1. Neurotransmitters are chemical substances released at synapses that transmit signals between neurons. The main neurotransmitters in the brain are acetylcholine, serotonin, dopamine, norepinephrine, glutamate, GABA, and endorphins. 2. Each neurotransmitter system is involved in regulating key brain functions and behaviors such as movement, mood, sleep, cognition, and pain perception. 3. Neurotransmitters act via membrane receptors on target neurons, including ionotropic receptors that are ligand-gated ion channels and metabotropic G-protein coupled receptors.