Tuesday, September 21, 2010

Against simple determinants of complex behavior: the case of oxytocin

Earlier, I argued against the view that complex behaviors and psychiatric diagnoses were caused by single genes. (In other words, that we will not find a gene for altruism, schizophrenia, or the propensity to drive slowly in the left lane).

A shy reader sent me the following private message:
“However, I have been amazed by the more I dig into the literature the more you find ‘one protein’ being responsible for very complex traits, one example is bonding and vasopressin and oxytocin.”

Oxytocin is a hormone produced in the brain by the posterior pituitary gland. It is released in quantity during child-birth to trigger lactation. Smaller amounts are released in both males and females following orgasm.

In sheep, the oxytocin released at birth seems necessary for bonding between a ewe and her lamb. In the absence of oxytocin release, the ewe is repelled by the odor of amniotic fluid, and will reject lambs approaching her.  However, the mechanism behind this hormonal influence on behavior is unknown.

However, the most famous case study for the striking behavioral effects of oxytocin comes from the prairie vole.  Prairie voles form strong and long-lasting bonds with single partners, typically following the first mating event. If a female vole is given oxytocin and then placed in the presence of a male she has not mated with, she will bond to him as if she had. Conversely, if oxytocin is blocked in the female after mating, she will not bond with the male. A similar pattern is shown in the male for the hormone vasopressin.

Different interests have capitalized on this data, calling oxytocin the “cuddle drug” and selling it for romantic courtship, curing social phobias, or by the military for interrogation enhancement.

But does oxytocin have such a clear role in changing the behavior of humans?  It is neither feasible nor ethical to manipulate the bonding between human parents and partners, so there is still much we don’t know. In a recent study, either oxytocin or placebo was given to participants playing an economic game requiring the trust of an unseen other player. Participants who received oxytocin demonstrated more trust in their partner, but only when the partner was behaving in a trustworthy way. In other words, oxytocin does facilitate trust, but not over-ride common sense.
It seems most likely that human behaviors are too complex to be completely modulated by ocytocin. However, I should also note in closing that even in animal models, the association between oxytocin and behavior is not 1:1. In the prairie vole, a strong pair bond can occur in the absence of mating (and hence an absence of oxytocin). And interestingly, genetically mutated mice without an oxytocin receptor gene have no trouble giving birth or lactating.

So, even in “textbook” cases of a single neurotransmitter causing a complex set of behaviors, we can have the following take-home messages:
  1. Even in animal models, the neurotransmitter might not be necessary and sufficient (e.g. the voles still forming pair bonds in the absence of mating).
  2. Cases of behavioral modification, in both humans and animals, cannot be divorced from context. Context can be biological (e.g. birth, mating) or social (the humans who were not gullible with oxytocin).

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