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Hemidactylus at my-dejanews.com Hemidactylus at my-dejanews.com
Mon Mar 1 16:01:39 EST 1999

In article <7be3iu$cg4$2 at denws02.mw.mediaone.net>,
  "Richard Norman" <rsnorman at mw.mediaone.net> wrote:

Note that I changed this post heading back to the original thread title. I
took the "evo, devo, and neuro" tangent based mainly on the "evolutionary
garbage" idea itself. I did make an appropriate thread title change though.
The broader issues I raised are still important IMO, perhaps not in the
context of neurotransmitters per se (receptors and transduction pathays
maybe). I was also thinking along the lines of *Hox* genes and such, very
divergent from the neurotransmitter topic. Sorry.

> A long history of posts goes back to the notion that the proliferation
> of so many neurotransmitters reflects "evolutionary garbage"
> >> >
> >> > As for the "evolutionary garbage" idea, I think what your prof
> may have
> >> > meant is that there's always an ongoing process of mutation and
> selection
> >> > among both neurotransmitters and receptors.
> >> >
> Fred Delcomyn's text, "Foundations of Neurobiology" (WH Freeman, 1998)
> discusses this on page 179.
> "The enormous variety of neurotransmitters presents an interesting
> problem
> for neurobiologists -- what are they all for? ...Why then, are there
> so many
> diferent neurotransmitters? It would seem that just a handful would be
> able
> to serve the needs of the nervous sytem.  It could be a case of
> evolutionary
> accident, a matter of mutations in membrane proteins yielding
> molecules
> that respond to various chemical substances that nerves happen to
> release.
> It is also possible that particular neurotransmitter systems evolved
> because
> different chemicals happened to best serve neurons that had specific
> functions."
> This is probably not the original source of the notion -- that
> evolution
> is not neat and tidy, that "efficiency" or "perfection" is often
> not the result is commonplace in biology.  And Delcomyn is
> a comparative physiologist with just this biological perspective.

It is interesting when looking at individuals of given species to note the
variety of transmitters, but it is also interesting that a certain degree of
conservation exists across phyla. From Shepherd's text _Neurobiology_ (p.
537-8): (begin quote) "With regard to neurotransmitters, it should already be
apparent that there is a strong tendency toward conservation across the
phyla. Thus, we have seen that glutamate is an excitatory transmitter and
GABA is an inhibitory transmitter...; indeed the earliest evidence for the
identification of these substances as transmitters was obtained in
invertebrates. Similarly, ACh is widespread as a neurotransmitter in many
invertebrate species; in fact, many insecticides have as their primary mode
of action a blocking of cholinergic synapses. Finally, we have noted the role
of 5HT in modulating behavioral states of some invertebrates... One can
conclude that there is a limited set of small molecules that have similar
functions across many phyla." (end quote)

This is yet another theme of evolution (i.e.- conservation). This does not
contradict your quote above, but complements it. There is diversity in
transmitters when looking at, lets say, a human brain, but when comparing
humans to other species one can appreciate that the same basic ideas are
continually re-used, perhaps in slightly different contexts(?).

I got a little too broad on the "evo, devo, and neuro" post wrt transmitters.
Hopefully the conservation subtheme is more appropriate in this context. I'll
get it right one of these days :-)


Shepherd GM. 1994. Neurobiology (3rd ed.). Oxford University Press. New York

Scott Chase

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