A role of melanin-concentrating hormone producing neurons in the central regulation of paradoxical sleep

Abstract

Background: Peptidergic neurons containing the melanin-concentrating hormone (MCH) and the hypocretins (or orexins) are intermingled in the zona incerta, perifornical nucleus and lateral hypothalamic area. Both types of neurons have been implicated in the integrated regulation of energy homeostasis and body weight. Hypocretin neurons have also been involved in sleep-wake regulation and narcolepsy. We therefore sought to determine whether hypocretin and MCH neurons express Fos in association with enhanced paradoxical sleep (PS or REM sleep) during the rebound following PS deprivation. Next, we compared the effect of MCH and NaCl intracerebroventricular (ICV) administrations on sleep stage quantities to further determine whether MCH neurons play an active role in PS regulation.

Results: Here we show that the MCH but not the hypocretin neurons are strongly active during PS, evidenced through combined hypocretin, MCH, and Fos immunostainings in three groups of rats (PS Control, PS Deprived and PS Recovery rats). Further, we show that ICV administration of MCH induces a dose-dependent increase in PS (up to 200%) and slow wave sleep (up to 70%) quantities.

Conclusion: These results indicate that MCH is a powerful hypnogenic factor. MCH neurons might play a key role in the state of PS via their widespread projections in the central nervous system.

Figure 1

Photomicrographs of double-immunostained sections from a rat belonging to the PS-Recovery condition. A, Three double-labeled MCH+/Fos+ cells with their cytoplasm colored in brown and their nucleus labeled in black and one single labeled MCH+ cell. B, Fos+ (with a black nucleus) and hypocretin (with a brown cytoplasm) single labeled cells. Scale bars: 20 μm.

Figure 2

Schematic distribution of Fos+ (grey dots), MCH+ (black dots) and MCH+/Fos+ (red dots) neurons on two coronal hemi-sections of the hypothalamus in a representative animal for PS-Control (A1, A2), PS-Deprivation (B1, B2) and PS-Recovery (C1, C2) conditions. Note the very large number of MCH+/Fos+ cells specifically in the PSR condition. Abbreviations: Arc, arcuate nucleus; DA, dorsal hypothalamic area; DMH, dorso-medial hypothalamic area; f, fornix; ic, internal capsule; LHA, lateral hypothalamic area; mt, mammillothalamic tract; opt, optic tract; PeF, perifornical area; VMH, ventro-medial hypothalamic area; ZI, zona incerta.

Figure 3

Increase of paradoxical sleep and slow wave sleep quantities by ICV administration of MCH. Administration of 0.2 μg (black line) and 1 μg (dashed line) of MCH induceda strong increase in the quantities of PS (A) and SWS (B) and a decrease in W (C) in male rats compared to NaCl (dotted line). The values are normalized to NaCl, NaCl corresponding to 100% of PS (A), SWS (B) or W (C). Significance values indicated for individual points are: 0.2 μg MCH versus NaCl ( * p < 0.05, **p < 0.01), 1 μg MCH versus NaCl (° p < 0.05, ° °p < 0.01).

Figure 4

Increase in the number of PS bouts after ICV administration of MCH. Histograms illustrating the number of PS bouts (A) and their duration (B) during the first four hours following ICV administration of NaCl, 0.02, 0.2,1, 5 or 10 μg of MCH (mean ± SEM). There is a significant difference in the number of PS bouts between NaCl and MCH injections for 0.2, 1 and 5 μg. No difference in PS bouts duration was observed. Significance values indicated are for MCH versus NaCl injections ( *p < 0.05, **p < 0.01, ***p < 0.001).