It’s no accident that we often refer to rejection or insult as a slap in the face — research shows the same brain signaling pathways are involved in both physical and emotional pain.

And morphine, a drug well known for its ability to dull physical pain, also reduces separation-induced emotional distress in monkeys, dogs, guinea pigs, rats and chickens.  This is thought to depend on changes in signaling through a morphine receptor called MOR (mu-opioid receptor) that is also implicated in emotional pain in humans. Studies have shown that signaling through MOR is reduced when women recall upsetting events like the death of a loved one or the breakup of a romantic relationship.

Given that MOR seems to be at the center of the overlap between physical and emotional pain, researchers from the UCLA Department of Psychology wondered whether a genetic variation in the gene that encodes MOR that is already known to impact sensitivity to physical pain might also affect how thick-skinned a person is.

Baldwin M. Way and colleagues surveyed 122 healthy young adults about their sensitivity to social rejection.  The assessment measured the tendency of study subjects to be fearful that social interactions will result in hurt feelings, criticism and being a burden to others.  The results, published online this week in the Proceedings of the National Academy of Sciences, show that people with one or two Gs at SNP rs1799971 had significantly higher levels of self-reported sensitivity to social rejection than those with an A at both copies.

The G version of rs1799971 has previously been associated with needing more morphine for pain relief after surgery.

(23andMe customers can check their data for rs1799971 using the Browse Raw Data feature.)

A subset of the study participants also had their brains scanned in an fMRI machine while playing an online ball-tossing game.  The subjects thought they were playing with two people, but in reality they were interacting with a pre-set computer program.  The game simulated social rejection by having the subjects’ virtual playmates leave them out of the fun.

Just as in the survey, the G version of rs1799971 was associated with heightened responses to social rejection.  Higher brain activity was recorded in the anterior cingulate cortex and the anterior insula, brain regions associated with the processing of both physical and emotional pain, in people with one or two Gs.

As is usually the case, the results of this study should be considered preliminary until other researchers replicate them. But based on their results, along with previous research, the authors conclude that, “… at multiple biological levels, including the neurochemical, neuroanatomical, and now genetic, feeling hurt physically shares more than just linguistic commonality with feeling hurt socially.”

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Redheads might have a better excuse than the rest of us for avoiding the dentist.

For several years now scientists have known that the same genetic variations that give redheads their fiery manes can increase the amount of general or local anesthetic a person needs in order to be properly put out or numbed up.

New research suggests that the effect of these variations is strong enough, and hasn’t been addressed by dentists well enough, that the people who carry them are more than twice as likely as those who don’t to avoid going to the dentist altogether.

Researchers at the University of Kentucky surveyed 67 redheads and 77 dark haired people about their general anxiety levels, their dental treatment-related anxiety and their fear of dental pain. The results, published in the July issue of the Journal of the American Dental Association, reveal that while hair color has no effect on general anxiety, redheads are more likely to be apprehensive about sliding into the dentist’s chair.

A closer look at the data showed that increased dental-treatment anxiety and fear of dental pain is found in all people with the variants in the MC1R gene associated with red hair, even those people who carry these variations despite having dark hair.

The protein encoded by the MC1R gene is found in melanocytes, the cells that give hair and skin their color. The variants associated with red hair alter the protein’s function, tipping the balance of pigment production in melanocytes from black-brown eumelanin to red-yellow pheomelanin. Researchers don’t yet understand how this same protein impacts pain sensitivity and anesthetic needs.

People carrying one or two of the MC1R variants had 2.46 times greater odds of avoiding routine dental care compared to those who carry none. The authors speculate that this might be because prior dental experiences have left them in pain.

(23andMe customers can use the table at the end of this post to see if they carry any of the MC1R variants associated with dental treatment-related anxiety, fear of dental pain and avoidance of dental treatment.)

The researchers recommend that dentists evaluate all patients, especially those with natural red hair, for dental procedure-related anxiety and take whatever steps are necessary to help them manage their feelings and make it to their regular check-ups, because as the saying goes, “Ignore your teeth and they’ll go away.”

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