TRANSMITTERS AND RECEPTORS FOR LIGAND GATED CHANNELS
In the previous chapters we considered the effects that opening ligand gated channels have on
the membrane potentials of neurons. In that chapter we also pointed out that the brain utilizes a
wide variety of neurotransmitters, i.e., ligands that bind to the receptors.
There are, however, only
three primary transmitters that directly open ligand gated channels in the brain.
One of the
transmitters, glutamate, is the universal excitatory transmitter.
Some type of glutamate receptor is
found in almost all neurons in almost all regions of the brain.
Two other transmitters, gamma
amino butyric acid (GABA) and glycine, are the primary inhibitory neurotransmitters in the brain.
Below we discuss some additional features of the operation of the ligand gated channels, where we
first consider glutamate receptors and then turn to some features of GABA receptors.
ACh receptors are directly gated ligand channels at the neuromuscular junction and also occur in
the central nervous system.
However, they are not nearly as common as glutamatergic ligand gated
More often, ACh receptors in the brain are muscarinic rather than nicotinic.
binds to muscarinic receptors, the ACh does not directly open channels but acts through second
messenger systems as discussed in the next chapter.
General comments about receptors and the channels they open
It is important to point out that there are two separate parts that comprise each ligand-gated
The first part is the receptor itself, the part of the channel that actually binds the
The second part is the pore, through which the ions pass when the transmitter
opens the channel.
Hence these kinds of receptors are also known as
when a ligand binds to the receptor, an ion channel, which is part of the receptor, opens.
There are two reasons for distinguishing between these two parts of a channel.
is that receptors that bind a particular transmitter may be different (Fig. 1).
That is, a receptor that
binds ACh, for example, can have different subunits than does another receptor that also binds
As discussed below, the same is true for glutamate as well as for other neurotransmitters.
discussed previously, the various types of receptors are distinguished by the way they react to
As an example, in the previous chapter we pointed out that there are two kinds of
ACh receptors: nicotinic ACh receptors that are found at the neuromuscular junction (among other
places) and muscarinic ACh receptors that are found in the heart.
They both bind and respond to
ACh, but the receptors on each channel are different since one type is opened by the drug nicotine
while the other is opened by the drug muscarine.
We shall have more to say about nicotinic and
muscarinic ACh channels in the next chapter.
For now, the important point is that the same
transmitter can bind to different kinds of receptors.