Congenital helpless rats as a genetic model for cortex metabolism in depression

Sex-Specific Effects of Early Life Stress on Brain Mitochondrial Function, Monoamine Levels and Neuroinflammation

Sex differences have been reported in the susceptibility to early life stress and its neurobiological correlates in humans and experimental animals. However, most of the current research with animal models of early stress has been performed mainly in males. In the present study, prolonged maternal separation (MS) paradigm was applied as an animal model to resemble the effects of adverse early experiences in male and female rats. Regional brain mitochondrial function, monoaminergic activity, and neuroinflammation were evaluated as adults. Mitochondrial energy metabolism was greatly decreased in MS females as compared with MS males in the prefrontal cortex, dorsal hippocampus, and the nucleus accumbens shell. In addition, MS males had lower serotonin levels and increased serotonin turnover in the prefrontal cortex and the hippocampus. However, MS females showed increased dopamine turnover in the prefrontal cortex and increased norepinephrine turnover in the striatum, but decreased dopamine turnover in the hippocampus. Sex differences were also found for pro-inflammatory cytokine levels, with increased levels of TNF-α and IL-6 in the prefrontal cortex and hippocampus of MS males, and increased IL-6 levels in the striatum of MS females. These results evidence the complex sex- and brain region-specific long-term consequences of early life stress.

Antagonism at the NR2B subunit of NMDA receptors induces increased connectivity of the prefrontal and subcortical regions regulating reward behavior

RATIONALE

Evidence indicates that ketamine's rapid antidepressant efficacy likely results from its antagonism of NR2B-subunit-containing NMDA receptors (NMDAR). Since ketamine equally blocks NR2A- and NR2B-containing NMDAR, and has affinity to other receptors, NR2B-selective drugs might have improved therapeutic efficiency and side effect profile.

OBJECTIVES

We aimed to compare the effects of (S)-ketamine and two different types of NR2B-selective antagonists on functional brain networks in rats, in order to find common circuits, where their effects intersect, and that might explain their antidepressant action.

METHODS

The experimental design comprised four parallel groups of rats (N = 37), each receiving (S)-Ketamine, CP-101,606, Ro 25-6981 or saline. After compound injection, we acquired resting-state functional magnetic resonance imaging time series. We used graph theoretical approach to calculate brain network properties.

RESULTS

Ketamine and CP-101,606 diminished the global clustering coefficient and small-worldness index. At the nodal level, all compounds induced increased connectivity of the regions mediating reward and cognitive aspects of emotional processing, such as ventromedial prefrontal cortex, septal nuclei, and nucleus accumbens. The dorsal hippocampus and regions involved in sensory processing and aversion, such as superior and inferior colliculi, exhibited an opposite effect.

CONCLUSIONS

The effects common to ketamine and NR2B-selective compounds were localized to the same brain regions as those reported in depression, but in the opposite direction. The upregulation of the reward circuitry might partially underlie the antidepressant and anti-anhedonic effects of the antagonists and could potentially serve as a translational imaging phenotype for testing putative antidepressants, especially those targeting the NR2B receptor subtype.

Prefrontal-limbic Functional Connectivity during Acquisition and Extinction of Conditioned Fear

This study is a new analysis to obtain novel metabolic data on the functional connectivity of prefrontal-limbic regions in Pavlovian fear acquisition and extinction of tone-footshock conditioning. Mice were analyzed with the fluorodeoxyglucose (FDG) autoradiographic method to metabolically map regional brain activity. New FDG data were sampled from the nuclei of the habenula and other regions implicated in aversive conditioning, such as infralimbic cortex, amygdala and periaqueductal gray regions. The activity patterns among these regions were inter-correlated during acquisition, extinction or pseudorandom training to develop a functional connectivity model. Two subdivisions of the habenular complex showed increased activity after acquisition relative to extinction, with the pseudorandom group intermediate between the other two groups. Significant acquisition activation effects were also found in centromedial amygdala, dorsomedial and ventrolateral periaqueductal gray. FDG uptake increases during extinction were found only in dorsal and ventral infralimbic cortex. The overall pattern of activity correlations between these regions revealed extensive but differential functional connectivity during acquisition and extinction training, with less functional connectivity found after pseudorandom training. Interestingly, habenula nuclei showed a distinct pattern of inter-correlations with amygdala nuclei during extinction. The functional connectivity model revealed changing interactions among infralimbic cortex, amygdala, habenula and periaqueductal gray regions through the stages of Pavlovian fear acquisition and extinction. This study provided new data on the contributions of the habenula to fear conditioning, and revealed previously unreported infralimbic-amygdala-habenula-periaqueductal gray interactions implicated in acquisition and extinction of conditioned fear.

Role of brain-derived neurotrophic factor in the molecular neurobiology of major depressive disorder

Major depressive disorder (MDD) is one of the most common neuropsychiatric disorders, which affects up to 20% of people in their lifetime in the United States. The exact neurobiological mechanisms of MDD remain elusive, and the diagnostics are still uncertain. Basic and clinical research from recent years demonstrated that the etiology of MDD might be associated with genetic changes of neurotrophins, particularly brain-derived neurotrophic factor (BDNF). BDNF plays a key role in neuronal development and neurogenesis. However, the detailed mechanisms related to depression and antidepressant responses are not fully understood. This review summarizes the current knowledge of the causal relationship between BDNF and MDD, and describes the important role of BDNF in the progress of depression in animal models and patients with depressive disorders.

Memory deficits with intact cognitive control in the methylazoxymethanol acetate (MAM) exposure model of neurodevelopmental insult

Cognitive impairments are amongst the most debilitating deficits of schizophrenia and the best predictor of functional outcome. Schizophrenia is hypothesized to have a neurodevelopmental origin, making animal models of neurodevelopmental insult important for testing predictions that early insults will impair cognitive function. Rats exposed to methylazoxymethanol acetate (MAM) at gestational day 17 display morphological, physiological and behavioral abnormalities relevant to schizophrenia. Here we investigate the cognitive abilities of adult MAM rats. We examined brain activity in MAM rats by histochemically assessing cytochrome oxidase enzyme activity, a metabolic marker of neuronal activity. To assess cognition, we used a hippocampus-dependent two-frame active place avoidance paradigm to examine learning and spatial memory, as well as cognitive control and flexibility using the same environment and evaluating the same set of behaviors. We confirmed that adult MAM rats have altered hippocampal morphology and brain function, and that they are hyperactive in an open field. The latter likely indicates MAM rats have a sensorimotor gating deficit that is common to many animal models used for schizophrenia research. On first inspection, cognitive control seems impaired in MAM rats, indicated by more errors during the two-frame active place avoidance task. Because MAM rats are hyperactive throughout place avoidance training, we considered the possibility that the hyperlocomotion may account for the apparent cognitive deficits. These deficits were reduced on the basis of measures of cognitive performance that account for motor activity differences. However, though other aspects of memory are intact, the ability of MAM rats to express trial-to-trial memory is delayed compared to control rats. These findings suggest that spatial learning and cognitive abilities are largely intact, that the most prominent cognitive deficit is specific to acquiring memory in the MAM neurodevelopmental model, and that hyperactivity can confound assessments of cognition in animal models of mental dysfunction.

Brain Systems Underlying Susceptibility to Helplessness and Depression

There has been a relative lack of research into the neurobiological predispositions that confer vulnerability to depression. This article reviews functional brain mappings from a genetic animal model, the congenitally helpless rat, which is predisposed to develop learned helplessness. Neurometabolic findings from this model are integrated with the neuroscientific literature from other animal models of depression as well as depressed humans. Changes in four major brain systems are suggested to underlie susceptibility to helplessness and possibly depression: (a) an unbalanced prefrontal-cingulate cortical system, (b) a dissociated hypothalamic-pituitary-adrenal axis, (c) a dissociated septal-hippocampal system, and (d) a hypoactive brain reward system, as exemplified by a hypermetabolic habenula-interpeduncular nucleus pathway and a hypometabolic ventral tegmental area-striatum pathway. Functional interconnections and causal relationships among these systems are considered and further experiments are suggested, with theoretical attention to how an abnormality in any one system could affect the others.

Treatment-resistant depression: are animal models of depression fit for purpose?

BACKGROUND

Resistance to antidepressant drug treatment remains a major health problem. Animal models of depression are efficient in detecting effective treatments but have done little to increase the reach of antidepressant drugs. This may be because most animal models of depression target the reversal of stress-induced behavioural change, whereas treatment-resistant depression is typically associated with risk factors that predispose to the precipitation of depressive episodes by relatively low levels of stress. Therefore, the search for treatments for resistant depression may require models that incorporate predisposing factors leading to heightened stress responsiveness.

METHOD

Using a diathesis-stress framework, we review developmental, genetic and genomic models against four criteria: (i) increased sensitivity to stress precipitation of a depressive behavioural phenotype, (ii) resistance to chronic treatment with conventional antidepressants, (iii) a good response to novel modes of antidepressant treatment (e.g. ketamine; deep brain stimulation) that are reported to be effective in treatment-resistant depression and (iv) a parallel to a known clinical risk factor.

RESULTS

We identify 18 models that may have some potential. All require further validation. Currently, the most promising are the Wistar-Kyoto (WKY) and congenital learned helplessness (cLH) rat strains, the high anxiety behaviour (HAB) mouse strain and the CB1 receptor knockout and OCT2 null mutant mouse strains.

CONCLUSION

Further development is needed to validate models of antidepressant resistance that are fit for purpose. The criteria used in this review may provide a helpful framework to guide research in this area.

Increased metabolic activity in the septum and habenula during stress is linked to subsequent expression of learned helplessness behavior

Uncontrollable stress can have a profound effect on an organism's ability to respond effectively to future stressful situations. Behavior subsequent to uncontrollable stress can vary greatly between individuals, falling on a spectrum between healthy resilience and maladaptive learned helplessness. It is unclear whether dysfunctional brain activity during uncontrollable stress is associated with vulnerability to learned helplessness; therefore, we measured metabolic activity during uncontrollable stress that correlated with ensuing inability to escape future stressors. We took advantage of small animal positron emission tomography (PET) and 2-deoxy-2[(18)F]fluoro-D-glucose ((18)FDG) to probe in vivo metabolic activity in wild type Sprague Dawley rats during uncontrollable, inescapable, unpredictable foot-shock stress, and subsequently tested the animals response to controllable, escapable, predictable foot-shock stress. When we correlated metabolic activity during the uncontrollable stress with consequent behavioral outcomes, we found that the degree to which animals failed to escape the foot-shock correlated with increased metabolic activity in the lateral septum and habenula. When used a seed region, metabolic activity in the habenula correlated with activity in the lateral septum, hypothalamus, medial thalamus, mammillary nuclei, ventral tegmental area, central gray, interpeduncular nuclei, periaqueductal gray, dorsal raphe, and rostromedial tegmental nucleus, caudal linear raphe, and subiculum transition area. Furthermore, the lateral septum correlated with metabolic activity in the preoptic area, medial thalamus, habenula, interpeduncular nuclei, periaqueductal gray, dorsal raphe, and caudal linear raphe. Together, our data suggest a group of brain regions involved in sensitivity to uncontrollable stress involving the lateral septum and habenula.

Synaptic abnormalities in the infralimbic cortex of a model of congenital depression

Multiple lines of evidence suggest that disturbances in excitatory transmission contribute to depression. Whether these defects involve the number, size, or composition of glutamatergic contacts is unclear. This study used recently introduced procedures for fluorescence deconvolution tomography in a well-studied rat model of congenital depression to characterize excitatory synapses in layer I of infralimbic cortex, a region involved in mood disorders, and of primary somatosensory cortex. Three groups were studied: (1) rats bred for learned helplessness (cLH); (2) rats resistant to learned helplessness (cNLH); and (3) control Sprague Dawley rats. In fields within infralimbic cortex, cLH rats had the same numerical density of synapses, immunolabeled for either the postsynaptic density (PSD) marker PSD95 or the presynaptic protein synaptophysin, as controls. However, PSD95 immunolabeling intensities were substantially lower in cLH rats, as were numerical densities of synapse-sized clusters of the AMPA receptor subunit GluA1. Similar but less pronounced differences (comparable numerical densities but reduced immunolabeling intensity for PSD95) were found in the somatosensory cortex. In contrast, non-helpless rats had 25% more PSDs than either cLH or control rats without any increase in synaptophysin-labeled terminal frequency. Compared with controls, both cLH and cNLH rats had fewer GABAergic contacts. These results indicate that congenital tendencies that increase or decrease depression-like behavior differentially affect excitatory synapses.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

Transient inactivation of the infralimbic cortex induces antidepressant-like effects in the rat

Affective disorders are among the main causes of disability worldwide, yet the underlying pathophysiology remains poorly understood. Recently, landmark neuroimaging studies have shown increased metabolic activity in Brodmann Area 25 (BA25) in depressed patients. Moreover, functional inactivation of this region using deep brain stimulation alleviated depressive symptoms in severely depressed patients. Thus, we examined the effect of a similar manipulation, pharmacological inactivation of the infralimbic cortex, the rodent correlate of BA25, in an animal model of antidepressant activity: the modified rat forced swim test. Transient inactivation of the infralimbic cortex using muscimol reduced immobility, an antidepressant-like effect in the test. Importantly, this activity was not the result of a general increase in locomotor activity. Activation of the infralimbic cortex using bicuculline did not alter behaviour. Finally, we examined the effect of muscimol in animals bred for high anxiety-related behaviour, which also display elevated depression-related behaviour. Transient inactivation of the infralimbic cortex decreased the high inborn depression-like behaviour of these rats. These results show that it is possible to replicate findings from a clinical trial in a rodent model. Further, they support the use of the forced swim test to gain greater understanding of the neurocircuitry involved in depression and antidepressant-action.

Behaviour of a genetic mouse model of depression in the learned helplessness paradigm

RATIONALE

H/Rouen (displaying a helpless phenotype in the tail suspension test) mice exhibiting features of depressive disorders and NH/Rouen (displaying non-helpless phenotype) mice were previously created through behavioural screening and selective breeding. Learned helplessness (LH), in which footshock stress induces a coping deficit, models some aspects of depression in rodents, but so far, fewer LH studies have been performed in mice than in rats.

OBJECTIVES

To study H/Rouen and NH/Rouen in the LH paradigm.

RESULTS

When CD1 mice were submitted to footshock with various training durations and shock intensities, the most suitable parameters to induce a behavioural deficit were 0.3 mA and four training sessions. A significantly longer latency to escape shocks was found in male H/Rouen mice compared to male NH/Rouen mice. On the other hand, once shocked, NH/Rouen mice showed more severe coping deficits than H/Rouen mice. In addition, a sub-chronic treatment with fluoxetine lacked efficacy in NH/Rouen mice, whereas it improved performances in H/Rouen mice. We also found that a shock reminder at day 8, subsequent to inescapable shocks, maintained helplessness for 20 days. Finally, female H/Rouen mice responded to chronic fluoxetine administration after 10 days of treatment, while a 20-day treatment was necessary to improve the behavioural deficit in H/Rouen male mice.

CONCLUSION

H/Rouen and NH/Rouen lines displayed different despair-related behaviour in the LH paradigm. Fluoxetine had beneficial effects after sub-chronic or chronic but not acute treatment of H/Rouen mice, thus providing a pharmacological validation of the protocols.

Mesolimbic effects of the antidepressant fluoxetine in Holtzman rats, a genetic strain with increased vulnerability to stress

This is the first metabolic mapping study of the effects of fluoxetine after learned helplessness training. Antidepressants are the most commonly prescribed medications, but the regions underlying treatment effects in affectively disordered brains are poorly understood. We hypothesized the antidepressant action of fluoxetine would produce adaptations in mesolimbic regions after 2 weeks of treatment. We used Holtzman rats, a genetic strain showing susceptibility to novelty-evoked hyperactivity and stress-evoked helplessness, to map regional brain metabolic effects caused by fluoxetine treatment. Animals underwent learned helplessness, and subsequently immobility time was scored in the forced swim test (FST). On the next day, animals began receiving 2 weeks of fluoxetine (5mg/kg/day) or vehicle and were retested in the FST at the end of drug treatment. Antidepressant behavioral effects of fluoxetine were analyzed using a ratio of immobility during pre- and post-treatment FST sessions. Brains were analyzed for regional metabolic activity using quantitative cytochrome oxidase histochemistry as in our previous study using congenitally helpless rats. Fluoxetine exerted a protective effect against FST-induced immobility behavior in Holtzman rats. Fluoxetine also caused a significant reduction in the mean regional metabolism of the nucleus accumbens shell and the ventral hippocampus as compared to vehicle-treated subjects. Additional networks affected by fluoxetine treatment included the prefrontal-cingulate cortex and brainstem nuclei linked to depression (e.g., habenula, dorsal raphe and interpeduncular nucleus). We concluded that corticolimbic regions such as the prefrontal-cingulate cortex, nucleus accumbens, ventral hippocampus and key brainstem nuclei represent important contributors to the neural network mediating fluoxetine antidepressant action.

Fluoxetine treatment induces EAAT2 expression in rat brain

Synaptic pathology and disturbed glutamatergic neurotransmission contribute to the neurobiology of depression. Reduced expression of glutamate transporters, most importantly excitatory amino acid transporter (EAAT2), was reported in human studies and animal models. We therefore assessed the effects of antidepressant treatment upon EAAT2 expression. Male Sprague-Dawley rats received daily intraperitoneal injections of the antidepressants desipramine (DES, N = 7), fluoxetine (FLU, N = 7), tranylcypromine (TRAN, N = 5) or a saline control (CON, N = 5) for a period of 14 days. The expression of the major glial glutamate transporter EAAT2 was evaluated by semi-quantitative in situ hybridizations using a (35)S-labeled cRNA probe. Treatment with FLU significantly induced EAAT2 expression in hippocampal and cortical regions in comparison with saline injections, while DES and TRAN-applications did not exert significant effects. It can be postulated that increased expression of EAAT2 may counterbalance the tonus of glutamatergic neurotransmission. Our findings are in concert with human post-mortem findings, valid animal models of depression, antidepressive effects of NMDA-antagonists, and the glutamatergic theory of depression. Further studies should examine the effects of antidepressant treatments upon EAAT2 expression in rodent models of depression to further elucidate the underlying molecular mechanisms.

Effects of maternal separation, early handling, and gonadal sex on regional metabolic capacity of the preweanling rat brain

This is the first study to assess the effects of mother-infant separation on regional metabolic capacity in the preweanling rat brain. Mother-infant separation is generally known to be stressful for rat pups. Holtzman adolescent rats show a depressive-like behavioral phenotype after maternal separation during the preweanling period. However, information is lacking on the effects of maternal separation on the brains of rat pups. We addressed this issue by mapping the brains of preweanling Holtzman rat pups using cytochrome oxidase histochemistry, which reflects long-term changes in brain metabolic capacity, following two weeks of repeated, prolonged maternal separation, and compared this to both early handled and non-handled pups. Quantitative image analysis revealed that maternal separation reduced cytochrome oxidase activity in the medial prefrontal cortex and nucleus accumbens shell. Maternal separation reduced prefrontal cytochrome oxidase to a greater degree in female pups than in males. Early handling reduced cytochrome oxidase activity in the posterior parietal cortex, ventral tegmental area, and subiculum, but increased cytochrome oxidase activity in the lateral frontal cortex. The sex-dependent effects of early handling on cytochrome oxidase activity were limited to the medial prefrontal cortex. Regardless of separation group, females had greater cytochrome oxidase activity in the habenula and ventral tegmental area compared to males. These findings suggest that early life mother-infant separation results in dysfunction of prefrontal and mesolimbic regions in the preweanling rat brain that may contribute to behavioral changes later in life.

Metabolic mapping of the effects of the antidepressant fluoxetine on the brains of congenitally helpless rats

Antidepressants require adaptive brain changes before efficacy is achieved, and they may impact the affectively disordered brain differently than the normal brain. We previously demonstrated metabolic disturbances in limbic and cortical regions of the congenitally helpless rat, a model of susceptibility to affective disorder, and we wished to test whether administration of fluoxetine would normalize these metabolic differences. Fluoxetine was chosen because it has become a first-line drug for the treatment of affective disorders. We hypothesized that fluoxetine antidepressant effects may be mediated by decreasing metabolism in the habenula and increasing metabolism in the ventral tegmental area. We measured the effects of fluoxetine on forced swim behavior and regional brain cytochrome oxidase activity in congenitally helpless rats treated for 2 weeks with fluoxetine (5mg/kg, i.p., daily). Fluoxetine reduced immobility in the forced swim test as anticipated, but congenitally helpless rats responded in an atypical manner, i.e., increasing climbing without affecting swimming. As hypothesized, fluoxetine reduced metabolism in the habenula and increased metabolism in the ventral tegmental area. In addition, fluoxetine reduced the metabolism of the hippocampal dentate gyrus and dorsomedial prefrontal cortex. This study provided the first detailed mapping of the regional brain effects of an antidepressant drug in congenitally helpless rats. All of the effects were consistent with previous studies that have metabolically mapped the effects of serotonergic antidepressants in the normal rat brain, and were in the predicted direction of metabolic normalization of the congenitally helpless rat for all affected brain regions except the prefrontal cortex.

Ambiguous-cue interpretation is biased under stress- and depression-like states in rats

Negative cognitive bias-the tendency to interpret ambiguous situations pessimistically-is a central feature of stress-related disorders such as depression. The underlying neurobiology of this bias, however, remains unclear, not least because of a lack of translational tools. We established a new ambiguous-cue interpretation paradigm and, with respect to the etiology of depression, evaluated if environmental and genetic factors contribute to a negative bias. Rats were trained to press a lever to receive a food reward contingent to one tone and to press another lever in response to a different tone to avoid punishment by electric foot-shock. In the ambiguous-cue test, the lever-press responses to tones with frequencies intermediate to the trained tones were taken as indicators for the rats' expectation of a positive or negative event. A negative response bias because of decreased positive and increased negative responding was found in congenitally helpless rats, a genetic animal model of depression. Moreover, treatment with a combined noradrenergic-glucocorticoid challenge, mimicking stress-related changes in endogenous neuromodulation, biased rats away from positive responding. This response shift was accompanied by neuronal activation in dentate gyrus and amygdala. Thus, environmental and genetic risk factors for depression induce a response bias, which resembles the pessimistic bias of patients suffering from depression. The behavioral paradigm described constitutes a useful tool to study the neuronal basis of decision making under ambiguous conditions and may promote innovative pharmaco- and psychotherapy for depression.

Inter-individual differences in neurobiology as vulnerability factors for affective disorders: implications for psychopharmacology

Susceptibility to affective disorders is individually different, and determined both by genetic variance and life events that cause significant differences in the CNS structure and function between individual subjects. Therefore it is plausible that search for the inter-individual differences in endophenotypes that mediate the effects of causal factors, both genetic and environmental, will reveal the substrates for vulnerability, help to clarify pathogenetic mechanisms, and possibly aid in developing strategies to discover better, more personalized treatments. This review first examines comparatively a number of animal models of human affect and affect-related disorders that rely on persistent inter-individual differences, and then highlights some of the neurobiological findings in these models that are compatible with much of research in human behavioural and personality traits. Many behaviours occur in specific combinations in several models, but often remarkable dissociations are observed, providing a variety of constellations of traits. It is concluded that more systematic comparative experimentation on behaviour and neurobiology in different models is warranted to reveal possible "building blocks" of affect-related personality common in animals and humans. Looking into the perspectives in psychopharmacology the focus is placed on probable associations of inter-individual differences with brain structure and function, personality and coping strategies, and psychiatric vulnerability, highlighting some unexpected interactions between vulnerability endophenotypes, adverse life events, and behavioural traits. It is argued that further studies on inter-individual differences in affect and underlying neurobiology should include formal modeling of their epistatic, hierarchical and dynamic nature.

Reduced expression of glutamate transporters vGluT1, EAAT2 and EAAT4 in learned helpless rats, an animal model of depression

BACKGROUND

It has been widely accepted that glial pathology and disturbed synaptic transmission contribute to the neurobiology of depression. Apart from monoaminergic alterations, an influence of glutamatergic signal transduction has been reported. Therefore, gene expression of glutamate transporters that strictly control synaptic glutamate concentrations have to be assessed in animal models of stress and depression.

METHODS

We performed in situ-hybridizations in learned helplessness rats, a well established animal model of depression, to assess vGluT1 and EAAT1-4. Sprague-Dawley rats of two inbred lines were tested for helpless behavior and grouped into three cohorts according to the number of failures to stop foot shock currents by lever pressing.

RESULTS

Helpless animals showed a significantly suppressed expression of the glial glutamate transporter EAAT2 (rodent nomenclature GLT1) in hippocampus and cerebral cortex compared to littermates with low failure rate and not helpless animals. This finding was validated on protein level using immunohistochemistry. Additionally, expression levels of EAAT4 and the vesicular transporter vGluT1 were reduced in helpless animals. In contrast, the transcript levels of EAAT1 (GLAST) and EAAT3 (EAAC1) were not significantly altered.

CONCLUSIONS

These results strongly suggest reduced astroglial glutamate uptake and implicate increased glutamate levels in learned helplessness. The findings are in concert with antidepressant effects of NMDA-receptor antagonists and the hypotheses that impaired astroglial functions contribute to the pathogenesis of affective disorders.

Strain, sex, and open-field behavior: factors underlying the genetic susceptibility to helplessness

Learned helplessness represents a failure to escape after exposure to inescapable stress and may model human psychiatric disorders related to stress. Previous work has demonstrated individual differences in susceptibility to learned helplessness. In this study, we assessed different factors associated with this susceptibility, including strain, sex, and open-field behavior. Testing of three rat strains (Holtzman, Long-Evans, and Sprague-Dawley) revealed that Holtzman rats were the most susceptible to helplessness. Holtzman rats not only had the longest escape latencies following inescapable shock, but also showed spontaneous escape deficits in the absence of prior shock when tested with a fixed-ratio 2 (FR2) running response. Moreover, when tested with fixed-ratio 1 (FR1) running - an easy response normally unaffected by helplessness training in rats - inescapable shock significantly increased the escape latencies of Holtzman rats. Within the Holtzman strain, we confirmed recent findings that females showed superior escape performance and therefore appeared more resistant to helplessness than males. However, regression and covariance analyses suggest that this sex difference may be explained by more baseline ambulatory activity among females. In addition, some indices of novelty reactivity (greater exploration of novel vs. familiar open-field) predicted subsequent helpless behavior. In conclusion, Holtzman rats, and especially male Holtzman rats, have a strong predisposition to become immobile when stressed which interferes with their ability to learn active escape responses. The Holtzman strain therefore appears to be a commercially available model for studying susceptibility to helplessness in males, and novelty-seeking may be a marker of this susceptibility.

Bipolar and major depressive disorder: neuroimaging the developmental-degenerative divide

Both major depressive disorder and bipolar disorder are the subject of a voluminous imaging and genetics literature. Here, we attempt a comprehensive review of MRI and metabolic PET studies conducted to date on these two disorders, and interpret our findings from the perspective of developmental and degenerative models of illness. Elevated activity and volume loss of the hippocampus, orbital and ventral prefrontal cortex are recurrent themes in the literature. In contrast, dorsal aspects of the PFC tend to display hypometabolism. Ventriculomegaly and white matter hyperintensities are intimately associated with depression in elderly populations and likely have a vascular origin. Important confounding influences are medication, phenotypic and genetic heterogeneity, and technological limitations. We suggest that environmental stress and genetic risk variants interact with each other in a complex manner to alter neural circuitry and precipitate illness. Imaging genetic approaches hold out promise for advancing our understanding of affective illness.

Chronic 13-cis-retinoic acid administration disrupts network interactions between the raphe nuclei and the hippocampal system in young adult mice

Previously, we showed that chronic administration of 13-cis-retinoic acid (13-cis-RA) induces depression-related behaviors in mice and that 13-cis-RA alters components of the serotonergic system in vitro. Work by others has shown that 13-cis-RA reduces hippocampal neurogenesis in mice and orbitofrontal cortex metabolism in humans. In the current study, we measured cytochrome oxidase activity, a metabolic marker that reflects steady state neuronal energy demand, in various regions of the brain to determine the effects of 13-cis-RA on neuronal metabolic activity and network interactions between the raphe nuclei and the hippocampal system. Brain cytochrome oxidase activity in young adult male mice was analyzed following 6 weeks of daily 13-cis-RA (1 mg/kg) or vehicle injection and behavioral testing. Chronic 13-cis-RA administration significantly decreased cytochrome oxidase activity only in the inferior rostral linear nucleus of the raphe. However, covariance analysis of interregional correlations in cytochrome oxidase activity revealed that 13-cis-RA treatment caused a functional uncoupling between the dorsal raphe nuclei and the hippocampus. Furthermore, a path analysis indicated that a network comprising lateral habenula to dorsal raphe to hippocampus was effectively uncoupled in 13-cis-RA treated animals. Finally, cytochrome oxidase activity in the dentate gyrus of 13-cis-RA treated mice was inversely correlated with depression-related behavior. Taken together, these data show that 13-cis-RA alters raphe metabolism and disrupts functional connectivity between the raphe nuclei and the hippocampal formation, which may contribute to the observed increase in depression-related behaviors.

Potential surgical targets for deep brain stimulation in treatment-resistant depression

OBJECT

The goal of this study was to evaluate the definition of treatment-resistant depression (TRD), review the literature regarding deep brain stimulation (DBS) for TRD, and identify potential anatomical and functional targets for future widespread clinical application.

METHODS

A comprehensive literature review was performed to determine the current status of DBS for TRD, with an emphasis on the scientific support for various implantation sites.

RESULTS

The definition of TRD is presented, as is its management scheme. The rationale behind using DBS for depression is reviewed. Five potential targets have been identified in the literature: ventral striatum/nucleus accumbens, subgenual cingulate cortex (area 25), inferior thalamic peduncle, rostral cingulate cortex (area 24a), and lateral habenula. Deep brain stimulation electrodes thus far have been implanted and activated in only the first 3 of these structures in humans. These targets have proven to be safe and effective, albeit in a small number of cases.

CONCLUSIONS

Surgical intervention for TRD in the form of DBS is emerging as a viable treatment alternative to existing modalities. Although the studies reported thus far have small sample sizes, the results appear to be promising. Various surgical targets, such as the subgenual cingulate cortex, inferior thalamic peduncle, and nucleus accumbens, have been shown to be safe and to lead to beneficial effects with various stimulation parameters. Further studies with larger patient groups are required to adequately assess the safety and efficacy of these targets, as well as the optimal stimulation parameters and long-term effects.

The effects of novelty-seeking phenotypes and sex differences on acquisition of cocaine self-administration in selectively bred High-Responder and Low-Responder rats

Individual differences in exploratory behavior can predictably influence psychostimulant self-administration behavior. Male rats that exhibit a high degree of locomotor activity in a novel environment (High Responders, HR) will self-administer cocaine more readily than males exhibiting low levels of novelty-induced locomotion (Low Responders, LR). The present experiment investigates the combined influences of the sex of an individual and individual phenotypes in novelty-induced locomotion to predispose animals to acquire cocaine self-administration behavior, in male and female rats selectively bred for the HR-LR phenotypes. We first established that HR females, like their male counterparts, exhibit a dramatically greater locomotor response to novelty and less anxiety-like behavior than do LR females. While locomotor behavior was subtly influenced by estrous stage, with both HR and LR females showing increased activity during metestrus and diestrus compared to proestrus and estrus, the effect did not obscure HR-LR differences. When male and female HR-LR animals were trained to self-administer cocaine (2 h/day, 5 days/wk x 3 wk, 0.2 mg cocaine/kg/infusion), HR males and females acquired cocaine self-administration significantly faster than their LR counterparts. Furthermore, HR females self-administered significantly more cocaine than all other groups. In conclusion, female rats, like males, exhibit HR-LR phenotypes that predict rapidity of acquiring cocaine self-administration. Moreover, HR females self-administer more cocaine than HR males and both LR groups.

Stress before puberty exerts a sex- and age-related impact on auditory and contextual fear conditioning in the rat

Adolescence is a period of major physical, hormonal, and psychological changes. It is also characterized by a significant increase in the incidence of psychopathologies and this increase is gender-specific. Stress during adolescence is associated with the development of psychiatric disorders later in life. In this study, we evaluated the impact of psychogenic stress (exposure to predator odor followed by placement on an elevated platform) experienced before puberty (days 28–30) on fear memories and hormonal response of male and female rats during adolescence and early adulthood. Stress before puberty impacted in a sex- and age-specific way on the responses to auditory and contextual fear conditioning in adolescence and adulthood: (a) increased conditioned fear to the tone in males during adolescence but not during adulthood; (b) impaired extinction to the tone in adult males; and (c) reduced freezing responses to the context in adolescent females. Stress before puberty did not influence the corticosterone levels 30 minutes after an additional stressor given in adulthood. These results indicate that stress experienced prior to puberty can exert a sex-related differential impact on fear-related behaviors displayed by individuals during late adolescence and early adulthood.

Reduced expression of complexins I and II in rats bred for learned helplessness

Disturbed synaptic transmission contributes to the pathophysiology of mood disorders. Post mortem studies reported reduced expression of the synaptic vesicle protein (SVP) complexins I and II in depression. Antidepressants were found to induce the expression of these genes. Since animals with congenital susceptibility to learned helplessness provide a valid animal model of depression, we investigated the expression of different SVPs in this system by semiquantitative in situ hybridization. Rats bred for congenital learned helpless behavior (cLH, N=6) failed to interrupt foot shock currents by lever pressing (mean 12.3 failures out of 15 trials). These animals showed significantly lower expression of complexins I and II mRNA in hippocampal, limbic and cortical brain areas compared to not helpless animals (cNLH, N=6) with a mean failure rate of 0.83 out of 15 trials. Expression levels of complexins I and II significantly correlated with the failure rate in the test paradigm. In contrast, the expressions of synaptotagmin I and synaptophysin were found unchanged. This investigation provides a further validation of the LH model of depression. The experimental data fit well into current pathogenetic concepts of mood disorders and support the hypothesis, that complexins are pivotal players in the pathophysiology of depression and tentative targets of antidepressants.

Methylene blue facilitates the extinction of fear in an animal model of susceptibility to learned helplessness

The objectives were to (1) extend previous findings on fear extinction deficits in male congenitally helpless rats (a model for susceptibility to learned helplessness) to female congenitally helpless rats, and (2) attempt a therapeutic intervention with methylene blue, a metabolic enhancer that improves memory retention, to alleviate the predicted extinction deficits. In the first experiment, fear acquisition (four tone-shock pairings in operant chamber) was followed by extinction training (60 tones in open field). Congenitally helpless rats showed fear acquisition similar to controls but had dramatic extinction deficits, and did not display the gradual extinction curves observed in controls. Congenitally helpless rats demonstrated greater tone-evoked freezing as compared to controls in both the acquisition and extinction contexts one week after extinction training, and also in the extinction probe conducted one month later. In the second experiment (which began one month after the first experiment) congenitally helpless subjects were further exposed to tones for 5 days, each followed by 4 mg/kg methylene blue or saline IP, and had a fear renewal test in the acquisition context. Methylene blue administration improved retention of the extinction memory as demonstrated by significant decreases in fear renewal as compared to saline-administered congenitally helpless subjects. The impaired ability to extinguish fear to a traumatic memory in congenitally helpless rats supports the validity of this strain as an animal model for vulnerability to post-traumatic stress disorder, and this study further suggests that methylene blue may facilitate fear extinction as an adjunct to exposure therapy.

Prefrontal mechanisms in extinction of conditioned fear

Interest in the medial prefrontal cortex (mPFC) as a source of behavioral inhibition has increased with the mounting evidence for a functional role of the mPFC in extinction of conditioned fear. In fear extinction, a tone-conditioned stimulus (CS) previously paired with a footshock is presented repeatedly in the absence of footshock, causing fear responses to diminish. Here, we review converging evidence from different laboratories implicating the mPFC in memory circuits for fear extinction: (1) lesions of mPFC impair recall of extinction under various conditions, (2) extinction potentiates mPFC physiological responses to the CS, (3) mPFC potentiation is correlated with extinction behavior, and (4) stimulation of mPFC strengthens extinction memory. These findings support Pavlov's original notion that extinction is new learning, rather than erasure of conditioning. In people suffering from posttraumatic stress disorder (PTSD), homologous areas of ventral mPFC show morphological and functional abnormalities, suggesting that extinction circuits are compromised in PTSD. Strategies for augmenting prefrontal function for clinical benefit are discussed.

Behavioral characteristics of rats predisposed to learned helplessness: reduced reward sensitivity, increased novelty seeking, and persistent fear memories

The congenitally helpless rat strain, which was selectively bred for increased susceptibility to learned helplessness, may model the predisposition to affective disorders, including depression and post-traumatic stress disorder. Other than the selected trait, the behavior of this strain is not well characterized. In this study, we assessed congenitally helpless rats on several behavioral tests. First, we assessed reward sensitivity by measuring their consumption of a 5% sucrose solution. Next, we assessed exploratory behavior and fearfulness in both a novel and familiar open field, and in a light-dark test. Finally, we assessed fear conditioning by exposing the animals to 4 tone-shock pairs on 1 day (acquisition) and then presenting 60 tones over the next 2 days (extinction). Compared to normal Sprague-Dawley controls, congenitally helpless rats showed less consumption of the sucrose solution and more exploratory behavior in the novel, but not the familiar, open fields. They also showed less fearfulness in the light-dark test, but more conditioned freezing to the tone predicting shock. Moreover, this freezing was resistant to extinction; congenitally helpless rats not only failed to show a fear decrement during extinction, but actually showed increased fear, a phenomenon termed "paradoxical enhancement." Thus, congenitally helpless rats appear to have a behavioral phenotype characterized by reduced sensitivity to reward, increased drive to explore novel environments, and increased propensity to form and maintain fear-associated memories. This behavioral phenotype is discussed as resembling the personality of humans vulnerable to post-traumatic stress disorder.

Behavioral correlates of differences in neural metabolic capacity

Cytochrome oxidase is a rate-limiting enzyme in oxidative phosphorylation, the major energy-synthesizing pathway used by the central nervous system, and cytochrome oxidase histochemistry has been extensively utilized to map changes in neural metabolism following experimental manipulations. However, the value of cytochrome oxidase activity in predicting behavior has not been analyzed. We argue that this endeavor is important because genetic composition and embryonic environment can engender differences in baseline neural metabolism in pertinent neural circuits, and these differences could represent differences in the degree to which specific behaviors are 'primed.' Here we review our studies in which differences in cytochrome oxidase activity and in behavior were studied in parallel. Using mammalian and reptilian models, we find that embryonic experiences that shape the propensity to display social behaviors also affect cytochrome oxidase activity in limbic brain areas, and elevated cytochrome oxidase activity in preoptic, hypothalamic, and amygdaloid nuclei correlates with heightened aggressive and sexual tendencies. Selective breeding regimes were used to create rodent genetic lines that differ in their susceptibility to display learned helplessness and in behavioral excitability. Differences in cytochrome oxidase activity in areas like the paraventricular hypothalamus, frontal cortex, habenula, septum, and hippocampus correlate with differences in susceptibility to display learned helplessness, and differences in activity in the dentate gyrus and perirhinal and posterior parietal cortex correlate with differences in hyperactivity. Thus, genetic and embryonic manipulations that engender specific behavioral differences produce specific neurometabolic profiles. We propose that knowledge of neurometabolic differences can yield valuable predictions about behavioral phenotype in other systems.

Stress models of depression: Forming genetically vulnerable strains

Among the most useful models for depressive disorders are those, which involve a stress induced change in behaviour. Learned helplessness is one such model and is induced through exposure to uncontrollable and unpredictable aversive events. Learned helplessness as induced in rats using foot shock is well characterized and has good face validity and predictive validity as a model of depression, including alterations in HPA axis activity and REM sleep characteristic of depression. The data concerning the validity will be briefly reviewed. The model can also be used to look at the role of genetics through selective breeding. These studies will be reviewed and the utility of the genetic strains for understanding the interaction of stress and affect will be examined. A second model of depression using exposure to chronic stress also has high face and predictive validity. A new form of this approach, recently described, also is suitable for the examination of genetic factors leading to depressive like behaviour and this will be presented.

Brain differences in newborn rats predisposed to helpless and depressive behavior

Inborn brain differences in metabolic capacity were mapped in congenitally helpless rats, a genetically selected strain predisposed to show helpless and depressive behavior. There are a number of brain regions showing abnormal metabolism in adult congenitally helpless rats. Some of these alterations may be innate while others may be due to environmental factors, such as maternal care and postnatal stress. To identify which brain structures show innate differences, brains of newborn rats from congenitally helpless and non-helpless strains were compared using cytochrome oxidase histochemistry, an endogenous marker of regional metabolic capacity. A smaller subset of regions affected in adults showed significantly less metabolic activity in the newborn brains, including paraventricular hypothalamus, habenula, hippocampus, subiculum, lateral septal nucleus, anterior cingulate cortex, infralimbic cortex, and medial orbitofrontal cortex. A covariance analysis further revealed a striking reduction of functional connectivity in the congenitally helpless brain, including a complete decoupling of limbic forebrain regions from midbrain/diencephalic regions. This pattern of brain metabolism suggests that helplessness vulnerability is linked to altered functioning of limbic networks that are key to controlling the hypothalamic-pituitary-adrenal axis. This implies that vulnerable animals have innate deficits in brain systems that would normally allow them to cope with stress, predisposing them in this manner to more readily develop helpless and depressive behaviors.

Cytochrome oxidase activity in the preoptic area correlates with differences in sexual behavior of intact and castrated male leopard geckos (Eublepharis macularius)

Although the utility of analyzing behavioral experience effects on neural cytochrome oxidase (CO) activity is well recognized, the behavioral correlates of endogenous differences in CO activity have rarely been explored. In male leopard geckos (Eublepharis macularius), the incubation temperature experienced during embryogenesis (IncT) and age affect CO activity in the preoptic area (POA), an area that modulates copulatory behavior. In this study, the authors assessed whether differences in POA CO activity correlate with differences in sexual behavior in intact and castrated geckos. Males with IncT- and age-dependent increases in POA CO activity mounted females with shorter latencies while intact and after castration and ejaculated more frequently after castration. The authors discuss the predictive value of CO activity and propose similar parallels in other species.

Limbic-frontal circuitry in major depression: a path modeling metanalysis

This paper reports the results of an across lab metanalysis of effective connectivity in major depression (MDD). Using FDG PET data and Structural Equation Modeling, a formal depression model was created to explicitly test current theories of limbic-cortical dysfunction in MDD and to characterize at the path level potential sources of baseline variability reported in this patient population. A 7-region model consisting of lateral prefrontal cortex (latF9), anterior thalamus (aTh), anterior cingulate (Cg24), subgenual cingulate (Cg25), orbital frontal cortex (OF11), hippocampus (Hc), and medial frontal cortex (mF10) was tested in scans of 119 depressed patients and 42 healthy control subjects acquired during three separate studies at two different institutions. A single model, based on previous theory and supported by anatomical connectivity literature, was stable for the three groups of depressed patients. Within the context of this model, path differences among groups as a function of treatment response characteristics were also identified. First, limbic-cortical connections (latF9-Cg25-OF11-Hc) differentiated drug treatment responders from nonresponders. Second, nonresponders showed additional abnormalities in limbic-subcortical pathways (aTh-Cg24-Cg25-OF11-Hc). Lastly, more limited limbic-cortical (Hc-latF9) and cortical-cortical (OF11-mF10) path differences differentiated responders to cognitive behavioral therapy (CBT) from responders to pharmacotherapy. We conclude that the creation of such models is a first step toward full characterization of the depression phenotype at the neural systems level, with implications for the future development of brain-based algorithms to determine optimal treatment selection for individual patients.

Rats with congenital learned helplessness respond less to sucrose but show no deficits in activity or learning

Inbred rat strains for congenital learned helplessness (cLH) and for congenital resistance to learned helplessness (cNLH) were investigated as a model to study genetic predisposition to major depression. Congenitally helpless rats respond less to sucrose under a progressive ratio schedule. This is not confounded by locomotor hypoactivity: in contrast, cLH rats show a slight hyperactivity during the first 5 min of an open field test. cLH rats acquire operant responding to sucrose as readily as cNLH rats and exhibit normal memory acquisition and retrieval in the Morris water maze, thus ruling out general learning deficits as the cause of the decreased response to sucrose. Reduced total responses and reduced breaking points for sucrose in the cLH strain argue for anhedonia, which is an analogue to loss of pleasure essential for the diagnosis of major depressive episodes, and thus confirm the validity of congenitally learned helpless rats as a model of major depression.

Congenitally learned helpless rats show abnormalities in intracellular signaling

BACKGROUND

Affective disorders and the drugs used to treat them lead to changes in intracellular signaling. We used a genetic animal model to investigate to what extent changes in intracellular signal transduction confer a vulnerability to mood or anxiety disorders.

METHODS

Levels of gene expression in a selectively bred strain of rats with a high vulnerability to develop congenitally learned helplessness (cLH), a strain highly resistant to the same behavior (cNLH) and outbred Sprague-Dawley (SD) control animals were compared using quantitative reverse transcription polymerase chain reaction.

RESULTS

Congenitally learned helpless animals had a 24%-30% reduced expression of the cyclic adenosine monophosphate response element binding protein messenger ribonucleic acid (mRNA) in the hippocampus and a 40%-41% increased level of the antiapoptotic protein bcl-2 mRNA in the prefrontal cortex compared to cNLH and SD rats. Other significant changes included changes in the expression levels of the alpha catalytic subunit of protein kinase A, glycogen synthase kinase 3beta, and protein kinase C epsilon.

CONCLUSIONS

Congenitally learned helpless animals show evidence of altered signal transduction and regulation of apoptosis compared to cNLH and SD control animals.

Opposite metabolic changes in the habenula and ventral tegmental area of a genetic model of helpless behavior

Congenitally helpless rats have been selectively bred to display an immediate helpless response to stress in order to model hereditary brain differences that contribute to depression vulnerability. Differences in regional brain metabolism between congenitally helpless and non-helpless rats were investigated using quantitative cytochrome oxidase histochemistry. The results indicated that congenitally helpless rats had 64-71% elevated metabolism in the habenula and a 25% elevation in the related interpeduncular nucleus. In contrast, helpless rats had 28% reduced metabolism in the ventral tegmental area (VTA) and 14-16% reductions in the basal ganglia and basolateral and central amygdala. The opposite metabolic changes in the habenula and ventral tegmental area may be especially important for determining the congenitally helpless rat's global pattern of brain activity, which resembles the metabolic activity pattern produced by dopamine antagonism.

Dissociation of septo-hippocampal metabolism in the congenitally helpless rat

Congenitally helpless rats, selectively bred to model features of endogenous depression, appear to have a paraventricular hypothalamic nucleus (PVH) that is markedly hyperactive. This study investigated septal and hippocampal regions purported to regulate the PVH. We found that cytochrome oxidase, an index of oxidative metabolism and neural activity, was significantly elevated in the hippocampus and subiculum of congenitally helpless rats. However, reduced activity was observed in the lateral and medial septal nuclei, the nucleus of the diagonal band, and the bed nucleus of the stria terminalis. This dissociation between hippocampal and septal activity may be a predisposing factor for the development of helpless behavior.

Environmental enrichment: effects on stereotyped behavior and regional neuronal metabolic activity

The present study evaluated whether environmental enrichment-related effects on the development of stereotyped behavior in deer mice were associated with alterations in neuronal metabolic activity. Deer mice were reared under either enriched or standard housing conditions for 60 days following weaning. All mice were then placed in automated photocell detectors and classified as either stereotypic or non-stereotypic. Neuronal metabolic activity was then assessed using cytochrome oxidase (CO) histochemistry. The results demonstrated that environmental enrichment significantly increased neuronal metabolic activity in the motor cortex. Furthermore, non-stereotypic mice exhibited significantly more CO activity than stereotypic mice in the cortex, striatum, nucleus accumbens, thalamus, hippocampus and amygdala. This latter effect was due to the enriched mice as evidenced by a significant interaction between housing condition and behavioral status in the cortex, striatum, nucleus accumbens, thalamus and hippocampus. Thus, the observed increase in CO activity reflected increased neuronal metabolic activity in non-stereotypic enriched mice relative to stereotypic enriched mice. These results suggest that, in a developmental model of spontaneous stereotypy, the enrichment-related prevention of stereotyped behavior is associated with increased CO activity.

The validity of animal models of predisposition to depression

Some animal models of depression, including the majority of the more recently introduced models, are better characterized as models of predisposition to depression. In the first part of this paper, we show that the basis for such a model could be either a procedure that increases the ease with which an analogue of major depression may be evoked, or a presentation analogous to dysthymia (chronic mild depression). We then consider how the concepts of predictive, face, and construct validity apply to such models. Next, we review the validity of the available models of predisposition to depression, which derive from genetics, genomics, developmental manipulations, and brain lesioning. Finally, we compare the performance of the different models, using a novel scoring system that formalizes the evaluation of animal models against each of the three sets of validation criteria.

Hypermetabolism of paraventricular hypothalamus in the congenitally helpless rat

The congenitally helpless rat, selectively bred to model behavioral features of depression, has shown metabolic activity patterns in frontal and cingulate cortex similar to those detected in human imaging studies of depression and sadness. This study extends the same metabolic mapping technique (quantitative cytochrome oxidase histochemistry) to the hypothalamus, where activity levels were assessed in six nuclei: paraventricular nucleus, medial preoptic area, lateral hypothalamic area, supraoptic nucleus, suprachiasmatic nucleus, and ventromedial nucleus. Helpless rats were compared with a strain of non-helpless rats selectively bred for stress resistance. Only the paraventricular nucleus showed a significant group difference, with helpless rats showing elevated metabolism and non-helpless rats showing reduced metabolism relative to normal rats. Thus, paraventricular nucleus activity may be associated with genetic susceptibility to helpless behavior.