© 2008 Nature Publishing Group http://www.nature.com/natureneuroscience

news and views
projection ­patterns to muscles7,8. However,essentially none of these genes are expressedin Foxp1 mutant mice. Counter-intuitively, the­projection ­patterns to most limb ­muscles and­intramuscular axonal arborizations in Foxp1
mutant mice are ­nevertheless ­similar to those ofwild-type mice3,4 (Fig. 1c). Notable ­exceptions
to this rule were the axonal arborizations in­muscles innervated by motor neurons pools
marked by the target-induced ETS ­transcription
factor Pea3. Dasen et al.3 showed that thesemuscles exhibited ­innervation defects in Foxp1
mutants that were comparable to those in Pea3mutant mice9 (Fig. 1c).
How can these rather modest changes inperipheral projection pattern be reconciled withthe marked alteration in molecular ­identity?
When looked at from the ­perspective of one
individual limb muscle, these new ­findings
demonstrate that coordinate ­elimination of allmolecular identity from a motor neuron pool
results in less severe innervation defects thanmutation of a single gene that contributes tothe complex code that controls the ­innervationof that same muscle. These results suggestthat in the complete absence of instructionsin LMC motor neurons, as observed in Foxp1
mutants, random pathfinding can neverthelessresult in seemingly intact innervation patterns.
Consistent with this, mice with a mutation in
the homeodomain transcription factor geneMnx1 (also known as Hb9), which exhibit even
earlier and more severe defects in ­motoneuronal­specification, also manage to innervate mostlimb muscle targets10,11. Furthermore, in­muscles innervated by Pea3-expressing motor
neurons, Hb9 mutant mice show projection

defects ­similar to Pea3 and Foxp1 mutants(E. Vrieseling and S.A., ­unpublished data).
Together, these findings suggest that genericaxonal ­projections to targets can still be­established in situations where many of the earlycell-­intrinsic genetic programs are lacking. In
the context of these missing cell-autonomous
programs, ­however, axons are no longer capableof reading target-derived cues that are essential
for axonal arborization in target muscles.
Despite the rather minor alterations of overall
peripheral projections to limb muscles, Foxp1
mutants showed severe deficiencies in the­topographical organization of motor ­neuron
cell bodies in the spinal cord. These defects
were revealed upon retrograde ­labeling of motor­neurons from individual muscles. Although a
motor neuron pool ­innervating one muscle­represents a tightly clustered cell body aggregate
in a defined position in the spinal cord in wild
types, corresponding cell bodies in Foxp1 mutant
mice showed a ­randomly dispersed pattern3,4.
These ­findings further underscore the arbitrary
nature of the motor axonal outgrowth process inFoxp1 mutants. As a consequence, these changes
are also expected to result in notable alterationsof central presynaptic connectivity onto these
motor neurons. The embryonic lethality of
Foxp1 mutants has so far prevented analysisof these later phenotypes, including expected­alterations in motor behavior, an interesting
avenue of research to pursue in the future.
Finally, Dasen et al.3 offer an intriguing­evolutionary interpretation of their results.
The motor neuronal spectrum generated in
Foxp1 mutant mice mirrors the composite thatis ­present in early aquatic vertebrates that lack

PGC and LMC, where simple ­locomotion isdriven entirely by MMC neurons and a set of
motor neurons innervating hypaxial muscles
that could correspond to the amniote HMC
set12. The recruitment of the Hox symphonic­repertoire and its maestro FoxP1 to suchan ancestral organism may thus have been
­instrumental in the evolutionary rise of theprimitive ­vertebrate motor system to the one
represented in ­mammals today, enabling them toperform refined motor behaviors. And, ­perhaps
not by chance, FoxP1’s sibling FoxP2 has been
linked to human speech and language13, which
depend on highly refined motor control of­larynx and mouth. Future work on when andhow the Hox symphony and its ­conductor
FoxP1 evolved and were brought together inthe evolution of motor circuits may give us deep
insights into the developmental and functional
aspects of ­spinal motor system evolution.
1. Dasen, J.S., Liu, J.P. & Jessell, T.M. Nature 425,
926–933 (2003).
2. Dasen, J.S., Tice, B.C., Brenner-Morton, S. & Jessell, T.M.
Cell 123, 477–491 (2005).
3. Dasen, J.S., De Camilli, A., Wang, B., Tucker, P.W. &Jessell, T.M. Cell 134, 304–316 (2008).
4. Rousso, D.L., Gaber, Z.B., Wellik, D., Morrisey, E.E. &Novitch, B.G. Neuron 59, 226–240 (2008).
5. Mann, R.S. & Affolter, M. Curr. Opin. Genet. Dev. 8,
423–429 (1998).
6. Wu, Y., Wang, G., Scott, S.A. & Capecchi, M.R.
Development 135, 171–182 (2008).
7. De Marco Garcia, N.V. & Jessell, T.M. Neuron 57,
217–231 (2008).
8. di Sanguinetto, S.A., Dasen, J.S. & Arber, S. Curr.
Opin. Neurobiol. 18, 36–43 (2008).
9. Livet, J. et al. Neuron 35, 877–892 (2002).
10. Arber, S. et al. Neuron 23, 659–674 (1999).
11. Thaler, J. et al. Neuron 23, 675–687 (1999).
12. Fetcho, J.R. Brain Behav. Evol. 40, 82–97 (1992).
13. Lai, C.S., Fisher, S.E., Hurst, J.A., Vargha-Khadem, F.
& Monaco, A.P. Nature 413, 519–523 (2001).
14. Tsuchida, T. et al. Cell 79, 957–970 (1994).

Regenerating your senses: multiple roles forneurogenesis in the adult brain
Paul W Frankland & Freda D Miller
The adult mouse brain continuously supplies new neurons to the olfactory bulb and hippocampus. A new study inthis issue shows that ongoing neurogenesis is essential for maintenance of the olfactory bulb and for spatial memory.
Since the ­initial discovery that the adultbrain ­contains stem cells that generate
new neurons1, an ­important question in
Paul W. Frankland is in the Neurosciences andMental Health Program, and Freda D. Miller is in
the Developmental and Stem Cell Biology Program,
The Hospital for Sick Children, 555 University
Avenue, Toronto, Ontario, Canada M5G 1X8.
e-mail: paul.frankland@sickkids.ca orfredam@sickkids.ca

1124­ eurobiology has been whether or not thisnadult ­neurogenesis is ­functionally ­important.
Although a ­substantial body of work has
shown that some of these new, adultborn neurons ­integrate into active neural­circuits, the magnitude and ­importanceof this ­integration has been unclear2. In a­definitive study, Imayoshi et al.3 ­demonstratethat adult neurogenesis is indeed ­importantand that, ­surprisingly, it may ­fulfill two very
distinct ­functions (Fig. 1). In one brain

region, the ­olfactory bulb, adult stem cell–­
generated ­progeny have a critical role in­tissue ­maintenance, much as they do in­tissues such as the gut and skin, whereas in asecond region, the ­hippocampus, they serveto add new ­neurons that are ­important for
adult behaviors.
Imayoshi et al.3 drew these conclusions on
the basis of an elegant transgenic ­strategy
that they used to permanently label andthen deplete stem cell–generated progeny

volume 11 | number 10 | OCTOBER 2008 nature neuroscience© 2008 Nature Publishing Group http://www.nature.com/natureneuroscience

news and viewsin the adult brain. Specifically, the authors­generated mice in which a tamoxifen­i nducible Cre ­recombinase (CreERT2)
was expressed under the promoter for the­n eural ­precursor marker nestin. Using
these mice, they first asked how many newstem cell–­derived ­neurons were added tothe adult brain. To do this, they crossed
their mice with ­various reporter lines andinduced ­recombinase ­activity in adults with­tamoxifen, thereby ­efficiently (up to 87%)
expressing the ­reporters in neural stem
cells (NSCs) and their progeny. Initially,
the authors followed the ­differentiation,­migration and ­maturation of new neurons
in the olfactory bulb. Previous work has
shown that neurons are generated from
NSCs in the subventricular zone of the lateral­ventricles and that these ­neurons migrate tothe ­olfactory bulb, where they mature andacquire ­features of mature granule ­neurons
after about 1 month4,5. The authors’ new­inducible transgenic approach confirmed thisearlier work and led to the conclusion that
almost the entire granule cell ­population was
replaced by new ­neurons over a 12-month
period. The rate of ­neuronal ­replacement
was nearly linear for the first 6 months,
with a decrease in the pace of ­addition
from 6–12 months, potentially reflecting adecline in ­olfactory neurogenesis in middle
age and beyond. The exception to this almost­complete replacement was in the ­superficial
layers, where only half of the ­neurons werelabeled, suggesting that there is at least one
subpopulation of persistent granule cells in
the olfactory bulb.
These data indicate that olfactory­neurogenesis provides a constant supply of
new neurons to replace cells in the olfactorybulb. What ­happens if this supply is turned
off? To answer this next critical question,
Imayoshi et al.3 generated mice in which
the majority of new adult stem cell–derived­neurons were ablated. To achieve this, theymated the nestin CreERT2 mice to mice inwhich a construct ­encompassing a floxed
stop codon followed by the ­catalytically
active ­fragment of ­diphtheria toxin wasinserted into the ­neuron-specific ­enolase
locus. When the ­resulting mice were treatedwith tamoxifen, ­diphtheria toxin ­expression
was induced, ­killing ­newborn cells as soon
as they ­differentiated into ­neuron-specificenolase–­expressing ­neurons. The authors­demonstrated the ­s pecificity of theirapproach, showing that it did not affect either­developmentally ­generated or ­earlier (before
tamoxifen) cohorts of adult-­generated ­granulecells. Using this elegant ­strategy, Imayoshi
et al.3 found that ­blocking adult ­neurogenesisa

Dentate gyrus

Olfactory bulb
and rostralmigratory streamb Olfactory bulb (maintenance)c Dentate gyrus (growth)

Glomerular
layer
Mitralcell layer
Adult-generated
granule cells

Developmentally
generated
granule cells

Adult
Blockade of adultneurogenesis
neurogenesis

Adult
Blockade ofneurogenesis adult neurogenesis

Figure 1 Two functions for adult neurogenesis in the olfactory bulb and hippocampus. (a) In the adultbrain, neurogenesis persists in two regions: the subventricular zone of the lateral ventricle (new neurons
migrate from here along the rostral migratory stream to populate the olfactory bulb) and the subgranularzone of the hippocampus (new neurons migrate the short distance from the subgranular zone to theinner granule cell layer of the dentate gyrus). (b) Adult-generated neurons are essential for maintaining
the olfactory bulb. When olfactory neurogenesis is blocked, there is extensive depletion of granule cells
in the olfactory bulb. (c) In contrast, in the dentate gyrus, adult-generated neurons contribute to tissue
growth. When hippocampal neurogenesis is blocked, dentate gyrus growth is attenuated.

led to an ­extensive ­depletion of granulecells in the olfactory bulb. This ­depletionwas ­detectable as early as 3 weeks post­tamoxifen and was even more ­pronounced
at 12 weeks post-tamoxifen. Thus, cell deathin the ­olfactory bulb is not a ­consequence ofneuron ­addition. Instead, ­olfactory bulb cell
death is an ­autonomous ­process that occurs
in the absence of ­olfactory ­neurogenesis
and that ­inevitably leads to ­substantial­tissue ­shrinkage when stem cell ­function is­perturbed. This ­situation is ­reminiscent ofthat in tissues such as the gut, skin or blood,
where adult stem cells have an essential rolein tissue ­maintenance and ­replacement.
The second place in the adult brain
where neurogenesis clearly persists is the­subgranular zone of the hippocampus, abrain region that is important in learning andmemory6. Does ­neurogenesis serve a similar­maintenance/replacement function in the­hippocampus? Imayoshi et al.3 took ­advantage
of a ­second ­nestin-CreERT2 line that had­substantially higher ­recombination efficiency
in the ­hippocampus to ask this question.
Consistent with previous observations2,4,5,
they found that these adult-generated­granule cells ­populated the ­innermost layersof the ­dentate gyrus, where they comprised
10% of the cells after 6 months, a numberthat remained constant for up to 12 months.
Surprisingly, ­however, ­ablating ­hippocampal­neurogenesis ­produced a very ­different

nature neuroscience volume 11 | number 10 | OCTOBER 2008

­ attern of results to the ­olfactory bulb.
pHere, ­blocking ­neurogenesis didn’t lead toshrinkage. Instead, the ­number and ­densityof granule cells remained ­constant in the­dentate gyrus for up to 24 weeks ­following­n eurogenesis ­blockade. This ­contrasted
with control ­animals, in which there wasan increase in cell number and ­density
over the same period of time, as ­previously
reported7. Thus, adult ­neurogenesis has
a quite ­different role in the ­hippocampus,­adding (rather than ­replacing) neuronsand ­contributing to ­hippocampal growth(rather than ­maintenance) ­throughout the­animal’s lifespan. Why is there this ­profound­difference? One ­possible ­explanation has to
do with ­differences in ­circuitry. The ­afferent
input to the olfactory bulb comes fromperipheral ­olfactory receptor ­neurons, a­population that is itself ­constantly dying
and being ­replenished. Perhaps the ­ongoing­deafferentation of the olfactory bulb affects
neuronal survival, much as it does for­developing neurons, ­leading to an increased
rate of neuronal death and ­necessitating a
steady supply of ­replacement neurons. In
contrast, although structural remodelingundoubtedly occurs in the ­hippocampus,
steady ­deafferentation does not.
How then do these profound changes in­structure affect neural function? The micein which adult-born neurons were depleted­provided Imayoshi et al.3 a ­definitive means1125

© 2008 Nature Publishing Group http://www.nature.com/natureneuroscience

news and viewsof addressing the ­functional role of adult­neurogenesis. However, their ­studies revealed
surprising differences when they ­examined
the impact of blocking ­hippocampal and­olfactory neurogenesis on ­behavior. To assess­hippocampal learning, they trained micein spatial (Barnes maze) and fear ­learning
tasks ­following ­ablation of ­hippocampal­neurogenesis. In both tasks, mice showedlearning and ­memory ­deficits, ­demonstratingthat the addition of new ­neurons to the ­dentate
gyrus is ­essential for normal ­functioningof hippocampal ­memory. Although this­conclusion is ­consistent with a number ofrecent studies8–10, the ­precise ­manner in whichthese new neurons ­contribute to ­hippocampal
memory is still unclear.
To assess olfactory bulb function, theauthors gave the same mice a battery of smell
tests. Most surprisingly, blocking olfactory­neurogenesis appeared to have little effecton olfactory-­mediated ­behaviors. Mice could
still readily ­discriminate between odors
and learn to ­associate specific odors with arewarding ­stimulus even at 6 months posttamoxifen, when neuronal depletion in theolfactory bulb was very pronounced. These
results suggest that, although the ­continuous
replacement of dying ­olfactory bulb ­neurons
is ­e ssential for ­m aintaining ­o lfactory
bulb structure, it is not necessary for the­maintenance of ­olfactory-mediated ­learningor behaviors. This ­conclusion is at odds withthose of some ­previous studies11, and, as theauthors ­acknowledge, it is impossible torule out the ­possibility that the behavioral­analyses were insufficiently ­comprehensive
to detect ­impairments. Indeed, evidence­indicates that olfactory ­neurogenesis is­important in ­maternal behaviors12 and inmate selection13 in female mice.
Together with previous work, this study­provides us with clear evidence for the­importance of adult neurogenesis in the­normal adult brain. Adult stem cell–derived
neurons are critical for the maintenance of theolfactory bulb, much as adult stem cell–­derived
progeny are essential for cell ­replacement inother parts of the body. When this ­process
is perturbed, the tissue itself ­degenerates.
In ­contrast, adult ­neurogenesis in the­hippocampus is not required for ­maintenance,

but is instead required for ­neuronal additionand hippocampal growth, thereby potentiallycontributing to the ­ability to accumulate
new memories throughout life. Whether or
not these, or other potential ­populations ofnestin-positive adult NSCs, can be recruitedfor neural repair14 can now also be addressedusing the same elegant approaches.
1. Reynolds, B.A. & Weiss, S. Science 255, 1707–1710(1992).
2. Zhao, C., Deng, W. & Gage, F.H. Cell 132, 645–660(2008).
3. Imayoshi, I. et al. Nat. Neurosci. 11, 1153–1161(2008).
4. Lledo, P.M., Alonso, M. & Grubb, M.S. Nat. Rev.
Neurosci. 7, 179–193 (2006).
5. Ming, G.L. & Song, H. Annu. Rev. Neurosci. 28,
223–250 (2005).
6. Squire, L.R. Psychol. Rev. 99, 195–231 (1992).
7. Bayer, S.A., Yackel, J.W. & Puri, P.S. Science 216,
890–892 (1982).
8. Dupret, D. et al. PLoS ONE 3, e1959 (2008).
9. Saxe, M.D. et al. Proc. Natl. Acad. Sci. USA 103,
17501–17506 (2006).
10. Zhang, C.L., Zou, Y., He, W., Gage, F.H. & Evans, R.M.
Nature 451, 1004–1007 (2008).
11. Gheusi, G. et al. Proc. Natl. Acad. Sci. USA 97,
1823–1828 (2000).
12. Shingo, T. et al. Science 299, 117–120 (2003).
13. Mak, G.K. et al. Nat. Neurosci. 10, 1003–1011 (2007).
14. Lindvall, O. & Kokaia, Z. Nature 441, 1094–1096(2006).

Pavlov’s moth: olfactory learning andspike timing–dependent plasticity
Julian P Meeks & Timothy E Holy
Spike-timing dependent plasticity is a favored synaptic mechanism for learning. However, a surprising new study by
Ito and colleagues in the insect mushroom body suggests that it cannot account for a paradigmatic form of learning.
Pavlov and many others have noticed
that ­timing is ­important in learning; for­example, any action (such as ringing a bell)that typically precedes feeding tends to bequickly ­associated with an impending meal.
Neuroscientists have long sought the ­circuitand cellular mechanisms that ­underlie
such learning. Much ­attention has been
focused on so-called Hebbian ­mechanisms1,­particularly a form known as spike timing–­
dependent plasticity (STDP) (reviewed
in ref. 2), as these ­mechanisms mimic, at a­cellular level, the requirement for a ­predictive­association between ­stimulus ­(ringing the

The authors are at the Department of Anatomy &Neurobiology, Washington University in St. Louis
School of Medicine, 660 S. Euclid, St. Louis,
Missouri 63110, USA.
e-mail: holy@wustl.edu

1126

bell/­presynaptic activity) and reward (food/postsynaptic spike). In this issue, Ito et al.3
report their studies of this question in a­setting, olfactory ­conditioning of moths,
that bears a notable similarity to Pavlov’s
original ­experiment. Surprisingly, they foundthat the timing of ­physiological activity inthe ­mushroom body, a center for olfactory­learning4, is all wrong: odor-induced ­activity
largely ­disappeared well before the optimum
time for delivery of the reward. This violates
the central prediction of STDP and might­indicate that, at least for this archetypal­example of ­learning, key pieces of the overall
puzzle of memory remain to be elucidated.
STDP typically (but not always5) ­manifests
as a strengthening of a synapse when­presynaptic activity precedes postsynaptic­depolarization and an attendant ­weakening of
the synapse when presynaptic activity follows­postsynaptic depolarization2,6. In modelingstudies, this ­simple rule endows synapses, cellsand ­circuits with a ­number of very attractive­properties for both ­development and ­learning,­allowing ­plasticity and circuit optimization­without ­‘runaway’ ­positive ­feedback7. Several­experiments have also shown that STDP canbe induced in vivo and correlative evidence
for STDP-induced ­alterations in sensory­processing has been obtained using natural
stimuli (reviewed in ref. 8).
The timing requirements for STDP
are fairly tight, typically a few tens of
milliseconds2,7. Although the evidence forneuronal ­activity with this degree of timing
precision is ­w idespread, the temporal gap
between ­‘stimulus’ and ‘reward’ in classical­conditioning can be many seconds, causing
some doubts about the ­ubiquity of STDP
for tasks on behavioral time scales9. One­attractive proposal, therefore, is the notionthat a short-term memory of the stimulus,

volume 11 | number 10 | OCTOBER 2008 nature neuroscience