In article <e.pietrzakXX.6.380A6055 at uq.net.au> Eva Pietrzak,
e.pietrzakXX at uq.net.au writes:
>Could anyone, please, explain what is a difference between receptors and transporters.
>I have lost intimate contact with neuroscience some years ago, and recently
>I've realised that some good old receptors (dopamine, serotonin and others)
>became transporters. Is it a new, fancy name for old receptor, or does it
>denotes some difference in function?
>>Thank you
>Eva
No, it's not a new name, it's a whole different thing. Remember the
neuromuscular junction (NMJ), where acetylcholine (ACh) is released into
the synapse, and activates postsynaptic nicotinic ACh receptors (nAChR)?
Well, studies at that synapse led to some "rules" for whether a
particular molecule could be considered a neurotransmitter or not. Some
of those rules are: it has to be present in nerve, and released upon
stimulation; applying the substance directly to the postsynaptic cell has
to mimic the effect of nerve stimulation (this is mediated by the
receptors); and there has to be some mechanism for getting rid of the
stuff once it's been released. At the NMJ, this last part is accomplished
by an enzyme called acetylcholinesterase (AChE). It turns out that AChE
is an incredibly efficient enzyme, and if you block it or poison it, then
the synaptic response to ACh becomes greatly prolonged because the ACh
hangs around for much longer than it normally would, so it can repeatedly
activate the receptors.
As people started to study synapses in the central nervous system, they
found for the most part that there weren't any degradative enzymes
similar to AChE at central synapses, whose job it was to clear away
transmitter. Instead, there were membrane-spanning proteins in the
neuronal and glial membranes that grabbed onto the transmitter in the
synapse, and then transported it across the membrane back into a cell.
This removed it from the synaptic space, so that it could no longer
activate receptors. These proteins are transporters, and cloning them and
studying their biophysical properties is all the rage these days. They've
been found for all of the "main" fast acting transmitters: glutamate,
GABA, glycine, dopamine, serotonin, norepinephrine, etc. Note that these
are all small molecules, the big peptide transmitters like opiods etc,
don't have transporters but are broken down by enzymes. That's because
it's very very difficult to move a big peptide across a lipid bilayer.
Incidentally, you've probably heard of Prozac. That works by blocking a
serotonin transporter, so that serotonin can hang around longer after its
release.
Cheers,
Matt Jones
