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Thirst and Evolution (was Re: Evolution, "adaptation")
Bryant (mycol1@unm.edu)
30 Aug 1996 06:09:24 -0600
In article <lpiotrow.389.3224AB99@magnus.acs.ohio-state.edu>,
Len Piotrowski <lpiotrow@magnus.acs.ohio-state.edu> wrote:
Bryant wrote:
>>>The neuropeptide causing
>>>the sensation of thirst has been identified (and, therefore, you would
>>>presumably agree that there are relevant genes for selection to have
>>>acted upon were thirst somehow evolutionarily adventageous).
>>
>>I can't figure out what your saying here. A neuropeptide causes the
>>"sensation" of thirst. Where does it "cause" this sensation, where does it
>>come from, and how is it produced and transmitted? (Excuse me for my ignorance
>>of this esoterica).
>
>I will respond at length, and carefully, after my approaching phys class.Ok?
Alright. I have a few minutes, and I think I'm awake. Here goes:
Neuropeptides are strings of amino acids. They were first found in the
brain (hence the name; 'peptides' would be less of a misnomer), but since
have been found to send msgs to and affect the behavior of the immune
system, etc. (This relationship is the basis of neuroimmunology, a young
field of medical science.)
The endorphins (the "brain's morphine") are neuropeptides. They appear
to mediate communication between cells throughout the body, including, of
course, the brain.
Cell surfaces have receptors for certain sets of neuropeptides,
reflecting the role the tissue they are part of plays in the organism.
Candace Pert, at Rutgers, is quoted in the perhaps dubiously 'scientific'
source _Healing & the Mind_, by Bill Moyers, as saying that she has
evidence that these neuropeptides mediate emotional states as well.
I believe that even prokaryotes use 'neuro'peptides for communication,
though it seems doubtful they mediate "emotions," of course, since those
seem to require more than one cell! :)
I will look up the specifics on the 'thirst' neuropeptide if necessary
for our discussion, but I can sketch this out now: the relevant peptide
is produced when the body approaches dehydration. It's been measured
experimentally in this context. Not only does it appear to mediate the
"emotion" or sensation of thirst, but when applied to renal or lung tissue,
for instance, it causes those tissues to retain water which would
normally be excreted. Give this peptide to rats and they'll drink
themselves silly. The peptide is not distributed just within the skull,
as neuroscientists long assumed--it's sent out throughout the body and
causes ~coordinated physiological responses for water retention and
water-seeking behavior.
Because it's a peptide, the product of amino acids, the direct products
of gene expression, there is little (if any) reasonable room for denying
that the neuropeptide I describe is a genetic product. Since there are
genes for this peptide, variation in its structure (and the structure and
placement of its receptors, also gene products) should have been subject
to natural selection through deep evolutionary time.
This, i hope, is a pretty clear example of a mental (and physiological)
state that is tied to "physical phenotype", and can reasonably be
described as a product of selection favoring water acquisition and
retention.
Cheers, Bryant
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