During pain experiments, mice show reduced pain responses in the presence of men compared to that of women, reports a study published in Nature Methods on April 28. The study, led by Jeffrey Mogil of McGill University in Montreal, Canada, found that odors given off by males—human or other species—induced stress responses in mice sufficient to trigger pain relief. The findings argue that the sex of experimenters is worth taking into account in rodent studies.
“This study puts the onus on us to declare the sex of the experimenters in our publications,” said David Finn of the National University of Ireland in Galway, who was not involved in the study. Finn studies how stress, mood, and anxiety influence pain responses.
Mogil’s group has previously found that the sex of the mouse influences how pain is processed (see PRF related news story). But the new study turns the tables by finding that manly scents elicit stress-induced analgesia, a phenomenon involving opioid and endocannabinoid signaling in the brain (Butler and Finn, 2009).
“We’ve known for some time that the presence of an experimenter can influence rodent behavior. But to my knowledge, this is the first demonstration of the importance of experimenter sex,” Finn said, adding that the results have prompted him to consider reanalyzing his lab’s past data.
The study stemmed from a suspicion in Mogil’s lab that the presence of an experimenter temporarily quashed pain responses. “People in my lab would report that it sometimes seemed like the mice were waiting for them to leave the room before showing pain responses,” Mogil told PRF.
Mogil and colleagues set out to study this systematically, expecting to find an experimenter-induced effect. But what they found was something specific to male experimenters. “That was a big shock to us,” he said.
When mice were injected with zymosan A, an inflammatory agent that produces pain, the presence of a male experimenter in the room produced pain responses 36 percent lower than those observed in mice with no human in the room; in contrast, pain responses in the presence of female experimenters were not significantly different from the no-observer condition. The male experimenter-induced analgesia lasted up to 60 minutes.
The effect might seem a minor wrinkle, because it mainly affects baseline responses, and pain studies mostly involve changes from baseline. More problematic, however, is the possibility that drug effects will be confused for the experimenter-induced effect.
More optimistically, the findings might explain discrepant results among different labs. “If one lab is using a male experimenter and another lab is using a female experimenter, they can come up with different results,” Mogil said. “Neither one is wrong, and it’s not a failure to replicate.”
Scent of a man
Using the mouse grimace scale developed by Mogil’s group to measure pain responses (Langford et al., 2010), first authors Robert Sorge and Loren Martin found that the male experimenter effect was mediated by olfactory cues. Shirts that had been worn by men, but not by women, placed in the room decreased mouse pain responses to zymosan injections, as did three different volatile compounds secreted more by males than females ([E]-3-methyl-2-hexenoic acid, androstenone, androstadienone).
Though the effect disappeared within an hour, it could be reinstated the next day by the same or different male. It also translated across species: Cage bedding from male mice, rats, guinea pigs, cats, and dogs all induced a similar analgesia. Bedding from cage mate mice, however, did not produce pain relief, indicating that mice can smell the difference between familiar and stranger male mice.
The researchers propose that the male experimenter effect originates from the animals’ innate fear of strange male mice. Because mammalian males secrete similar odorants, the fear extends to humans.
In the absence of pain, these odors increased markers of stress, including the hormone corticosterone. These stress responses attenuated pain by activating opioid and cannabinoid 1 receptors, and this effect was plain in the spinal cord: Exposure to male experimenters or their shirts halved the number of activated neurons, as measured by expression of the immediate-early gene c-fos, in the dorsal horn after zymosan injection.
When the researchers sifted through earlier data, they confirmed the male experimenter effect on other measures of pain. Mice tested by male experimenters had significantly longer tail or paw withdrawal latencies when exposed to noxious heat than those tested by females; similarly, those tested by males had higher mechanical stimulation thresholds than those tested by females.
Beyond pain, the researchers also found that male odors increased anxiety, as measured by increased time mice stayed against the wall in an open-field test. “And who knows what else?” Finn asked. “This is likely to have implications across a range of behaviors and paradigms.”
Michele Solis is a science writer and former neuroscientist who lives in Seattle, Washington, US.
Image: Alexander H. Tuttle