Neurotransmitters play crucial roles in transmitting signals across synapses in the brain and are integral to various psychological processes. One neurotransmitter, known for its association with mood regulation and feelings of happiness, is often targeted in the treatment of depression. However, its effects extend beyond mood; it also influences certain cognitive functions and bodily systems. Which of the following neurotransmitters is primarily linked to feelings of well-being and is often a focal point in pharmacological treatments for depression?