Electrical impulses move between nerve cells in an effort to conduct essential sensory and motor information by way of synapses, the spaces between nerve cells. Most synapses are chemical and, due to depolarization, whenever an action potential comes to the end of the neuron's axon, it cannot move across the gap until electrical signals are transformed into those that are chemical. Neurotransmitters are the chemical messengers that work to conduct nerve signals across the synaptic cleft between neurons to affect the functioning of other nerve cells or glands. Some neurotransmitters in the brain regulate the action of other neurotransmitters, further enhancing or decreasing their effects postsynaptically.
Discovered before all other neurotransmitters, acetylcholine is excitatory and inhibitory, meaning that it can increase or decrease nerve cell functioning. Acetylcholine affects vegetative functioning, including heart rate, breathing, and muscle cell activity, depending on where it is released. If it is acting on the central nervous system, namely the brain, its action is excitatory; if in the peripheral nervous system, it's inhibitory.
Norepinephrine and other neurotransmitters in the brain referred to as serotonin and dopamine are classified as biogenic amines, or, more commonly, catecholamines. Biogenic amines influence mood as their alterations are associated with certain disorders including the mental disease schizophrenia, and Parkinson's disease, which occurs due to destruction of dopamine-producing neurons. Psychotropic medications such as antidepressants are used primarily for their effects on the brain's biogenic amines.
Amino acids gamma-aminobuytric acid (GABA) and glycine are neurotransmitters in the brain which prevent neural firing in the brain and spinal cord. Anxiety-reducing drugs such as the barbiturates and benzodiazepines increase the action of GABA. The most available of all the brain's inhibitory neurotransmitters, GABA is important to sight and control of the action of skeletal muscle.
Opiates such as codeine and morphine are potent analgesics, a group of drugs prescribed for pain relief that do not cause a complete loss of consciousness. Natural endorphins are produced by the body in the form of endogenous opioids such as beta-endorphin and eukephalin, which act by binding to opioid receptors in the brain, inhibiting pain impulses. Opioids also function as neuromodulators by regulating the effects of other types of neurotransmitters in the brain.
Certain drugs do have their effect on neurotransmitters in the brain which can cause or lead to addiction, also referred to as drug abuse. Agonists are substances that are capable of working the same way or can increase the effects of certain neurotransmitters. This is accomplished by either increasing the effects on receptors or by prohibiting re-uptake. Antagonists are drugs that sit on receptors without inducing an effect, which stops neurotransmitters from binding with receptors.