Whole-brain irradiation increases NREM sleep and hypothalamic expression of IL-1β in rats

Although it has mainly been described qualitatively, whole brain irradiation induces somnolence in patients with malignant diseases. Therefore, we used a rat model to quantify the effects of irradiation of healthy brain tissue on both sleep-wake patterns and the expression of the pro-inflammatory cytokine interleukin-1β (IL-1β), which is known to induce sleep.

Different groups were examined at three time points after irradiation (1 day, 30 days and 60 days). Polysomnographic recordings were performed on each rat before and after total cranial irradiation (12 Gy). IL-1β protein levels in several brain regions were assessed by enzyme-linked immunosorbent assays, and site-specific immunoreactivity was observed by immunofluorescence.

We found that both non-rapid eye movement sleep and IL-1β protein expression in the hypothalamus increased 30 days after irradiation.

Whole brain irradiation increases sleep in our rat model, and this finding is similar to qualitative reports from patients. Because IL-1β has been proposed as a sleep-promoting molecule, we propose that the polysomnographic results may be attributable, at least in part, to the delayed overexpression of IL-1β in the hypothalamus.