Researchers from the Department of Neurobiology at the Weizmann Institute of Science discovered that female mice tend to behave quite maternally, even if they have not yet given birth to pups.
Most female mammals give birth and care for their offspring, while males play a lesser role in parenting, sometimes even switching to other females shortly after mating (a behavior known as polygamy). And yet, scientists find it difficult to pinpoint the exact area of the brain where these differences between the sexes are expressed - or from which they arise. Because of this, the question to what extent "parental behavior" is burned into the structure of the brain has so far remained open.
Dr. Tali Kimchi and research student Niv Scott, in collaboration with Dr. Ofer Yizhar and postdoctoral researcher Dr. Mathias Friga, from the Department of Neurobiology at the Weizmann Institute of Science, set out to find a solution to this basic question, focusing on laboratory mice. Female mice tend to behave quite maternally, even if they have not yet given birth to pups. They will return a puppy that was left in the corner of the cage to the safe place in the breeding nest, devote time to the care of a day-old puppy, and more. This tendency is amplified in maternal mice. The males, on the other hand, are usually aggressive and guard their territory. In many cases they ignore residences that are not theirs, and sometimes even attack them. But for a short period of time after mating and giving birth, they still exhibit parental behavior.
To investigate how the brain controls this parental behavior (in both males and females), the scientists focused on a small part of the brain (in the hypothalamus) called the AVPV, which is larger in females than in males; And especially in nerve cells that express the protein
(Tyrosine Hydroxylase) TH, which plays a role in the production of the neurotransmitter dopamine. Thus, it was found that mice have more neurons that produce TH compared to males; And also, that mother mice have more such cells compared to females that have not yet given birth.
The scientists used genetic and neuro-biochemical methods - among other things through light stimulation (opto-genetic) - to increase and decrease the expression of TH in the neurons of the defined brain area, as well as to activate this group of neurons; and document the behavioral effects that resulted from this - both in males and females. In this way, it became clear that increasing TH levels in these nerve cells particularly affects the maternal behavior of the mice. Short opto-genetic stimulation of the TH-secreting neurons caused a distinct increase in maternal behavior. In a follow-up experiment, it became clear that these manipulations also increased the levels of oxytocin (a hormone that helps contract the uterine muscles during childbirth as well as during breastfeeding and which is known as the "love hormone") in the blood, which was manifested in more devoted parental behavior. Conversely, reducing the number of TH-expressing neurons in the mice resulted in lower oxytocin levels, and the disruption of their parental behavior.
Using optogenetic methods (activating nerve cells through light) to increase the activity of the cells that secrete TH in male mice, did not lead to a change in oxytocin levels and treatment of pups, but surprisingly, greatly reduced the aggressive behavior of non-father males towards foreign pups, as well as towards adult males Strangers. Reducing the number of these nerve cells, resulted in an increase in aggression towards puppies, and a significant increase in aggression in general. That is, this group of nerve cells is apparently responsible for reducing aggressive behavior. in males (different than in females).
"Controlling the activity and quantity of these unique nerve cells allowed us to direct the degree of maternal behavior of the females, as well as the aggression of the males," says Dr. Kimchi. "The results of the study imply that maternal behavior is controlled by structured (innate) nerve cell networks, which differ to some extent in males, compared to females, and which are controlled, at least partially, by the hormone oxytocin."
Dr. Tali Kimchi and the members of her research group intend to continue researching the networks of nerve cells in the brain that regulate parental behavior, alongside other behaviors that differ between males and females, such as social communication, aggression and sexual behaviors. In this way they hope to gain a better understanding of the relationship between innate brain properties (genetic, physiological and structural) and environmental influences to which we are exposed during life; And also regarding the question of how all these shape social behavior in males and females. These insights could perhaps, in the future, help in understanding the biological factors involved in brain disorders of a social nature, and which are expressed in different ways in men and women, such as postpartum depression, aggressiveness and autism.
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