"So we've been working on the stretch of a receptor for a neurotransmitter called GABA,
gamma-aminobutyric acid.
This transmitter is the major inhibitory transmitter in the brain.
So what it does is it binds to the receptor and that will then reduce the firing rate
of neurons.
The receptor itself is known to be made from five sub-units, so it's a pentamer, and there
are various different types of sub-units from which the receptor can be made.
And the one we've been interested in is one that contains a unit called the delta sub-unit.
What's interesting about this is that the receptor is not localised at synapses, which
are the junctions between the neurons.
The receptor is actually spread all over the surface of the neuron, and what it does is
it seems to damp down neuronal activity generally.
So the receptor, although it's not a major type of GABA receptor, is very widely distributed
in the brain.
It seems to have this general role of what's called tonic inhibition, this damping-down
neuronal activity.
The technique we use to look at the structure of the receptor is a technique called atomic
force microscopy, and this works in a very different way from a normal light microscope.
It scans a very sharp tip over the surface of the sample.
The sample is stuck to a mica disk, which is a very flat surface, and as the tip is
moving back and to over the surface, if it encounters a particle it is deflected and
the deflections of the tip are picked up and used to create an image of the sample that
we're looking at.
The image, which you can see on the screen, is being built up line by line and you may
be able to see that there are white blobs on the screen.
These are the individual receptors that we're imaging.
Each of these receptors is about 10 or 20 nanometers in diameter, so it's very tiny
and we're able to see things at a higher resolution than any light microscope for example.
It's almost approaching the resolution that you get with an electron microscope.
In this way, by looking at the geometry of the complexes, we can figure out the arrangements
of the sub-units around the receptor.
So we now know what the arrangement is and this is of interest because we know that a
lot of drugs that interact with these GABA receptors are binding at the interfaces, the
junctions, between the various sub-units.
We have now defined what these drug-binding sites look like because we know now what all
the interfaces are around the receptor.
So it's possible now for us to think about modelling the structure of the receptor, and
particularly the potential drug-binding sites at the interfaces, between the sub-units.
So in the long-term it's possible that this new information will allow us to think about
a rational design of new drugs that will act specifically on this particular type of receptor,
the delta-containing receptor.
It's been implicated in a number of conditions.
The first condition is pre-menstrual tension.
It's known in rats that the levels of the receptor change during the ovarian cycle.
Now in humans before menstruation there is a sharp fall in the levels of progesterone,
which is acting upon these receptors, and it's been shown in rats that the numbers of
the delta-containing receptors increase during the equivalent period in the ovarian cycle.
So, as a consequence of this, the excitability of the rats increases at this time.
Now it's a long jump between rats and humans, but if we can make that jump, it's possible
that there's a similar thing going on in humans, where, just before menstruation, the levels
of these receptors will rise, and this might contribute to some of the symptoms of pre-menstrual
tension.
Another condition where the delta sub-unit has been implicated is epilepsy.
In epilepsy what's happening is the neuronal activity is uncontrolled so there's an excessive
amount of activity, and in various animal models of this condition, the levels of the
delta sub-unit-containing receptors fall, which may account for this over-excitability.
Also in some human forms of epilepsy, variations in the sequence of the delta sub-unit seem
to be occurring.
So it's possible that in epilepsy as well, there is an involvement of the delta-containing
receptor.
Finally there is some evidence that this particular type of receptor is extremely sensitive to
alcohol.
Most of the GABA receptors are sensitive to alcohol, but the delta-containing ones are
especially sensitive.
And indeed, sensitive to levels of alcohol that are present in the brain, even during
social drinking.
So it's possible that the delta-containing receptor is mediating some of the effects
of even quite moderate amounts of alcohol.
The control of neuronal function depends upon a balance between what are called excitatory
and inhibitory inputs, so each neuron is working by firing what are called action potentials.
So the activity of an individual neuron depends upon inputs it's getting from its neighbours.
So some of the neurons are releasing transmitters that are excitatory, and they will act on
the receptors and increase the firing rate of the target neuron.
Other neurons are releasing inhibitory messengers, inhibitory transmitters, for example GABA,
which will then bind to the receptor and reduce the firing rate and reduce the activity of
the target neuron.
So normal brain function depends on a controlled balance between these excitatory and inhibitory
effects, and if we go one way or the other, that will produce abnormal brain function.
For example, in epilepsy excitation predominates and therefore all of the neurons are firing
too rapidly.
Then you see the effects of epilepsy, which are things like convulsions.
So we've got to maintain this balance between excitatory and inhibitory input into each
neuron.
And the GABA is the inhibitory messenger.