Reward and unease are closely linked in the brain -- ScienceDailyOne of the key components of the reward system is the signal substance dopamine, which acts as a chemical messenger between nerve cells. Dopamine stimulates motivation and causes animals and humans to exert themselves to achieve anything that is experienced as rewarding. When the researchers examined the dopamine-based signalling in the brain, they saw that the dopamine level in normal mice fell in the reward centre of the brain when the animals experienced something unpleasant. In contrast, it increased slightly in the mice that lacked melanocortin 4 receptors. "It seems that this receptor in some way prevents danger signals from activating the reward system. If the receptor is missing, the danger signals will gain access to the reward system and activate it. This means that mice that lack the receptor will seek out things that are associated with danger or discomfort," says David Engblom.