Adaptation under constant light or dark: a challenge to the pineal

The pineal gland synchronizes the body's circadian rhythms by producing melatonin in response to changes in the light-dark cycle. Our study evaluated how prolonged exposure to constant lighting (LL) or darkness (DD) affects pineal physiology by monitoring the transcription of genes critical for rhythmic endocrine activity. To achieve this, rats were exposed to LL, DD, or LD (control) from P21 to P90. LL and DD impacted the rats' physiology, as evidenced by the shift from a bimodal voluntary activity pattern to a free-running one. Serum melatonin and the transcription of genes encoding key enzymes involved in melatonin synthesis and adrenergic receptors changed their characteristic diurnal pattern observed in the LD group. The transcription of clock genes important for pineal rhythmicity was disturbed under both LL and DD: LL attenuated or flattened the expression of core clock genes, while DD showed a weaker effect and shifted the peak of expression. In LL, altered expression of clock genes was associated with increased transcription of regulators of mitochondrial biogenesis, and markers of mitophagy and mitochondrial dynamics, resulting in elevated ATP production. Oppositely, in DD conditions, ATP decreased. Principal Component Analysis (PCA) revealed distinct clusters comprising clock and mitochondria-related genes, indicating a close association between the circadian clock and mitochondrial function. These findings suggest that long-term exposure to LL environment poses more significant challenges to the pineal gland than DD. This might be associated with the heightened blood corticosterone levels observed in LL conditions, indicating potential chronobiological stress.