Ameliorative effects of melatonin on radiotherapy-induced apoptotic and inflammatory responses in rat parotid gland.

Radiotherapy-induced parotid gland injury is a major contributor to salivary hypofunction and xerostomia in head-and-neck cancer management. This study aimed to evaluate the differential radiobiological effects of flattening filter (FF) and flattening filter-free (FFF) 6 MV photon beams, delivered as single-dose or fractionated-dose regimens, on apoptotic, inflammatory, and angiogenic gene expression in rat parotid tissue, and to assess the potential radioprotective action of melatonin. Female Sprague-Dawley rats (8-10 weeks old) were randomized into ten groups and irradiated with either a single 16 Gy dose or a fractionated-dose protocol (6.4 Gy × 5) using FF or FFF beams. Melatonin (10 mg/kg, intraperitoneally) was administered 15 min before irradiation. Parotid glands were collected 10 days post-irradiation, and mRNA expression levels of Bax, Bid, Casp3, Bcl2, Bcl2l1, Il1b, Tnf, and Vegfa were quantified by RT-qPCR. Irradiation significantly upregulated pro-apoptotic (Bax, Bid, Casp3) and pro-inflammatory (Il1b, Tnf) transcripts, along with Vegfa, while downregulating anti-apoptotic markers (Bcl2, Bcl2l1). These molecular alterations were more pronounced following FFF-beam and fractionated-dose. Melatonin pretreatment markedly attenuated radiation-induced gene expression changes, restored Bcl2 and Bcl2l1 levels, and normalized the Bcl2/Bcl2l1 ratio. Melatonin effectively mitigates key apoptotic, inflammatory, and angiogenic responses associated with radiation-induced parotid gland injury. These findings support melatonin as a potential radioprotective adjunct during head-and-neck irradiation.