Institutionen för neurovetenskap / Department of Neuroscience
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
