User:GavynBackus4

Gavyn Backus

UWO Biology 4931G - Seminar in Physiology

Article Choices for Wikipedia Update Assignment

1. There is currently no Wikipedia page for “venom glands.” Small amounts of information can be found on the snake venom page; however, none of this information addresses the physiological aspects of venom production or secretion within the gland. I would like to address where and how these toxins are produced/secreted and what physiological elements might regulate these processes. In addition, I would like to discuss the similarities between parietal cells and venom secreting cells as well as some of the important protective qualities of the gland that prevent snakes from being poisoned by their own venom.

2. The Snake venom page does not address the metabolic constraints that are associated with venom production. I would like to discuss this topic by providing physiological and behavioural evidence for increased metabolic rate. I would also like to use this theme as a segway to discuss the concepts behind venom metering.

3. The animal migration page has virtually no information on the physiological constraints associated with high intensity exercise such as long distance migration. I would like to discuss the link between optimal exercise intensity and the particular fuel mixtures that different animals use.

Wikipedia update: Venom Gland

Article to review: Snake venom

Critique

The poor quality and small amount of information that is available on snake venom glands is laughable. Two sentences are certainly not adequate to fully describe the venom gland; considering that this structure is one of the major factors that differentiates toxic and non-toxic snakes. There is sparse information on venom gland morphology. In addition, the physiological processes behind venom production and regulation are completely absent. A comprehensive review of venom glands is therefore necessary to make the Snake venom page a more complete analysis of snake physiology and morphology.

This Wiki page states that venom glands are modified from the parotid salivary gland of other vertebrates; however, they offer no theoretical or empirical support for this statement. In addition, it neglects to mention that there are also similarities between venom gland and digestive function. It is always important to provide evidence because simply stating a fact (without the proper support) does not make it true. This is one of the major criticisms of Wikipedia that prevent this website from being seen as a credible and reliable source of information.

The most descriptive part of this page describes the glands morphology, although it only gives a superficial overview and does not mention any of the important cells that are involved in venom synthesis, such as the columnar secretory cells and mitochondria-rich cells. Consequentially, there is also no description of the physiological mechanisms underlying venom production or the regulatory processes that are involved. It is important to give a proper and detailed description of the venom gland morphology, function and control in order to better understand how this structure actually produces and secretes venom. Furthermore, understanding the manner and speed at which synthesis can occur highlights the ecological relevance of venom as a predatory and defensive mechanism.

The snake venom page states that snakes have other glands that secrete “antivenom-like” components that allow venomous snakes to tolerate the toxicity of their own venom. The article gives no examples or explanations of how this is actually accomplished. Furthermore, it does not discuss the importance of the venom gland itself with regards to “self-resistance.” This is a very important function of the venom gland because it explains how venomous animals are unaffected by their own toxins. It also helps explain why certain components of snake venom are used to develop antivenom treatment for humans.

Three key references for this wikipedia update include:

1) Fine structure of the venom gland epithelium of the South American rattlesnake and radioautographic studies of protein formation by the secretory cells

This study gives an excellent description of venom gland morphology and the physiology behind venom synthesis and secretion. The relevant findings are:

• There are two important types of cell that make up the myoepithelium of the venom gland: columnar secretory (granular) cells and mitochondrial cells.
• Granular cells contain large quantities of rough endoplasmic reticulum (RER) and Golgi apparatus and are therefore responsible for venom production and secretion.
• Protein synthesis does not occur when secretory cells are cuboidal in shape and the RER is flattened. Synthesis is initiated when the secretory cells are columnar in shape and the RER is distended
• After venom proteins are synthesized, the Golgi apparatus packages them into vesicles. These vesicles then secrete their contents into the central lumen.
• Protein synthesis in the venom gland is quite similar to that which is seen in mammalian salivary glands.

This study gives a nice description of the of the venom gland and should improve the vague and poorly described morphological description that is found on the Snake venom page.

2) Control of venom production and secretion by sympathetic outflow in the snake Bothrops jararaca.

This is the first study to demonstrate that the venom production in viviperid snakes is regulated by the sympathetic nervous system. The relevant findings are:

• Venom production is not constitutive and acitivity is turned on by the sympathetic nervous system.
• Treating snakes with reserpine pharmacologically denervates the secretory cells thereby blocking protein synthesis at the RER and vesicle formation at the Golgi.
• Treating the secretory cells with noradrenergic agonists, such as isoprenaline, restores protein synthesis and vesicle formation.
• These agonists target B- and a-adrenorecptors on the secretory cells.

This study shows that venom production is associated with a “fight or flight” response and may therefore have evolved for defensive purposes, although further research is necessary to support this argument.

3) Bioweapons synthesis and storage: the venom gland of front-fanged snakes

Mackessy and Baxter (2006) discuss how snakes produce, store and deliver venom without poisoning themselves or losing the biological potency of the venom. The researchers give a nice summary of the different protective mechanisms that are involved in self resistance but direct most of their focus towards mitochondria-rich cells and their role in venom storage. The relevant findings are:

• The abundance of mitochondria, the elongated microvilli and the acidophilic nature of mitochondria-rich cells make them morphologically and functionally similar to the parietal cells found in the gastric pit of rattlesnakes
• Freshly sampled rattlesnake venom is acidic (pH=5.4) and therefore inhibits the “venom activating enzymes.” These enzymes function best under less acidic conditions (pH=7.2-7.4) and will become activated the instant they are injected into the preys tissue.
• Mitochondria-rich cells acidify stored venom in order to inhibit “venom activating enzymes” in the central lumen.
• Tripeptide inhibitors of venom metalloproteases and organic acid inhibitors, such as citrate, compliment the storage and protective processes that take place in the gland
• Many of the protective mechanisms have pharmacological applications

This article provides pertinent information about “self-resistance,” venom gland structure and pharmacological application. In addition, it has an excellent figure that compares mitochondria-rich cells to parietal cells elucidating their structural and functional similarities.

• Citations