BDNF and colorectal carcinoma: A strong connection is suggested between brain–gut axis dysfunctions, the expression of a range of Neurotransmitters (Nts) (including Neuropeptides, NPs), as well as Nerve Growth Factor (NGF) family of proteins called Neurotrophins (Ntt) and the progression of CRC (including distant metastases) [17,18,28,29,30].