Newsgroups: comp.ai.neural-nets
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From: cati@cogsci.ed.ac.uk (Catherine Collin)
Subject: Re: neurotransmitters
Message-ID: <DBEAJ5.799@cogsci.ed.ac.uk>
Organization: Centre for Cognitive Science, Edinburgh, UK
Date: Sat, 8 Jul 1995 11:28:15 GMT
Lines: 46


About neurotransmitters...It's possible that there's something of a
misuse of terms going around in this thread. Currently there are four
criteria all of which must be fulfilled by a substance if it's to be
considered a neurotransmitter. (No doubt, this is subject to debate,
but...) These are -

1. It is synthesised in the neuron.
2. It is present in the pre-synaptic terminal and released in amounts 
   sufficient to exert its supposed action on the postsynaptic neuron
   or an effector organ.
3. When applied exogenously it mimics exactly the action of the 
   endogenous substance.
4. A specific mechanism exists for removing it from its site of
   action.

I'm guessing that when matt austen and tony zeigler talk of
neurotransmitters, they're talking of the fast ones - the ones that,
yep, get released probabilistically from vesicles (though also subject
to debate) - e.g acetylcholine, gaba...

However, any substance which fulfills those four criteria are
considered to be neurotransmitters, and these include many 'hormones'.
e.g adrenocorticotrophins, lutenizing hormones, growth hormones...

As for:

>Incidentally, I'm not talking about hormones.  Hormones probably do
>cause some sort of slow modulation involving large volumes of neural
>tissue.  That's an entirely separate problem, though: there's very
>little relation between hormones and neurotransmitters.
>--

There's quite a lot of relation in fact - classical neurotransmitters
and 'hormones' coexist in the same neuron, can be released together,
and work synergistically on the same target cells e.g vasoactive
intestinal peptide and acetylcholine.

Basically, i'd be careful about saying 'there's little relation...'
about any two things in the brain, somehow, somewhere, they're
probably doing something together...

catherine

(Thanks to Kandel et al. 'Principles of Neural Science' for examples.)

