Reproductive system pharmacology

Introduction

Several sites in the human reproductive system are either vulnerable to chemicals or can be manipulated by drugs. Within the central nervous system, sensitive sites include the hypothalamus (and adjacent areas of the brain) and the anterior lobe of the pituitary gland. Regions outside the brain that are vulnerable include the gonads (i.e., ovary or testis, the uterus in the female, and the prostate in the male).

The body has anatomical or physiological barriers that tend to protect the system. The so-called placental barrier and the blood-testis barrier impede certain chemicals, although both allow most fat-soluble chemicals to cross. Drugs that are more water soluble and that possess higher molecular weights tend not to cross either the placental or the blood-testis barrier. In addition, if a drug or chemical binds to a large molecule such as a blood-borne protein, it is less likely to be transported into the testes or less likely to come in contact with the fetus. There appears to be little, if any, barrier to chemicals or drugs gaining entry to breast milk or semen.

If the fetus is exposed in the uterus to certain environmental chemicals, infections, or drugs, it may develop abnormalities. The toxic substance is described as teratogenic (literally, "monster-producing"), and the study of this type of toxicity is called teratology. About 3 percent of developmental abnormalities have been proved to be drug-induced. It is wise to avoid all drugs (including nicotine) during pregnancy, unless the medicine is well tried and essential. Drugs taken by male partners may be teratogenic if they damage the genetic material (chromosomes) of the spermatozoa.

Female reproductive system

Oral contraceptives, universally known as "the Pill", constitute a class of steroid hormones. They are capable of suppressing the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior lobe of the pituitary gland. Known collectively as gonadotrophic hormones, FSH and LH are capable of stimulating the release of progesterone and esterogen from the gonads, or ovaries, all of these hormones are responsible for modulating the female menstrual cycle. Ovulation is believed to be related to a midcycle release of LH, which can be effectively suppressed or blocked by the systematic administration of systemic hormones. There are many commercial preparations of oral contraceptives, but most of them contain a combination of an estrogen (usually ethynyl estradiol) and a progestin (commonly norethindrone). In general, oral contraceptives are taken in a monthly regiment that parallels the menstrual cycle. Protection from pregnancy is often unreliable until the second or third drug cycle, and during this time certain side effects such as nausea, breast tenderness, or breakthrough bleeding may be evident. More serious side effects, including venous and arterial thromboembolism and a rise in blood pressure, are possible, especially in women over 34 years of age. Normal ovulation usually commences two to three months after stopping the Pill.

Progestin-only preparations (the so-called Minipill) thicken the mucus lining of the cervix and make it more acidic, thereby rendering it hostile to the male spermatozoa. Progestin-only preparations are somewhat less reliable than the combination preparations, but produce fewer side effects. Under certain circumstances, the progestin may be administered as an intramuscular deposit that gradually releases the hormone over the course of one to three months.

Short courses of a high-dose estrogen (the so-called Morning-After Pill) may be taken after coitus. It increases the activity of the fallopian tubes so that the fertilized egg is expelled into the uterus before the uterus has undergone the modification necessary to receive it. This type of contraceptive produces a great deal of nausea and vomiting.

Apart from oral contraceptives, no other drugs are used therapeutically to affect ovarian function. Some drugs may have undesirable side effects on the ovary, which often culminate in menstrual irregularities. Certain tranquilizers (e.g., reserpine chlorpromazme, narcotics, and anti-cancer drugs) can adversely affect the hormonal secretions of the ovary.