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Volume 16, Number 9,
Issue of May 1, 1996
pp. 3019-3025
Copyright ©1996 Society for Neuroscience
Chronic 3-Nitropropionic Acid Treatment in Baboons Replicates the
Cognitive and Motor Deficits of Huntington's Disease
Stéphane Palfi1, 2,
Robert J. Ferrante3,
Emmanuel Brouillet1,
M. Flint Beal4,
Robert Dolan1,
Marie Caroline Guyot1,
Marc Peschanski2, and
Philippe Hantraye1
1 URA CEA-CNRS 1285, Service Hospitalier
Frédéric Joliot, DRIPP, CEA-DSV, 91401 ORSAY Cedex,
France, 2 INSERM U 421, Faculté de Médecine,
94010 CRETEIL Cedex, France, 3 GRECC Unit, 182B, Bedford
Veteran Administration Medical Center, Bedford, Massachusetts 01730, and Department of Neurology, Boston University Medical School, Boston,
Massachusetts 02118, and 4 Neurochemistry Laboratory,
Department of Neurology, Massachusetts General Hospital, Boston,
Massachusetts 02114
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
We showed recently that chronic administration of the mitochondrial
inhibitor 3-nitropropionic acid (3NP) in primates produces various
dyskinetic movements and dystonic postures associated with selective
striatal lesions displaying many similarities with the pathological
features of Huntington's disease (HD). In the present study, we
examined whether such a toxic treatment could also induce frontal-type
deficits similar to those observed in HD patients. Cognitive
performances of 3NP-treated and control baboons were compared using the
object retrieval detour task (ORDT), a test designed to assess the
functional integrity of the frontostriatal pathway in human and
nonhuman primates. During the same time, the motor function of each
animal was assessed under spontaneous ``no drug'' conditions, and
time-sampled neurological observations were used after apomorphine
administration. A significant impairment in the ORDT was observed in
the 3NP animals after 3-6 weeks of treatment, occurring in the absence
of spontaneous abnormal movements but in the presence of
apomorphine-inducible dyskinesias. Prolonged 3NP treatment
resulted in the progressive appearance of spontaneous abnormal
movements. Histological evaluation of these animals showed selective
bilateral caudate-putamen lesions with sparing of the cerebral cortex,
notably the prefrontal cortex. The present study demonstrates that
chronic 3NP treatment replicates in primates the basic
pathophysiological triad of HD, including spontaneous abnormal
movements, progressive striatal degeneration, and a frontostriatal
syndrome of cognitive impairment.
Key words:
Huntington's disease;
basal ganglia;
caudate
nucleus;
frontal cortex;
baboon;
3-nitropropionic acid;
object
retrieval detour task;
cognitive behavior;
motor behavior;
apomorphine
testing
INTRODUCTION
Huntington's disease (HD) is a dominantly
inherited neurodegenerative disorder characterized by involuntary
choreiform movements and progressive cell loss in brain regions
belonging to the basal ganglia, particularly the striatum (Martin and
Gusella, 1986 ; Harper, 1991 ; Kremer et al., 1992 ). In addition, most HD
patients display memory disturbances, psychiatric manifestations,
frontal-type cognitive abnormalities, decreased verbal fluency, and
poor recall of recently learned information (Butters et al., 1978 ;
Brandt et al., 1988 ; Massman et al., 1990 ; Pillon et al., 1991 ;
Randolph, 1991 ; Diamond et al., 1992 ). Most of these cognitive
alterations resemble those observed in patients with lesions of the
dorsolateral prefrontal cortex (Cummings, 1993 ).
Despite the recent discovery of the genetic mutation associated with HD
(The Huntington's Disease Collaborative Group, 1993 ), the cause of
this disorder is still unknown. One possibility is that the genetic
defect may lead to an impairment of energy metabolism resulting in slow
excitotoxic neuronal death (Albin and Greenamyre, 1992 ; Beal, 1992 ). In
line with this, several studies have reported decreased striatal
glucose metabolism (Kuhl et al., 1982 ; Young et al., 1986 ; Mazziotta et
al., 1987 ; Berent et al., 1988 ; Grafton et al., 1990 ; Kuwert et al.,
1990 ; Martin et al., 1992 ), abnormalities in mitochondrial electron
transport chain (Brennan et al., 1985 ; Mann et al., 1990 ; Parker et
al., 1990 ), and increased concentrations of lactate in the cerebral
cortex and the striatum as detected by nuclear magnetic resonance
spectroscopy, providing in vivo evidence for impaired
oxidative energy metabolism in HD (Jenkins et al., 1993 ). Furthermore,
in support of the energy impairment hypothesis, chronic blockade of
energy metabolism induced in rats by systemic administration of
mitochondrial toxins induces selective excitotoxic-like lesions of the
striatum (Beal et al., 1993 ; Brouillet et al., 1994 ). We recently
extended this observation to primates by demonstrating that chronic
systemic administration to baboons of 3-nitropropionic acid (3NP), an
irreversible inhibitor of succinate dehydrogenase (for review, see
Ludolph et al., 1991 ), can produce various dyskinetic movements and
dystonic postures associated with selective striatal lesions (Brouillet
et al., 1993 , 1995 ; Ferrante et al., 1993 ). To date, however, no
studies have examined the effect of such a neurotoxic treatment on
cognitive aspects of behavior.
In the present study, the cognitive performances of 3NP-treated
and control baboons were compared using the object retrieval detour
task (ORDT), a test designed to assess the functional integrity of the
frontostriatal pathway in both humans and nonhuman primates (Diamond,
1990 ). At the same time, the motor function in these animals was
studied under spontaneous ``no drug'' conditions and in the
apomorphine test, a test designed to unmask presymptomatic striatal
dysfunction in excitotoxically lesioned baboons (Hantraye et al., 1990 )
and 3NP-treated baboons (Brouillet et al., 1995 ). This combination of
tests was used to study a possible association of frontal-type deficits
with presymptomatic or clinically detectable striatal dysfunction.
Finally, histological analysis of brain sections of control and
3NP-treated baboons was conducted to correlate cognitive and motor
deficits with anatomical lesions.
MATERIALS AND METHODS
Animals
Fourteen Papio anubis baboons (Charles River Labs,
Wilmington, MA) (7-10 kg body weight) were included in this study.
Four baboons received a chronic systemic 3NP treatment, whereas the
remaining 10 received saline injections and served as control animals
for the behavioral studies. All animals were housed individually in
standard primate cages with free access to water and food, except for
bananas, which were available only while the animals were performing
the ORDT. Studies were carried out in accordance with the European
convention for animal care and the guide for the care and use of
laboratory animals adopted by the National Institutes of Health
(Bethesda, MD).
3NP lesion
3NP (Sigma, St. Louis, MO) was dissolved at a concentration of
80 mg/ml in deionized water and adjusted to pH 7.4 with NaOH (1 M). The four baboons (young adolescents) received
repeated intramuscular injections of 3NP (administered in daily
injections at 10 A.M. and 4 P.M.) for 20 weeks. In accordance with
previous studies (Brouillet et al., 1995 ), the dose regimen for the
administration of 3NP was designed to yield a linear increase in doses
until obvious spontaneous abnormal movements could be observed (mean
starting dose, 14 mg·kg 1·d 1; mean
final dose, 33 mg·kg 1·d 1). As
shown in Figure 1, administered doses were adjusted
slightly according to the physical status of each baboon, but these
alterations did not diverge significantly from the linear dose schedule
(r = 0.95; p < 0.001).
Fig. 1.
Schematic representation of the experimental
design used in this study. The average daily dose of 3NP administered
to the four baboons is indicated as a function of time. Dose regimen
was adjusted every week according to the physical status of each animal
to avoid any acute intoxication (Brouillet et al., 1995 ). The various
behavioral tests used to assess the presence of motor deficits
(apomorphine tests, APO1-3) or frontostriatal cognitive
dysfunction (ORDT) in the 3NP-treated baboons, are
represented as open boxes. Note that APO1 and APO2 were
performed immediately before and after the ORDT testing to assess the
functional status of the striatum at that particular time period and
possibly correlate cognitive deficits with presymptomatic striatal
dysfunction.
[View Larger Version of this Image (20K GIF file)]
Behavioral assessment
Clinical examinations were performed before each injection to
detect any signs of acute intoxication and the presence of spontaneous
abnormal movements. In addition, two behavioral methods were used:
time-sampled neurological observations after intramuscular
administration of apomorphine (Hantraye et al., 1990 , 1992b ; Brouillet
et al., 1995 ) and the ORDT (Diamond and Goldman-Rakic, 1985 ; Diamond et
al., 1989 , 1990; Taylor et al., 1990a ,b; Schneider, 1992 ).
The apomorphine test. We previously used apomorphine
administration in excitotoxically lesioned baboons to reveal
preclinical dysfunction after partial striatal lesions. In these
initial studies, the apomorphine test results were shown to be highly
reproducible for all symptoms observed: individual animals responding
with a similar set and incidence of dyskinesias over several successive
test sessions with no change in symptoms for at least 1 year (Isacson
et al., 1991 ; Hantraye et al., 1992b ). In the present study,
time-sampled neurological observations were obtained after
intramuscular administration of 0.5 mg/kg apomorphine, as described
previously (Hantraye et al., 1990 , 1992b ). Briefly, four different
categories of abnormal movements (orofacial dyskinesia, dyskinesia of
extremities, dystonia, and choreiform movements) were monitored after
video-recording from front-view images and were rated as being present
(= 1) or absent (= 0) during each 5 min time period of a 40 min test
session. A dyskinesia index (sum of incidences) was computed by adding
together the incidence of each symptom during the 40 min test period
(maximum score = 32, minimum score = 0). Five normal baboons selected
randomly among the 10 controls at the beginning of the study and the
four 3NP-treated baboons received apomorphine testing. As shown in
Figure 1, the first apomorphine test (APO 1) was performed
the week before the first ORDT test session (corresponding to 3 weeks
of treatment). The second apomorphine test (APO 2) was
performed immediately after the last ORDT test session (corresponding
to 7 weeks of treatment). Finally, the last apomorphine test (APO
3) was performed 6-10 weeks after the last ORDT test-session,
i.e., after 13-17 weeks of treatment (Fig. 1).
The ORDT. The ORDT was used to assess the ability of the
animals to retrieve an object (a slice of banana) from inside a
transparent box open on only one side. We selected this task because it
requires complex sequential motor planning and is particularly
sensitive for detecting frontal cortex or striatal dysfunction (Diamond
and Goldman-Rakic, 1985 ; Diamond et al., 1989 , 1990; Taylor et al.,
1990a ,b; Schneider, 1992 ). In addition, the task involves features
implicated in motor skills; however, in the majority of the trials in
which the opening of the box is not facing the animal, the primates
must inhibit their natural tendency to reach straight for what they
want (prepotent response tendencies) and must make a detour around the
transparent side of the box into the open side of the box. Inhibition
of this natural tendency specifically requires the functional integrity
of the frontostriatal system (Diamond, 1990 ) and is not affected by
lesions of other brain regions such as the hippocampal formation or the
parietal cortex (Diamond and Goldman-Rakic, 1985 ; Diamond et al.,
1989 ).
The Plexiglas box (8 × 8 × 9 cm) was fixed on a tray adaptable to the
home cage of the animal. The cognitive level and motor skills required
to solve the task and retrieve the banana slice were modified by the
experimenter by varying the location of the box relative to the
subject, the location of the reward into the box, and finally the
orientation of the open side of the box relative to the subject. As
shown in Figure 2, each test session consisted of 15 trials presented randomly to the animals. The reward was visible only
after an opaque screen placed between the animal and the box was
raised. Subjects were then allowed 1 min to retrieve the reward, a
period after which the screen was put back in place and the box set up
for the next trial. The movements and responses of the animal involving
the tray or the box were not restrained in any way.
Fig. 2.
ORDT experimental setup. Diagrammatic
representation showing the position of the box on the tray, reward on
the box, and open side of the box on the 15 trials that constituted
each test session. In six of these configurations (1-6),
the open side of the box is facing the subject so that no detour is
required to reach the reward (``easy'' trials). In nine of these
configurations (7-15), a closed (transparent) side of the
box is facing the subject, so that a detour is required to retrieve the
reward (``difficult'' trials).
[View Larger Version of this Image (20K GIF file)]
Responses of the subject were recorded on videotape. Measures of
performance included number of ``success'' responses (retrieval of
the reward on the first reach of the trial), and number of
``correct'' responses (retrieval of the reward within the 1 min time
period; the correct responses also include the success responses),
number of ``barrier hits'' responses (hitting the closed transparent
side of the box instead of making a detour), and number of ``motor
problems'' responses [reaching the correct (open) side of the box but
failing to retrieve the reward]. The number of correct and success
responses are expressed as a percentage of the total number of trials.
Barrier hits and motor problems are expressed as percentage of
responses observed. Animal performances were also analyzed as a
function of the level of cognitive difficulty (``easy'' and
``difficult''). The easy trials were defined as the trials in which
the opening of the box was facing the baboon (no detour to make to
reach the reward; configurations 1-6 in Fig. 2), whereas difficult
trials were defined as the configurations in which the baboon had to
make a detour around a closed side of the box to reach the reward
(configurations 7-15 in Fig. 2). Animal testing consisted of four
consecutive test sessions separated by a 1 week interval.
Anatomical studies
Anatomical studies were carried out in the four 3NP-treated
primates, which were killed 4 d after termination of the 3NP treatment,
and in two control animals. Under chemical restraint (ketamine 10 mg/kg), animals were killed by intravenous administration of
pentobarbital (120 mg/kg). Their brains were removed rapidly from the
skull, cut in 2- to 3-cm-thick frontal blocks, postfixed for 48 hr in
paraformaldehyde 2% sodium-m-periodate lysine, cryoprotected in
phosphate buffer glycerol 20%, and kept frozen at 70°C.
Histochemical and immunocytochemical procedures [calbindin
immunodetection and nicotinamide adenine dinucleotide phosphate (NADPH)
histochemistry] were performed on 50 µm frozen sections as described
previously (Ferrante et al., 1993 ; Brouillet et al., 1995 ). Detailed
immunocytochemical and anatomical descriptions of the lesions observed
in these animals will also appear in a separate paper (R.J. Ferrante,
P. Hantraye, E. Brouillet, M.F. Beal, unpublished observations).
Magnetic resonance imaging (MRI)
Animals were anesthetized with ketamine/xylazine, and MRI
was performed on a 0.5 T MR magnet (General Electric) using a homemade
Helmotz receive-only probe as described previously (Hantraye et al.,
1992a ).
Statistical analyses
Tests of variance homogeneity, normality, and distribution were
performed to ensure that the assumptions required for a standard
parametric ANOVA were satisfied. Statistical analysis of the ORDT test
results was carried out using ANOVA (two factors repeated-measures
ANOVA) to study interactions between groups (3NP-treated vs control
subjects) or interactions between groups and session for each of the
measures examined. Factorial ANOVA was used to study possible
interactions between groups in the apomorphine testing as well as a
possible effect of repetition of the test on the ORDT performances. All
significant interactions were reanalyzed post hoc using Scheffe's
F test.
RESULTS
Histological observations
Chronic treatment with 3NP produced a marked, bilateral, neuronal
cell loss restricted to the caudate and putamen nuclei in the four
3NP-treated baboons, confirming previous findings (Brouillet et al.,
1995 ). As shown in Figure 3A,B on
representative frozen cut frontal sections, there was a bilateral
dorsoventral gradient of pathological effect within the striatum in
each of the animals. Lesions were restricted to the dorsal aspect of
the caudate nucleus and the putamen; the ventral striatum and the
nucleus accumbens were relatively spared (Fig. 3A). There
was also a rostrocaudal gradient of pathological damage; the rostral
part of the striatum (caudate and putamen) was more affected than its
most caudal part (Fig. 3A,B). MRI showed increased signal on
T2-weighted images in both the caudate and putamen, at completion of
the 3NP-treatment (Fig. 4A).
Immunocytochemically, the striatal lesions were characterized by a
severe loss in calbindin-positive neurons, a subset of medium-size
spiny projection neurons, and a relative sparing of the
NADPH-diaphorase-positive interneurons (Fig. 4C), as
compared with control specimen (Fig. 4B). No additional
damage was identified in any other area of the brain, including the
prefrontal cortex (Fig. 3 and data not shown).
Fig. 3.
Photographs of fixed frozen-cut frontal section of
the brain in a systemically treated primate using 3NP at the level of
the caudate nucleus, putamen, and nucleus accumbens (A) and
at the level of the anterior commissure and globus pallidus
(B). At each of the levels considered, lesions were
restricted to the dorsal aspect of the caudate nucleus and putamen; the
ventral striatum and the nucleus accumbens were relatively spared. No
other damage was identified in any other area of the brain. The lesion
loci appear well delineated as dark areas on these frozen
specimens. Scale bar, 1 cm.
[View Larger Version of this Image (138K GIF file)]
Fig. 4.
Striatal pathology in 3NP-treated primate.
A, MR T2-weighted image obtained after completion of the 3NP
in the same animal as in Figure 3. High signal intensity within the
caudate nucleus and putamen corresponds to tissue damage seen in
C. B and C, Double-stained sections
reacted to reveal the presence of calbindin, which labels a
subpopulation of medium-size spiny neurons, and NADPH-diaphorase
enzyme, which labels somatostatin/neuropeptide Y-interneurons in the
caudate nucleus of the 3NP-treated animal (C) in comparison
with a normal (control) specimen (B). Note the severe loss
in calbindin-positive neurons and the relative sparing of the
NADPH-diaphorase neurons (dark neurons with dendritic arbors) in the
treated animal as compared with the control. [Same magnification in
B and C; scale bar (shown in C): 100 µm.]
[View Larger Version of this Image (100K GIF file)]
Behavioral observations
Motor function under spontaneous ``no drug'' conditions
As previously noted in baboons with partial excitotoxic striatal
lesions (Hantraye et al., 1990 ), no spontaneous dyskinesias were
observed in the 3NP-treated animals during the initial part of the
neurotoxic treatment (weeks 1-11); however, in the final part of the
neurotoxic treatment, spontaneous abnormal movements, primarily foot
and leg dystonias, and some dyskinesias of the hindlimb were observed,
starting at ~11 weeks of treatment, which then persisted unchanged
until the end of the study (i.e., for at least 9 weeks).
Apomorphine-induced dyskinesias
As early as 3 weeks after treatment, apomorphine
administration elicited abnormal movements in the 3NP-treated animals.
These abnormal movements consisted of irregular and partial movements
of the extremities and the trunk resembling choreiform movements,
orofacial dyskinesias, and dystonias. Orofacial dyskinesias consisted
of repetitive jaw movements accompanied in some instances by protrusion
of the tongue. The most common forms of dystonia observed in these
baboons were twisting of the trunk and unilateral (occasionally
bilateral) foot dystonia. In control animals, only slow jaw movements
and repetitive licking behavior were elicited by apomorphine, which
were taken as a baseline for comparison with the orofacial dyskinesias
observed in the 3NP-treated animals. The dyskinesia index for abnormal
movements was significantly higher in the treated animals as compared
with the five control animals (Fig. 5)
(p < 0.02) tested under the same
conditions at the two latest time points (APO2 and
APO3, in Fig. 5). Although a clear tendency toward an
increased dyskinesia index was already visible at APO1
between controls and the 3NP-treated baboons, the difference between
the two groups of animals was not statistically significant at this
time point (p > 0.05).
Fig. 5.
Apomorphine test in control and 3NP-treated
baboons. The dyskinesia index in controls and the 3NP-treated animals
was determined at various time points during the chronic treatment with
3NP. APO1: 3 weeks of treatment; APO2: 7 weeks of
treatment; APO3: 13-17 weeks of treatment. Values are mean ± SEM. Black bars represent control animals
(CTRL, n = 5). Cross-hatched bars
represent 3NP-treated animals (n = 4). *p < 0.02; **p < 0.0002; ns (nonsignificant), ANOVA
analyzed post hoc using Scheffe's F test.
[View Larger Version of this Image (25K GIF file)]
Cognitive function
The ORDT testing was performed between APO 1 and APO 2 (see Fig.
1), i.e., when the animals were essentially nonsymptomatic under ``no
drug'' conditions but starting to show abnormal movements in the
apomorphine test (Fig. 5). At this time point, ANOVA repeated-measures
analysis indicated that 3NP-treated subjects were significantly less
successful in obtaining the reward on the first reach during testing
(F = 7.53; p < 0.02) and made significantly more
barrier hits when performing the task (F = 16.3;
p < 0.002) as compared with controls (Fig.
6A,C). These differences could not be
attributed to a physical disability of the 3NP-treated subjects,
because motor function (Fig. 6D) did not differ
significantly between the two groups (ANOVA, F = 0.492;
p > 0.49). In addition, 3NP-treated baboons were not
impaired significantly (F = 0.749; p > 0.40) in
the acquisition of the task, because their correct responses did not
differ significantly from controls (Fig. 6B).
Fig. 6.
ORDT performances in 4 3NP-treated baboons and 10 age-matched control animals. The percentage of ``SUCCESS''
(A), ``CORRECT'' (B),
``BARRIER HITS'' (C), and ``MOTOR
PROBLEMS'' (D) responses are represented as
black bars for control animals (n = 10) and
cross-hatched bars for the 3NP-treated animals (n = 4). Values are mean ± SEM. Statistical analysis was carried out
using ANOVA repeated-measures and then reanalyzed post hoc using
Scheffe's F test to determine significant interactions
between groups (3NP-treated vs control subjects). Success and correct
responses are expressed as a percentage of the total number of trials.
Correct responses include success responses. Barrier hits and motor
problems are expressed as a percentage of the total responses
registered. As compared with controls, 3NP animals seemed significantly
impaired in their ability to reach the reward in the first attempt
(success responses), which resulted in a higher number of barrier hits
that reflect a lack of inhibition of their natural prepotent tendency
and a dysfunction of the frontostriatal pathway.
[View Larger Version of this Image (30K GIF file)]
Statistical analysis of the session effect (Fig. 7)
showed a significant improvement between the first and last session in
the control group for success and correct responses (success:
F = 8.66, p < 0.001; correct: F = 3.23, p < 0.04) but not in the 3NP-treated group (success:
F = 0.09, p > 0.96; correct: F = 0.02, p > 0.99). A similar pattern was observed for barrier
hits responses (controls: F = 8.2, p < 0.001;
3NP-treated: F = 0.4, p > 0.75), indicating that
3NP-treated animals persisted in making errors in the ORDT test,
whereas control animals were learning to make a detour around the
closed transparent side of the box.
Fig. 7.
Session effect in the performances of 3NP-treated
baboons and control animals in the ORDT. The percentage of
``SUCCESS,'' ``BARRIER HITS,'' and
``CORRECT'' responses is represented for each
consecutive test session (S1, S2, S3,
S4). Black bars represent control animals
(n = 10), and cross-hatched bars represent
3NP-treated animals (n = 4). Values are mean ± SEM.
Statistical analysis was carried out using ANOVA factorial measures and
then reanalyzed post hoc using Scheffe's F test to
determine significant interactions between sessions (S1,
S2, S3, S4) in 3NP-treated animals and
controls. As compared with control animals whose performances in
success, barrier hits, and correct responses improved significantly
with time, the deficits in the ORDT observed in the 3NP animals
remained constant from one session to another. Significant differences
between groups were also analyzed for each test session by factorial
ANOVA: **p < 0.01; *p < 0.05 (3NP-treated
animals vs controls).
[View Larger Version of this Image (26K GIF file)]
Finally, when the level of cognitive difficulty was considered (Fig.
8), barrier hits performances of 3NP-treated animals
were significantly different from those of controls in the difficult
(F = 19.5; p < 0.001) but not in the easy trials
(F = 1.29; p > 0.25).
Fig. 8.
``Barrier hits'' responses of 3NP-treated
baboons and control animals according to the level of difficulty in the
ORDT. The percentage of barrier hits responses are represented for
easier front-reach trials (open side of the box directly facing the
animal) and for difficult trials (transparent side of the box
interposing the reward with the animal). Black bars
represent control animals. Cross-hatched bars represent
3NP-treated animals. Values are mean ± SEM. Statistical analysis was
carried out using factorial ANOVA and post hoc analysis using
Scheffe's F test to determine significant interactions
between groups. 3NP-treated animals seemed significantly different from
controls in the difficult but not the easy trials.
[View Larger Version of this Image (21K GIF file)]
DISCUSSION
We showed recently that chronic administration of the irreversible
succinate dehydrogenase inhibitor 3NP to nonhuman primates can produce
selective bilateral degeneration of the caudate-putamen complex
accompanied by spontaneous and apomorphine-induced abnormal movements
resembling those observed in HD (Brouillet et al., 1995 ). The striatal
lesions were strikingly reminiscent of those occurring in HD, with
selective loss of medium-size spiny neurons as compared with other
striatal cell types, and a dorsoventral gradient of neuronal
degeneration within both caudate and putamen. Additional histological
observations included a sparing of the nucleus accumbens, a
preservation of the patch-matrix compartmentalization of the striatum,
and the presence of growth-related neuronal alterations in spiny
striatal neurons (Ferrante et al., 1993 ; Brouillet et al., 1995 ; R.J.
Ferrante, P. Hantraye, E. Brouillet, M.F. Beal, unpublished
observations).
In the present study we aimed to characterize further this primate
model as a model of HD by studying whether such a chronic 3NP treatment
would also produce HD-like cognitive deficits in primates.
As shown here, chronic 3NP produced significant deficits in the ORDT.
Thus, success responses were significantly reduced, and barrier hits
responses were increased significantly in the 3NP-treated animals, as
compared with control baboons. When the level of cognitive difficulty
was taken into account, 3NP animals seemed significantly different from
controls only in the difficult trials, i.e., configurations for which a
detour around a closed transparent side of the box is necessary to
retrieve the reward. In contrast, the correct responses, which
represent the ability of each animal to finally reach the reward, were
not significantly different between the two groups, and no detectable
motor problems were noted in the 3NP-treated animals as compared with
controls.
Altogether these findings indicate that (1) alterations in motor
control or a disinterest of the animals in the task are not responsible
for the differences observed between the two groups and (2) 3NP animals
differ from controls in trials specifically requiring the inhibition of
a natural tendency to reach straight for a reward, which as discussed
by Diamond (1990) involves the frontostriatal pathway. Therefore, a
specific functional and/or anatomical dysfunction of the frontostriatal
pathway had to be present in the 3NP-treated baboons. Interestingly,
these observations were obtained in the first phase of the toxic
treatment during which the 3NP-treated animals did not show any
abnormal movements under spontaneous conditions. This indicates further
that the frontal-type cognitive deficits associated with the
3NP-treatment precede the onset of spontaneous dyskinesias in this
chronic model of striatal degeneration.
Similar cognitive deficits in the ORDT have been observed in
monkeys with lesions of the dorsolateral prefrontal cortex (Diamond and
Goldman-Rakic, 1985 ) or with severe striatal dopamine depletion
attributable to MPTP treatment (Taylor et al., 1990a ,b; Schneider,
1992 ). It is interesting to note that these two experimental conditions
as well as the 3NP-model described here directly affect either the
anatomical or the functional integrity of the frontal cortex or the
striatum. The cognitive deficit observed in the 3NP-treated baboons
after 5-7 weeks of treatment (ORDT sessions 3 and 4) is thus
reminiscent of frontal-type deficits resulting from a dysfunction of
the striatum, especially the dorsomedial caudate nucleus. This
interpretation is supported by two types of observations. First,
histological observations in these and other animals have demonstrated
that chronic 3NP treatment in primates is associated with lesions
restricted to the striatum, involving the dorsal and medial aspects of
both the caudate and putamen but sparing the nucleus accumbens and the
cerebral cortex, especially the prefrontal cortex. Second, by
performing the apomorphine test immediately before and after the
cognitive test sessions, we have shown that despite the absence of
spontaneous abnormal movements, there was a presymptomatic striatal
dysfunction in the 3NP-treated baboons at the time of ORDT testing.
Because previous experimental and clinical studies (Battig et al.,
1960 ; Divac et al., 1967 ; Cummings, 1993 ) have shown that striatal
lesions, notably those involving the medial caudate nucleus, which
receives dense projections from the orbitofrontal and anterior
cingulate cortex, and those involving the dorsolateral caudate, which
receives projections from the dorsolateral prefrontal cortex, can
produce personality alterations and cognitive impairments
indistinguishable from those accompanying frontal syndromes (Richfield
et al., 1987 ; Mendez et al., 1989 ), these observations indicate that
the frontal-type deficits in the 3NP-treated baboons have a striatal,
probably caudate, origin.
The time course analysis of the success and barrier hits showed
that 3NP-treated baboons did not improve their performances
significantly from the first test day to the fourth, whereas controls
significantly improved their performances from one test session to the
other. Such alterations in the acquisition of the task in the
3NP-treated animals may be interpreted as a learning deficit, but as in
the frontal syndrome observed in HD, which is primarily characterized
by deficits in executive function and organizational strategies for
learning tasks, the present results may also be interpreted as an
impairment in organizational strategy in the 3NP-treated animals.
Additional studies that would use more specific cognitive tasks
designed to assess more specifically frontal-type deficits (Battig et
al., 1960 ; Divac et al., 1967 ; Roberts et al., 1994 ), memory
disturbances (Chavoix et al., 1994 ), or organizational strategies are
required to characterize further the exact nature of the cognitive
deficit associated with a chronic 3NP treatment.
In conclusion, this new primate model offers several advantages for the
study of normal striatal function, as well as for the pathophysiology
of HD and the testing of new therapeutic strategies as compared with
the previously developed unilateral excitotoxic lesion model (Hantraye
et al., 1990 ). Because 3NP readily crosses the blood-brain barrier,
the toxin can be delivered by intramuscular injection. This allows the
generation of a model of progressive striatal degeneration associated
with selective bilateral (symmetrical) striatal lesions, which do not
require repeated intrastriatal injections. In addition, the chronic
nature of this lesion model enables one to study the time course of
cognitive deficits associated with various degrees of striatal atrophy
and their relationship with the onset of spontaneous dyskinesias. The
chronic administration of the mitochondrial toxin also produces the
progressive appearance of spontaneous abnormal movements, which persist
over time, and frontal-type deficits, which are particularly suitable
for assessment of the efficacy of neurotrophic/neuroprotective agents
(Beal, 1994 ; Beal et al., 1994 ) or neural grafting procedures (Hantraye
et al., 1992b ) on motor and cognitive aspects of the 3NP-induced
syndrome.
FOOTNOTES
Received Sept. 11, 1995; revised Jan. 23, 1996; accepted Feb. 9, 1996.
This study was supported by grants from CNRS-NSF and CNAMTS/INSERM.
S.P. was supported by fellowships from Bayer-Pharma (France),
Association Française contre les Myopathies (AFM), and
Association Huntington France. We thank Dr. M. Mazière and
Professor Y. Kéravel for continuing support and Dr. P. C. Jedynak
for expert behavioral observations. Jérôme Cayla and
Christophe Jouy are greatly acknowledged for the outstanding care of
the primate colony and for their help in the behavioral
experiments.
Correspondence should be addressed to Dr. Philippe Hantraye, URA
CEA-CNRS 1285, Service Hospitalier Frédéric Joliot, DRIPP,
CEA-DSV, 4 Place du Général Leclerc, 91401 Orsay Cedex,
France.
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