Opioid dependence

Worldwide, opioids are the most common cause of drug related deaths.

The number of individuals who use them has quadrupled in the last 20 years, with an uptick in heroin use, an even bigger uptick in prescription opioid use, and a large number of people using both.

Because of their potential for addiction and overdose, opioids are regulated substances in many countries.

As a class, opioids share one thing in common—they bind to opioid receptors in the brain, spinal cord, and gastrointestinal tract.

Some are endogenous, meaning that they are produced naturally by the body, like endorphin, short for endogenous morphine.

But others are exogenous, meaning that they come from the environment, like heroin and morphine, which come from the opium poppy—a flowering plant that oozes a milky white liquid—while others like fentanyl are synthesized in the laboratory.

To understand how opioids work, let’s zoom into a region of the brain tissue that has opioid receptors.

Normally, in the absence of endorphins, inhibitory neurons secrete a neurotransmitter that prevents nearby neurons from releasing the neurotransmitter dopamine.

Now, let’s say someone goes to play a rigorous game of badminton.

Exercise releases endorphins which activate the three major opioid receptors located on the inhibitory neurons, called the mu, kappa, and delta receptors.

As endorphins bind to these receptors, they block the inhibitory neuron from releasing neurotransmitters, allowing the dopamine secreting neurons to freely unload dopamine.

The dopamine then gets picked up by a third neuron in the same area.

When dopamine release takes place in pain processing regions of the brain like the thalamus, brainstem, and spinal cord, the result is feeling less pain.

When dopamine release takes place in reward pathway regions like the ventral tegmental area, nucleus accumbens, and prefrontal cortex, the result is a calming effect that feels good.