CCL2 and fibromyalgia: Substance P is the neuropeptide most consistently demonstrated to be elevated fibromyalgia [46,47], although other neuropeptides such as MCP-1, brain-derived neurotrophic factor, corticotrophin-releasing hormone (CRH), hemokinin-1 (HK-1), and calcitonin gene-related peptide are also implicated [48,49,50,51].