Homing In With Heat - Vampire Bats find Blood

Chris -   How do blood thirsty vampire bats home in on the best place to bite and therefore guarantee achieving a trouble-free feed?  Well the answer is that they've evolved their own built-in infrared detectors to pinpoint where the best blood vessels are and David Julius from the University of California San Francisco is behind the discovery.  Hello, David.

David -   Hello.

Chris -   What made you think that bats might actually be resorting to temperature to guide them to where they should sink their teeth in this way?

David -   So, it's been known for several decades that bats have these so-called pit organs on their face that are heavily innervated with nerve fibres that allow them to detect infrared radiation.  What we've done is to ask what the molecular underpinnings of that system might be.

Chris -   So how these special pit structures on their faces can actually pick up infrared or heat?

David -   That's right.

Chris -   How did you approach it?  What did you do?

David -   We're more generally interested in the whole mechanism of temperature sensation, how we, as humans for example detect things like hot and cold, and we've been interested in finding out how this works in animals that really take thermal sensation to the extreme in a way and use this in a different but generally related manner.  And what we did was use some new methods in genomics, what they call deep sequencing or DNA sequencing where we can profile all the genes that are expressed in different tissues, and we ask what kind of molecules are expressed in the nerve cells that send their projections to these heat-sensing pits, and are known to be involved in the infrared detection mechanism.  And we look through those to find molecules that might be involved in this form of what turns out to really be heat sensation.

Chris -   Okay, so we know that our skin is sensitive to heat and we have a pretty good idea how it detects heat.  There are various chemicals which are on the surface of nerve cells that sensitise those nerve cells when the temperature goes up.  So, are you saying that a variant of one of those is being used by the bats on their face in order to not just detect temperature but to specifically detect temperature relevant to body heat?

David -   Yes, that's exactly the case.  So, what we've shown is that the bat expresses a form of a protein that we also use to detect heat.  But the form of the protein that the bat expresses is in some ways optimised where the temperature required for activation is lower than it is for our heat sensors and enables them to detect body heat coming from their prey, from a blood supply from a cow or a pig, what have you.  So the basic underlying mechanism is the same as the one we use, but they have some little bit of genetic trickery that enables them to modify the protein so that it's more sensitive to heat and can pick up radiant heat from their blood supply.

Chris -   So they're using the same genetic machinery that they would use elsewhere in the body to pick up when they're being burned or things are getting too hot.  But in these special facial regions, they are tweaking the gene a bit so that it becomes more sensitive at a lower temperature so they can use those organs to see where there is heat radiating from the right bit of an animal they want to bite, so they can infer where the blood vessels must be.

David -   Right, exactly.  In a body of a mammal, the sensory nerve fibres for example that allow you to sense temperature, touch or pain are distributed into different - what we call ganglia - that contain clusters of nerve cells.  And those that innervate everything from the neck down are in one set of ganglia and those that innervate everything from the neck up are in another set.  And in our bodies those two sets of neurons are more or less the same.  There are some slight differences in the expression of genes, but pretty much what you see in one cluster of neurons is the same as in the others.  And in the vampire bat, what we found is that in this particular gene that expresses this heat sensor, the nerve clusters that send nerve fibres to everything from the neck, up to the facial area, which includes the heat-sensing pits, the expression of the heat sensors are different,  and the protein coming from gene is modified so that it takes on this different form.  And in fact, that's one of the big clues that tells us that this gene is likely involved in this specialised function of the vampire bat, namely infrared sensation, because it is modified and it's modified only in those clusters of nerve cells that send their nerve fibres to this region of the body that is involved in infrared detection.

Chris -   There are other animals that also home in on heat.  There are some snakes and vipers for example that aim for the hot spot because that's where they want to invenomate because they, I guess, figure that if they put the venom where the heat is, that's where the blood is so it will act most quickly, and then also guarantee a strike on the animal.  Do they use the same mechanism as your bats then?

David -   They use a mechanism that's related but in detail, different.  One of the great example of this in terms of pit vipers is the snake that lives out around my area here called the Western Diamond Back Rattlesnake, and it also has what we call "facial pits."  They're somewhat different in structure that a vampire bat but generally have a similar plan and they detect radiant heat say, from a squirrel or a mouse that they're trying to find in a dark burrow at night, so it allows them to see the animal as a radiant illuminated figure.  They use a protein molecule that detects temperature that's a member of the same protein or gene family as the one found in the vampire bat, and the one that we use for heat detection.  It's encoded by a different gene, but they're part of the same gene family.  And so overall, the mechanism is similar but the exact molecule that's used is different in its detail and in its structure.

Chris -   And just to finish up, David.  Given that you've got this new insight into how this gene can change its behaviour if you do what the vampire bats are doing to it, in other words, make it sensitive at a lower temperature.  How does this inform our understanding of how pain is signalled in the nervous system and could there therefore be some uses of what you've discovered?

David -   Yeah, so that's an excellent question, and a molecule that we express, which by the way I should say is the target for things like chilli peppers.  So, the molecule that we're talking about here that's involved in temperature sensation in the bats and in our own nervous system is what allows us to appreciate sort of that hot zing from chilli peppers.  We're interested in that molecule, as are many other labs, because there's evidence to suggest that its also modified by agents that are produced during inflammation and tissue injury that then sensitise the whole system so that you now for example would appreciate a lower temperature as being sometimes painful.  So the example would be, if you have a sun burn and then you get in the shower and the temperature is normally what you'd consider to be warm and very comfortable, you might consider that or perceive that as being noxiously or painfully hot and that has to do with the fact that these inflammatory agents are acting on this molecule to lower the threshold to heat and therefore, generate a perception of pain even in temperatures that normally you wouldn't consider painful.  And understanding how that occurs and how changes in these molecule and how the structure of this molecule is involved in those sensitisation mechanisms is very important for understanding pain hypersensitivity, especially in the context of tissue injury.  And looking at this structure of these special bat receptors gives us some clues about what parts of the molecule might be involved in those kinds of temperature shifts.

Chris -   Who would've thought it?!  We know that garlic drives vampires away but now, maybe the chilli attracts them.  David, thank you very much.  That's David Julius.  He's from UCSF and you can find the work that he was talking about published this week, it's in the journal Nature.