henry copeland:

How relapse happens: Opiates reduce the brain's ability to form, maintain synapses: Preclinical research was focused on revealing the molecular mechanisms behind addiction and relapse -- ScienceDaily

In experiments with rodents, the UB team determined that exposure to heroin and morphine reduced drebrin levels in the nucleus accumbens, a key part of the brain's reward pathway. Synaptic rewiring The researchers found that opiate exposure causes synaptic rewiring in this part of the brain, as well as a decrease in drenditic spines, the protrusions on neurons that play key roles in neuronal transmission, learning and memory. "Opiates fundamentally change how the brain communicates with itself," Dietz said.
- www.sciencedaily.com