Orexinergic signaling mediates light-induced neuronal activation in the dorsal raphe nucleus

Abstract

Seasonal affective disorder (SAD), a major depressive disorder recurring in the fall and winter, is caused by the reduction of light in the environment, and its depressive symptoms can be alleviated by bright light therapy. Both circadian and monoaminergic systems have been implicated in the etiology of SAD. However, the underlying neural pathways through which light regulates mood are not well understood. The present study utilized a diurnal rodent model, Arvicanthis niloticus, to explore the neural pathways mediating the effects of light on brain regions involved in mood regulation. Animals kept in constant darkness received light exposure in early subjective day, the time when light therapy is usually applied. The time course of neural activity following light exposure was assessed using Fos protein as a marker in the following brain regions/cells: the suprachiasmatic nucleus (SCN), orexin neurons in the perifornical-lateral hypothalamic area (PF-LHA) and the dorsal raphe nucleus (DRN). A light-induced increase in Fos expression was observed in orexin neurons and the DRN, but not in the SCN. As the DRN is densely innervated by orexinergic inputs, the involvement of orexinergic signaling in mediating the effects of light on the DRN was tested in the second experiment. The animals were injected with the selective orexin receptor type 1 (OXR1) antagonist SB-334867 prior to the light exposure. The treatment of SB-334867 significantly inhibited the Fos induction in the DRN. The results collectively point to the role of orexin neurons in mediating the effects of light on the mood-regulating monoaminergic areas, suggesting an orexinergic pathway that underlies light-dependent mood fluctuation and the beneficial effects of light therapy.

Fig. 1

Light-induced Fos expression in the SCN, PF-LHA and DRN. (A), number of Fos-ir nuclei and representative images in the SCN. (B), number of Fos-ir and representative images in the PF-LHA. (C), the percent of orexin cells double-labeled with Fos and (D), Fos double-labeled with orexin in the PF-LHA. (E), number of Fos-ir nuclei in the DRN and representative images at the rostral-caudal level 1–2 of the DRN. Higher magnification images are shown in the insets. Animals were treated with a light pulse (LP, 300lux), and analyzed prior to the LP (0 min) and 30, 60 and 120 min after. Results are displayed as mean ± SEM. * indicated p<0.05. 3v, 3^rd ventricle; Aq, aqueduct; oc, optic chiasm. Scale bar, 100μm.

Fig. 2

Orexinergic innervations in the DRN. Orexin and 5-HT staining in the rostral and caudal portion of the DRN in adjacent sections. Aq, aqueduct; mlf: medial longitudinal fasciculus. Scale bar, 250μm.

Fig. 3

Effects of orexinergic signaling on Fos expression in the SCN (A), PF-LHA (B) and DRN (C). (A), number of Fos-ir nuclei in the SCN. (B), number of Fos-ir nuclei and the percent of orexin cells double-labeled with Fos in the PF-LHA. (C), number of Fos-ir nuclei in the DRN and representative images showing Fos/5-HT double-label in the lateral DRN at rostral-caudal level 3. Animals were treated with vehicle or OXR1 antagonist SB-334867 before exposed to the LP (120min, 300lux). Results are displayed as mean ± SEM. * indicated p<0.05. mlf: medial longitudinal fasciculus. Scale bar, 100μm.