Repeated electrical brain stimulation for pain relief caused tolerance similar to opioid drug tolerance — it could be overcome by increasing intensity but was blocked by naloxone.
Tolerance from depletionOpioid peptide stores in the PAG were depleted by repeated stimulation
What the researchers found
Electrical stimulation of the periaqueductal grey (PAG, a brain region that controls pain) initially produced strong pain relief in rats. With repeated stimulation, this relief gradually disappeared.
The tolerance was not from opioid depletion. Brain levels of beta-endorphin, met-enkephalin, and dynorphin were unchanged in tolerant rats compared to controls.
It was not a learned (conditioned) response either. Tolerant rats exposed to all the cues associated with stimulation showed no compensatory pain increase. And repeated exposure to the cues without stimulation did not restore pain relief.
The tolerance behaved exactly like drug tolerance: the dose-response curve shifted right (needed more stimulation for the same effect), it was reversed by naloxone, and it recovered spontaneously over time. Tolerant rats also showed cross-tolerance to morphine, confirming the same opioid receptor system was involved.
Rats showed no signs of stress during the protocol: normal body weight, food intake, temperature, adrenal weight, and hormone levels.
Why it matters
This study clarified why brain stimulation for chronic pain loses effectiveness over time. The tolerance is a real pharmacological phenomenon at the receptor level, not a psychological adaptation or a running out of the brain's painkiller supply. This has implications for people who use neurostimulation devices for pain management.
How the study worked
Freely moving rats received repeated electrical stimulation of the ventral PAG. Pain was measured using heat and pressure tests. Opioid peptides were measured in multiple brain regions and the pituitary. Conditioning experiments tested whether tolerance was a learned response. Stress markers were measured. Cross-tolerance to morphine was tested. All tested in rats, not people.
What this study cannot tell us
Tested in rats, not people. Only one brain stimulation target was tested. The study could not directly measure receptor-level changes. The exact molecular mechanism of tolerance was not identified.
How to read the evidence
Preliminary animal study with comprehensive pharmacological and biochemical characterization.
When this study was published
Published in 1987 — informed the development of clinical deep brain stimulation protocols.
The bigger picture
Deep brain stimulation for chronic pain is a clinical reality. Understanding the tolerance mechanisms helps optimize stimulation protocols and potentially combine with strategies to maintain effectiveness.
Questions still open
- Can drug-free intervals reverse stimulation tolerance?
- Would exogenous opioid peptide supplementation prevent tolerance?
Common questions
What is the periaqueductal grey?
Why does tolerance develop?
Read the original research
An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats.
Brain research, 435(1-2), 97-111
Citation
Millan, M J; Członkowski, A; Herz, A. (1987). An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats.. Brain research, 435(1-2), 97-111.