Further testing showed that the rodents exposed to male odors were actually feeling less pain, rather than simply hiding the pain they were in. The male aroma ramped up their stress levels, which deadened the hurt. “It’s really astounding that such a robust effect could have been missed for so many years,” Mogil says.
Though, to be fair, they don't go into detail about how exactly they determined that the rodent was "actually feeling less pain"...
True SIA should be accompanied by decreased immediate-early gene expression in the pain-processing neurons of the spinal cord dorsal horn; we observed a dramatic (≥50%) decrease in Fos protein–positive neurons in mice injected with zymosan and exposed to male but not female experimenters or shirts (Kruskal-Wallis test statistic: 12.0, degrees of freedom = 4, P < 0.05; Fig. 2d). Much is known about the neurochemistry of SIA, which exists in opioid (naloxone-reversible9), non-opioid (cannabinoid-1 (CB1) receptor–mediated10) and mixed opioid/non-opioid11 forms. The male experimenter SIA seen here was found to be the latter, as it was blocked (observer × drug interaction: F3,56 = 2.8, P < 0.05) by the broad-spectrum opioid receptor antagonist naloxone (1 mg per kg body weight (mg/kg); P < 0.05), by the inverse CB1 receptor agonist AM-251 (10 mg/kg; P < 0.05) and by a combination of the two drugs (P < 0.01), at doses not affecting pain sensitivity per se (Fig. 2e).
In other words, pain in mice is associated with a particular protein binding neurons in a particular part of the brain. The researchers showed that in mice handled by men, the proportion of those neurons bound by the pain-associated protein was significantly decreased, supporting the hypothesis that male-handled mice are actually experiencing less pain.
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The article addresses this:
Though, to be fair, they don't go into detail about how exactly they determined that the rodent was "actually feeling less pain"...
It's properly addressed in the actual paper [0]:
True SIA should be accompanied by decreased immediate-early gene expression in the pain-processing neurons of the spinal cord dorsal horn; we observed a dramatic (≥50%) decrease in Fos protein–positive neurons in mice injected with zymosan and exposed to male but not female experimenters or shirts (Kruskal-Wallis test statistic: 12.0, degrees of freedom = 4, P < 0.05; Fig. 2d). Much is known about the neurochemistry of SIA, which exists in opioid (naloxone-reversible9), non-opioid (cannabinoid-1 (CB1) receptor–mediated10) and mixed opioid/non-opioid11 forms. The male experimenter SIA seen here was found to be the latter, as it was blocked (observer × drug interaction: F3,56 = 2.8, P < 0.05) by the broad-spectrum opioid receptor antagonist naloxone (1 mg per kg body weight (mg/kg); P < 0.05), by the inverse CB1 receptor agonist AM-251 (10 mg/kg; P < 0.05) and by a combination of the two drugs (P < 0.01), at doses not affecting pain sensitivity per se (Fig. 2e).
In other words, pain in mice is associated with a particular protein binding neurons in a particular part of the brain. The researchers showed that in mice handled by men, the proportion of those neurons bound by the pain-associated protein was significantly decreased, supporting the hypothesis that male-handled mice are actually experiencing less pain.
http://www.nature.com/nmeth/journal/vaop/ncurrent/full/nmeth...