Like proteins, peptides are formed by chains of amino acids. The difference between both substances lies in the length of the chains: peptides are made up out of shorter chains than proteins. Brain peptides called endorphins act like opium to kill pain or cause sleepiness.

In the early seventies, scientists discovered opiate receptors in several regions of the brain, suggesting that the brain might make substances similar to opium. A little while later, the first opiate produced by the brain was discovered. It resembled morphine and was named enkephalin. Soon after this, the endorphins, another type of opioid peptide, whose name comes from endogenous morphine, were discovered.

The Mystery Role of Peptides

The precise role of opioid peptides in the body remains unclear. A plausible guess is that they are released by neurons in times of stress to minimize pain and enhance adaptive behavior. This might explain why, for example, injuries received during the stress of combat are often not noticed until hours later.

These peptides and their receptors are closely associated with pathways in the brain that are activated by painful or tissue-damaging stimuli. This type of signal is transmitted to the central nervous system by special sensory nerves, small myelinated fibers and tiny unmyelated C fibers.

It has been discovered that these C fibers contain a peptide called substance P that causes the sensation of burning pain. Unsurprisingly, it has been found that the active component in chili peppers, capsaicin, causes the release of substance P.

Gases: The Odd Class

Very recently, a new class of neurotransmitters has been discovered. What is so odd about them, is that they are gases. These molecules, nitric oxide and carbon monoxide, do not adhere to the ‘laws’ governing the behavior of other neurotransmitters. Being gases, they cannot be stored in any structure, certainly not in synaptic structures. Instead, they are made by enzymes when needed and are released from the neurons by diffusion. Furthermore, they do not act at receptor sites, but rather diffuse into adjacent neurons and act on chemical targets, which may be enzymes.

The Role of Nitric Oxide

Even though it has been characterized only recently as a neurotransmitter, nitric oxide has already been shown to play an important role in several processes. For example, it governs erection through stimulating the neurons in the penis. In the nerves of the digestive tract, it governs the relaxation that contributes to the normal movements of digestion. In the brain, it is a major regulator of the intracellular messenger molecule cyclic GMP. In stroke, as a result from excess glutamate release, part of the ensuing neuronal damage may be attributed to nitric oxide.

Exact functions for carbon monoxide have not yet been shown.