To answer this, Dr. Barres has developed a "tis- 
sue print" preparation that allows acute isolation of 
optic nerve glia for electrophysiological studies. En- 
zymatically treated tissue is gently pressed against a 
nitrocellulose-coated surface and removed, leaving 
a thin layer of adherent, viable cells still bearing 
^_ processes. Retention of processes is important, be- 
cause some ion channel types appear to be local- 
ized to processes. For instance, sodium channels 
are found in only —20% of dissociated type 1 astro- 
cytes (lacking processes) but in 100% of those in 
tissue prints. Each optic nerve glial cell type has 
now been studied in these tissue prints, and in 
some cases different ion channels are found in 
these cells than were found in vitro. 
The next step in these studies of glial function is 
to reconstitute the in vivo phenotypes in vitro. In 
addition to the culture medium, the presence of 
certain cells may be essential for reconstitution. 
Sodium current is expressed by 100% of type 1 
astrocytes in tissue prints but by only 15% of those 
in culture. However, if type 1 astrocytes are 
cocultured with purified retinal ganglion cells, then 
these astrocytes form processes and, in preliminary 
experiments, all develop sodium currents. 
B. The 02A glial progenitor cell The type 2 as- 
trocyte and oligodendrocyte are generated by a 
common bipotential cell, the 02A progenitor cell. 
The electrophysiological properties of the 02A 
have been studied in a new serum-free culture sys- 
tem developed by Drs. Laura Lillien and Martin 
Raff Dr. Barres has found that 02A progenitors in 
these cultures have many properties characteristic 
of neurons: they express the neuronal form of 
the sodium channel, they fire single regenerative 
potentials, they have glutamate-activated ion 
channels, and they synthesize the neurotransmitter 
7-aminobutyric acid (GABA). Nearly identical 
properties were observed in acutely isolated 02A 
progenitors, indicating that this phenotype is not 
an artifact of culture. The 02A did not express a 
simple subset of channel types found in its descen- 
dant cells, the type 2 astrocyte and the 
oligodendrocyte, studied in the same culture sys- 
tem. Thus during development these active mem- 
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brane properties may contribute to 02A function 
in vivo. 
C. GABA in glia. Although GABA, a major inhibitory 
neurotransmitter in the brain, is thought to be syn- 
thesized solely by neurons. Dr. Barres has recently 
observed that 02A progenitors and type 2 astro- 
cytes in culture express a surface antigen found on 
a subset of GABAergic neurons, recognized by the 
monoclonal antibody VCl.l. Antibodies to GABA 
also labeled these cells. The presence of GABA in 
these cultures was confirmed using high-perfor- 
mance liquid chromatography (HPLC). Because these 
cultures are both neuron- and serum-free and the 
culture medium does not contain GABA, these cells 
appear to be synthesizing it. However, Dr. Barres 
was unable to detect the presence of the GABA syn- 
thetic enzyme, glutamic acid decarboxylase, which 
in neurons converts glutamate to GABA. Instead, 
they appear to synthesize it by an alternative path- 
way that uses putrescine as a precursor. When pu- 
trescine was omitted from the culture medium, 
GABA-like immunoreactivity disappeared from both 
02A progenitors and type 2 astrocytes, and GABA 
could no longer be detected by HPLC. Preliminary 
experiments suggest that this GABA synthesis is not 
an artifact of putrescine-containing tissue culture 
conditions: acutely isolated 02A lineage cells in tis- 
sue prints are labeled by GABA antibodies. 
D. Function. The unique and complex electrophysi- 
ological properties of each optic nerve glial cell 
type and the presence of glutamate-activated ion 
channels and the neurotransmitter GABA in 02A 
progenitors and type 2 astrocytes suggest that glial 
cells— despite their passive reputations— may be ca- 
pable of some surprisingly interactive behavior. 
Since type 2 astrocytes have processes that closely 
appose nodes of Ranvier, it appears plausible that 
some glial types may even participate in local mod- 
ulatory circuits. 
Dr. Corey is also Assistant Physiologist in the 
Department of Neurology, Massachusetts General 
Hospital, and Assistant Professor of Neuroscience at 
Harvard Medical School. 
Articles 
Assad, J.A., Hacohen, N., and Corey, D.R 1989. Voltage dependence of adaptation and active bundle move- 
ment in bullfrog saccular hair cells. Proc Natl Acad Sci USA 86:2918-2922. 
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