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How Do Sperm Develop?

A fundamental discovery is reported this week about the way the brain controls the delicate biochemical chain reaction which determines the pattern of ovulation in women and the development of sperm in men.

The finding could have an important influence on methods for regulating fertility and contraception.

Scientists have found a molecule that belongs to the family of substances classed as neurotransmitters, or the chemical messengers of the brain and central nervous system.

Meet The Molecule Called GAP

The new peptide molecule has been given the initials GAP, and it is produced at the base of the brain where it regulates a cascade of hormones that control reproduction. These include the luteinizing hormone and follicle-stimulating hormone, which ensure that the tissues in the ovaries and in the testes develop for fertility.

But there are two other key hormones, prolactin, and gonadotropin, in the cascade secreted by the pituitary gland. Prolactin is the hormone that prevents women from having periods when they are breastfeeding.

According to the discovery by a team working in California with Dr. Peter Seeburg, the new molecule, GAP, instructs the gland to stop producing prolactin and tells it when to secrete gonadotropin. The full name of the peptide, which consists of a chain of 56 amino acids, is “gonadotropin-releasing hormone-associated peptide”.

The identification and isolation of the molecule were made by using the latest methods of genetic engineering, whereby the gene which carries the code for the synthesis of GAP was inserted into a micro-organism and pure samples of the peptide were synthesized in the laboratory cultures.

The group of five scientists from the Genetech laboratories, in San Francisco, and the University of California at San Francisco, describe the research in a paper published in the current issue of Nature.

Their account includes the response of laboratory animals injected with extracts of GAP for regulating reproduction.

In addition to mothers who breastfeed, there are some illnesses in which large amounts of prolactin are made.

At present, the level of prolactin can be reduced in infertile women by a drug called bromocriptine, which lowers the amount in the blood serum, but it does not stop the brain from producing the molecule. Women treated by in vitro fertilization also have supplements of gonadotropins.

A Simpler Method For Controlling Fertility

The conclusion of Dr. Seeburg’s team is that the discovery of the basic brain chemical, which is the discovery of the sought-after factor that inhibits prolactin, should simplify understanding of the control of gonadotropin and prolactin secretion.

They believe more effective and simpler methods of controlling fertility through genetically engineered GAP should now be possible.

In a review of the paper in the same issue of Nature, a more cautious opinion is offered by Dr. George Fink, director of the Medical Research Council’s brain metabolism research unit at Edinburgh.

He describes the work of synthesizing GAP and testing it by immunizing animals as brilliant and exciting. But he has reservations about how effective it might be in humans because of some observed basic differences in hormonal control between species which are not fully understood, and which are not made clearer by work on the new peptide.

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