Reponse from neuronal network to applied stimuli ?

Lars Thomsen lthomsen at aix1.danadata.dk
Fri Aug 25 19:34:42 EST 1995


I work with electrophysiological characterization of enteric neurones
in the small intestine. I (as probably everybody else) is interested
in understanding the network processes in the system. 

There exists several very good methods to investigate the cells,
their synaptic inputs and the transmitters they contain (patch-clamp,
current-clamp, receptor labelling, immunohistochemistry etc).

The small intestine has an electrogenic transport of electrolytes
across the epithelia. The transepithelial generated can be measured by
mounting the tissue in a Ussing chamber. A measure of the netto active
transepithelial transport can be obtained by short circuiting the
setup with a opposite directed potential. The current used to generate
the opposite potential is a measure of the netto transport of
electrolytes across the epithelia. 

Wholemounts of epithelia and enteric nervous system can easily be
obtained. Several authors has shown that agonist applied to the
basolateral side of the Ussing chamber causes a change in the
transepithelia transport caused by effects on the enteric nervous
system. Otten the effects can be blocked by tetrodotoxin (TTX) and
hexamethonium. This indicates clearly that neuronal circuits
participates in the observed responses.

I have no doubt about the truth in the above mentioned. My concern is
directed against the conclusion made from this kind of experiments.

 As an example 5-HT (only one of many other neurotransmitters) is
claimed to be involved in Cl- secretions of the small intestine. The
evidence is given by a concentration dependent change in short-circuit
current following application of the agonist. The effects can be
blocked by 5-HT receptor antagonist. The responses can be blocked by
TTX.

This experiment shows that 5-HT receptors are located in the small
intestine. But  it does not show that 5-HT is involved in secretion,
as far as I can see. I would like a comment from people who has more
knowledge about neuronal networks that I have. 

I would like to note that the secretory response of the small
intestine is a well known protective mechanism to flush the
gastrointestinal tract for toxic substances.















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