The Corrective Role of Melatonin in Synergism of Dark Deprivation and CCl4 Intoxication in the Pathogenesis of Liver Damage a in Rats

Circadian rhythm disruption induced by exposure to light—excessive in duration and intensity (dark deprivation)—and the impact of hepatotoxins are both significant risk factors for liver pathology. The purpose of this research was to evaluate the potentially synergistic effects of continuous lighting and carbon tetrachloride (CCl4) toxicity on the structural and functional organization and daily (circadian) rhythmicity of the liver in rats, as well as to look at the corrective capability of exogenous melatonin under such influences. The experiment was conducted on 200 outbred 6-month-old Wistar rat males, which were distributed into five groups, including a control (normal light/dark cycle), dark deprivation (constant light), CCl4 intoxication, and combined exposure to CCl4 and dark deprivation with or without melatonin administration (0.3 mg/kg). Histological, immunohistochemical (Ki-67, Per2, and Bmal1), biochemical, and ELISA methods were used. Circadian rhythms were analyzed using cosinor. It was shown that dark deprivation and CCl4 intoxication act synergistically, potentiating liver damage. The most severe necrosis (54.17 ± 9.13%), steatosis (57.85 ± 12.14%), and suppression of regenerative potential (decreased proportion of binucleated hepatocytes to 2.17 ± 0.21%) were observed in the group with combined exposure. This correlated with a substantial decline in melatonin content in blood plasma (7.85 ± 2.1 pg/mL) and a profound disruption in circadian rhythms. Administration of exogenous melatonin exerted pronounced hepatoprotective and chronotropic effects: it significantly reduced pathological changes (necrosis reduced to 16.35 ± 6.17%), stimulated regeneration (binucleated hepatocytes increased to 13.57 ± 0.81%), and restored the circadian rhythms of the studied parameters to levels close to those of the control. The key pathogenetic link in the potentiation of CCl4 hepatotoxicity under dark deprivation is light-induced deficiency of endogenous melatonin. Exogenous melatonin demonstrated high efficacy in correcting both structural and functional damage and liver desynchronosis, confirming its therapeutic potential under conditions of combined exposure to chronodisruptors and toxins.