The Journal of Neuroscience, October 15, 2003 • 23(28):9289 –9296 • 9289
Behavioral/Systems/Cognitive
Suppression of Adult Neurogenesis Impairs Olfactory
Learning and Memory in an Adult Insect
Sophie Scotto-Lomassese, Colette Strambi, Alain Strambi, Aı̈cha Aouane, Roger Augier, Geneviève Rougon, and
Myriam Cayre
Laboratoire NMDA, Institut de Biologie du Développement de Marseille, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 6156,
Campus de Luminy, 13288 Marseille Cedex 9, France
Although adult neurogenesis has now been demonstrated in many different species, the functional role of newborn neurons still remains
unclear. In the house cricket, a cluster of neuroblasts, located in the main associative center of the insect brain, keeps producing new
interneurons throughout the animal’s life. Here we address the functional significance of adult neurogenesis by specific suppression of
neuroblast proliferation using ␥ irradiation of the insect’s head and by examining the impact on the insect’s learning ability. Forty gray
irradiation performed on the first day of adult life massively suppressed neuroblasts and their progeny without inducing any noticeable
side effect. We developed a new operant conditioning paradigm especially designed for crickets: the “escape paradigm.” Using olfactory
cues, visual cues, or both, crickets had to choose between two holes, one allowing them to escape and the other leading to a trap. Crickets
lacking adult neurogenesis exhibited delayed learning when olfactory cues alone were used. Furthermore, retention 24 hr after conditioning was strongly impaired in irradiated crickets. By contrast, when visual cues instead of olfactory ones were provided, performance
of irradiated insects was only slightly affected; when both olfactory and visual cues were present, their performance was not different from
that of controls. From these results, it can be postulated that newborn neurons participate in the processing of olfactory information
required for complex operant conditioning.
Key words: adult neurogenesis; mushroom body; neuroblasts; insect; house cricket; learning and memory; olfactory and visual conditioning; irradiation
Introduction
The dogma that the adult brain produces no new neurons has
been overturned, but the question still remains of whether adult
neurogenesis is a biologically relevant phenomenon. Observed in
some invertebrates, as well as in all mammalian species examined
so far, adult neurogenesis is conserved through evolution, and
newly formed neurons are integrated in structures where information is processed and temporarily stored: the dentate gyrus of
the hippocampus (Bayer et al., 1982; Gould et al., 1998; Eriksson
et al., 1998) and the olfactory bulb (Corotto et al., 1993; Lois and
Alvarez-Buylla, 1994) in vertebrates and the mushroom bodies in
insects (Cayre et al., 1994, 1996a, Gu et al., 1999). Striking interrelations between neurogenesis and behavior have been reported
in birds (Barnea and Nottebohm, 1996; Li et al., 2000). Likewise
in rodents, factors that increase progenitor cell proliferation
(running) or newborn neuron survival (enriched environment
and learning) also improve learning performance (Kempermann
et al., 1997; Gould et al., 1999a,b; Nilsson et al., 1999; van Praag et
Received June 6, 2003; revised Aug. 20, 2003; accepted Aug. 21, 2003.
This work was supported by the Fyssen Foundation. We thank Dr. P. Mouchy and Dr. C. Boyer for providing access
to the irradiator, A. de Moya for building the escape maze, Dr. M. Giurfa for advice and enriching discussions, and F.
Haigler for careful editing of this manuscript.
Correspondence should be addressed to Myriam Cayre, Laboratoire NMDA, Institut de Biologie du Développement de Marseille, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 6156, Campus de Luminy,
Case 907, 13288 Marseille Cedex 9, France. E-mail: cayre@ibdm.univ-mrs.fr.
Copyright © 2003 Society for Neuroscience 0270-6474/03/239289-08$15.00/0
al., 1999; Rochefort et al., 2002). Conversely, glucocorticoids and
stress, which inhibit neurogenesis, deteriorate learning and
memory (Gould et al., 1997, 1998; Krugers et al., 1997; Lemaire et
al., 2000). However, evidence of a direct causal link between the
two phenomena is still scarce because of the difficulty of stopping
neurogenesis in adults with high specificity. Using an antimitotic
agent, Shors et al. (2001) showed that reduction of hippocampal
neurogenesis impaired rat performance specifically in a
hippocampus-dependent conditioning test. By contrast, impairment of neurogenesis by deletion of the presinilin-1 gene did not
cause a learning deficit, leading Feng et al. (2001) to suggest that
new hippocampal neurons might be necessary to forget outdated
information after cortical memory consolidation has occurred.
In songbirds, destruction of adult-born neurons led to song deterioration with a recovery concomitant to compensatory neuronal production (Scharff et al., 2000). On the basis of these data,
the precise role of adult neurogenesis in vertebrates still remains
enigmatic.
No study has yet been devoted to the function of newborn
neurons in invertebrates, although they constitute suitable and
simple models. In insects, mushroom bodies are the main integrative centers of the brain because they receive multimodal sensory information (Mobbs, 1982, 1984; Gronenberg, 2001) and
are involved in learning and memory processes (for review, see
Heisenberg, 1994, 1998; Hammer and Menzel, 1995). In the
house cricket Acheta domesticus, mushroom body neuroblasts are
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Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
grouped in a cluster. By asymmetric cell division, the neuroblast
self renews and forms a ganglion mother cell (GMC), which divides symmetrically to give rise to new mushroom body interneurons (Kenyon cells) throughout the insect’s life (Cayre et al.,
1994). As in vertebrates, both hormones (Cayre et al., 1994, 1997)
and sensory stimuli (Scotto-Lomassese et al., 2000, 2002) regulate
adult insect neurogenesis. To assess the putative participation of
newly formed interneurons to learning processes, mushroom
body neuroblasts were specifically destroyed by ␥ irradiation, and
learning abilities of control versus neuroblast-deprived animals
were evaluated in a new learning paradigm, using olfactory conditioning, visual conditioning, or both.
Materials and Methods
Animal treatments
Adult females were isolated from the colony at the imaginal molt [day 0
(D0)], maintained under a 16/8 hr light/dark cycle in the rearing room,
where the relative humidity was 55% and the temperature was 29 ⫾ 1°C.
One group was used as a control, and the other was submitted to radioactive 137Cs ␥ irradiation on D0. Crickets were cold-anesthetized and
individually immobilized in a small assay tube. The tubes were then stuck
on a 5 cm lead plate so that only the insect’s head protruded and could be
exposed to the ␥ rays. The lead plate was placed in an irradiator (IBL 637;
CIS Bio International) containing a source of 137Cs. To determine the
optimal ␥ ray dose, exposure ranging from 5 to 120 gray (Gy) was used,
and neuroblast proliferation was examined either on D5 or D21 after
irradiation.
Figure 1. Experimental device used to record and quantify locomotor activity and oviposition movements (adapted with permission from Cayre et al., 1996b). The horizontal probe
received ␥-rays only when the tip of the ovipositor was inserted into the egg-laying substrate.
The vertical probe received levels of radiation inversely proportional to its distance from the
source. Pb, Lead wall; 192Ir, iridium wire that emits ␥-rays.
Estimation of cell proliferation and cell death after irradiation
5-Bromo, 2⬘-deoxyuridine labeling. The thymidine analog 5-bromo, 2⬘deoxyuridine (BrdU), a marker of cell proliferation, was used on mushroom body spreadings as a fast method to evaluate the efficiency of
irradiation on neuroblast destruction and to determine the optimal ␥-ray
dose.
Ten microliters of BrdU (40 mg/ml; Sigma, St. Louis, MO) were injected 2 hr before insect dissection. Then the insect brain was dissected
out; the neural sheath was removed; and each mushroom body cortex
was excised and spread as a cell monolayer onto a polylysinated glass
slide. Immunocytochemical detection of BrdU was then performed as
described elsewhere (Scotto-Lomassese et al., 2000). BrdU-labeled cells
in each spreading (i.e., per mushroom body) were counted under a microscope (Axiophot; Zeiss, Oberkochen, Germany).
Feulgen–Rossenbeck nuclear staining. Because the “spreading” technique does not preserve the integrity of the structure, we also performed
histological serial sections of the brain followed by Feulgen–Rossenbeck
nuclear staining, which allows visualization and counting of both pyknotic and mitotic cells.
Control and irradiated females were examined at the ages of 1, 4, and
7 d. Cerebral ganglia were quickly dissected out in saline and then fixed
for 6 hr in Carnoy’s fixative. After three 24 hr washes in 95% ethanol and
three 24 hr washes in 1-butanol, tissues were embedded in paraffin and
cut in 6 m serial sections. Sections were deparaffinized, rehydrated, and
treated for DNA staining according to the method of Feulgen–Rossenbeck: DNA was hydrolyzed using 6N HCl for 1 hr and stained with
Schiff’s reagent (1 hr) at room temperature. Sections were counterstained in 0.4% picro-indigo-carmin. Mitotic cells and pyknotic nuclei
were counted on serial sections of mushroom bodies. To correct the
overestimation linked to counting the same nucleus on two adjacent
sections, the following formula (Abercrombie, 1946) was applied: N ⫽
[(n ⫻ t)/(d ⫹ t)], where t was section thickness; d, nucleus diameter; and
n, number of counted nuclei.
Data acquisition for ovipositor movements and locomotor
activity recordings
Seven days after irradiation (D7), ovipositor movements and locomotor
activity were continuously detected by radio tracking (Fig. 1) over 8
consecutive days. A 192Ir wire (1 mm length, 0.3 mm diameter, 1.5–2
MBq activity) was glued to the tip of the female cricket’s ovipositor (Rage
Figure 2. Experimental device of the escape paradigm used to evaluate the learning and
retention abilities of control and irradiated crickets. The cricket, placed in the center of the
arena, has to choose between two open holes to find the one that will allow it to escape into a
large cage. Olfactory cues, visual cues, or both are proposed. Here, only olfactory cues are
represented. The legs of the arena have not been represented for clarity.
and Renucci, 1985), and the ␥-rays emitted by the radio element were
detected and converted to electrical impulses by two scintillation probes
(SMIG/E 724; Nardeux) recording locomotor activity or ovipositor
movements, respectively (Renucci et al., 1992). The scintillation probes
were interfaced with a personal computer (IPC), which stored the data at
fixed 6 sec intervals. Data were then pooled in 30 sec sequences.
Escape paradigm
The device was a circular arena, the wall of which was perforated by 12
equally spaced holes allowing the cricket to escape (Fig. 2). For the
present study, only two holes were opened, one leading to a reward, a
large dim cage with food and water, and the other leading to a punishment, a small tube where animals were trapped. Olfactory cues, visual
cues, or both were available to enable the animal to differentiate the two
holes. Filter papers impregnated with odorant cues were stuck inside the
tubes leading either to the escape cage or to the trap, 7 cm away from the
hole in the arena wall (Fig. 2). Orange odor (Pranarom-natessence,
1:500) was associated with the reward hole, and vanilla odor (natural
food extract of vanilla) was associated with the punishment hole. These
associations were chosen because crickets usually exhibit a spontaneous
Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
preference for vanilla over orange. They were thus forced to reverse their
initial preference. Associating a preferred odor with the punishment and
a less attractive odor with the reward strengthens the significance of the
animal’s choice on the basis of learning rather than at random. Visual
cues were rectangles (5.5 ⫻ 5 cm), which were black, white, or striped,
glued on the gray wall of the arena right above the holes. To rule out the
possibility that other visual stimuli outside the arena could affect the
animal’s behavior, the arena was rotated by 45° between each trial, and
experiments were performed in a dark room with a strong light immediately above the apparatus, specifically illuminating the arena.
Two days before the beginning of the experiment (D7), controls and
irradiated animals were deprived of food. The initial spontaneous preferences of crickets for orange or vanilla were tested 4 d before the beginning of the training session using olfactory cues, in two sessions of 10 min
each according to the protocol of Matsumoto and Mizunami (2000), and
the very few crickets that exhibited an initial preference for orange over
vanilla were discarded.
One training session was performed each day and consisted of 10 trials.
In the first two trials, only one hole was opened (leading randomly to
punishment or to reward); therefore, we made sure that the insect successively visited the two holes. In the next eight trials, the two holes were
opened, and the crickets had to choose between them. Usually, crickets
spent a few minutes exploring the arena before choosing a hole (1 min 24
sec ⫾ 11 sec for control crickets vs 1 min 46 sec ⫾ 13 sec for irradiated
crickets; p ⫽ 0.196); insects that remained in the arena ⬎10 min without
entering any hole were discarded from the experiment (in total, only two
control and four irradiated females had thus been discarded). Punishment and reward lasted 8 min each. The daily training session was repeated (with a maximum of 5 d) until the animal’s score reached 7 of 8 or
8 of 8, indicating that the task had been learned.
Two retention sessions were performed 24 hr and 7 d after the last
learning session and consisted of one choice trial without reward or
punishment so that reactivation of learning could be excluded. For each
trial, we calculated the retention index (RI), which is the number of
successful animals minus the number of unsuccessful animals divided by
the total number of crickets. Therefore, an RI of 1 means that all animals
chose the correct hole; a null RI means that half the animals tested were
successful and half were wrong; and an RI of –1 means that all animals
chose the wrong hole (i.e., the hole leading to the trap).
Data analysis
The number of BrdU-labeled cells was expressed as mean ⫾ SEM per
brain, and the significance of differences was evaluated using the nonparametric Mann–Whitney U test. For behavioral analysis, Cox’s test of
survival and a variant analysis (t test) were chosen to compare the rate of
acquisition and the retention abilities between control and irradiated
animals.
Results
Determination of the optimum dose of ␥ irradiation
The effect of irradiation on neuroblast proliferation was examined 5 and 21 d after exposure to the cesium source. Whatever the
delay, irradiation induced a dose-dependent reduction in the
number of BrdU labeled neuroblasts and ganglion mother cells.
As shown in Figure 3, the values draw asymptote-shaped curves,
pointing out that 40 Gy was the lowest ␥-ray dose allowing a
prominent effect. At this dose, only 10% of the neuroblasts kept
proliferating after 5 d, and these values dropped to only 1% 3
weeks after irradiation. Consequently, the dose of 40 Gy was
chosen for all the following experiments.
Furthermore, to check a posteriori the efficiency of irradiation
for each irradiated insect subjected to the “escape paradigm,” all
the insects used in behavioral studies were injected with BrdU on
day 4, and at the end of the experiment, they were killed, and the
number of BrdU-labeled cells in their mushroom bodies was
counted. We observed that all irradiated crickets used in the behavioral experiments exhibited a strong decrease in neuroblast
J. Neurosci., October 15, 2003 • 23(28):9289 –9296 • 9291
Figure 3. Dose–response effect of ␥ irradiation with 137Cs on neuroblast proliferation. Irradiation was performed on the first day of adult life (D0). BrdU was injected either on D5 (solid
line) or on D21 (dotted line), and crickets were killed 2 hr after BrdU injection. Error bars indicate
SEM. At 5 d, n ⫽ 6, 6, 6, 3, 5, 8, and 6, respectively, for increasing doses; at 21 d, n ⫽ 6, 2, 3, 6,
6, 4, and 3, respectively, for increasing doses. The arrow indicates that the dose of 40 Gy was
chosen for the following experiments.
proliferation (22.3 ⫾ 5.8 BrdU-labeled cells in irradiated insects
vs 718.9 ⫾ 169.4 cells in control animals).
Effects of irradiation on mushroom body cells
After Feulgen–Rossenbeck staining, mitotic cells were easily recognizable by their condensed chromatin, showing metaphase or
anaphase figures, whereas small, dense nuclei characterized degenerating cells (Fig. 4). Neuroblasts are grouped in a cluster of
cells localized at the apex of the mushroom body cortex (Fig. 4 A);
their nucleus is twice as big as that of differentiated Kenyon cells.
GMCs lie immediately beneath the neuroblasts and are of intermediate size (Cayre et al., 1994). Newly born Kenyon cells fill the
most central part of the cortex, whereas older mature interneurons are located more laterally (Cayre et al., 1996a; Malaterre et
al., 2002).
In control brains, a few pyknotic nuclei were randomly scattered among the mature Kenyon cells (6.3 ⫾ 1.7, 4.2 ⫾ 2.0, and
4.9 ⫾ 1.8 at days 1, 4, and 7, respectively; n ⫽ 3, 6, and 5). By
contrast, 1 d after 40 Gy irradiation, although only a few degenerating neuroblasts could be seen, numerous pyknotic nuclei
(197.4 ⫾ 35.0; n ⫽ 5), corresponding to GMCs and newly generated interneurons, were observed at the bottom of the proliferative area (Fig. 4 B). Only rare pyknotic figures were found among
mature Kenyon cells. Four days after irradiation, the proliferative
area looked disorganized, and pyknotic figures (40.4 ⫾ 7.5 nuclei;
n ⫽ 5) corresponded to neuroblasts (Fig. 4C). Seven days after
irradiation, the number of pyknotic cells, mostly neuroblasts,
decreased (19.5 ⫾ 6.6; n ⫽ 4). No difference in the number of
pyknotic mature interneurons could be demonstrated between
irradiated and control crickets. The mitotic index, defined as the
number of mitotic figures observed in the proliferative area 1–7 d
after irradiation, was strongly and significantly reduced compared with that of controls (day 1, 0.2 ⫾ 0.2 vs 32.5 ⫾ 2.2; p ⬍
0.0001; day 2, 2.2 ⫾ 0.2 vs 29.5 ⫾ 1.8; p ⬍ 0.002; day 7, 0.3 ⫾ 0.3
vs 23.0 ⫾ 2.5; p ⬍ 0.008). These results emphasize that although
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Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
40 Gy ␥-ray irradiation was very efficient in killing proliferating
cells or immature neurons, it did not induce apoptosis of differentiated interneurons.
Evaluation of the general effect of irradiation
Ovarian maturation in young adult females requires extensive
cell proliferation. We thus explored the effect of irradiation on
ovarian maturation by measuring the length of the longest distal
oocytes and by checking the presence of vitellogenic mature oocytes in the abdomen. In both control and irradiated crickets,
ovaries appeared to be fully developed, with ⬎10 vitellogenic
chorionated, free oocytes in the abdominal cavity. The longest
nonchorionated distal oocytes exceeded 850 m in all cases.
Therefore, no significant difference could be observed between
the two experimental groups. This suggests that the lead plate
used to protect the insect body was highly efficient; therefore,
only the insect head was exposed to noxious irradiation.
Moreover, neither locomotor activity (1011 ⫾ 99.9 movements/d for control females vs 930 ⫾ 137.1 for irradiated ones;
p ⫽ 0.64), used as an index of good health, nor the innate egglaying behavior (83.62 ⫾ 58.44 oviposition sequences/d for control females vs 50.35 ⫾ 30.05 for irradiated ones; p ⫽ 0.63) were
altered in irradiated animals.
In addition, we checked that irradiated crickets were still able
to smell and discriminate between orange and vanilla, the two
odors used as olfactory cues in our associative learning paradigm.
Using the odor preference test described by Matsumoto and Mizunami (2000), we observed that irradiated crickets spent more
time on the vanilla than on the orange source, indicating a clear
preference for vanilla over orange, as already observed in control
crickets (93 ⫾ 3% of the time spent on an odor source was spent
on vanilla for control crickets vs 92 ⫾ 8% for irradiated crickets).
Thus, as with a 40 Gy dose, the general physiology of the
irradiated females was not affected; irradiation appears to be a
method compatible with behavioral studies.
Effects of irradiation on learning performance in the
escape paradigm
Olfactory cues
As previously quoted, a majority of both control and irradiated
crickets exhibited a stronger preference for vanilla than for orange. The few animals that presented a spontaneous preference
for orange were not used in the escape paradigm because the
orange odor was associated with the reward, and the vanilla odor
was associated with the punishment.
Compared with control animals, irradiation induced a significant impairment of learning abilities when crickets were provided with olfactory cues only ( p ⫽ 0.016). As shown in Figure
5A, acquisition was delayed in irradiated animals compared with
control ones. As early as the first session, 35% of control animals
were successful, whereas none of the irradiated females succeeded
( p ⫽ 0.024). On day 2, 76% of control crickets had learned the
task versus only 15% of irradiated ones ( p ⫽ 0.003). After four
sessions, although the task was mastered by 94% of control animals, only 77% of irradiated animals succeeded.
Figure 4. Effect of 40 Gy ␥-irradiation on mushroom body cells. Histological sections of the
mushroom body cortex stained with the nuclear coloration of Feulgen–Rossenbeck are shown.
The proliferative area is delineated by black asterisks. Irradiation was performed on the first day
of adult life (D0). A, Before irradiation. Arrows indicate dividing neuroblasts in M-phase. B, One
day after irradiation. The arrowhead indicates pyknotic nuclei of one neuroblast; small arrows
point to pyknotic ganglion mother cells; and long arrows indicate pyknotic immature neurons.
C, Four days after irradiation. Numerous neuroblasts exhibit pyknotic morphology, indicating
their degenerescence (arrowheads). No mitotic figures can be observed. Scale bars, 20 m.
Visual cues
Acquisition in control animals was slower than for olfactory discrimination, but 100% of them learned the task in four sessions
(Fig. 5B). Conversely, irradiated animals learned faster with visual than with olfactory cues. Therefore, there was no significantdifference in the time course of learning between control and
irradiated crickets provided with visual cues alone ( p ⫽ 0.098).
Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
J. Neurosci., October 15, 2003 • 23(28):9289 –9296 • 9293
not disturbed (Fig. 6) because these postlearning irradiated animals remembered the
task almost as well as control animals (RI,
0.64). This observation suggests that irradiation did not affect the neural networks
involved in the formation of the memory.
By contrast, suppressing neuron production by irradiation on D0 produced adult
crickets with retention deficiencies in our
olfactory conditioning.
Visual cues
Although crickets were able to learn the
visual task, as early as 24 hr after training,
neither irradiated nor control animals
could perform it correctly (Fig. 5E). This
performance was not improved after 7 d.
Figure 5. Effect of irradiation on learning and retention performance of crickets in the escape paradigm when olfactory ( A, D),
visual ( B, E ), or both olfactory and visual ( C, F ) cues were provided. Irradiation was performed on the first day of adult life (D0);
behavioral study started 7 d after irradiation. Retention tests were performed 1 and 7 d after the last learning session. The RI is the
number of successful animals minus number of unsuccessful animals divided by total number of animals tested. *Significant
difference ( p ⬍ 0.05) between the two experimental groups (control vs irradiated).
However, whereas all control females were successful after five
training sessions, 22% of irradiated females never mastered the
task properly.
Olfactory and visual cues
When the animals were provided with both olfactory and visual
cues, acquisition was very fast, and 100% of the animals learned
the task in three or four training sessions (Fig. 5C). The performances for the two groups were thus better than for visual or
olfactory cues alone. No difference between control and irradiated groups was observed either in the speed of acquisition or in
the number of successful insects ( p ⫽ 0.371).
Our results suggest that the destruction of mushroom body
neuroblasts by irradiation affects learning performance of the
insects specifically when the conditioning relies on olfactory cues.
Effects of irradiation on retention performance
Olfactory cues
Twenty-four hours after the end of acquisition, control crickets
presented a high retention score (RI, 0.82; Fig. 5D). By contrast,
the RI of irradiated crickets was greatly reduced (RI, 0.2; p ⫽
0.05). Seven days after learning, the RI of irradiated females was
null, indicating a random choice of the holes by the animals.
Control crickets presented a positive but low RI, suggesting that
retention of the conditioning was weak after this time lapse.
To check whether irradiation disturbed the mature Kenyon
cell networks, successful control animals, which were provided
with olfactory cues, were exposed to ␥-rays 6 hr after the last
learning session. Their retention ability, evaluated 24 hr later, was
Olfactory and visual cues
When both visual and olfactory cues were
present during conditioning, the RI at 24
hr after learning was twice as high in the
control group as in the irradiated group
(Fig. 5F ). However, 7 d after the last training session, the RI of control crickets was
slightly negative, suggesting that they
could not remember the task properly. Interestingly, in irradiated crickets, the RIs
even reached highly negative scores, indicating that most of them chose the odor
they initially preferred.
Discussion
To examine the functional significance of
adult neurogenesis in the adult cricket
brain, we developed a method for selectively killing mushroom body neuroblasts and examined the behavioral performance of crickets lacking adult neurogenesis. Our
results show for the first time that newly generated neurons improve learning speed in operant olfactory conditioning.
Hydroxyurea, an antimitotic drug commonly used to prevent
mushroom body formation in larval Drosophila and honeybees
(de Belle and Heisenberg, 1994; Malun, 1998), failed to give satisfactory results in adult crickets because doses necessary to obtain robust reduction of neurogenesis presented toxic effects (M.
Cayre, unpublished results). We therefore searched for a more
selective way to destroy neuroblasts. Taking advantage of the
greater sensitivity of proliferating cells to ionizing irradiation
compared with postmitotic cells, we performed insect head irradiation to kill progenitor cells. Progenitor cells of the hippocampus and subventricular zone may be destroyed by irradiation
(Peißner et al., 1999; Tada et al., 1999, 2000). Irradiation had
previously been used successfully during the development of the
moth Manduca sexta to reduce the population of glial cells in the
antennal lobe (Oland et al., 1988). Apart from a few dividing cells
present close to the medulla of the optic lobe, previous extensive
BrdU labeling of the adult cricket brain demonstrated that the
mushroom body cortex was the main brain structure where neural progenitors persisted (Cayre et al., 1996a). Our results clearly
demonstrated the efficiency of ␥-ray exposure because a dose of
40 Gy was necessary and sufficient to trigger an irreversible suppression of cell proliferation. In irradiated crickets, pyknotic figures were restricted to neuroblasts and to newly generated cells;
mature Kenyon cells were not affected by 40 Gy irradiation. Fur-
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Figure 6. Effect of postlearning irradiation on retention performances. Irradiation was performed 6 hr after the last learning session; retention performances were tested 24 hr later. A,
Learning performances of untreated crickets provided with olfactory cues (n ⫽ 11). B, Retention performances 1 d after irradiation. The RI of these crickets irradiated after learning is
compared here with the RI of the control crickets of Figure 5D. No significant difference is
observed between the two groups.
thermore, locomotion and ovarian maturation of irradiated females were similar to those of control females. The efficiency of
irradiation for neuroblast destruction together with the absence
of side effects validated its use for subsequent behavioral analysis.
Mechanical, chemical, and genetic manipulations have demonstrated the involvement of mushroom bodies in learning and
memory processes (for review, see Erber et al., 1987; Davis, 1996).
We thus hypothesized that newly generated Kenyon cells might
play a role in learning tasks. Proboscis extension reflex (PER)
conditioning and electric conditioning are commonly used, respectively, in honeybees and Drosophila (Takeda, 1961; Quinn et
al., 1974; Bitterman et al., 1983). In both species, mushroom body
neuroblasts degenerate before adulthood (Ito and Hotta, 1992;
Farris et al., 1999; Ganeshina et al., 2000). Therefore, because
adult neurogenesis is not found in these species, the cricket is
currently one of the best invertebrate model species for studying
the functional role of newly generated neurons in the adult animal. The learning tests used in flies and bees were not easily
transposable to the house cricket. We first tried the Tennessee
Williams paradigm used for the cockroach (Mizunami et al.,
1993, 1998), itself adapted from the water maze paradigm used in
rodents and consisting of one unheated platform hidden in a
heated surface. Unfortunately, it did not work because the first
motivation of crickets was to escape from the closed arena. This
escape behavior was repeatedly observed in different types of tests
we tried. This led us to develop a new learning paradigm based on
the natural tendency of the house cricket to escape from a closed
box, which we named the escape paradigm. The insect was asked
to choose between two holes, one allowing it to escape and the
other one leading to a trap. Either olfactory or visual conditioning
can be achieved. To minimize the possibility of success by luck,
we considered that crickets had learned the task only if they made
zero ( p ⫽ 0.004) or one mistake ( p ⫽ 0.035) in eight consecutive
trials. Whatever the sensory mode used for conditioning (visual
or olfactory cues), control crickets appeared to learn the task in a
Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
few days because after five learning sessions, almost all of them
were successful. These results validate this new operant conditioning for our insect model. The retention index indicated that,
whereas 90% of control crickets were able to remember the task
24 hr after acquisition when olfactory cues were used, they forgot
the task very quickly when visual cues alone were provided. These
data suggest that the visual cues that we provided failed to induce
strong memory of the conditioning. We do not exclude that other
visual patterns might be more efficient to establish stable memory. It is noteworthy that crickets are nocturnal insects that rely
on olfaction for foraging, and visual information might not be
particularly relevant for them. Seven days later, control crickets
failed to remember the task, whatever the sensory mode used. By
contrast, in another cricket (Gryllus bimaculatus), the retention
of elementary olfactory conditioning has been shown to last up to
10 weeks (Matsumoto and Mizunami, 2002). In contrast to the
honeybee and Drosophila, in which specific molecular mechanisms underlying short-, medium-, and long-term memory have
been identified, nothing is known about the different phases of
memory in Gryllidae. Because the escape paradigm is developed
over several days, the kinetics are completely different from those
involved in the PER conditioning used to study memory phases.
It is therefore hazardous to establish parallels between these two
models.
In contrast to control animals, learning performance of irradiated crickets depended on the type of sensory cues used. When
olfactory cues alone were provided, learning was significantly
retarded compared with that of control crickets, and retention
was strongly reduced. These learning and memory deficiencies
were unlikely to be attributable to altered olfaction ability of irradiated crickets because they still presented a clear preference for
vanilla over orange, as control crickets did. When visual cues
alone were provided, irradiated crickets exhibited slightly slower
learning, but it was not significantly different from that of controls, suggesting that visual abilities were not highly affected by
irradiation. When both sensory cues were concomitantly supplied, acquisition speed and the success rate of irradiated crickets
were identical to those of control animals. Interestingly, whereas
acquisition was improved by the association of visual and olfactory cues, retention performances of irradiated insects seemed to
be impaired by this association. This suggests that different processes and neural networks are involved during acquisition and
memory storage, because it has been shown that different molecular mechanisms underlie short-, medium-, and long term memory (for review, see Menzel, 1999, 2001). The fact that irradiated
crickets learned as well as control insects when both cues were
provided allows us to rule out the hypothesis that altered performances could be ascribed to nonspecific deleterious effects of
irradiation. In addition, to definitely rule out unspecific irradiation effects, we performed a control experiment in which intact
crickets were allowed to learn the olfactory task and were then
irradiated between the end of acquisition and the retention test.
Because these animals exhibited normal retention scores, it seems
that irradiation per se did not affect neuronal transmission.
In all our behavioral analyses, training sessions started 1 week
after irradiation and lasted for up to 5 d; consequently, retention
tests were performed up to 19 d after irradiation. Thus, irradiated
crickets lacked all the neurons that should have been generated
during this period. Previous immunohistological studies combining BrdU and lachesin labeling (a marker of insect immature
neurons) showed that newborn Kenyon cells begin to extend
neurites into the peduncle a few days after their birth and project
into specific areas of the pedunculus and of the lobes (Malaterre
Scotto-Lomassese et al. • Impaired Learning in Crickets Lacking Adult Neurogenesis
et al., 2002). Our results clearly show that adult-born neurons are
not essential for learning but that they do influence learning kinetics. Reasonable assumptions can be made about immature
neurons compared with mature neurons: because they are structurally plastic, they are highly susceptible to environmental
changes and to different life experiences (Gould and Gross 2002).
In cricket mushroom bodies, immunostaining and electron microscopic studies showed that the subpopulation of immature
Kenyon cells expressed different neurotransmitters and presented specific arborization patterns (Strambi et al., 1998; Schürmann et al., 2000; Malaterre et al., 2002). Thus, newly born Kenyon cells probably present higher plasticity in terms of
synaptogenesis and modulation of synaptic strength in response
to environmental cues. In adult rodents, newborn hippocampal
neurons display more robust long-term potentiation than mature interneurons (Wang et al., 2000). Thus, the continual influx
of neurons that are temporarily immature (rather than a total
increase in neuron number) could underlie behavioral abilities
(Gould and Gross, 2002). We suggest that these immature neurons could increase the acquisition rate in this complex olfactory
operant paradigm.
We cannot totally rule out the possibility that the behavioral
effects observed could be the consequence of the diminution of
the total mushroom body neuron number rather than of the
absence of newly generated neurons. However, this hypothesis
seems unlikely. Numerous studies emphasize that the mushroom
body cortex is not constituted of a homogenous group of interneurons but is composed of different populations of Kenyon
cells, characterized by the expression of different transcription
factors and signaling proteins (Yang et al., 1995; Crittenden et al.,
1998; Sinakevitch et al., 2001). Such mushroom body subdivisions are highly suggestive of different functional roles for these
interneurons.
Using hydroxyurea during larval development, Malun et al.
(2002) obtained honeybees with partial mushroom body ablation
and demonstrated that it did not affect acquisition and retention
of PER conditioning. These authors concluded that mushroom
body integrity was not necessary for elementary olfactory discrimination tasks, for which antennal lobes might be sufficient.
In contrast to the PER conditioning, our olfactory conditioning
does not rely on a simple reflex but is a more complex conditioning in which the cricket has to make a choice. Mushroom bodies
might be involved in complex learning tasks but not in simple
elementary tasks, as supported by the work of Liu et al. (1999),
showing that mushroom bodies are not required for visual learning in Drosophila but are needed for context generalization in
visual conditioning. In future studies, it would therefore be interesting to investigate whether irradiated cricket performance
would be impaired in more complex visual conditioning (e.g.,
with more than two opened holes) or whether the sensory modality used for conditioning is really a determinant in crickets
lacking adult neurogenesis, as suggested by the present results.
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