Metabotropic Pathways

mat mats_trash at hotmail.com
Wed Jan 16 18:28:01 EST 2002

"yan king yin" <y.k.y@(dont spam)lycos.com> wrote in message news:<PVd18.935$na5.123627 at news.xtra.co.nz>...
> There could be two types of diffusion. One is retrograde, from
> postsynaptic to presynaptic. Another type is from one postsynaptic
> site to another postsynaptic site nearby (inside the same dendrite).
> Im suggesting this latter form might lead to some interesting learning
> rules.

There is actually spillover of transmitter itself at some synapses. 
Another thing you might want to investigate for your learning rules is
the role of Glial cells.  THey are not passive structural cells but
actually participate in transmitter uptake from synpases etc. further
a system of 'volume transmission' of molecules such as NO over broad
areas of the brain has been found to be due to glia.  Its role in
higher function is unclear though.
> The secondary messengers of metabotropic receptors should
> serve some purpose, otherwise they could be replaced entirely
> by electrical synapses. Thats why Im suggesting that there
> could be some downstream events after their activation, and
> 2nd messengers might diffuse to a nearby synapse.

Well of course they do... Metabotropic receptors belong to the
superfamily of G-protein coupled receptors which constitutes the
largest receptor class in the body, and they definitely do have
effects - IP3 liberation by group I mGluR increases cytosolic calcium
- the possible consequences of this are various, at a presynaptic site
transmitter release may be increased for example.  I've just read an
article on calcium signalling and there are some very interesting
conjectures on how IP3 may serve as a coincidence detector of pre and
post synaptic spiking.

Electrical synapses? do you really mean those? or chemical/ionotropic
synapses?  Direct electrical synapses are very different (and much
rarer).  I think you're getting tempted to view the brain as a
designed computer - why would metabotroipic receptors get selected
out?  Evolution only gets rid of deleterious differenes.
> Lets say there are 1-3 synapses per um (micrometer) of dendrite.
> That means synapses are about 1-0.3 um apart. "Fast" axonal
> transport is 2 to 5 um per second. Diffusion might even be faster
> than this.

Diffusion is very definitely faster than that, but cells (neurones or
otherwise) are not just bags of water - they have discrete
compartments and the spatial distribution of things like second
messengers is highly regulated, otherwise they wouldn't be of any use.

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