Age‐dependent alterations of the kynurenine pathway in the YAC128 mouse model of Huntington disease

Indoleamine 2,3 dioxygenase (Ido1), the first and rate‐limiting enzyme of the kynurenine pathway (KP), is a striatally enriched gene with increased expression levels in the YAC128 mouse model of Huntington disease (HD). Our objective in this study was to delineate age‐related KP alterations in this model. Three enzymes potentially catalyze the first step of the KP; Ido1 and Indoleamine 2,3 dioxygenase‐2 were highly expressed in the striatum and Tryptophan 2,3 dioxygenase (Tdo2) in the cerebellum. During development, Ido1 mRNA expression is dynamically regulated and chronically up‐regulated in YAC128 mice. Kynurenine (Kyn) to tryptophan (Trp) ratio, a measure of activity in the first step of the KP, was elevated in YAC128 striatum, but no change in Tdo2 mRNA levels or Kyn to Trp ratio was detected in the cerebellum. Ido1 induction was coincident with Trp depletion at 3 months and Kyn accumulation at 12 months of age in striatum. Changes in downstream KP metabolites of YAC128 mice generally followed a biphasic pattern with neurotoxic metabolites reduced at 3 months and increased at 12 months of age. Striatally specific induction of Ido1 and downstream KP alterations suggest involvement in HD pathogenesis, and should be taken into account in future therapeutic developments for HD.     

Age‐related deterioration of motor function in male and female 5xFAD mice from 3 to 16 months of age

Alzheimer's disease (AD) is a neurodegenerative disorder that leads to age‐related cognitive and sensori‐motor dysfunction. There is an increased understanding that motor dysfunction contributes to overall AD severity, and a need to ameliorate these impairments. The 5xFAD mouse develops the neuropathology, cognitive and motor impairments observed in AD, and thus may be a valuable animal model to study motor deficits in AD. Therefore, we assessed age‐related changes in motor ability of male and female 5xFAD mice from 3 to 16 months of age, using a battery of behavioral tests. At 9‐10 months, 5xFAD mice showed reduced body weight, reduced rearing in the open‐field and impaired performance on the rotarod compared to wild‐type controls. At 12‐13 months, 5xFAD mice showed reduced locomotor activity on the open‐field, and impaired balance on the balance beam. At 15‐16 months, impairments were also seen in grip strength. Although sex differences were observed at specific ages, the development of motor dysfunction was similar in male and female mice. Given the 5xFAD mouse is commonly on a C57BL/6 × SJL hybrid background, a subset of mice may be homozygous recessive for the Dysf ^im mutant allele, which leads to muscular weakness in SJL mice and may exacerbate motor dysfunction. We found small effects of Dysf ^im on motor function, suggesting that Dysf ^im contributes little to motor dysfunction in 5xFAD mice. We conclude that the 5xFAD mouse may be a useful model to study mechanisms that produce motor dysfunction in AD, and to assess the efficacy of therapeutics on ameliorating motor impairment.     

Impaired GAD1 expression in schizophrenia‐related WISKET rat model with sex‐dependent aggressive behavior and motivational deficit

After peri‐adolescence isolation rearing (IS) and subchronic ketamine (KET) treatment, adult, selectively bred Wistar rats (named WISKET) mimic abnormal behaviors reminiscent of human schizophrenia, including reduced prepulse‐inhibition of startle reflex, disturbances in cognition, locomotor activity and thermoregulation, decreased pain sensitivity and electrophysiological alterations. To further validate our WISKET rat line, regarding its translational utility in schizophrenia research, we examined their social behavior and introduced a short and simple holeboard (HB)‐like test to investigate their motivational deficit that predicts the cognitive disturbance. Sex‐dependent alterations in schizophrenia may yield important insights into its etiology; thus, male and female WISKET rats were also investigated and compared with their naive Wistar counterparts. Considering the contribution of the hippocampal and cortical GABAergic inhibitory circuitry in these behavioral alterations, molecular‐biology studies were also performed regarding the GAD1 gene products. Impaired social activity with increased aggression, stress‐related behavior, active social withdrawal, motivation deficit and decreased exploration were observed, especially in male WISKET rats, compared with Wistar ones and their corresponding females. These alterations were accompanied by sex‐dependent alterations regarding GAD67 mRNA and protein expression in the prefrontal cortex and hippocampus. In conclusion, the WISKET animals are valuable tools for animal‐based preclinical drug discovery studies for predictive screening of novel compounds improving negative symptoms with potential antipsychotic efficacy.     

Reduced acoustic startle response and peripheral hearing loss in the 5xFAD mouse model of Alzheimer's disease

Hearing dysfunction has been associated with Alzheimer's disease (AD) in humans, but there is little data on the auditory function of mouse models of AD. Furthermore, characterization of hearing ability in mouse models is needed to ensure that tests of cognition that use auditory stimuli are not confounded by hearing dysfunction. Therefore, we assessed acoustic startle response and pre‐pulse inhibition in the double transgenic 5xFAD mouse model of AD from 3–4 to 16 months of age. The 5xFAD mice showed an age‐related decline in acoustic startle as early as 3–4 months of age. We subsequently tested auditory brainstem response (ABR) thresholds at 4 and 13–14 months of age using tone bursts at frequencies of 2–32 kHz. The 5xFAD mice showed increased ABR thresholds for tone bursts between 8 and 32 kHz at 13–14 months of age. Finally, cochleae were extracted and basilar membranes were dissected to count hair cell loss across the cochlea. The 5xFAD mice showed significantly greater loss of both inner and outer hair cells at the apical and basal ends of the basilar membrane than wild‐type mice at 15–16 months of age. These results indicate that the 5xFAD mouse model of AD shows age‐related decreases in acoustic startle responses, which are at least partially due to age‐related peripheral hearing loss. Therefore, we caution against the use of cognitive tests that rely on audition in 5xFAD mice over 3–4 months of age, without first confirming that performance is not confounded by hearing dysfunction.     

Regulation of emotional memory by hydrogen sulfide: role of GluN2B‐containing NMDA receptor in the amygdala

As an endogenous gaseous molecule, hydrogen sulfide (H₂S) has attracted extensive attention because of its multiple biological effects. However, the effect of H₂S on amygdala‐mediated emotional memory has not been elucidated. Here, by employing Pavlovian fear conditioning, an animal model widely used to explore the neural substrates of emotion, we determined whether H₂S could regulate emotional memory. It was shown that the H₂S levels in the amygdala of rats were significantly elevated after cued fear conditioning. Both intraamygdala and systemic administrations of H₂S markedly enhanced amygdala‐dependent cued fear memory in rats. Moreover, it was found that H₂S selectively increased the surface expression and currents of NMDA‐type glutamate receptor subunit 2B (GluN2B)‐containing NMDA receptors (NMDARs) in lateral amygdala of rats, whereas blockade of GluN2B‐containing NMDARs in lateral amygdala eliminated the effects of H₂S to enhance amygdalar long‐term potentiation and cued fear memory. These results demonstrate that H₂S can regulate amygdala‐dependent emotional memory by promoting the function of GluN2B‐containing NMDARs in amygdala, suggesting that H₂S‐associated signaling may hold potential as a new target for the treatment of emotional disorders.

     

Behavioral flexibility in a mouse model for obsessive‐compulsive disorder: Impaired Pavlovian reversal learning in SAPAP3 mutants

Obsessive‐compulsive disorder (OCD) is characterized by obsessive thinking, compulsive behavior and anxiety, and is often accompanied by cognitive deficits. The neuropathology of OCD involves dysregulation of cortical‐striatal circuits. Similar to OCD patients, SAPAP3 knockout mice 3 (SAPAP3^?/?) exhibit compulsive behavior (grooming), anxiety and dysregulated cortical‐striatal function. However, it is unknown whether SAPAP3^?/? display cognitive deficits and how these different behavioral traits relate to one another. SAPAP3^?/? and wild‐type (WT) littermates were trained in a Pavlovian conditioning task pairing visual cues with the delivery of sucrose solution. After mice learned to discriminate between a reward‐predicting conditioned stimulus (CS+) and a non‐reward stimulus (CS?), contingencies were reversed (CS+ became CS? and vice versa). Additionally, we assessed grooming, anxiety and general activity. SAPAP3^?/? acquired Pavlovian approach behavior similarly to WT, albeit less vigorously and with a different strategy. However, unlike WT, SAPAP3^?/? were unable to adapt their behavior after contingency reversal, exemplified by a lack of re‐establishing CS+ approach behavior (sign tracking). Surprisingly, such behavioral inflexibility, decreased vigor, compulsive grooming and anxiety were unrelated. This study shows that SAPAP3^?/? are capable of Pavlovian learning, but lack flexibility to adapt associated conditioned approach behavior. Thus, SAPAP3^?/? not only display compulsive‐like behavior and anxiety, but also cognitive deficits, confirming and extending the validity of SAPAP3^?/? as a suitable model for the study of OCD. The observation that compulsive‐like behavior, anxiety and behavioral inflexibility were unrelated suggests a non‐causal relationship between these traits and may be of clinical relevance for the treatment of OCD.     

Autism‐related behaviors in the cyclooxygenase‐2‐deficient mouse model

Prostaglandin E2 (PGE2) is an endogenous lipid molecule involved in normal brain development. Cyclooxygenase‐2 (COX2) is the main regulator of PGE2 synthesis. Emerging clinical and molecular research provides compelling evidence that abnormal COX2/PGE2 signaling is associated with autism spectrum disorder (ASD). We previously found that COX2 knockout mice had dysregulated expression of many ASD genes belonging to important biological pathways for neurodevelopment. The present study is the first to show the connection between irregular COX2/PGE2 signaling and autism‐related behaviors in male and female COX2‐deficient knockin, (COX)‐2^?, mice at young (4‐6 weeks) or adult (8‐11 weeks) ages. Autism‐related behaviors were prominent in male (COX)‐2^? mice for most behavioral tests. In the open field test, (COX)‐2^? mice traveled more than controls and adult male (COX)‐2^? mice spent less time in the center indicating elevated hyperactive and anxiety‐linked behaviors. (COX)‐2^? mice also buried more marbles, with males burying more than females, suggesting increased anxiety and repetitive behaviors. Young male (COX)‐2^? mice fell more frequently in the inverted screen test revealing motor deficits. The three‐chamber sociability test found that adult female (COX)‐2^? mice spent less time in the novel mouse chamber indicative of social abnormalities. In addition, male (COX)‐2^? mice showed altered expression of several autism‐linked genes: Wnt2, Glo1, Grm5 and Mmp9. Overall, our findings offer new insight into the involvement of disrupted COX2/PGE2 signaling in ASD pathology with age‐related differences and greater impact on males. We propose that (COX)‐2^? mice might serve as a novel model system to study specific types of autism.     

Directional protein secretion by the retinal pigment epithelium: roles in retinal health and the development of age‐related macular degeneration

The structural and functional integrity of the retinal pigment epithelium (RPE) is fundamental for maintaining the function of the neuroretina. These specialized cells form a polarized monolayer that acts as the retinal–blood barrier, separating two distinct environments with highly specialized functions: photoreceptors of the neuroretina at the apical side and Bruch's membrane/highly vascularized choriocapillaris at the basal side. The polarized nature of the RPE is essential for the health of these two regions, not only in nutrient and waste transport but also in the synthesis and directional secretion of proteins required in maintaining retinal homoeostasis and function. Although multiple malfunctions within the RPE cells have been associated with development of age‐related macular degeneration (AMD), the leading cause of legal blindness, clear causative processes have not yet been conclusively characterized at the molecular and cellular level. This article focuses on the involvement of directionally secreted RPE proteins in normal functioning of the retina and on the potential association of incorrect RPE protein secretion with development of AMD. Understanding the importance of RPE polarity and the correct secretion of essential structural and regulatory components emerge as critical factors for the development of novel therapeutic strategies targeting AMD.     

Walking deficits and centrophobism in an α‐synuclein fly model of Parkinson's disease

Parkinson's disease (PD) is a movement neurodegenerative disorder, characterized by bradykinesia, rigidity and tremor, constituting difficulties in walking and abnormal gait. Previous research shows that Drosophila expressing human α‐synuclein A30P (A30P) develop deficits in geotaxis climbing; however, geotaxis climbing is a different movement modality from walking. Whether A30P flies would exhibit abnormal walking in a horizontal plane, a measure more relevant to PD, is not known. In this study, we characterized A30P fly walking using a high‐speed camera and an automatic behavior tracking system. We found that old but not young A30P flies exhibited walking abnormalities, specifically decreased total moving distance, distance per movement, velocity, angular velocity and others, compared with old control flies. Those features match the definition of bradykinesia. Multivariate analysis further suggested a synergistic effect of aging and A30P, resulting in a distinct pattern of walking deficits, as seen in aged A30P flies. Psychiatric problems are common in PD patients with anxiety affecting 40–69% of patients. Central avoidance is one assessment of anxiety in various animal models. We found old but not young A30P flies exhibited increased centrophobism, suggesting possible elevated anxiety. Here, we report the first quantitative measures of walking qualities in a PD fly model and propose an alternative behavior paradigm for evaluating motor functions apart from climbing assay.     

The relationship between socio‐sexual behavior and salivary cortisol in bonobos: tests of the tension regulation hypothesis

Bonobos are known for their pacifistic behavior and their large repertoire of behaviors that are thought to serve conflict resolution. One is an unusual form of ventro‐ventral mounting that facilitates genital contacts (GC). Various hypotheses have been proposed to explain its function. In this study we tested predictions of the tension regulation hypothesis using salivary cortisol as a marker for social stress. The results indicate a temporal relationship between GC and cortisol levels. Compared with baseline data and matched samples of unrestricted food access, rates of GC increased when access to food sources was restricted. Cortisol levels were highest when access to food was constrained. However, because the behavioral and hormonal responses occurred when viewing the stimulus at a distance and preceded the physical presence of the stimulus, we conclude that the anticipation of a competitive situation was sufficient to induce social stress. Contrary to our prediction, targets of aggression did not have higher rates of GC nor did they solicit GC more often than others. Furthermore, higher GC rates did not correlate with a more pronounced decrease in cortisol levels. Not all results obtained in this study supported the predictions concerning the regulatory function of GC on social tension and further research is needed to explore this question. However, the results indicate that the anticipation of competition may be sufficient to induce a costly physiological response, and that high levels of resource competition may have lasting effects on physical stress and stress management. Am. J. Primatol. 71:223–232, 2009. © 2008 Wiley‐Liss, Inc.     

Early postnatal behavioral changes in the Mecp2‐308 truncation mouse model of Rett syndrome

In a mouse model of Rett syndrome (RTT) which expresses a truncated form of methyl‐CpG‐binding protein 2 (Mecp2) gene (Mecp2‐308), we performed a neurobehavioral evaluation across the life span, starting from soon after birth till adulthood. A focus was made on those developmental phases and behavioral domains which have not been previously investigated. The results evidenced subtle anomalies on postnatal days (pnds) 3 to 9 (so‐called presymptomatic phase) in spontaneous movements by hemizygous neonatal male mice. Specifically as early as pnd 3, mutant pups exhibited more intense curling and more side responses and on pnd 9 more pivoting and head rising behaviors than wild type (wt) littermates. A significant decrease in ultrasonic vocalization rate, also emerged in Mecp2‐308 pups. The same mice were also characterized by increased anxiety‐like behaviors (open‐field and zero‐maze tests) during the early symptomatic phase, in the absence of changes in cognitive passive‐avoidance task and rotarod performances. Upon the clearly symptomatic stage, 5‐month‐old Mecp2‐308 mice were also associated with reduced spontaneous home‐cage motor activity, motor coordination impairments (rotarod and dowel tests), and a more marked profile of d ‐amphetamine (10 mg/kg) released stereotyped behavioral syndrome than wt mice. Present results provide an interesting timeline of the progression of symptoms in the Mecp2‐308 model and emphasize the need for increased attention to the presymptomatic phase which may be especially informative in mouse models of human neurodevelopmental disorders. This analysis has provided evidence of precocious behavioral markers of RTT and has identified an early developmental window of opportunities on which potential therapies could be investigated.