question: job of a neuron

Matt Jones jonesmat at physiology.wisc.edu
Wed Sep 12 16:38:49 EST 2001


In article <%0Mn7.344021$TM5.48519406 at typhoon.southeast.rr.com> Glen
M. Sizemore, gmsizemore at triad.rr.com writes:
>GS: Interesting.....I have believed for a long time
>that spontaneous locomotion (spontaneous at the
>level of behavior - i.e., no eliciting stimulus) is close
>to the first kind of behavior in existence. I believe
>that it is more fundamental than the unconditioned
>reflex which almost certainly requires excitatory
>synaptic drive. So, the notion that inhibitory
>neurotransmission is, evloutionarily, the most
>primitive, makes some sense to me, given what you
>write about oscillations which are almost certainly
>the backbone of most central pattern generators.
>BTW, what is the evidence that inhibitory
>connections are the first to arise?
>
>Cordially,
>Glen
>

Hi Glen,

First off, I think I owe everybody reading this thread an apology for
spreading misinformation.

My previous comment...

"the "earliest" ligand-gated ion channel may be an ancestor of
the modern glycine receptor" 

...was misleading. After posting it, I went and dug up the review
article that I was quoting, to make sure I got it right, and sure
enough, I screwed it up.

The article is "Evolutionary history of the ligand-gated ion-channel
superfamily of receptors." by Ortells & Lunt, TINS (1995) 18, 121-127.
The authors studied the genomic sequences of about 100 related channel
genes, and did a phylogenetic analysis to estimate a family tree.
Then, using molecular clock assumptions, they estimated the times of
divergence of the different genes and families. Their -actual-
conclusion was as follows:

1) The two "earliest" precursors appear to be the GABA-A receptor
delta subunit (inhibitory) -AND- the 5HT3 receptor (excitatory). That
is, it was not possible through this analysis to tell whether an
excitatory or inhibitory subunit arose first. Both came from some
common ancestor, the ionic selectivity of which is unknown.

2) Inhibitory glycine receptors appear to have derived -from- GABA-A
beta subunits.

A later study, however, by Xue (1998) J. Mol. Evolution, 47:323-333,
failed to find compelling evidence for the descent of Glycine
receptors from GABA receptors, rather than the opposite order of
descent.

Either way, they're all pretty damn old. In the Ortells & Lunt review,
they estimate that GABA-A delta subunit and 5HT3 subunit diverged from
a common ancestor about 2.5 billion years ago!

Yep, 2.5 billion years, that's what they say. So in light of the issue
of whether excitatory or inhibitory -neurotransmission- came first,
this really throws a wrench into it:

According to the evolution timeline at
http://www.talkorigins.org/origins/geo_timeline.html, the estimated
date of 2.5 billion years ago places the origin of modern ligand-gated
 ion channels about halfway in between the origin of prokaryotes and
eukaryotes. That is, both the excitatory and inhibitory versions of
these  channels appear to have evolved -long- before there was any
such thing as a nervous system or synaptic transmission. Since brains
are really smart, they must've seen a good thing and simply
appropriated it for their own evil purposes...

Anyway, I agree that spontaneous locomotion must have come really
early. After all, what's the use of have goal-seeking or escape
behaviors unless you can already locomote?

The most popular experimental model for the neural basis of
spontaneous locomotion is probably the isolated lamprey spinal cord.
When completely severed from the rest of the animal, including the
brain, the neurons in the cord fire in a pattern similar to what they
would do in the intact animal. So this shows that all the machinery is
there and functional for generating neural firing that -would- produce
swimming, even in the complete absence of sensory drive,
proprioceptive feedback, or descending signals from "higher" centers.

Interestingly, people have a hard time finding "pacemaker" neurons,
per se, and generally conclude that the oscillations arise from
network interactions between -both- excitatory and inhibitory neurons.

Cheers,

Matt




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