The researchers studied CB1, a "cannabinoid" receptor that binds the main active chemical for marijuana, delta-9-tetrahydrocannabinol (THC).
In pregnant mice that lacked the gene for the receptor, or in which the receptor was blocked, the embryo failed to go through the oviduct - the tube leading from the ovaries to the uterus. The same thing happened in normal mice when the receptor was over-stimulated.
The study, published in the current issue of the journal Nature Medicine, describes for the first time how the CB1 receptor in the mouse regulates muscle contraction to move the embryo down the oviduct.
It is not known whether drugs that block or, in the case of marijuana, over-stimulate the CB1 receptor can cause ectopic pregnancy in humans. However, "our results raise caution for women of reproductive ages regarding the chronic use of marijuana for recreation or pain alleviation," the researchers concluded.
The report's senior author, Sudhansu K. Dey, Ph.D., said he also was concerned about the potential impact of an anti-obesity drug, now in clinical trials, that suppresses appetite by blocking the CB1 receptor. Such a drug, if approved, would likely be taken by young women of reproductive age.
"What will happen if they consume anti-CB1 drugs?" asked Dey, Dorothy Overall Wells Professor of Pediatrics and professor of Cell & Developmental Biology and Pharmacology at Vanderbilt University Medical Center.
According to the U.S. Centers for Disease Control and Prevention, about 100,000 ectopic pregnancies occur in the United States each year (out of more than 6 million total pregnancies) and account for about 9 percent of all pregnancy-related deaths in the country. Risk factors include pelvic inflammatory disease, which can scar the fallopian tubes, and smoking.
At the same time, surveys indicate that a significant percentage of young women regularly smoke marijuana. According to the 2002 National Survey on Drug Use and Health, 13.6 percent of women aged 18 to 25 said they had used the drug within the past month. The federal survey interviews approximately 67,500 people each year.
Marijuana exerts its effects in the brain and peripheral organs through two cannabinoid receptors, CB1 and CB2. Lipid molecules made by the body, called "endocannabinoids," activate these receptors, and are involved in several important physiological functions, including memory, pain and appetite.
In the 1990s, while at the University of Kansas Medical Center in Kansas City, Dey and his colleagues found that the mouse pre-implantation embryo, or blastocyst, had much higher levels of the CB1 receptor than did the brain. They also found that the uterus produced one of the activating molecules, called anandamide.
After moving to Vanderbilt two years ago, the researchers found that anandamide regulates implantation of the mouse embryo in the wall of the uterus. At low concentrations, it synchronizes embryo development with uterine receptivity so that successful implantation occurs. At higher levels, however, implantation is blocked. Elevated levels of anandamide also have been shown to cause spontaneous pregnancy loss in women.
"I feel that it has a fundamental physiological function in reproduction," said Dey, who directs the division of Reproductive and Developmental Biology at Vanderbilt.
The current study began with the observation that mice commonly used in genetic experiments often fail to deliver pups, even though they become pregnant. Upon flushing the oviducts, the researchers found the embryos, which for some reason were unable to reach the uterus.
This strain of mouse lacks the gene for the CB1 receptor. When the researchers gave a drug that blocked CB1 in normal mice, they observed the same phenomenon - in most of the mice the embryos failed to reach the uterus. The same thing happened when the mice were given an amandamide-like drug to over-stimulate the receptor.
In a search to unravel this mystery, Haibin Wang, Ph.D., research assistant professor of Pediatrics, found that the CB1 receptor is located near two other receptors (adrenergic receptors) that - through the action of noradrenaline, a neurotransmitter - regulate muscle contraction. Wang, who led the study, has received two Lalor Foundation fellowships supporting research in reproductive biology.
Subsequent experiments supported a new concept: the CB1 receptor modulates the release of noradrenaline, which in turn stimulates waves of muscle contraction and relaxation in the oviduct, nudging the embryo toward the uterus. Too much or too little of this "endocannabinoid tone" can result in pregnancy failure.
In support of this hypothesis, abnormal muscular contraction that prevents human embryos from reaching the uterus is one known cause of ectopic pregnancy, Dey said.
Polymorphisms (different forms) of the CB1 gene have been associated with drug dependency in humans. Dey and his colleagues plan to study whether certain polymorphisms also may reflect an increased risk for ectopic pregnancy. "That will be the ultimate test" for proving the receptor's role in ectopic pregnancy in humans, he says.
Other members of the research team were Dingzhi Wang, Ph.D., Philip J. Kingsley, Lawrence J. Marnett, Ph.D., Sanjoy K. Das, Ph.D., Raymond N. DuBois, M.D., Ph.D., and Yong Guo, Ph.D., a visiting scholar from China. "This work would not have been realized without the participation of these individuals and their labs," Dey said.
The study was supported by the National Institute on Drug Abuse and the National Institute of Child Health and Human Development.
Vanderbilt University Medical Center is a top-20 NIH-funded academic research institution. It consists of Vanderbilt University Hospital; Vanderbilt University School of Medicine; The Vanderbilt-Ingram Cancer Center, an NCI Comprehensive Cancer Center; the Vanderbilt University School of Nursing and Vanderbilt Children's Hospital.
VUMC faculty members have been awarded two Nobel Prizes for Medicine and Physiology.