Glial Neuronal Interactions in the Mammalian Brain

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Glial/ Neuronal Interactions in the Mammalian
Brain Glenn I. Hatton Department of Cell Biology
Neuroscience University of California, Riverside
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Introduction Glial cells in the mammalian CNS
come in a variety of types, shapes and sizes.
There are astroglia, ependymoglia, microglia and
oligodendroglia, and at least some variation
within each of these. Most variant, most
intimately associated with all parts of neurons
and, thus, most interesting functionally are the
astroglia or astrocytes. For these reasons, this
presentation will focus mainly on
astrocyte-neuron interactions.
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3. Electron micrograph showing a single astrocyte
in intimate contact with axon terminals (At)
making asymmetric synapses with four spines (Sp).
(From Peters et al. The Fine Structure of the
Nervous System, 3rd ed. 1991, Oxford, p. 165.)
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4. Electron micrograph of neurovascular contact
zone showing pituitary astrocytes (P) and neural
terminals (T, arrow) occupying the basal lamina
surrounding the perivascular space (PV).
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5. Primary astrocyte cultures immunostained for
GFAP. A. Flattened confluent control culture. B.
Process-bearing appearance was produced by 30 min
treatment of this culture with 100 nM
epinephrine, a pure b2 agonist. Effect was
blocked by IPS339, b2 antagonist.
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Morphological plasticity Astrocyte shape changes
and glial/neuronal interactions Model system
Function-related glial-neuronal morphological
plasticity is best illustrated by the model
system in which the phenomenon was discovered
the magnocellular neuroendocrine system of the
hypothalamus. The next series of slides gives an
overview of this system in the rat.
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6. Diagram of rat brain cut at the level of the
supraoptic (SON) and paraventricular (PVN) nuclei
showing the principal axonal projections of the
magnocellular neurons to the neurohypophysis.
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7. Coronal section through the SON and PVN
illustrating the dense packing of the
magnocellular neurons in these nuclei. OC
posterior optic chiasm scale bar 500 µm.
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8. Diagram of the SON and the pial-glial limitans
in coronal plane. Profiles representing somata
are lateral to the myelinated fibers of the optic
tract (OT) in the somatic zone (SZ). Ventral to
the SZ are the parallel-projecting dendrites
(unfilled small circles), depicted in cross
section, and constituting the dendritic zone
(DZ). Mingling with only the most ventral
dendrites are the astroglial cell nuclei (larger
filled circles), whose ventrally projecting
processes (shown in Figure 11) fill the clear
space between the basal lamina (small arrows) and
the most ventral dendrites. These glial cell
bodies and processes constitute the ventral glial
lamina (VGL). Dorsally projecting processes from
these glia fill most of the space not occupied by
the somata and dendrites. Ventrolaterally
projecting dendrites are not included here. Pia
mater is indicated by open arrows.
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9. Electron micrograph showing the extensive
astroglial membrane in the ventral glial lamina
of the SON. Dendrites (d), some of which are in
bundles, are cut in cross section. As astrocyte
nucleus. Bar 2 µm.
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10. GFAP-ir astrocytes and astrocytic-neuronal
appositions. A GFAP-ir (green) near the SON,
indicated by the presence of oxytocin-ir neurons
(red), and located in a rich astrocytic area. B
Astrocytic-neuronal appositions in the SON, under
conditions of low demand for the release of
peptides. Somata of these astrocytes are located
in the vgl. N magnocellular neuron, As
astrocytic process. C GFAP-ir astrocytes and
vimentin-ir cells in the SON and underlying
meninges. Note the presence of both vimentin-ir
and GFAP-ir in the processes of some astrocytes
(arrowheads). c capillaries. E Vimentin-ir
only, in the bottom part of C. Brain surface is
indicated by white lines. D New model of
organization including the S100b meningeal,
vascular and parenchymal cellular organization.
BL basal lamina. Scale bars. A 100 mm, B 20
mm, C 50 µM.
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11. Electron micrographs of SON neurons
illustrating the changes in somasomatic direct
apposition that occur with activation of this
system. A. Day 1 postpartum small direct
apposition (open arrow). B. Lactating animal
large areas of apposition (filled arrows).
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12. Electron micrographs of SON dendritic zones
of animals under basal (A) and activated (B)
conditions. In A, dendrites tend to be separated
from one another and wrapped by glial processes
(arrows). In B, dendritic membrane tends to be in
apposition with that of other dendrites, forming
dendritic bundles. Those dendrites with similar
numbers are in bundles. A multiple synapse is
indicated (arrows). As astrocyte nucleus. Bar
2 µm.
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13. Longitudinal section of SON dendrite (D)
containing neurosecretory granules. At the
plasmalemmal surface can be seen exocytotic
figures (arrows) indicating release of peptide.
Bar 1µm. (From Pow and Morris Neuroscience
1989, 32435-439.)
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14. Electron micrographs of SON neurons showing
several types of synapses common in lactating but
rare in virgin rats. a. Terminal containing dense
core (arrow) and clear vesicles and
simultaneously synapsing onto two somata
(arrowheads). b. Axodendritic and axosomatic
multiple synapse. c. Somasomatic membrane
specialization (open arrow) and axon terminals
with dense core and clear vesicles. d. Axon
terminal making synaptic contact with one soma
and separated from another only by a thin
astrocytic process. From Hatton Tweedle, 1982.
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15. Electron micrographs of taurine-like IR in
thin glial processes in the SON somatic zone. A.
Electron-dense particulate labeling can be
followed in one of the thin layered glial
processes (arrows) interposed between two
immunonegative magnocellular neuronal perikarya
(P). Bar 380 nm. B. A non-IR bouton (b)
contacting a neuronal perikaryon (P) is
surrounded by IR glial processes (). One of
these immunoreactive glial processes is also
located around a somatic spine (arrow). Bar 250
nm. From Decavel Hatton, 1995.
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16. Electron micrograph of a taurine-immunoreactiv
e astrocyte completely surrounding an axon (ax)
and its bouton (b) making contact with a
supraoptic dendrite (D). Visible in this
astrocyte are glial filaments (gf). Note that
although the astrocyte is replete with taurine,
the dendrite and the afferent process contain
little or none. GSSP gold substituted silver
periodate. Bar 290 nm. From Decavel Hatton,
1995.
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17. Electron micrographs of neurohypophysial
astrocytes from prepartum (A) and postpartum rats
(B). Note the neurosecretory axon profiles ()
surrounded by glial cytoplasm in A. The astrocyte
in B engulfs no axon profiles, some of which
contact the basal lamina (arrowheads) and are
devoid of secretory granules. For clarity, the
membranes of the astrocytes have been highlighted.
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18. High power electron micrograph of a
neurosecretory axon containing both dense cored
vesicles and clear microvesicles, making a
synaptoid contact with an astrocyte (P). Bar
224 nm. From Hatton, 1999.
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Some functional implications of morphological
plasticity Effects on spatial buffering of
Ko, neuron-neuron interactions (new synapses,
gap junction formation, efficacy of neuroactive
factors released from dendrites, etc.), glutamate
uptake from the synapse and thus, excitability.
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Direct astroglial/neuronal signaling Since
astrocytes have receptors for many
neurotransmitters and neuromodulators,
neuronal/glial signaling is well established.
What is new, is that the astrocytes have now been
shown to release neuroeffector molecules. This
then establishes the glial/neuronal signaling
pathway. In addition to the astrocytic release of
taurine mentioned earlier, astrocytes have now
been shown to release glutamate in response to a
variety of signaling molecules that induce
significant rises in intracellular calcium. The
next series of slides illustrates this signaling
pathway.
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19. Cultures of forebrain astrocytes loaded with
the fluorescent indicator, fura-2 and ratio
imaged for calcium response to bradykinin (10 nM)
stimulation. Ordinate ratio responses to two
excitation wavelengths, 240 and 280 nm.
Yellow-red pseudocolor 1 µM Ca2.
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20. Caged calcium (NP EGTA). Method for
selectively raising intracellular Ca2 in
astrocytes and direct measurement of effects on a
neuron placed on a small group of astrocytes.
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21. Neuronal inward currents (lower trace) and
glial intracellular Ca2 (upper trace) produced
by photolysis of caged Ca2 producing two levels
of intracellular Ca2 in the astrocytes.
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22. Intracellular Ca2 rises in astrocytes
stimulated with dopamine or norepinephrine.
Glutamate release from astrocytes is accomplished
by intracellular Ca2 rises that are well
within the physiological range.
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23. Neuronal inward currents in response to
raised intracellular Ca2 in associated
astrocytes are blocked by glutamate receptor
antagonists.
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24. Bradykinin (10 nM) releases measurable
quantities of glutamate from astrocytes.
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Conclusions Glial/neuronal interactions are a
two-way street, as it were, with signaling going
in both directions, and probably of nearly equal
importance. Recent evidence suggests that
astrocytes engage in complex feedback
relationships with various parts of neurons,
which itself implies some compartmentalization of
the neuron. Such feedback loops have the
potential to be accomplished not only via
receptor-mediated ion fluxes (e.g., Ca2 or K),
but also by synthesis and release of effector
molecules for which the neuronal elements have
receptors. Further research is clearly needed to
help us understand this fascinating dialogue.
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