Summary

       This study involved 75 male albino rats. Animals were weighing about 150–250g housed under standard laboratory conditions (12h light: 12h dark photoperiod), 22 ± 4 Cº, and the animals were given standard rat chaw and tap water ad libitum. The in vivo study aims to investigate the effects of melatonin and its agonist (Ramelteon) on systolic blood pressure (SBP (, serum endothelin-1 (ET-1) level, nitric oxide (NO), oxidative stress marker malondialdehyde (MDA), lipid profile, liver, and renal function test parameters, and hematological parameters in continuous light emitting diode (LED) exposure and pinealectomized rats. While the in vitro study aims to investigate the impact of melatonin and ramelteon on both the ET-1 dose-response curve and time-dependent ET-1 relaxation curve in isolated aortic rings of rats.

         Two sets of experiments were performed in both in vivo and in vitro studies. Pinealectomies were performed using the standard procedure described by Hoffman and Reiter and SBP was measured by tail cuff plethysmography every two weeks in both experiments till the end of the experimental period. In the histological part, the pineal gland and thoracic aorta were removed & prepared for paraffin sections. However, in vitro study cumulative doses of ET-1 (10^-12–10^-7 M) were applied to both continuous LED light exposure and pinealectomy experimental groups.

       In the in vivo study, the results revealed that continuous LED exposure, pinealectomy, and pinealectomy with continuous LED exposure all caused a significant rise in SBP after two weeks. In contrast, both melatonin and ramelteon reduced SBP. Moreover, the continuous LED exposure slightly reduced ET-1 and NO levels, whereas MDA increased significantly compared to the control rats.

 On the other hand, melatonin slightly elevated plasma ET-1 but ramelteon significantly, conversely melatonin elevated NO significantly but ramelteon slightly, and both of them reduced MDA compare to LED exposure rats. In the second experiment, pinealectomy and pinealectomy with LED exposure non-significantly reduced ET-1 and NO while, MDA increased compared to sham-operated rats. Besides, melatonin increased plasma ET-1 and NO but ramelteon slightly, and MDA was reduced by melatonin significantly but ramelteon slightly changed it.

        Moreover, serum red blood cells (RBC), hemoglobin (HGB), and hematocrit (HCT) were reduced significantly in contiguous light LED exposure rats. While non-significant changes were observed in white blood cell (WBC), leukocyte account, and platelet indices, also no significant change was found in serum levels of blood glucose concentration, lipid profiles, liver and renal function parameters. On the other hand, melatonin administration reduced Alanine transaminase (ALT) and gamma-glutamyl transferase (GGT) significantly. While ramelteon administration reduced mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH) compare with continuous light exposure rats. In the second experiment, pinealectomy didn’t cause a significant change in all biochemical and hematological parameters. The blood glucose concentration and serum GGT were elevated after pinealectomy with continuous LED exposure. Melatonin in pinealectomized rats reduced alkaline phosphatase (ALP), but ramelteon administration didn’t significantly change all other biochemical and hematological parameters compared with pinealectomized rats.

        In vitro study, the results indicated that in LED light exposure of the rats for 10 weeks, the ET-1-induced aortic ring contraction was markedly reduced. Conversely, melatonin and its agonist (Ramelteon) markedly elevated the ET-1-induced aortic ring contraction. Neither, melatonin nor its agonist supplementation significantly increased ET-1 maximal efficacy (Emax). Similarly, in the second experiment, after 6 weeks of pinealectomy and pinealectomy with continuous LED exposure, significantly decreased aortic ring contraction. Moreover, ET-1 Emax significantly and ET-1 efficiency and area under the curve (AUC) were slightly elevated in pinealectomy with light exposure group compared to the pinealectomized rats. On the other hand, ramelteon shifted the dose-response curve of ET-1 to the left side.

       Moreover, the present study demonstrated that rats continuously exposed to light considerably shifted the time-dependent relaxation curve of ET-1 to the right side but not significantly compared to the control group. Supplementation of melatonin significantly shifted the time-dependent relaxation of ET-1 to the left side. Moreover, ramelteon slightly and melatonin significantly reduced ET-1 Emax compared to LED-exposure rats. On the other hand, the AUC values of the ramelteon administration group compared to the continuous light group were significantly elevated in the ramelteon group.

       In Conclusion, the results from the In vivo study suggested that continuous LED light exposure and pinealectomy increase SBP. While melatonin and ramelteon caused a decreased in SBP may be due to increasing NO production and reducing oxidative stress. However, in vitro results suggeste that most of the melatonin actions on vascular modulation are due to its binding to the Melatonin receptor type 1 (MT1) and Melatonin receptor type 2 (MT2).