Drugs That Alter Neurotransmitter Levels
Mechanism oF Action
Stimulates neurotransmitter synthesis
Decreases packaging of neurotransmitter into vesicles
Decreases blood pressure
Blocks receptor binding
Enhances receptor binding
Sense of pleasure
Blocks enzymatic degradation of neurotransmitter in presynaptic cell
Dual reuptake inhibitors
*Others may be aF
ected as well.
Events Leading to
1. Action potential passes along an axon and over the surface of its
2. Synaptic knob membrane becomes more permeable to calcium
ions, and they diF
3. In the presence of calcium ions, synaptic vesicles fuse to synaptic
4. Synaptic vesicles release their neurotransmitter by exocytosis into
the synaptic cleft.
5. Synaptic vesicles become part of the membrane.
6. The added membrane provides material for endocytotic
To Chapter 3, Exocytosis, page 97.
After a vesicle releases its neurotransmitter, it becomes
part of the cell membrane. Endocytosis eventually returns
it to the cytoplasm, where it can provide material to form
new secretory vesicles.
summarizes this process,
called vesicle trafF
To keep signal duration short, enzymes in synaptic
clefts and on postsynaptic membranes rapidly decompose
some neurotransmitters. The enzyme
for example, decomposes acetylcholine on postsynaptic
membranes. Other neurotransmitters are transported back
into the synaptic knob of the presynaptic neuron or into
nearby neurons or neuroglia, a process called
inactivates the mono amine
neurotransmitters epinephrine and norepinephrine after
reuptake. This enzyme is found in mitochondria in the syn-
aptic knob. Destruction or removal of neurotransmitter pre-
vents continuous stimulation of the postsynaptic neuron.
Neurons in the brain or spinal cord synthesize
These peptides act as neurotransmitters or as
—substances that alter a neuron’s response to a
neurotransmitter or block the release of a neurotransmitter.
Among the neuropeptides are the
throughout the brain and spinal cord. Each enkephalin mol-
ecule is a chain of F ve amino acids. Synthesis of enkephalins
increases during periods of painful stress, and they bind
to the same receptors in the brain (opiate receptors) as the
narcotic morphine. Enkephalins relieve pain sensations and
probably have other functions. Another morphinelike pep-
is found in the brain and cerebrospinal
uid. It acts longer than enkephalins and is a much more
potent pain reliever (Clinical Application 10.4).
is a neuropeptide that consists of eleven
amino acids and is widely distributed. It functions as a neu-
rotransmitter (or perhaps as a neuromodulator) in the neu-
rons that transmit pain impulses into the spinal cord and on
to the brain. Enkephalins and endorphins may relieve pain
by inhibiting the release of substance P from pain-transmit-
The way the nervous system processes and affects nerve
impulses reﬂ ects, in part, the organization of neurons and
axons in the brain and spinal cord.
Interneurons, the neurons completely in the CNS, are orga-
These are groups of neurons that