Differences in the stress response of Wistar-Kyoto (WKY) rats from different vendors

GLP-1 in the Hypothalamic Paraventricular Nucleus Promotes Sympathetic Activation and Hypertension

Glucagon-like peptide-1 (GLP-1) and its analogs are widely used for diabetes treatment. The paraventricular nucleus (PVN) is crucial for regulating cardiovascular activity. This study aims to determine the roles of GLP-1 and its receptors (GLP-1R) in the PVN in regulating sympathetic outflow and blood pressure. Experiments were carried out in male normotensive rats and spontaneously hypertensive rats (SHR). Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded. GLP-1 and GLP-1R expressions were present in the PVN. PVN microinjection of GLP-1R agonist recombinant human GLP-1 (rhGLP-1) or EX-4 increased RSNA and MAP, which were prevented by GLP-1R antagonist exendin 9-39 (EX9-39) or GLP-1R antagonist 1, superoxide scavenger tempol, antioxidant N-acetylcysteine, NADPH oxidase (NOX) inhibitor apocynin, adenylyl cyclase (AC) inhibitor SQ22536 or protein kinase A (PKA) inhibitor H89. PVN microinjection of rhGLP-1 increased superoxide production, NADPH oxidase activity, cAMP level, AC, and PKA activity, which were prevented by SQ22536 or H89. GLP-1 and GLP-1R were upregulated in the PVN of SHR. PVN microinjection of GLP-1 agonist increased RSNA and MAP in both WKY and SHR, but GLP-1 antagonists caused greater effects in reducing RSNA and MAP in SHR than in WKY. The increased superoxide production and NADPH oxidase activity in the PVN of SHR were augmented by GLP-1R agonists but attenuated by GLP-1R antagonists. These results indicate that activation of GLP-1R in the PVN increased sympathetic outflow and blood pressure via cAMP-PKA-mediated NADPH oxidase activation and subsequent superoxide production. GLP-1 and GLP-1R upregulation in the PVN partially contributes to sympathetic overactivity and hypertension.

Unmoving and uninflamed: Characterizing neuroinflammatory dysfunction in the Wistar-Kyoto rat model of depression

Reductionistic research on depressive disorders has been hampered by the limitations of animal models. Recently, it has been hypothesized that neuroinflammation is a key player in depressive disorders. The Wistar-Kyoto (WKY) rat is an often-used animal model of depression, but no information so far exists on its neuroinflammatory profile. As such, we compared male young adult WKY rats to Wistar (WS) controls, with regard to both behavioral performance and brain levels of key neuroinflammatory markers. We first assessed anxiety- and depression-like behaviors in a battery consisting of the Elevated Plus Maze (EPM), the Novelty Suppressed Feeding (NSFT), Open Field (OFT), Social Interaction (SIT), Forced Swim (FST), Sucrose Preference (SPT), and Splash tests (ST). We found that WKY rats displayed increased NSFT feeding latency, decreased OFT center zone permanence, decreased EPM open arm permanence, decreased SIT interaction time, and increased immobility in the FST. However, WKY rats also evidenced marked hypolocomotion, which is likely to confound performance in such tests. Interestingly, WKY rats performed similarly, or even above, to WS levels in the SPT and ST, in which altered locomotion is not a significant confound. In a separate cohort, we assessed prefrontal cortex (PFC), hippocampus and amygdala levels of markers of astrocytic (GFAP, S100A10) and microglial (Iba1, CD86, Ym1) activation status, as well as of three key proinflammatory cytokines (IL-1β, IL-6, TNF-α). There were no significant differences between strains in any of these markers, in any of the regions assessed. Overall, results highlight that behavioral data obtained with WKY rats as a model of depression must be carefully interpreted, considering the marked locomotor activity deficits displayed. Furthermore, our data suggest that, despite WKY rats replicating many depression-associated neurobiological alterations, as shown by others, this is not the case for neuroinflammation-related alterations, thus representing a novel limitation of this model.

Analysis of salivary flow rate, biochemical composition, and redox status in orchiectomized spontaneously hypertensive rats

OBJECTIVE

This study aimed to analyze the salivary flow rate, biochemical composition, and redox status in orchiectomized spontaneously hypertensive rats (SHR) compared to normotensive Wistar rats.

DESIGN

Thirty-two young adult male SHR and Wistar (3-months-old) rats were randomly distributed into four groups; either castrated bilaterally (ORX) or underwent fictitious surgery (SHAM) as Wistar-SHAM, Wistar-ORX, SHR-SHAM, and SHR-ORX. Two months beyond castration, pilocarpine-induced salivary secretion was collected from 5-month-old rats to analyze salivary flow rate, pH, buffer capacity, total protein, amylase, calcium, phosphate, sodium, potassium, chloride, thiobarbituric acid reactive substances (TBARs), carbonyl protein, nitrite, and total antioxidant capacity.

RESULTS

The salivary flow rate was higher in the Wistar-ORX compared to the Wistar-SHAM group, while remaining similar between the SHR-SHAM and SHR-ORX groups. ORX did not affect pH and salivary buffer capacity in both strains. However, salivary total protein and amylase were significantly reduced in the Wistar-ORX and SHR-ORX compared to the respective SHAM groups. In both ORX groups, salivary total antioxidant capacity and carbonylated protein were increased, while lipid oxidative damage (TBARs) and nitrite concentration were higher only in the Wistar-ORX than in the Wistar-SHAM group. In the Wistar-ORX and SHR-ORX, the salivary calcium, phosphate, and chloride were increased while no change was detected in the SHAM groups. Only salivary buffering capacity, calcium, and chloride in the SHR-ORX adjusted to values similar to Wistar-SHAM group.

CONCLUSION

Hypertensive phenotype mitigated the orchiectomy-induced salivary dysfunction, since the disturbances were restricted to alterations in the salivary biochemical composition and redox state.

Decreased sensitivity to antidepressant drugs in Wistar Hannover rats submitted to two animal models of depression

The Wistar Hannover rat (WHR) is a strain commonly used for toxicity studies but rarely used in studies investigating depression neurobiology. In this study, we aimed to characterise the behavioural responses of WHR to acute and repeated antidepressant treatments upon exposure to the forced swim test (FST) or learned helplessness (LH) test. WHR were subjected to forced swimming pre-test and test with antidepressant administration (imipramine, fluoxetine, or escitalopram) at 0, 5 h and 23 h after pre-test. WHR displayed high immobility in the test compared to unstressed controls (no pre-swim) and failed to respond to the antidepressants tested. The effect of acute and repeated treatment (imipramine, fluoxetine, escitalopram or s-ketamine) was then tested in animals not previously exposed to pre-test. Only imipramine (20 mg/kg, 7 days) and s-ketamine (acute) reduced the immobility time in the test. To further investigate the possibility that the WHR were less responsive to selective serotonin reuptake inhibitors, the effect of repeated treatment with fluoxetine (20 mg/kg, 7 days) was investigated in the LH model. The results demonstrated that fluoxetine failed to reduce the number of escape failures in two different protocols. These data suggest that the WHR do not respond to the conventional antidepressant treatment in the FST or the LH. Only s-ketamine and repeated imipramine were effective in WHR in a modified FST protocol. Altogether, these results indicate that WHR may be an interesting tool to investigate the mechanisms associated with the resistance to antidepressant drugs and identify more effective treatments.

The Wistar Kyoto Rat: A Model of Depression Traits

There is an ongoing debate about the value of animal research in psychiatry with valid lines of reasoning stating the limits of individual animal models compared to human psychiatric illnesses. Human depression is not a homogenous disorder; therefore, one cannot expect a single animal model to reflect depression heterogeneity. This limited review presents arguments that the Wistar Kyoto (WKY) rats show intrinsic depression traits. The phenotypes of WKY do not completely mirror those of human depression but clearly indicate characteristics that are common with it. WKYs present despair- like behavior, passive coping with stress, comorbid anxiety, and enhanced drug use compared to other routinely used inbred or outbred strains of rats. The commonly used tests identifying these phenotypes reflect exploratory, escape-oriented, and withdrawal-like behaviors. The WKYs consistently choose withdrawal or avoidance in novel environments and freezing behaviors in response to a challenge in these tests. The physiological response to a stressful environment is exaggerated in WKYs. Selective breeding generated two WKY substrains that are nearly isogenic but show clear behavioral differences, including that of depression-like behavior. WKY and its substrains may share characteristics of subgroups of depressed individuals with social withdrawal, low energy, weight loss, sleep disturbances, and specific cognitive dysfunction. The genomes of the WKY and WKY substrains contain variations that impact the function of many genes identified in recent human genetic studies of depression. Thus, these strains of rats share characteristics of human depression at both phenotypic and genetic levels, making them a model of depression traits.

Anxiety-like Behavior and GABAAR/BDZ Binding Site Response to Progesterone Withdrawal in a Stress-Vulnerable Strain, the Wistar Kyoto Rats

Stress susceptibility could play a role in developing premenstrual anxiety due to abnormalities in the hypothalamus–pituitary–adrenal (HPA) axis and impairments in the GABAA receptors’ benzodiazepine (BDZ) site. Hence, we studied the stress-vulnerable Wistar Kyoto rat strain (WKY) to evaluate progesterone withdrawal (PW) effects on anxiety, HPA axis response, and to explore indicators of GABAA functionality in the BDZ site. For five days, ovariectomized WKY rats were administered 2.0 mg/kg of progesterone. Twenty-four hours after the last administration, rats were tested in the anxiety-like burying behavior test (BBT) or elevated plus maze test (EPM), and corticosterone was determined. [3H]Flunitrazepam binding autoradiography served as the BDZ binding site index of the GABAA receptor in amygdala nuclei and hippocampus’s dentate gyrus (DG). Finally, different doses of diazepam in PW-WKY rats were tested in the BBT. PW induced anxiety-like behaviors in both BBT and EPM compared with No-PW rats. PW increased corticosterone, but was blunted when combined with PW and BBT. PW increased [3H]Flunitrazepam binding in the DG and central amygdala compared with No-PW rats. Diazepam at a low dose induced an anxiogenic-like response in PW rats, suggesting a paradoxical response to benzodiazepines. Overall, PW induced anxiety-like behavior, a blunted HPA axis response, and higher GABAAR/BZD binding site sensitivity in a stress-vulnerable rat strain. These findings demonstrate the role of stress-susceptibility in GABAAR functionality in a preclinical approximation of PMDD.

Whole genome sequencing of nearly isogenic WMI and WLI inbred rats identifies genes potentially involved in depression and stress reactivity

The WMI and WLI inbred rats were generated from the stress-prone, and not yet fully inbred, Wistar Kyoto (WKY) strain. These were selected using bi-directional selection for immobility in the forced swim test and were then sib-mated for over 38 generations. Despite the low level of genetic diversity among WKY progenitors, the WMI substrain is significantly more vulnerable to stress relative to the counter-selected WLI strain. Here we quantify numbers and classes of genomic sequence variants distinguishing these substrains with the long term goal of uncovering functional and behavioral polymorphism that modulate sensitivity to stress and depression-like phenotypes. DNA from WLI and WMI was sequenced using Illumina xTen, IonTorrent, and 10X Chromium linked-read platforms to obtain a combined coverage of ~ 100X for each strain. We identified 4,296 high quality homozygous SNPs and indels between the WMI and WLI. We detected high impact variants in genes previously implicated in depression (e.g. Gnat2), depression-like behavior (e.g. Prlr, Nlrp1a), other psychiatric disease (e.g. Pou6f2, Kdm5a, Reep3, Wdfy3), and responses to psychological stressors (e.g. Pigr). High coverage sequencing data confirm that the two substrains are nearly coisogenic. Nonetheless, the small number of sequence variants contributes to numerous well characterized differences including depression-like behavior, stress reactivity, and addiction related phenotypes. These selected substrains are an ideal resource for forward and reverse genetic studies using a reduced complexity cross.

Hypothalamic Gene Expression and Postpartum Behavior in a Genetic Rat Model of Depression

Postpartum depression is a complex illness that often occurs in genetically predisposed individuals. Closely related inbred rat strains are a great resource to identify novel causative genes and mechanisms underlying complex traits such as postpartum behavior. We report differences in these behaviors between the inbred depression model, Wistar Kyoto (WKY) More Immobile (WMI), and the isogenic control Wistar Kyoto Less Immobile (WLI) dams. WMI dams showed significantly lower litter survival rate and frequency of arched back and blanket nursing, but increased pup-directed licking, grooming, and retrieval during postpartum days (PPD) 1-10, compared to control WLIs. This increased pup-directed behavior and the frequency of self-directed behaviors segregated during selective breeding of the progenitor strain of WKY, which is also a depression model. These behaviors are manifested in the WMIs in contrast to those of WLIs. Furthermore, habitual differences in the self-directed behavior between light and dark cycles present in WLIs were missing in WMI dams. Hypothalamic transcript levels of the circadian rhythm-related gene Lysine Demethylase 5A (Kdm5a), period 2 (Per2), and the maternal behavior-related oxytocin receptor (Oxtr), vasopressin (Avp), and vasopressin receptor 1a (Avpr1a) were significantly greater in the post-weaning WMI dams at PPD 24 compared to those of WLIs, and also to those of WMI dams whose litter died before PPD 5. Expression correlation amongst genes differed in WLI and WMI dams and between the two time-points postpartum, suggesting genetic and litter-survival differences between these strains affect transcript levels. These data demonstrate that the genetically close, but behaviorally disparate WMI and WLI strains would be suitable for investigating the underlying genetic basis of postpartum behavior.

Facilitating Complex Trait Analysis via Reduced Complexity Crosses

Genetically diverse inbred strains are frequently used in quantitative trait mapping to identify sequence variants underlying trait variation. Poor locus resolution and high genetic complexity impede variant discovery. As a solution, we explore reduced complexity crosses (RCCs) between phenotypically divergent, yet genetically similar, rodent substrains. RCCs accelerate functional variant discovery via decreasing the number of segregating variants by orders of magnitude. The simplified genetic architecture of RCCs often permit immediate identification of causal variants or rapid fine-mapping of broad loci to smaller intervals. Whole-genome sequences of substrains make RCCs possible by supporting the development of array- and targeted sequencing-based genotyping platforms, coupled with rapid genome editing for variant validation. In summary, RCCs enhance discovery-based genetics of complex traits.

Genetic Model to Study the Co-Morbid Phenotypes of Increased Alcohol Intake and Prior Stress-Induced Enhanced Fear Memory

Posttraumatic Stress Disorder (PTSD) is a complex illness, frequently co-morbid with depression, caused by both genetics, and the environment. Alcohol Use Disorder (AUD), which also co-occurs with depression, is often co-morbid with PTSD. To date, very few genes have been identified for PTSD and even less for PTSD comorbidity with AUD, likely because of the phenotypic heterogeneity seen in humans, combined with each gene playing a relatively small role in disease predisposition. In the current study, we investigated whether a genetic model of depression-like behavior, further developed from the depression model Wistar Kyoto (WKY) rat, is a suitable vehicle to uncover the genetics of co-morbidity between PTSD and AUD. The by-now inbred WKY More Immobile (WMI) and the WKY Less Immobile (WLI) rats were generated from the WKY via bidirectional selective breeding using the forced swim test, a measure of despair-like behavior, as the functional selector. The colonies of the WMIs that show despair-like behavior and the control strain showing less or no despair-like behavior, the WLI, are maintained with strict inbreeding over 40 generations to date. WMIs of both sexes intrinsically self-administer more alcohol than WLIs. Alcohol self-administration is increased in the WMIs without sucrose fading, water deprivation or any prior stress, mimicking the increased voluntary alcohol-consumption of subjects with AUD. Prior Stress-Enhanced Fear Learning (SEFL) is a model of PTSD. WMI males, but not females, show increased SEFL after acute restraint stress in the context-dependent fear conditioning paradigm, a sexually dimorphic pattern similar to human data. Plasma corticosterone differences between stressed and not-stressed WLI and WMI male and female animals immediately prior to fear conditioning predict SEFL results. These data demonstrate that the WMI male and its genetically close, but behaviorally divergent control the WLI male, would be suitable for investigating the underlying genetic basis of comorbidity between SEFL and alcohol self-administration.

Differences in Stress-Induced Modulation of the Auditory System Between Wistar and Lewis Rats

Many aspects of stress-induced physiological and psychological effects have been characterized in people and animals. However, stress effects on the auditory system are less explored and their mechanisms are not well-understood, in spite of its relevance for a variety of diseases, including tinnitus. To expedite further research of stress-induced changes in the auditory system, here we compare the reactions to stress among Wistar and Lewis rats. The animals were stressed for 24 h, and subsequently we tested the functionality of the outer hair cells (OHCs) using distortion product otoacoustic emissions (DPOAEs) and auditory neurons using evoked auditory brainstem responses (ABR). Lastly, using Western blot, we analyzed the levels of plasticity-related proteins in the inferior colliculus, confirming that the inferior colliculus is involved in the adaptive changes that occur in the auditory system upon stress exposure. Surprisingly, the two strains reacted to stress quite differently: Lewis rats displayed a lowering of their auditory threshold, whereas it was increased in Wistar rats. These functional differences were seen in OHCs of the apical region (low frequencies) and in the auditory neurons (across several frequencies) from day 1 until 2 weeks after the experimental stress ended. Wistar and Lewis rats may thus provide models for auditory threshold increase and decrease, respectively, which can both be observed in different patients in response to stress.

Estrous Cycle Phase-Dependent Changes in Anxiety- and Depression-Like Profiles in the Late Adolescent Wistar-Kyoto Rat

BACKGROUND

Depression often manifests during adolescence when the development and networking of social and emotional brain areas is being influenced by hormones. The Wistar Kyoto (WKY) rat has been proposed as an animal model of adolescent depression with various face, construct, and predictive validities for clinical depression having been established.

PURPOSE

The influence of the estrous cycle on anxiety- and depression-like behaviors in female adolescents may be tested out further using this model.

METHODS

Female adolescent WKY rats were tested for anxiety- and depression-like behaviors in the elevated plus maze and forced swim test (FST) during different phases of the estrous cycle with inbred, age-, and phase-matched Wistar rats as controls.

RESULTS

Wistars in proestrus-estrus demonstrated reduced anxiety levels. WKY also demonstrated increased open arm time and entries and closed arm time, but less than Wistars, and as closed arm entries remained unaffected, it did not translate into a lowering of the anxiety levels. Risk taking and risk assessment behaviors were not affected by estrous phases in WKY, though exploratory behavior was reduced in proestrus-estrus. In Wistars, increased risk taking and decreased risk assessment behaviors were observed during proestrus-estrus. Increased immobility in the FST, indicative of learned helplessness was not influenced by phase in the WKY, which was at variance with Wistars that demonstrated phase-specific differences.

CONCLUSION

Results indicate a masking effect of indicative hormones in this putative model of adolescent depression, with implications for an unravelling of the steroid milieu in predisposed adolescent depression and for taking phase-specific time windows into account for therapeutic interventions.

Immediate and delayed anxiety- and depression-like profiles in the adolescent Wistar-Kyoto rat model of endogenous depression following postweaning social isolation

In order to understand links that exist between inherited risk or predisposition, brain and behavioural development, endocrine regulation and social/environmental stimuli, animal models are crucial. The Wistar-Kyoto (WKY) rat has been shown to have validity as a model of adult and adolescent depression. While sex- and age-specific differences in some of the face, predictive and construct validities of the model such as depression-like behaviours have been established, anhedonia and anxiety using other induced anxiety paradigms such as elevated plus maze remain equivocal. First, post-weaning social isolation effects on inherent and induced anxiety behaviours were tested during two critical time periods, early- and mid-adolescence. Isolation induced immediate effects on novel environment-induced hyperactivity and anxiety-related behaviours. Adolescent WKYs demonstrated reduced 50-kHz ultrasonic vocalizations suggesting agoraphobia-like behaviours. Second, isolated rats, despite being subsequently social-/group-housed demonstrated longer lasting effects on social interaction measures and anhedonia. This establishes that the depression-like profile observed during early- and mid-adolescence persists into late adolescence and early adulthood in WKY. Further, that interventions at a later stage during adolescence may not be able to reverse early adolescent effects in the context of pre-disposition, thus highlighting the irreversibility of being double-hit during critical time periods of brain and behavioural development and maturation.

Adolescent d-amphetamine treatment in a rodent model of attention deficit/hyperactivity disorder: impact on cocaine abuse vulnerability in adulthood

RATIONALE

Stimulant medications for attention-deficit/hyperactivity disorder (ADHD) in adolescents remain controversial with respect to later development of cocaine abuse. Past research demonstrated that adolescent methylphenidate treatment increased several aspects of cocaine self-administration during adulthood using the spontaneously hypertensive rat (SHR) model of ADHD. Presently, we determined effects of the alternate stimulant medication, d-amphetamine, on cocaine self-administration.

OBJECTIVES

We tested the hypothesis that adolescent d-amphetamine would not increase cocaine self-administration in adult SHR, given that d-amphetamine has a different mechanism of action than methylphenidate.

METHODS

A pharmacologically relevant dose of d-amphetamine (0.5 mg/kg) or vehicle was administered throughout adolescence to SHR and two control strains, Wistar-Kyoto (WKY) and Wistar (WIS). Three aspects of cocaine abuse vulnerability were assessed in adulthood after discontinuing adolescent treatments: acquisition rate and dose-related responding under fixed (FR) and progressive (PR) ratio schedules.

RESULTS

Adult SHR acquired cocaine self-administration faster and self-administered more cocaine across multiple doses compared to WKY and WIS under FR and PR schedules, indicating that SHR is a reliable animal model of comorbid ADHD and cocaine abuse. Relative to vehicle, SHR and WIS with adolescent d-amphetamine treatment self-administered less cocaine upon reaching acquisition criteria, and WIS additionally acquired cocaine self-administration more slowly and had downward shifts in FR and PR cocaine dose-response curves. WKY with adolescent d-amphetamine treatment acquired cocaine self-administration more quickly relative to vehicle.

CONCLUSIONS

In contrast to methylphenidate, adolescent d-amphetamine did not augment cocaine self-administration in SHR. Adolescent d-amphetamine treatment actually protected against cocaine abuse vulnerability in adult SHR and WIS.

Hippocampal and motor fronto-cortical neuroligin1 is increased in an animal model of depression

Neuroligins (NLGNs) regulate synaptic excitability, neuronal signaling and sleep. We hypothesize that alteration of NLGNs is involved in the pathology of depression and tested the hypothesis in a model of depression using Wistar Kyoto (WKy) rat and its control, the Wistar (Wis) rat. We first evaluated behavioral deficits using the forced swim test and then characterized alterations of NLGN1 and NLGN2 with RT-PCR and Western Blotting in the prefrontal cortex, motor frontal cortex and hippocampus. Compared with controls of Wis rats, (1) the WKy rats had significantly shorter swim time and longer immobile time; (2) NLGN1 mRNA levels was higher in the motor frontal cortex and hippocampus in the WKy model; (3) NLGN1 protein was significantly higher in the motor frontal cortex, the prefrontal cortex and the hippocampus in the WKy model; (4) NLGN2 mRNA was significantly higher in the motor frontal cortex but significantly lower in the hippocampus in the WKy model. We concluded that NLGN1 gene and protein expression is higher in the motor frontal cortex, hippocampus and in the prefrontal cortex in the WKy rats suggesting that alterations of NLGN1 is involved in the pathology of depression but need to be further evaluated in human.

Environmental enrichment prevents anxiety-like behavior induced by progesterone withdrawal in two strains of rats

Stress vulnerability could influence the treatment response to anxiety associated with abrupt hormonal suppression. The present study explored the effects of different treatments on experimental anxiety induced by progesterone withdrawal (PW) in a stress-sensitive rat strain, Wistar Kyoto (WKY), in the burying behavior test (BBT). The following experimental series was conducted using independent groups of Wistar (control strain) and WKY ovariectomized rats: Experiment 1: Rats were treated for 5days with oil, a constant dose of progesterone (0.5mg/rat, s.c) or a combination of progesterone (0.5mg/rat, s.c) plus fluoxetine (10 mg/kg, i.p); on day 6, all rats were subjected to BBT. Experiment 2: Rats received corn oil or decreasing doses of progesterone (0.84, 0.67, 0.5, 0.33 and 0.17mg/rat; one dose daily); on day 6, the rats were subjected to BBT. Experiment 3: Rats were divided into two groups that were subjected to 30days of standard conditions or environmental enrichment (EE); from days 25 to 30, all rats received a fixed dose of progesterone (0.5mg/rat, s.c.) or vehicle. On day 31, the rats were tested with BBT. Results showed that PW increased anxiety in both strains, and fluoxetine prevented anxiety in WKY rats. In contrast, a gradual reduction of progesterone prevents the anxiety in Wistar but not in WKY. EE was preventive against the anxiety induced by PW in both strains of rats. Thus, the results suggest that anxiety induced by PW is prevented by EE while the anxiolytic effect of pharmacological treatments depends on stress vulnerability.

Behavioral differences of male Wistar rats from different vendors in vulnerability and resilience to chronic mild stress are reflected in epigenetic regulation and expression of p11

Outbred rat lines such as Wistar rats are commonly used for models of depressive disorders. Such rats arise from random mating schedules. Hence, genetic drift occurs in outbred populations which could lead to genotypic and phenotypic heterogeneity between rats from different vendors. Additionally, vendor specific rearing conditions could contribute to intrastrain variability. In the present study differences in behavioral responses to the chronic mild stress (CMS) model of depression within Wistar rat strains from different vendors are described. DNA methylation studies and mRNA expression analysis of p11 revealed that the behavioral differences between the substrains are reflected at the epigenetic and genetic level. The results suggest that there are breeder-dependent differences in vulnerability to stress in the CMS model of depression, which might bear on the validity of the model and contribute to contradictory findings and difficulties of replication between laboratories. P11 mRNA expression seems to be differently regulated depending on the quality of the stress response evoked by CMS exposure.

Nature and nurture: environmental influences on a genetic rat model of depression

In this study, we sought to learn whether adverse events such as chronic restraint stress (CRS), or 'nurture' in the form of environmental enrichment (EE), could modify depression-like behavior and blood biomarker transcript levels in a genetic rat model of depression. The Wistar Kyoto More Immobile (WMI) is a genetic model of depression that aided in the identification of blood transcriptomic markers, which successfully distinguished adolescent and adult subjects with major depressive disorders from their matched no-disorder controls. Here, we followed the effects of CRS and EE in adult male WMIs and their genetically similar control strain, the Wistar Kyoto Less Immobile (WLI), that does not show depression-like behavior, by measuring the levels of these transcripts in the blood and hippocampus. In WLIs, increased depression-like behavior and transcriptomic changes were present in response to CRS, but in WMIs no behavioral or additive transcriptomic changes occurred. Environmental enrichment decreased both the inherent depression-like behavior in the WMIs and the behavioral difference between WMIs and WLIs, but did not reverse basal transcript level differences between the strains. The inverse behavioral change induced by CRS and EE in the WLIs did not result in parallel inverse expression changes of the transcriptomic markers, suggesting that these behavioral responses to the environment work via separate molecular pathways. In contrast, 'trait' transcriptomic markers with expression differences inherent and unchanging between the strains regardless of the environment suggest that in our model, environmental and genetic etiologies of depression work through independent molecular mechanisms.

Adolescent methylphenidate treatment differentially alters adult impulsivity and hyperactivity in the Spontaneously Hypertensive Rat model of ADHD

Impulsivity and hyperactivity are two facets of attention deficit/hyperactivity disorder (ADHD). Impulsivity is expressed as reduced response inhibition capacity, an executive control mechanism that prevents premature execution of an intermittently reinforced behavior. During methylphenidate treatment, impulsivity and hyperactivity are decreased in adolescents with ADHD, but there is little information concerning levels of impulsivity and hyperactivity in adulthood after adolescent methylphenidate treatment is discontinued. The current study evaluated impulsivity, hyperactivity as well as cocaine sensitization during adulthood after adolescent methylphenidate treatment was discontinued in the Spontaneously Hypertensive Rat (SHR) model of ADHD. Treatments consisted of oral methylphenidate (1.5mg/kg) or water vehicle provided Monday-Friday from postnatal days 28-55. During adulthood, impulsivity was measured in SHR and control strains (Wistar Kyoto and Wistar rats) using differential reinforcement of low rate (DRL) schedules. Locomotor activity and cocaine sensitization were measured using the open-field assay. Adult SHR exhibited decreased efficiency of reinforcement under the DRL30 schedule and greater levels of locomotor activity and cocaine sensitization compared to control strains. Compared to vehicle, methylphenidate treatment during adolescence reduced hyperactivity in adult SHR, maintained the lower efficiency of reinforcement, and increased burst responding under DRL30. Cocaine sensitization was not altered following adolescent methylphenidate in adult SHR. In conclusion, adolescent treatment with methylphenidate followed by discontinuation in adulthood had a positive benefit by reducing hyperactivity in adult SHR rats; however, increased burst responding under DRL compared to SHR given vehicle, i.e., elevated impulsivity, constituted an adverse consequence associated with increased risk for cocaine abuse liability.

Adolescent D-amphetamine treatment in a rodent model of ADHD: Pro-cognitive effects in adolescence without an impact on cocaine cue reactivity in adulthood

Attention-deficit/hyperactivity disorder (ADHD) is comorbid with cocaine abuse. Whereas initiating ADHD medication in childhood does not alter later cocaine abuse risk, initiating medication during adolescence may increase risk. Preclinical work in the Spontaneously Hypertensive Rat (SHR) model of ADHD found that adolescent methylphenidate increased cocaine self-administration in adulthood, suggesting a need to identify alternatively efficacious medications for teens with ADHD. We examined effects of adolescent d-amphetamine treatment on strategy set shifting performance during adolescence and on cocaine self-administration and reinstatement of cocaine-seeking behavior (cue reactivity) during adulthood in male SHR, Wistar-Kyoto (inbred control), and Wistar (outbred control) rats. During the set shift phase, adolescent SHR needed more trials and had a longer latency to reach criterion, made more regressive errors and trial omissions, and exhibited slower and more variable lever press reaction times. d-Amphetamine improved performance only in SHR by increasing choice accuracy and decreasing errors and latency to criterion. In adulthood, SHR self-administered more cocaine, made more cocaine-seeking responses, and took longer to extinguish lever responding than control strains. Adolescent d-amphetamine did not alter cocaine self-administration in adult rats of any strain, but reduced cocaine seeking during the first of seven reinstatement test sessions in adult SHR. These findings highlight utility of SHR in modeling cognitive dysfunction and comorbid cocaine abuse in ADHD. Unlike methylphenidate, d-amphetamine improved several aspects of flexible learning in adolescent SHR and did not increase cocaine intake or cue reactivity in adult SHR. Thus, adolescent d-amphetamine was superior to methylphenidate in this ADHD model.

Cocaine self-administration in Wistar-Kyoto rats: a behavioral and biochemical analysis

Depression and cocaine abuse disorders are common concurrent diagnoses. In the present study, we employed Wistar-Kyoto (WKY) rats that showed a depressive-like phenotype to study intravenous cocaine self-administration and extinction/reinstatement procedures. We also investigated the basal tissue level of neurotransmitters, their metabolites and plasma corticosterone (CORT) concentrations in WKY rats, bulbectomized (OBX) rats, and control rats. The WKY rats exhibited an attenuation of the cocaine-associated lever presses and cocaine intake during the acquisition/maintenance of cocaine self-administration only under specific conditions. Active lever presses exhibited by the WKY rats and control animals did not differ during the extinction training and cocaine-seeking behaviors. The WKY rats demonstrated alterations in the basal levels of dopamine, norepinephrine, and serotonin in selected brain structures involved in depression and drug addiction. The changes in the level of neurotransmitters in these animals refer not only to the control (Wistar) rats but also to bulbectomized animals, which represent another depression model. Furthermore, we identified unchanged levels of CORT in the WKY and OBX rats during the light phase and free-stress conditions. This finding suggests that WKY rats should not be used to investigate the co-occurrence of depression and cocaine addiction, as this rat strain does not show an enhanced risk of relapse.

Antidepressant-like effects of buprenorphine in rats are strain dependent

The prevalence of major depressive disorder and the limited efficacy of conventional drug treatments provide significant impetus to develop novel and more rapidly acting antidepressants for individuals with treatment resistant forms of depression. The primary goal of these studies was to ascertain whether buprenorphine (BPN), a medically available drug with mixed effects at opioid receptors, was effective in behavioral tests using the Wistar Kyoto (WKY) rat strain, a rodent model of exaggerated depressive and anxiety behaviors that demonstrates resistance to certain antidepressants. As WKY rats are maintained by different sources, we assessed the behavioral effects of BPN using the modified rat forced swim test (FST) and the emergence test in WKY rat colonies obtained from different vendors. BPN dose-dependently reduced immobility and increased swimming behavior in the FST and reduced emergence latencies in two WKY lines (Charles River (WKY/NCrl) and Harlan laboratories (WKY/NHsd)) that also showed high baseline immobility in the FST. WKY rats from Taconic (WKY/NTac) did not show high baseline immobility in the FST or anxiety as had been previously reported, suggesting a drift in the phenotype of rats from this supplier. Furthermore, BPN did not reduce immobility in the FST or reduce latencies in the emergence test in WKY rats from Taconic. BPN also failed to produce antidepressant-like effects in Wistar and Sprague-Dawley rats. These results indicate a striking strain-selectivity for the effects of BPN, producing antidepressant and anxiolytic-like responses in WKY/NCrl and WKY/NHsd lines but not in the normosensitive control Wistar and Sprague-Dawley strains.

The spontaneously hypertensive rat/Izm (SHR/Izm) shows attention deficit/hyperactivity disorder-like behaviors but without impulsive behavior: therapeutic implications of low-dose methylphenidate

The spontaneously hypertensive rat (SHR) has been used as a genetic animal model of attention deficit/hyperactivity disorder (ADHD). SHR/Izm is derived from stroke-resistant SHR as SHR/NIH and SHR/NCrl but from 22nd to 23rd generation descendants of the SHR/NIH ancestor and therefore may show different behavioral phenotypes compared to other SHR sub-strains. In this study, ADHD-like behaviors in SHR/Izm were evaluated compared to Wistar rats. SHR/Izm showed high locomotor activity in the habituation phase in a novel environment, although locomotor activity in the initial exploratory phase was low. In a behavioral test for attention, spontaneous alternation behavior in the Y-maze test was impaired in SHR/Izm. However, impulsive behavior in the elevated-plus maze test, which is designed to detect anxiety-related behavior but also reflects impulsivity for novelty seeking, was comparable to Wistar rats. Hyperactivity and inattention, detected as ADHD-like behaviors in SHR/Izm, were ameliorated with methylphenidate at a low dose (0.05mg/kg, i.p.). Therefore, SHR/Izm represents a unique animal model of ADHD without anxiety-related impulsive behavior.

Using the five-choice serial reaction time task to examine the effects of atomoxetine and methylphenidate in the male spontaneously hypertensive rat

Attention deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder and is normally treated with either stimulant or non-stimulant medication such as methylphenidate and atomoxetine respectively. The impact of these drugs on attention and impulsivity has been explored extensively in healthy animals but there is little research into their effects in an animal model of ADHD. In the present study we investigated the effects of both drugs on the spontaneously hypertensive rat (SHR) model of ADHD using the five-choice serial reaction time task (5CSRTT). We found a number of difficulties associated with training this vulnerable strain on such a complex task. However, where rats were able to learn the task we found very small effects of methylphenidate; increased incorrect responding and therefore decreased accuracy, a marker of attention at a single dose. There were no significant effects of atomoxetine on accuracy once multiple comparisons were taken into consideration. We found no effects of either drug on premature responding, a marker of impulsivity. These results indicate that the 5CSRTT may not be most sensitive to the impulsivity displayed in the SHR. Furthermore, they suggest that the SHR may lack predictive validity and further investigation is needed to optimise use of this model.

Changes in behavior and ultrasonic vocalizations during antidepressant treatment in the maternally separated Wistar-Kyoto rat model of depression

Genetic predisposition and stress are major factors in depression. The objective of this study was to establish a robust animal model of depression by selecting the appropriate substrain of the Wistar-Kyoto (WKY) rat, and subjecting these rats to the stress of maternal separation during the early stages of development. The initial experiment identified WKY/NCrl as the appropriate substrain of WKY to use for the study. In the second part of the study, depression-like behavior and ultrasonic vocalizations (USVs) were recorded in WKY/NCrl and maternally separated WKY/NCrl rats during the course of reversal of depression-like behavior. Wistar rats served as the reference strain. In adulthood, non-separated WKY/NCrl, maternally separated WKY/NCrl and Wistar rats were injected intraperitoneally with either saline or desipramine (15 mg/kg/day) for 15 days and their behavior recorded. Desipramine decreased immobility and increased active swimming and struggling behavior of WKY/NCrl in the FST and also decreased their USVs in response to removal of cage mates. The USVs in this study appeared to signal an attempt to re-establish social contact with cage mates and provided a measure of social dependence. Maternally separated WKY/NCrl rats displayed more anxiety than normally reared WKY/NCrl rats and responded to the anxiolytic effects of desipramine. The present findings support the use of WKY/NCrl as an animal model of depression. Maternal separation increased the anxiety-like behavior of the WKY/NCrl, thus providing a robust model to study depression- and anxiety-related behavior.

Autism-related behavioral phenotypes in an inbred rat substrain

Behavioral and genetic differences among Wistar-Kyoto (WKY) rats from different vendors and different breeders have long been observed, but generally overlooked. In our prior work, we found that two closely related WKY substrains, the WKY/NCrl and WKY/NHsd rats, differ in a small percentage of their genome which appeared to be highly enriched for autism risk genes. Although both substrains have been used widely in studies of hypertension, attention deficit/hyperactivity disorder (ADHD) and depression, they have not been tested for any autism-related behavioral phenotypes. Furthermore, these two substrains have often been used interchangeably in previous studies; no study has systematically examined the phenotypic differences that could be attributed by their small yet potentially meaningful genetic differences. In this paper we compared these two substrains on a battery of neurobehavioral tests. Although two substrains were similar in locomotor activity, WKY/NCrl rats were significantly different from WKY/NHsd rats in the elevated plus maze test, as well as measures of social interaction and ultrasonic vocalization. These strains were also compared with Sprague Dawley (SD) rats, a common outbred strain, and spontaneous hypertensive rats (SHR), an inbred rat model for ADHD and hypertension, which were derived from the same ancestor strain as the WKY strains. Our behavioral findings suggest that WKY/NCrl rats may be useful as a model autism spectrum disorders due to their lower social interest, lower ultrasonic vocalization and higher anxiety levels when WKY/NHsd rats are used as the control strain. Given the small genetic difference between the two inbred substrains, future studies to identify the exact gene and sequence variants that differ between the two may be useful for identifying the genetic mechanisms underlying these behaviors.

Variation in the form of Pavlovian conditioned approach behavior among outbred male Sprague-Dawley rats from different vendors and colonies: sign-tracking vs. goal-tracking

Even when trained under exactly the same conditions outbred male Sprague-Dawley (SD) rats vary in the form of the Pavlovian conditioned approach response (CR) they acquire. The form of the CR (i.e. sign-tracking vs. goal-tracking) predicts to what degree individuals attribute incentive salience to cues associated with food or drugs. However, we have noticed variation in the incidence of these two phenotypes in rats obtained from different vendors. In this study, we quantified sign- and goal-tracking behavior in a reasonably large sample of SD rats obtained from two vendors (Harlan or Charles River), as well as from individual colonies operated by both vendors. Our sample of rats acquired from Harlan had, on average, more sign-trackers than goal-trackers, and vice versa for our sample of rats acquired from Charles River. Furthermore, there were significant differences among colonies of the same vendor. Although it is impossible to rule out environmental variables, SD rats at different vendors and barriers may have reduced phenotypic heterogeneity as a result of genetic variables, such as random genetic drift or population bottlenecks. Consistent with this hypothesis, we identified marked population structure among colonies from Harlan. Therefore, despite sharing the same name, investigators should be aware that important genetic and phenotypic differences exist among SD rats from different vendors or even from different colonies of the same vendor. If used judiciously this can be an asset to experimental design, but it can also be a pitfall for those unaware of the issue.

Genetic architecture of Wistar-Kyoto rat and spontaneously hypertensive rat substrains from different sources

The spontaneously hypertensive rat (SHR) has been widely used as a model for studies of hypertension and attention deficit/hyperactivity disorder. The inbred Wistar-Kyoto (WKY) rat, derived from the same ancestral outbred Wistar rat as the SHR, are normotensive and have been used as the closest genetic control for the SHR, although the WKY has also been used as a model for depression. Notably, however, substantial behavioral and genetic differences among the WKY substrains, usually from the different vendors and breeders, have been observed. These differences have often been overlooked in prior studies, leading to inconsistent and even contradictory findings. The complicated breeding history of the SHR and WKY rats and the lack of a comprehensive understanding of the genetic background of different commercial substrains make the selection of control rats a daunting task, even for researchers who are mindful of their genetic heterogeneity. In this study, we examined the genetic relationship of 16 commonly used WKY and SHR rat substrains using genome-wide SNP genotyping data. Our results confirmed a large genetic divergence and complex relationships among the SHR and WKY substrains. This understanding, although incomplete without the genome sequence, provides useful guidance in selecting substrains and helps to interpret previous reports when the source of the animals was known. Moreover, we found two closely related, yet distinct WKY substrains that may provide novel opportunities in modeling psychiatric disorders.

Effects of atomoxetine on locomotor activity and impulsivity in the spontaneously hypertensive rat

Atomoxetine (ATX) is a commonly used non-stimulant treatment for Attention deficit hyperactivity disorder (ADHD). It primarily acts to increase noradrenalin levels; however, at higher doses it can increase dopamine levels. To date there has been no investigations into the effects of orally-administered ATX in the most commonly used model of ADHD, the spontaneously hypertensive rat (SHR). The aim of this study was to describe the effects of doses thought to be selective (0.15 mg/kg) and non-selective (0.3 mg/kg) for noradrenalin on behavioural measures in the SHR. Firstly, we examined the effects of acute and chronic ATX on locomotor activity including sensitisation and cross-sensitisation to amphetamine. Secondly, we measured drug effects on impulsivity using a T-maze delay discounting paradigm. We found no effect of ATX on locomotor activity and no evidence for sensitisation or cross-sensitisation. Furthermore, there were no differences in T-maze performance, indicating no effects on impulsivity at these doses. The absence of behavioural sensitisation supports previous claims of superior safety relative to psychostimulants for the doses administered. There was also no effect on impulsivity; however, we suggest that was confounded by stress specific to SHRs. Implications for future studies, behavioural assessment of SHRs and their use as a model of ADHD are discussed.

Alcohol induced depressive-like behavior is associated with a reduction in hippocampal BDNF

Strong positive correlation between depression and alcoholism is evident in epidemiological reports. However, a causal relationship for this co-morbidity has not been established. We have observed that chronic daily exposure to a relatively high dose of alcohol can induce depressive-like behavior in rats and that pretreatment with nomifensine or imipramine can block the "depressogenic" effects of alcohol. Since brain derived neurotrophic factor (BDNF) is considered to play an important role in depressive-like behaviors and its elevation, particularly in the hippocampus, appears to be critical for the action of many antidepressants, we hypothesized that: 1. WKY rats, a putative animal model of depression, will show a lower hippocampal BDNF compared to their control Wistar rats, 2. Alcohol-induced depressive like behavior will be associated with a significant decrease in hippocampal BDNF and 3. Treatments with antidepressants will normalize hippocampal BDNF. These postulates were verified by measuring hippocampal BDNF in Wistar and WKY rats at baseline, following chronic (10 day) treatment with alcohol and combination of alcohol with nomifensine or imipramine. Alcohol was administered via inhalation chamber (3 h/day) such that a blood alcohol level of approximately 150 mg% was achieved. Nomifensine (10 mg/kg) or imipramine (10 mg/kg) was administered i.p. daily immediately after alcohol exposure. BDNF was measured by standard ELISA kit. The results support a role for central BDNF in depressogenic effects of alcohol and antidepressant effects of nomifensine and imipramine. Moreover, depression per se as manifested in WKY rats may be associated with a reduction in hippocampal BDNF.

The age-dependent effects of selective serotonin reuptake inhibitors in humans and rodents: A review

The selective serotonin reuptake inhibitor (SSRI) Prozac® (fluoxetine) is widely prescribed for the treatment of depression and anxiety-related disorders. While extensive research has established that fluoxetine is safe for adults, safety is not guaranteed for (unborn) children and adolescents. Some clinical studies have reported adverse outcomes, such as premature birth, neonatal cardiovascular abnormalities, and pulmonary hypertension in children whose mothers used SSRIs during pregnancy. In addition, several reports show that adolescent fluoxetine treatment increases risk for suicidal behavior. Despite these studies, fluoxetine is not contraindicated in the treatment of depressed pregnant women and adolescents. Longitudinal research in humans is limited because of ethical reasons and time constraints, and to overcome these limitations, rodents are used to increase insight in the age-dependent effects of fluoxetine exposure. It has been established that neonatal and adolescent fluoxetine exposure leads to paradoxical anxiety- and depression-like features in later life of rats and mice, although in some studies adolescent fluoxetine exposure was without effects. These age-dependent outcomes of fluoxetine may be explained by serotonin's neurotrophic effects, which may vary according to the developmental stage of the brain due to epigenetic modifications. Here we review the existing evidence for the age-dependent effects of fluoxetine in humans and rodents, address the gaps in our current knowledge and propose directions for future research. Given the overlap between human and rodent findings, rodents provide heuristic value in further research on the age-dependent effects of SSRIs.

Feeding and reward: perspectives from three rat models of binge eating

Research has focused on understanding how overeating can affect brain reward mechanisms and subsequent behaviors, both preclinically and in clinical research settings. This work is partly driven by the need to uncover the etiology and possible treatments for the ongoing obesity epidemic. However, overeating, or non-homeostatic feeding behavior, can occur independent of obesity. Isolating the variable of overeating from the consequence of increased body weight is of great utility, as it is well known that increased body weight or obesity can impart its own deleterious effects on physiology, neural processes, and behavior. In this review, we present data from three selected animal models of normal-weight non-homeostatic feeding behavior that have been significantly influenced by Bart Hoebel's 40+-yr career studying motivation, feeding, reinforcement, and the neural mechanisms that participate in the regulation of these processes. First, a model of sugar bingeing is described (Avena/Hoebel), in which animals with repeated, intermittent access to a sugar solution develop behaviors and brain changes that are similar to the effects of some drugs of abuse, serving as the first animal model of food addiction. Second, another model is described (Boggiano) in which a history of dieting and stress can perpetuate further binge eating of palatable and non-palatable food. In addition, a model (Boggiano) is described that allows animals to be classified as having a binge-prone vs. binge-resistant behavioral profile. Lastly, a limited access model is described (Corwin) in which non-food deprived rats with sporadic limited access to a high-fat food develop binge-type behaviors. These models are considered within the context of their effects on brain reward systems, including dopamine, the opioids, cholinergic systems, serotonin, and GABA. Collectively, the data derived from the use of these models clearly show that behavioral and neuronal consequences of bingeing on a palatable food, even when at a normal body weight, are different from those that result from simply consuming the palatable food in a non-binge manner. These findings may be important in understanding how overeating can influence behavior and brain chemistry.

Schedule-induced polydipsia in the spontaneously hypertensive rat and its relation to impulsive behaviour

Eight Spontaneously Hypertensive Rat (SHR), 8 Wistar-Kyoto (WKY) and 8 Wistar rats, all male, maintained at 80-85% of their free-feeding weight by controlled access to food, were exposed to a series of fixed time (FT) schedules whereby food pellets were regularly delivered regardless of the animals' behaviour. The FT values used were 9, 15, 30, 60, 120 and 180 s, with the order of presentation of the schedules among the animals being counterbalanced (except under the FT 120-s and 180-s schedules, which were successively presented as the last two of the series). Due to freely available access to water, the animals developed schedule-induced drinking under all FT schedules, marked by the characteristic bitonic function that relates the number of licks and amount of water drunk to the length of the inter-food interval. Wistar and WKY rats displayed maximum drinking under an FT 15-s schedule, with WKY rats registering lower quantities across all FT values. Among SHR rats, maximum schedule-induced polydipsia was observed under the FT 30-s schedule, with a rightward shift in the bitonic function compared to controls. For long FT values, the temporal distribution of licks within inter-food intervals was shifted slightly towards the right in the SHR rats. In a subsequent study, only the SHR and Wistar rats were used, and the animals were exposed to a delay-discounting procedure. The rats were faced with successive choices, in which they could choose between an immediate reward of one food pellet and another of four food pellets at a delay of 3, 6, 12 or 24s. In the case of the longer delays, SHR rats chose the immediate reward of lower magnitude more often than did their Wistar counterparts, and also committed a greater number of omissions during the forced-choice trials of the procedure. The results indicate that differences in schedule-induced polydipsia are related to indexes of cognitive rather than motor impulsivity, a finding in line with the theoretical idea that adjunctive behaviour is linked to operant reinforcement processes.

The genetic basis of delay discounting and its genetic relationship to alcohol dependence

Delay discounting is steeper for individuals who drink heavily or are alcohol dependent, but the reasons for this are unclear. Given the substantial genetic component for alcohol dependence it is not unreasonable to ask whether discounting and alcohol dependence have a genetic relationship. For there to be a genetic relationship, delay discounting must have a genetic component (heritability). A review of the human and animal literature suggests that this is the case. Other literature examining whether discounting is a correlated phenotype in individuals who are genetically predisposed to drink (family history positive individuals and selected lines of rats and mice) is mixed, suggesting that networks of genes are critical for the relationship to be seen. The identities of the genes in this network are not yet known, but research examining polymorphisms associated with differences in discounting is beginning to address this issue.

Medial temporal lobe functioning and structure in the spontaneously hypertensive rat: comparison with Wistar-Kyoto normotensive and Wistar-Kyoto hypertensive strains

The spontaneously hypertensive rat (SHR) is used as an animal model of attention deficit hyperactivity disorder (ADHD). It displays deficits in frontostriatal functioning, but it is unclear if medial temporal lobe functioning and structure are affected. We used behavioral tasks that evaluate functioning of the amygdala and hippocampus to compare male SHR to male rats from two inbred comparator strains, the normotensive Wistar-Kyoto (WKY) and the hypertensive Wistar-Kyoto (WKHT) rat (n = 8/strain). The three strains showed similar levels of amygdala-related stimulus-reward learning during conditioned cue preference testing. In the ambiguous T-maze task, which dissociates between spatial and habit learning, significantly more WKHT than SHR or WKY used a response (indicative of habit learning) versus a place (indicative of spatial learning) strategy during an early probe test on day 8. During a later probe test on day 24, WKY progressed significantly from using a place strategy to a response strategy. Throughout all probe tests, a place strategy was used predominately by SHR and a response strategy by WKHT. Thus, SHR exhibited deficits in dorsal striatum-related habit learning, whereas WKHT exhibited deficits in hippocampus-related spatial learning. Following behavioral testing, fluid attenuated inversion recovery (FLAIR) magnetic resonance imaging scans were conducted in subgroups of rats from each strain (n = 4/strain). FLAIR imaging detected bilateral hippocampal hyperintensities in three of four WKHT and unilateral hippocampal atrophy in one of four SHR. The association between response strategy use during the initial probe test to forage for food in the ambiguous T-maze task and bilateral hippocampal abnormalities was significant. Collectively, while medial temporal lobe functioning appears to be normal in SHR exhibiting an ADHD-like phenotype, WKHT rats display both hippocampal functioning deficits and signs of bilateral hippocampal cell loss. The latter characteristics might be used to develop a new animal model of age- or disease-related decline in hippocampal functioning.

Comparison of the kappa-opioid receptor antagonist DIPPA in tests of anxiety-like behavior between Wistar Kyoto and Sprague Dawley rats

RATIONALE

Recent evidence suggests a role for the dynorphin/kappa-opioid receptor (KOR) system in the expression of stress-induced behaviors. Wistar Kyoto (WKY) rats exhibit increased depression-like and anxiety-like responses in behavioral tests compared to other strains and may be a model of comorbid depression and anxiety characterized by increased activity within the dynorphin/KOR system. Though KOR antagonists produce antidepressant-like effects in WKY rats, their effects in tests of anxiety-like behavior have not been examined in the WKY strain.

OBJECTIVE

The aim of the current study was to investigate the effects of the KOR antagonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide hydrochloride (DIPPA) on the behavior of WKY rats and Sprague Dawley (SD) rats in tests of anxiety-like behavior.

METHODS

The novelty-induced hypophagia and defensive burying tests were used to measure anxiety-like behavior in WKY and SD rats and determine the effects of DIPPA on anxiety-like behavior in both strains.

RESULTS

WKY rats displayed greater amounts of anxiety-like behavior compared to SD rats. DIPPA produced anxiolytic-like effects in both tests in both strains.

CONCLUSIONS

WKY rats display more anxiety-like behavior at baseline compared to SD rats, and DIPPA produced anxiolytic-like effects in both WKY and SD rats. These findings support previous research suggesting that KOR antagonists possess anxiolytic-like properties and may potentially represent a novel class of treatments for mood disorders.

Intrastrain differences in seizure susceptibility, pharmacological response and basal neurochemistry of Wistar rats

Reliable well-characterised animal models of seizures are necessary in order to better understand the underlying pathophysiological mechanisms as well as to screen potential anticonvulsant drugs. We currently use the focal pilocarpine model as an acute limbic seizure model. Due to breeding problems at the vendor, and apparent changes in pilocarpine-induced seizure susceptibility, we were forced to change breeding locations and vendors over a period of 2 years. Male Wistar rats were either purchased from two breeding locations of Charles River Laboratories (France and Germany), or obtained from Harlan Laboratories (The Netherlands). In the present retrospective study we evaluated the impact of these vendor changes on ketamine dosing to establish anaesthesia, on pilocarpine-induced seizure susceptibility, and on basal extracellular hippocampal noradrenaline, dopamine, serotonin, gamma-amino butyric acid, and glutamate levels of all pilocarpine-treated rats included in our studies. Significant differences were present in all of the parameters analyzed. This study clearly illustrates that intrastrain differences do exist from one vendor/breeding location to another, or even between rats from the same breeding location.

Sympathetic innervation of the spleen in male Brown Norway rats: a longitudinal aging study

Aging leads to reduced cellular immunity with consequent increased rates of infectious disease, cancer, and autoimmunity in the elderly. The sympathetic nervous system (SNS) modulates innate and adaptive immunity via innervation of lymphoid organs. In aged Fischer 344 (F344) rats, noradrenergic (NA) nerve density in secondary lymphoid organs declines, which may contribute to immunosenescence with aging. These studies suggest there is SNS involvement in age-induced immune dysregulation. The purpose of this study was to longitudinally characterize age-related change in sympathetic innervation of the spleen and sympathetic activity/tone in male Brown Norway (BN) rats, which live longer and have a strikingly different immune profile than F344 rats, the traditional animal model for aging research. Splenic sympathetic neurotransmission was evaluated between 8 and 32 months of age by assessing (1) NA nerve fiber density, (2) splenic norepinephrine (NE) concentration, and (3) circulating catecholamine levels after decapitation. We report a decline in NA nerve density in splenic white pulp (45%) at 15 months of age compared with 8-month-old (M) rats, which is followed by a much slower rate of decline between 24 and 32 months. Lower splenic NE concentrations between 15 and 32 months of age compared with 8M rats were consistent with morphometric findings. Circulating catecholamine levels after decapitation stress generally dropped with increasing age. These findings suggest there is a sympathetic-to-immune system dysregulation beginning at middle age. Given the unique T-helper-2 bias in BN rats, altered sympathetic-immune communication may be important for understanding the age-related rise in asthma and autoimmunity.

Antidepressants and REM sleep in Wistar–Kyoto and Sprague–Dawley rats

Compared to other rat strains, the Wistar-Kyoto rats show increased amount of REM sleep, one of the characteristic sleep changes observed in depressed patients. The aims of this study were firstly to validate a simple sleep stage discriminator and then compare the effect of antidepressants on suppression of rapid eye movement (REM) sleep in Wistar-Kyoto rats and an outbred rat strain (Sprague-Dawley). Rats were implanted with telemetry transmitters with electroencephalogram/electromyogram electrodes. Following recovery, the animals were orally dosed at light onset with either desipramine (20 mg/kg), fluoxetine (10 mg/kg), citalopram (10 or 40 mg/kg) or vehicle in a cross-over design. Every 12-s epoch was automatically scored as WAKE, NREM or REM sleep. Results confirm that Wistar-Kyoto rats show increased amount of REM sleep and decreased REM latency compared with Sprague-Dawley rats. All antidepressants significantly suppressed REM sleep in Sprague-Dawley rats, but only the high dose of citalopram suppressed REM sleep in Wistar-Kyoto rats. These findings suggest that the enhanced REM activity in Wistar-Kyoto rats is less sensitive to the effect of antidepressants and therefore does not provide any additional predictive validity for assessing antidepressant efficacy.

Gender-specific effects of thyroid hormones on cardiopulmonary function in SHHF rats

Spontaneously hypertensive heart failure (SHHF) rats develop hypertension and heart failure. We hypothesized that induction of hyperthyroidism should accelerate development of heart failure in male SHHF rats. Male and female SHHF rats received diets containing desiccated thyroid glands (DTG) or a control diet for 8 wk. Male and female Wistar-Kyoto rats were used as normotensive controls. DTG treatment reduced body weight in male, but not female, SHHF rats but increased body temperature and heart weight-to-body weight ratio in both genders. In DTG-treated male SHHF rats, serum triiodothyronine levels doubled relative to SHHF controls, whereas O2 consumption increased in DTG-treated SHHF rats. Frequency of breathing in air increased in DTG-treated female rats, and ventilation increased in DTG-treated male rats. Ventilatory equivalents exhibited gender differences in SHHF rats, were decreased in both genders by DTG treatment, and reached levels similar to those of Wistar-Kyoto rats. DTG increased heart rate, right ventricular pressure, and contractility in both genders and increased left ventricular pressure in SHHF male rats. These results refute our hypothesis and suggest that cardiopulmonary function of SHHF male rats may be improved by DTG treatment.

Selectively bred Wistar-Kyoto rats: an animal model of depression and hyper-responsiveness to antidepressants

The Wistar-Kyoto (WKY) rat strain demonstrates endogenous hormonal and behavioral abnormalities that emulate many of those found in symptom-presenting depressive patients. Evidence suggests that the WKY strain may harbor heterogeneity not found in other inbred strains, including greater behavioral and genetic variability. We took advantage of this variability and selectively bred WKY for 'depressive' behavior using immobility in the forced swim test (FST) as a functional selector. Successive generations of selective breeding resulted in rats that exhibited the extremes of immobility in the FST: 'WKY most immobile' (WMI) and 'WKY least immobile' (WLI). Male WMI rats also showed significantly decreased activity in the open field test (OFT). Plasma corticosterone (CORT) response to restraint stress was significantly lower and less variable in WMI compared to WLI males. Subacute treatment of males with several classes of antidepressant had different effects on FST behavior in the two substrains. Both desipramine (10 mg/kg body weight), a tricyclic antidepressant, and phenelzine (7.5 mg/kg), a monoamine oxidase inhibitor, significantly and drastically decreased FST immobility in WMI. In contrast, WLI showed a limited response to these antidepressants. Neither substrain responded to fluoxetine (10 mg/kg), a selective serotonin reuptake inhibitor. These data show that selective breeding of WKY rats has resulted in two substrains with reduced variability and differing responsiveness to antidepressants, which represent a novel means to dissect the molecular mechanisms underlying depressive behavior.

Differences in the renal dopaminergic system activity between Wistar rats from two suppliers

AIM

Dopamine of renal origin reduces tubular sodium reabsorption and controls blood pressure. The present study evaluated renal dopaminergic activity and its response to uninephrectomy in Wistar Han rats from two suppliers, Harlan (W-H) and Charles River (W-CR).

RESULTS

After uninephrectomy, the fractional excretion of sodium (FENa+) increased in both W-CR and W-H rats (W-CR: from 0.17 +/- 0.01 to 0.27 +/- 0.02%; W-H: from 0.39 +/- 0.04 to 0.54 +/- 0.04%, P < 0.05); however, in W-CR rats the FENa+ was lower than in W-H rats in both Sham and uninephrectomized (Unx) animals (P < 0.05). Systolic blood pressure in Unx W-CR rats was higher than in Unx W-H animals (131 +/- 3 vs. 122 +/- 2 mmHg, P < 0.05). Uninephrectomy was accompanied in W-H rats by increases in urinary levels (nmol g kidney(-1) day(-1)) of dopamine (10.3 +/- 0.5 vs. 8.3 +/- 0.7, P < 0.05) and 3,4-dihydroxyphenylacetic acid (DOPAC) (30.5 +/- 3.7 vs. 21.3 +/- 1.4, P < 0.05) and increases (P < 0.05) in maximal velocity values (Vmax in nmol mg prot(-1) 15 min(-1), 325 +/- 12 vs. 265 +/- 3) for renal aromatic L-amino acid decarboxylase (AADC), the enzyme responsible for the synthesis of renal dopamine. By contrast, in W-CR rats uninephrectomy did not change either the urinary levels of dopamine (7.1 +/- 0.5 vs. 7.6 +/- 0.7) and DOPAC (25.0 +/- 1.9 vs. 24.8 +/- 4.1) or AADC activity (Vmax 199 +/- 3 vs. 193 +/- 9). The Vmax values for renal AADC in W-CR rats were lower than those found in corresponding W-H animals.

CONCLUSION

Wistar rats from different suppliers represent an important source of variability in the renal dopaminergic system activity. This may contribute to differences in sodium balance and blood pressure control in response to uninephrectomy.

Strain-dependent modification of behavior following antidepressant treatment

The effects of repeated antidepressant drug treatment on behavioral outcome in Wistar Kyoto (WKY) rats, a putative animal model of depressive behavior, were compared to Wistar and Sprague-Dawley (SD) rats. Rats were treated with desipramine (norepinephrine [NE] uptake blocker), nomifensine (NE and dopamine [DA] uptake blocker), paroxetine (serotonin [5-HT] uptake blocker) or saline, for 12 days. On Day 11, rats were tested in the Porsolt forced swim test (FST). On Day 12, rats were tested in the open field test (OFT). Stress reactivity was assessed on Day 13 when all rats were exposed to water-restraint ulcerogenic stress. Significant strain differences in behavioral responses to the drug treatments were observed. Control WKY rats showed the typical freezing behavior in the OFT and excessive floating behavior in the FST as compared to Wistar and SD rats. Desipramine and nomifensine decreased immobility and increased swim time in the FST in WKY rats. Nomifensine reduced response latency in the OFT in WKY rats and increased activity in the OFT in WKY and SD rats. None of the drugs altered the FST in SD rats. Following ulcerogenic stress, desipramine was the only antidepressant that decreased ulcer incidence in all rat strains compared to saline controls. These results suggest that the "depressive behavior" in WKY rats may be modified by antidepressants that alter synaptic levels of NE and/or DA, but not 5-HT.

Altered hormone levels and circadian rhythm of activity in the WKY rat, a putative animal model of depression

The Wistar Kyoto (WKY) rat is hyperreactive to stress and exhibits depressive-like behavior in several standard behavioral tests. Because patients with depressive disorders often exhibit disruptions in the circadian rhythm of activity, as well as altered secretory patterns of the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-thyroid hormones, we tested the hypothesis that these phenomena occur in the WKY rat. Plasma ACTH and corticosterone levels remained significantly higher after the diurnal peak for several hours in WKY rats relative to Wistar rats. Also, plasma levels of thyroid-stimulating hormone were significantly higher in WKY relative to Wistar rats across the 24-h period, despite normal or slightly higher levels of 3,5,3'-triiodothyronine. In addition, under constant darkness conditions, WKY rats exhibited a shorter free running period and a decreased response to a phase-delaying light pulse compared with Wistar rats. In several ways these results are similar to those seen in other animal models of depression as well as in depressed humans, suggesting that the WKY rat could be used to investigate the genetic basis for these abnormalities.