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 youWe’ve heard it before: mobilephones aren’t good for yourbrain. First it was a risk ofdeveloping tumours, and nowit seems that mobile phonescan kill neurons. Referring towork by Leif Salford and hiscolleagues (Lund University,Sweden), BBC News (UK, 5 February 2003) reported that“mobile phones damage keybrain cells and could triggerthe early onset of Alzheimer’sdisease.” In the study,published in EnvironmentalHealth Perspectives, Salfordand his group exposed rats totwo hours of radiation at alevel equivalent to that emittedby mobile phones. Fifty dayslater, the scientists “foundlong exposure to operatinghandsets destroys cells inareas of the brain importantfor memory, movement andlearning, and fear it couldcause the premature onset ofillnesses usually linked toageing” (The Herald Sun,Australia, 7 February 2003).

Speaking to BBC News,Salford is quoted as sayingthat mobile phones mighthave the same effect inpeople, “A rat’s brain is verymuch the same as a human’s.They have the same blood–brain barrier and neurons.”And in a statement that willhorrify teenagers around theworld, he added, “maybe weshould think about restrictingour use of mobile phones.”

Not surprisingly, the mobilephone industry received thenews with scepticism. In theUK, “a spokeswoman for theMobile Operators Associationdismissed this latest study”(BBC News). In South Africa,the chairman of the CellularTelecommunicationsAssociation is quoted assaying that “governmentsworldwide had adoptedcomprehensive internationalsafety guidelines” for theoperation of mobile phones(The Mercury, South Africa, 6 February 2003), a statementthat will probably not reassureall consumers. Just in case, itmight be better to stick to textmessaging for now, at leastuntil someone models itseffects on rat’s paws.

Juan Carlos López

IN THE NEWS

The development of imaging systems involvingcalcium-sensitive fluorescent dyes has provided anunprecedented opportunity to observe the activity ofneurons 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 structureand function in the Drosophila olfactory system.

In Drosophila, each olfactory sensory neuronexpresses one of around 80 different odorant receptorsubtypes. Projections from neurons that express thesame receptor converge in structures called glomeruliin the antennal lobe. The glomeruli are innervated bythe dendrites of projection neurons, which relayinformation to the mushroom bodies andprotocerebrum. The fly olfactory system has become apopular model for studying olfactory coding because itis much simpler and more accessible than that ofvertebrates, yet the glomerular anatomy of the primaryrelay centres is strikingly similar.

Wang et al. imaged the heads of flies in which eitherthe projection neurons or the sensory neuronsexpressed the G-CaMP protein. The fluorescentintensity of this protein reflects the intracellularcalcium level (a signature of electrical activity), and theauthors detected the fluorescence using two-photonmicroscopy. This sensitive detectionsystem enabled them to generate ahigh-resolution map of theglomeruli that were activated bydifferent odours at concentrationsthat the fly would encounter in itsnatural environment.

The authors showed that eachodour activated a specificcombination of glomeruli. Theresponse patterns were highlyreproducible, not only betweendifferent trials in the same fly, butalso between different flies.Interestingly, imaging of sensoryand projection neurons producedthe same odour-evoked patterns ofglomerular activity, indicating thatthe pattern generated by thestimulation of sensory neurons istransmitted intact to higherprocessing centres in the brain.

Wang et al. also used thisimaging technique to examine themolecular basis of olfactory codingin the fly. Sensory neurons thatexpress the or43a receptor geneproject to the DA4 glomerulus, andthe authors identified a range ofodours that activate DA4, but not

another glomerulus, VA1lm. However, when theyexpressed or43a ectopically in the neurons that projectto VA1lm, this glomerulus now responded to the samerange of odours as DA4. This implies that the responsepatterns of individual glomeruli are probablydetermined by single receptor subtypes.

By combining calcium imaging with two-photonmicroscopy, Wang et al. have generated a model to testvarious principles of olfactory coding in flies, many ofwhich might also be relevant to vertebrates. In additionto providing new insights into olfaction, this imagingtechnique is also likely to have more generalapplications for measuring neuronal activity in the flybrain in relation to various behaviours.

Heather Wood

References and linksORIGINAL RESEARCH PAPER Wang, J. W. et al. Two-photon calciumimaging reveals an odor-evoked map of activity in the fly brain. Cell 112,271–282 (2003)FURTHER READING Nakai, J. et al. A high signal-to-noise Ca2+ probecomposed of a single green fluorescent protein. Nature Biotech. 19, 137–141(2001) | Vosshall, L. B. Olfaction in Drosophila. Curr. Opin. Neurobiol. 10,498–503 (2000)WEB SITESAxel lab: http://cpmcnet.columbia.edu/dept/neurobeh/axel/overview.html Flybase: http://flybase.bio.indiana.edu/or43aMovies online: http://www.cell.com/cgi/content/full/112/2/271/DC1

Seeing how flies smell

O L FA C TO R Y C O D I N G

A fly antennal lobe, in which G-CaMP — a calcium-sensitive green fluorescent protein — is expressed onlyin projection neurons that innervate the lobe. The high signal-to-noise ratio of G-CaMP provides arepresentation, at cellular resolution, of a defined population of neurons in the brain as the fly is stimulated byodorants at physiological concentrations. Different odours elicit different patterns of activation in the antennallobe. Courtesy of J. W. Wang, Center for Neurobiology and Behavior, Columbia University, New York, USA.