Considering the importance of the gut–brain interconnections, we evaluated the levels of 5-HT, of the receptors 5-HT3B and 5-HT4 and the 5-HT transporter SERT, of the neurotrophin BDNF and its cognate receptor TrkB in colon and brain samples of MD exposed rats fed with a standard diet (IBS-Std) or KD (IBS-KD) for ten weeks. The gene discussed is SLC6A4; the disease is irritable bowel syndrome.