No loss of orexin/hypocretin, melanin-concentrating hormone or locus coeruleus noradrenergic neurons in a rat model of chronic sleep restriction.

Chronic sleep restriction (CSR) is common in modern society, adversely affecting cognitive performance and health. Yet how it impacts neurons regulating sleep remains unclear. Several studies using mice reported substantial losses of wake-active orexin/hypocretin and locus coeruleus (LC) noradrenergic neurons, but not rapid eye movement sleep-active melanin-concentrating hormone (MCH) neurons, following CSR. Here, we used immunohistochemistry and stereology to examine orexin, MCH and LC noradrenergic neurons in a rat model of CSR that uses programmed wheel rotation (3 h on/1 h off; '3/1' protocol). Adult male Wistar rats underwent one or four cycles of the 4-day 3/1 CSR protocol, with 2-day recovery between cycles in home cages. Time-matched control rats were housed in locked wheels/home cages. We found no significant differences in the numbers of orexin, MCH and LC noradrenergic neurons following either one- or four-cycle CSR protocol compared to respective controls. Similarly, the four-cycle CSR protocol had no effect on the densities of orexin axon terminals in the LC, noradrenergic dendrites in the LC and noradrenergic axon terminals in the frontal cortex. Body weights, however, decreased after one cycle of CSR and then increased with diminishing slope over the next three cycles. Thus, we found no evidence for loss of orexin or LC noradrenergic neurons following one and four cycles of the 4-day 3/1 CSR protocol in rats. Differences in CSR protocols and/or possible species differences in neuronal vulnerability to sleep loss may account for the discrepancy between the current results in rats and previous findings in mice.