January 12, 2010 – How does PZP work? (Part 1, Reproduction)

In order to understand how PZP is able to lower the rates at which wild horses reproduce, it is important to understand how reproduction actually works. Because PZP works on females, we will focus on mares. To help illustrate the steps in reproduction, please consider the mare Graciana. She had her first foal in 2009. What steps led to this foal being born? Also, as we go along, consider how reproduction could have been stopped by disrupting the different processes that end up leading to the birth of the foal. (Feel free to click on the following images for larger versions of them.)

Reproduction starts in the brain with the release of certain hormones. In the lower portion of the  brain is the hypothalamus. At the hypothalamus, there is a hormone produced called gonadotropin-releasing hormone. This hormone is commonly referred to as GnRH. GnRH ends up coming in contact with the pituitary gland, and this causes two more hormones to get released. These hormones are follicle stimulating hormone (or FSH) and luteinizing hormone (or LH). Thus, sometime in the early summer of 2008, Graciana’s hypothalamus produced and released GnRH. As a result, the pituitary gland released FSH and LH.

FSH and LH move through the body to interact with structures in the ovaries.

As its name implies, FSH stimulates ovarian follicles to mature. Ovarian follicles are structures that develop eggs. Thus, ovarian follicles must mature before they can release a mature egg during ovulation. LH stimulates ovulation, or the release of a mature egg from an ovarian follicle.

Please note that the function of these different hormones really is complex, and they even do a few more things than what I described. For example, they are involved in some feedback loops, such as one involving estrogen, that I won’t discuss here. The main point to remember is that GnRH leads to the release of FSH and LH which leads to the release of a mature egg.

The FSH and LH produced by Graciana’s pituitary gland moved through her body to her ovaries. The FSH stimulated the ovarian follicles to mature so that a mature egg could be produced.

The LH initiates ovulation; the mature egg is released.

After a mature egg is released, it is possible for fertilization to occur. Fertilization occurs following the union of an egg with sperm.

Thus, in early summer of 2008, Graciana successfully mated with a stallion; and Graciana’s foal was conceived.

About 11 months after Graciana conceived her foal, she gave birth to Jedediah, a striking black colt.

We have now seen how and where the processes of reproduction occur. To summarize this, here is a diagram demonstrating what I discussed above:

With this, we can see some possible steps that could be disrupted to prevent pregnancy. For example, if the release of GnRH is disrupted, then there would not be a release of FSH and LH. This in turn would prevent these two hormones from working toward the development and release of a mature egg. If there is no mature egg, then it is impossible for fertilization to occur. This would thus prevent pregnancy from occurring.

However, when thinking about stopping the processes that control reproduction, there is something else we should consider. Notice how toward the top of the chart is the brain and the source of reproductive processes. This all starts with the release of GnRH from the hypothalamus and continues as we move down until a mature egg is finally released. Moving down the chart is like moving downstream in a river. As we go downstream, we reach steps that are more and more specific to different processes. This is important to understand when thinking about disrupting certain processes as there is the possibility that the disruption of these processes can lead to the disruption of other processes that may have nothing to do with reproduction.

PZP works by disrupting fertilization. As you can see, this is very far downstream. As far as females go, the only way that pregnancy could be prevented further downstream would be an abortion.  How is PZP able to prevent fertilization from occurring? This is what I will be describing in tomorrow’s post.

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Published in: on January 12, 2010 at 10:53 pm  Comments (6)  

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6 CommentsLeave a comment

  1. Matt,
    This is great stuff. To approach the dilemma of controlling herd populations from an honest and scientific viewpoint is what is needed. Right now, emotions are running high because of such huge roundups as the one underway in Nevada. If only a moratorium could be imposed, while such controls as PZP are explored for all herds.
    I look forward to your future postings, as to how it has been effective on other horse herds, and the various pros and cons.
    It may be too late to help those horses already removed, but if this could eventually eliminate or reduce future roundups, it is certainly worthwhile.
    Do you know if the BLM are on-side in using PZP on all mustang herds, or is it up to each BLM regional office? I know PZP was used on the Pryors in September, but it is a relatively small herd. Is it considered feasible to use it on herds that number in the thousands, like the Calico herd?
    Wendy D

    • Thanks for your message, Wendy! The BLM is bringing fertility control into planning. In the Calico Complex, fertility control could be used in conjunction with skewing the sex ratio toward males. There is a similar proposal in the preliminary gather plan for the upcoming Eagle HMA gather.

  2. You can probably scratch the thought of PZP being used in the Eagle round up. Both the EA and conversations with BLM say that the number of horses gathered up is not expected to allow any releases. No releases….. no PZP

    • Thank you very much for your information. This is a really good point you have brought up; it seems to be the case that the use of PZP is determined by the number of horses that are actually able to be gathered. Both the Calico and Eagle EAs discuss how at least 80% gather efficiency must be reached before PZP is used. It sounds as though your research has found that they do not anticipate reaching 80% at the Eagle HMA.

  3. Matt,
    I really understand the reason for birth control for mares in the wild horse population, but I am still worried about the foals born out of season. Horses have a reason for foaling when they do. I don’t know if we have a right to try to outsmart them.

    • Well, as we go along in this discussion, and any future discussions we’ll hopefully be having with other methods of management, keep a couple things in mind. First off, think about what is backing up statements that are made. With PZP, we have a lot of research that has been done. This will be coming out a lot more as we get into more specific topics. Second, ask yourself about the alternatives that exist. That is, if you aren’t so sure about something that could be caused by one method of management, think about if the alternative would be any better. Please do keep reading, though; I think you’ll find a lot of your concerns are addressed in the research I will be discussing in days to come.


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