Final answer:
Serotonin is a neurotransmitter that helps regulate mood, social behavior, appetite, digestion, sleep, memory, and sexual desire and function. Alterations in serotonin function can contribute to affective disorders, anxiety states, and obsessive-compulsive disorders.
Step-by-step explanation:
The neurotransmitter believed to help regulate mood and social behavior, appetite and digestion, sleep, memory, and sexual desire and function is serotonin. Serotonin is a monoamine neurotransmitter found in the central nervous system and is involved in various neurological functions. It plays a key role in mood control and alterations in its function have been implicated in several clinical states, including affective disorders, anxiety states, and obsessive-compulsive disorders.