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Neurotransmitter questions

Robert John Butera rbutera at owlnet.rice.edu
Mon Oct 5 23:35:34 EST 1992

In article <1aqqt4INNi4c at calvin.usc.edu> alves at calvin.usc.edu (William Alves) writes:
>First, this whole neurotransmitter business. As I see it, there are only
>two possible effects a neurotransmitter can have on a neuron - it can
>inhibit or excite the cell. Then doesn't it seem logical that there would
>be only two types of neurotransmitters, instead of the pharmacopeia that
>has been discovered? What is the difference between inhibition from one
>neurotransmitter and another?
I'll try to keep this nontechnical, which should be easy, since I
prefer to avoid technical terms whenever possible :)

The problem lies with how you define "excitatory" and "inhibitory"
While these have general meanings, they are better defined by the
context in which they are being used.  In the simplest sense, we
could describe excitatory as a synaptic current that has a
depolarizing effect, and inhibitory that has a hyperpolarizing effect.
Even by this definition, biphasic synaptic currents have been seen
for quite some time (for example, one known neuron has a synaptiic
input that is excitatory for a short period time, followed by a much
longer inhibitory phase).

Getting back to the definition thing.  What is excitatory?  Is it
a hyperpolarization of the action potential threshold?  Is it an
increase in spike frequency?  It could be either (or others).
Likewise, an inhibitory effect could mean a lowering of spike
frequency, a higher activation threshold, an increase in the neuron's
spike frequency adaption.

Some neurotransmitters migght have one effect at an elevated 
concentration, and an opposite effect at an even higher concentration.

To think of another variation, what about bursting neurons (ones that
are hyperpolarized for a period of time and firing action potentials
for a period of time).  A neurtransmitter might effect the frequency
of the spikes within the burst, yet make the interburst interval

Basically, it is possible to have a good "feel" for excitatory or
inhibitory effects.  However, the manifestation of that effect may
vary from neoron to neuron.   

More importantly, I think one must consider the mechanism of the 
neurotransmitter. Does it simply activate an ion channel that is normally
silent?  Does it modulate an existing channel?  Is it involved in a
more complex biochemical scheme that effects enzymatic reactions
within the cell  whose products, in turn, effect certain changes on
cell behavior?  Extend this line of questioning and you can get into
all sorts of second messenger effects, short term memory studies, etc.

I've tried to keep it simple and have glossed over/over-simplified
some things, but hope I kept it brief.  I'd reccomend the following

Principles of Neuroscience, by Kandel, Schwartz, and ???.  Its a huge
reference on all aspects of neuroscience, and recently in its 3rd 
edition.  I've seen it in many college bookstores, both as a reference
or as a textbook for a senior/graduate course in neurobiology or
neuroscience.  Also, for the $$$ (under $75), this book delivers more
pages or pounds per $ than any other textbook I've seen recently 
(its biiig)..

Also check out the September 1992 issue of Scientific American.  Its
primarily about learning and memory, but might provide a good starting
point / references for you.

>I haven't gone to med school, but I have been to the library, and that's

All the better.  Med students often seem to be preprogrammed with
standard responses they've been forced to memorize :)  Hunt down
a grad student in physiology, neuroscience, neurobiology, etc.

Rob Butera - ECE grad student   Rice University, Houston, TX   rbutera at rice.edu

"...this thing of separation of church and state is a figment of some infidel's
 imagination." - Rev. W. A. Criswell

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