Animal models of depression: parallels and correlates to severe depression in humans

M6A RNA Methylation-Based Epitranscriptomic Modifications in Plasticity-Related Genes via miR-124-C/EBPα-FTO-Transcriptional Axis in the Hippocampus of Learned Helplessness Rats

BACKGROUND

Impaired synaptic plasticity has been linked to dynamic gene regulatory network changes. Recently, gene regulation has been introduced with the emerging concept of unique N6-methyladenosine (m6A)-based reversible transcript methylation. In this study, we tested whether m6A RNA methylation may potentially serve as a link between the stressful insults and altered expression of plasticity-related genes.

METHODS

Expression of plasticity genes Nr3c1, Creb1, Ntrk2; m6A-modifying enzymes Fto, methyltransferase like (Mettl)-3 and 14; DNA methylation enzymes Dnmt1, Dnmt3a; transcription factor C/ebp-α; and miRNA-124-3p were determined by quantitative polymerase chain reaction (qPCR) in the hippocampus of rats that showed susceptibility to develop stress-induced depression (learned helplessness). M6A methylation of plasticity-related genes was determined following m6A mRNA immunoprecipitation. Chromatin immunoprecipitation was used to examine the endogenous binding of C/EBP-α to the Fto promoter. MiR-124-mediated post-transcriptional inhibition of Fto via C/EBPα was determined using an in vitro model.

RESULTS

Hippocampus of learned helplessness rats showed downregulation of Nr3c1, Creb1, and Ntrk2 along with enrichment in their m6A methylation. A downregulation in demethylating enzyme Fto and upregulation in methylating enzyme Mettl3 were also noted. The Fto promoter was hypomethylated due to the lower expression of Dnmt1 and Dnmt3a. At the same time, there was a lower occupancy of transcription factor C/EBPα on the Fto promoter. Conversely, C/ebp-α transcript was downregulated via induced miR-124-3p expression.

CONCLUSIONS

Our study mechanistically linked defective C/EBP-α-FTO-axis, epigenetically influenced by induced expression of miR-124-3p, in modifying m6A enrichment in plasticity-related genes. This could potentially be linked with abnormal neuronal plasticity in depression.

Inappropriate modeling of chronic and complex disorders: How to reconsider the approach in the context of predictive, preventive and personalized medicine, and translational medicine

Preclinical investigations such as animal modeling make the basis of clinical investigations and subsequently patient care. Predictive, preventive, and personalized medicine (PPPM) not only highlights a patient-tailored approach by choosing the right medication, the right dose at the right time point but it as well essentially requires early identification, by the means of complex and state-of-the-art technologies of unmanifested pathological processes in an individual, in order to deliver targeted prevention early enough to reverse manifestation of a pathology. Such an approach can be achieved by taking into account clinical, pathological, environmental, and psychosocial characteristics of the patients or an individual who has a suboptimal health condition. Inappropriate modeling of chronic and complex disorders, in this context, may diminish the predictive potential and slow down the development of PPPM and consequently modern healthcare. Therefore, it is the common goal of PPPM and translational medicine to find the solution for the problem we present in our review. Both, translational medicine and PPPM in parallel, essentially need accurate surrogates for misleading animal models. This study was therefore undertaken to provide shreds of evidence against the validity of animal models. Limitations of current animal models and drug development strategies based on animal modeling have been systematically discussed. Finally, a variety of potential surrogates have been suggested to change the unfavorable situation in medical research and consequently in healthcare.

What is stressful for females? Differential effects of unpredictable environmental or social stress in CD1 female mice

Stressful life events are a major factor in the etiology of several diseases, such as cardiovascular, inflammatory and psychiatric disorders (i.e., depression and anxiety), with the two sexes greatly differing in vulnerability. In humans and other animals, physiological and behavioral responses to stress are strongly dependent on gender, and conditions that are stressful for males are not necessarily stressful for females. Hence the need of an animal model of social chronic stress specifically designed for females. In the present study we aimed to compare the effects of two different chronic stress procedures in female mice, by investigating the impact of 4weeks of nonsocial unpredictable, physical stress by the Chronic Mild Stress paradigm (CMS; Exp.1) or of Social Instability Stress (SIS; Exp.2) on physiological, endocrine and behavioral parameters in adult female mice. CMS had a pronounced effect on females' response to novelty (i.e., either novel environment or novel social stimulus), body weight growth and hormonal profile. Conversely, 4weeks of social instability did not alter females' response to novelty nor hormonal levels but induced anhedonia. Our findings thus showed that female mice were more sensitive to nonsocial stress due to unpredictable physical environment than to social instability stressors. Neither of these stress paradigms, however, induced a consistent behavioral and physiological stress response in female mice comparable to that induced by chronic stress procedures in male mice, thus confirming the difficulties of developing a robust and validated model of chronic psychosocial stress in female mice.

Altered Expression of Endoplasmic Reticulum Stress Associated Genes in Hippocampus of Learned Helpless Rats: Relevance to Depression Pathophysiology

The unfolded protein response (UPR) is an evolutionarily conserved defensive mechanism that is used by cells to correct misfolded proteins that accumulate in the endoplasmic reticulum. These proteins are misfolded as a result of physical stress on a cell and initiate a host of downstream effects that govern processes ranging from inflammation to apoptosis. To examine whether UPR system plays a role in depression, we examined the expression of genes that are part of the three different pathways for UPR activation, namely GRP78, GRP94, ATF6, XBP-1, ATF4, and CHOP using an animal model system that distinguishes vulnerability (learned helpless, LH) from resistance (non-learned helpless, NLH) to develop depression. Rats were exposed to inescapable shock on days 1 and 7 and were tested for escape latency on day 14. Rats not given shock but tested for escape latency were used as tested control (TC). Plasma corticosterone (CORT) levels were measured. Expression levels of various UPR associated genes were determined in hippocampus using qPCR. We found that the CORT level was higher in LH rats compared with TC and NLH rats. Expression of GRP78, GRP94, ATF6, and XBP-1 were significantly upregulated in LH rats compared with TC or NLH rats, whereas NLH rats did not show such changes. Expression levels of ATF4 and CHOP showed trends toward upregulation but were not significantly altered in LH or NLH group. Our data show strong evidence of altered UPR system in depressed rats, which could be associated with development of depressive behavior.

Roles of olfactory system dysfunction in depression

The olfactory system is involved in sensory functions, emotional regulation and memory formation. Olfactory bulbectomy in rat has been employed as an animal model of depression for antidepressant discovery studies for many years. Olfaction is impaired in animals suffering from chronic stress, and patients with clinical depression were reported to have decreased olfactory function. It is believed that the neurobiological bases of depression might include dysfunction in the olfactory system. Further, brain stimulation, including nasal based drug delivery could provide novel therapies for management of depression.

Repeated lysergic acid diethylamide in an animal model of depression: Normalisation of learning behaviour and hippocampal serotonin 5-HT2 signalling

A re-balance of postsynaptic serotonin (5-HT) receptor signalling, with an increase in 5-HT1A and a decrease in 5-HT2A signalling, is a final common pathway multiple antidepressants share. Given that the 5-HT1A/2A agonist lysergic acid diethylamide (LSD), when repeatedly applied, selectively downregulates 5-HT2A, but not 5-HT1A receptors, one might expect LSD to similarly re-balance the postsynaptic 5-HT signalling. Challenging this idea, we use an animal model of depression specifically responding to repeated antidepressant treatment (olfactory bulbectomy), and test the antidepressant-like properties of repeated LSD treatment (0.13 mg/kg/d, 11 d). In line with former findings, we observe that bulbectomised rats show marked deficits in active avoidance learning. These deficits, similarly as we earlier noted with imipramine, are largely reversed by repeated LSD administration. Additionally, bulbectomised rats exhibit distinct anomalies of monoamine receptor signalling in hippocampus and/or frontal cortex; from these, only the hippocampal decrease in 5-HT2 related [(35)S]-GTP-gamma-S binding is normalised by LSD. Importantly, the sham-operated rats do not profit from LSD, and exhibit reduced hippocampal 5-HT2 signalling. As behavioural deficits after bulbectomy respond to agents classified as antidepressants only, we conclude that the effect of LSD in this model can be considered antidepressant-like, and discuss it in terms of a re-balance of hippocampal 5-HT2/5-HT1A signalling.

Effect of five-consecutive-day exposure to an anxiogenic stressor on sleep-wake activity in rats

Repeated exposure to an anxiogenic stressor (AS) is a known environmental factor for the development of depression, yet the progression of sleep-wake (S-W) changes associated with the onset of AS-induced depression (ASID) is not completely understood. Thus, the aim of this study was to identify these progressive S-W changes by developing ASID in rats, via repeated exposure to an AS, and compare this ASID-associated sleep phenotype with the sleep phenotype of human depression. To achieve this aim, rats were first recorded for a 6 h period of baseline S-W activity without AS. Then, rats were subjected to 5 days of AS [Day 1: inescapable foot-shock; 5 trials of 3 s foot-shocks (1.0 mA) at 3 min intervals; Days 3–5: 15 trials of 5 s foot-shocks at 45 s intervals]. S-W activity was recorded for 6 h immediately after each AS treatment session. Two days later rats were again recorded for 6 h of S-W activity, but with no exposure to the AS (NASD). Compared to the baseline day: Day 1 of AS (ASD-1) increased wakefulness, slow-wave sleep (SWS) latency, and rapid-eye movement (REM) sleep latency, but decreased the total amount of REM sleep; ASD-2 animals remained awake throughout the 6 h S-W recording period; ASD-3, ASD-4, and ASD-5 (ASDs-3–5) decreased wakefulness, SWS latency, and REM sleep latency, but increased the total amount of REM sleep. Interestingly, these results reveal that initial exposure to the AS versus later, repeated exposure to the AS produced opposing S-W changes. On NASD, animals exhibited baseline-like S-W activity, except slightly less REM sleep. These results suggest that repeated AS produces a sleep phenotype that resembles the sleep phenotype of depression in humans, but consistent re-exposure to the AS is required. These results are promising because the methodological simplicity and reversibility of the ASID-associated S-W phenotype could be more advantageous than other animal models for studying the pathophysiological mechanisms that underlie the expression of ASID.

Animal models of depression and neuroplasticity: assessing drug action in relation to behavior and neurogenesis

Depression is among the most prevalent forms of mental illness and a major cause of morbidity worldwide. Diagnosis of depression is mainly based on symptomatic criteria, and the heterogeneity of the disease suggests that multiple different biological mechanisms may underlie its etiology. Animal models have been important for recent advances in experimental neuroscience, including modeling of human mood disorders, such as depression and anxiety. Over the past few decades, a number of stress and neurobiochemical models have been developed as primary efficacy measures in depression trials, which are paving the way for the discovery of novel therapeutic targets. Recent data indicates that stress-related mood disorders have influence on neuroplasticity and adult neurogenesis. In this chapter, several currently available animal models are presented as powerful tools for both mechanistic studies into the neurobiology of the antidepressant response and for drug discovery.

Galanin receptor 2 overexpressing mice display an antidepressive-like phenotype: possible involvement of the subiculum

The behavioral phenotype of a transgenic mouse overexpressing a galanin receptor 2 (GalR2)-enhanced, green fluorescent protein (EGFP)-construct under the platelet-derived growth factor-B promoter, and of controls, was assessed in various behavioral tests, such as the Porsolt forced swim test, as well as the open field, elevated plus maze and passive avoidance tests. In addition, the distribution of GalR2-EGFP expressing cell bodies and processes was studied in the brain of these mice using histochemical methods. Three age groups of the transgenic mice demonstrated decreased levels of immobility in the forced swim test, indicative of antidepressive-like behavior and/or increased stress resistance. Anxiety-like behaviors, measured in two different tests, did not differ between the GalR2-overexpressing and the wild-type mice, nor did motor activity levels, emotional learning or memory behaviors. High levels of GalR2 mRNA and protein expression were observed in the presubiculum, subiculum, cingulate cortex, retrosplenial granular and agranular cortices, subregions of prefrontal cortex, and the olfactory bulb, regions which are directly or indirectly implicated in depression-like behavior. These results may contribute to the understanding of the pathophysiology of major depressive disorder and the role of GalR2 in the regulation of mood, and suggest a potential therapeutic effect by targeting the GalR2 for treatment of depressive disorders.

Neuropharmacological effects of Nigerian honey in mice

Honey is a natural sweet substance that bees produce by transforming flower nectar or other sweet secretions of plants. It has widespread use in traditional medicine in various parts of the world. It has been reported to assist in building the entire central nervous system. The beneficial effects of honey have been attributed to the possible polyphenolic contents and some other constituents. The geographical locations and the sources of plant nectars may contribute to the effects of honey samples. Thus, we evaluated the neuropharmacological effects of six samples of honey (10%, 20% and 40%(V)/v, p.o.) from three geographical locations of Nigeria using the following behavioral models: Novelty-induced behaviors (NIB), learning and memory, pentobarbital-induced hypnosis, anxiolytic, anticonvulsant, analgesic and antidepressant models in mice. The results showed that honey significantly (p< 0.05) decreased locomotion and rearing behaviors in NIB and amphetamine-induced locomotor activity when compared to the control group. Exploratory behavior was significantly increased in both holeboard and elevated plus maze but had no significant effect on spatial working memory. Honey sample from Umudike has significant hypnotic and anticonvulsant effects. The antinociceptive models (hot plate and tail flick tests) showed that the honey samples significantly increased the pain reaction time and naloxone blocked these central antinociceptive effects. The force swimming test showed that only the Idanre (ID) honey sample had antidepressant effect. In conclusion, some of these honey samples have central inhibitory property, anxiolytic, antinociceptive, anticonvulsant and antidepressant effects, thus may be used as nutraceutic. It can also be inferred that some of these effects are probably mediated through dopaminergic and opioidergic systems.

Differential sensitivity to SSRI and tricyclic antidepressants in juvenile and adult mice of three strains

Clinical studies have shown differential efficacy of several antidepressants in children and adolescents compared to adults, yet few animal studies have sought to characterize this phenomenon. We compared effects of fluoxetine and imipramine in two common behavioral assays that hold high predictive validity for antidepressant activity, tail suspension and forced swim test, using juvenile (5 weeks) and adult (12 weeks) mice from 3 strains. C57BL/6J-Tyr(c-Brd) (C57), hybrid C57BL/6J-Tyr(c-Brd)x129S5/SvEvBrd (F2), and Balb/cAnNTac (Balb/C) mice were tested in forced swim test and tail suspension after i.p. dosing with either fluoxetine or imipramine. Brain tissues were analyzed to evaluate levels of VMAT2, a possible modulator of age-dependent sensitivity to antidepressants. Imipramine had more consistent antidepressant effect across age groups and strains. Imipramine increased struggle in mice of both ages. Fluoxetine did not have an effect on immobility in Balb/C of both ages in tail suspension. Fluoxetine also did not increase forced swim struggle behavior in juvenile mice of all strains, but was effective in increasing struggle in adults. Juvenile mice had higher immobility and lower struggle than adults in forced swim, and juveniles also had higher immobility in tail suspension test for Balb/C and C57. In addition, VMAT2 levels were increased in juveniles. These results confirm that standard antidepressants produce effects in both juveniles and adults but age-related differences were evident in both tests. Further examination of these effects is needed to determine whether it may be related to age-dependent difference in the clinical response to antidepressants of these classes.

Divergent maternal behavioral patterns in two genetic animal models of depression

Maternal behavior was examined in Flinders Sensitive-Line (FSL) and Wistar-Kyoto (WKY) rats, two different genetic animal models of depression. Behavioral patterns were assessed by undisturbed observations in the nest [Post-Partum Days (PPD) 4 and 9] and post-disturbance observations ("retrieval tests") on PPD 10. Litters were randomly allocated to a mild chronic-stress condition (limiting available bedding between PPD 2 and 9) or a standard rearing condition. The findings indicated that FSL dams did not differ from control dams in the undisturbed observations. However, in the post-disturbance observations FSL dams exhibited less pup-directed behaviors, a shorter latency to first pup carrying/retrieval and more self-directed behaviors than controls (the latter effect only in dams' interaction with whole litter). In contrast, WKY dams performed more pup-directed activities (e.g., nursing and licking) and less self-directed activities in both the undisturbed and post-disturbance observations (in both dams' interaction with single-pup and with the whole-litter) compared to controls. Accordingly, WKY dams exhibited a shorter latency for first pup-licking bout (in both post-disturbance observations). The early life mild chronic-stress used in the study ('limited-bedding') had a minor effect on the dams' behavior. Overall, the study investigated for the first time the maternal behavior of WKY dams and suggests that these dams show an almost opposite behavioral pattern to that of FSL dams. The results are discussed with regard to earlier findings in the FSL strain and behavioral patterns documented in depressed human mothers.

A mouse model of depression induced by repeated corticosterone injections

A rat model of depression has been recently developed by exogenous corticosterone administration. In this study, we further determined whether corticosterone administration also increased depression-like behavior in mice and explored the brain biochemical consequences of exposure to this administration paradigm. Mice received repeated injections of vehicle and 20 mg/kg of corticosterone for 1, 3 and 5 weeks, and then were subjected to the forced-swim and tail suspension tests. The results showed that repeated corticosterone injections increased immobility behavior in the forced-swim and tail suspension tests in a time-dependent manner. Meanwhile, this injection paradigm produced a time-related effect on tyrosine hydroxylase (TH) levels in the hippocampus of mice. These results are consistent with correlations in stress-induced depression models, and suggest that the repeated corticosterone injection paradigm provides a useful and reliable mouse model within which to further study the role of stress and glucocorticoids in depressive illness, as well as screen for antidepressants or preventive drugs.

Role of N-methyl-D-aspartate receptors in antidepressant-like effects of sigma 1 receptor agonist 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA-4503) in olfactory bulbectomized rats

In the present study, we aimed to investigate the role of N-methyl-D-aspartate (NMDA) receptors in the antidepressant-like effects of a sigma(1) receptor agonist, 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA-4503), in the olfactory bulbectomized (OB) rat model of depression. A symptomatology-based behavioral investigation was made by reconstructing in OB rats the symptoms of depression, such as psychomotor agitation, loss of interest, and cognitive dysfunction, using a typical antidepressant, desipramine, as a positive control. Repeated treatment with SA-4503 ameliorated the behavioral deficits in OB rats resembling depression symptoms in the open-field test, sexual behavior test, and cued and contextual fear-conditioning test. SA-4503 displayed advantages over desipramine in the sexual behavior test. SA-4503 also reversed the decrease in the protein expression of NMDA receptor subunit (NR)1, but not NR2A or NR2B, in the prefrontal cortex, hippocampus, and amygdala of OB rats. The behavioral and neurochemical effects of SA-4503 were blocked by combined treatment with a specific sigma(1) receptor antagonist, N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)ethylamine monohydrochloride (NE-100). Furthermore, the effects of SA-4503 on the performance of OB rats in the behavioral tests were abrogated by acute treatment with an NMDA receptor antagonist, (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801). The present study indicated for the first time that the sigma(1) receptor agonist SA-4503 may have effects on depressive symptoms such as agitation, loss of interest, and impaired cognition, which are mediated by NMDA receptors.

Behavioural and neurochemical features of olfactory bulbectomized rats resembling depression with comorbid anxiety

In order to probe the nature and validity of olfactory bulbectomized (OB) rats as a model of depression, we reevaluated their behavioural and neurochemical deficits in relation to the symptoms and neurochemical abnormalities of depression using our protocols, which distinguish anhedonia-resembling behaviour in sexual behavioural test, the hippocampus (Hip)-dependent long-term memory and anxiety-resembling behaviour specially. Besides exploratory hyperactivity in response to a novel environmental stress resembling the psychomotor agitation, OB rats showed a decrease of libido, and a deficit of long-term explicit memory, resembling loss of interest and cognitive deficits in depressive patients, respectively. OB rats also exhibited the anxiety symptom-resembling behaviour in social interaction and plus-maze tests. In the OB rats, we found degenerated neurons in the piriform cortex, decreased protein expression of NMDA receptor subunit 1 (NR1), but not NR2A or NR2B, in the prefrontal cortex (PFC), Hip and amygdala (Amg), and decreased phosphorylation of cAMP-response element-binding protein (CREB) in the PFC and Hip, but not Amg. The behavioural and neurochemical abnormalities in OB rats, except for the performance in the plus-maze task and neuronal degeneration, were significantly attenuated by repeated treatment with desipramine (10 mg/kg), a typical antidepressant. The present study indicated that OB rats may be a model of depression with comorbid anxiety, characterized by agitation, sexual and cognitive dysfunction, neuronal degeneration, decreased protein expression of NR1, and decreased phosphorylation of CREB.

Experimental animal models for the simulation of depression and anxiety

An impressive number of animal models to assess depression and anxiety are available today. However, the relationship between these models and the clinical syndromes of depression and anxiety is not always clear. Since human anxiety disorders represent a multifactorial phenomenon frequently comorbid with major depression and/or other psychiatric problems, the chance of creating animal models which consistently reflect the human situation is quite poor. When using experimental models to understand homologies between animal and human behavior, we have to consider the context in which an animal is investigated, and both the functional significance and relevance of the behavioral parameters that are quantified. Moreover, gender and interindividual and interspecies variabilities in behavioral responses to the test situation and in the sensitivity to pharmacological treatments are potential sources for confounding results. In the past, these aspects have been often neglected in preclinical approaches to behavioral pharmacology and psychopharmacology. A pragmatic approach of combined preclinical and clinical efforts is necessary to imitate one or more aspects relevant to pathological anxiety disorders and depression. The resulting models may identify central nervous processes regulating defined behavioral output, with the potential to develop more effective treatments.

Critical prenatal and postnatal periods for persistent effects of dexamethasone on serotonergic and dopaminergic systems

Glucocorticoid administration to preterm infants is associated with neurodevelopmental disorders. We treated developing rats with dexamethasone (Dex) at 0.05, 0.2, or 0.8 mg/kg, doses below or spanning the range in clinical use, testing the effects of administration during three different stages: gestational days 17-19, postnatal days 1-3 or postnatal days 7-9. In adulthood, we assessed the impact on synaptic biomarkers for serotonin (5-hydroxytryptamine (5HT)) systems. Across all three regimens, Dex administration evoked upregulation of cerebrocortical 5HT1A and 5HT2 receptors and the presynaptic 5HT transporter, greatest for 5HT1A receptors. The effects were fully evident even at the lowest dose. In contrast, 5HT levels in the cerebral cortex and hippocampus showed disparate patterns of temporal sensitivity, with no change after gestational treatment, an increase with the early postnatal regimen, and a decrease with the later postnatal exposure. None of the changes in 5HT concentrations were offset by adaptive changes in the fractional 5HT turnover rate. Furthermore, the critical period of sensitivity seen for 5HT levels differed from that of dopamine even within the same brain region. These findings suggest that developmental exposure to Dex during the critical neurodevelopmental period corresponding to its use in preterm infants, elicits selective changes in 5HT and dopaminergic synaptic function over and above its effects on general aspects of neural cell development, below the threshold for somatic growth impairment, and even at doses below those used clinically. Accordingly, adverse neurobehavioral consequences may be inescapable in glucocorticoid therapy of preterm infants.

Two different putative genetic animal models of childhood depression

BACKGROUND

In an attempt to model childhood depression, we examined whether existing genetic animal models of depression in adult rats are also valid in prepubertal rats.

METHODS

Two different "depressed" rat lines were studied: the Flinders Sensitive Line (FSL) and their controls, Sprague-Dawley (SD); and the Wistar Kyoto (WKY) line and their controls, Wistar. We hypothesized that male prepubertal FSL and WKY rats would show increased swim test immobility and different patterns of social play and of basal plasma levels of corticosterone and adrenocorticotropic hormone (ACTH) compared with control rats.

RESULTS

Prepubertal FSL and WKY rats exhibited significantly longer duration of immobility than control rats in the swim test. The FSL rats demonstrated significantly higher levels of social play behaviors and lower levels of corticosterone and ACTH compared with SD control rats, whereas WKY rats demonstrated significantly lower levels of social play behaviors and higher plasma levels of corticosterone and ACTH compared with Wistar control rats.

CONCLUSIONS

The results might suggest that prepubertal FSL and WKY rats are both putative genetic animal models of childhood depression, exhibiting separate patterns and symptoms of childhood depression.

NDN, VOLUME TRANSMISSION, AND SELF-ORGANIZATION IN BRAIN DYNAMICS

Fields of neural activity are seen in synchronized oscillations that are detected at mesoscopic scales in syntheses of multicellular recordings of action potentials and electroencephalograms (EEGs) over broad areas of cerebral cortex. The waves often have large-scale, highly textured spatial patterns of cortical activity, formed in the context of associative learning under classical and operant conditioning in rabbits. The patterns show spatial amplitude modulation of shared oscillations of carrier waves in the beta and gamma ranges of the EEG, with recurrence at frame rates in the alpha and theta ranges. The frames also show spatial phase modulation that is inconsistent with driving of the oscillations by focal pacemakers. The hypothesis is developed that the synchronization manifests continuous distributions of activity in cortical neuropil that modulate firings of selected neural networks embedded in the neuropil. Five interactive agencies have been postulated to explain the mechanism for the field synchrony: electric fields; magnetic fields; electromagnetic fields (radio waves); diffusion chemical gradients; and order parameters that control self-organization of large populations of neurons by widespread synaptic interaction constituting negative and positive feedback. Only the last interactive agency fits the data. The points are emphasized that these field patterns in frames require interactive neural dynamics that is modulated in respect to global operations mediating arousal, attention, selective emotional stance, wake, sleep, learn, habituate, dishabituate, etc., and that these operations require differing but complementary fields that form by massive parallel feed-forward architectures of brainstem neuromodulatory nuclei. An example is given using histamine of the neural discharges of brainstem nuclei that do not require fine spatiotemporal texturing of their firing; they operate by nonsynaptic release of neuromodulators that effect changes in background state, such that textured patterns of cortical activity can form and update in flexible adaptations of brains to their environments. These systems instantiate volume transmission by nonsynaptic diffusion transmission, in concert with the self-organization of the textured neural activity that supports cognition.

What should animal models of depression model?

In this article, we discuss what animal models of depression should be attempting to 'model'. One must first determine if the goal is to model the regulatory mechanisms by which antidepressant treatments alleviate the various symptoms of depression, or to model the dysregulatory mechanisms underlying the etiology of those symptoms. When modeling the mechanisms of antidepressant effects, a key feature that is often overlooked is the time course required for behavioral efficacy. Even in the clinical literature, there is considerable confusion and inconsistency in defining and identifying 'time of onset' of clinical effect. Although the 'therapeutic lag' may not be as long as has been commonly believed, it does occur. Observable improvement in either global symptomatology or specific symptoms becomes evident after 7-14 days of treatment, and more complete recovery takes considerably longer. Thus, any model addressing potential mechanisms of antidepressant action should exhibit a similar time-dependency. Second, whether attempting to address mechanisms underlying behavioral effects of antidepressants, or the neurobiological substrates underlying the development and manifestation of depression, it is essential to recognize that the syndrome of depression is a diagnostic construct that includes a variety of disparate symptoms, some of which may be related mechanistically, and others that may not be specific to depression, but may cut across categorical diagnostic schemes. Further, it is critical to recognize the close relationship of depression and anxiety. Psychological studies have suggested that the myriad symptoms of depression and anxiety may be subsumed within a more limited number of distinct behavioral dimensions, such as negative affect (neuroticism), positive affect, or physiologic hyperarousal. These dimensions may be related to the functioning of specific neurobiological systems. Thus, rather than trying to recreate or mimic the entire spectrum of symptoms comprising the syndrome of depression, it may be more informative to develop animal models for these behavioral dimensions. Such models may then provide access not only to the neural regulatory mechanisms underlying effective antidepressant treatment, but may also provide clues to the processes underlying the development and manifestation of depression.

Permanent deficits in serotonergic functioning of olfactory bulbectomized rats: an in vivo microdialysis study

BACKGROUND

Bilateral removal of the olfactory bulbs (OBX) in rats results in a complex constellation of behavioral, neurochemical, neuroendocrine, and neuroimmune alterations, many of which are also reported in patients with major depressive disorder (MDD). Drawing on clinical findings, there has been considerable interest in the role of serotonin in the mechanism of action of OBX. However, to date, there has been no report of direct measurement of serotonergic functioning of bulbectomized animals using microdialysis. The present study describes the effects of olfactory bulbectomy on functioning of the serotonergic system.

METHODS

In vivo microdialysis was performed in conscious rats that underwent OBX or sham surgery. Alterations in the functioning of the serotonergic system were assessed by administration of fluvoxamine, fenfluramine, and 3-hydroxybenzylhydrazine (NSD-1015). Animals were also repeatedly tested in an open field.

RESULTS

Bilateral removal of the olfactory bulbs decreased basal extracellular levels by decreasing the releasable pool of serotonin (5-HT) in the basolateral amygdala 2 weeks after surgery and in the dorsal hippocampus 2 weeks and 5 months after surgery. Olfactory bulbectomized animals showed a lower rate of 5-HT synthesis under basal conditions. However, the capacity of the system to synthesize 5-HT was not affected. Olfactory bulbectomized rats were hyperactive in the open field. This hyperactivity remained after successive testing, indicating permanent behavioral changes.

CONCLUSIONS

This microdialysis study shows that OBX has profound and long-lasting effects on serotonergic functioning and on activity levels and is therefore considered an intriguing and promising animal model for affective processes in the brain.

Developmental exposure of rats to chlorpyrifos leads to behavioral alterations in adulthood, involving serotonergic mechanisms and resembling animal models of depression

Developmental exposure to chlorpyrifos (CPF) causes persistent changes in serotonergic (5HT) systems. We administered 1 mg/kg/day CPF to rats on postnatal days 1-4, a regimen below the threshold for systemic toxicity. When tested in adulthood, CPF-exposed animals showed abnormalities in behavioral tests that involve 5HT mechanisms. In the elevated plus maze, males treated with CPF spent more time in the open arms, an effect seen with 5HT deficiencies in animal models of depression. Similarly, in an anhedonia test, the CPF-exposed group showed a decreased preference for chocolate milk versus water. Developmental CPF exposure also has lasting effects on cognitive function. We replicated our earlier finding that developmental CPF exposure ablates the normal sex differences in 16-arm radial maze learning and memory: during acquisition training, control male rats typically perform more accurately than do control females, but CPF treatment eliminated this normal sex difference. Females exposed to CPF showed a reduction in working and reference memory errors down to the rate of control males. Conversely, CPF-exposed males exhibited an increase in working and reference memory errors. After radial-arm acquisition training, we assessed the role of 5HT by challenging the animals with the 5HT2 receptor antagonist ketanserin. Ketanserin did not affect performance in controls but elicited dose-dependent increases in working and reference memory errors in the CPF group, indicating an abnormal dependence on 5HT systems. Our results indicate that neonatal CPF exposures, classically thought to be subtoxic, produce lasting changes in 5HT-related behaviors that resemble animal models of depression.

Subsyndromal depression in the elderly: underdiagnosed and undertreated

Major depressive illness is present in about 5.7% of US residents aged>or=65 years, whereas clinically significant nonmajor or "subsyndromal" depression affects approximately 15% of the ambulatory elderly. Risk of developing subsyndromal depression increases as elderly people get older. Because they have numerous distressing ailments, everyday life can be burdensome for many elderly persons. Almost one third of Americans aged 75 years or older rate their health as "fair to poor." Yet, the physical discomforts experienced by so many elderly individuals are unlikely to generate a clinically significant depression unless other ingredients such as loneliness, impairment of mobility, loss of a spouse, a serious financial reverse, and--probably most important--genetic susceptibility are added to the psychophysiological mix. Because depression damages quality of life and is usually eminently treatable, it is essential that physicians and other health professionals be trained to recognize true depression and distinguish it from confounding conditions caused by medications, organic brain disease, or short-term grief reactions. In the medically ill elderly, depressive symptoms may be overlooked because of the assumption that they are a part of the concurrent medical illness. Diagnosis of depression in the elderly can be greatly assisted by use of age-specific screening instruments such as the Geriatric Depression Scale. Ultimately, brain imaging and biochemical and physiological measurements may prove useful in diagnosis. The presence of somatic concomitants of depression such as severe neck and low back pain should alert the clinician to the possibility of an underlying mood disorder. Suicide and suicide attempts occur all too frequently in the depressed elderly; therefore, screening for late-life depression is urgently required among the elderly in primary and residential health care settings.

Single and repeated stress-induced modulation of phospholipase C catalytic activity and expression: role in LH behavior

PI-PLC, a critical enzyme of the phosphoinositide (PI) signaling pathway, mediates many physiological functions in the brain, including cellular plasticity. Stress-induced learned helplessness (LH) in animals serves as a model of behavioral depression. Recently, we observed that repeated stress prolongs the duration of LH behavior in rats, enabling us to compare neurobiologic abnormalities in acute and chronic depression. Here we examine whether LH behavior is associated with alterations in phospholipase C (PLC), and whether repetition of inescapable shock has similar or dissimilar effects on PLC to those of the single-stress paradigm. Rats were exposed to inescapable shock either once on day 1, or twice, on days 1 and 7. Rats were tested for escape latency on days 2 and 4 after day 1 inescapable shock or on days 2, 8, and 14 after day 1 and 7 inescapable shock. PI-PLC activity and mRNA and protein expression of three different PLC isozymes were determined in the frontal cortex and hippocampus. Higher escape latencies were observed in LH rats tested on day 2 after single inescapable shock and on day 14 after repeated inescapable shock. Single inescapable shock reduced PI-PLC activity in the frontal cortex and hippocampus of LH rats. On the other hand, repeated inescapable shock not only reduced PI-PLC activity in these brain areas of LH rats but also selectively decreased the expression of PLC beta1 and PLC gamma1 isozymes. Our results suggest different responsiveness at the level of PI-PLC after single vs repeated stress, and that reductions in PLC may be critical in the pathophysiology of depression and other stress-related disorders.

Social stress in tree shrews as an animal model of depression: an example of a behavioral model of a CNS disorder

Animal models are invaluable in preclinical research on human psychopathology. Valid animal models to study the pathophysiology of depression and specific biological and behavioral responses to antidepressant drug treatments are of prime interest. In order to improve our knowledge of the causal mechanisms of stress-related disorders such as depression, we need animal models that mirror the situation seen in patients. One promising model is the chronic psychosocial stress paradigm in male tree shrews. Coexistence of two males in visual and olfactory contact leads to a stable dominant/subordinate relationship, with the subordinates showing obvious changes in behavioral, neuroendocrine, and central nervous activity that are similar to the signs and symptoms observed during episodes of depression in patients. To discover whether this model, besides its "face validity" for depression, also has "predictive validity," we treated subordinate animals with the tricyclic antidepressant clomipramine and found a time-dependent recovery of both endocrine function and normal behavior. In contrast, the anxiolytic diazepam was ineffective. Chronic psychosocial stress in male tree shrews significantly decreased hippocampal volume and the proliferation rate of the granule precursor cells in the dentate gyrus. These stress-induced changes can be prevented by treating the animals with clomipramine, tianeptine, or the selective neurokinin receptor antagonist L-760,735. In addition to its apparent face and predictive validity, the tree shrew model also has a "molecular validity" due to the degradation routes of psychotropic compounds and gene sequences of receptors are very similar to those in humans. Although further research is required to validate this model fully, it provides an adequate and interesting non-rodent experimental paradigm for preclinical research on depression.

Differential regulation of serotonin (5HT)2A receptor mRNA and protein levels after single and repeated stress in rat brain: role in learned helplessness behavior

Stress-induced learned helplessness in animals serves as a model of behavioral depression and other stress-related disorders. Our recent report that repeated stress prolongs the duration of learned helplessness behavior in rats may be important since acute and recurrent disorders may have different responsive mechanisms. To examine the role of serotonergic (5HT) mechanisms in such behavior, we studied the expression of 5HT2A receptors in different brain areas of rats, and further investigated whether the alterations in expression of 5HT2A receptors are similar after single versus repeated stress. Rats exposed to inescapable shock once on day 1, or twice, on day 1 and day 7, were tested for escape latency on days 2 and 4, or day 14, respectively. Higher escape latencies were observed on day 2 after single, and on day 14 after repeated shock. Whereas the single-stress paradigm produced a significant decrease of 5HT2A receptor mRNA and protein expression in hippocampus of non-learned helpless and learned helpless rats as compared with tested controls, repeated stress resulted in increase in frontal cortex but decrease in hippocampus and hypothalamus of learned helpless rats only, as compared with tested control rats. These results demonstrate differential regulation of 5HT2A receptors in LH rats after single and repeated stress, which may be critical in the pathophysiology of depression/other stress-related disorders.

Region-specific dysregulation of allopregnanolone brain content in the olfactory bulbectomized rat model of depression

Allopregnanolone (ALLO) is one of the most potent positive endogenous allosteric modulators of the type A gamma-aminobutyric acid (GABA(A)) receptors. While the robust anxiolytic profile of ALLO has been extensively characterized in rodents and its antidepressant-like effect was recently demonstrated in mice, there have been only few reports on alterations of brain ALLO levels in putative animal models of depression and anxiety. Removal of the olfactory bulbs of rats produces one of the most predictive animal models with which to screen for drugs with potential antidepressant activity following repeated treatment. We therefore investigated whether the olfactory bulbectomized (OB) rat model of depression may be associated with alterations of ALLO levels in whole brain tissue and in different brain regions. We determined ALLO levels in whole brain, amygdala, frontal cortex, hippocampus, and whole cerebral cortex of OB or sham-operated rats at 7, 14, or 28 days following bulbectomy or sham surgery. We observed a significant increase of whole brain ALLO content at 7 and 28 days post-surgery in the OB rats. At days 7 and 14 following olfactory bulb removal, ALLO levels were significantly decreased in amygdala and frontal cortex and significantly increased in whole cerebral cortex. In the hippocampus we observed only a tendency for decreased ALLO levels at day 14. Our data indicates a strong region-specific dysregulation of ALLO homeostasis in brains of OB rats which may contribute to the formation of the bulbectomy syndrome via a sustained reduction in physiological GABA-ergic tone in amygdala and frontal cortex.

Effects of imipramine on T cell subsets in olfactory bulbectomized mice

To elucidate the immunological outcome in antidepressant therapy, the effects of acute or chronic imipramine (IMP) treatment on T cell subsets were examined in sham-operated (SO) and olfactory bulbectomized (OB) mice. Olfactory bulbectomy decreased the ratio of Lyt2-positive suppressor T cells to L3T4-positive T helper cells. Acute IMP treatment did not exert any effect on the ratio in SO nor in OB mice. Chronic IMP administration was found to significantly increase the ratio in SO and OB mice and, as a result, the ratio was significantly higher in SO mice compared with the control, while the ratio was normalized in OB mice. The present study may be suggestive of the immune activation in depression and of the immunosuppressive effects of antidepressants.

Multiple MPEP administrations evoke anxiolytic- and antidepressant-like effects in rats

Several lines of evidence suggest a crucial involvement of glutamate in the mechanism of action of anxiolytic and antidepressant drugs. The involvement of group I mGlu receptors in anxiety and depression has also been proposed. In view of the recent discovery of anxiolytic- or antidepressant-like effects of acute injections of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective and brain penetrable mGlu5 receptor antagonist, we designed the present study to examine anxiolytic- and/or antidepressant-like effects of multiple administrations of this drug. The anxiolytic-like effects of MPEP were evaluated in rats using the conflict drinking test. The antidepressant-like effect was estimated using the rat olfactory bulbectomy model of depression. Seven subsequent injections of MPEP (1 mg/kg) significantly (by 320%) increased the number of shocks accepted during the experimental session in the Vogel test. MPEP given once daily at a dose of 10 mg/kg, restored the learning deficit of bulbectomized rats after 14 days of treatment, remaining without any effect in the sham-operated animals. N-methyl-D-aspartic acid (NMDA)-induced convulsions in mice were not affected by a single injection of MPEP (30 mg/kg) indicating that at this dose MPEP did not block NMDA receptors. The results indicate that the prolonged blockade of mGlu5 receptors exerts anxiolytic- and antidepressant-like effects in rats. No tolerance to anxiolytic-like action occurs. The previously mentioned results further indicate that antagonists of group I mGlu receptors may play a role in the therapy of both anxiety and depression.

Effects of bilateral olfactory bulbectomy on N-methyl-D-aspartate receptor function: autoradiographic and behavioral studies in the rat

Rat bilateral olfactory bulbectomy (OBX) serves as a useful model in the study of depression and the mechanisms of action of antidepressant treatments. Considering the evidence of NMDA receptors involvement in depression, the present study was undertaken in order to investigate the time-course effects of OBX on the NMDA receptor function. Following bilateral olfactory bulbectomy, rats display an increase in locomotor activity and changes in other types of behavior in a novel environment. Autoradiographic experiments using the noncompetitive NMDA antagonist [(125)I]-iodo-MK-801 as the labeling agent showed that this increase in behavioral activities corresponds to a decrease in [(125)I]-iodo-MK-801 binding in a number of brain regions. In most regions, this reduction reached significance by the third week following OBX. However, in some cortical areas-a nucleus of the thalamus (AV) and one of the amygdala (LA)-this reduction was already significant in the first or second week following OBX and lasted throughout the 4 weeks of the study. We also compared the behavioral modifications induced by a challenge injection of MK-801 (0.2 mg/kg i.p.) in OBX and sham-operated rats. This challenge is known to induce hyperlocomotion and a number of stereotypies in naive rats. These effects were drastically reduced in OBX as compared to sham-operated rats. These data are consistent with the above-mentioned decrease in cerebral binding of MK-801 to NMDA receptors.

Alterations in serotonin transporter expression in brain regions of rats exposed neonatally to chlorpyrifos

Chlorpyrifos (CPF), one of the most widely-used organophosphate pesticides, is a suspected neuroteratogen. We administered CPF to neonatal rats on postnatal days (PN) 1-4 (1 mg/kg) or PN11-14 (5 mg/kg), treatments devoid of overt toxicity. At the end of the treatment period (PN5 and 15, respectively) and 5-7 days later, we then examined the effects on paroxetine (PXT) binding to the presynaptic 5HT high-affinity transporter, a marker for serotonin (5HT) projections. In males, we found a persistent decrease in PXT binding across the two different treatment regimens, with deficits apparent in a brain region containing 5HT terminal fields (forebrain) as well as in a region containing 5HT cell bodies (brainstem). In contrast, females given the early treatment regimen (PN1-4) showed deficits in the brainstem but transient elevations in the forebrain; the later treatment regimen (PN11-14) had no significant effect on PXT binding in females. These data are consistent with earlier work showing brainstem cell injury resulting from neonatal CPF exposure, and indicate specific damage to 5HT neurons, with a consequent loss of transporter expression in both terminal fields and perikarya. In females, the damage may be temporarily offset by initial trophic effects in the terminal region, consequent to the cholinergic stimulation evoked by cholinesterase inhibition via the active metabolite, CPF oxon. The gender-selective effects on 5HT systems are likely to contribute to similar gender dimorphism in behavioral performance. Because the CPF effects involve 5HT, a neurotransmitter intimately involved in the control of mood, we suggest the need to evaluate behaviors that typify animal models of depression.

Response dynamics of entorhinal cortex in awake, anesthetized, and bulbotomized rats

The generation of oscillatory activity may be crucial to brain function. The coordination of individual neurons into rhythmic and coherently active populations is thought to result from interactions between excitatory and inhibitory cells mediated by local feedback connections. By using extracellular recording wires and silicon microprobes to measure electrically evoked damped oscillatory responses at the level of neural populations in the entorhinal cortex, and by using current-source density analysis to determine the spatial pattern of evoked responses, we show that the propagation of activity through the cortical circuit and consequent oscillations in the local field potential are dependent upon background neural activity. Pharmacological manipulations as well as surgical disconnection of the olfactory bulb serve to quell the background excitatory input incident to entorhinal cortex, resulting in evoked responses without characteristic oscillations and showing no signs of polysynaptic feedback. Electrical stimulation at 200 Hz applied to the lateral olfactory tract provides a substitute for the normal background activity emanating from the bulb and enables the generation of oscillatory responses once again. We conclude that a non-zero background level of activity is necessary and sufficient to sustain normal oscillatory responses and polysynaptic transmission through the entorhinal cortex.

Social stress in mice: gender differences and effects of estrous cycle and social dominance

A large discrepancy in the possibility of inducing social stress in the two genders exists. Since generalizations of findings from one sex to the other appear not to be valid, reliable models of social stress in females are needed. We examined the effects of social context in the housing environment, as a possible source of stress, on exploration and anxiety in male and female mice, taking into account the estrous phase for females and the social status for males as additional variables. Mice housed individually or with siblings were tested in a free-exploratory paradigm of anxiety (where test animals have a choice to stay in their home cage or to explore an open field, OF). Individually housed females did not leave their home cage for long periods, explored less the unfamiliar area and displayed higher risk assessment, a behavioral profile suggestive of lower propensity for exploration and higher level of anxiety compared with group-housed females. Individually housed males tended to show an opposite profile. Proestrus mice were less sensitive to the decrease of exploratory propensity induced by individually housing compared to estrus and diestrus mice. Social dominants and social subordinates in sibling groups did not differ in their exploratory responses to the OF. Different housing procedures, as means to provide different social environment, may differentially induce mild social stress in male and female mice.

Effects of sertraline on regional neuropeptide concentrations in olfactory bulbectomized rats

Corticotropin-releasing factor (CRF) and thyrotropin-releasing hormone (TRH) are two neuropeptides that exhibit increased cerebrospinal fluid (CSF) concentrations during major depressive episodes while somatostatin (somatotropin-release inhibiting factor, SRIF) is decreased. Clinical and basic research findings indicate that clinically effective antidepressant therapies often normalize the indicators of CRF and TRH hypersecretion as well as SRIF hyposecretion. The olfactory bulbectomized (OBX) rat is used to screen potential antidepressant drugs for clinical efficacy. This model requires chronic administration of the antidepressant drug to normalize OBX-induced behaviors such as increased locomotion in a novel environment. This report describes the regional brain concentration changes in CRF, TRH and SRIF produced by OBX and demonstrates the ability of the selective serotonin re-uptake inhibitor and antidepressant drug, sertraline (10 mg/kg), to normalize certain of these alterations in regional neuropeptide concentrations as well as normalizing OBX-induced increases in locomotor activity. OBX-induced increases in CRF concentrations in the hypothalamus and bed nucleus of the stria terminalis were specifically and significantly decreased by sertraline. OBX-induced increases in TRH concentrations in the hypothalamus were reversed by sertraline. The concentration of SRIF was significantly reduced by OBX in the anterior caudate and the piriform cortex, but sertraline reversed these changes only in the anterior caudate.

How valuable are animal models in defining antidepressant activity?

Animal models of depression have been utilised to screen novel compounds with antidepressant potential although uncertainty lingers concerning their clinical relevance. In order for a model to be considered of any value, it must possess predictive validity (does drug action in the model correspond to that in the clinic?), face validity (are there phenomenological similarities between the model and the clinic?) and construct validity (does the model possess a strong theoretical rationale?). On the one hand, there are models based on stress such as the learned helplessness model, the forced swimming test and the chronic mild stress model and, on the other hand, models based on neuronal deficits such as the olfactory bulbectomy model. To date, among models more frequently used in depression, none of them meet all these criteria. Moreover, improvements to tests are often poorly validated and estimating time of onset of action of antidepressants remains a major challenge in animal model research. Finally, reproducing the tests outside the laboratory of origin continues to be problematic and leads to variability in results. Although animal models of depression fail to be unequivocally valid, they represent the best tool to define potential antidepressant activity of drugs, to investigate their mechanism of action and, to a greater extent, explore this complex heterogeneous illness. Copyright 2001 John Wiley & Sons, Ltd.

An in-vivo magnetic resonance imaging study of the olfactory bulbectomized rat model of depression

The olfactory bulbectomized (OB) rat is a well-accepted animal model of depression. The present magnetic resonance imaging (MRI) investigation demonstrates alterations in signal intensities in cortical, hippocampal, caudate and amygdaloid regions in OB animals, but not in sham operated controls. Ventricular enlargement was also evident in OB animals. These alterations have implications with regard to the face and construct validity of this model.

Gender and gonadal hormone effects in the olfactory bulbectomy animal model of depression

Major depressive disorder (MDD) affects women to a greater extent then men; however, the few studies that have examined the role of gender in an animal model of depression have shown inconsistent results. The purpose of the present study was to determine if the gonadal hormone milieu of the animal modulated behavioral changes following olfactory bulbectomy (OBX), a well-documented animal model of depression. Body weight, sucrose preference levels and open-field activity levels were measured once a week for a period of 2 weeks in gonadally intact and gonadectomized male and female rats. Following these baseline measurements, animals underwent either OBX or sham surgery. Body weight, sucrose preference and activity levels were assessed for 4 weeks post-OBX surgery. OBX-gonadectomized animals exhibited higher activity levels than OBX gonadally intact and control animals. This effect of gonadectomy was more robust in males. OBX-females (both intact and gonadectomized) exhibited significantly lower sucrose preference levels than OBX-males (both intact and gonadectomized) and control animals. These results suggest that the gonadal hormone milieu of the animal plays a role in modulating sucrose preference and activity levels following OBX.

Prolonged c-Jun expression in the basolateral amygdala following bulbectomy: possible implications for antidepressant activity and time of onset

Olfactory bulbectomy is a well established animal model of depression. Neurochemical and behavioral alterations observed following olfactory bulbectomy, are due, in part, to the neurodegeneration of specific brain structures. Amygdaloid dysfunction in particular, is known to play a substantial role in the syndrome of the olfactory bulbectomized rat. The present study examined both short- and long-term alterations in immediate early gene expression, tyrosine hydroxylase and serotonin immunoreactivity, and classical silver staining, following olfactory bulbectomy in the basolateral amygdala. The results indicated no consistent change in Fos expression observed over the experimental period. Following bulbectomy, long term (up to 64 days post-lesion) Jun expression, not coincident with silver staining, was observed in the basolateral nucleus. The basolateral nucleus was also intensely immunoreactive for serotonin at this timepoint post-bulbectomy. Thus, following bulbectomy long term alterations in Jun expression occurs in the serotonin rich basolateral amygdala. As a site of action for antidepressant compounds, alterations at the immediate early gene level in this region may have implications both for the model, and antidepressant drug action therein.