New Chapt 13-direct gated channels(1)

New Chapt 13-direct gated channels(1) - CHAPTER 13...

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127 CHAPTER 13 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. (Nicotinic 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 channels. More often, ACh receptors in the brain are muscarinic rather than nicotinic. When ACh 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 channel. The first part is the receptor itself, the part of the channel that actually binds the neurotransmitter. 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 ionotropic receptors, because 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. The first reason 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 ACh. As discussed below, the same is true for glutamate as well as for other neurotransmitters. As discussed previously, the various types of receptors are distinguished by the way they react to certain drugs. 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. The s
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This note was uploaded on 09/19/2011 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas.

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New Chapt 13-direct gated channels(1) - CHAPTER 13...

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