Vasopressin and oxytocin management might soon join the ranks of testosterone and serotonin management for treatment of aggressive behavior in dogs. Current interventions for aggressive behavior among our canine friends often target the hormones testosterone and serotonin. Veterinarians often recommend the neutering of male dogs in order to address aggressive behaviors like leash aggression, growling, lunging, and biting because neutering a male can reduce testosterone levels. Meanwhile, selective serotonin reuptake inhibitors (SSRIs) have also been prescribed to address canine aggression because serotonin is believed to help reduce aggressive behaviors. New research indicates that two other hormones might also play a significant role in aggressive behavior.
Researchers at the University of Arizona, led by Evan MacLean, say that high levels of vasopressin and low levels of oxytocin, hormones also found in people, might affect social behavior among dogs. Their research findings are published in the journal Frontiers in Psychology. MacLean is an assistant professor of anthropology at the University of Arizona and is the director of the Arizona Canine Cognition Center at the university's School of Anthropology.
"Dog aggression is a huge problem. Thousands of people are hospitalized every year for dog bites, especially kids, and aggression is one of the main reasons that dogs get relinquished to shelters," MacLean explained, according to Science Daily. "If there are ways to intervene and affect biological processes that produce aggression, that could have a huge benefit both for people and dogs."
The research paper reported that earlier research in humans found a positive association between arginine vasopressin and aggression. Arginine vasopressin and oxytocin "are closely related nonapeptides with wide ranging effects on social behavior, cognition, and stress responses," the researchers wrote. Most people know that oxytocin is considered the "love hormone." For example, oxytocin levels rise when we hug or kiss. While vasopressin is most known for its relationship with water retention, this hormone also has been seen among people with chronic aggression.
In their first experiment, the researchers compared the behavior and oxytocin and vasopressin levels of two groups of dogs. One group consisted of dogs with a known history of aggression toward unfamiliar situations and things and another group with no history of aggression. These dogs were shown dog models. The dog models were three-dimensional replicas of three different dog breeds. The models resembled a Jack Russell Terrier, a Shetland Sheepdog, or an Old English Sheepdog. The models were hidden behind a curtain, then exposed momentarily just after an audio clip of a dog barking was played. They were also walked in a room with common control items like a trash bag or a yoga ball. The dogs' hormone levels and responses were measured by the research team before and after each situation.
In a second analysis, researchers also measured the hormone concentration of assistance dogs and compared them to the dogs from the first experiment. Though both sets of dogs from the first experiment had similar oxytocin levels, they were lower than the oxytocin levels of the assistance dogs.
Dogs that had been identified as aggressive towards other dogs showed significantly lower levels of free plasma arginine vasopressin, but significantly higher total vasopressin levels than the control dogs. Although oxytocin levels did not differ between the two dog groups, when they compared the oxytocin levels to those of assistance dogs, they found lower oxytocin levels among the leash aggressive dogs.
"Seeing high oxytocin levels in assistance dogs is completely consistent with their behavioral phenotype -- that they're very, very friendly dogs that are not aggressive toward people or other dogs," MacLean explained.
"It would be reasonable to think that if vasopressin facilitates aggression, you could develop pharmaceuticals that could target the vasopressin system to help in cases where dogs are really aggressive," MacLean said. "Oxytocin and vasopressin are being used extensively as therapeutics in humans right now. Regulation of the oxytocin system has been implicated in things ranging from autism to schizophrenia to post-traumatic stress disorder, and there are clinical trials looking at administering oxytocin as a drug to create some kind of behavioral response. It's interesting to think that maybe some of these same therapies we're trying with people could be useful in dogs."
The researchers speculate that vasopressin might be a primary activator of the hypothalamic-pituitary-adrenal axis. The HPA axis is a complex set of interactions between these three endocrine glands. The HPA axis is believed to facilitate the "fight or flight" response in people and dogs.
The researchers called for further studies that will examine the interactions between serotonin, testosterone, oxytocin, and vasopressin. New research testing the effects of administering peptides into the noses of dogs has already found that this potential therapy "has led to notable changes in social behavior and cognition" in some animals.
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