162 | MARCH 2003 | VOLUME 4 www.nature.com/reviews/neuro
H I G H L I G H T S
Don't call us, we'll call you
We've heard it before: mobile
phones aren't good for your
brain. First it was a risk of
developing tumours, and now
it seems that mobile phones
can kill neurons. Referring to
work by Leif Salford and his
colleagues (Lund University,
Sweden), BBC News (UK,
5 February 2003) reported that
"mobile phones damage key
brain cells and could trigger
the early onset of Alzheimer's
disease." In the study,
published in Environmental
Health Perspectives, Salford
and his group exposed rats to
two hours of radiation at a
level equivalent to that emitted
by mobile phones. Fifty days
later, the scientists "found
long exposure to operating
handsets destroys cells in
areas of the brain important
for memory, movement and
learning, and fear it could
cause the premature onset of
illnesses usually linked to
ageing" (The Herald Sun,
Australia, 7 February 2003).
Speaking to BBC News,
Salford is quoted as saying
that mobile phones might
have the same effect in
people, "A rat's brain is very
much the same as a human's.
They have the same blood­
brain barrier and neurons."
And in a statement that will
horrify teenagers around the
world, he added, "maybe we
should think about restricting
our use of mobile phones."
Not surprisingly, the mobile
phone industry received the
news with scepticism. In the
UK, "a spokeswoman for the
Mobile Operators Association
dismissed this latest study"
(BBC News). In South Africa,
the chairman of the Cellular
Telecommunications
Association is quoted as
saying that "governments
worldwide had adopted
comprehensive international
safety guidelines" for the
operation of mobile phones
(The Mercury, South Africa,
6 February 2003), a statement
that will probably not reassure
all consumers. Just in case, it
might be better to stick to text
messaging for now, at least
until someone models its
effects on rat's paws.
Juan Carlos López
IN THE NEWS
The development of imaging systems involving
calcium-sensitive fluorescent dyes has provided an
unprecedented opportunity to observe the activity of
neurons and circuits in real time.In a report in Cell,
Wang et al.describe how they have used a dye called
G-CaMP to study the relationship between structure
and function in the Drosophila olfactory system.
In Drosophila,each olfactory sensory neuron
expresses one of around 80 different odorant receptor
subtypes.Projections from neurons that express the
same receptor converge in structures called glomeruli
in the antennal lobe.The glomeruli are innervated by
the dendrites of projection neurons,which relay
information to the mushroom bodies and
protocerebrum.The fly olfactory system has become a
popular model for studying olfactory coding because it
is much simpler and more accessible than that of
vertebrates,yet the glomerular anatomy of the primary
relay centres is strikingly similar.
Wang et al. imaged the heads of flies in which either
the projection neurons or the sensory neurons
expressed the G-CaMP protein.The fluorescent
intensity of this protein reflects the intracellular
calcium level (a signature of electrical activity),and the
authors detected the fluorescence using two-photon
microscopy.This sensitive detection
system enabled them to generate a
high-resolution map of the
glomeruli that were activated by
different odours at concentrations
that the fly would encounter in its
natural environment.
The authors showed that each
odour activated a specific
combination of glomeruli.The
response patterns were highly
reproducible,not only between
different trials in the same fly,but
also between different flies.
Interestingly,imaging of sensory
and projection neurons produced
the same odour-evoked patterns of
glomerular activity,indicating that
the pattern generated by the
stimulation of sensory neurons is
transmitted intact to higher
processing centres in the brain.
Wang et al. also used this
imaging technique to examine the
molecular basis of olfactory coding
in the fly.Sensory neurons that
express the or43a receptor gene
project to the DA4 glomerulus,and
the authors identified a range of
odours that activate DA4,but not
another glomerulus,VA1lm.However,when they
expressed or43a ectopically in the neurons that project
toVA1lm,this glomerulus now responded to the same
range of odours as DA4.This implies that the response
patterns of individual glomeruli are probably
determined by single receptor subtypes.
By combining calcium imaging with two-photon
microscopy,Wang et al. have generated a model to test
various principles of olfactory coding in flies,many of
which might also be relevant to vertebrates.In addition
to providing new insights into olfaction,this imaging
technique is also likely to have more general
applications for measuring neuronal activity in the fly
brain in relation to various behaviours.
Heather Wood
References and links
ORIGINAL RESEARCH PAPER Wang, J. W. et al. Two-photon calcium
imaging reveals an odor-evoked map of activity in the fly brain. Cell 112,
271­282 (2003)
FURTHER READING Nakai, J. et al. A high signal-to-noise Ca2+
probe
composed of a single green fluorescent protein. Nature Biotech. 19, 137­141
(2001) | Vosshall, L. B. Olfaction in Drosophila. Curr. Opin. Neurobiol. 10,
498­503 (2000)
WEB SITES
Axel lab: http://cpmcnet.columbia.edu/dept/neurobeh/axel/overview.html
Flybase: http://flybase.bio.indiana.edu/
or43a
Movies online: http://www.cell.com/cgi/content/full/112/2/271/DC1
Seeing how flies smell
OLFACTO RY CODI NG
A fly antennal lobe, in which G-CaMP -- a calcium-sensitive green fluorescent protein -- is expressed only
in projection neurons that innervate the lobe. The high signal-to-noise ratio of G-CaMP provides a
representation, at cellular resolution, of a defined population of neurons in the brain as the fly is stimulated by
odorants at physiological concentrations. Different odours elicit different patterns of activation in the antennal
lobe. Courtesy of J. W. Wang, Center for Neurobiology and Behavior, Columbia University, New York, USA.