Genetic Stress Vulnerability Changes How Repeated Stress Affects Pain and the Endocannabinoid System | Research

The takeaway

Stress-vulnerable and stress-normal rat strains showed opposite pain responses after repeated swim stress, with corresponding differences in endocannabinoid system changes in the spinal cord and amygdala.

Read this if you are interested in how genetic factors influence the relationship between stress, pain, and the endocannabinoid system.

Genetic background reversed the stress-pain relationship entirely

What the researchers found

Chronic stress can make pain worse, but this study showed that genetic background dramatically changes this relationship. Two rat strains with different stress vulnerabilities showed opposite outcomes after 10 days of swim stress.

Normal Sprague-Dawley rats developed enhanced pain sensitivity after repeated stress, as expected. But stress-vulnerable Wistar Kyoto rats actually showed reduced pain sensitivity after the same stress protocol.

The endocannabinoid system changes matched these behavioral differences. Stressed normal rats showed increased MAGL (an enzyme that breaks down endocannabinoids) in the spinal cord and decreased anandamide in the amygdala. These changes were absent in the stress-vulnerable strain. Additional strain differences in CB1 receptor expression and 2-AG levels were found in both the spinal cord and amygdala.

Why it matters

This study demonstrates that the interaction between stress and pain is not universal but depends on genetic background. The endocannabinoid system sits at this intersection, and understanding how it mediates different stress-pain outcomes in different genetic contexts could lead to more personalized approaches to pain treatment.

The numbers in context

10 days of forced swim stress. SD rats: enhanced pain (late phase formalin response). WKY rats: reduced pain. SD rats showed increased MAGL mRNA in ipsilateral spinal cord and decreased anandamide in contralateral amygdala. These changes absent in WKY rats.

How the study worked

Sprague-Dawley (stress-normal) and Wistar Kyoto (stress-vulnerable) rats were subjected to 10 days of forced swim stress. Inflammatory pain was assessed using the formalin test. Endocannabinoid levels, receptor expression, and enzyme levels were measured in the dorsal spinal cord and amygdala using RT-PCR and mass spectrometry.

What this study cannot tell us

WKY rats are an inbred strain that may not directly model human genetic variation. The forced swim stress protocol is specific and may not generalize to other stress types. The formalin test measures acute inflammatory pain, which may differ from chronic pain conditions.

How to read the evidence

Well-designed animal study comparing two genetically distinct strains with multiple molecular endpoints, but limited to rodent models.

When this study was published

Published in 2016. Research on individual differences in endocannabinoid-mediated stress and pain responses has continued.

The bigger picture

The finding that genetic background completely reverses the stress-pain relationship challenges one-size-fits-all approaches to stress-related pain conditions. It also suggests that the endocannabinoid system could be a key mediator of individual differences in stress-pain interactions.

Questions still open

Do similar genetic differences in stress-pain-endocannabinoid interactions exist in humans? Could endocannabinoid profiles predict who will develop stress-related chronic pain? Would cannabinoid-based treatments work differently in stress-resilient versus stress-vulnerable individuals?

Common questions

Does stress always make pain worse?

Not necessarily. This study found that stress-normal rats developed increased pain after repeated stress, but stress-vulnerable rats actually showed decreased pain, with corresponding endocannabinoid system differences.

How does the endocannabinoid system relate to stress and pain?

The endocannabinoid system showed different changes in stress-normal versus stress-vulnerable rats, suggesting it mediates the genetically dependent relationship between stress exposure and pain sensitivity.

Read the original research

Repeated forced swim stress differentially affects formalin-evoked nociceptive behaviour and the endocannabinoid system in stress normo-responsive and stress hyper-responsive rat strains.

Progress in neuro-psychopharmacology & biological psychiatry, 64, 181-9

View on PubMed ↗Publisher’s paper ↗

Citation

Jennings, Elaine M; Okine, Bright N; Olango, Weredeselam M; Roche, Michelle; Finn, David P. (2016). Repeated forced swim stress differentially affects formalin-evoked nociceptive behaviour and the endocannabinoid system in stress normo-responsive and stress hyper-responsive rat strains.. Progress in neuro-psychopharmacology & biological psychiatry, 64, 181-9. https://doi.org/10.1016/j.pnpbp.2015.05.008