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| 4. ROCs | |
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Receptor operated channels are generally found at chemical synapses. They can be excitatory or inhibitory.
You will not be surprised to learn that these also
form a superfamily of so-called ligand-gated channels,
that share elements of a common structure and which are likely
to have evolved from a small number of ancestral genes.
The reason that our nervous systems must have ROCs
or ligand-gated channels is the individual elements (neurones)
are not connected electrically. If an excitable event is to be
passed from one cell to the next, there must be a way to cause
excitation in the next cell. ROCs serve that function.
Perhaps you are asking, why could not the G protein
coupled receptors serve this function? If so, good. There
is not a very good answer. ROCs don't have to rely on several
stages of biochemistry and diffusion to cause ion channels to
open: having the ion channel in the same structure makes them
very fast! There is much to be gained by having a fast nervous
system.
The ROC that we will consider in the element 3 lectures
is the nicotinic acetylcholine receptor (nAChR).
The name is drawn from the natural endogenous ligand, acetylcholine.
However, receptors that bind acetylcholine are not all ROCs,
some are G protein coupled. These are referred to as subtypes
of acetylcholine receptor. Fortunately they differ in their sensitivity
to other ligands. The ROC acetylcholine receptor binds and is
stimulated by the plant alkaloid, nicotine, and
the G protein coupled acetylcholine receptor binds and activated
by a fungal alkaloid, muscarine. To distinguish
between these receptors it is customary to prefix acetylcholine
receptor with nicotinic or muscarinic.
Occasionally you will find receptor subtypes being defined only
by numbers, the histamine type 1 (H1) receptor responsible
for hayfever symptoms and local swelling and inflammation, and
the histamine type 2 (H2) receptor responsible for
stimulating gastric acid secretion. |
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| Copyright © 1998 University of Bristol. All rights reserved.
Author: Phil Langton |
Last modified: 9 Jun 1999 20:11 Authored in CALnet |