High-Yield Neuroanatomy 5e

164 Chapter 20
2. Enkephalins are the most widely distributed and abundant opiate peptides. They are found in the
highest concentration in the globus pallidus. They play a role in pain suppression.
3. Dynorphins are opioid peptides that are synthesized widely throughout the CNS. They moderate
pain response.
E. Nonopioid Neuropeptides
1. Substance P is a neuropeptide that functions in pain modulation and in signaling the intensity of
noxious stimuli, more recently its role in psychologic stress has been described. It plays a role in
movement disorders—substance P levels are reduced in patients with Huntington disease.
2. Somatostatin (somatotropin-release–inhibiting factor). Somatostatinergic neurons from
the anterior hypothalamus project their axons to the median eminence, where somatostatin enters
the hypophyseal portal system and regulates the release of growth hormone and thyroidstimulating
hormone. The concentration of somatostatin in the neocortex and hippocampus
is significantly reduced in patients with Alzheimer disease. Striatal somatostatin levels are
increased in patients with Huntington disease.
F. Amino Acid Transmitters
1. Inhibitory amino acid transmitters
a. GABA (Figure 20-6) is the major inhibitory neurotransmitter of the brain. Purkinje, stellate,
basket, and Golgi cells of the cerebellar cortex are GABA-ergic.
i. GABA-ergic striatal neurons project to the globus pallidus and substantia nigra.
ii. GABA-ergic pallidal neurons project to the thalamus.
iii. GABA-ergic nigral neurons project to the thalamus.
iv. GABA receptors (GABA-A and GABA-B) are intimately associated with benzodiazepinebinding
sites. Benzodiazepines enhance GABA activity.
b. Glycine is a major inhibitory neurotransmitter of the spinal cord, brainstem, and retina. It is
used by the Renshaw cells of the spinal cord.
2. Excitatory amino acid transmitters
a. Glutamate (Figure 20-7) is the major excitatory neurotransmitter of the CNS. Neocortical
glutamatergic neurons project to the striatum, subthalamic nucleus, and thalamus.
i. Glutamate is the transmitter of the cerebellar granule cells, of nonnociceptive, large, primary
afferent fibers that enter the spinal cord and brainstem, and of the corticonuclear and corticospinal
tracts.
Figure 20-6 Distribution of γ-aminobutyric acid (GABA)-containing neurons and their projections. GABA-ergic neurons
are the major inhibitory cells of the central nervous system. GABA local circuit neurons are found in the neocortex,
hippocampal formation, and cerebellar cortex (Purkinje cells). Striatal GABA-ergic neurons project to the thalamus and
subthalamic nucleus (not shown).