Sleep in the Wistar-Kyoto rat, a putative genetic animal model for depression

N-methyl-d-aspartic acid receptors are altered by stress and alcohol in Wistar-Kyoto rat brain

Previous studies have shown that the Wistar-Kyoto (WKY) rat strain is more sensitive to stressors and consumes significant quantities of alcohol under basal as well as stressful conditions when compared to other strains. Given that the glutamate neurotransmitter system has been implicated in depression and addiction, the goals of the present study were to investigate the effects of stress and stress-alcohol interactions on N-methyl-d-aspartate (NMDA) receptors in the rat brain. Thus this study measured the binding of [(3)H] MK-801 to NMDA receptors in the prefrontal cortex (PFC), caudate putamen (CPu), nucleus accumbens (NAc), hippocampus (HIP) and basolateral amygdala (BLA) in WKY rats in comparison to the Wistar (WIS) rat strain. Our results suggested that while voluntary alcohol consumption did not alter NMDA receptors in the PFC, CPu or NAc in either rat strain, it increased NMDA receptors in the HIP and BLA in both strains. In contrast, chronic stress increased NMDA receptors in the PFC, CPu, NAc in WKY rats but not in WIS rats. Chronic stress also decreased NMDA receptors in the HIP and increased NMDA receptors in the BLA in both strains. Alcohol co-treatment with stress increased NMDA receptors in the PFC, CPu and NAc in WKY rats but not in WIS rats. Interestingly, while alcohol co-treatment did not reverse stress induced decreases in NMDA receptors in the HIP, it reduced the binding of NMDA receptors in the BLA to control levels in both strains. Thus it appears that NMDA receptors in the PFC, CPu and NAc may be more sensitive to the effects of stress and could be implicated in the stress-induced alcohol consumption behavior seen in WKY rats. In contrast, NMDA receptors in the HIP and BLA may reflect an adaptive response and may not be responsible for the stress susceptible phenotype of the WKY rat strain.

Wake and sleep EEG provide biomarkers in depression

Both wake and sleep electroencephalogram (EEG) provide biomarkers of depression and antidepressive therapy, respectively. For a long time it is known that EEG activity is altered by drugs. Quantitative EEG analysis helps to delineate effects of antidepressants on brain activity. Cordance is an EEG measure with a superior correlation with regional brain perfusion. Prefrontal quantitative EEG cordance appears to be a predictor of the response to antidepressants. Sleep EEG shows characteristic changes in depression as impaired sleep continuity, desinhibition of REM sleep and changes of nonREM sleep. Elevated REM density (a measure for frequency of rapid eye movements) characterizes an endophenotype in family studies of depression. REM-sleep changes including a more distinct REM rebound after sleep deprivation are found in animal models of depression. Most antidepressants suppress REM sleep in depressed patients, normal controls and laboratory animals. REM suppression appears to be a distinct, but not an absolute requirement for antidepressive effects of a compound. Sleep-EEG variables like REM latency or certain clusters of variables were shown to predict the response to the treatment with a certain antidepressant or even the course of the disorder for several years. Some of these predictive sleep-EEG markers of the longterm course of depression appear to be closely related to hypothalamo-pituitary-adrenocortical system activity.

Conditional corticotropin-releasing hormone overexpression in the mouse forebrain enhances rapid eye movement sleep

Impaired sleep and enhanced stress hormone secretion are the hallmarks of stress-related disorders, including major depression. The central neuropeptide, corticotropin-releasing hormone (CRH), is a key hormone that regulates humoral and behavioral adaptation to stress. Its prolonged hypersecretion is believed to play a key role in the development and course of depressive symptoms, and is associated with sleep impairment. To investigate the specific effects of central CRH overexpression on sleep, we used conditional mouse mutants that overexpress CRH in the entire central nervous system (CRH-COE-Nes) or only in the forebrain, including limbic structures (CRH-COE-Cam). Compared with wild-type or control mice during baseline, both homozygous CRH-COE-Nes and -Cam mice showed constantly increased rapid eye movement (REM) sleep, whereas slightly suppressed non-REM sleep was detected only in CRH-COE-Nes mice during the light period. In response to 6-h sleep deprivation, elevated levels of REM sleep also became evident in heterozygous CRH-COE-Nes and -Cam mice during recovery, which was reversed by treatment with a CRH receptor type 1 (CRHR1) antagonist in heterozygous and homozygous CRH-COE-Nes mice. The peripheral stress hormone levels were not elevated at baseline, and even after sleep deprivation they were indistinguishable across genotypes. As the stress axis was not altered, sleep changes, in particular enhanced REM sleep, occurring in these models are most likely induced by the forebrain CRH through the activation of CRHR1. CRH hypersecretion in the forebrain seems to drive REM sleep, supporting the notion that enhanced REM sleep may serve as biomarker for clinical conditions associated with enhanced CRH secretion.

Animal models of narcolepsy

Narcolepsy is a debilitating sleep disorder with excessive daytime sleepiness and cataplexy as its two major symptoms. Although this disease was first described about one century ago, an animal model was not available until the 1970s. With the establishment of the Stanford canine narcolepsy colony, researchers were able to conduct multiple neurochemical studies to explore the pathophysiology of this disease. It was concluded that there was an imbalance between monoaminergic and cholinergic systems in canine narcolepsy. In 1999, two independent studies revealed that orexin neurotransmission deficiency was pivotal to the development of narcolepsy with cataplexy. This scientific leap fueled the generation of several genetically engineered mouse and rat models of narcolepsy. To facilitate further research, it is imperative that researchers reach a consensus concerning the evaluation of narcoleptic behavioral and EEG phenomenology in these models.

Role of orexin in central regulation of gastrointestinal functions

Orexins are neuropeptides that are localized in neurons within the lateral hypothalamus and regulate feeding behavior. The lateral hypothalamus plays an important role in not only feeding but also in the central regulation of gut function. Along this line, accumulating evidence has shown that orexins act in the central nervous system to regulate gastrointestinal functions. The purpose of this review is to summarize recent relevant findings on brain orexins and the digestive system, and discuss the pathophysiological roles of these peptides. Centrally administered orexin or endogenously released orexin in the brain potently stimulates gastric acid secretion in rats. The vagal cholinergic pathway is involved in the orexin-induced stimulation of acid secretion. Because of its stimulatory action on feeding, it can be hypothesized that orexin in the brain is a candidate mediator of cephalic phase gastric secretion. In addition, brain orexin may be involved in the development of depression and functional gastrointestinal disorders, which are frequently accompanied by inhibition of gut function, because lack of orexin activity might cause the inhibition of gastric physiological processes and evoke a depressive state. These lines of evidence suggest that orexin in the brain is a potential molecular target for treatment of functional gastrointestinal disorders.

Effects of rapid eye movement sleep deprivation on hypocretin neurons in the hypothalamus of a rat model of depression

Hypocretin (Hcrt, also known as orexin) is a hypothalamic neuropeptide linked to narcolepsy, a disorder diagnosed by the appearance of rapid eye-movement sleep (REMS)-state characteristics during waking. Major targets of Hcrt-containing fibers include the locus coeruleus and the raphe nucleus, areas with important roles in regulation of mood and sleep. A relationship between REMS and mood is suggested by studies demonstrating that REMS-deprivation (REMSD) ameliorates depressive symptoms in humans. Additional support is found in animal studies where antidepressants and REMSD have similar effects on monoamiergic systems thought to be involved in major depression. Recently, we have reported that Wistar-Kyoto (WKY) rats, an animal model of depression, have reduced number and size of hypothalamic cells expressing Hcrt-immunoractivity compared to the parent, Wistar (WIS) strain, suggesting the possibility that the depressive-like attributes of the WKY rat may be determined by this relative reduction in Hcrt cells [Allard, J.S., Tizabi, Y., Shaffery, J.P., Trouth, C.O., Manaye, K., 2004. Stereological analysis of the hypothalamic hypocretin/orexin neurons in an animal model of depression. Neuropeptides 38, 311-315]. In this study, we sought to test the hypothesis that REMSD would result in a greater increase in the number and/or size of hypothalamic, Hcrt-immunoreactive (Hcrt-ir) neurons in WKY, compared to WIS rats. The effect of REMSD, using the multiple-small-platforms-over-water (SPRD) method, on size and number of Hcrt-ir cells were compared within and across strains of rats that experienced multiple-large-platforms-over-water (LPC) as well as to those in a normal, home-cage-control (CC) setting. In accord with previous findings, the number of Hcrt-ir cells was larger in all three WIS groups compared to the respective WKY groups. REMSD produced a 20% increase (p<0.02) in the number of hypothalamic Hcrt-ir neurons in WKY rats compared to cage control WKY (WKY-CC) animals. However, an unexpected higher increase in number of Hcrt-ir cells was also observed in the WKY-LPC group compared to both WKY-CC (31%, p<0.001) and WKY-SPRD (20%, p<0.002) rats. A similar, smaller, but non-significant, pattern of change was noted in WIS-LPC group. Overall the data indicate a differential response to environmental manipulations where WKY rats appear to be more reactive than WIS rats. Moreover, the findings do not support direct antidepressant-like activity for REMSD on hypothalamic Hcrt neurons in WKY rats.

Reduced orexin levels in the cerebrospinal fluid of suicidal patients with major depressive disorder

Orexins are neuropeptides selectively expressed in a small number of neurons in the lateral-posterior hypothalamus. We measured orexin-A in the cerebrospinal fluid (CSF) of 66 patients with major depressive disorder (MDD), dysthymia and adjustment disorder after a suicide attempt. Blood samples confirmed that the patients were free from antidepressive and neuroleptic medication at the time of the lumbar punctures. CSF levels of orexin-A were significantly lower in patients with MDD than in patients with adjustment disorder and dysthymia. Orexin correlated significantly with CSF levels of somatostatin, delta sleep inducing peptide-like immunoreactivity (DSIP-LI) and corticotrophin releasing factor (CRF), but not with leptin or vasopressin. Plasma levels of thyroid-stimulating hormone (TSH) were not reduced in MDD patients, and did not correlate with CSF-orexin. Our results suggest that suicidal patients with MDD have distinct neurobiological features, involving compromised levels of hypothalamic peptides regulating the state of arousal.

Orexin and psychiatric symptoms in suicide attempters

BACKGROUND

The orexins (hypocretins) are recently discovered hypothalamic peptides that are involved in the regulation of sleep, appetite and state of arousal. In the present study, we investigated the relationship between cerebrospinal fluid (CSF) orexin and specific psychiatric symptoms in suicidal patients.

METHODS

A total of 101 patients were enrolled in the study shortly after a suicide attempt. All patients underwent a lumbar puncture after a wash-out period during which they did not receive any antipsychotic or antidepressive medication. Structured interviews were performed using the Comprehensive Psychopathological Rating Scale (CPRS). CSF-orexin-A was measured and correlated with ratings of psychiatric symptoms.

RESULTS

There were significant and negative correlations between CSF-orexin and the symptoms lassitude (difficulty to initiate activities) and slowness of movement, as well as the ratings of global illness (p<0.005 for all three items, Spearman's rho).

LIMITATIONS

Correlation analysis is an indirect method of investigation and does not demonstrate causal relationships.

CONCLUSION

Low CSF-orexin levels are related to pronounced symptoms of inertia and reduced motor activity in suicidal patients. Interestingly, the lower the orexin levels, the higher were ratings of overall illness, as observed by a specialist in psychiatry. Our results suggest that reduced orexin levels are involved in the etiology of specific psychiatric symptoms.

The neurobiological characteristics of rapid eye movement (REM) sleep are candidate endophenotypes of depression, schizophrenia, mental retardation and dementia

Animal models are a promising method to approach the basic mechanisms of the neurobiological disturbances encountered in mental disorders. Depression is characterized by a decrease of REM sleep latency and an increase of rapid eye movement density. In schizophrenia, electrophysiological, tomographic, pharmacological and neurochemical activities are all encountered during REM sleep. Mental retardation and dementia are characterized by rather specific REM sleep disturbances. Identification of the genetic support for these abnormalities (endophenotypes) encountered during REM sleep could help to develop specific treatments.

Identifying genes in monoamine nuclei that may determine stress vulnerability and depressive behavior in Wistar-Kyoto rats

The Wistar-Kyoto (WKY) rat is stress sensitive and exhibits depressive-like behavior. The locus coeruleus (LC)-norepinephrine and dorsal raphe (DR)-serotonin systems mediate certain aspects of the stress response and have been implicated in depression. Microarray technology was used to identify gene expression differences in the LC and DR between WKY vs Sprague-Dawley (SD) rats that might account for the WKY phenotype. RNA was isolated from microdissected LC and DR, amplified, and hybridized to microarrays (1 array/subject, n = 4/group). Significance of microarray (SAM) analysis revealed increased expression of 66 genes in the LC and 19 genes in the DR and decreased expression of 33 genes in the DR of WKY rats. Hierarchical clustering identified differences in gene expression profiles of WKY vs SD rats that generally concurred with SAM. Notably, genes that encoded for enzymes involved in norepinephrine turnover, amino-acid receptors, and certain G-protein-coupled receptors were elevated in the LC of WKY rats. The DR of WKY rats showed decreased expression of genes encoding several potassium channels and neurofilament genes. The chromosomal locations of 15 genes that were differentially expressed in WKY rats were near loci identified as contributing to depressive-like behaviors in the rat. The specific genes revealed by the present analysis as being differentially expressed in WKY rats may contribute to their unique behavioral profile and suggest targets that confer susceptibility to stress-related psychiatric disorders.

Homeostatic regulation of sleep in a genetic model of depression in the mouse: effects of muscarinic and 5-HT1A receptor activation

In depressed patients, sleep undergoes marked alterations, especially sleep onset insomnia, sleep fragmentation, and disturbances of the Rapid Eye Movement (REM) sleep. Abnormalities of rest-activity rhythms and of hypothalamic-pituitary-adrenocortical function have also been described in these patients. In the present study, we examined the presence of such abnormalities in a recently developed line of mice (Helpless mice-H) that exhibit depression-like behaviors in validated tests, compared to the nonhelpless (NH) line derived from the same colony. Experiments were essentially carried out in females for which previous studies showed marked differences between H and NH lines. Compared to NH mice, the H line exhibited (i) lower basal locomotor activity, (ii) sleep fragmentation, shift towards lighter sleep stages, and facilitation of REM sleep reflected by increased amounts and decreased latency, (iii) larger response to the REM sleep promoting effect of muscarinic receptor stimulation (by arecoline). In contrast, H and NH mice were equally responsive to the REM sleep inhibitory effect of 5-HT1A receptor stimulation (by 8-OH-DPAT). In addition, a deficiency in delta power enhancement after sleep deprivation was observed in the H group, and acute immobilization stress in this group failed to elicit a REM sleep rebound and was associated with a long-lasting raise in serum corticosterone levels. These results further validate H mice as a depression model and suggest they might be of particular interest for investigating the neurobiological mechanisms and possibly genetic substrates underlying sleep alterations associated with depression.

Animal models of depression in drug discovery: a historical perspective

Over the course of the last 50 years many models of major depressive disorder have been developed on the basis of theoretical aspects of this disorder. These models and procedures have been crucial in the discovery and development of clinically-effective drugs. Notwithstanding, there is presently great concern about the discrepancy between positive outcomes of new candidate drugs in animal models and apparent lack of efficacy in humans i.e., the predictive validity of animal models. Some reasons for this concern lie in the over-reliance in the face value of behavioural models, design of clinical trials, placebo responses, genetic variations in response to drugs, species differences in bioavailability and toxicology, and not least, disinterest of pharmaceutical sponsors to continue developing certain drugs. Present model development is focusing on endophenotypic aspects of behaviours rather than trying to model whole syndromes. This essay traces the origins and theoretical bases of our animal models of depression or depressed-like behaviours in humans and indicates how they have evolved from behavioural assays used to measure the potency and efficacy of potential candidate drugs to tools by which endophenotypes of depression may be identified and verified pharmacologically. A cautionary note is included though to indicate that the true predictive validity of our models will not be fully assessed until we can determine the attrition rate of molecules discovered from new drug targets translating into clinically-effective drugs.

Contribution of 5-HT2 receptor subtypes to sleep-wakefulness and respiratory control, and functional adaptations in knock-out mice lacking 5-HT2A receptors

Serotonin (5-hydroxytryptamine; 5-HT) plays key roles in sleep-wakefulness regulation. Evidence indicates that 5-HT2 receptors are involved mainly in non-rapid eye movement sleep (NREMS) regulation and respiratory control. Here, we investigated the relative contribution of 5-HT(2A), 5-HT(2B), and 5-HT(2C) receptor subtypes to NREMS and breathing during sleep, using 5-HT2 subtype-selective ligands in wild-type (5-HT(2A)+/+) and knock-out (5-HT(2A)-/-) mice that do not express 5-HT(2A) receptors. Acute blockade of 5-HT(2A) receptors induced an increase in NREMS in 5-HT(2A)+/+ mice, but not 5-HT(2A)-/- mutants, which spontaneously expressed less NREMS than wild-type animals. In 5-HT(2A)+/+ mice, 5-HT(2B) receptor blockade produced a reduction of NREMS, whereas receptor activation induced an increase in this sleep stage. These effects were less pronounced in 5-HT(2A)-/- mice, indicating a lower sensitivity of 5-HT(2B) receptors in mutants, with no change in 5-HT(2B) mRNA. Blockade of 5-HT(2C) receptors had no effect on NREMS in both strains. In addition, an increase in EEG power density after sleep deprivation was observed in 5-HT(2A)+/+ mice but not in 5-HT(2A)-/- mice. Whole-body plethysmographic recordings indicated that 5-HT(2A) receptor blockade in 5-HT(2A)+/+ mice reduced NREMS apneas and bradypneas that occurred after sighs. In contrast, in 5-HT(2A)-/- mutants, NREMS apneas were not modified, and bradypnea after sighs were more pronounced. Our results demonstrate that 5-HT exerts a 5-HT(2B)-mediated facilitation of NREMS, and an influence respectively inhibitory on NREMS and facilitatory on sleep apnea generation, via 5-HT(2A) receptors. Moreover, 5-HT(2A) gene knock-out leads to functional compensations yielding adaptive changes opposite to those caused by pharmacological blockade of 5-HT(2A) receptors in 5-HT(2A)+/+ mice.

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.

Changes in sleep-wake cycle after chronic exposure to uranium in rats

Uranium is a heavy metal known to induce toxicity in kidneys. It is also known to enter the central nervous system, thus inducing neurophysiological effects, after exposure to relatively high concentrations. The effect of chronic uranium exposure (40 mg l(-1) in drinking water, for 90 days) on electroencephalographic architecture has been studied on freely moving rats using a telemetry technique. The main effects of uranium on the sleep-wake cycle were an increase in rapid eye movement sleep (REM-sleep) and theta band power during the light period, as early as Day 30 after exposure commenced. The most probable explanation for these effects is that uranium directly affects the brain. This increase in REM-sleep was previously described in human depression or models of chronically stressed rats and it may be assimilated with some protective or compensatory mechanisms.

Genetics and genomics of depression

Depressive disorders are among the most common psychiatric diseases, with prevalence estimates ranging from 5% to a maximum of 20%. Despite their high prevalence and socioeconomic impact, little is known about their etiology. Heritability estimates demonstrate up to a 50% genetic component based on family aggregation and contrasting monozygotic and dizygotic twin studies. The low relative risk to siblings ( lambda sib <1.5) makes the search for their genetic determinants very tedious. Gene-environment interaction has been recognized for a long time in the pathophysiology of depression, and its best biological substratum at present is represented by the serotonin transporter (5-HTT) gene, where several copies of its short allele culminate in depression and suicide in response to lifelong stress events. Many total genome scans have been performed with variable results, the most authoritative being the one of Utah pedigrees with a strong family history of major depression. It identified a locus on chromosome 12 encompassing a gene cluster and sex-specific predisposition. Nevertheless, recent genome scan meta-analysis yielded somewhat disappointing conclusions with a relatively low significance for quantitative trait loci on chromosomes 9, 10, 14, and 18. Studies on animal models have contributed to the chromosomal mapping of many behavioral traits, including anxiety, the stress response, and depression. Although F2 crosses constitute a classical approach, novel models of recombinant inbred strain and recombinant congenic strain animals allow for a rapid initial localization of any traits. This type of analysis has led us to uncover significant loci for the stress response and anxiety in rats and mice.

Role of anxiety levels in memory performance of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) show behavioural differences when compared to their strain-matched controls. These differences include decreased anxiety-like behaviour in SHR, while both improved performance and behavioural deficits have been reported in learning/memory studies. Considering that alterations in anxiety levels during the training session can modify retention performance in animal models of memory, the aim of the present study was to investigate the role of anxiety levels in the performance of SHR rats in the plus-maze discriminative avoidance task (PM-DAT), in which memory and anxiety are evaluated simultaneously. Adult (5-month-old) and young (45-day-old) SHR and normotensive Wistar rats (NWR) were treated with chlordiazepoxide (CDZ) or saline. Thirty minutes later, rats were submitted to the PM-DAT training session. After 24 h, the test session was performed. The results showed that: (1) adult SHR showed lower anxiety levels compared to adult NWR; (2) adult SHR and NWR, as well as young NWR, showed significant retention of the task, while young SHR showed impaired performance; (3) 5.0 mg/kg CDZ decreased anxiety levels in adult NWR and young and adult SHR; (4) 5.0 mg/kg CDZ impaired retention in adult SHR and NWR and increased retention in young SHR. Our data suggest an important role of anxiety levels in the performance of SHR in a plus-maze discriminative avoidance task.

Stereological analysis of the hypothalamic hypocretin/orexin neurons in an animal model of depression

Affective disorders often occur in combination with disrupted sleep-wake cycles and abnormal fluctuations in hypothalamic neurotransmitters. Hypocretin (orexin) is a hypothalamic neuropeptide linked to narcolepsy, a sleep-related disorder characterized by profound disturbances in the normal sleeping pattern and variable degrees of depression. Wistar-Kyoto (WKY) rats exhibit depressive characteristics and patterns of sleep disruption similar to that observed in depressed human patients. In this study we sought to determine whether the total number or the size of hypothalamic hypocretin neurons in WKY rats differ from their control, Wistar (WIS) rats. Immunocytochemical and stereological methods were applied to quantify hypocretin-1 containing neurons in the hypothalamus. The study revealed 18% fewer hypocretin-1 positive neurons as well as a 15% decrease in average neuronal soma size of hypocretin-1 producing cells in the hypothalamus of WKY rats compared to WIS rats. These findings support the view that reduced number or size of hypothalamic hypocretinergic neurons may underlie the disrupted sleep pattern associated with depressive characteristics in WKY rats.

Chronic mild stress affects sucrose intake and sleep in rats

Depression in humans is associated with sleep abnormalities of three types: altered rapid eye movement (REM) sleep, fragmented sleep, and reduced delta sleep. In an animal model of depression, chronic exposure to mild stressors (CMS, e.g. periods of soiled cage, reversed light/dark cycle, grouped housing, food and/or water deprivation) causes behavioral and hormonal changes which, in humans, often are associated with depression. In the CMS model, a reduced sucrose intake has been defined as one of the core symptoms of depression, anhedonia, although this finding is not consistent among various laboratories. In the present study, we investigated if the CMS procedure, in our laboratory, would cause decreased sucrose intake and, also, give sleep changes similar to what is found in depressed patients. Exposure to CMS decreased sucrose intake in our rats. The largest effect was obtained after 2 weeks of the stress protocol. CMS rats spent more time in REM sleep and showed more fragmented sleep compared to their baseline recording, while there were no changes in the control rats. Increased sleep fragmentation in CMS rats was particularly evident by increased number of arousals, and increased REM sleep and slow-wave-sleep-1 (SWS-1) episodes. The duration of sleep stage episodes was decreased. The amount of slow-wave-sleep-2 (SWS-2) was not decreased, however SWS-2 in percent of total SWS was reduced. Correlation analysis showed that animals that had less consumption of sucrose spent more time in REM sleep and had increased number of REM sleep episodes. In this study, CMS appears to be a model of depression.

Perspectives for the development of animal models of bipolar disorder

Bipolar disorder (BD) has been a particularly challenging illness for the development of adequate animal models for neurobiological studies. These difficulties are largely related to the peculiar clinical characteristics of this illness, with an intriguing alternation of mania, depression, euthymia, and mixed states. The etiology and brain mechanisms involved in this several mental illness remain unknown. Preclinical studies with animal models of mania or depression have been developed to evaluate the potential efficacy of new psychotropic drugs and generate information concerning the biochemical effects of these drugs on specific targets. These models try to mimic the behavioral components of mania and depression in human subjects and examine the pharmacological responses and mechanisms of action of potentially new therapeutic agents. The main limitation is that there is currently no model that would mimic mood cyclicity, which is a hallmark feature of BD. Thus, these models do not represent valid paradigms for the study of this illness, because they do not address key questions regarding cyclicity. In this review, we propose that new genetics approaches involving potential animal models of BD are a promising new area for further development.

Similar genetic mechanisms may underlie sleep-wake states in neonatal and adult rats

Genetic differences in the characteristics of sleep-wake states in adult animals offer a potential window for examining how the neonatal and adult behavioural states are related to one another. Our recent finding that adult Wistar-Kyoto (WKY) rats show pronounced genetic differences in sleep-wake patterns relative to the Wistar (WIS) control strain led us to investigate the relationship between these behavioural states in neonates and adults in a longitudinal study in these two strains of rats. Similar pronounced differences in the sleep-wake states were observed between WKY and WIS rats in neonatal and in adult animals. At both ages, WKY rats spent more time in activesleep (AS) and rapid eye movement sleep (REMS) and less time in quiet sleep (QS) and non-REM sleep (NREMS) than WIS rats, and the sleep-wake states were more fragmented in neonatal and adult WKY rats. While it is not known how neonatal AS and QS are physiologically related to adult REMS and NREMS, respectively, the finding of similar differences in the amounts of sleep-wake states in neonatal and adult WKY and WIS rats argues strongly that at some level they are controlled by similar genetic as well as cellular/physiological mechanisms.

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.

Orexin A immunoreactivity and preproorexin mRNA in the brain of Zucker and WKY rats

The primary role of the orexins was originally believed to be appetite regulation, but is now believed to be the regulation of sleep, arousal and locomotor activity. Orexin A immunoreactivity (orexin A-IR) and prepro-orexin mRNA were measured in the CNS of obese and lean Zucker rats. There were no differences in orexin A-IR or prepro-orexin mRNA levels between obese and lean Zucker rats. The orexins are therefore unlikely to be important in this model of obesity. Levels of orexin A-IR and prepro-orexin mRNA were measured in the CNS of Wistar-Kyoto (WKY) rats, which are hypoactive and have abnormal sleep architecture. Compared to Wistar rats, WKY rats had significantly lower orexin A-IR (with differences of up to 100% in some brain regions) and prepro-orexin mRNA levels. These observations suggest that the sleep and activity phenotype of the WKY strain may be related to orexin deficiency and that this strain may be a useful model of partial orexin deficiency.