Summary: Optogenetics and brain imaging techniques allow researchers to identify a small cluster of neurons in the hypothalamus that trigger attraction, a new study reports.
With a whiff of the opposite sex, these hormone-sensitive neurons trigger pro-social behavior in mice and could play roles in anxiety, depression, and other mood-related conditions in humans.
The ancient impulse to procreate is necessary for survival and must be hardwired into our brains. Now scientists from the University of North Carolina School of Medicine have discovered an important clue about the neurons involved in that wiring.
Using advanced deep brain imaging techniques and optogenetics, the UNC scientists found that a small cluster of sex-hormone-sensitive neurons in the mouse hypothalamus are specialized for inducing mice to “notice” the opposite sex and trigger attraction.
This study, led by Garret D. Stuber, PhD, associate professor of psychiatry and cell biology & physiology, and Jenna A. McHenry, PhD, a postdoctoral research associate in Stuber’s lab, identified a hormone-sensitive circuit in the brain that controls social motivation in female mice.
“These neurons essentially take sensory and hormonal signals and translate them into motivated social behavior,” said Stuber, who is also a member of the UNC Neuroscience Center.
The findings, reported in Nature Neuroscience, illuminate the neural roots of opposite-sex social behavior in mammals, and may also be relevant to certain psychiatric illnesses.
“These neural circuits that bridge social and reward processing should also provide important insights for disorders that impair social motivation,” said McHenry, the first author of the paper.
In the study, Stuber and colleagues examined the medial preoptic area (mPOA) of the brain. This clump of neurons sits within the hypothalamus, an evolutionarily ancient structure at the bottom-center of the brain. Prior research showed that the mPOA is important for social and reproductive behavior in all vertebrate species studied from fish to human, but it has been unclear whether this area drives social motivation through circuit connections with reward systems in the brain.
The researchers focused on one of the mPOA’s major connections, through which it sends neural signals to another brain structure called the ventral tegmental area (VTA) – known to be a powerful contributor in motivating behavior and the release of the neurotransmitter dopamine.
The researchers injected the VTAs of female mice with special fluorescent beacon molecules that, like some viruses, tend to move “upstream” along nerve connections. When these tiny beacons reached the mPOA, they ended up highlighting VTA-projecting neurons that express a gene called neurotensin. Analyses of these VTA-projecting neurons showed that most of them also express estrogen receptors and are therefore likely to be sensitive to rises and falls of ovarian hormones in the mouse fertility cycle, also known as the estrus cycle.
The researchers next studied this specific set of mPOA neurons in living mice, which was a considerable challenge. The microscopy techniques that permit imaging of brain cells in awake mice generally can’t visualize anything deeper than a fraction of a millimeter below the brain‘s surface, whereas the mPOA is several millimeters deep. To get around this problem, Stuber’s team used tiny tubular lenses connected from their microscope, in effect, to the mPOA. With a technique known as two-photon calcium imaging, the scientists were then able to visualize the activity of mPOA neurons in awake, behaving female mice. To enhance the accuracy of the technique, the researchers used mice that had been genetically engineered so that only their mPOA neurotensin neurons could be imaged.
“With our setup, we could image the mice a couple of times a week and each time find the same cells that we previously recorded brain activity from,” Stuber said.
The team found that when the female mice were exposed to the odor of male mouse urine – but not the odor of female mouse urine or other attractive odors, like appetizing food – a large subset of the mPOA neurotensin neurons was excited into greater activity. The researchers also found that these neurons responded more robustly to male mouse urine when females had high circulating levels of estrogen or a combination of estrogen/progesterone, which surges before the mice become fertile.
About this neuroscience research article
The research involved collaboration between Stuber’s team and the lab of David R. Rubinow, MD, who is the Assad Meymandi Distinguished Professor and Chair of the Department of Psychiatry at UNC, and directs the UNC Center for Women’s Mood Disorders.
Other co-authors are James M. Otis, PhD, and Mark A. Rossi, PhD, both postdoc in the Stuber lab; J. Elliott Robinson, PhD, former graduate student in the Stuber lab; Oksana Kosyk, Stuber Lab manager; UNC undergraduate Noah W. Miller, Zoe A. McElligott, PhD, assistant professor of psychiatry at UNC; and Evgeny A. Budygin, PhD, associate professor at Wake Forest School of Medicine..
Funding: Support was provided by the Postdoctoral Fellowship in Reproductive Mood Disorders at UNC (co-directed by Rubinow and Susan Girdler, PhD, professor of psychiatry and psychology at UNC), the National Institute of Mental Health, the National Institute on Drug Abuse, the Foundation for Alcohol Research, the National Institute of Alcohol Abuse and Alcoholism, the National Institute of Diabetes and Digestive and Kidney Diseases, the Foundation of Hope, the Brain and Behavior Research Foundation, and the Simons Foundation.
Source: Mark Derewicz – UNC Image Source: NeuroscienceNews.com image is credited to Jenna McHenry, PhD, UNC School of Medicine. Original Research: The study will appear in Nature Neuroscience during the week of January 30 2017.
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[cbtabs][cbtab title=”MLA”]UNC “Illuminating the Neurons of Social Attraction.” NeuroscienceNews. NeuroscienceNews, 30 January 2017. <https://neurosciencenews.com/social-attraction-neuroscience-6025/>.[/cbtab][cbtab title=”APA”]UNC (2017, January 30). Illuminating the Neurons of Social Attraction. NeuroscienceNew. Retrieved January 30, 2017 from https://neurosciencenews.com/social-attraction-neuroscience-6025/[/cbtab][cbtab title=”Chicago”]UNC “Illuminating the Neurons of Social Attraction.” https://neurosciencenews.com/social-attraction-neuroscience-6025/ (accessed January 30, 2017).[/cbtab][/cbtabs]