Metabolic Controls of Feeding Behavior
By Barbara Spector
The reward mammals derive from food motivates them to initiate feeding behavior, a life-sustaining activity, notes mammalian neuroscientist W. Sue Ritter, M.A. '71, Ph.D. '73. While investigators believe many of these reward mechanisms are activated in the forebrain, Ritter — a professor of veterinary and comparative anatomy, pharmacology and physiology at Washington State University's College of Veterinary Medicine in Pullman — studies neurons located in the hindbrain, the most primitive area of the brain.
W. Sue Ritter,
M.A. 71, Ph.D. 73
"Animals need the forebrain to control complex behaviors involved in activities such as looking for food and identifying food," Ritter explains. "But the animal needs to be motivated to engage in these behaviors." Some of the glucose-sensing cells that activate the animal's motivation circuits are located in the hindbrain.
Ritter focuses on the neural circuitry involved in detection of glucose deficit, which drives appetite. Glucose is the brain's essential metabolic fuel. "Glucose has to be supplied on a continual basis from the blood," Ritter explains, "because glucose is not stored in the brain." Her research suggests that catecholamine neurons in the hindbrain help mediate responses to glucose deficit by linking glucoreceptor cells to forebrain and spinal neurons capable of stimulating behavioral and hormonal responses that elevate blood glucose. Her group is working to identify the specific catecholamine neurons involved.
Ritter's work has implications for studies of obesity, diabetes and reproduction. "When the glucose supply is low, animals suppress their reproductive behaviors as a way of preserving the metabolic fuels," she notes. Suppression of the estrus cycle in rats when glucose levels fall may be related to the suppression of the menstrual cycle in women with anorexia, she notes.
Researchers investigating the neural connections through which these protective mechanisms are controlled have found that an episode of severe glucose deficit can cause glucoreceptor cells to become desensitized, such that they do not elicit normal feeding and hormonal responses. This appears to be related to a condition in diabetics known as hypoglycemia-associated autonomic failure. While insulin treatment is essential for keeping blood glucose from going too high, a side effect of this treatment is that blood glucose may fall too far, leading to temporary desensitization of the brain's glucoreceptor cells. A subsequent fall in glucose levels will not be detected. If this happens, a diabetic can become hypoglycemic without being aware of it. "Their normal physiological responses, including increased appetite, are not elicited," Ritter says. "Brain function becomes increasingly impaired and they can fall into a coma and may die."
New Approaches to Old Questions
Ritter, a member of the Washington State faculty since 1974, co-founded the school's undergraduate neuroscience major. She teaches undergraduates, graduate students and veterinary students.
"What I like about having an academic career are the wonderful opportunities for interacting with colleagues and sharing ideas," she says. "I value the opportunity to travel to meetings, where I can meet new friends and keep up close friendships with people whose careers parallel my own."
Yet Ritter had not always planned to become a scientist. As an undergraduate at Valparaiso University in Indiana, she thought she would go into education. But her husband, whom she married on the day after his college graduation — Robert C. Ritter, now also a professor at Washington State's veterinary college — encouraged her to pursue her budding interest in biology. "It didn't occur to me that I could be a scientist," she recalls. "That insight came partly from his interest in science and was reinforced by the mentoring I received while working in a research laboratory in my senior year."
The couple moved to Philadelphia so Robert Ritter could attend veterinary school at the University of Pennsylvania. A year after he began his studies, Sue Ritter started graduate school at Bryn Mawr. "Bryn Mawr was a very supportive, enriching intellectual environment," she remembers. "I felt free to choose my area of interest; they didn't try to funnel me into anything."
Ritter's master's thesis dealt with classical conditioning of postural reactions to motor cortical stimulation in cats, but she decided to investigate the areas of the brain that mediate rewarding experiences for her Ph.D. studies. Her thesis adviser was Larry Stein, then an adjunct professor of psychology who had a research lab at Wyeth Laboratories in Radnor, Pa. "I haven't really gone too far from my Ph.D. thesis," Ritter says. "Catecholamine neurons are a critical part of the brain's reward and motivation circuitry. I'm still studying the way these neurons respond to physiological need and help the brain prioritize and motivate the appropriate behavioral responses."
Although researchers in the field are still addressing some questions that were first posed 80 to 100 years ago, there have been significant recent advancements, Ritter notes. "The field has become much more cellular in its orientation," she says. "There are more molecular biological tools, and the discovery of leptin has brought a tremendous influx of molecular and genetic strategies."
The Ritters have two sons: Josh, a singer-songwriter, and Lincoln, who is studying computer science in graduate school. Sue Ritter says faith has played an integral part in the family's life. "My faith is so much a part of the structure of my brain that I find it hard to isolate it from my thinking on any topic," she says.
Barbara Spector writes on science and technology as well as business topics. She is the editor-in-chief of Family Business magazine and former editor of The Scientist.