If it existed, it might also be profoundly autistic and … diabetic? So science cannot disprove the existence of a soul, but one thing we’re learning is how much valued human properties such as love and attachment and awareness of others are a product of our biology — emotions like love are an outcome of chemistry, and can’t be separated from our meaty natures.
The latest issue of BioEssays has an excellent review of the role of the hormone oxytocin in regulating behaviors. It highlights how much biochemistry is a determinant of what we regard as virtues.
Anyone with a little familiarity with physiology will know the simple function of oxytocin. It’s a short peptide, only nine amino acids long, that is produced by neurons of the hypothalamus. These neurons send a process down into the stalk of the pituitary gland, where they terminate on blood vessels and secrete their hormonal payload into the bloodstream for circulation to the rest of the body. This is a potent hormone: it’s responsible for inducing muscle contractions in the uterus (it’s the hormone used to induce labor in pregnant women), and release also triggers lactation. Those are the overt functions that we expect our physiology students to memorize, and it’s also what’s diagrammed below: the big pink neuron on the left is sending black dots, packets of oxytocin, down its axon to be disseminated to the rest of the body.
Ah, but that simple story is incomplete. The diagram also shows dendrites emerging from the cell body of that hypothalamic neuron; these dendrites are also secreting oxytocin deeper into the brain, into the hippocampus, amygdala, striatum, and nuclei in the mid- and hind-brain. It’s oozing all over the place, and these other roles are almost certainly just as important as its role as an endocrine signal — after all, we males also have these oxytocin-secreting cells, yet we neither lactate nor go into labor. What do these signals do in our brains?
There is a long literature showing an association between oxytocin and affiliative behavior. The differences between prairie and montane voles are well-known: montane voles show little pair-bonding behavior and males do not share in raising the young compared to prairie voles, and the montane voles also show reduced levels of oxytocin receptors in areas associated with reinforcement and conditioning. The idea is that release of oxytocin occurs in social conditions, giving the voles with receptors a little natural high that they seek to repeat, while the surlier montane voles get no biochemical reward, and social behavior is not reinforced.
Experiments in other animals, such as sheep and mice, with application of oxytocin or oxytocin antagonists (also with gene knockout experiments in mice) have had similar effects. Oxytocin encourages nurturing and the measurable symptoms of affection, while blocking oxytocin weakens social behavior. These experiments would be impossible to justify in human subjects — can you imagine mad scientists prowling the maternity wards to see if any new mothers would like to try a drug that would reduce their attachment to their baby? — but some experiments on other behaviors, which involve intranasal oxytocin (that’s right, they’re snorting it) show interesting effects. For instance, oxytocin increased trust between men playing an investment game, and also increased the ability to infer the mental status of others, or elevated empathy.
The diagram above also shows some other complicated interactions going on. Oxytocin is the effector molecule, the one that when released causes changes in other neurons, but other molecules are important regulators of oxytocin. This paper focuses primarily on CD38, a transmembrane receptor that has ADP-ribosyl catalase activity — that is, it creates internal second messenger molecules, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAD) that mobilize calcium stores and increase various activities of the cell. The identification of a regulatory molecule allows more fine-tuned analysis of how oxytocin works. Knocking out oxytocin in mice, for instance, is going to cause obvious problems in the animals with lactation and birth — oxytocin knock-out mice don’t lactate. Knocking out CD38 has more subtle effects: the mother mice lactate, but have greater latencies and spend less time nurturing their pups.
Here’s a science-fiction scenario for you, too: the mice with mutant CD38 showed reduced mothering. They were infected with a virus carrying a wild-type copy of CD38, and a few weeks later, their maternal behaviors were restored. Think about using infectious agents to intentionally modify behavior — you should be a little bit concerned about the possibilities.
The paper mentions a few other interesting correlations. The deficiencies in social behaviors in oxytocin-deficient animals is suggestive of a human disorder, autism. Studies have found reduced plasma levels of oxytocin in autistic individuals, but one has to worry about confusing cause and effect: oxytocin levels are modified by social interaction, too, and the diminished sociability of autistic people may be indirectly reducing their oxytocin secretion, rather than reduced oxytocin secretion impairing their social interactions. There are a few studies that show some amelioration of the effects of autism by intravenous transfusion of oxytocin, though, which is suggestive. Don’t get too hopeful, though, since at best what was seen was a mild reduction of some of the symptoms. (Hey! Maybe they have to try the mind-altering virus approach!)
The other interesting correlation is that cADPR is a second messenger used to regulate insulin production in pancreatic cells. One study of familial type II diabetes identified a common mutation that downregulated cADPR; a few patients in this study also showed an autism-related phenotype, although the numbers looked so small to me that we’re well into the realm of anecdote with this one.
Now I know some people are peculiarly offended by the idea that something like love can be reduced to “just chemicals”, but I’m not one of them. I find it absolutely wonderful that beautiful human feelings are not the product of ineffable invisible spirits, but are a consequence of our splendidly earthly humanity — hug someone, and little peptides tickle regions of their brains, and they feel good and happy, and they might just hug you back … and that’s all right with everyone. Who we are is inseparable from what we are, and we’re all complicated conglomerations of intricate biochemistry with a long, long history of natural change, and we should revel in that. Share some chemistry with your neighbors today!
Bartz JA, McInnes LA (2007) CD38 regulates oxytocin secretion and complex social behavior. BioEssays 29(9):837-841.