If you were to hear the words ‘opioid peptides’, they might not trigger much within your brain, other than that the former sounds a bit like opium and together they sound quite scientific. Opium (also known as poppy tears) is a dried substance or latex that originates, as the alternative name suggests, from the opium poppy. Beautifully intricate pipes of bamboo, ivory, silver, jade and porcelain have been carved over the centuries and used to vaporise and inhale the latex traditionally obtained by scratching immature poppy seed pods by hand. Numerous Empires including the Egyptian, Greek, Roman, Persian and Arab made widespread use of the drug, which was then the most potent form of pain relief available. This analgesic property is conferred by morphine, which constitutes approximately twelve per cent of opium and is chemically processed to produce heroin. Commonly known by the street names H, smack, horse and brown, among others, the effects of heroin will be well known by any ‘Trainspotting’ fans. What writer Irvine Welsh did not reveal, however, is that opiates such as heroin mimic the effects of naturally occurring molecules that can be generated inside our own bodies.
Opioid peptides are small molecules that are produced in the central nervous system (the brain and spinal cord) and in various glands throughout the body such as the pituitary and adrenal glands. These peptides can be divided into three categories (enkephalins, endorphins, and dynorphins), depending on the type of larger precursor molecule from which they are derived. Opioid peptides function both as hormones and as neuromodulators; the former are secreted in the blood system by glands and are delivered to a variety of target tissues where they induce a response, while the later are produced and secreted by nerve cells (or neurons) and act in the central nervous system to modulate the actions of other neurotransmitters.
Neurons are electrically excitable cells that process and transmit information through electrical and chemical signals that travel via synapses, specialised connections with other cells. These signals are transmitted across a synapse from one neuron to another by neurotransmitters. By altering the electrical properties of their target neurons and making them difficult to excite, opioid peptides can influence the release of various neurotransmitters.
Through these two different mechanisms, opioid peptides can produce many effects including pain relief, euphoria and altered behaviour such as food and alcohol consumption. The apparent connection between exercise and happiness has been explained at least somewhat by the release of endorphins, for example. Exercise is commonly recommended as a strategy for stress-relief and mood improvement, but less widely accepted forms of therapy might also be connected to opioid peptides. Evidence suggests that pain relief induced by acupuncture results from stimulation of opioid peptides – these peptides act through receptors on their target neurons, and chemicals that inhibit opioid receptor function have been found to reverse acupuncture-induced analgesia. Painful, stressful or traumatic events or stimuli can induce the release of opioid peptides, with the resulting euphoria and pain relief making the sufferer less sensitive to noxious events. Opioid peptides have been reported to affect the release of specific neurotransmitters such as dopamine and serotonin, but the response of the neurons that receive opioid-peptide stimulation depends on their excitatory versus inhibitory nature, making the outcome difficult to predict.
The words ‘opioid peptides’ may not have left a dazzling feeling of recognition within your memory upon first encounter, but these peptides act within the brain and wider body to influence a number of important functions. Although it is not easy to predict the effect of neuromodulators that alter the release of other neurotransmitters, there is little question that opioid systems play a critical role in modulating a large number of sensory, motivational, emotional and cognitive functions. Alterations in opioid peptide systems may contribute to a variety of clinical conditions, including alcoholism, obesity, depression, diabetes and epilepsy. Many questions still remain, particularly those concerning the exact role of opioid peptides produced within the body in relation to addictive and emotional behaviour and psychiatric disorders. Since these disorders are typically of a complex nature, seeking the answers to these questions is not a simple feat. Advances in genetics and genomics research that aim to explain function by studying our DNA are helping to pave the way. But perhaps if there is one thing that can help motivate our talented scientists to reach their challenging goals, a healthy dose of opioid peptide might be just the thing.
Emily Brown PhD