Amino Acids as Neurotransmitters

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Excitatory Neurotransmitters

The amino acids glutamic acid and aspartic acid function as excitatory neurotransmitters in the CNS. Glutamic acid (or glutamate), indeed, is the major excitatory neurotransmitter in the brain, producing excitatory postsynaptic potentials (EPSPs). Research has revealed that each of the glutamate receptors encloses an ion channel, similar to the arrangement seen in the nicotinic ACh receptors (see fig. 7.23).

Among these EPSP-producing glutamate receptors, three subtypes can be distinguished. These are named according to the molecules (other than glutamate) that they bind, and include:

(1) NMDA receptors (named for N-methyl-D-aspartate);

(2) AMPA receptors; and (3) kainate receptors. NMDA and AMPA receptors are illustrated in chapter 8, figure 8.15.

The NMDA receptors for glutamate are involved in memory storage, as will be discussed more fully in the section on long-term potentiation. These receptors are quite complex, because the ion channel will not open simply by the binding of glutamate to its receptor. Instead, two other conditions must be met at the same time: (1) the NMDA receptor must also bind to glycine (or D-serine, which has recently been shown to be produced by astrocytes); and (2) the membrane must be partially depolarized at this time by a different neurotransmitter molecule that binds to a different receptor (for example, by glutamate binding to the AMPA receptors). Once open, the NMDA receptor channels permit the entry of Ca2+ and Na+ (and exit of K+) into the dendrites of the postsynaptic neuron.

Inhibitory Neurotransmitters

The amino acid glycine is inhibitory; instead of depolarizing the postsynaptic membrane and producing an EPSP, it hyperpolar-izes the postsynaptic membrane and produces an inhibitory postsynaptic potential (IPSP). The binding of glycine to its receptor proteins causes the opening of chloride (Cl-) channels in the postsynaptic membrane. As a result, Cl- diffuses into the postsynaptic neuron and produces the hyperpolarization. This inhibits the neuron by making the membrane potential even more negative than it is at rest, and therefore farther from the threshold depolarization required to stimulate action potentials.

The inhibitory effects of glycine are very important in the spinal cord, where they help in the control of skeletal movements. Flexion of an arm, for example, involves stimulation of the flexor muscles by motor neurons in the spinal cord. The motor neurons that innervate the antagonistic extensor muscles are inhibited by IPSPs produced by glycine released from other neurons. The importance of the inhibitory actions of glycine is revealed by the deadly effects of strychnine, a poison that causes spastic paralysis by specifically blocking the glycine receptor proteins. Animals poisoned with strychnine die from asphyxiation because they are unable to relax the diaphragm.

The neurotransmitter gamma-aminobutyric acid (GABA) is a derivative of another amino acid, glutamic acid. GABA is the most prevalent neurotransmitter in the brain; in fact, as many as one-third of all the neurons in the brain use GABA as a neuro-transmitter. Like glycine, GABA is inhibitory—it hyperpolarizes the postsynaptic membrane by opening Cl- channels. Also, the effects of GABA, like those of glycine, are involved in motor control. For example, the large Purkinje cells mediate the motor functions of the cerebellum by producing IPSPs in their post-synaptic neurons. A deficiency of GABA-releasing neurons is responsible for the uncontrolled movements seen in people with Huntington's chorea.

g Benzodiazepines are drugs that act to increase the ability of GABA to activate its receptors in the brain j and spinal cord. Since GABA inhibits the activity of

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