Scientists who work with rodents in laboratories know that putting animals from different litters together in the same environment is usually a bad idea. Adult males will usually turn against each other and eliminate the youngest pups, even when adults and newborns all come from the same parents. This behavior, called infanticide, is frequent among rats and mice. According to a study published in Science in 2014, it is shared by slightly over a hundred species of mammals – from predators like bears and lions to primates such as chimpanzees, baboons and gorillas. Experiments conducted by biologist Fabio Papes and his team at the Institute of Biology at the University of Campinas (IB-Unicamp) have started to elucidate part of the mystery surrounding this behavior and identify the molecular mechanisms that drive mice to commit infanticide in certain situations.
Papes and his group are confident that the signal for males to kill their newborn pups arrives through the air. In its first weeks of life, the body of a rodent pup releases odor molecules (not yet identified) that activate a particular group of cells in the nose of older animals. These cells, identified by the research group at Unicamp and described in the journal BMC Biology in February 2016, transform chemical information into electric signals that mobilize areas of the brain associated with aggression.
The cells that specialize in recognizing pup odors are neurons, as are the other sensory cells that enable mammals to identify aromas in the environment. But some differences make them unique within the olfactory system. The first and most important is that they only exist in adult mice and are more active in virgin males, who have never reproduced, than in females (regardless of having given birth) or males that have sired offspring. “This is the first documented case of cells in the olfactory system showing this type of activation pattern,” says Papes. “This difference helped shape our hypotheses on roles that they could play.”
Cells that detect pup odor express a surface protein called OLFr692, an acronym for member 692 of the olfactory receptor family. This protein crosses the neural cell membrane and captures chemical compounds given off by newborn rodents. Olfactory receptors make up an extensive family of 1,300 proteins specializing in the identification of small, volatile organic molecules known as odorants. With the exception of OLFr692, these receptors are usually found in sensory cells within the largest olfactory organ of the nose, the main olfactory epithelium. In mice, this epithelium contains 1,300 different types of sensitive cells that line the deepest portion of the nasal cavity like a carpet, allowing the mouse to recognize its environment and create an aromatic memory of it.
A peculiar aspect of the cells described in BMC Biology is that they include proteins from the olfactory receptor family, but are not located in the main olfactory epithelium. Instead, biologist Thiago Nakahara, a PhD student advised by Papes, found them only in the vomeronasal organ, a different kind of olfactory tissue in the nose, with distinctive molecular features. In mice, this organ consists of two cylindrical structures about 2 millimeters long, one on each side of the nose. Researchers once thought that the organ’s cell surface proteins were exclusively those in the vomeronasal receptor family (VRs), which specialize in detecting pheromones, organic molecules that trigger instinctive behaviors related to defense, mating, aggression and alarm.
“More sensitive techniques are starting to show that cells expressing OLFr proteins can also be found in tissues far from the nose,” says biochemist Bettina Malnic, a researcher at the University of São Paulo who helped decipher how odor molecules interact with cells in the olfactory epithelium and trigger the signals that will be interpreted by the brain. As an example, Malnic recalls that in late 2015, U.S. researchers identified cells with the OLFr78 receptor in a sensitive structure of the carotid artery, where they monitor oxygen levels in the blood.
Mediators of agression
Having identified the population of OLFr692-expressing cells in the vomeronasal organ, the researchers in Campinas faced a bigger challenge: discovering the role played by these cells in the rodent olfactory system. Their first clue, namely the fact that newborn mice do not have these cells, led the team to imagine that they might be involved in signaling for behaviors that are exclusive to adults.
Papes’ group then started testing mice of different ages to analyze the activation patterns of OLFr692-expressing cells. In some experiments, males were made to cohabit with females to check if these cells were mediators of sexual attraction. In others, males interacted for a given period with other males in the same cage. The goal in this case was to determine if OLFr692-expressing cells were active in inducing the aggressive behavior commonly seen when males come into contact. Other mice were exposed to the odors of predators (cats, rats, snakes and spiders), to determine whether the cells are involved in instinctive reactions of defense and fear (see Pesquisa FAPESP Issue nº 172). However, OLFr692 was not activated in any of these cases.
The last hypothesis was that cells expressing this protein might be modulating some kind of interaction between adults and young. Like other rodents, adult male and female mice engage in parental care: they clean their young by licking them, and they bring pups back to the nest when they try to escape. Mothers also feed their offspring. Nakahara measured the activation of OLFr692-expressing cells in adult mice and found that, in virgin males, these cells participate in detecting newborn odor. When interacting with pups, these males will not protect them – they will kill them instead. “This is a common behavior in mice,” says Papes. “Vivarium workers know that you can’t put an unfamiliar male near the pups.”
The researchers are now seeking to prove that OLFr692-expressing cells are necessary in triggering this behavior. To that end, they will repeat their tests on mice genetically modified not to produce OLFr692. “We have a hunch that the infanticidal behavior will disappear,” says Papes. “Studying how a neuron circuit like the one activated by OLFr692 triggers specific behaviors,” says Malnic, “can help us understand how the brain is organized and can reveal how altered neuron circuits can lead to neurological and behavioral disorders in humans.”
Molecular biology of the olfactory system in mammals: study on the detection of odors and their neural representation in the brain (nº 2009/00473-0); Grant Mechanism Young Investigators in Emerging Institutions; Principal Investigator Fabio Papes (IB-Unicamp); Investment R$780,405.02.
NAKAHARA, T. S. et al. Detection of pup odors by non-canonical adult vomeronasal neurons expressing an odorant receptor gene is influenced by sex and parenting status. BMC Biology. V. 14. Feb. 15, 2016.