Opioids are substances that attach to opioid receptor sites found on neurons, cells that transmit information between the brain and the rest of the body. This transmission of information is known as neurotransmission. During neurotransmission, neurotransmitters, or chemical messengers, spread information between neurons by either activating or inhibiting those neurons from producing their own chemical messages. Neurotransmission is naturally regulated by a series of inhibitory and activation signals that control neuronal activation; some neurotransmitters cause neurons to activate while some neurotransmitters prevent neurons from activating. The signals received by neurons from neurotransmitters regulate pathways of neuron signaling.
According to the National Institutes of Health (NIH), opioids function as neurotransmitters capable of activating certain pathways of neuron signaling by binding to their specific opioid receptor sites. The body naturally produces endogenous opioids that bind to and activate opioid receptor sites on neurons to release “feel-good” chemicals such as endorphins, serotonin, oxytocin, and dopamine that control our reaction to painful stimuli.
However, the NIH states that externally-administered, opioid drugs not produced by the body can disrupt the natural regulation of neurotransmission. These opioid drugs mimic the molecular structure of the brain’s natural opioids to bind to natural opioid receptor sites, activate neurons, and cause neurons to overproduce the “feel-good” neurotransmitters. In this way, these opioid drugs “fool” the nervous system into a state of overstimulation that ultimately results in the excess release of neurotransmitters that produce feelings of relaxation and pleasure. For this reason, clinicians can prescribe opioid analgesics as a form of pain-managing medical treatment because of the drugs’ ability to induce the release the chemicals that alleviate painful feelings.
According to the National Institute on Drug Abuse (NIDA), one risk of using prescription opioids is addiction: opioid usage particularly activates the production of dopamine, the neurotransmitter that cause the desire to repeat behavior. Dopamine signaling activates the brain’s reward system and causes the desire to repeat pleasurable experiences. As a result, a small percentage of patients develop an unwanted addiction to opioids that begins with the usage of prescription opioids. The usage of heroin, an illegal opioid drug, is nineteen times higher for individuals who began using opioids as prescription medication than those who had no prior prescription access to opioids.
Despite the link between prescription opioid usage and illicit opioid usage, the prescription rate for opioids has significantly grown over the past few years, perhaps over three times higher than it was three decades ago.
Opioids and opioid medications can be classified into three main categories, distinguished by their synthetic properties:
- Natural opioid: opioids derived from the opium poppy (e.g., morphine, codeine, thebaine);
- Semi-synthetic opioids: opioids created from natural opiates (e.g., hydromorphone, hydrocodone, oxycodone, heroin); and
- Fully synthetic opioids: opioids created entirely synthetically (e.g., fentanyl, pethidine, levorphanol).