Brain graft causes hypertension in rats.

About 90 percent of the time, physicians do not really know why a patient develops high blood pressure, a problem that affects some 63 million people in the United States. Diet, exercise, and emotions play a role (SN: 10/16/93, p.246; 12/4/93, p.380), as does genetics. But researchers now think they may have come closer to identifying the real reason behind primary hypertension.

By transplanting brain tissue from specially bred rats into normal rats, neurobiologists have homed in on a brain region that can cause hypertension.

The transplant procedure also provides researchers with a new way to study the selective loss of nerve cells, says Raya Eilam, a neurobiologist at the Weizmann Institute of Science in Rehovot, Israel. Cell losses seem to underlie many neurological disorders that affect adults, including Alzheimer's, Parkinson's, and Huntington's diseases. "It's possible that, in humans, primary hypertension is also caused by the death of specific cell groups," she says.

Eilam hopes to use this transplant procedure to test whether growth factors or other substances protect these cells from dying. "From this model, we can learn many things about degeneration in the brain," she predicts.

Researchers already knew that the area of the brain known as the hypothalamus produces a substance called vasopressin, which helps regulate blood pressure. The hypothalamus consists of nuclei, or groups of nerve cells, that perform various roles.

For this study, Eilam and her colleagues removed the hypothalamus from 15-day-old embryos belonging to a hypertensive strain of rats. They divided this tissue into front (rostral) and back (caudal) pieces and implanted the pieces into normal 8-week-old rats.

By the time the animals reached 4 months of age, the scientists noticed increased blood pressure in the rats with rostral implants but not in those receiving caudal tissue or no tissue at all, Eilam says. Also, the heart and kidneys of rats with rostral tissue got much bigger, the researchers report in the August JOURNAL OF NEUROSCIENCE. Enlargement of the heart tends to occur in people and animals with high blood pressure, she notes.

During the 8 months after transplantation, her group also examined the different types of brain cells in the rats with rostral tissue. The researchers counted the number of each type present in a rat's own hypothalamus as well as in its graft. The rat's own hypothalamus lost most of the nerve cells that make and use vasopressin as a neurotransmitter, while cells involved in making this molecule for use as a hormone remained intact, she reports.

"I think the [rostral grafts] produce something that causes damage to the adult cells," Eilam says. She speculates that the lack of these cells then causes the body's feedback system for regulating blood pressure to malfunction.