Lateral hypothalamic kindling induces manic-like behavior in rats: a novel animal model

Pushing the Frontiers: Optogenetics for Illuminating the Neural Pathophysiology of Bipolar Disorder

Bipolar disorder (BD), a disabling mental disorder, is featured by the oscillation between episodes of depression and mania, along with disturbance in the biological rhythms. It is on an urgent demand to identify the intricate mechanisms of BD pathophysiology. Based on the continuous progression of neural science techniques, the dysfunction of circuits in the central nervous system was currently thought to be tightly associated with BD development. Yet, challenge exists since it depends on techniques that can manipulate spatiotemporal dynamics of neuron activity. Notably, the emergence of optogenetics has empowered researchers with precise timing and local manipulation, providing a possible approach for deciphering the pathological underpinnings of mental disorders. Although the application of optogenetics in BD research remains preliminary due to the scarcity of valid animal models, this technique will advance the psychiatric research at neural circuit level. In this review, we summarized the crucial aberrant brain activity and function pertaining to emotion and rhythm abnormities, thereby elucidating the underlying neural substrates of BD, and highlighted the importance of optogenetics in the pursuit of BD research.

Dysfunction of ventral tegmental area GABA neurons causes mania-like behavior

The ventral tegmental area (VTA), an important source of dopamine, regulates goal- and reward-directed and social behaviors, wakefulness, and sleep. Hyperactivation of dopamine neurons generates behavioral pathologies. But any roles of non-dopamine VTA neurons in psychiatric illness have been little explored. Lesioning or chemogenetically inhibiting VTA GABAergic (VTAVgat) neurons generated persistent wakefulness with mania-like qualities: locomotor activity was increased; sensitivity to D-amphetamine was heightened; immobility times decreased on the tail suspension and forced swim tests; and sucrose preference increased. Furthermore, after sleep deprivation, mice with lesioned VTAVgat neurons did not catch up on lost sleep, even though they were starting from a sleep-deprived baseline, suggesting that sleep homeostasis was bypassed. The mania-like behaviors, including the sleep loss, were reversed by valproate, and re-emerged when treatment was stopped. Lithium salts and lamotrigine, however, had no effect. Low doses of diazepam partially reduced the hyperlocomotion and fully recovered the immobility time during tail suspension. The mania like-behaviors mostly depended on dopamine, because giving D1/D2/D3 receptor antagonists reduced these behaviors, but also partially on VTAVgat projections to the lateral hypothalamus (LH). Optically or chemogenetically inhibiting VTAVgat terminals in the LH elevated locomotion and decreased immobility time during the tail suspension and forced swimming tests. VTAVgat neurons help set an animal's (and perhaps human's) mental and physical activity levels. Inputs inhibiting VTAVgat neurons intensify wakefulness (increased activity, enhanced alertness and motivation), qualities useful for acute survival. In the extreme, however, decreased or failed inhibition from VTAVgat neurons produces mania-like qualities (hyperactivity, hedonia, decreased sleep).

Circadian Rhythms of the Hypothalamus: From Function to Physiology

The nearly ubiquitous expression of endogenous 24 h oscillations known as circadian rhythms regulate the timing of physiological functions in the body. These intrinsic rhythms are sensitive to external cues, known as zeitgebers, which entrain the internal biological processes to the daily environmental changes in light, temperature, and food availability. Light directly entrains the master clock, the suprachiasmatic nucleus (SCN) which lies in the hypothalamus of the brain and is responsible for synchronizing internal rhythms. However, recent evidence underscores the importance of other hypothalamic nuclei in regulating several essential rhythmic biological functions. These extra-SCN hypothalamic nuclei also express circadian rhythms, suggesting distinct regions that oscillate either semi-autonomously or independent of SCN innervation. Concurrently, the extra-SCN hypothalamic nuclei are also sensitized to fluctuations in nutrient and hormonal signals. Thus, food intake acts as another powerful entrainer for the hypothalamic oscillators' mediation of energy homeostasis. Ablation studies and genetic mouse models with perturbed extra-SCN hypothalamic nuclei function reveal their critical downstream involvement in an array of functions including metabolism, thermogenesis, food consumption, thirst, mood and sleep. Large epidemiological studies of individuals whose internal circadian cycle is chronically disrupted reveal that disruption of our internal clock is associated with an increased risk of obesity and several neurological diseases and disorders. In this review, we discuss the profound role of the extra-SCN hypothalamic nuclei in rhythmically regulating and coordinating body wide functions.

Prolonged functional cerebral asymmetry as a consequence of dysfunctional parvocellular paraventricular hypothalamic nucleus signaling: An integrative model for the pathophysiology of bipolar disorder

Approximately 45 million people worldwide are diagnosed with bipolar disorder (BD). While there are many known risk factors and models of the pathologic processes influencing BD, the exact neurologic underpinnings of BD are unknown. We attempt to integrate the existing literature and create a unifying hypothesis regarding the pathophysiology of BD with the hope that a concrete model may potentially facilitate more specific diagnosis, prevention, and treatment of BD in the future. We hypothesize that dysfunctional signaling from the parvocellular neurons of the paraventricular hypothalamic nucleus (PVN) results in the clinical presentation of BD. Functional damage to this nucleus and its signaling pathways may be mediated by myriad factors (e.g. immune dysregulation and auto-immune processes, polygenetic variation, dysfunctional interhemispheric connections, and impaired or overactivated hypothalamic axes) which could help explain the wide variety of clinical presentations along the BD spectrum. The neurons of the PVN regulate ultradian rhythms, which are observed in cyclic variations in healthy individuals, and mediate changes in functional hemispheric lateralization. Theoretically, dysfunctional PVN signaling results in prolonged functional hemispheric dominance. In this model, prolonged right hemispheric dominance leads to depressive symptoms, whereas left hemispheric dominance correlated to the clinical picture of mania. Subsequently, physiologic processes that increase signaling through the PVN (hypothalamic-pituitaryadrenal axis, hypothalamic- pituitary-gonadal axis, and hypothalamic-pituitary-thyroid axis activity, suprachiasmatic nucleus pathways) as well as, neuro-endocrine induced excito-toxicity, auto-immune and inflammatory flairs may induce mood episodes in susceptible individuals. Potentially, ultradian rhythms slowing with age, in combination with changes in hypothalamic axes and maturation of neural circuitry, accounts for BD clinically presenting more frequently in young adulthood than later in life.

Ablation of Ventral Midbrain/Pons GABA Neurons Induces Mania-like Behaviors with Altered Sleep Homeostasis and Dopamine D2R-mediated Sleep Reduction

Individuals with the neuropsychiatric disorder mania exhibit hyperactivity, elevated mood, and a decreased need for sleep. The brain areas and neuronal populations involved in mania-like behaviors, however, have not been elucidated. In this study, we found that ablating the ventral medial midbrain/pons (VMP) GABAergic neurons induced mania-like behaviors in mice, including hyperactivity, anti-depressive behaviors, reduced anxiety, increased risk-taking behaviors, distractibility, and an extremely shortened sleep time. Strikingly, these mice also showed no rebound sleep after sleep deprivation, suggesting abnormal sleep homeostatic regulation. Dopamine D₂ receptor deficiency largely abolished the sleep reduction induced by ablating the VMP GABAergic neurons without affecting the hyperactivity and anti-depressive behaviors. Our data demonstrate that VMP GABAergic neurons are involved in the expression of mania-like behaviors, which can be segregated to the short-sleep and other phenotypes on the basis of the dopamine D₂ receptors.

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Hyperactivity and anti-depressive behaviors are induced by loss of VMP GABA neurons

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Homeostatic sleep rebound is lost together with largely shorten daily sleep

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Dopamine D₂ receptors mediate the daytime sleep loss

Nitrated meat products are associated with mania in humans and altered behavior and brain gene expression in rats

Mania is a serious neuropsychiatric condition associated with significant morbidity and mortality. Previous studies have suggested that environmental exposures can contribute to mania pathogenesis. We measured dietary exposures in a cohort of individuals with mania and other psychiatric disorders as well as in control individuals without a psychiatric disorder. We found that a history of eating nitrated dry cured meat but not other meat or fish products was strongly and independently associated with current mania (adjusted odds ratio 3.49, 95% confidence interval (CI) 2.24-5.45, p < 8.97 × 10^-8). Lower odds of association were found between eating nitrated dry cured meat and other psychiatric disorders. We further found that the feeding of meat preparations with added nitrate to rats resulted in hyperactivity reminiscent of human mania, alterations in brain pathways that have been implicated in human bipolar disorder, and changes in intestinal microbiota. These findings may lead to new methods for preventing mania and for developing novel therapeutic interventions.

Sex difference in the progression of manic symptoms during acute hospitalization: A prospective pilot study

OBJECTIVES

Acute mania is a serious medical condition that impacts men and women equally. Longtime presentation of manic symptoms is sex-dependent; however, little is known about acute symptoms of mania. The objective of this study is to track and compare acute manic symptoms for sex differences during inpatient hospitalization.

METHODS

All patients with bipolar mania admitted to a large university hospital between January and October 2017 were invited to participate in this longitudinal naturalistic follow-up study. Manic (YMRS), depressive (MADRS), and psychotic (PAS) symptoms were tracked daily from admission to discharge.

RESULTS

The total YMRS scores decreased significantly overtime (p < .0001) in both male (n = 34) and female (n = 23) patients (p = .7). However, male patients scored significantly higher in sexual interest (p = .01), disruptive and aggressive behavior (p = .01), and appearance (p < .001) while females had better insight into their illness (p = .01). Males and females received similar doses of lithium (p = .1), but males received significantly higher doses of valproic acid (VPA) in comparison with females (p = .003). However, plasma lithium and VPA concentrations at discharge were not significantly different between sexes.

CONCLUSION

Our results show sex differences in the progression of certain domains of manic symptoms in a cohort of 23 female and 34 male patients admitted to a large academic center in Turkey. Males, in this sample, exhibited more sexual interest, disruptive and aggressive behaviors, better grooming, and less insight compared to females. While these results are concordant with our preclinical findings and with anecdotal clinical observations, replication in larger samples is needed.

Sexual behavior and testis morphology in the BACHD rat model

BACKGROUND

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene, which results in brain neurodegeneration and peripheral pathology affecting different organs including testis. Patients with HD suffer from motor and cognitive impairment, and multiple psychiatric symptoms. Among behavioral abnormalities in HD, sexual disturbances have often been reported, but scarcely investigated in animal models. The BACHD rat model of HD carries the human full-length mutated HTT (mHTT) genomic sequence with 97 CAG-CAA repeats and displays HD-like alterations at neuropathological and behavioral level.

OBJECTIVE

This study aims to phenotype the BACHD rats' sexual behavior and performance as well as testis morphology because alterations in these aspects have been associated to HD.

METHODS

Two rat cohorts at the age of 3 and 7 months were subjected to mating tests to assess different parameters of sexual behavior. Histological analyses for testis morphology were performed in different rat cohorts at 1.5, 7 and 12 months of age whereas immunohistochemical analyses were carried out at 7 and 12 months of age to visualize the presence of mHTT in testicular tissue. Furthermore, western blot analyses were used to assess HTT and mHTT expression levels in striatum and testis at three months of age.

RESULTS

At 3 months, BACHD rats showed a decreased time exploring the female anogenital area (AGA), decreased latency to mount, increased number of intromissions and ejaculations and enhanced hit rate. At 7 months, all sexual parameters were comparable between genotypes with the exception that BACHD rats explored the AGA less than wild type rats. Testis analyses did not reveal any morphological alteration at any of the examined ages, but showed presence of mHTT limited to Sertoli cells in transgenic rats at both 7 and 12 months. BACHD rat HTT and mHTT expression levels in testis were lower than striatum at 3 months of age.

CONCLUSIONS

The testis phenotype in the BACHD rat model does not mimic the changes observed in human HD testis. The altered sexual behavior in BACHD rats at three months of age could be to a certain extent representative of and share common underlying pathways with some of the sexual disturbances in HD patients. Further investigating the biological causes of the sexual phenotype in BACHD rats may therefore contribute to clarifying the mechanisms at the base of sexual behavior changes in HD.

No differences in manic-like behaviors between voluntary ethanol and water drinking in Lateral Hypothalamic Kindled (LHK) alcohol preferring rats

This study investigates phenotypic differences of manic-like behaviors during voluntary ethanol (vs. water) intake in alcohol preferring (P) rats, using the lateral hypothalamus kindled (LHK) rat mania model. Alcohol preferring (P) rats exhibited classic manic-like behaviors during kindling and the post-kindling intervals in all coded behaviors (p < 0.0001), lending further validity to the lateral hypothalamus kindled rat as a useful model to study mania. However, there was no significant phenotypic difference between alcohol and water drinking alcohol preferring (P) rats.

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

The lateral hypothalamus to lateral habenula projection, but not the ventral pallidum to lateral habenula projection, regulates voluntary ethanol consumption

The lateral habenula (LHb) is an epithalamic brain region implicated in aversive processing via negative modulation of midbrain dopamine (DA) and serotonin (5-HT) systems. Given the role of the LHb in inhibiting DA and 5-HT systems, it is thought to be involved in various psychiatric pathologies, including drug addiction. In support, it has been shown that LHb plays a critical role in cocaine- and ethanol-related behaviors, most likely by mediating drug-induced aversive conditioning. In our previous work, we showed that LHb lesions increased voluntary ethanol consumption and operant ethanol self-administration and blocked yohimbine-induced reinstatement of ethanol self-administration. LHb lesions also attenuated ethanol-induced conditioned taste aversion suggesting that a mechanism for the increased intake of ethanol may be reduced aversion learning. However, whether afferents to the LHb are required for mediating effects of the LHb on these behaviors remained to be investigated. Our present results show that lesioning the fiber bundle carrying afferent inputs to the LHb, the stria medullaris (SM), increases voluntary ethanol consumption, suggesting that afferent structures projecting to the LHb are important for mediating ethanol-directed behaviors. We then chose two afferent structures as the focus of our investigation. We specifically studied the role of the inputs from the lateral hypothalamus (LH) and ventral pallidum (VP) to the LHb in ethanol-directed behaviors. Our results show that the LH-LHb projection is necessary for regulating voluntary ethanol consumption. These results are an important first step towards understanding the functional role of afferents to LHb with regard to ethanol consumption.

Corticosterone oscillations during mania induction in the lateral hypothalamic kindled rat-Experimental observations and mathematical modeling

Changes in the hypothalamic-pituitary-adrenal (HPA) axis activity constitute a key component of bipolar mania, but the extent and nature of these alterations are not fully understood. We use here the lateral hypothalamic-kindled (LHK) rat model to deliberately induce an acute manic-like episode and measure serum corticosterone concentrations to assess changes in HPA axis activity. A mathematical model is developed to succinctly describe the entwined biochemical transformations that underlay the HPA axis and emulate by numerical simulations the considerable increase in serum corticosterone concentration induced by LHK. Synergistic combination of the LHK rat model and dynamical systems theory allows us to quantitatively characterize changes in HPA axis activity under controlled induction of acute manic-like states and provides a framework to study in silico how the dynamic integration of neurochemical transformations underlying the HPA axis is disrupted in these states.

Predictive Validity of Some Common Animal Models of Bipolar Disorder Using Lithium and Lamotrigine Therapy: An Attempt towards a Battery-Based Approach for the Evaluation of Mood Stabilizers

OBJECTIVE

To determine the predictive validity of some of the commonly employed models of mania and depression using standard drugs i.e. lithium (70 mg/kg) and lamotrigine (5 mg/kg) in male Wistar rats.

METHODS

The depression facet of bipolar disorder was evaluated using forced swim test, tail suspension test, and chronic mild stress test. The models used to evaluate the mania facet of bipolar disorder were isolation-induced aggression test, saccharine preference test, and morphine-sensitized hyperlocomotion test.

RESULTS

The immobility time was significantly (p<0.05) reduced by lamotrigine in the tail suspension test and the forced swim test, while lithium caused significant (p<0.05) reduction only in the tail suspension test. Rats exposed to chronic mild stress showed the maximal increment of 1% sucrose consumption at the 3rd week of treatment in both the lithium (p<0.001) and lamotrigine (p<0.01) groups. In the isolation-induced aggression test, the aggressive behaviour of rats was significantly reduced by both lithium [approach (p<0.001), attack (p<0.01), and bite (p<0.01)] and lamotrigine [approach (p<0.001), and attack (p<0.05)]. Neither of the drugs were effective in the saccharine preference test. Only lithium was able to significantly (p<0.05) reduce the crossing parameter in morphine-sensitized rats.

CONCLUSION

Our study identifies the chronic mild stress test and isolation-induced aggression test of having the highest predictive validity in the depression and mania facets of bipolar disorder, respectively, and should be a part of a battery of tests used to evaluate novel mood stabilizers.

Chronic treatment with mood stabilizer lithium inhibits amphetamine-induced risk-taking manic-like behaviors

A lack of behavioral tests and animal models for manic-depressive bipolar disorder is recognized as an important factor limiting development of novel pharmaceutical treatments for the disorder. Repeated amphetamine-induced hyperactivity is a commonly used animal model for mania. However, hyperactivity represents only one facet of mania and is also seen in other disorders. Increased engagement in risk taking behavior is frequently observed in the manic phase of bipolar disorder. In the present study, we analyzed the effect of the most commonly used mood stabilizer lithium on repeated amphetamine treatment-induced risk-taking behaviors in rats using elevated plus maze and wire-beam bridge tests. We found that repeated amphetamine treatment not only increased locomotor activity, but also increased risk taking behaviors in rats, and further that chronic lithium treatment inhibited the amphetamine-increased risk taking behavior. Our studies suggest that these tests may be useful tools to analyze the pharmacological validity of new and improved anti-manic drugs in animals.

Sex differences in mania phenotype and ethanol consumption in the lateral hypothalamic kindled rat model

Sex differences have been observed in mania phenotypes in humans. However the mechanisms underlying this difference are poorly understood. Activating the lateral hypothalamus is implicated in manic-like behaviors in rodents. Using newly established lateral hypothalamus kindled (LHK) rat mania model, we investigated sex differences of manic-like behaviors and its correlation with voluntary ethanol intake. We stimulated the lateral hypothalamus bilaterally in the male and female Wistar rats over five consecutive days. We recorded and quantified kindling-induced behaviors for each individual animal. We also assessed ethanol consumption using a two-bottle choice ethanol drinking as well as circadian locomotor activity counts daily throughout the experiment. We found notable sex differences in several aspects of manic-like behaviors during kindling. Males exhibited a significantly increased locomotor activity during the light phase, and reduced rest interval. On the other hand, females displayed significantly higher ethanol consumption and more frequent rearing behavior. However, no sex differences were present in the duration of sexual, feeding or grooming behaviors or in dark-phase activity counts. The excessive alcohol intake in LHK female rats is reminiscent of clinically reported sex differences in bipolar patients while the other phenotypic sex differences such as rearing and locomotor activity are less clearly described in clinical studies. Overall, our results lend further evidence for the validity of the LHK rat as a useful model to study brain region-specific molecular changes during mania and its correlation with alcohol use disorders.