Impact of daytime light intensity on the central orexin (hypocretin) system of a diurnal rodent (Arvicanthis niloticus)

Abstract

The neuropeptide orexin/hypocretin is implicated in sleep and arousal, energy expenditure, reward, affective state and cognition. Our previous work using diurnal Nile grass rats (Arvicanthis niloticus) found that orexin mediates the effects of environmental light, particularly daytime light intensity, on affective and cognitive behaviours. The present study further investigated how daytime light intensity affects the central orexin system in male and female grass rats. Subjects were housed for 4 weeks in 12:12 hr dim light:dark (50 lux, dimLD) or in 12:12 hr bright light:dark cycle (1000 lux, brightLD). Day/night fluctuations in some orexin measures were also assessed. Despite similar hypothalamic prepro-orexin mRNA expression across all conditions, there were significantly more orexin-immunoreactive neurons, larger somata, greater optical density or higher orexin A content at night (ZT14) than during the day (ZT2), and/or in animals housed in brightLD compared to dimLD. Grass rats in brightLD also had higher cisternal CSF levels of orexin A. Furthermore, orexin receptor OX1R and OX2R proteins in the medial prefrontal cortex were higher in brightLD than dimLD males, but lower in brightLD than dimLD females. In the CA1 and dorsal raphe nucleus, females had higher OX1R than males without any significant effects of light condition, and OX2R levels were unaffected by sex or light. These results reveal that daytime light intensity alters the central orexin system of both male and female diurnal grass rats, sometimes sex-specifically, and provides insight into the mechanisms underlying how daytime light intensity impacts orexinregulated functions.

Effects of bright light therapy on neuroinflammatory and neuroplasticity markers in a diurnal rodent model of Seasonal Affective Disorder

“…Using the diurnal grass rat model of SAD we demonstrated that early morning BLT given daily for 4 weeks leads to changes in the expression of neuroinflammatory and neuroplasticity markers in corticolimbic brain regions involved in regulating affective and cognitive functions. Consistent with our previous findings in grass rats housed in winter- or summer-like lighting conditions [ 54 , 90 ] or following a BLT paradigm [ 52 ], the effects of BLT are brain region-specific and differ between males and females. The results from our present and previous studies provide novel insights into how seasonal light conditions and light therapy influence a variety of neural functions including neuropeptides and neurotransmitter systems, neuroinflammation, and neuroplasticity with concomitant effects on numerous behavioural outcomes highly relevant to SAD and other depressive disorders in humans [ 41 ].…”

Avian Orexin: Feed Intake Regulator or Something Else?

“…In diurnal Nile grass rats, the levels of orexin-A and orexin immuno-reactive neurons was increased at night and in animals housed in bright light compared to dim. Additionally, bright light environments resulted in an increased protein expression of both orexin receptors in the medial prefrontal cortex in males compared to dim light [ 43 ]. These studies demonstrate orexin’s role in sleep-wake cycles in mammals, as well as the orexin response to stimuli, in addition to their influence on feeding behavior.…”