How do opiates cause euphoria and addiction?

into the blood stream the drug gets around brain cells. Between one another (synapses) there are normal discharges of neurotransmitters (GABA), the opiates simulate the same effect of those chemistry mediators and you get euphoric. The addiction comes when the brain gets accustomed with that extra amount of neurotransmitters and stop to produce its own. When you stop using the drug the brain do not get happy and starts to complain (abstinence syndrome).

Here's what I know about the subject:

Addictive properties have long been thought to result from activation of the "mesolimbic pathway" which is one of the dopaminergic pathways in the brain.

Most addictive drugs (such as amfetamin, cocaine and nicotine) increase dopamine release from the "nucleus accumbens".

Some addictive drugs (such as opioids and THC) inhibit GABA release from GABAergic neurons.

The pain-relieving effects of opioids are mediated by separate mechanism (activation of both peripheral and central mu-opioid-receptors).

Methadone doesn't usually have the same euphoria as the other opiates, I'm a recovering heroin addict whos on methadone and it doesn't give me any euphoric feelings. The first couple days I was on it, it did, but still not as much as other opiates. Methadone conversion is tricky, it really all depends on the person. I'm on 145mgs daily, but that could kill even a experinced addict, I had to slowly work up to it. Be careful with it.

It depends on many things