Several different types of peptide neurotransmitters exist in the peripheral and central nervous system, including opioids, somatostatins and secretins. Other groups of peptide neurotransmitters include neurohypophyseals, gastrins and insulin. Distinguished from other neurotransmitters by the presence of amino acids, peptide neurotransmitters may have as few as two amino acids or as many as 100 amino acids linked in short chains; most have fewer than 30 amino acids. Some peptide neurotransmitters are also considered hormones.
Often called neuropeptides, peptide neurotransmitters are most active in the gastrointestinal (GI) region. Like other neurotransmitters, neuropeptides are released from vesicles at the end of nerve cells and travel across the synaptic cleft to other neurons. In the case of neuropeptides that are hormones, these hormones are first released from a gland and then corralled in vesicles of neurons inside that gland where they are often matched with carrier proteins before being released from the vesicle. Controllers of physiology and behavior, peptide neurotransmitters are known for rendering their effects slowly but over a long period.
Considered natural analgesics, opioid neurotransmitters participate in pain perception and sexual attraction. They are so named because they attach to the same receptors activated by opium. Divided into three classes, opioid peptide neurotransmitters include endorphins, dynorphins and enkephalins. In addition to pain and attraction, opioid peptide neurotransmitters are also necessary for memory, motion and seizure control. The majority of opioid neurotransmitters found in the body are located in the brain region.
Somatostatins are active in the pancreas and stomach region. These peptide neurotransmitters are most known for their ability to suppress other hormones, such as those secreted by the pituitary gland and those affecting the GI tract, such as gastrin and insulin. This suppression helps create balance in the GI region.
Secretins are another type of peptide neurotransmitter that aids digestion. Specifically, secretin triggers bile production in the liver. Additionally, this chemical messenger controls when the stomach and pancreas produce pepsin and digestive juices.
Neurohypophyseals, most active in the brain and in the blood, are peptide neurotransmitters that modulate cognition, social behavior, and some bodily functions, such as lactation and urination. They include such chemicals as vasopressin and oxytocin. Psychiatrists credit oxytocin for triggering protective and accommodating behavior toward trusted individuals and for encouraging aggression toward threatening individuals. Vasopression helps the kidneys restrict how much water is released during urination, thereby serving as an anti-diuretic.
Gastrin and insulin are two types of neuropeptides that work in tandem. Insulin, a chemical hormone and messenger that modulates blood sugar levels, can be spiked by gastrin, which determines when increases in insulin occur. Four different types of gastrin also control the levels of hydrochloric acid produced in the GI tract.