Ganong's Review of Medical Physiology, 23rd Edition

132 SECTION II Physiology of Nerve & Muscle Cells
FIGURE 7–2 Four diffusely connected systems of central neurons using modulatory transmitters. A) Norepinephrine-containing neurons.
B) Serotonin-containing neurons. C) Dopamine-containing neurons. D) Acetylcholine-containing neurons. (Reproduced with permission from
Boron WF, Boulpaep EL: Medical Physiology. Elsevier, 2005.)
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 include
transporters for norepinephrine, dopamine, serotonin,
GABA, and glycine, as well as transporters for proline, taurine,
and the acetylcholine precursor choline. In addition, there