Analgesic Effects of Formononetin: An Investigation of the Role of Hypothalamic Corticotrophin Releasing Hormone, Orexin, and Melanin Concentrating Hormone Signaling Pathways in a Pain Model of Rats

Background: Formononetin is a phytoestrogen that exhibits antioxidant, analgesic, anxiolytic, and anti-inflammatory properties. However, there is limited information about the central molecular mechanisms mediating the analgesic effects of formononetin. Objectives: The present study aimed to assess the impacts of formononetin on hypothalamic mRNA levels of hypocretin (HCRT), corticotropin-releasing hormone (CRH), and melanin-concentrating hormone (MCH). Methods: Twenty male Wistar rats weighing 200 ± 10 g were divided into four groups (n = 5). Groups 1 and 2 were the control and the pain model groups, respectively, which received saline. Groups 3, 4, and 5 were the pain model rats that received 20 and 40 µg of formononetin and 20 µg of diclofenac via the third cerebral ventricle, respectively. To induce pain, formalin (50 µL of 5%) was injected into the plantar surface of the hind paw subcutaneously. Behavioral tests were performed. Hypothalamic samples were removed, and gene expression was measured using the real-time polymerase chain reaction (RT-PCR) method. Results: Formalin-induced pain caused a significant increase in the mRNA levels of HCRT, CRH, and MCH compared to the control. Administration of 20 and 40 µg of formononetin significantly decreased the mRNA levels of HCRT, CRH, and MCH in comparison to the formalin group. Conclusions: Downregulation of hypothalamic CRH and blocking the effects of neuropeptide orexin and MCH signaling pathways upstream of CRH neurons may mediate the antinociceptive effects of formononetin.