night. The cyclic variation was abolished and the protein levels were increased in rats held in constant darkness. The postnatal development of the IFN- gammaR protein was studied. At postnatal day (P) 1 the protein was distributed throughout the entire nuclei, but relocated to the ventrolateral retinorecipient subdivisions of the SCN between P l 1 and P20.

In conclusion, the present findings demonstrate that T. b. brucei dysregulates the endogenous rhythm in SCN activity, which probably alters the circadian output and may be manifested as a fragmentation of the sleep-wake cycle. Further, the SCN contain glutamatergic synapses that display an increase in activity during the subjective day in vitro. This activity is decreased in slices from trypanosome-infected rats, possibly explaining the observed alteration in spontaneous firing. Cytokines released during trypanosome- infections, such as IFN-gamma may affect protein expression of glutamate receptors and glutamatergic postsynaptic transmission via its receptor, which is located in the ventralateral regions of the SCN.