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SECTION II
Physiology of Nerve & Muscle Cells
region of the receptor by a specific
β
-adrenergic receptor
kinase
(
β
-ARK)
or binding
β
-arrestins.
Four
β
-arrestins have
been described in mammals. Two are expressed in rods and
cones of the retina and inhibit visual responses. The other two,
β
-arrestin 1 and
β
-arrestin 2, are more ubiquitous. They
desensitize
β
-adrenegic receptors, but they also inhibit other
heterotrimeric G protein-coupled receptors. In addition, they
foster endocytosis of ligands, adding to desensitization.
REUPTAKE
Neurotransmitters are transported from the synaptic cleft
back into the cytoplasm of the neurons that secreted them, a
process referred to as
reuptake
(Figure 7–3). The high-affinity
reuptake systems employ two families of transporter proteins.
One family has 12 transmembrane domains and cotransports
the transmitter with Na
+
and Cl- . Members of this family in-
clude transporters for norepinephrine, dopamine, serotonin,
GABA, and glycine, as well as transporters for proline, taurine,
and the acetylcholine precursor choline. In addition, there
FIGURE 7–2
Four diffusely connected systems of central neurons using modulatory transmitters. A)
Norepinephrine-containing neu-
rons.
B)
Serotonin-containing neurons.
C)
Dopamine-containing neurons.
D)
Acetylcholine-containing neurons.
(Reproduced with permission from
Boron WF, Boulpaep EL:
Medical Physiology.
Elsevier, 2005.)