Neuropeptides

Neuropeptides are small, protein-like signaling molecules (amino acid chains) synthesized and released by neurons in the central (CNS) and peripheral (PNS) nervous systems. They regulate diverse brain and bodily functions, such as mood, stress, sleep, appetite, pain, and memory. Neuropeptides either function as neurotransmitters (directly affecting synapses) or as neuromodulators (altering the effect of other neurotransmitters). Beyond the brain, many act as hormones or signaling molecules in the immune, endocrine, and digestive systems. Examples  include oxytocin, endorphins, neuropeptide Y, and substance P. Another key example is orexin-A and orexin-B (also known as hypocretin), produced in the hypothalamus to regulate arousal, wakefulness, appetite, and energy homeostasis. Also see Circadian Rhythms