Continuous Light and Pinealectomy: Two Models of Melatonin Deficiency Revealing Distinct Interactions with Vitamin D Deficiency on Blood Parameters

Modern lifestyles reduce sunlight exposure and increase artificial light at night, potentially promoting vitamin D deficiency (VDD) and melatonin (MEL) suppression. The two hormones regulate immunity, erythropoiesis, and platelet (PLT) indices, but their combined hematological effects are not well characterized. We investigated the independent and combined effects of VDD and MEL deficiency on leukocyte profiles, erythrocyte indices, and PLT parameters in albino rats using two 12-week models (n = 8/group): continuous light (CL)-induced MEL suppression (Control, CL, VDD, and CL + VDD) and pinealectomy (Px)-induced MEL deficiency (Sham. Px, VDD, Px + VDD). Complete blood counts (CBCs) and inflammation-related ratios were assessed at study end. In the CL model, CL increased total white blood cells (WBCs) (p = 0.0135), whereas VDD increased the neutrophil to lymphocyte ratio (NLR) and Systemic Immune-Inflammation Index (SII) (p = 0.0021 and p = 0.0034, respectively). Notably, CL + VDD decreased SII (p < 0.0001), indicating non-additive interactions. In the Px model, Px increased granulocyte (GRA), NLR, and SII (p = 0.0136, p < 0.01, and p = 0.0011, respectively), with Px + VDD showing the highest elevations in WBC (p = 0.0482, SII p < 0.0001). Erythrocyte indices showed selective changes: The hematocrit (HCT) decreased after Px and Px + VDD compared to Sham (p < 0.0002 and p = 0.0429, respectively), and the mean corpuscular hemoglobin concentration (MCHC) decreased with VDD (p < 0.0001), consistent with partially distinct effects on red cell volume/turnover versus hemoglobinization. Overall, VDD favored a pro-inflammatory and hypochromic profile, while MEL deficiency, especially via Px, enhanced innate immune activation, together supporting early adaptive hematologic remodeling rather than overt pathology.