[Catalyst 2017]

astrocytes
SHINING THE SPOTLIGHT ON THE BRAIN’S RISING STAR
KELSEY SAnDERS
WE HAVE WITHIN US the most complex
and inspiring stage to ever be set: the
human brain. The cellular components
of the brain act as players, interacting
through chemical and electrical signaling
to elicit emotions and convey information.
Although most of our attention has in the
past been focused on neurons, which were
erroneously presumed to act alone in their
leading role, scientists are slowly realizing
that astrocytes—glial cells in the brain that
were previously assumed to only have a
supportive role in association with neurons—
are so much more than merely supporting
characters .
Though neurons are the stars, most of the
brain is actually composed of supportive cells
like microglia, oligodendrocytes, and, most
notably, astrocytes. Astrocytes, whose formal
name is a misnomer given that modern
imaging technology reveals they actually
maintain a branch-like shape rather than a
star-like one, exist as one of three mature
types in the grey matter, white matter, or
retina. Structurally, the grey matter astrocyte
variant exhibits bushy, root-like tendrils and
a spherical shape. The white matter variant,
commonly found in the hippocampus, favors
finer extensions called processes. The retinal
variant features an elongated structure.¹
Functionally, astrocytes were previously
believed to play a solely supportive role, as
they constitute a large percentage of the
glial cells present in the brain. Glial cells are
essentially all of the non-neural cells in the
brain that assist in basic functioning; they
themselves are not electrically excitable.
However, current research suggests that
astrocytes play far more than merely a
supporting role in the brain. Astrocytes and
neurons directly interact to interpret stimuli
and store memories⁴, among many other yet
undiscovered tasks.
Although astrocytes are not electrically
excitable, astrocytes communicate with
neurons via calcium signaling and the
neurotransmitter glutamate.² Calcium
signaling works whereby intracellular
calcium in astrocytes is released upon
excitation and is propagated in waves that
move through neighboring astrocytes and
neurons. Neurons experience a responsive
increase in intracellular calcium if they are
directly touching affected astrocytes, as the
signal is communicated via gap junctions
rather than synaptically. Such signaling is
unidirectional; calcium excitation can move
from astrocyte to neuron, but not from
neuron to astrocyte.³ The orientation of
astrocytes in different regions of the brain
and their proximity to neurons allows them
to form close communication networks
that help information travel throughout the
central nervous system.
Astrocytes in the hippocampus play a role
in memory development. They act as an
intermediary cell in a neural inhibitory circuit
that utilizes acetylcholine, glutamate, and
Gamma-Aminobutyric Acid (GABA) to solidify
experiential learning and memory formation.
Disruption of cholinergic signaling, signaling
relating to acetylcholine, prohibits the
formation of memories in the dentate gyrus
of the hippocampal formation. Astrocytes act
as mediators that convert cholinergic inputs
into glutamatergic activation of neurons.⁴
Without the assistance of astrocytic networks
in close association with neurons, memory
formation and long-term potentiation would
be far less efficient if even still possible.
Astrocytes’ ability to interpret and release
chemical neurotransmitters, especially
glutamate, allows them to regulate the
intensity of synaptic firing in neurons.⁵
Increased glutamate uptake by astrocytes
reduces synaptic strength in associated
neurons by decreasing neuronal
concentration of glutamate.⁶ Regulation
of synaptic strength in firing is crucial
for healthy brain function. If synapses
fire too much or too powerfully, they
may overwhelm the brain. Conversely,
if synapses fire too infrequently or not
strongly enough, messages might not make
their way throughout the central nervous
system. The ability of astrocytes to modulate
synaptic activity through selective glutamate
interactions puts them in an integral
position to assist in consistent and efficient
transmission of information throughout the
human body.
Through regulation of neurotransmitters
and psychoactive chemicals in the brain,
astrocytes are able to maintain homeostasis
in the central nervous system. Potassium
buffering and balancing of pH are the major
ways that astrocytes assist in maintaining
optimal conditions for brain function.⁷
Astrocytes are able to compensate for the
slow re-uptake of potassium by neurons,
thus decluttering the extracellular space
of free potassium in response to neuronal
activity. Re-uptake of these ions is extremely
important to brain function as synaptic
transmission by neurons relies on electrically
switching membrane potentials along
neuronal axons.
Due to their role in synaptic regulation and
their critical position in the brain network,
astrocytes also have the potential to aid
in therapies for dealing with neurological
disorders. For example, epileptic seizures
have been found to be related to an
excitatory loop between neurons and
astrocytes. Focal ictal discharges, the brain
activity responsible for epileptic seizures,
are correlated to hyperactivity in neurons
as well as an increase in intracellular
calcium in nearby astrocytes; the calcium
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