Chronic morphine increased dynorphin in the addiction-related nucleus accumbens and altered enkephalin levels across brain regions, with distinct patterns during withdrawal.
Dynorphin increasedIn the nucleus accumbens (reward center) after 8 days of chronic morphine, explaining tolerance development
What the researchers found
Chronic morphine increased dynorphin A and B in the nucleus accumbens and met-enkephalin in the striatum, with distinct withdrawal-phase patterns across brain regions.
Why it matters
Changes in endogenous opioid peptides during morphine use and withdrawal help explain tolerance, dependence, and the difficulty of quitting opioids. The nucleus accumbens changes are particularly relevant to addiction.
How the study worked
Male Sprague-Dawley rats received twice-daily morphine injections for 8 days. Opioid peptide levels were measured in multiple brain regions and pituitary by radioimmunoassay during treatment and withdrawal.
What this study cannot tell us
Animal study in rats. Specific morphine dosing regimen may not represent all human opioid exposure patterns. Only measured peptide levels, not receptor changes.
How to read the evidence
Moderate — controlled animal study with systematic measurement across brain regions, though limited to one dosing regimen.
When this study was published
Published in 1995. The dynorphin/anti-reward theory of addiction has since become a major framework in addiction neuroscience, substantially supported by subsequent research.
The bigger picture
The nucleus accumbens is central to addiction. Rising dynorphin there during chronic morphine use may explain the decreasing pleasure (tolerance) and increasing distress that drives compulsive opioid use — the brain's own anti-reward system fighting back.
Questions still open
- Do dynorphin increases in the nucleus accumbens directly drive the negative emotional state of opioid withdrawal?
- Could blocking dynorphin signaling reduce opioid dependence without affecting pain relief?
Common questions
Why does morphine stop working as well over time?
What happens to these peptides during withdrawal?
Read the original research
The effects of morphine treatment and morphine withdrawal on the dynorphin and enkephalin systems in Sprague-Dawley rats.
Psychopharmacology, 118(4), 391-400
Citation
Nylander, I; Vlaskovska, M; Terenius, L. (1995). The effects of morphine treatment and morphine withdrawal on the dynorphin and enkephalin systems in Sprague-Dawley rats.. Psychopharmacology, 118(4), 391-400.