Agonist vs. Antagonist Opioids: How Painkillers, Heroin and Opiate Medications Work in the Brain

Agonist vs. Antagonist Opioids Painkillers Heroin Opiate Medications Work in Brain - FAN

Even if you live under a rock, you have probably heard that we have a nationwide opioid crisis in America. The death toll from opioid misuse, abuse and overdose are staggering.

Use of prescription opioids – such as Vicodin, OxyContin and morphine – as well as the street opioid heroin is skyrocketing. In fact, the epidemic is so bad that a half a million people are expected to die from opioid use within the next decade.

What Are Opioids?

Originating from the poppy plant, current opioids are natural, partially synthetic or synthetic drugs. From Victorian opium lounges to the Wild West surgeon’s table, variations of these drugs have been around for a long time.

Opioids are very good at controlling pain. They are also very good at addicting people.

Opioids are addictive because of the way the drug attaches to receptors in the brain. Our brains have opioid-specific receptors, and when those are activated, we feel pleasure as well as relief from pain. This feeling is desirable and motivates our brains to seek it out again once experienced.

Agonist vs. Antagonist Opioids

An agonist in biochemistry is a substance that mimics another substance and activates a physiological response when combined with a receptor (cells that receive stimuli).

A full agonist activates a full-action response, resulting in a full effect of the substance being mimicked. A partial agonist activates the receptors to action, but to a much lesser degree.

An antagonist is a substance that inhibits and blocks or dampens a physiological action. An antagonist, also termed a blocker or a blocking agent, binds to and blocks a receptor, preventing a substance of similar structure from attaching to the receptor.

How Opioid Addiction Starts

So, as it pertains to how opioids affect the brain, an agonist is a drug that activates the opioid receptors in the brain, causing that euphoric feeling people get when taking drugs like hydrocodone, oxycodone or heroin. A full agonist response elicits a rush of dopamine to the brain’s reward system. For most people, this creates a high level of pleasure or excitement.

The brain is made to repeat rewarding activities, so once you take opioids, your brain wants to repeat that pleasure – and that’s how addiction begins.

Treatment with Opioid Antagonists

An opioid antagonist binds to the opioid receptor and forms something like a seal to cover the entry point where the drug hits the brain. By preventing opioids from crossing the blood-brain barrier, even if people use opioids at the same time, an antagonist will reverse the effects (like euphoria and slow breathing).

Naloxone and naltrexone are opioid antagonists that block the effects of opioid binding. The nasal spray version of naloxone, Narcan, is a fast-acting emergency treatment for someone who has stopped breathing because of opioid overdose. It has saved numerous lives.

Naloxone reverses respiratory depression as a result of too much heroin, OxyContin or other opioid drugs. Both naloxone and naltrexone are also used to treat other conditions, such as drug and alcohol addiction and chronic pain.

What Is Buprenorphine?

Buprenorphine is a mild or partial agonist in that it is an opioid, but it acts as both an agonist and an antagonist. Buprenorphine activates the opioid receptors in the brain, but to a much lesser degree than full agonists like Vicodin or fentanyl. At the same time, buprenorphine blocks other opioids from attaching to the brain’s opioid receptors.

This makes buprenorphine unique and a good choice for addiction treatment. It gives the person addicted to drugs or alcohol a little bit of the pleasure of opioid feelings, which quiets the cravings and suppresses withdrawal symptoms. Buprenorphine is also prescribed for chronic pain. It has much less potential for addiction than full agonist opioids do.

How Heroin Affects the Brain

Everyone has opioids in their brains. They are a naturally occurring substance meant to calm the body and manage the reward and pleasure circuitry in the brain.

Studies have shown that even after taking prescription painkillers for only a few weeks, the changes in the brain’s structure are evident in MRIs. Patients taking pain meds have a reduction in the gray matter responsible for the regulation of pain, cravings and emotions.

What are the implications of a reduction of gray matter that regulate emotions? People taking painkillers over a long time can have a harder time controlling their emotions. Additionally, painkillers reduce your body’s ability to control pain, making you more sensitive to it.

Other effects of the binding of synthetic opioids to the brain’s opioid receptors:

  • Slows down the central nervous system
  • Depresses respiration and slows breathing

A depressed respiratory function is what puts opioid users at serious risk of death. It is easy to overdose on opioids and stop breathing. Heroin, one of the strongest opioids, which is partially why it is illegal, affects the brain deeply and can take years to reverse.

What Happens to the Brain After Someone Stops Using Opioids?

Even after someone has stopped using opioids, their brain still shows effects of the drug. It can take a long time for the brain to restabilize – many months or years. The longer a person uses prescription drugs, the more ingrained brain changes are, resulting in a longer recovery period. Use of stronger drugs, such as heroin or fentanyl, also result in a longer amount of time for the brain to adjust back.

The physical dependence on the opioid can be reduced by gradually tapering off. During this time, the patient can make behavioral changes that will rewire the brain. Cognitive therapy also helps to deal with cravings and negative behaviors.

Post-acute opioid withdrawal symptoms include:

  • Anxiety and irritability
  • Mood swings
  • Increased pain
  • Cravings
  • Low energy and enthusiasm, fatigue

The long-term effects of opioids on the brain are substantial. For a long time after opioid use ceases, the individual may experience learning issues, memory problems and other cognitive impairments. The recovery process will be a lengthy one, and it is common to experience many challenges during the process.

Is There a Difference Between Addiction and Dependence?

Yes. People with chronic pain that take painkillers over time develop a dependence on their medication. If they suddenly stop taking their medication, they will go through physical withdrawal symptoms due to the changes the body undergoes when on painkillers.

Physical acute withdrawal symptoms include:

  • Vomiting
  • Diarrhea
  • Tremors
  • Cramps
  • Weakness
  • Possible suicidal thoughts

Addiction is an abnormal condition, which is classified as a disease. Furthermore, it may or may not come with a physical dependency. Addiction is characterized by compulsive behavior, uncontrollable cravings and participation in drug use (or the addictive behavior of choice) despite harmful life consequences to oneself or others.

Your Turn

Nearly everyone in America knows someone who has been affected by the opioid crisis.

Has someone in your family experienced heroin/opioid use or overdose? Do you have personal experience with medically assisted therapy? Discuss opioids and share your experiences on related topics with other members in our online forum who have traveled paths similar to yours.

See Our Heroin Addiction Fact Sheet

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