A receptor for cannabinoids was found in the human brain in the late 1980s. This finding suggested that the body must produce its own chemical version of THC to activate these receptors, and a substance called anandamide was found to be that chemical. Schuel and collaborators were the first to report cannabinoid receptors in sperm, using sea urchins as a model, and that anandamide also activated those receptors. The regulatory mechanisms that prepare sperm to fertilize eggs remain, for the most part, a scientific mystery. Human sperm are bathed in male reproductive-tract secretions when they are ejaculated into the female's vagina, and are not immediately capable of fertilizing eggs.
Capacitated sperm exhibit a pattern of vigorous swimming called hyperactivated motility.
When they bind to a specific protein in the egg's surface coat, sperm can be stimulated to secrete enzymes that enable them to penetrate the egg coat during fertilization, a secretory process called the acrosome reaction. We also know that the cannabinoid receptor found in the human brain is expressed in the human testis, and that anandamide is produced in the testis and uterus of mammals. Within the uterus, anandamide regulates early development of the fertilized egg, and determines where the embryo will implant to initiate pregnancy. Now, Schuel and collaborators have the first evidence that anandamide can regulate directly human sperm's ability to fertilize an egg. Too much inhibits swimming while a lower dose stimulates swimming.