Light rescues circadian behavior and brain dopamine abnormalities in diurnal rodents exposed to a winter-like photoperiod

Mar 22, 2018Brain structure & function

Light improves daily rhythms and brain dopamine problems in day-active rodents exposed to winter-like light conditions

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Abstract

Exposure to a winter-like photoperiod significantly altered locomotor activity and dopamine levels in the diurnal rodent Arvicanthis ansorgei.

Arvicanthis ansorgei exposed to a light-dark cycle of 8 hours of light and 16 hours of darkness displayed changes in their daily activity patterns.
Dopaminergic neurotransmission was altered in the nucleus accumbens and dorsal striatum under winter-like conditions.
Light exposure at either early or late day reversed dopamine disturbances, but only late-day light exposure restored normal locomotor rhythms.
The expression of the Per2 gene in the suprachiasmatic nucleus was also influenced by late-day light exposure in winter-like conditions.
These findings indicate that light affects behavior through both circadian and non-circadian mechanisms involving dopaminergic pathways.

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Seasonal affective disorder (SAD), beyond mood changes, is characterized by alterations in daily rhythms of behavior and physiology. The pathophysiological conditions of SAD involve changes in day length and its first-line treatment is bright light therapy. Animal models using nocturnal rodents have been studied to elucidate the neurobiological mechanisms of depression, but might be ill suited to study the therapeutic effects of light in SAD since they exhibit light-aversive responses. Here Arvicanthis ansorgei, a diurnal rodent, was used to determine behavioral, molecular and brain dopamine changes in response to exposure to a winter-like photoperiod consisting of a light-dark cycle with 8 h of light, under diminished light intensity, and 16 h of darkness. Furthermore, we evaluated whether timed-daily bright light exposure has an effect on behavior and brain physiology of winter-like exposed animals. Arvicanthis under a winter-like condition showed alterations in the synchronization of the locomotor activity rhythm to the light-dark cycle. Moreover, alterations in day-night activity of dopaminergic neurotransmission were revealed in the nucleus accumbens and the dorsal striatum, and in the day-night clock gene expression in the suprachiasmatic nucleus. Interestingly, whereas dopamine disturbances were reversed in animals exposed to daily light at early or late day, altered phase of the daily rhythm of locomotion was reverted only in animals exposed to light at the late day. Moreover, Per2 gene expression in the SCN was also affected by light exposure at late day in winter-like exposed animals. These findings suggest that light induces effects on behavior by mechanisms that rely on both circadian and rhythm-independent pathways influencing the dopaminergic circuitry. This last point might be crucial for understanding the mechanisms of non-pharmacological treatment in SAD.

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Light rescues circadian behavior and brain dopamine abnormalities in diurnal rodents exposed to a winter-like photoperiod

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