Final answer:
The correct neurotransmitter that the brain releases as a reward is dopamine. It increases the concentration of cAMP when binding to its G-protein-linked receptor in the brain.
Step-by-step explanation:
The brain rewards certain activities by releasing the neurotransmitter dopamine. When dopamine binds its receptor, which is a G-protein-linked receptor, we can expect an increase in the concentration of cyclic AMP (cAMP). This is because the binding of dopamine to its receptor leads to an activation of an enzyme called adenylate cyclase, which converts adenosine triphosphate (ATP) into cAMP, a secondary messenger that plays a significant role in transmitting the signal inside the cell.
Dopamine is also part of the dopaminergic system, which has specific dopamine receptors, and it is involved in various neurological processes. In contrast, the neurotransmitter epinephrine, also known as adrenaline, is released into the bloodstream mainly by the adrenal glands and affects the body differently, such as in the fight-or-flight response by increasing heart rate and blood glucose levels. Despite epinephrine also being a neurotransmitter in the brain, it is not the correct answer in this context.