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Determination of Synaptic Type ?

Thomas Chimento chimento at neuron.arc.nasa.gov
Wed Feb 7 13:03:29 EST 1996

In article <4f5lji$kes at itssrv1.ucsf.edu>, paul at phy.ucsf.edu (Paul Bush) wrote:

> In article <4eq6fn$2i6 at helios.herts.ac.uk>, AliStair G Rust
<a.g.rust at herts.ac.uk> writes:
> |> However, how is the type of synapse determined ?  I've not found anything
> |> specific in the literature I've read so I've got a few questions.
> |> > 
> Type is determined by the presynaptic cell. Excitatory cells (in the
cortex this
> means spiny cells) send out axons which always make excitatory synapses on the
> neurons they contact. Similarly, inhibitory cells (smooth or sparsely spiny
> cortical cells) always make the same type of synapse on their postsynaptic
> targets. Note that while inhibitory neurons will always release GABA, the
> inhibitory cortical neurotransmitter, the effect on the postsynaptic cell
> (inhibitory or excitatory) depends on its membrane potential at the time.
> Kandel means that we can infer whether the synapse is excitatory
> or inhibitory from its position on the postsynaptic cell, not that the site of
> synaptic contact developmentally determines the type of synapse. Thus, in
> cortex, an axo-axonic synapse is inhibitory, an axo-somatic synapse is
> if on an excitatory cell and can be either if on an inhibitory cell. An
> axo-dendritic synapse can be either excitatory or inhibitory. 
> Examine the synapse at the EM (electron microscope) level. Inhibitory synapses
> are symmetrical, meaning they have an equally dense staining layer on
both the 
> pre- and post-synaptic side of the synapse, and the presynaptic bouton
> flattened, eliptical vesicles (packets of neurotransmitter). Excitatory
> are asymmetrical and have round vesicles. Inhibitory are type II,
excitatory are
> type I (opposite of what you said).
> Excitatory/inhibitory is detemined by the presynaptic cell, as above.
'Tuning' of
> the densities and types of ion channels on the postsynaptic side
probably occurs
> in response to electirical activity/usage, but this is most likely just
> the strength of the synapse and not its sign.
> Paul

I would respectfully submit that the nervous system is not so cleanly
arranged. I only
wish that looking at synapses with an EM could answer all these questions
and that each
of the synaptic types were so obvious and simple. There are in the range
of dozens of 
presynaptic neurotransmitters identified and lately the number of
postsynaptic receptor
types has gotten even worse. The possible permutations make the whole
system depressingly
complex. That is one reason there are hundreds of articles published in
the patch clamp arena
examining the electrical responses of cells - you cant just look at where
they are or even 
what they look like and know how they behave. Even excitatory and
inhibitory are severe
oversimplifications. The magnitude and time course of the responses varies
The sign of the synapse can indeed change depending on the state of the
neuron and what
other events are occuring near in time. The original set of questions has
occupied the entire
career of scores of investigators - not a few of whom are at UCSF, Paul.
The above information
is a good place to start - it covers the introductory Neuroscience text
presentation of whats
what - but if you want to know something about a particular synaptic class
in a specific site
in the brain of a species of animal - you've got a lot more work to do.
You could never publish
an article in J. Neuroscience or J. Neurophysiology saying you knew the
function of a synapse based
on what it looked like or where it resided, unless the details had already
been worked out with
numerous other techniques (Immunohistochemistry, cloning the gene for the
receptor, electrophysiology
to name a few). Have fun in your endeavors - youve just begun.

defining with anything other than

Thomas Chimento                 chimento at neuron.arc.nasa.gov
                        Keep it simple, do it right.

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