DOI: 10.1126/science.1214713
, 608 (2011);334Science
, et al.Matthieu Maroteaux
Synaptic Switch and Social Status
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DSCs are extremely good at separating
photogenerated charge carriers and taking
them to the electrodes. If an incoming photon
is absorbed by the dye in a state-of-theart
DSC, the probability of the charge carriers
reaching the electrodes is nearly 100%. Until
recently, the world record for power conversion
was 11.1% (4). Although the sensitizing
molecules in these cells have a relatively
large energy gap, the cells do not generate as
high a voltage as one could reasonably expect
because the redox potential of the most commonly
used redox couple, which is based on
iodide ions, is too low for the best sensitizing
dyes (see the figure, panel B). Energy is
wasted as the redox couple reduces a chargedsensitizing
dye back to its neutral state.
Many researchers have been trying for
more than a decade to find a redox couple
with a more ideally suited redox potential in
order to increase the voltage of DSCs, but
the process has been slow and frustrating.
The iodide mediator is special as its oxidized
form, I3
–
, does not readily accept electrons
from the titania surface, which would be an
unwanted recombination process (5). The
I3
–
ions can persist for 1 ms or more before
undergoing an electron-transfer reaction in
solution, which gives them sufficient time to
diffuse to the counterelectrode for reduction.
Several alternative metal complexes with better
redox potentials have been found, but solar
cells made with them typically have unacceptably
high recombination rates and lower
open-circuit voltages.
Last year, Boschloo, Hagfeldt, and coworkers
achieved a power-conversion efficiency
of 6.7% in DSCs with iodide-free
electrolytes using Co complexes (6). The
key to making good cells was adding just
enough electrically inactive bulk to the
periphery of the sensitizing dyes and the
Co complexes to slow down recombination
without blocking the necessary electrontransfer
processes.
The approach Yella et al. took to slowing
down recombination was to use a relatively
new family of dyes that connect an electron
donor (D) moiety to an electron acceptor
moiety (A) through a conjugated (π−bonded)
bridge (D−π−A) provided by a zinc porphyrin
complex. These D−π−A dyes do not contain
any expensive, rare metal atoms, such as
Ru, and tend to absorb light more strongly.
Most important, they attached alkoxy chains
to the sides of these molecules to provide
a very effective barrier to recombination
between electrons in the titania and holes in
the Co complexes.
One of the shortcomings of the Co complexes
is that they diffuse through the electrolyte
more slowly than the conventional
iodide ions because they are larger. Yella et
al. found that they could get efficiency as
high as 13.1% by reducing the illumination
intensity by 50% because it is less important
for the ions to diffuse to the electrode quickly
when the carrier density is lower. This efficiency
might be obtained under normal solar
lighting by reducing the distance the ions
need to diffuse through the use of thinner
films. Complete absorption in these thinner
films could be achieved by using even better
D−π−A dyes that absorb more strongly or
advanced light-trapping techniques such as
plasmonics (7, 8). Long-term stability studies
must be performed on Co complexes in
dye-sensitized solar cells to determine if they
are as stable as iodide-based electrolytes.
For many years, dozens of researchers
around the world tried to develop new sensitizing
dyes and redox couples for DSCs
but inevitably concluded that the best sensitizing
dyes needed to contain Ru and the
best mediators needed to contain iodide.
These paradigms are now shattered. One of
the next developments that needs to occur is
reducing the energy gap of sensitizing dyes
so that light can be harvested efficiently
further into the infrared region of the spectrum.
One approach would be to use sensitizing
dyes to absorb infrared light and energy
relay dyes that absorb visible light, and then
transfer energy to the sensitizing dyes (9).
As scientists around the world develop new
D−π−A dyes and redox couples and combine
them with energy relay dyes and new lighttrapping
techniques, we can expect to see the
efficiency climb toward 15%, which could
make DSC technology competitive with
other kinds of solar cells.
References
1. B. O’Regan, M. Gratzel, Nature 353, 737 (1991).
2. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson,
Chem. Rev. 110, 6595 (2010).
3. A. Yella et al., Science 334, 629 (2011).
4. Y. Chiba et al., Jpn. J. Appl. Phys. 45, L638 (2006).
5. G. Boschloo, A. Hagfeldt, Acc. Chem. Res. 42, 1819
(2009).
6. S. M. Feldt et al., J. Am. Chem. Soc. 132, 16714 (2010).
7. H. A. Atwater, A. Polman, Nat. Mater. 9, 205 (2010).
8. I.-K. Ding et al., Adv. Energy Mater. 1, 52 (2011).
9. B. E. Hardin et al., Nat. Photonics 3, 406 (2009).
10.1126/science.1212818
Synaptic Switch and Social Status
NEUROSCIENCE
Matthieu Maroteaux and Manuel Mameli
The strength of synaptic connections in the mammalian brain can influence social status.
I
n 1859, Charles Darwin introduced the
key concept of natural selection—that
in the struggle for survival and reproduction,
individuals compete for the same
resources. Hence, animals living in a social
environment can establish dominance hierarchies
within a short time, which remain
stable during the existence of the group
(1). This ranking within social communities
has a fundamental advantage—it eliminates
conflict in the group, which minimizes
energy expenditure and violence, thereby
allowing resource sharing (2). On page
693 of this issue, Wang et al. (3) demonstrate
that encoding of social dominance in
mice involves specific synapses in cortical
regions of the brain.
Wang et al. used a behavioral experimental
model (4) that provides a quantitative
measure of aggressiveness without allowing
physical contact between competing mice,
thereby preventing injuries. In each trial, one
mouse forces its opponent outside of a neutral
area, permitting identification of a dominant
and a subordinate mouse. The authors
show that in a cohort of four mice, a social
hierarchy was quickly organized and once
established, persisted over time. Another
important aspect is the complete independence
of social dominance from other factors,
including sensorimotor and learning
skills. This makes hierarchy formation a
unique and distinct trait of animal behavior.
When and where does nature decide that
an individual will dominate in a community?
Early in life, mice as well as humans
are embedded in a community that in the
simplest scenario consists of siblings. An
interesting possibility is that innate traits and
genetic programming that shape neuronal circuits
from the postnatal period are responsible
for the allocation of dominance or suborINSERM
UMR-S 839, Institut du Fer à Moulin, Université
Pierre et Marie Curie, F75005 Paris. E-mail: manuel.
mameli@inserm.fr
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www.sciencemag.org SCIENCE VOL 334 4 NOVEMBER 2011 609
PERSPECTIVES
dination. But in the case of newly established
cohorts, where animals meet randomly, how
does social dominance adjust? In one plausible
sequence of events, a dominant individual
suddenly facing subordination experiences a
range of emotions, including disappointment
or even fear. Stress has been proposed to
affect social relationships in this context (5).
Indeed, by acting both on neuronal circuits
implicated in cognition and emotions and on
hormone levels, stress amplifies memories
associated with the previously established
social status (6).
Changes in the synaptic strength of excitatory
connections in the rodent hippocampus,
amygdala, and midbrain represent a cellular
substrate of experience-driven behaviors
(7). This suggests the engagement of similar
mechanisms during the encoding of social
dominance. In humans, the dorsolateral prefrontal
cortex (PFC), the homolog of the
rodent medial PFC (mPFC), has been implicated
in hierarchy-related behaviors. Wang
et al. focused on excitatory synapses onto
layer V pyramidal neurons of the mPFC (the
mainoutputofthisstructure)andshowthatthe
strength of synaptic transmission mediated by
the neurotransmitter glutamate matches social
ranking in mice (see the figure).
The stochastic release of individual neurotransmitter
vesicles (quanta) has been
used to probe the basic function of synapses
in the neuromuscular junction and central
nervous system (8). Wang et al. observed
that the size of quantal release onto cortical
neurons was higher in dominant than subordinate
adult mice (rank 1 versus rank 4
among four mice). As an outcome, higher
synaptic efficiency will potentially translate
into a stronger output signal from the
cortex to downstream brain structures.
The authors also established a causal link
between strength of excitatory synapses and
social dominance. Decreasing the synaptic
transmission mediated by AMPA receptors
(which are postsynaptically activated by
the release of glutamate) switched the initial
ranking—the dominant mouse became
the subordinate member of the cohort (from
rank 1 to rank 4). Likewise, an increase in
AMPA receptor–mediated synaptic transmission
switched the subordinate mouse to a
dominant rank (rank 4 to rank 1). Thus, synaptic
efficacy, specifically in cortical layer V,
is sufficient to tune social status. Although
the amount of AMPA receptors at synapses
is unlikely to be the only mechanism that
discriminates a dominant versus a subordinate
mouse, these findings provide a further
argument that manipulation or the synaptic
strength by controlling the number or the
trafficking of postsynaptic AMPA receptors
directly modifies specific behaviors.
Wang et al. provide two conceptual
advances: the idea that a neurobiological
substrate for social ranking is located in the
mPFC, and that synaptic efficacy represents
a cellular substrate determining social status.
Although the mPFC has an established
role in social behavior, it cannot be considered
the only structure where dominance
is encoded. Indeed, the amygdala and lateral
septum are key players in establishing
animal hierarchy (9). Future studies will
be necessary to determine the hierarchical
organization among brain structures underlying
this complex behavior.
An interesting aspect arising from the
study of Wang et al. concerns the mechanisms
that establish glutamatergic transmission
upon the first encounter of a stranger. It
is plausible that mice might be differently
programmed for leadership from birth.
However, it cannot be ruled out that animals
may also acquire their social status based
on biological factors and external cues,
thereby learning to be dominant or subordinate.
If that is the case, this process might
share traits with experience-driven behavior,
where synaptic adaptations are crucial.
How does the synaptic strength adapt in
conditions where dominancy is challenged?
A network activity might be in part responsible
for driving these processes or alternatively,
synaptic scaling may take place more
slowly over time (10, 11). Wang et al. demonstrate
that impairing AMPA receptor trafficking
to the synapse devalues the status of
a dominant mouse. This raises the interesting
possibility that AMPA receptors constitutively
insert at synapses and maintain
dominance over time.
Although Wang et al. demonstrate that
social rank correlates with synaptic strength,
they also highlight the need to further understand
the differences between individuals
at the cellular level. This could depend on
the number of AMPA receptors, dendritic
spines, or synaptic inputs—or a broader network
tuning could be involved.
References and Notes
1. R. J. Blanchard, K. J. Flannelly, D. C. Blanchard, Physiol.
Behav. 43, 1 (1988).
2. D. van Kreveld, Ned. Tijdschr. Psychol. 25, 55 (1970).
3. F. Wang et al., Science 334, 693 (2011); 10.1126/
science.1209951.
4. G. Lindzey, H. Winston, M. Manosevitz, Nature 191, 474
(1961).
5. R. M. Sapolsky, Science 308, 648 (2005).
6. M. I. Cordero, C. Sandi, Front. Neurosci. 1, 175 (2007).
7. H. W. Kessels, R. Malinow, Neuron 61, 340 (2009).
8. M. R. Bennett, J. L. Kearns, Prog. Neurobiol. 60, 545
(2000).
9. M. Timmer, M. I. Cordero, Y. Sevelinges, C. Sandi,
Neuropsychopharmacology 36, 2349 (2011).
10. R. Malinow, R. C. Malenka, Annu. Rev. Neurosci. 25, 103
(2002).
11. G. Turrigiano, Annu. Rev. Neurosci. 34, 89 (2011).
12. We thank M. C. T. Brown and J.-C. Poncer for helpful
comments and the Ecole de Neuroscience de Paris,
INSERM, and Ville de Paris for support.
10.1126/science.1214713
Spine head
Pyramidal neuron
AMPAR NMDAR
Behavioral switch DominantSubordinate
Synapses and rank. Excitatory synaptic drive onto cortical pyramidal neurons in the mouse brain is stronger
in dominant individuals than subordinates. Modulating synaptic strength by increasing or decreasing AMPA
receptor–mediated transmission switches the initial social ranking.
CREDIT:B.STRAUCH/SCIENCE
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