Contribution of the parafascicular nucleus in the spontaneous object

Contribution of the parafascicular nucleus in the spontaneous object

Neurobiology of Learning and Memory 96 (2011) 272–279
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Neurobiology of Learning and Memory
journal homepage: www.elsevier.com/locate/ynlme
Contribution of the parafascicular nucleus in the spontaneous object
recognition task
Edwin Castiblanco-Piñeros, Maria Fernanda Quiroz-Padilla ⇑,
Carlos Andres Cardenas-Palacio, Fernando P. Cardenas
Laboratorio de Bases Biológicas del Comportamiento, Facultad de Psicología, Universidad de la Sabana, Bogotá, Colombia
Laboratorio de Neurociencias y Comportamiento, Departamento Psicología, Universidad de los Andes, Bogotá, Colombia
a r t i c l e
i n f o
Article history:
Received 24 February 2011
Revised 11 May 2011
Accepted 13 May 2011
Available online 23 May 2011
Keywords:
Intralaminar thalamic nuclei
NMDA lesions
Spontaneous object recognition task
Prefrontal cortex
Relational memory
a b s t r a c t
The parafascicular (PF) nucleus, a posterior component of the intralaminar nuclei of the thalamus, is considered to be an essential structure in the feedback systems of basal ganglia–thalamo-cortical circuits
critically involved in cognitive processes. The specific role played by multimodal information encoded
in PF neurons in learning and memory processes is still unclear. We conducted two experiments to investigate the role of the PF in the spontaneous object recognition (SOR) task. The behavioral effects of pretraining rats with bilateral lesions of PF with N-methyl-D-aspartate (NMDA) were compared to vehicle
controls. In the first experiment, rats were tested on their ability to remember the association immediately after training trials and in the second experiment after a 24 h delay. Our findings provide evidence
that PF lesions critically affect both SOR tests and support its role in that non-spatial form of relational
memory.
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
The parafascicular nucleus (PF) in rats and the centromedian
parafascicular complex (CM–PF) in primates and other mammals
are the so-called posterior intralaminar nuclei (pIL) of the thalamus.
These nuclei are a critical component of the ascending reticular activating system (ARAS) and the basal ganglia–thalamocortical circuit
(Groenewegen & Berendse, 1994; Parent & Parent, 2005; Van der
Werf, Witter, & Groenewegen, 2002). In general, damage to intralaminar nuclei has been associated with signs of amnesia (Bailey &
Mair, 2005; Mitchell & Dalrymple-Alford, 2005; Newman & Burk,
2005; Quiroz-Padilla, Marti-Nicolovius, & Guillazo-Blanch, 2010;
Savage, Castillo, & Langlais, 1998; Van der Werf, Jolles, Witter, & Uylings, 2003), and functions as arousal (Mancia & Marini, 1995;
Marini, Tredici, & Mancia, 1998; Shirvalkar, Seth, Schiff, & Herrera,
2006; Steriade & Deschenes, 1984; Van der Werf et al. 2002), attention (Bailey & Mair, 2005; Burk & Mair, 2001; Kinomura, Larsson,
Gulyas, & Roland, 1996; Minamimoto & Kimura, 2002; Newman &
Burk, 2005; Newman & Mair, 2007) and pain (Cheng et al., 2009;
Dupouy & Zajac, 1997; Gao et al., 2010; Liu, Qiao, & Dafny, 1993;
Vogt, Hof, Friedman, Sikes, & Vogt, 2008), in humans and other
animals.
⇑ Corresponding author. Address: Facultad de Psicología, Universidad de la
Sabana. Campus Universitario del Puente del Común, Autopista Norte de Bogotá,
DC, Colombia. Fax: +57 861 2721.
E-mail address: [email protected] (M.F. Quiroz-Padilla).
1074-7427/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.nlm.2011.05.004
Single-axon tracing studies have indicated that virtually all PF
neurons project to both cerebral cortex and striatum (Deschenes,
Bourassa, Doan, & Parent, 1996). Anatomically, the PF is a major
source of excitatory projections to striatum and prefrontal cortices
(PFC), mainly prelimbic, and less abundantly to cingulate regions
(Berendse & Groenewegen, 1991; Deschenes et al., 1996; Marini,
Pianca, & Tredici, 1996; Parent & Parent, 2005; Smith, Raju, Pare,
& Sidibe, 2004; Vercelli, Marini, & Tredici, 2003). In relation to striatum projections, the lateral PF projects to the putamen, the lateral
globus pallidus and more diffusely to dorsolateral caudate, areas
that are related to sensory-motor function. The medial PF projects
to the ventral pallidus and in lesser amounts to the accumbens and
olfactory tubercle, regions both related to associative-limbic functions (Otake & Nakamura, 1998; Smith et al., 2004; Van der Werf
et al., 2002). The cognitive deficits observed after PF lesions are
thought to arise from the combined deafferentation of PFC and striatum targets (Van der Werf et al., 2003). The effect of PF manipulations on memory functions should be specifically related to the
neural systems innerved. The PF has a strategic location to be considered an excellent candidate for investigating memory processes.
However, before concluding PF participation in non-spatial relational memory it is necessary to rule out any possible deficits in
locomotion induced by the excitotoxic lesion. So an open field locomotion assessment was carried out before the spontaneous object
recognition (SOR).
The link between PF and cognitive processes has traditionally
been investigated through rat studies of radiofrequency (Nyakas,
E. Castiblanco-Piñeros et al. / Neurobiology of Learning and Memory 96 (2011) 272–279
Veldhuis, & De Wied, 1985; v Wimerdma Greidanus, Bohus, & de
Wied, 1974) or electrolytic (Cardo & Valade, 1965; GuillazoBlanch et al., 1995; Massanes-Rotger, Aldavert-Vera, SeguraTorres, Marti-Nicolovius, & Morgado-Bernal, 1998; Redolar-Ripoll
et al., 2003; Shapovalova, Pominova, & Dyubkacheva, 1997;
Thompson, Kao, & Yang, 1981; Tikhonravov, 2000) lesions. There
are only three studies (M’Harzi, Jarrard, Willig, Palacios &
Delacour, 1991; Quiroz-Padilla, Guillazo-Blanch, Vale-Martinez, &
Marti-Nicolovius, 2006; Quiroz-Padilla, Guillazo-Blanch, ValeMartinez, Torras-Garcia, & Marti-Nicolovius, 2007) evaluating the
effects of excitotoxic lesions restricted to PF area in four different
memory tasks: two of procedural memory (the appetitively motivated odor discrimination and the aversively motivated two-way active avoidance tasks) (Quiroz-Padilla et al., 2007), and two of
relational memory (the object recognition and the social transmission of food preference (M’Harzi et al., 1991; Quiroz-Padilla et al.,
2006). The lack of knowledge of the specific participation of PF in
memory processes urges the need to continue researching on its role
in cognitive function.
The main purpose of the present study is to identify the contribution of the PF nuclei to non-spatial relational memory by using
the SOR task. Ennaceur and Delacour (1988) proposed differential
patterns of exploration for familiar and novel objects maybe as a
result of the natural tendency to explore novel objects. An important advantage of SOR is that no aversive/stressful stimuli are
needed. Other feature of the task is the requirement of perirhinal
(PRh) cortex integrity to discriminate novel objects (Aggleton,
Albasser, Aggleton, Poirier, & Pearce, 2010; Brown & Aggleton,
2001; Clark & Squire 2010; Murray & Bussey, 1999; Winters,
Saksida, & Bussey, 2008). Previous studies (Bussey, Duck, Muir,
and Aggleton, 2000; Ennaceur, Neave, & Aggleton, 1996) reported
impaired object recognition memory in the SOR task following
neurotoxic PRh damage in rat. The PRh cortex is responsible for
familiarity discrimination however, this kind of memory involves
interaction with other structures outside the medial temporal lobe
as PFC. According to this idea we can speculate that PF lesions
decreased the cortical activation required to discriminate significant events in the SOR task, therefore memory can be affected. This
could presumably be a result of PF deafferentation of important
system components of basal ganglia–thalamic-cortex circuit, particularly PFC (Berendse & Groenewegen, 1991; Deschenes et al.,
1996; Heidbreder & Groenewegen, 2003; Hsu & Price, 2007;
Macchi & Bentivoglio, 1986; Marini et al., 1996; Otake &
Nakamura, 1998; Parent & Parent, 2005; Sadikot & Rymar, 2009;
Smith et al., 2004; Van der Werf et al., 2002; Vercelli et al., 2003;
Vertes, 2004).
The finding of impairments in two different retention delays:
immediately and/or 24 h after training, in rats submitted to bilateral lesion with N-methyl-D-aspartate (NMDA) should support
the contribution of PF in the memory process of the SOR task.
273
(60 60 40 cm high) with the floor covered with an acrylic
surface. The objects were made of plastic, aluminum or glass with
differences in color, height (between 8 and 15 cm), weight (between
55 and 226 g), shape and texture (Fig. 1). After each session, the
objects and surface of the open box were cleaned with acetic acid
(10%) to avoid olfactory cues.
2.1.2. Analysis of behavior
The animals were videotaped while performing both the
locomotion tests and the SOR task. Object exploration was operationally defined as physical contact with the object through
forepaws or snout. Distances >2 cm, as well as sitting or standing
on the object with the nose facing the ceiling were not considered
exploratory behavior. For the SOR task training the dependent
variables were: the mean time spent exploring the objects and
the mean average frequency of contact with the objects. For the
SOR task test, a discrimination ratio was used (discrimination
ratio = novel object exploration/total interaction with all the
objects) (see Bevins & Besheer, 2006).
The frequency of following behaviors were analyzed in the locomotion tests: crossing the lines that divided the cylindrical open
field bearing in mind that the animal’s hind paws crossed the line
between each quadrant; standing on their hind paws in the field
(rearing); licking or scratching themselves (grooming); elongation
of the head and shoulders followed by retraction to its original
position (stretching); and remaining stationary showing piloerection (freezing). Each of these behaviors was measured before and
after of the surgery, and a delta score was obtained; this delta score
is defined as: frequency of the behavior before surgery - frequency
of the behavior after surgery. These dates were collected with the
software X-Plo-Rat 2005 1.1. (Taverna-Chaim & Morato, 2008).
2.1.3. Measurements and statistical analysis
All statistical analyses were carried out by using SPSS for
Windows, version 17.0. The comparison of the averages of the
groups for all SOR task measures was done using Student t test
for independent samples. In order to compare the results in locomotion tests, a Student t test for independent samples was done
using delta scores for each behavior (crossing, rearing, grooming
and stretching). Other analyses were carried out to assess the effect
of bilateral PF lesions on object exploratory behaviors and therefore discard any effect of lesion on the SOR task. In this regard,
General Lineal Model for repeated measures was used with corresponding contrasts (simple for between-groups effects and polynomial and repeated for within-group effects), in which the
independent variables were Group (Lesion and Vehicle) and the
2. Materials and methods
2.1. General methods: Experiments 1 and 2
Both experiments were performed taking into account the
ethical and legal standards required for laboratory animal research
in Colombia (Estatuto Nacional de Protecciòn Animal – Law 84 of
1989 and Resolution Number 008430 of 1993 of the Ministerio de
la Salud).
2.1.1. Apparatus
A cylindrical open field (55 60 cm; diameter and height) was
used to assess locomotion before and 6 days after surgery. The SOR
task was carried out in an open box made of wood
Fig. 1. Photograph of the objects used in the spontaneous recognition task.
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dependent variables were: approaches 62 cm, contact with forepaws, and snout. In the statistical procedures used in this study
an alpha of p 6 0.05 were assumed, with a reliability of 95%.
2.2. Experiment 1
2.2.1. Subjects
Twenty-four male Wistar rats obtained from Inmunopharmos
laboratories were used in the experiment. At the beginning of the
study the mean age of the animals was 98 days (SE = 3.87) and
mean weight of 291.48 g (SE = 8.09). Throughout the period of
the experiment the animals were housed individually in plasticbottomed cages (48 38 20 cm) with sawdust bedding. During
all stages of the investigation the animals were kept under controlled temperature settings (20–24 °C), humidity (40–70%) and a
12 h light–dark cycle (lights on at 8:00 a.m.). All experimental procedures were performed during artificial light cycle. During the
experiment the animals were supplied with food and water ad libitum, and their weight and cleanliness were monitored.
2.2.2. Surgical procedure
Before surgical procedure, the animals were randomly assigned
to Lesion or Vehicle groups. Rats were anaesthetized with
intraperitoneal injection of ketamine hydrochloride (ketamine
50Ò, Holliday-Scott SA, 75 mg/kg) and xylazine (SetonÒ, Calier;
10 mg/kg), and placed in a stereotaxic instrument (Stoelting Co.,
Model 51725) with the incisor bar set 3.3 mm below the interaural
line. Before the skin incisions, local anesthesia was administered
by subcutaneous application of lidocaine (Roxicaina 1%Ò, Ropsohn
Therapeutics Ltda). The skull was exposed through a midline incision and leveled along the bregma-lambda axis. Two holes were
drilled in the cranium of the animals and a 26-gauge needle Hamilton syringe was lowered to the targets according to the following
stereotaxic coordinates (Paxinos & Watson, 1997): anteroposterior
(AP) – 4.1 mm from bregma, lateral (L) ± 0.6 mm from midline, and
dorsoventral (DV) – 6.1 mm and – 5.6 mm from the skull surface.
Before the infusion, the needle was left in place for 30 s. Animals
belonging to the lesion group were infused bilaterally at the PF nucleus the excitotoxin NMDA (Sigma–AldrichÒ, 0.15 M in sterile
phosphate-buffered, saline, pH 7.4). A digital microinjector (Stoelting Quintessential Injector, model 5331) was used for all
infusions. The total volume of infusion for each hemisphere was
1.2 ll. At the first depth 0.8 at a rate of 0.133 ll per minute were
infused. While 0.4 at a rate of 0.1 ll per minute were infused at
the second depth. The first infusion was made in the DV deeper
coordinate (DV – 0.61 mm). The needle was allowed to sit for
10 min before being raised to avoid the spread of NMDA up the
needle tract. The scalp was then sutured and a topical antiseptic
was administered to prevent infections (IsodineÒ, Boehringer
Ingelheim S.A.). Throughout the procedure, the animaĺs body
temperature was kept constant with a thermal blanket. Vehicle
rats underwent the same procedure, except that sterile phosphate-buffered saline was infused.
2.2.3. Behavioral tasks
In the first phase of the experiment, each rat was kept in individual plastic cages. In order to animals get familiarized to the
investigator, all rats were handled for 5-min each day for a period
of 4 days. On the fifth day the locomotion pre surgery test was
done by placing the animals in the cylindrical open field for
5 min. Then the rats had 2 days without treatment before the surgery. All animals were allowed to recover for 7 days previous to
locomotion post surgery test, which was performed for 5 min in
the same cylindrical open field used in the pre surgery tests. Next
day, the SOR task was applied to the animals. First, each rat had a
habituation session for five minutes in the open field (above
described) without objects. Then, three 5-min training trials were
done, with inter-trial intervals of 2 min. In each trial, the animals
were placed in the corner of the open field where there was not
object, looking at the vertex of the walls. All other three corners
had a different object. These three objects were the same in all
three trials for each subject, but among trials the objects were
rotated taking care of do not to repeat the location, to reduce place
preferences. The test session took place two minutes after completion of training trials and lasted five minutes. In this session, one of
the three objects previously used (familiar objects) was replaced
by a new object with which the animal had never had contact.
2.2.4. Histology
After the behavioral procedures, all animals were deeply anesthetized with an overdose of sodium pentobarbital (EuthanexÒ,
Invet s.a., 200 mg/kg) and were perfused transcardially with 0.9%
saline followed by 10% formalin-saline. Brains were extracted
and post-fixed in formalin for at least 24 h and then submerged
in a 30% sucrose solution prior to sectioning. Coronal 40-lm
sections were cut out on a cryostat (Leica MicrosystemsÒ, model
CM1850), mounted and stained with Cresyl violet. The sections
were examined under a light microscope (NikonÒ Eclipse 80i)
and microphotographs were taken with a digital camera (Nikon
Ds-Fi1). The tissue evaluation was done using a blind strategy:
two observers who were not aware of the behavioral results independently examined the brain sections. The lesions were reconstructed on standardized sections of the rat brain (Paxinos &
Watson, 1997).
2.3. Experiment 2
2.3.1. Subjects
Twenty-four male Wistar rats obtained from Inmunopharmos
laboratories were used in the experiment. At the beginning of the
study the mean age of the animals was 100 days (SE = 4.7) and
mean weight of 295,98 g (SE = 12.16). During all stages of the
study, the animals were handled with the same parameters for
maintenance and control described in Experiment 1.
2.3.2. Surgical procedure
Before surgical procedure, rats were randomly assigned to
LESION or VEHICLE groups. The surgical protocol was the same
as in Experiment 1,
2.3.3. Behavioral tasks
The experimental conditions were similar to those used in
Experiment 1, except that the test session took place 24 h after
completion of training trials.
2.3.4. Histology
The histological procedures and the criteria applied were the
same as in Experiment 1.
3. Results
3.1. Histology
Fig. 2A and B shows the maximum and minimum extents of
successful PF lesions for each plate (figures modified from Paxinos
& Watson, 1997). Final sample included only rats with bilateral
lesions P40% in the PF, damage that extended from 3.80 mm to
4.52 mm posterior to bregma (Paxinos & Watson, 1997). The
lesions were characterized by loss of neurons, glyosis and traces
of necrosis adjacent to the tracks made by the needle (Fig. 3A–
D). The axons in the fasciculus retroflexus remained visible in all
E. Castiblanco-Piñeros et al. / Neurobiology of Learning and Memory 96 (2011) 272–279
275
Fig. 2. Schematic drawing of the smallest (gray area) and the largest (striped area) PF lesions in successive anterior/posterior coronal sections in Experiment 1 (A) and
Experiment 2 (B). The extent of the lesions is superimposed on figures modified from Paxinos and Watson (1997).
subjects included in the final sample. For both experiments, histological analyses were performed by two independent blind
observers.
3.2. Behavior
3.2.1. Experiment 1
The final sample was made up of 18 rats distributed into LESION
(n = 8) and VEHICLE (n = 10) groups.
3.2.1.1. Locomotion test. In order to assess any possible effect of surgery upon locomotion and motility, Student t tests were conducted
on delta scores (differences between data at the pre and post surgery
sessions) (see page 8 for description) for crossings, grooming, rearing, stretching and freezing. Independent samples t-test analysis
showed that there were not differences between the groups in delta
scores for crossings [t(16) = 0563; p = 0581], rearing [t(16) = 0.07;
p = 0945], grooming [t(16) = 0989; p = 0337] and stretching
[t(16) = 0243; p = 0811]. None of the subjects regardless of the group
showed immobility or piloerection (freezing). These results indicate
that lesions did not affect the locomotor activity of animals.
3.2.1.2. Spontaneous object recognition task. Fig. 4A shows the
discrimination ratio of the novel object for all the groups in the
test. Student t-test showed significant differences between the
groups in the discrimination ratio [t(16) = 2162; p = 0.0461
< 0.05]. This finding suggests that lesions in PF change the normal
functioning of object recognition memory when performing the
test 2 min. after training. There were no significant differences between groups in the total time exploring objects during the trial
sessions [t(16) = 1983; p = 0065], in the total frequency of contact
with objects in the trial sessions [t(16) = 0587; p = 0566] and in
the total time exploring objects during the test session
[t(16) = 1277; p = 0220] (Fig. 4 B). General lineal model for repeated measures showed no significant differences in the exploration time [F(1,16) = 3935, P = 0.65], as well as the frequencies of
approaches 62 cm [F(1,16) = 0136, P = 0, 717], and contacts with
forepaws [F(1,16) = 2947, P = 0, 105], and the snout [F(1,16) = 0330,
P = 0, 574].
3.2.2. Experiment 2
The final sample was made up of 17 rats distributed into LESION
(n = 7) and VEHICLE (n = 10) groups.
3.2.2.1. Locomotion test. Independent samples t-test analysis
showed that there were not differences between the groups in delta
scores for crossings [t(15) = 0245; p = 0810], rearing [t(15) = 0095;
p = 0926], grooming [t(15) = 0373; p = 0715] and stretching
[t(15) = 0967; p = 0349]. None of the subjects regardless of the
group showed immobility or piloerection (freezing). The findings
are consistent with those found in Experiment 1, confirming that
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Fig. 3. Photomicrographs of Cresyl violet-stained coronal sections (40 lm thick), about 4.30 mm posterior to bregma, showing the appearance of a typical NMDA bilateral PF
lesion (A) and (B) compared to a vehicle-infused subject (C) and (D). Scale bar = 1.00 mm in (A), (B), (C) and (D). PF, parafascicular; fr, fasciculus retroflexus.
Fig. 4. Experiment 1, (A) Discrimination ratio of the novel object two minutes after training trials for both groups. (B) Differences between groups in the total time exploring
objects during the trial sessions (⁄P < 0.05).
the PF nucleus lesions did not affect the locomotor activity of the
experimental subjects.
3.2.2.2. Spontaneous object recognition task. Analysis of the discrimination ratio revealed that there are significant differences
between the control group and lesion group [t(15) = 4890;
p < 0.001] (Fig. 5 A). This result suggests that lesions in PF produce
a deficit in object recognition memory at 24 h. Moreover, There
were no significant differences between groups in the total time
exploring objects during the trial sessions [t(15) = 0238;
p = 0815], in the total frequency of contact with objects in the trial
sessions [t(15) = 0.091; p = 0929] and in the total time exploring
objects during the test session [t(15) = 1707; p = 0108] (Fig. 5 B).
To complement the analysis of exploratory behavior, general
lineal model for repeated measures was conducted for the trial sessions. There were no significant differences between groups
[F(1,15) = 0057, P = 0815]. This result supports the evidence that
the PF nucleus lesion did not affect exploratory behavior. In the
analysis of object exploration during the trial phases there were
no differences groups for the frequency of approaches 6 2 cm
[F(1,15) = 0031, P = 0863], the frequency of contacts with snout
[F(1,15) = 0.41, P = 0842], and with forepaws [F(1,15) = 1282,
P = 0275]. These results, together with those obtained in Experiment 1, suggest that bilateral lesions of the PF nucleus did not
affect object exploration and, therefore, did not influence animal
performance in SOR task.
E. Castiblanco-Piñeros et al. / Neurobiology of Learning and Memory 96 (2011) 272–279
277
Fig. 5. Experiment 2, (A) Discrimination ratio of the novel object 24 h after training trials for both groups. (B) Differences between groups in the total time exploring objects
during the trial sessions (⁄⁄⁄P < 0.001).
4. Discussion
The present study evaluated the effects of bilateral NMDA
lesions of the PF nucleus on a SOR task. In this paradigm, rats were
required to remember familiar objects immediately or 24 h after.
The main results showed that PF lesions impair SOR at both retention delays. These findings suggest that PF is involved in the object
recognition memory.
Regarding to exploration or motor behavior in both experiments, there were no significant differences between groups in
crossing, rearing and stretching frequency in the open field. There
were no significant differences between groups in both frequency
or time spent exploring the sample objects in the SOR task or in
other exploratory behaviors (approaches 6 2 cm, contact with forepaws and snout). No significant changes were observed in the overall exploration time in the test session.
Altogether, these findings dismiss the incidence of variables
other than memory and maybe learning on the performance of lesioned animals in the SOR task. The explanations for the results obtained in this investigation are based on the fact that the PF
nucleus participates in the ARAS, as well as in the basal ganglia–
thalamic-cortex circuit; both systems related to generalized activation or arousal (Jones, 2003; Quiroz-Padilla et al., 2010), attention
(Heidbreder & Groenewegen, 2003; Hulme, Whiteley, & Shipp,
2010; Matsumoto, Minamimoto, Graybiel, & Kimura, 2001; Minamimoto, Hori, & Kimura, 2005; Minamimoto & Kimura, 2002; Raeva, 2006; Smith et al., 2004), learning and memory (GuillazoBlanch et al., 1995; Massanes-Rotger et al., 1998; Quiroz-Padilla
et al., 2006; Quiroz-Padilla et al., 2007).
According to this evidence it is hypothesized that the PF lesions
decreased the cortical activation required to discriminate significant events in the SOR task, therefore memory can be affected. This
could presumably be a result of PF deafferentation of important
system components of basal ganglia–thalamic-cortex circuit, particularly PFC (Berendse & Groenewegen, 1991; Deschenes et al.,
1996; Heidbreder & Groenewegen, 2003; Hsu & Price, 2007; Macchi & Bentivoglio, 1986; Marini et al., 1996; Otake & Nakamura,
1998; Parent & Parent, 2005; Sadikot & Rymar, 2009; Smith
et al., 2004; Van der Werf et al., 2002; Vercelli et al., 2003; Vertes,
2004).
There are some reports on memory impairment after lesions in
the intralaminar nuclei, including the PF nucleus, very similar to
those caused by hypofunction of PFC (Burk & Mair, 1998; Van
der Werf et al., 2003). Indeed, there have been reported deficits
in working memory after thalamic damage, including the PF
nucleus, in different animal models used to evaluate this kind of
memory (Burk & Mair, 1999; Langlais & Savage, 1995; Porter, Koch,
& Mair, 2001; Savage et al., 1998). Furthermore, Smith, Countryman, Sahuque, and Colombo (2007) found that the acquisition
and remembering in the socially transmitted food preference task
(model of non-spatial relational memory) induces activation of the
medial prefrontal cortices (PFCm), specifically in the prelimbic and
infralimbic cortex.
Concerning to object recognition memory, there are studies
showing connection between this kind of memory and PFCm
cortex, especially with the prelimbic area (Barker, Bird, Alexander,
& Warburton, 2007; Christoffersen et al., 2008; DeVito &
Eichenbaum, 2010; Levallet, Hotte, Boulouard, & Dauphin, 2009).
Barker et al. (2007), suggest that object recognition memory in rats
requires judgments on the previous occurrence of stimuli made on
the basis of the relative familiarity with each object. It has been
proposed that PFCm may be related to the discrimination required
for object recognition test in order to establish an order or timing
of the objects presentation (Dere, Huston, & De Souza Silva, 2007).
Our results contrast with those reported by M’Harzi, et al.
(1991), who found no impairment on object recognition memory
after ibotenic acid lesion of PF in rats. This divergence in results
may be due to some methodological differences such as the kind
of the object recognition task used. In the experiments reported
here, three trials before the test session were done using three objects per session, whereas in the study of M’Harzi et al. (1991),
there was only one trial before test and two objects per session.
Our variation increases the difficulty of the SOR task, demanding
so a greater degree of arousal and attention in order to properly acquire and store it. Moreover, this study adds to emerging evidence
of the involvement of PF non-spatial relational memory, as was
suggested in the study reported by Quiroz-Padilla et al. (2006),
and complements the work involving PF with other kinds of relational memory (Bailey & Mair, 2005; Burk & Mair, 1998; Lopez
et al., 2009; Porter et al., 2001; Savage, Sweet, Castillo, & Langlais,
1997; Savage et al., 1998; Van der Werf et al., 2003).
Given that an optimal level of alertness is required for learning
and behavior to occur, and that the PFCm is critically involved in
alertness (Robbins & Everitt, 1995; Valdés & Torrealba, 2006), it is
possible that the deficit observed in object recognition test could
be the result of alterations in glutamatergic neurons projecting from
PF to PFCm, particularly to prelimbic cortex – region directly related
to ARAS and relational memory processes. The deafferentation of the
prelimbic cortex could have decreased the cortical activation required for encoding information and memory formation. This idea
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is consistent with the findings reported by DeCoteau, McElvaine,
Smolentzov, and Kesner (2009), according to which the animals with
prelimbic/infralimbic cortex lesions displayed a profound and sustained deficit, in a go/no-go visual discrimination task.
According to Minamimoto and Kimura (2002), PF is involved in
tasks requiring shifts in response patterns. Differential exploratory
behavior on near objects could be understood as a response pattern.
It could be the case that PF lesioned animals failed to shift the
exploratory pattern between new and familiar objects resembling
so an attentional failure leading to alterations in memory formation. From this stand, it is possible that the SOR task used here could
include some components similar to those found in flexibility tasks
(i.e. enhancement of exploration of novel object and decrease of
familiar objects exploration) and, as a result of PF lesions, the subject is unable to make a behavioral shift between ongoing behavior
and appropriated responses to each kind of object.
The crucial role of PFC has been well documented in this kind of
behavioral shifts (Egerton et al., 2008; Floresco, Block & Tse, 2008;
Parsegian, Glen, Lavin, & See, 2011). As already stated by Brown,
Baker, and Ragozzino (2010), the role of PF on PFC activation could
be indirectly mediated by striatal cholinergic neurons related to
behavioral flexibility. They found that PF lesions cause a decrease
in acetylcholine release which concomitantly impaired tasks requiring flexibility as reversal learning. This effect could be mediated by
the decreased cholinergic activity in the striatal-cortical projection
to PFC. In fact, it has been proposed that one of the functions of PF relates to the processing of stimuli that were previously irrelevant,
requiring the selection of different actions (Brown et al., 2010).
It could be also the case that PF lesions lead to a possible attention deficit interfering with the performance in object recognition
task and not exactly a failure in cortical arousal. As above mentioned, PF is also related to attentional processes; in fact, some
authors have proposed that the preference for novelty (traditionally assessed by SOR tasks) could reflect deficits in attention rather
than alterations of relational memory (Desimone & Duncan, 1995;
Gaskin et al., 2010). However, as already mentioned, in this study
some changes in the SOR task made it more complex and, therefore, demanded greater attention for the acquisition and retention
of the task. In addition, the analysis of exploratory behavior during
the three trials and the test session did not show any differences
between control group and lesioned animals, so, it is possible that
PF lesion did not imply attention deficits as the only explanation
for the memory deficits observed.
In conclusion, our results suggest that PF may be involved in
modulation of object recognition memory. The contribution of PF
in memory may be linked to its role in the basal ganglia–
thalamic-cortex circuit. Hence, PF damage may have reduced the
activation of specific brain regions necessary for helping animals
to cope with changing task demands.
Acknowledgments
This research was supported by funds provided by the Fondo
financiero de investigación de la Universidad de la Sabana and by
FAPA-GRANT from Universidad de los Andes. The authors thank
Dr. Angela Trujillo and Dr. Claudio Da Cunha for their helpful comments. These data were presented at the 2009 European Brain and
Behaviour Society Meeting (Rhodas-Grecia, 13th and 14th
September).
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