Vole-entine’s Day: A Tale of Two Hormones
Two well studied hormones seem to play a very important role in amorous social behavior in vertebrates and are therefore perfect subjects for a Valentine’s Day post. These two hormones are vasopressin and oxytocin. Go to a neuroendocrinology conference and you’ll see talks and posters from vast herds of vasopressin and oxytocin studies (usually on the same few model organisms). Why are these two hormones so fascinating to researchers?
In all vertebrates, the family of oxytocin nonapeptide hormones supports reproductive functions and the various vasopressin nonapeptide hormones are involved in osmoregulation (control of water loss by affecting urine dilution). The two genes are always found on the same chromosome and are quite proximate (less than 15,000 base pairs apart). The two genes are likely the result of a gene duplication event from early in the evolution of chordates and just such a putative ancestral gene is found in modern agnathans.
Besides the important physiological regulatory mechanisms, vasopressin and oxytocin also have significant behavioral effects. In mammals oxytocin is most strongly associated with lactation and reproductive cycles and is secreted from the pituitary gland after orgasms and during parturition. It has been demonstrated to play a role in sexual arousal, pair-bonding, and maternal behavior. Vasopressin has similar behavioral effects in males and seems to play an important role in pair-bonding behavior (and especially male-male aggression).
The most famous study looked at two closely related species of voles; the prairie vole (Microtus orchogaster) and the meadow vole (Microtus pennsylvanicus). Prairie voles are known for being very monogamous with the pair sometimes mating for life. Males show a great deal of affiliative and grooming behavior, share parenting duties, and even assist as a midwife. Meadow voles on the other hand are polygamous and show no bonding behavior at all. Researchers discovered that the two species differed in the sequence of a gene coding for vasopressin receptors in the brain. Now here’s the cool part. They created transgenic meadow voles with the homologous gene sequence from prairie voles in place of that of their own species and the males showed elevated levels of social grooming and pair-bonding behavior. Obviously these receptors are playing a large role in the differences in social behavior between the two species. Moreoever, a more recent study has been able to explain different degrees of pair-bonding within just the prairie voles. They found that the most devoted mates and fathers tended to be those voles with long sequences of repeated microsattelites flanking the receptor gene. These sequences which had been previously thought of as “junk DNA” seem to be modifying the expression level of the receptor and thereby influencing paternal behavior. Of course it would be simplistic to say that these two hormones are entirely responsible for pair-bond formation in either humans or voles, but they do play an important role. And yes, correlative studies on humans have demonstrated elevated oxytocin and vasopressin involved in "falling in love". Does that take anything away from the experience? I don’t think so. Better understanding the hows and whys of behavior doesn’t make them any less remarkable to me.
In all vertebrates, the family of oxytocin nonapeptide hormones supports reproductive functions and the various vasopressin nonapeptide hormones are involved in osmoregulation (control of water loss by affecting urine dilution). The two genes are always found on the same chromosome and are quite proximate (less than 15,000 base pairs apart). The two genes are likely the result of a gene duplication event from early in the evolution of chordates and just such a putative ancestral gene is found in modern agnathans.
Besides the important physiological regulatory mechanisms, vasopressin and oxytocin also have significant behavioral effects. In mammals oxytocin is most strongly associated with lactation and reproductive cycles and is secreted from the pituitary gland after orgasms and during parturition. It has been demonstrated to play a role in sexual arousal, pair-bonding, and maternal behavior. Vasopressin has similar behavioral effects in males and seems to play an important role in pair-bonding behavior (and especially male-male aggression).
The most famous study looked at two closely related species of voles; the prairie vole (Microtus orchogaster) and the meadow vole (Microtus pennsylvanicus). Prairie voles are known for being very monogamous with the pair sometimes mating for life. Males show a great deal of affiliative and grooming behavior, share parenting duties, and even assist as a midwife. Meadow voles on the other hand are polygamous and show no bonding behavior at all. Researchers discovered that the two species differed in the sequence of a gene coding for vasopressin receptors in the brain. Now here’s the cool part. They created transgenic meadow voles with the homologous gene sequence from prairie voles in place of that of their own species and the males showed elevated levels of social grooming and pair-bonding behavior. Obviously these receptors are playing a large role in the differences in social behavior between the two species. Moreoever, a more recent study has been able to explain different degrees of pair-bonding within just the prairie voles. They found that the most devoted mates and fathers tended to be those voles with long sequences of repeated microsattelites flanking the receptor gene. These sequences which had been previously thought of as “junk DNA” seem to be modifying the expression level of the receptor and thereby influencing paternal behavior. Of course it would be simplistic to say that these two hormones are entirely responsible for pair-bond formation in either humans or voles, but they do play an important role. And yes, correlative studies on humans have demonstrated elevated oxytocin and vasopressin involved in "falling in love". Does that take anything away from the experience? I don’t think so. Better understanding the hows and whys of behavior doesn’t make them any less remarkable to me.