Analgesics are a class of drugs that help relieve the body from the sensation of pain by blocking chemicals in the pain sensor neurons of the brain. Other neurons throughout the body send messages to the brain on a multitude of topics (eg, temperature, pressure, pH).
Nociceptors, nerve receptors for pain, send messages of noxious stimuli to the brain that something is potentially hurting the body. Nociceptors send messages about such things as pressure, sharp objects, noxious smells, bad tastes, and pain, and the brain interprets those for immediate response to protect the body.
There are several neurotransmitters involved in pain signals, the main ones being glutamine and substance P. When noxious stimuli trigger the primary neuron through the skin or muscle, the message is relayed by a secondary neuron to the spinal cord’s dorsal root ganglion and toward the brain for interpretation. These chemical neurotransmitters are relayed to the thalamus in the brain and then onto the limbic system for an emotional response. Ideally, the message to the limbic area of the brain promotes learning so as to avoid the cause of the noxious substance in the future.
Opioids inhibit pain signals at multiple areas in this pathway. They affect the brain, the spinal cord, and even the peripheral nervous system. Opioids work on both directions of messages in the nervous system, including the ascending pathways in the spinal cord, which they inhibit, and the descending pathways, by which they block inflammatory responses to noxious stimuli.
Our bodies have three receptors called mu, kappa, and delta, that can be activated by opioid agonists like morphine, hydrocodone, or heroin. When mu receptors are activated, dopamine, the natural brain chemical for pleasure, is also increased. Pleasurable feelings are experienced as inherently worth repeating, which drives the user to repeat the drug use.
Opioid receptors are found on both the primary and secondary neurons, and when an opioid binds to these receptors no other pain signals are sent up to the brain—making opioids very effective against pain. In the brain, opioids cause sedation and decrease the emotional response to pain. Heroin, like morphine, passes through the liver and then is released back into the blood, where it crosses the blood–brain barriers. Heroin is converted to morphine where it connects with mu receptors, so fast that heroin is three times more potent than morphine.
Short-term sensations of opioids
- Warmth sensation through skin and body
- A feeling of heaviness in arms and legs
- Pain relief
- Dry mouth
- Possible dry mouth
- Slowed heart functioning
- Slowed breathing
- Sense of well being
These opioid agonists come with additional noxious side effects. When a kappa receptor is stimulated, it can also produce hallucinations, anxiety, and restlessness. Delta and mu receptors can cause respiratory depression, because as the midbrain is stimulated it suppresses the body’s ability to detect carbon dioxide levels in the body, which is the main stimulus for breathing.
Other negative side effects include constipation, sedation, nausea, dizziness, urinary retention and tolerance. Tolerance is the requirement of the body for increased amounts of the drug to reach its desired effects; this is why opioids can become addictive as the person continues taking more and more of the drug to achieve the desired pain relieving and sedating effects (Dunphy & Winland-Brown, 2016). The key ingredient in opium is morphine, which began to be produced formally by the pharmaceutical company Merck. It was also discovered that when administered by IV, morphine is 3 times more potent than administered by other methods such as smoking or snorting.
Long-term Side Effects of Opioid Use
Long-term use of opioids has been shown to cause deterioration of the brain’s white matter and includes effects of insomnia, chronic constipation, sexual dysfunction, irregular menstrual cycles in women, and kidney disease, as well as physical damage resulting from administration techniques such as snorting, smoking, or IV drug use.
Although cocaine and morphine both have effects on the neurotransmitter dopamine, they work in different ways. Whereas the opioids increasing dopamine stimulation, cocaine blocks the reuptake of existing dopamine and makes it last longer, producing a longer state of pleasure. Both opioids and cocaine do, however, influence the brain’s interpretation of pleasure—reinforcing the repeated drive to get the drug. In addition to short-term withdrawal symptoms,
Long-term opioid use causes
- Decreased ability in decision making
- Decreased ability for self-reflection and discipline
- Decreased ability to respond to stress effectively
Test Your Knowledge
How do opioids work?
- They block pain at the site of the noxious signal.
- Opioids block the neurotransmitter dopamine.
- Opioids bind to receptors in the peripheral and CNS to block pain signals.
- Opioids increase serotonin throughout the brain.
Apply Your Knowledge
Q: What is the fastest acting opioid that is used in medicine for pain relief? How is it administered?
A: IV morphine and fentanyl