Some Possible Excitatory Neurotransmitters

The chemical neurotransmitter at the neuromuscular junction is ACh, as discussed in Chapter 8. Acetylcholine is also used as a neurotransmitter at some neuron-to-neuron synapses. In addition, many other substances act as neurotransmitters at excitatory synapses in the nervous system. The molecular structures of a representative sample of excitatory neurotransmitter substances are shown in Figure 9-5. Many excitatory neurotransmitters are relatively small molecules, often derived from amino acids by simple chemical

20 mV

30 msec

Figure 9-3 Intracellular recordings of e.p.s.p.'s in a neuron, showing summation of successive e.p.s.p.'s. Each set of traces shows three superimposed responses. The arrow indicates the electrical stimulus used to trigger action potentials in the presynaptic neurons that make excitatory synapses onto the recorded cell. Trace a in each set shows a stimulus that failed to trigger the presynaptic input. Trace b shows e.p.s.p.'s that failed to reach threshold. Trace c shows summated e.p.s.p.'s that reach threshold and produce a postsynaptic action potential. In this figure, the postsynaptic cell is a motor neuron of the sympathetic nervous system (which is described in Chapter 11). (Data provided by H.-S. Wang and D. McKinnon of the State University of New York at Stony Brook.)

modifications. Amino acids are more commonly thought of as the basic building blocks for the construction of proteins. In the nervous system, however, amino acids are also often used for cell-to-cell signaling in neurotransmission. For example, glutamate and aspartate are unmodified amino acids, norepinephrine and dopamine are derived from the amino acid tyrosine, and serotonin is derived from tryptamine. Glutamate is thought to be the excitatory transmitter at the synapse between the sensory and motor neurons in the patellar reflex.

Other neurotransmitters are more structurally complex than the small amino-acid derivatives. These substances called peptide neurotransmit-ters, or more simply neuropeptides are formed from a series of individual amino acids linked by peptide bonds, like a small piece of a protein molecule. Indeed, neuropeptides are synthesized by neurons as larger protein precursors, which are then processed proteolytically to release the embedded neuropeptide fragment. An example of a neuropeptide is substance P, whose amino-acid sequence is shown in Figure 9-5.

The list of excitatory neurotransmitters in Figure 9-5 is by no means exhaustive. As our knowledge of the brain grows, it is likely that new candidates will be added to the list.

Em of motor neuron (mV)

Postsynaptic action potential

Postsynaptic action potential

Stimulate action potential in one sensory neuron

Stimulate action potentials in all four sensory neurons

Stimulate action potential in one sensory neuron

Stimulate action potentials in all four sensory neurons

Figure 9-4 Spatial summation of excitatory inputs to a motor neuron. (a) A diagram of the synaptic circuitry and recording arrangement. (b) An illustration of synaptic responses in the postsynaptic motor neuron.

Norepinephrine

Serotonin (5-hydroxytryptamine)

Serotonin (5-hydroxytryptamine)

Norepinephrine

0 H

H

0

-NH2

\ I

/

0—0 —

0 —

0

/ I

\

HO H

NH2

OH

Aspartic acid

(Aspartate)

H

HO H

s

\ /

I

H

H

HoV/ V

-C —

■0 — I

)-(

H

H

H H

Dopamine

Arg-Pro-Lys-Pro-Gln-GIn- Phe-Phe-Gly-Leu-Met-NH,, Substance P

(a string of 11 amino acids attached by peptide bonds)

Arg-Pro-Lys-Pro-Gln-GIn- Phe-Phe-Gly-Leu-Met-NH,, Substance P

(a string of 11 amino acids attached by peptide bonds)

Figure 9-5 Structures of some excitatory neurotransmitter substances in the nervous system.

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