The olfactory bulbectomized rat as a model of depression: an update

Brain lesions that impair vocal imitation in adult budgerigars

Vocal imitation is a complex form of imitative learning that is well developed only in humans, dolphins, and birds. Among birds, only some species are able to imitate sounds in adulthood. Of these, the budgerigar (Melopsittacus undulatus) has been studied in most detail. Previous studies suggested that the vocal motor system in budgerigars receives auditory information from the lateral frontal neostriatum (NFl). In the present study, we confirm this hypothesis by showing that infusions of the GABA agonist muscimol into NFl reduce the strength of auditory responses in a telencephalic vocal motor nucleus, the central nucleus of the lateral neostriatum (NLc). To test whether the auditory information conveyed from NFl to NLc plays a role in vocal imitation, we lesioned parts of NFl and the overlying ventral hyperstriatum (HVl) in seven adult male budgerigars and then examined whether the lesioned males would imitate the calls of females with whom they were paired. We found that, compared to sham-lesioned controls, the lesioned birds were significantly impaired in their imitation of female calls. Yet, the lesioned males were clearly not deaf (e.g., their previously learned calls did not degrade as they do after deafening). Therefore, the data suggest that NFl/HVl lesions impair vocal imitation by reducing the amount of auditory information that reaches the vocal motor system. Interestingly, the females that were paired with lesioned males displayed more vocal plasticity than the females in the control group, and some even imitated their male's prepairing calls.

Intravenous self-administration of amphetamine is increased in a rat model of depression

Affective disorders and substance abuse frequently coexist, yet few previous studies have examined drug self-administration using animal models of depression. The olfactory-bulbectomized rat is a well-established model that exhibits a high degree of neurochemical similarity to depression. Olfactory bulbectomy (OBX) increases dopamine receptor densities in the ventral striatum, which may increase the reinforcing effects of drugs of abuse. Experiments were designed to test the hypotheses that acquisition and stable self-administration of amphetamine would be increased in bulbectomized rats. In the first experiment, rats underwent bilateral OBX or sham surgery and intravenous jugular catheters were implanted 12-14 days later. Acquisition was examined using a standard operant paradigm involving a nose-poke response for a very low dose of D-amphetamine sulfate (12 microg/infusion, IV). A separate group of rats received coinfusions of sulpiride. In a second experiment designed to minimize differences in acquisition and examine stable self-administration, lever pressing for a low (0.10 mg/kg, IV) or high (0.25 mg/kg, IV) dose of D-amphetamine sulfate was measured in rats pretrained to lever press for food. Bulbectomized rats acquired the self-administration of very low dose amphetamine faster than sham-operated rats and this effect was reversed by sulpiride coinfusion. Stable self-administration of the low dose of amphetamine was also markedly increased in bulbectomized rats. The findings reveal the utility of the OBX model for studying the neurobiological basis of depression and drug abuse comorbidity and support the hypothesis that neurochemical abnormalities associated with depression may enhance the addictive properties of some drugs of abuse.

Regulation of neuropeptide Y in the rat amygdala following unilateral olfactory bulbectomy

While the mechanisms are not fully understood, olfactory bulbectomy (OBX) is a well-known rat model of depression and depression-related disorders such as anxiety and aggression. Alterations in neuropeptide Y (NPY) levels in the brain have been linked to depression and have been shown to be involved in the response to stress. This study explored the possible regulation of NPY immunoreactivity in specific regions of the amygdala 14 days after OBX in adult male Sprague-Dawley rats (n=6). Unilateral OBX and immunohistochemistry permitted comparisons of NPY in the ipsilateral amygdala with NPY in the contralateral (sham) amygdala. OBX resulted in significant increases (P<0.05) in NPY immunoreactivity in the anterior medial amygdala (threefold) and the posterior medial amygdala (2.5-fold). These regions receive projections from the accessory olfactory bulb (AOB). In contrast, the anterior and posterolateral cortical nuclei of the amygdala receive projections from the main olfactory bulb (MOB). NPY was not increased in these nuclei. These data show that not only does OBX increase NPY immunoreactivity in the amygdala, but also suggest that the AOB plays a prominent role in this regulation.

Determination of the dose of agomelatine, a melatoninergic agonist and selective 5-HT(2C) antagonist, in the treatment of major depressive disorder: a placebo-controlled dose range study

Agomelatine (S 20098) has a unique and new pharmacological profile. It is a melatoninergic agonist and selective antagonist of 5-HT2C receptors, and has been shown to be active in several animal models of depression. The aim of this study was to determine the active dose of agomelatine in the treatment of major depressive disorder (DSM-IV criteria). The methodology used was a conventional double-blind design comparing three different doses of agomelatine (1, 5 and 25 mg once a day) with placebo over an 8-week treatment period. Paroxetine was used as the study validator. Seven hundred and eleven patients with a baseline mean score of 27.4 on the 17-item Hamilton Rating Scale for Depression (HAM-D) were included. On the pivotal analysis, the mean final HAM-D total score (Full Analysis Set LOCF) demonstrated agomelatine 25 mg to be statistically more effective than placebo. This was confirmed by other analyses and criteria (responders, remission, subpopulation of severely depressed patients, Montgomery-Asberg Depression Rating Scale, Clinical Global Impression-Severity of Illness). Agomelatine 25 mg alleviated the anxiety associated with depression, as measured on Hamilton Anxiety Scale. Paroxetine was found to be effective on pivotal analysis and most of the secondary criteria used to validate the study methodology and population. Agomelatine, whatever the dose, showed good acceptability with a side-effects profile close to that of placebo. In conclusion, this study demonstrates that agomelatine is efficient in the treatment of major depressive disorder and that 25 mg is the target dose.

Stress, norepinephrine and depression

Experimental and clinical evidence implicates stress as a major predisposing factor in depression and other severe psychiatric disorders. In this review, evidence is presented to show how the impact of stress on the central sympathetic system leads to changes in the endocrine, immune and neurotransmitter axes which underlie the main clinical symptoms of depression. Thus it can be shown that the noradrenergic system is dysfunctional in depression, a situation which reflects the chronic hypersecretion of glucocorticoids and inflammatory mediators within the brain in addition to an enhanced activity of the locus ceruleus. With regard to the actions of antidepressants in modulating the stress response and alleviating depression it is now evident that, irrespective of the presumed specificity of the antidepressants for the noradrenergic or serotonergic systems, they all normalize noradrenergic function. This action is due partly to the regulation of tyrosine hydroxylase activity in the locus ceruleus but also enhances neuronal sprouting which counteracts the neurodegenerative effects of chronic stress.

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.

Assessing antidepressant activity in rodents: recent developments and future needs

Animal models are indispensable tools in the search to identify new antidepressant drugs and to provide insights into the neuropathology that underlies the idiopathic disease state of depression. As new targets are developed, both serendipitously and through hypothesis-driven research, existing animal paradigms are being modified and new tests are being developed to detect antidepressant actions of compounds acting on a broad range of neural and genetic targets. This review focuses on recent findings regarding some of the most widely employed animal models used currently to predict antidepressant potential. Emphasis is placed on recent modifications to such paradigms that have increased their utility and reliability. Furthermore, some key issues that need to be addressed for future discovery of novel antidepressant agents are examined, and the available data on genetically altered mice that might lead to the discovery of novel targets for antidepressant action are collated.

Animal models of psychiatric disorder and their validity. From the perspective of behavioral pharmacology

Animal models of psychiatric disorders are indispensable tools to gain insights into neural mechanisms underlying these disorders and to assess potential therapeutic actions of novel compounds in preclinical settings. However, it is difficult to establish appropriate animal models for these disorders because there is no proof on whether or not what occurs in the animal brain is comparable to what occurs in the human brain. The initial development of animal models of psychiatric disorders is often based on "face validity" as reflected in the similarity of behavioral signs and symptoms observed in humans and animal and subsequently based on "construct validity" as measured by strong correlation in behaviors and neural events between the animal model and patients of the disorder. The practical value of such animal models is reflected ultimately in their "predictive validity" in predicting the therapeutic efficacy of new treatments for psychiatric disorders. The present review will focus on animal models of schizophrenia and depression by the use of mainly environmental stress and pharmacological treatments, which are expected to induce signs and symptoms analogous to those of patients with such disorders.

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment

Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.

Novel treatments for anxiety and depression: hurdles in bringing them to the market

Anxiety and depressive disorders are the most common psychiatric conditions. The medical need for newer, better-tolerated and more efficacious treatments remains high. However, drug development is time-consuming and has a high rate of failed or inconclusive trials. Improvements in study design, investigator training and early proof-of-concept studies are being discussed as means to decrease failure rates and the duration of development. So far, no uniformly applicable 'magic formulas' for success have been discovered. The most promising approach to overcome these hurdles appears to be a sound study design carried out by experienced professionals in the clinic and in industry.

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.

Reduced olfactory performance in patients with major depression

The aim of the present study is to investigate olfactory sensitivity and odor evaluations in a homogeneous sample of unipolar depressive patients using pure olfactory odors. Twenty-four in-patients with major depressive disorder (MDD) were investigated during their acute depressive phase. Eighteen of them participated a second time after successful treatment. A group of healthy subjects, matched by age, sex, and smoking behavior, served as a control. Olfactory sensitivity, as measured by threshold tests, was strongly reduced in patients with severe depression. Additional correlative analyses revealed that the lowered sensitivity could partly be predicted by high depression scores. After successful medical treatment, these sensitivity differences were reduced and did not reach the significance level. The subjective odor evaluations (valence and intensity ratings) were not markedly changed in general. The results reveal that olfactory performance in MDD patients is reduced at an early perceptional level of stimulus processing. It is discussed whether this effect can be attributed to the close functional connection between the main olfactory bulb and the amygdala.

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.

Changes in defensive behaviors following olfactory bulbectomy in male and female rats

The present study examined if olfactory bulbectomy (OBX) altered defensive behaviors on the elevated plus-maze and the open-field differently in male and female rats. Similar increases in defensive behaviors in male and female rats were observed in both tests following OBX. No significant correlations were detected between defensive behaviors and activity, supporting the hypothesis that some behavioral changes following OBX may be due to decreased defensive behaviors and not increased activity.

Changes in the immune system in depression and dementia: causal or co-incidental effects?

It is now widely accepted that psychological stress and psychiatric illness can compromise immune function. Furthermore the mechanisms whereby such changes occur are probably associated with the activities of the cytokines and other inflammatory mediators of the immune system which are known to initiate changes in behaviour. This review aims to summarise the experimental and clinical evidence that implicates the pro-inflammatory cytokines in the pathological changes seen in major depression and in Alzheimer's disease (AD). In major depression, evidence is provided to show that both activation (e.g., macrophage activity, acute phase proteins) and inhibition (e.g., natural killer cell activity) of the immune system occur. Many of the behavioural changes seen in depression are simulated by three pro-inflammatory cytokines (IL-1, IL-6 and TNF-alpha), which may produce their impact on the brain by activating cyclooxygenase, nitric acid synthase and corticotrophin releasing factor. Effective antidepressant treatments largely attenuate the immune changes thereby raising the possibility that the normalisation of central biogenic amine function that are conventionally implicated in the cause of depression may be secondary to those of the pro-inflammatory cytokines. With respect to AD, while the cause(s) are unknown, there is both experimental and clinical evidence to suggest that inflammatory processes in the brain caused in particular by TNF-alpha together with the subsequent rise in free radicals, are instrumental in causing the pathological changes which underlie the disease. Evidence in favour of the inflammatory hypothesis is supported by the finding that nonsteroidal anti-inflammatory drugs slow down the progression of the disease.Although, more research is needed into the inter-relationships between the various pro-inflammatory cytokines and the behavioural changes invoked in major depression and AD, the immunological hypothesis has been important in stimulating new concepts regarding the causes of the pathological changes in these diseases and how effective drug treatments may attenuate them.

Effects of olfactory bulbectomy on NMDA receptor density in the rat brain:

Olfactory bulbectomy (OBX) transects the glutamatergic efferents from the olfactory bulbs, and the changes of glutamatergic N-methyl-D-aspartate (NMDA) receptor-mediated function are though to be involved in the behavioral deficits seen in OBX rats. In the present study, irritability scores in OBX male Wistar rats were correlated with discrete regional effects on NMDA receptor function measured using a [3H] MK-801 binding assay. Irritability scores, measured before and for 2 weeks after OBX, showed a gradual increase in irritability after OBX. A reduction of the NMDA receptor density was observed in the cerebral cortex and amygdala 16 days after OBX, but not in the striatum, olfactory tubercle, entorhinal cortex, and hippocampus. These results demonstrate that OBX causes changes in the NMDA receptor system in certain brain regions and suggest that these changes may be responsible for the behavioral deficits of OBX rats.

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.

Platelet glutamate receptor supersensitivity in major depressive disorder

Dysregulation of glutamate has been described in depression, and supersensitivity of platelet glutamate receptors has been found in both psychotic major depression and schizophrenia. The aim of this study was to examine the platelet glutamate receptor sensitivity in patients with nonpsychotic, unipolar major depression to assess whether this is a marker of depression or of psychosis. Glutamate receptor sensitivity was assessed using the platelet intracellular calcium response to glutamate (0-100 micromol) measured by spectrofluorometry. The depression group showed a significantly greater platelet intracellular calcium response to glutamate stimulation than the control group, both in terms of absolute values (p = 0.007) and percentage of response from baseline (p = 0.030). These data suggest that platelet glutamate receptors may be supersensitive in depression and that the platelet may be a possible peripheral marker of glutamate function in depression.

Glutamate and its role in psychiatric illness

Glutamate, a dicarboxylic amino acid, is the most abundantly active neurotransmitter in the mammalian brain; it is also the principal excitatory neurotransmitter in the cerebral cortex. As our knowledge of this neurotransmitter deepens, it is increasingly being implicated in the pathophysiology of mental illness. This review begins by examining the physiology of glutamate and its receptors. Its role in memory, movement, perception and neuronal development is discussed. The development of the glutamate hypothesis of schizophrenia is traced, and the emerging lines of evidence for attenuated function of the N-methyl-D-aspartate receptor in schizophrenia are examined. For ease of discussion, these are divided into pharmacological, post-mortem, imaging, platelet and genetic studies. Interactions between glutamate and other neurotransmitters are discussed, as are possible mechanisms by which such altered receptor activity might result in the clinical expression of schizophrenia. The possible role of glutamate in major depression and bipolar disorder is explored. The review concludes by highlighting the importance of avoiding a reductionist approach to the pathophysiology of any mental illness. Copyright 2001 John Wiley & Sons, Ltd.

Antidepressant activity and calcium signaling cascades

Although antidepressant treatments produce clear effects on monoaminergic neuronal function, the link between these effects and therapeutic response to treatment is controversial. Previous studies have demonstrated that antagonists of the NMDA receptor-gated calcium ionophore result in antidepressant-like responses in rodents and humans. Likewise, antidepressant treatments produce regionally selective adaptation of the NMDA receptor suggestive of diminished capacity to gate calcium into receptive neurons. Similarly, voltage-dependent calcium channel antagonists have been reported to produce antidepressant-like effects in rodents. A major target of increases in subcellular calcium concentration is nitric oxide synthase (NOS) which liberates NO in response to stimulation. Recently, we have demonstrated that nitric oxide synthase antagonists produced antidepressant-like response in both in vivo preclinical screening procedures and in post-mortem in vitro studies of beta-adrenoceptor density. We propose: 1) that interruption of the Ca(2+)-calmodulin-NOS-guanylyl cyclase subcellular signaling pathway at any point will produce antidepressant-like effects; 2) that the acute actions of antidepressants in preclinical screening procedures are a consequence of their ability to disrupt Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling; 3) that chronic but, not acute treatment with antidepressants results in adaptation of the Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling pathway; 4) that this adaptation is necessary for the achievement of the therapeutic actions of antidepressants and; 5) that major depression is accompanied by an alteration (hyperactivity?) of subcellular Ca(2+) signaling. Copyright 2001 John Wiley & Sons, Ltd.

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.

Regional differences in brain monoamine oxidase subtypes in an animal model of geriatric depression: effects of olfactory bulbectomy in young versus aged rats

Geriatric depression is often associated dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis, and with poor responsiveness to antidepressants that work through inhibition of monoamine reuptake; accordingly, it has been suggested that MAO inhibitors may represent a therapeutic alternative in this group. In the current study, we evaluated expression of MAO subtypes in brain regions of young and aged rats subjected to olfactory bulbectomy (OBX), a procedure that reproduces many of the biochemical and functional changes associated with human depression. Activities of both MAO A and B were elevated in aged rats as compared to young rats in most regions, but not in the midbrain, and the OBX lesion failed to produce any change in this pattern. These results stand in contrast to the differential effects of glucocorticoids, which reduce brain MAO in young animals but induce activity in aged rats. Our results support the view that the aged brain possesses biochemical characteristics that distinguish its monoamine biochemistry from that of young brain, and that these distinctions may work in conjunction with HPA axis dysregulation to influence the etiology and therapy of geriatric depression. The use of appropriate animal models for depression and for disruption of HPA axis function can allow for the testing of potential human biomarkers (such as platelet MAO) that may serve to predict treatment outcome.

Olfactory bulbectomy increases met-enkephalin- and neuropeptide-Y-like immunoreactivity in rat limbic structures

Bilateral olfactory bulbectomy (OBX) in rats produces a well-characterized syndrome of behavioral, physiological, and neurochemical changes identical to those seen in depression. Previous experiments using in situ hybridization histochemistry have demonstrated that OBX increases prepro-neuropeptide-Y (NPY) and prepro-enkephalin (ENK) mRNA levels in limbic structures. The present experiments determined whether increases in peptide immunoreactivity occur in conjunction with increases in mRNA levels following OBX. In situ hybridization analyses in olfactory bulbectomized and sham-operated rats revealed increased prepro-ENK mRNA in the piriform cortex (PIR) and olfactory tubercles (OTs) of bulbectomized rats. Prepro-NPY mRNA levels were significantly increased in the PIR of bulbectomized rats as comapred to controls. Radioimmunoassays (RIAs) revealed significant elevations in ENK-like immunoreactivity in the OTs following OBX. NPY-like immunoreactivity was significantly elevated in the PIR following OBX. These data reveal that OBX-induced increases in ENK-like immunoreactivity occur concomitantly with increases in prepro-ENK mRNA, and NPY-like immunoreactivity occur concomitantly with increases in prepro-NPY mRNA.

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.

The specificity of platelet glutamate receptor supersensitivity in psychotic disorders

Hypoglutamatergic function is implicated in the pathogenesis of schizophrenia, and supersensitivity of platelet NMDA receptors has been reported in schizophrenia. The aim of this study was to examine the platelet glutamate receptor sensitivity in patients with schizophrenia (n=12), mania with psychotic features (n=10) and depression with psychotic features (n=10) and matched controls (n=12) in order to assess if this is a marker of schizophrenia or occurs in other psychotic conditions. Glutamate receptor sensitivity was assessed using the intracellular calcium response to glutamate measured with spectrofluorometry. The percentage response of the schizophrenic and depressed psychotic subjects to glutamate stimulation was significantly greater than control subjects (p<0.005). The mania with psychotic features group was not significantly different to controls. This data suggests that platelet glutamate receptors may be supersensitive in schizophrenia and depression with psychotic features. Furthermore, the platelet may be a possible peripheral marker of glutamate function in schizophrenia and depression with psychotic features.

Olfactory bulbectomy alters NMDA receptor levels in the rat prefrontal cortex

Olfactory bulbectomized (OBX) rats show a variety of behavioral and biochemical deficits that parallel human depression. We investigated the expression of glutamate receptor subtypes in cortical and subcortical brain regions following bilateral olfactory bulbectomy in adult rats. Quantitative receptor autoradiography using [(125)I]MK-801 (NMDA receptor), [(3)H]AMPA (AMPA receptor), and [(3)H]kainate (kainate receptor) was performed on brain sections at 1-5 weeks following olfactory bulbectomy. Our results show an elevation of NMDA receptors in the medial prefrontal cortex within 1 week following bulbectomy, which persisted up to at least 5 weeks post-bulbectomy. Neither kainate nor AMPA receptors were altered in any brain region examined. The potential significance of these results is discussed in light of experimental findings supporting a role for NMDA receptors in the mechanism of action of antidepressant drugs and the pathophysiology of major depression.

Reboxetine: a pharmacologically potent, selective, and specific norepinephrine reuptake inhibitor

BACKGROUND

Reboxetine is a potent antidepressant, with efficacy comparable to that of imipramine, desipramine, and fluoxetine, and has improved side-effect profile. The basis of its efficacy and improved tolerability is sought through studies of reboxetine in a number of pharmacological models of depression.

METHODS

Pharmacological selectivity for uptake systems was defined by uptake and binding assays for the three monoamine uptake sites. Specificity was determined in 39 different receptor and 6 enzyme assays. In vivo selectivity was defined by measurement of neuronal firing rates in the locus coeruleus, dorsal raphe, and substantia nigra. Reserpine-induced blepharospasm and hypothermia, clonidine-induced hypothermia, defined reboxetine's in vivo pharmacology. Reboxetine's antidepressant potential was evaluated behaviorally by the tail-suspension test, forced swimming, and the DRL72 operant responding test.

RESULTS

Reboxetine is a potent, selective, and specific norepinephrine reuptake inhibitor (selective NRI) as determined by both in vitro and in vivo measurements. Unlike desipramine or imipramine, reboxetine has weak affinity (Ki > 1,000 nmol/L)for muscarinic, histaminergic H1, adrenergic alpha1, and dopaminergic D2 receptors. In vivo action of reboxetine is entirely consistent with the pharmacological action of an antidepressant with preferential action at the norepinephrine reuptake site. Reboxetine showed an antidepressant profile in all tests of antidepressant activity used. Significant decreases in immobility were observed in the tail suspension test and behavioral despair test. Increased efficiency in responding was observed in the DRL72 test.

CONCLUSIONS

Reboxetine is a potent, selective, and specific noradrenergic reuptake inhibitor. It has a superior pharmacological selectivity to existing tricyclic antidepressants and selective serotonin reuptake inhibitors when tested in a large number of in vitro and in vivo systems. Given the pharmacological profile, reboxetine is expected to be a selective and potent tool for psychopharmacological research. The use of reboxetine in the clinic will also help clarify the role norepinephrine plays in depression.

Animal models of depression: utility for transgenic research

The utility of available animal models of depression for transgenic research is reviewed. Criteria for usefulness are non-dependence on a mechanism of action, pharmacological validity, existence of genetic determinants, availability of a mouse version, procedural simplicity, and reproducibility. The following models are reviewed: behavioral despair, tail suspension, learned helplessness, chronic mild stress, olfactory bulbectomy, DRL behavior and conditioned place preference. It is concluded that the behavioral despair and tail suspension models satisfy the criteria most closely. On the other hand, despite its procedural complexity and poor reproducibility, the chronic mild stress model shows high promise for the future.

Geruchs- und emotionale Reiz-Bewertung bei Patienten mit Major Depression

Zusammenfassung. Die Untersuchung der affektiven Reaktion auf olfaktorische und visuelle Reize bei Patienten mit Major Depression (MD) hatte zum Ziel, aufzuklären, ob eine abweichende Reizbewertung bei MD-Patienten modalitätsspezifisch ist und/oder von der Reiz-Valenz abhängig ist. Es wurden 26 MD-Patienten untersucht und mit einer nach Alter und Geschlecht parallelisierten Kontroll- Stichprobe verglichen. Die Teilnehmer hatten zur Aufgabe, ihre emotionale Reaktion auf 10 Gerüche und 20 Bilder aus dem International Affective Picture System (IAPS) auf drei Dimensionen (Valenz, Arousal, Dominanz) zu beschreiben. Alle Reize wurden entsprechend ihrer Valenz in positive, neutrale und negative klassifiziert. Depressive Patienten reagierten mit stärkerer Erregung auf negative Reize als die nicht-depressiven Kontrollen. Dieser Effekt war unabhängig von der Reizmodalität. Außerdem fühlten sich MD-Patienten bei beiden Reizqualitäten und unabhängig von der Valenz-Klasse submissiver als die Kontrollen. Auf der Valenz-Dimension ergaben sich keine Gruppen-Unterschiede.

Role of aversively motivated behavior in the olfactory bulbectomy syndrome

The aim of the present studies was to determine the extent to which changes in defensive behaviors could account for some of the behavioral effects of bilateral olfactory bulbectomy (OBX) in rats. Four tests of aversively-motivated behavior were conducted in bulbectomized and sham-operated rats: activity in a dimly lit or brightly lit open field, passive avoidance, foot shock-induced freezing, and defensive withdrawal. OBX reduced the duration of immobility in the open field. Bulbectomized rats exhibited less freezing in response to foot shock than sham-operated rats. In the defensive-withdrawal test, bulbectomized rats made more transitions into and spent less time inside the covered enclosure than sham-operated rats. The experiments thus reveal two novel paradigms for assessing the behavioral effects of OBX. The results also suggest that deficits in aversively-motivated behavior, specifically defensive freezing, may comprehensively explain the putative "hyperactivity" and "passive-avoidance learning deficits" widely associated with the OBX behavioral syndrome.

An investigation of serotonergic involvement in the regulation of ACTH and corticosterone in the olfactory bulbectomized rat

The bilateral olfactory bulbectomy resulted in significantly higher plasma concentration of corticosterone, but not of ACTH in basal conditions and much higher plasma ACTH and corticosterone concentrations after 15 min of immobilization stress than were observed in sham-operated animals. Daily treatment with fluoxetine-a specific serotonin reuptake inhibitor-(15 mg/kg/day) had no effect on basal ACTH and corticosterone concentrations in OB rats. Fluoxetine treatment caused lower levels of ACTH, but not of corticosterone secretion, in response to immobilization stress. Bulbectomy significantly reducing 5-HT concentration in the amygdala. Stress increased serotonergic activity in the hypothalamus but not in the amygdala of OB rats. Chronic fluoxetine treatment of both unstressed and stressed OB rats resulted in a lower turnover rate in the two structures. Our results suggest that the hypercorticosteronemia observed after bulbectomy in unstressed OB rats is independent of the serotonergic system in both hypothalamus and amygdala. In contrast, they also demonstrate hypothalamic 5-HT changes in the HPA hyperactivity of OB rats in response to stress. Chronic fluoxetine treatment may normalize pituitary ACTH secretion in response to stress, possibly desensitization of the 5-HT receptors in the hypothalamus due to 5-HT being move available at the synapses.

Onset of the effects of the 5-HT1A antagonist, WAY-100635, alone, and in combination with paroxetine, on olfactory bulbectomy and 8-OH-DPAT-induced changes in the rat

5-HT1A receptor antagonists have recently been shown to accelerate the effects of some antidepressant drugs in clinical trials. In this study we investigate the effects of combining a full antagonist at the 5-HT1A receptor, WAY 100635 (0.2 mg/kg, SC) with the selective serotonin reuptake inhibitor (SSRI) paroxetine (5 mg/kg. SC) in the olfactory bulbectomized (OB) rat, an animal model of chronic (but not acute) antidepressant activity. Ambulation scores were measured in the open-field apparatus, following 3, 7, and 14 days of treatment. Further to the OB study, we simultaneously studied adaptive changes in 5-HT1A receptor function, utilizing alterations in the hypothermic response to the 5-HT1A receptor agonist 8-OH-DPAT. Paroxetine, in combination with WAY 100635, attenuated the hypothermic effects of 8-OH-DPAT as early as 3 days, with a full reversal evident following 7 days, whereas paroxetine, although attenuating the hypothermic effects in OB group by day 7, only reversed it fully after 14 days. Paroxetine alone and in combination with the antagonist reversed the olfactory bulbectomy-induced hyperactivity in the open field following 14 days of treatment only, this being the normal time of an "antidepressant" response in this model. However, there was no significant attenuation at any of the earlier time points. This further demonstrates that the reversal of this aspect of the olfactory bulbectomy-induced behavioral syndrome is insensitive to the potential faster onset of antidepressant action induced by 5-HT1A receptor antagonists. Nonetheless, WAY 100635, unlike previous studies with pindolol, did not interfere with the effects of the antidepressant in the model. The ability of the combination group to attenuate the hypothermic effects of 8-OH-DPAT faster than paroxetine alone, further emphasizes the role of the 5-HT1A receptor in the mechanism of action of antidepressants, and as a target for the development of faster acting antidepressants.

Olfactory bulbectomy increases prepro-enkephalin mRNA levels in the ventral striatum in rats

The effects of bilateral olfactory bulbectomy (OBX) on prepro-enkephalin, thyrotropin-releasing hormone, and D-2 receptor mRNA levels in the ventral striatum were examined by in situ hybridization histochemistry. Pre- pro-enkephalin mRNA levels were significantly increased in the olfactory tubercle (OT), but not in the nucleus accumbens, 14 days following bilateral OBX. Levels of D-2 receptor mRNA were also increased in the OT, though to a lesser degree. Prepro-thyrotropin-releasing hormone mRNA was unaffected by OBX. A separate experiment revealed no effect of OBX on enkephalin gene expression 7 days following surgery but a comparable elevation in pre- pro-enkephalin mRNA 14 and 28 days post-surgery. The findings are consistent with previously-reported effects of dopamine lesions on striatal gene expression, suggesting that the observed effects may be mediated by deafferentation-induced alterations in dopaminergic transmission in the OT. Altered dopaminergic function in the OT may be particularly relevant to the 'anhedonia' that has been associated with the olfactory bulbectomized rat model of depression.

Stressor-induced alterations in serotonergic activity in an animal model of depression

The present study examined the effect of two neurogenic stressors (air puff and restraint) and a metabolic stressor (lipopolysaccharide; LPS 100 microg/kg, i.p.) on accumbal serotonergic neurotransmission in the olfactory bulbectomized (OB) rat model of depression. Both air puff and restraint stress caused greater increases in accumbal 5-HIAA in OB than in sham-operated rats. In contrast, bulbectomy resulted in a blunted serotonergic response to a challenge with LPS (a metabolic stressor). In addition, OB rats displayed significantly lower basal levels of 5-HIAA than sham-operated counterparts, a finding consistent with previous reports of the OB rat being a model of hyposerotonergic depression. The relevance of these findings to stressor-provoked depressive-like behaviors in the OB rat are discussed.

Regulation of neuropeptide Y mRNA and peptide concentrations by copper in rat olfactory bulb

Neuropeptide Y is highly abundant in both the peripheral and central nervous systems and is known to have diverse functions including regulation of feeding behavior, blood pressure, circadian rhythms, reproductive behavior and the response to stress. Northern analysis showed that copper deficiency increased brain NPY mRNA abundance particularly in the olfactory bulb (OB). These increases were not accompanied by alterations in food intake or blood pressure. After 4 weeks of a copper-restricted diet, OB copper concentrations decreased to 44% of control and NPY mRNA increased 1.5-fold. Addition of a copper chelator to the restricted diet, resulted in a two-fold increase in OB NPY mRNA over copper adequate controls. These results were confirmed in primary cultures of OB neurons suggesting that the regulation of NPY mRNA is at the level of the bulb rather than by a hormonal or other copper-regulated factor external to the OB. Immunoreactive NPY (IR-NPY) levels were not, however, increased following the 4 weeks of copper deficiency. Addition of the chelator resulted in a 1.4-fold increase in IR-NPY that, while statistically significant, was not proportional to the two-fold increase in NPY mRNA in the same study. This may suggest that copper deficiency inhibits the translational mechanisms responsible for the synthesis of NPY or that NPY is exported from the bulb in copper deficiency.

Combining pindolol and paroxetine in an animal model of chronic antidepressant action--can early onset of action be detected?

The realisation that pindolol may accelerate the effects of some antidepressant drugs in clinical trials has added extra impetus to the search for faster acting antidepressants. Currently, no animal model of depression can identify potential faster acting antidepressant drugs or drug combinations. In this study, we investigate the effects of combining pindolol (2 mg/kg, s.c., bid) with the antidepressant paroxetine (2.5 mg/kg, i.p., bid) in the olfactory bulbectomised rat, an animal model of chronic (but not acute) antidepressant activity. Ambulation scores were measured in separate groups of rats, following 3, 7 and 14 days of treatment. Further, we simultaneously study adaptive changes in 5-HT1A receptor function, utilising alterations in the hypothermic response to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Pindolol in combination with paroxetine attenuated the hypothermic effects of 8-OH-DPAT as early as 3 days with a full reversal evident following 7 days, whereas paroxetine alone did so after 14 days only. Likewise, paroxetine alone reversed the olfactory bulbectomy-induced hyperactivity in the open field following 14 days of treatment only, this being the normal time of an 'antidepressant' response in this model. However, the group treated with both paroxetine and pindolol failed to reverse the hyperactive response. This suggests that a factor intrinsic to pindolol antagonises the behavioural effects of paroxetine in the olfactory bulbectomised rat. It also demonstrates that the reversal of this aspect of the olfactory bulbectomy-induced behavioural syndrome is insensitive to the potential faster onset of antidepressant action induced by pindolol. The ability of the combination group to attenuate the hypothermic effects of 8-OH-DPAT much faster further emphasises the role of the 5-HT1A receptor in the mechanism of action of antidepressants and as a target for the development of faster acting antidepressants. However, an animal model sensitive to the effects of any such compound and the actions of pindolol remains elusive.