Skip to main content
NCBI home page
British Journal of Pharmacology logoLink to British Journal of Pharmacology
. 2010 Apr;159(8):1704–1715. doi: 10.1111/j.1476-5381.2010.00669.x

Soluble βamyloid1-42: a critical player in producing behavioural and biochemical changes evoking depressive-related state?

M Colaianna 1, P Tucci 1, M Zotti 1, MG Morgese 1, S Schiavone 1, S Govoni 2, V Cuomo 3, L Trabace 1
PMCID: PMC2925493  PMID: 20218978

Abstract

Background and purpose:

Depression is common in early phases of Alzheimer's disease (AD) and may represent prodromal symptoms of dementia. Recent reports suggest that early memory deficits and neuropsychiatric symptoms are caused by soluble rather than aggregated βamyloid (Aβ). Thus, we investigated the effects of soluble Aβ1-42 on working memory and depressive/anxiety-related behaviour in rats and on 5-hydroxytryptaminergic neurotransmission and neurotrophin content in various brain regions.

Experimental approach:

Behavioural reactivity to novel object recognition, open field, elevated plus maze and forced swimming test were assessed 7 days after i.c.v. injection of Aβ1-42 or its vehicle. BDNF (brain-derived neurotrophic factor) and NGF (nerve growth factor) mRNA and protein levels and 5-hydroxytriptamine (5-HT) content were measured in the prefrontal cortex (PFC), striatum (STR) and nucleus accumbens (NAc).

Key results:

Aβ1-42 did not affect the ability to distinguish between familiar and novel objects, but Aβ-treated rats exhibited an increase in forced swimming immobility. No differences were revealed between experimental groups in the elevated plus maze test or in self-grooming (evaluated in the open field). In the PFC, but not STR or NAc, Aβ-injected rats exhibited a selective reduction in 5-HT content, BDNF and NGF expression.

Conclusions and implications:

Our data suggest that soluble Aβ-treated rats have a depressive, but not anxiogenic-like, profile, accompanied by brain region-dependent alterations in the expression of neurotrophins and 5-hydroxytryptaminergic neurotransmission. Hence, these alterations induced by soluble Aβ might be sensitive indicators of early phases of AD and possible risk factors for the expression of neuropsychiatric symptoms in AD.

Keywords: soluble βamyloid, depression, prefrontal cortex, 5-hydroxytryptamine, BDNF, NGF, rats

Introduction

Mounting evidence suggests that depression is a risk factor for Alzheimer's disease (AD) (Ownby et al., 2006; Sun et al., 2008). Indeed, depression is common in pre-clinical AD, and may represent an early manifestation of this disease before any cognitive impairments appear (Geerlings et al., 2000; Visser et al., 2000). In particular, subjects with mild cognitive impairment and depression have more than twice the risk of developing Alzheimer-type dementia than patients without depression (Modrego and Ferrandez, 2004). Neuropsychiatric symptoms are not just an emotional reaction to the awareness of the dementing diseases, but may derive from neurobiological changes in specific brain areas and may be prodromal symptoms of dementia (Andersen et al., 2005).

Recent studies suggest that early memory deficits and neuropsychiatric symptoms may be explained by the presence of soluble forms of βamyloid (Aβ) rather than the aggregated form which develops into insoluble plaques (Rowan et al., 2005). Accordingly, in the early stages of AD, significant cognitive deficits have been directly attributed to soluble Aβ fragments (Mattson, 2004; Cleary et al., 2005) and increased levels of soluble Aβ oligomers caused synaptic dysfunction (Hardy and Selkoe, 2002; Selkoe and Schenk, 2003), raising the possibility that increased brain soluble Aβ levels may contribute to the development of non-cognitive symptoms.

The preservation of key neurotransmitters and neurotrophic factors is considered crucial for the regulation of synaptic plasticity and neuronal survival, and a decline in their function is related to neurodegenerative disorders like AD (Mattson, 2004). Several studies suggest that a decrease in the levels of brain-derived neurotrophic factor (BDNF) could be associated with the pathogenesis of AD (Siegel and Chauhan, 2000; Fumagalli et al., 2006; Tapia-Arancibia et al., 2008), but the role for this factor is still to be established. BDNF has a pleiotropic influence on nerve cells, with effects ranging from synaptic transmission and neuronal plasticity, to regulation of differentiation and survival of specific neuronal populations (Thoenen, 2000; Burbach et al., 2004). Moreover, BDNF has been frequently linked to mood disorders (Altar, 1999; Castren et al., 2007; Kalueff et al., 2007; Monteggia et al., 2007) and, in animal models of depression, centrally administered BDNF produces antidepressant-like activity (Shirayama et al., 2002). Therefore, BDNF has been proposed as a bridge between depression and AD (Tsai, 2003). Other neurotrophins have also been linked to depression and AD, such as nerve growth factor (NGF) (Schulte-Herbruggen et al., 2007). A significant decrease in serum NGF has been observed in patients with mild cognitive impairment, suggesting that the availability of NGF might be reduced at the onset of any neurodegenerative processes (Schaub et al., 2002). Neurotrophins and 5-hydroxytryptamine (5-HT) are close and reciprocally regulated signals and the central 5-hydroxytryptaminergic system has been shown to be altered in AD patients (Garcia-Alloza et al., 2004); hence, changes in the levels of neurotrophin might contribute to degenerative disorders through modifications of the 5-hydroxytryptaminergic system (Tapia-Arancibia et al., 2008).

In addition, Christensen et al. (2008) observed changes in BDNF levels and the function of the 5-hydroxytryptaminergic system 83 days after a single intra-hippocampal injection of Aβ1-42 peptide in its aggregated form; in particular, the lower hippocampal 5-HT2A receptor levels and frontal cortical BDNF concentrations were affected.

The aim of the present study was to investigate whether an acute intracerebroventricular (i.c.v.) injection of soluble Aβ might induce alterations in non-cognitive domains as well as impair short-term memory in rats. Although the evaluation of mood-related disturbances in animal models is not straightforward, several paradigms for investigating symptoms related to depression and anxiety in rodents have been developed (Pellow et al., 1985; Pellow and File, 1986; Choleris et al., 2001; Cryan et al., 2005). Therefore, the current study was designed to evaluate the effects of soluble Aβ on the working memory, motor activity, anxiety- and depression-related behaviours of young adult male rats. The levels of BDNF and NGF mRNA and protein were also measured in rat prefrontal cortex (PFC), striatum (STR) and nucleus accumbens (NAc). In addition, as the 5-hydroxytryptaminergic system may be primarily involved in the development of non-cognitive symptoms (Gage and Springer, 1981; Steckler and Sahgal, 1995), we investigated whether 5-hydroxytryptaminergic neurotransmission in the rat PFC, STR and NAc was affected by soluble Aβ treatment.

Methods

Animals

The experiments were conducted using 193 young-adult male Wistar rats (Harlan, S. Pietro al Natisone, Udine, Italy) weighing 250–300 g. The animals were randomly assigned to the experimental groups, one for each behavioural, neurochemical and biochemical analysis and they were allowed to acclimatize to the animal house for at least 7 days before the experiments. They were housed in pairs with food and water available ad libitum and handled during the weekly care procedures. They were housed at a constant room temperature (22 ± 1°C) and relative humidity (55 ± 5%) under a 12-h light/dark cycle (lights on from 7 h 00 min to 19 h 00 min). The experiments involving the animals and their care conformed to the institutional guidelines in compliance with national (D. L. N°. 116, G. U., Suppl. 40, February 18, 1992, Circ N°. 8, G. U., July 14, 1994) and International laws and policies (EEC Council Directive 86/609, OJ L 358, 1, 12 December 1987; Guide for the Care and Use of Laboratory Animals, U.S. National Research Council, 1996). Every effort was made to minimize the number of animals used and their suffering.

Aβ1-42 administration

The Aβ1-42 peptide was obtained from Tocris (Bristol, UK). The peptide was dissolved daily in sterile double-distilled water (vehicle) at a concentration of 4 µM as previously described (Trabace et al., 2007). Briefly, rats were anaesthetized by administration of Equithesin (composition: 1.2 g sodium pentobarbital; 5.3 g chloral hydrate; 2.7 g MgSO4; 49.5 mL propylene glycol; 12.5 mL ethanol and 58 mL distilled water), 3.6 mL·kg−1 i.p., and secured in a stereotaxic frame (David Kopf Instruments, Tujunga, CA, USA). Skin was shaved, disinfected and cut with a sterile scalpel and a hole was drilled to insert the injection needle (30-gauge stainless steel tubing; Cooper's Needles, Birmingham, UK). The i.c.v. injections were made using the following coordinates relative to bregma: AP = −0.5, ML = +1.2, DV = −3.2 with the incisor bar set at −3.3 mm (Paxinos and Watson, 1998).

Aβ1-42 (5 µL) was delivered through a 25 µL Hamilton microsyringe at an infusion rate of 2 µL·min−1 for 2.5 min. Control rats were injected with vehicle only, as reverse Aβ42-1, used in preliminary experiments, was indistinguishable from vehicle alone (unpublished observations). The injection needle was left in place for an additional 5 min to prevent reflux. Placement of the needle track was verified at the time of dissection. All experimental procedures (behavioural and biochemical experiments) were performed 7 days after i.c.v. administration (sham-operated or Aβ-treated groups) or i.p. injection (Equithesin, intact group).

Novel object recognition test

The test was performed according to Giustino et al. (1996). Briefly, 30 rats were submitted to two habituation sessions (intersession interval: 24 h) where they were allowed 5 min to explore the apparatus. Twenty-four h after the last habituation, a session of two 3-min trials separated by a 1-min inter-trial interval was carried out. In the first trial (T1), rats were exposed to two identical objects (white glasses or light bulbs). During the second trial (T2), rats were exposed to one familiar (F) object but the second familiar object was replaced by a new, differently shaped, object (N). In each trial each object was placed at an equidistant position between the centre and the wall of the arena. The position of the two objects was counterbalanced and randomly permutated during T2. At the beginning of each trial the rats were placed near the centre of the arena with their heads oriented in the opposite direction to the objects. Exploration of the objects was defined as sniffing or touching the object with the nose. Turning around or sitting on the object was not considered to be exploration. Object exploration was quantified as: exploratory activity, total time spent exploring both objects during each trial (T1 and T2); index of discrimination, expressed as ratio between the net time spent exploring the new (N–F) over the total exploration time (N–F/N+F). Objects and arena were carefully cleaned between each session to avoid confounding olfactory stimuli.

Open field spontaneous locomotor activity

The apparatus consisted of a circular arena, 75 cm diameter, made of dark plastic under dim lighting, as previously described by Monteggia et al. (2007). The experimental sessions were videotaped by a camera fixed above the arena. Thirty animals were acclimatized to the test room for 1 h before each test. Motor activity was measured by placing the rat into the centre of the arena before a 20 min session. The scoring was performed using a video-tracking motion analysis system (Ethovision, Noldus Information Tecnology, Wageningen, the Netherlands). To assess general locomotor activity, the following behavioural parameters (expressed as frequency on 5 min counts) were scored: number of square limit crossings with both forepaws, rearing (standing with the body inclined vertically, forequarters raised), and wall rearing (standing on the hind-limbs and touching the walls of the apparatus with the forelimbs). To investigate anxiety-related behaviour, we measured time spent performing general grooming activity consisting of: face grooming (strokes along the snout), head washing (semicircular movements over the top of the head and behind the ears) and body grooming (body fur licking) (Choleris et al., 2001).

Forced swimming test

On the first of the two test days, thirty animals were placed individually in inescapable Perspex cylinders (diameter 23 cm; height 70 cm) filled with a constant maintained 25°C temperature water at a height of 30 cm (Cryan et al., 2005). During the preconditioning period, the animals were observed for 15 min. Then, rats were removed and dried before returning them to their home cages. Twenty-four h later, each rat was returned to the water-filled cylinder for 5 min. This session was video-recorded and subsequently scored by an observer blind to the treatment groups. During the test sessions, the time that rats spent performing the following behaviours was measured: struggling (time spent in tentative of escaping), swimming (time spent moving around the cylinder) and immobility (time spent remaining afloat making only the necessary movements to keep its head above the water).

Elevated plus-maze test

The experimental procedures were performed according to Pellow et al. (1985). Briefly, the Plexiglas apparatus consisted of two opposite open arms (50 × 10 cm) without side walls and two closed arms (50 × 10 × 40 cm) extending horizontally at right angles from a central area (10 × 10 cm). Our maze was situated in a separate brightly lit room illuminated with four, 32-W fluorescent overhead lights each, which produce consistent illumination within the room. The apparatus had similar levels of illumination on both open and closed arms as reported by Walf and Frye (2007). The maze was elevated to a height of 50 cm in this lit room. At the beginning of the experiment, thirty animals were placed at the centre of the plus-maze, facing the open arm. During a 5-min observation period the following parameters were measured: number of open and closed arm entries and percentage of time spent on open arms. An arm entry was counted when both rat forepaws were placed into the given arm. The % open arm time, an inverse measure of anxiety-like behaviour, was calculated as (time in open arms/total time in arms) × 100.

Tissue 5-hydroxytryptamine content

Eighteen rats were killed by decapitation and brain immediately removed for dissection. Brains were placed dorsal side up in an ice-chilled rat brain matrix (World Precision Instruments, Inc., Aston, Stevenage, UK) with slits spaced at 1 mm. Using an ice-chilled razor blade, the target regions (PFC, STR and NAc) were dissected according to the atlas of Paxinos and Watson (1998). Tissues were frozen and stored at −80°C until analysis was performed. Samples were homogenized in 10 volumes (w·V−1) of perchloric acid 0.1 M. The homogenates were stored on ice for 30 min and then centrifuged at 10 000 g for 10 min at 4°C. The supernatants were filtered and properly diluted for analysis. 5-HT concentrations were determined by high performance liquid chromatography (HPLC) coupled with a coloumetric detector (ESA, Coulochem II, Bedford, MA, USA). Analysis was performed through a LC18 reversed phase cartridge column (15 cm × 4.6 mm, 3 micron; Supelco, Milan, Italy). The mobile phase was 130 mM CH3COONa, 13 mM citric acid, 0.1 mM EDTA, acetonitrile 6%, in distilled water, buffered at pH 5.1 with CH3COOH and properly filtered. The flow rate, maintained by an isocratic pump (Shimadzu LC-10AD, Kyoto, Japan), was 1 mL·min−1. Cell potentials were set at E1=+50 mV and E2=+250 mV.

BDNF and NGF mRNA measurement

Brain areas from 18 animals were collected as described above. mRNA was isolated using RNeasy mini kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions. Residual genomic DNA was removed using RNase-Free DNase set (Qiagen, Hilden, Germany). One µg total mRNA was reverse transcribed using the superscript II kit according to the manufacturer's instructions (Invitrogen, Milan, Italy). RT-PCR for BDNF and NGF was performed at 35 cycles. The primers used for BDNF were F 5′ GCG GCA GAT AAA AAG ACT GC 3′ and R 5′ GCC AGC CAA TTC TCT TTT TG 3′; for NGF were F 5′ GGA CGC AGC TTT CTA TCC TG 3′ and R 5′ AAA CAG TTT GGG GTC CAC AG 3′ (Invitrogen, Milan, Italy). RT-PCR gel (1%) was run at 100 V. Optical band density was analysed using ImageJ software (http://rsb.info.nih.gov/ij/).

BDNF and NGF protein measurements

PFC, STR and NAc were collected from 37 rats as described above. They were rapidly dissected, frozen on dry ice and stored at −80°C until assayed for neurotrophin content. BDNF and NGF protein was measured by ELISA using commercially available kits (BDNF and NGF Emax Immunoassay System, respectively, Promega, Milan, Italy) according to manufacturer's instructions. Briefly, tissues were homogenized and centrifuged at 12 000 g at 4°C for 30 min, and supernatants were diluted with a blocking buffer. ELISA was performed in 96-well plates (Iwaki, Funahashi, Japan). Colorimetric detection of peroxidase activity was achieved by adding TMB solution and peroxidase substrate, and incubating for 10 min at room temperature according to the manufacturer's instructions. The enzymatic reaction was stopped with HCl (1.0 M) and the optical density of each well was measured at 450 nm using a PowerWave XS plate reader (Bio-Tek, Winooski, VT, USA). Each analysis was performed in duplicate in the same assay to avoid inter-assay variations. Neurotrophin levels are expressed as percentage of intact controls.

Statistical analysis

All statistical analyses were performed using SigmaStat® 3.1 and Graph Pad® 5.0 for Windows. Data were tested for normality by the selection of parametric and non-parametric tests. Behavioural data were analysed by a two-way analysis of variance (anova) for repeated measures or one-way anova, followed by a Newman-Keuls Multiple Comparison's test, as required. For the post-mortem tissue analyses, data were analysed by one-way anova followed by a Tukey's test. Differences were considered significant only when P-values were less than 0.05.

Results

Effect of Aβ on the novel object recognition test

Two-way anova revealed no differences between the treatment groups, whereas a significant difference was found only in time (Ft(1,27)= 23.23, P < 0.001, Figure 1A). A post-hoc test showed that all experimental groups spent less time in exploring objects in T2. When rats were re-exposed to the familiar and to the novel object, they did not show any significant difference between groups in total time spent for exploration of both objects during T2 (Figure 1B). In T2 two-way anova revealed that all rats, independently of their respective group, spent more time in exploring the new than the familiar object (Ft(1,46)= 33.64, P < 0.001; Figure 1B). As shown in Figure 1C, no differences were found in discrimination index between the experimental groups (one way anova, F(2,23)= 0.44, n.s.; Figure 1C). However, statistical analysis revealed that Aβ-treated animals spent less total time exploring both objects (N and F) in both trials (T1 and T2) than controls (F(2,23)= 6.01; P < 0.01; Figure 1D).

Figure 1.

Figure 1

Open in a new tab

(A) Exploratory activity during Trial 1 (T1) and Trial 2 (T2) (B) exploratory activity of novel and familiar objects during T2 (C) discrimination index and (D) total exploration time in the novel object recognition test of male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) or Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM of total frequency counts in 5 min intervals and total time in s (n= 10 per group). (One-way anova followed by Newman-Keuls multiple comparison's Test, *P < 0.01 vs. intact and sham).

Effect of Aβ on the open field test

As shown in Figure 2, analysis of data revealed that there were no significant differences in the frequencies of crossing (two-way anova for repeated measures: Ftr(2,69)= 1.89, n.s., Ft(3,69)= 84.98, P < 0.001, Ftxtr(6,69)= 0.41, n.s.; Figure 2A), rearing (Ftr(2,81)= 0.12, n.s., Ft(3,81)= 6.83, P < 0.01, Ftxtr(6,81)= 0.84, n.s.; Figure 2B) and wall rearing (Ftr(2,81)= 2.22, n.s., Ft(3,81)= 70.08, P < 0.001, Ftxtr(6,81)= 0.76, n.s.; Figure 2C) between the experimental groups. Similarly, no differences were found across the groups with regard to the self-grooming behaviour (one way anova, F(2,25)= 0.096, n.s.; Figure 2D).

Figure 2.

Figure 2

Open in a new tab

(A) Crossing (B) rearing and (C) wall rearing frequency, and (D) self-grooming time, in the open field test in male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM of total frequency counts in 5 min and total time in s (n= 10 per group).

Effect of Aβ on the forced swimming test

Data showed that immobility frequency was significantly increased (one way anova, F(2,25)= 5.59, P < 0.01; Figure 3A) in Aβ-injected rats compared with either sham-operated or intact rats. Conversely, frequency in swimming activity was significantly decreased in Aβ-treated animals (one way anova, F(2,25)= 3.63, P < 0.05; Figure 3B). No difference in struggling activity was found between all the experimental groups considered (Figure 3C). Although a decrease in motor strength or endurance may affect the forced swimming test, no apparent motor deficit was noticed during habituation, consisting of 15 min of swimming in the cylinder 24 h before the test session (data not shown).

Figure 3.

Figure 3

Open in a new tab

(A) Immobility (B) swimming and (C) struggling frequency in the forced swimming test in male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM (n= 10 per group). (One-way anova followed by Newman-Keuls multiple comparison's test, *P < 0.01, #P < 0.05 vs. both intact and sham).

Effect of Aβ on the elevated plus-maze test

As shown in Figure 4, analysis of data revealed no significant differences in the percentage of time spent in the open arms, a measure of anxiety-like behaviour (one way anova: F(2,22)= 0.05, n.s., Figure 4A), or in the number of open arm entries (F(2,20)= 0.10, n.s., Figure 4B) between the groups. Moreover, no significant differences were found in the number of entries into the closed arms, a measure of non-specific motor activity (F(2,24)= 0.06, n.s.; Figure 4C) between the experimental groups.

Figure 4.

Figure 4

Open in a new tab

(A) Amount of time (% of total time) spent in open arm (B) number of open arm entries and (C) number of closed arm entries in the elevated plus-maze test in male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM (n= 10 per group).

Effect of Aβ on tissue 5-HT levels

One-way anova revealed a significant difference in 5-HT content between intact or sham-operated and soluble Aβ-injected rats. Indeed, Tukey's post hoc test showed that soluble Aβ induced a significant reduction in 5-HT tissue concentrations in the PFC of rats (F(2,17)= 3.64, P < 0.05; Figure 5A). Conversely, no significant differences in 5-HT content between the groups were found in the STR and NAc (Figure 5B,C).

Figure 5.

Figure 5

Open in a new tab

5-hydroxytryptamine (5-HT) levels in (A) PFC (B) STR and (C) NAc of male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM (n= 6 per group). (One-way anova followed by Tukey's test, *P < 0.05 vs. intact or sham).

Effect of Aβ on BDNF and NGF mRNA and protein levels

As shown in Figure 6, one-way anova followed by Tukey's comparison test revealed a significant reduction in BDNF mRNA and protein levels in PFC of soluble Aβ-injected compared with intact or sham-operated rats (F(2,15)= 53.55, P < 0.01 for mRNA; F(2,20)= 7.74, P < 0.01 vs. intact and P < 0.05 vs. sham for protein; Figure 6A,B). On the other hand, no difference was found in STR of Aβ-treated rats compared with both intact and sham-operated animals (F(2,15)= 0.45, n.s. for mRNA; F(2,15)= 0.09, n.s. for protein; Figure 6C,D).

Figure 6.

Figure 6

Open in a new tab

Brain-derived neurotrophic factor (BDNF) expression of mRNA in (A) PFC and (C) STR, and protein in (B) PFC and (D) STR of male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM (n= 6 per group). (One-way anova followed by Tukey's test, *P < 0.01 for mRNA vs. intact and sham; *P < 0.01 vs. intact and #P < 0.05 vs. sham for protein).

NGF analysis revealed the same pattern showing a significant reduction in PFC mRNA and protein levels only in Aβ-injected rats (F(2,15)= 13.58, P < 0.01 for mRNA; F(2,15)= 6.51, P < 0.01 for protein; Figure 7A,B), while no differences were found in STR in all experimental groups (F(2,15)= 1.12, n.s. for mRNA; F(2,15)= 0.09, n.s. for protein; Figure 7C,D).

Figure 7.

Figure 7

Open in a new tab

Nerve growth factor (NGF) expression of mRNA in (A) PFC and (C) STR, and protein in (B) PFC and (D) STR of male Wistar rats 7 days after administration of Equithesin (3.6 mL·kg−1 i.p.; INTACT), water (5 µL i.c.v.; SHAM) and Aβ1-42 (4 µM, 5 µL i.c.v.; Aβ). Data are expressed as mean ± SEM (n= 6 per group). (One-way anova followed by Tukey's test, *P < 0.01 vs. intact and sham for mRNA; *P < 0.05 vs. intact and sham for protein).

As previously reported in the literature (Conner et al., 1997), the levels of BDNF and NGF mRNA in the NAc were barely detectable (data not shown). However, there were no significant differences in the protein levels of these neurotrophins in this area between the three experimental groups (BDNF: intact, 307 ± 44 pg·mg−1 of tissue, sham, 384 ± 94, Aβ, 307 ± 22, F(2,9)= 0.53, n.s.; NGF: intact, 3269 ± 459, sham, 7727 ± 2207, Aβ, 8114 ± 1882, F(2,9)= 2.52, n.s.).

Discussion and conclusions

The present study suggests that soluble Aβ-treated rats show a depressive-, but not anxiogenic-like, phenotype. These outcomes are accompanied by alterations in the expression of neurotrophins and 5-hydroxytryptaminergic neurotransmission in selective brain regions without an impairment in working memory.

Although the relationship among soluble Aβ, brain neurochemistry and depression remains complex, several studies have demonstrated an increased risk for the development of AD in individuals with late-life depression, suggesting a prodromal state of AD (Steffens et al., 1997; Dal Forno et al., 2005; Sun et al., 2008). Aβ may have an effect on mood not limited to non AD patients, depression-like symptoms may indeed precede or accompany dementia (Starkstein et al., 2008). On the other hand, Aβ42/Aβ40 ratio has been reported to correlate with severity of signs of AD pathogenesis in elderly individuals with late life major depression (Pomara et al., 2006). Hence, the identification of early events in the development of AD-related depression would be an advance in our understanding of pathophysiological mechanisms underlying the prodromal phase of AD.

From a behavioural point of view, we found that soluble Aβ did not alter the ability of rats to distinguish between a familiar and a novel object. Surprisingly, Aβ-treated animals showed lower exploratory activity. Indeed, they spent less time exploring both objects in all experimental sessions than controls, suggesting these animals are less inclined to explore but can still recognize a novel object when encountered, in the context of normal levels of general motor activity. A possible interpretation of the reduced explorative behaviour during working memory evaluation is that soluble Aβ, before the appearance of cognitive impairments, might induce motivational deficits. Accordingly, others have reported that non-cognitive symptoms often accompany behavioural changes in AD (Assal and Cummings, 2002; Egashira et al., 2005). Interestingly, we found that soluble Aβ significantly affected rat behaviour when they were placed in a cylinder of water, during the FST, a method for inducing a behavioural state resembling depression in rats by exposing them to a mildly aversive situation from which there is no possibility of escape (Porsolt et al., 1977). Aβ-treated rats exhibited a marked increase in forced swimming test-induced immobility time compared with controls, reflecting a state of behavioural despair or hopelessness. Although obtained in a different animal model, our results are in line with those from studies reporting that mice over-expressing APPswe/PS1, at an age characterized by high levels of soluble Aβ (Marutle et al., 2002), showed an increased duration of immobility in FST (Filali et al., 2009). Nevertheless, in our experimental conditions, a possible anxiogenic effect of soluble Aβ is doubtful, as it had no effect in the elevated plus-maze test, a behavioural model of anxiety-like behaviour (Pellow et al., 1985; Pellow and File, 1986). In addition, under the appropriate conditions, behavioural reactivity in the open field test can be used as an index of increased anxiety (Britton and Britton, 1981; Choleris et al., 2001), because rats engage in repetitive grooming in response to anxiogenic situation (Spruijt et al., 1988). Again, in this test, no differences were revealed between Aβ-treated and control animals confirming a lack of anxiety-like behaviour. Therefore, the enhanced immobility time of Aβ-treated animals is unlikely to have been caused by a deficit in motor function or a change in their anxiety level.

The mechanism by which soluble Aβ peptide induces depressive-like behaviour is not yet clear, but the present results raise the possibility that the modulation of 5-hydroxytryptaminergic neurotransmission is involved. Soluble Aβ-treated rats showed a selective reduction in swimming, and not struggling, behaviour in the FST. Accordingly, selective 5-HT reuptake inhibitors share the common feature of selectively increase swimming, but not struggling, behaviour, whereas selective noradrenaline reuptake inhibitors and tricyclic antidepressants preferentially increase struggling, but not swimming behaviour (Detke et al., 1995; Cryan et al., 2005).

To assess directly whether soluble Aβ inhibits 5-hydroxytryptaminergic neurotransmission as the above data suggest, we measured 5-HT levels in several brain areas. As expected, we found that 5-HT content was selectively reduced in the PFC of Aβ-treated animals, but not in the STR or NAc. These results are supported by demonstrations that impairments of the 5-hydroxytryptaminergic system are present in the very early stages of AD (Versijpt et al., 2003; Egashira et al., 2005; Kepe et al., 2006). In this regard, it is well known that impaired 5-hydroxytryptaminergic neurotransmission in the prefrontal area is central to depressive disorders (Krishnan and Nestler, 2008), but could also have an important role in the pathogenesis of several neurodegenerative diseases (Mattson et al., 2004; Egashira et al., 2005). In particular, there is substantial post-mortem and clinical evidence of disruptions of the 5-hydroxytryptaminergic system in AD (Morgan et al., 1987; Lanctot et al., 2001). Furthermore, the risk of developing AD is higher in individuals with a history of depression (Kessing and Andersen, 2004). This area needs to be evaluated further in molecular studies of 5-hydroxytryptaminergic function and by investigating the responsiveness of soluble Aβ-treated animals to antidepressant-like activity of selective 5-HT reuptake inhibitors.

The Aβ-induced changes may result in dysfunction in multiple neurotransmitter systems and their associated interactions. Indeed, our and other previous studies have shown deficits in dopaminergic system in soluble Aβ-treated rats both in PFC (Trabace et al., 2007) and in NAc (Preda et al., 2008). Of note, functional interactions between dopaminergic and 5-hydroxytryptaminergic neuronal systems in the rat PFC have been observed. In particular, dopamine release was facilitated by fluoxetine, a drug which increases 5-HT concentrations, administration (Matsumoto et al., 1999). Basic science research has shown that 5-HT receptors modulate dopaminergic function (Alex and Pehek, 2007). Therefore, it is conceivable that the neuromodulatory action of soluble Aβ on both the 5-hydroxytryptaminergic and dopaminergic system in PFC might profoundly disrupt the functioning of this area, potentially leading to impairment of mood control.

Another important factor that undergoes significant changes in AD is represented by BDNF (Lanctot et al., 2001). Many reports have documented evidence of decreased expression of BDNF in neurological diseases (Murer et al., 2001; Lang et al., 2004). Moreover, relative levels of BDNF mRNA and proteins are decreased in frontal (Ferrer et al., 1999), temporal (Connor et al., 1997), parietal (Garzon et al., 2002; Michalski and Fahnestock, 2003) and entorhinal cortex (Narisawa-saito et al., 1996) in severe AD.

To determine whether the observed BDNF decrease is an early event during the progression of cognitive decline in AD, we measured the BDNF expression of either its mRNA or protein in the PFC, STR and NAc after an acute injection of soluble Aβ, and observed a selective reduction of both in the PFC of Aβ-injected rats, but not in the STR.

These data on the Aβ-induced decrease in BDNF and 5-HT content in PFC are supported by previous observations, in which exogenous delivery of BDNF promoted the function and sprouting of 5-hydroxytryptaminergic neurons in adult rat brains (Mamounas et al., 1995), and BDNF-deficient mice were also deficient in 5-hydroxytryptaminergic innervation (Lyons et al., 1999). On the other hand, neurotrophins are known to regulate synaptic plasticity, neurogenesis and neuronal survival in the adult brain. These two signals co-regulate each other such that 5-HT stimulates the expression of BDNF, and BDNF enhances the growth and survival of 5-HT neurons (Mamounas et al., 1995).

In addition, the content of NGF in the brain has been shown to change during the time course of neurodegeneration in AD. Post mortem studies have indicated a lack of NGF action in early stages of AD, whereas NGF concentrations were found to be enhanced in brains of patients with severe AD, thought to be partly due to the pathologically altered axonal transport of NGF in neurons (Schulte-Herbruggen et al., 2008). In the present study, we found a reduction in the expression of NGF in PFC of soluble Aβ-injected animals. No alterations were observed in the STR. In accord with previous published data (Conner et al., 1997), mRNA neurotrophin levels in the NAc were below the limit of detection in the present study, but no differences were observed in either the BDNF or NGF protein content.

Our findings support the concept that Aβ has detrimental effects in its soluble form even before plaque formation and before the occurrence of neurodegeneration (Moechars et al., 1999). Previously, using Hoechst immunocytochemistry, we showed that no gross signs of neurodegeneration occur within the area of Aβ diffusion in the periventricular parenchyma (site of injection) and in the PFC (Trabace et al., 2007). However, the possibility that Aβ treatment induces more subtle signs of toxicity, such as synaptic degeneration and neurite retraction, cannot be exclude from our present results and future studies are warranted in this regard. The actions of soluble Aβ on the central nervous system are complex, and much more detailed assessments of these phenomena, accounting for length of exposure, route of administration and experimental periods, are clearly needed.

The findings from the present study differ in some important aspects from previous data on the role of soluble Aβ in mediating behavioural, neurochemical and biochemical alterations. Indeed, the novelty of this study is apparent from the observed acute effect of Aβ in its soluble form and, by showing that soluble Aβ1-42 might have pro-depressive properties, our data provide some novel outcomes that contribute to research on the non-cognitive symptoms of AD.

To the best of our knowledge, this is the first report showing that soluble Aβ can selectively inhibit the expression of BDNF and NGF, and can selectively reduce 5-HT content in the PFC, while inducing a state of despair in healthy adult rats. In addition, motor function, working memory and anxiety-related emotional response were normal. We interpret these results to signify that the soluble Aβ-induced impairments might be considered as sensitive markers of an early dysfunction observed in adult animals without neuropathological plaques, thus suggesting that these changes might be risk factors for the expression of neuropsychiatric symptoms in AD.

In conclusion, the findings described here suggest that soluble Aβ represents a critical player in producing functional and biochemical deficits in rats showing a depressive-like, but not an anxiogenic-like, phenotype. Such an approach should help to provide a fruitful basis for questioning not only the mechanisms underlying the effects of soluble Aβ, but also possible targets for therapeutic intervention very early in the disease process.

Acknowledgments

This study was supported by PRIN 2007 (to L.T.) from MIUR and by Fondazione ‘Siniscalco Ceci’.

Glossary

Abbreviations:

5-HT

5-hydroxytryptamine

Aβ

βamyloid1-42

AD

Alzheimer's disease

BDNF

brain-derived neurotrophic factor

NAc

nucleus accumbens

NGF

nerve growth factor

PFC

prefrontal cortex

STR

striatum

Conflict of interest

The authors declare no conflict of interest.

References

  1. Alex KD, Pehek EA. Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission. Pharmacol Ther. 2007;113:296–320. doi: 10.1016/j.pharmthera.2006.08.004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Altar CA. Neurotrophins and depression. Trends Pharmacol Sci. 1999;20:59–61. doi: 10.1016/s0165-6147(99)01309-7. [DOI] [PubMed] [Google Scholar]
  3. Andersen K, Lolk A, Kragh-Sorensen P, Petersen NE, Green A. Depression and the risk of Alzheimer disease. Epidemiology. 2005;16:233–238. doi: 10.1097/01.ede.0000152116.32580.24. [DOI] [PubMed] [Google Scholar]
  4. Assal F, Cummings JL. Neuropsychiatric symptoms in the dementias. Curr Opin Neurol. 2002;15:445–450. doi: 10.1097/00019052-200208000-00007. [DOI] [PubMed] [Google Scholar]
  5. Britton DR, Britton KT. A sensitive open field measure of anxiolytic drug activity. Pharmacol Biochem Behav. 1981;15:577–582. doi: 10.1016/0091-3057(81)90212-4. [DOI] [PubMed] [Google Scholar]
  6. Burbach GJ, Hellweg R, Haas CA, Del Turco D, Deicke U, Abramowski D, et al. Induction of brain-derived neurotrophic factor in plaque-associated glial cells of aged APP23 transgenic mice. J Neurosci. 2004;24:2421–2430. doi: 10.1523/JNEUROSCI.5599-03.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Castren E, Voikar V, Rantamaki T. Role of neurotrophic factors in depression. Curr Opin Pharmacol. 2007;7:18–21. doi: 10.1016/j.coph.2006.08.009. [DOI] [PubMed] [Google Scholar]
  8. Choleris E, Thomas AW, Kavaliers M, Prato FS. A detailed ethological analysis of the mouse open field test: effects of diazepam, chlordiazepoxide and an extremely low frequency pulsed magnetic field. Neurosci Biobehav Rev. 2001;25:235–260. doi: 10.1016/s0149-7634(01)00011-2. [DOI] [PubMed] [Google Scholar]
  9. Christensen R, Marcussen AB, Wortwein G, Knudsen GM, Aznar S. Abeta(1-42) injection causes memory impairment, lowered cortical and serum BDNF levels, and decreased hippocampal 5-HT(2A) levels. Exp Neurol. 2008;210:164–171. doi: 10.1016/j.expneurol.2007.10.009. [DOI] [PubMed] [Google Scholar]
  10. Cleary JP, Walsh DM, Hofmeister JJ, Shankar GM, Kuskowski MA, Selkoe DJ, et al. Natural oligomers of the amyloid-beta protein specifically disrupt cognitive function. Nat Neurosci. 2005;8:79–84. doi: 10.1038/nn1372. [DOI] [PubMed] [Google Scholar]
  11. Conner JM, Lauterborn JC, Yan Q, Gall CM, Varon S. Distribution of brain-derived neurotrophic factor (BDNF) protein and mRNA in the normal adult rat CNS: evidence for anterograde axonal transport. J Neurosci. 1997;17:2295–2313. doi: 10.1523/JNEUROSCI.17-07-02295.1997. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Connor B, Young D, Yan Q, Faull RL, Synek B, Dragunow M. Brain-derived neurotrophic factor is reduced in Alzheimer's disease. Brain Res Mol Brain Res. 1997;49:71–81. doi: 10.1016/s0169-328x(97)00125-3. [DOI] [PubMed] [Google Scholar]
  13. Cryan JF, Valentino RJ, Lucki I. Assessing substrates underlying the behavioral effects of antidepressants using the modified rat forced swimming test. Neurosci Biobehav Rev. 2005;29:547–569. doi: 10.1016/j.neubiorev.2005.03.008. [DOI] [PubMed] [Google Scholar]
  14. Dal Forno G, Palermo MT, Donohue JE, Karagiozis H, Zonderman AB, Kawas CH. Depressive symptoms, sex, and risk for Alzheimer's disease. Ann Neurol. 2005;57:381–387. doi: 10.1002/ana.20405. [DOI] [PubMed] [Google Scholar]
  15. Detke MJ, Rickels M, Lucki I. Active behaviors in the rat forced swimming test differentially produced by serotonergic and noradrenergic antidepressants. Psychopharmacology (Berl) 1995;121:66–72. doi: 10.1007/BF02245592. [DOI] [PubMed] [Google Scholar]
  16. Egashira N, Iwasaki K, Takashima A, Watanabe T, Kawabe H, Matsuda T, et al. Altered depression-related behavior and neurochemical changes in serotonergic neurons in mutant R406W human tau transgenic mice. Brain Res. 2005;1059:7–12. doi: 10.1016/j.brainres.2005.08.004. [DOI] [PubMed] [Google Scholar]
  17. Ferrer I, Marin C, Rey MJ, Ribalta T, Goutan E, Blanco R, et al. BDNF and full-length and truncated TrkB expression in Alzheimer disease. Implications in therapeutic strategies. J Neuropathol Exp Neurol. 1999;58:729–739. doi: 10.1097/00005072-199907000-00007. [DOI] [PubMed] [Google Scholar]
  18. Filali M, Lalonde R, Rivest S. Cognitive and non-cognitive behaviors in an APPswe/PS1 bigenic model of Alzheimer's disease. Genes Brain Behav. 2009;8:143–148. doi: 10.1111/j.1601-183X.2008.00453.x. [DOI] [PubMed] [Google Scholar]
  19. Fumagalli F, Racagni G, Riva MA. The expanding role of BDNF: a therapeutic target for Alzheimer's disease? Pharmacogenomics J. 2006;6:8–15. doi: 10.1038/sj.tpj.6500337. [DOI] [PubMed] [Google Scholar]
  20. Gage FH, Springer JE. Behavioral assessment of norepinephrine and serotonin function and interaction in the hippocampal formation. Pharmacol Biochem Behav. 1981;14:815–821. doi: 10.1016/0091-3057(81)90366-x. [DOI] [PubMed] [Google Scholar]
  21. Garcia-Alloza M, Hirst WD, Chen CP, Lasheras B, Francis PT, Ramirez MJ. Differential involvement of 5-HT(1B/1D) and 5-HT6 receptors in cognitive and non-cognitive symptoms in Alzheimer's disease. Neuropsychopharmacology. 2004;29:410–416. doi: 10.1038/sj.npp.1300330. [DOI] [PubMed] [Google Scholar]
  22. Garzon D, Yu G, Fahnestock M. A new brain-derived neurotrophic factor transcript and decrease in brain-derived neurotrophic factor transcripts 1, 2 and 3 in Alzheimer's disease parietal cortex. J Neurochem. 2002;82:1058–1064. doi: 10.1046/j.1471-4159.2002.01030.x. [DOI] [PubMed] [Google Scholar]
  23. Geerlings MI, Schoevers RA, Beekman AT, Jonker C, Deeg DJ, Schmand B, et al. Depression and risk of cognitive decline and Alzheimer's disease. Results of two prospective community-based studies in The Netherlands. Br J Psychiatry. 2000;176:568–575. doi: 10.1192/bjp.176.6.568. [DOI] [PubMed] [Google Scholar]
  24. Giustino A, Beckett S, Ballard T, Cuomo V, Marsden CA. Perinatal cocaine reduces responsiveness to cocaine and causes alterations in exploratory behavior and visual discrimination in young-adult rats. Brain Res. 1996;728:149–156. doi: 10.1016/0006-8993(96)00229-6. [DOI] [PubMed] [Google Scholar]
  25. Hardy J, Selkoe DJ. The amyloid hypothesis of Alzheimer's disease: progress and problems on the road to therapeutics. Science. 2002;297:353–356. doi: 10.1126/science.1072994. [DOI] [PubMed] [Google Scholar]
  26. Kalueff AV, Wheaton M, Ren-Patterson R, Murphy DL. Brain-derived neurotrophic factor, serotonin transporter, and depression: comment on Kaufman et al. Biol Psychiatry. 2007;61:1112–1113. doi: 10.1016/j.biopsych.2006.07.001. author reply 1113-1115. [DOI] [PubMed] [Google Scholar]
  27. Kepe V, Barrio JR, Huang SC, Ercoli L, Siddarth P, Shoghi-Jadid K, et al. Serotonin 1A receptors in the living brain of Alzheimer's disease patients. Proc Natl Acad Sci U S A. 2006;103:702–707. doi: 10.1073/pnas.0510237103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Kessing LV, Andersen PK. Does the risk of developing dementia increase with the number of episodes in patients with depressive disorder and in patients with bipolar disorder? J Neurol Neurosurg Psychiatry. 2004;75:1662–1666. doi: 10.1136/jnnp.2003.031773. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Krishnan V, Nestler EJ. The molecular neurobiology of depression. Nature. 2008;455:894–902. doi: 10.1038/nature07455. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Lanctot KL, Herrmann N, Mazzotta P. Role of serotonin in the behavioral and psychological symptoms of dementia. J Neuropsychiatry Clin Neurosci. 2001;13:5–21. doi: 10.1176/jnp.13.1.5. [DOI] [PubMed] [Google Scholar]
  31. Lang UE, Jockers-Scherubl MC, Hellweg R. State of the art of the neurotrophin hypothesis in psychiatric disorders: implications and limitations. J Neural Transm. 2004;111:387–411. doi: 10.1007/s00702-003-0100-0. [DOI] [PubMed] [Google Scholar]
  32. Lyons WE, Mamounas LA, Ricaurte GA, Coppola V, Reid SW, Bora SH, et al. Brain-derived neurotrophic factor-deficient mice develop aggressiveness and hyperphagia in conjunction with brain serotonergic abnormalities. Proc Natl Acad Sci U S A. 1999;96:15239–15244. doi: 10.1073/pnas.96.26.15239. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Mamounas LA, Blue ME, Siuciak JA, Altar CA. Brain-derived neurotrophic factor promotes the survival and sprouting of serotonergic axons in rat brain. J Neurosci. 1995;15:7929–7939. doi: 10.1523/JNEUROSCI.15-12-07929.1995. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Marutle A, Unger C, Hellstrom-Lindahl E, Wang J, Puolivali J, Tanila H, et al. Elevated levels of Abeta1-40 and Abeta1-42 do not alter the binding sites of nicotinic receptor subtypes in the brain of APPswe and PS1 double transgenic mice. Neurosci Lett. 2002;328:269–272. doi: 10.1016/s0304-3940(02)00546-3. [DOI] [PubMed] [Google Scholar]
  35. Matsumoto M, Togashi H, Mori K, Ueno K, Miyamoto A, Yoshioka M. Characterization of endogenous serotonin-mediated regulation of dopamine release in the rat prefrontal cortex. Eur J Pharmacol. 1999;383:39–48. doi: 10.1016/s0014-2999(99)00636-6. [DOI] [PubMed] [Google Scholar]
  36. Mattson MP. Pathways towards and away from Alzheimer's disease. Nature. 2004;430:631–639. doi: 10.1038/nature02621. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Mattson MP, Maudsley S, Martin B. BDNF and 5-HT: a dynamic duo in age-related neuronal plasticity and neurodegenerative disorders. Trends Neurosci. 2004;27:589–594. doi: 10.1016/j.tins.2004.08.001. [DOI] [PubMed] [Google Scholar]
  38. Michalski B, Fahnestock M. Pro-brain-derived neurotrophic factor is decreased in parietal cortex in Alzheimer's disease. Brain Res Mol Brain Res. 2003;111:148–154. doi: 10.1016/s0169-328x(03)00003-2. [DOI] [PubMed] [Google Scholar]
  39. Modrego PJ, Ferrandez J. Depression in patients with mild cognitive impairment increases the risk of developing dementia of Alzheimer type: a prospective cohort study. Arch Neurol. 2004;61:1290–1293. doi: 10.1001/archneur.61.8.1290. [DOI] [PubMed] [Google Scholar]
  40. Moechars D, Dewachter I, Lorent K, Reverse D, Baekelandt V, Naidu A, et al. Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain. J Biol Chem. 1999;274:6483–6492. doi: 10.1074/jbc.274.10.6483. [DOI] [PubMed] [Google Scholar]
  41. Monteggia LM, Luikart B, Barrot M, Theobold D, Malkovska I, Nef S, et al. Brain-derived neurotrophic factor conditional knockouts show gender differences in depression-related behaviors. Biol Psychiatry. 2007;61:187–197. doi: 10.1016/j.biopsych.2006.03.021. [DOI] [PubMed] [Google Scholar]
  42. Morgan DG, May PC, Finch CE. Dopamine and serotonin systems in human and rodent brain: effects of age and neurodegenerative disease. J Am Geriatr Soc. 1987;35:334–345. doi: 10.1111/j.1532-5415.1987.tb04641.x. [DOI] [PubMed] [Google Scholar]
  43. Murer MG, Yan Q, Raisman-Vozari R. Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease. Prog Neurobiol. 2001;63:71–124. doi: 10.1016/s0301-0082(00)00014-9. [DOI] [PubMed] [Google Scholar]
  44. Narisawa-Saito M, Wakabayashi K, Tsuji S, Takahashi H, Nawa H. Regional specificity of alterations in NGF, BDNF and NT-3 levels in Alzheimer's disease. Neuroreport. 1996;7:2925–2928. doi: 10.1097/00001756-199611250-00024. [DOI] [PubMed] [Google Scholar]
  45. Ownby RL, Crocco E, Acevedo A, John V, Loewenstein D. Depression and risk for Alzheimer disease: systematic review, meta-analysis, and metaregression analysis. Arch Gen Psychiatry. 2006;63:530–538. doi: 10.1001/archpsyc.63.5.530. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Paxinos G, Watson CRR. The Rat Brain in Stereotaxic Coordinates. New York: Elsevier Academic Press; 1998. [Google Scholar]
  47. Pellow S, Chopin P, File SE, Briley M. Validation of open : closed arm entries in an elevated plus-maze as a measure of anxiety in the rat. J Neurosci Methods. 1985;14:149–167. doi: 10.1016/0165-0270(85)90031-7. [DOI] [PubMed] [Google Scholar]
  48. Pellow S, File SE. Anxiolytic and anxiogenic drug effects on exploratory activity in an elevated plus-maze: a novel test of anxiety in the rat. Pharmacol Biochem Behav. 1986;24:525–529. doi: 10.1016/0091-3057(86)90552-6. [DOI] [PubMed] [Google Scholar]
  49. Pomara N, Doraiswamy PM, Willoughby LM, Roth AE, Mulsant BH, Sidtis JJ, et al. Elevation in plasma Abeta42 in geriatric depression: a pilot study. Neurochem Res. 2006;31:341–349. doi: 10.1007/s11064-005-9029-z. [DOI] [PubMed] [Google Scholar]
  50. Porsolt RD, Bertin A, Jalfre M. Behavioral despair in mice: a primary screening test for antidepressants. Arch Int Pharmacodyn Ther. 1977;229:327–336. [PubMed] [Google Scholar]
  51. Preda S, Govoni S, Lanni C, Racchi M, Mura E, Grilli M, et al. Acute beta-amyloid administration disrupts the cholinergic control of dopamine release in the nucleus accumbens. Neuropsychopharmacology. 2008;33:1062–1070. doi: 10.1038/sj.npp.1301485. [DOI] [PubMed] [Google Scholar]
  52. Rowan MJ, Klyubin I, Wang Q, Anwyl R. Synaptic plasticity disruption by amyloid beta protein: modulation by potential Alzheimer's disease modifying therapies. Biochem Soc Trans. 2005;33:563–567. doi: 10.1042/BST0330563. [DOI] [PubMed] [Google Scholar]
  53. Schaub RT, Anders D, Golz G, Gohringer K, Hellweg R. Serum nerve growth factor concentration and its role in the preclinical stage of dementia. Am J Psychiatry. 2002;159:1227–1229. doi: 10.1176/appi.ajp.159.7.1227. [DOI] [PubMed] [Google Scholar]
  54. Schulte-Herbruggen O, Hellweg R, Chourbaji S, Ridder S, Brandwein C, Gass P, et al. Differential regulation of neurotrophins and serotonergic function in mice with genetically reduced glucocorticoid receptor expression. Exp Neurol. 2007;204:307–316. doi: 10.1016/j.expneurol.2006.11.009. [DOI] [PubMed] [Google Scholar]
  55. Schulte-Herbruggen O, Jockers-Scherubl MC, Hellweg R. Neurotrophins: from pathophysiology to treatment in Alzheimer's disease. Curr Alzheimer Res. 2008;5:38–44. doi: 10.2174/156720508783884620. [DOI] [PubMed] [Google Scholar]
  56. Selkoe DJ, Schenk D. Alzheimer's disease: molecular understanding predicts amyloid-based therapeutics. Annu Rev Pharmacol Toxicol. 2003;43:545–584. doi: 10.1146/annurev.pharmtox.43.100901.140248. [DOI] [PubMed] [Google Scholar]
  57. Shirayama Y, Chen AC, Nakagawa S, Russell DS, Duman RS. Brain-derived neurotrophic factor produces antidepressant effects in behavioral models of depression. J Neurosci. 2002;22:3251–3261. doi: 10.1523/JNEUROSCI.22-08-03251.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Siegel GJ, Chauhan NB. Neurotrophic factors in Alzheimer's and Parkinson's disease brain. Brain Res Brain Res Rev. 2000;33:199–227. doi: 10.1016/s0165-0173(00)00030-8. [DOI] [PubMed] [Google Scholar]
  59. Spruijt BM, Welbergen P, Brakkee J, Gispen WH. An ethological analysis of excessive grooming in young and aged rats. Ann N Y Acad Sci. 1988;525:89–100. doi: 10.1111/j.1749-6632.1988.tb38598.x. [DOI] [PubMed] [Google Scholar]
  60. Starkstein SE, Mizrahi R, Power BD. Depression in Alzheimer's disease: phenomenology, clinical correlates and treatment. Int Rev Psychiatry. 2008;20:382–388. doi: 10.1080/09540260802094480. [DOI] [PubMed] [Google Scholar]
  61. Steckler T, Sahgal A. The role of serotonergic-cholinergic interactions in the mediation of cognitive behaviour. Behav Brain Res. 1995;67:165–199. doi: 10.1016/0166-4328(94)00157-b. [DOI] [PubMed] [Google Scholar]
  62. Steffens DC, Plassman BL, Helms MJ, Welsh-Bohmer KA, Saunders AM, Breitner JC. A twin study of late-onset depression and apolipoprotein E epsilon 4 as risk factors for Alzheimer's disease. Biol Psychiatry. 1997;41:851–856. doi: 10.1016/S0006-3223(96)00247-8. [DOI] [PubMed] [Google Scholar]
  63. Sun X, Steffens DC, Au R, Folstein M, Summergrad P, Yee J, et al. Amyloid-associated depression: a prodromal depression of Alzheimer disease? Arch Gen Psychiatry. 2008;65:542–550. doi: 10.1001/archpsyc.65.5.542. [DOI] [PMC free article] [PubMed] [Google Scholar]
  64. Tapia-Arancibia L, Aliaga E, Silhol M, Arancibia S. New insights into brain BDNF function in normal aging and Alzheimer disease. Brain Res Rev. 2008;59:201–220. doi: 10.1016/j.brainresrev.2008.07.007. [DOI] [PubMed] [Google Scholar]
  65. Thoenen H. Neurotrophins and activity-dependent plasticity. Prog Brain Res. 2000;128:183–191. doi: 10.1016/S0079-6123(00)28016-3. [DOI] [PubMed] [Google Scholar]
  66. Trabace L, Kendrick KM, Castrignano S, Colaianna M, De Giorgi A, Schiavone S, et al. Soluble amyloid beta1-42 reduces dopamine levels in rat prefrontal cortex: relationship to nitric oxide. Neuroscience. 2007;147:652–663. doi: 10.1016/j.neuroscience.2007.04.056. [DOI] [PubMed] [Google Scholar]
  67. Tsai SJ. Brain-derived neurotrophic factor: a bridge between major depression and Alzheimer's disease? Med Hypotheses. 2003;61:110–113. doi: 10.1016/s0306-9877(03)00141-5. [DOI] [PubMed] [Google Scholar]
  68. Versijpt J, Van Laere KJ, Dumont F, Decoo D, Vandecapelle M, Santens P, et al. Imaging of the 5-HT2A system: age-, gender-, and Alzheimer's disease-related findings. Neurobiol Aging. 2003;24:553–561. doi: 10.1016/s0197-4580(02)00137-9. [DOI] [PubMed] [Google Scholar]
  69. Visser PJ, Verhey FR, Ponds RW, Kester A, Jolles J. Distinction between preclinical Alzheimer's disease and depression. J Am Geriatr Soc. 2000;48:479–484. doi: 10.1111/j.1532-5415.2000.tb04992.x. [DOI] [PubMed] [Google Scholar]
  70. Walf AA, Frye CA. The use of the elevated plus maze as an assay of anxiety-related behavior in rodents. Nat Protoc. 2007;2:322–328. doi: 10.1038/nprot.2007.44. [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from British Journal of Pharmacology are provided here courtesy of The British Pharmacological Society

RESOURCES