Animal models and human depressive disorders

Evaluation of depression in rats exposed to chronic (unpredictable) electric shock

The present study was undertaken to evaluate the applicability of a proposed behavioral stress paradigm as an animal model for depression. Rats were trained to press a lever under a fixed ratio (FR) 5 schedule in a Skinner box for 10 days and were subsequently exposed to a daily regimen of 20 cycles of FR 5 and 10 cycles of variable ratio (VR) 10 for about a week. This exposure resulted in a reduction of the number of lever presses and successful escapes compared to the level achieved after training. In addition, weight gain was significantly suppressed compared with other treatments. Acute and chronic administration of psychotropic drugs (imipramine and chlordiazepoxide) showed that treatment with imipramine increased both the number of lever presses and successful escapes while chlordiazepoxide increased only the number of lever presses. The results suggest that this simplified animal model utilizing chronic unpredictable electric shock may be useful in the study of human depression.

The Flinders sensitive line rats: a genetic animal model of depression

The Flinders Sensitive Line (FSL) rat, selectively bred for increased responses to the anticholinesterase DFP, was originally proposed as an animal model of depression because, like depressed humans, it is supersensitive to the behavioral and hormonal effects of cholinergic (muscarinic) agonists. The present review critically examines earlier and recent data collected on FSL rats to assess whether the model has good face, construct and/or predictive validity. With respect to face validity, FSL rats resemble depressed humans, at least superficially, in that they demonstrate: (a) reduced locomotor activity, (b) reduced body weight, (c) increased REM sleep, and (d) cognitive (learning) difficulties. So far, studies designed to assess the presence of anhedonia, a cardinal symptom of melancholic depression, have been inconclusive, but there are trends for the FSL rats to be more anhedonic than their control counterparts, the Flinders Resistant Line (FRL) rats, when exposed to chronic mild stress. Thus, FSL rats fulfill the criterion of face validity. Because FSL rats also are more sensitive to cholinergic agonists and have phase advanced circadian rhythms, they meet the criteria for the cholinergic and circadian rhythm models of depression and, therefore, have good construct validity. A key behavioral symptom exhibited by the FSL rat is demonstration of an exaggerated immobility when exposed to stressors such as foot shock and forced swimming. This behavioral abnormality has been normalized by a number of well-recognized antidepressant drugs such as imipramine and desipramine, as well as newer generation antidepressants with promising clinical effects such as sertraline and rolipram. However, several treatments that have not been routinely used to treat depression (lithium, exposure to bright light, the anticholinesterase DFP) have been ineffective in reversing the exaggerated immobility. Thus, the evidence in the present review indicates that the FSL rat model of depression fulfills the criteria of face, construct, and predictive validities.

Repeated stressful experiences differently affect the time-dependent responses of the mesolimbic dopamine system to the stressor

The increase in mesolimbic dopamine (DA) release observed during the first 40 min of 120 min restraint in naive rats is not evident in repeatedly stressed animals (daily 60 min restraint, for 5 days). However, repeatedly stressed rats show a significant decrease in DA release from 80 min of restraint onwards which is not observable in naive rats. These results indicate that repeated stressful experiences do not produce habituation but alter the response of mesolimbic DA system to the stressor. Moreover, they point to a possible neuronal mechanism underlying stress-induced depression.

Neonatal treatment with clomipramine increased immobility in the forced swim test: an attribute of animal models of depression

The forced swimming test in rats has been identified as a suitable model for detecting antidepressant activity of several drugs regardless of their mode of action. On the other hand, a number of animal models of human endogenous depression have been proposed. Recently, it has been reported that perinatal administration of clomipramine in rats elicits behavioral changes in adulthood that resemble human endogenous depression. In the present study, we showed that in this new animal model of depression immobility was increased when animals were submitted to the forced swimming test. This finding supports the notion that the amount of immobility during the forced swimming test is directly proportional to a depressive state in the rat.

Maternal behaviour in the Wistar rat under atypical Zeitgeber

In previous experiments, neonatal rats subjected to atypical Zeitgeber (light and temperature) produce adult animals that present a different reactivity in some test situations. On the other hand, a great body of evidence indicates that physiological and psychological processes, including the maturation of the circadian system, are regulated by maternal behaviour. In order to investigate in which manner the atypical Zeitgeber, mentioned above, influences maternal behaviour, mothers with their young were observed systematically. More precisely, the question was: is the changed reactivity due to the fact that the young experience a situation of classical early privation (e.g., a decrease in social or other sensory stimulation or in nutrition) or is it due to a situation in which only the temporal pattern of the maternal behaviour is modified and thus disturbs the proper development of the circadian system of the young. Results tend to show that the only difference between the experimental and the control group is a modification in the circadian rhythm of several behavioural items. Only one activity (licking of the young) shows a significant difference in the overall mean value but in favour of the experimental group. We would suggest that the modified temporal pattern of the maternal care could be a poor timegiver for the young, in such a way that the development of their circadian rhythms is disturbed. The unusual reactivity in adulthood could depend on this abnormal development of the circadian system and not on a care privation.

Effects of BCH 325 (Pro-D-Phe-Pro-Gly) on open field behavior after chronic stress procedure

The chronic stress model is an animal model relevant to clinical depression. The chronic stress rat group showed a significant decrease in motor activity. The reduced open field activity could be restored by subchronic treatment with imipramine and doxepine. Subchronic treatment with haloperidol, amphetamine and methylglucamine orotate was not able to alter the reduced open field activity. Previous treatment with a beta-casomorphin derivative (BCH 325) significantly antagonized the effect of chronic stress. The findings point out an antidepressant-like effect of BCH 325.

Social model of depression in mice of C57BL/6J strain

Long experience of defeat in daily social intermale confrontations and permanent living with aggressive males under sensory contact conditions [Kudryavtseva (8)] has been shown to produce changes in the patterns of submissive behavior of male mice of C57BL/6J strain. The submissive males after 20 defeats demonstrated passive defense postures instead of active defense and withdrawal which they had displayed in first encounters. Moreover, new immobile postures appeared, which were very rare in the first confrontations. Submissive animals displayed a decrease of ambulation in the open-field test and increase the immobility time in the Porsolt's test. Chronic treatment with imipramine prevented the increase of "depressiveness" estimated by means of the Porsolt's test. There was a loss of weight and some disturbances in gastrointestinal functions. The data are discussed in terms of the development of depression in submissive male C57BL/6J mice as a result of chronic unavoidable social stress.

Criterion for learned helplessness in the rat: a redefinition

Numerous investigators have reported difficulty obtaining reliable learned helplessness. Various laboratories have used differing test environments and criteria, making comparisons among experiments difficult. Some use an escape deficit criterion, in which escape is slowed down in a shuttle box, while others have used an escape failure criterion, in which rats do not escape at all on most test trials. Little work has been done to test the validity of LH, i.e., the prediction of persistence of escape failure after exposure to uncontrollable shock. The present studies demonstrate that the reliability and validity of learned helplessness can be improved by 1) modifying the shuttle box to increase task difficulty and decrease random escape behavior and 2) adopting a new escape failure criterion for helpless behavior which is based on statistical prediction of the persistence of escape deficits.

Neonatal chronic stress induces subsensitivity to chronic stress in adult rats. I. Effects on forced swim behavior and endocrine responses

An influence of early stimulation on sensitivity to acute stress in adulthood has been reported. The purpose of the present work was to determine the effect of exposure of male and female rats to three models of chronic stress (unpredictable stress, cold stress and handling) from day 2 to day 15 of life on behavioral and endocrine sensitivity to chronic stresses in adulthood. The chronic stresses applied in adulthood were a model of intermittent cold stress (daily 30-min sessions at -20 degrees C for 15 days) and the Katz's model of unpredictable chronic stress (15 days). Forced swim behavior and serum concentration of the stress-sensitive hormones, corticosterone and prolactin, were chosen to investigate stress sensitivity. It was found that all neonatal treatments stimulated body weight gain, did not cause infant mortality and did not affect forced swim behavior as adult. The repetitive exposure to cold stress in adulthood did not cause major impairment of forced swim behavior and did not affect basal levels of serum corticosterone and prolactin in either control or experimental rats. These findings support the view that repeated stressors can induce behavioral and endocrine adaptation in rats. The neonatal treatments did not affect this characteristic. The exposure of control rats to the unpredictable stress model severely impaired forced swim behavior and increased basal levels of serum corticosterone and prolactin. This observation conforms to the view that standard laboratory rats cannot adapt to unpredictable chronic stress. This has been reported to cause a behavioral depression syndrome comprising forced swim deficit and endocrine alterations.(ABSTRACT TRUNCATED AT 250 WORDS)

Is it possible to predict the activity of a new antidepressant in animals with simple psychopharmacological tests?

Behavioural tests for predicting antidepressant activity in the animal provide a closer approximation than other tests of states of depression in man but are often long and costly to perform (except the behavioural despair test). The tests proposed here presuppose a pharmacological interaction (except the Porsolt test) but are simple enough to allow screening: included are antagonism of reserpine hypothermia, ptosis and akinesia; antagonism of effects induced by oxotremorine; antagonism of high-dose apomorphine; and potentiation of yohimbine toxicity. In combination with the study of motor activity in the mouse, these tests allow assessment of the specificity of antidepressant activity by establishing a ratio between the "antidepressant" dose and the "stimulant" or "sedative" dose. It can be predicted that a substance will be antidepressant and sedative or stimulant at the same dose if the ratio is close to 1; if the ratio is less than 1, at antidepressant doses the substance will be very sedative or stimulant according to the case. The specificity of the tests discussed can be debatable. Antagonism of reserpine-induced hypothermia indicates substances with direct or indirect beta-mimetic activity, ptosis antagonism, substances with alpha-adrenergic (not antidepressants) or serotoninergic (possibly antidepressants) activity; and akinesia antagonism, a direct or indirect dopaminergic activity (sometimes found in antidepressants) with psychostimulant activity. The oxotremorine test is related to the anticholinergic activity of substances, except in the case of hypothermia antagonism. The high-dose apomorphine test seems to be specific for substances inhibiting norepinephrine reuptake. The yohimbine test is simple to carry out, relatively inexpensive and does not fail to screen any molecule known to be effective to-date. The behavioural despair test is a good complement for screening except for drugs having a beta-agonist activity, it appears that this test is dependent on functional relationships between alpha 2 and serotonergic systems.

Effects of TRH and prolactin in the behavioral despair (swim) model of depression in rats

The neuropeptides thyrotropin releasing hormone (TRH) and prolactin (PRL), which affect various behaviors in animals, showed "antidepressant" properties in an experimental model of depression. Subcutaneous administration of TRH reduced the total immobility time of rats tested in the despair (constrained swim) test and potentiated the anti-immobility effect of intraperitoneally administered desimipramine (DMI). This effect was not mimicked by the peripheral injection of TSH, T3 or T4. Hyperprolactinemia induced by pituitary homografts under the kidney capsule and the intracerebroventricular injection of PRL also potentiated the DMI-induced reduction of total immobility time of rats in the despair test and exerted "antidepressant" effects in aged rats.

Effects of chronic clonidine administration and withdrawal on free-running circadian activity rhythms

This study examined the effects of the alpha 2-adrenergic agonist clonidine on free-running circadian activity rhythms in rats, using a dosing regimen similar to one previously shown to induce behavioral depression in the swim test. In constant light, clonidine consistently shortened the free-running circadian period, reduced circadian amplitude, and reduced the overall level of locomotor activity. These effects were reversed after the termination of clonidine treatment. In constant darkness, clonidine reduced circadian amplitude, but both increases and decreases in free-running period and activity level were observed. Clonidine-induced changes in free-running period and activity level were systematically related to individual differences in baseline activity in both constant light and constant darkness. These results demonstrate that clonidine can alter both the rhythmicity and the level of spontaneous activity, and are consistent with the hypothesis that monoaminergic systems may mediate relationships between behavioral state and circadian rhythmicity.

MIF-1 is active in a chronic stress animal model of depression

MIF-1 was tested in an animal model of depression that used unpredictable chronic stress. In this paradigm, rats received either no stressors or a daily protocol of a variety of stressors for 20 days, during which time daily, intraperitoneal injections of various compounds were given. The tricyclic antidepressant imipramine (5 mg/kg) and low doses (0.1 and 1.0 mg/kg) of MIF-1 significantly increased activity and decreased defecation in an open field on day 21. No dose of naloxone (0.01-10.0 mg/kg) acted as an antidepressant. A high dose (10.0 mg/kg) of MIF-1 significantly increased the effects of chronic stress and produced hyperalgesia. Chronically-stressed rats were significantly more analgesic than controls. The results indicate that MIF-1 can act as an antidepressant in this model.

Distress call reactivation in isolated chicks: a behavioral indicator with high selectivity for antidepressants

Four-day-old chicks respond to isolation with distress calling. A decrease in distress calling during isolation seems to reflect some general aspect of depressive syndrome and appears to be sensitive to pharmacological manipulation. Diverse clinically active antidepressants are able to counteract this decrease with high selectivity; non-antidepressants have either no influence or further inhibit the distress calling.

Genetic and pharmacological models of cholinergic supersensitivity and affective disorders

Increased muscarinic sensitivity has been associated with altered hormonal states (hypothyroidism and hyperadrenocorticism), chronic administration of muscarinic antagonists or antidepressants with muscarinic actions, selective breeding for anticholinesterase sensitivity, and certain inbred strains of rats and mice. Thus, both genetic and environmental factors may influence muscarinic receptor sensitivity. The reasonably detailed studies on the selectively-bred rats have revealed that the Flinders Sensitive Line (FSL) rats weigh less, are less active, are more sensitive to muscarinic agonists and to stressors, and have higher concentrations of hippocampal and striatal muscarinic receptors than 'normal', or the selectively-bred, Flinders Resistant Line (FRL) rats. Thus, there are a number of parallels between FSL rats and depressed humans. The FSL rats may be the first animal model of depression to mimic the actual trait of depression, and not just the state.

Cerebral glucose utilization after aversive conditioning and during conditioned fear in the rat

Regional cerebral glucose utilization (rCMRglu) was studied in rats with and without previous aversive conditioning. Four groups of rats were studied. Two groups of rats were aversely conditioned by placing them in a shock chamber (conditioned stimulus) where they received random footshocks. The two remaining groups were placed in the shock chamber but not conditioned. Regional CMRglu and systemic parameters (heart rate, mean arterial blood pressure (MABP), blood gases and pH, plasma catecholamines, and plasma glucose) were measured in unconditioned and conditioned rats in the presence and in the absence of the conditioned stimulus. The changes in rCMRglu described below appeared to be global and not limited to specific regions. Results are as follows: (1) transferring unconditioned rats to the shock chamber had no significant effect on rCMRglu even though the systemic parameters indicated a stress response. It appears that stress capable of inducing changes in heart rate, MABP, and plasma catecholamines is not necessarily accompanied by increases in cerebral glucose utilization. (2) Conditioned rats not exposed to the shock chamber at the time rCMRglu was measured had decreased rates of rCMRglu compared to rats that were not conditioned. Except for plasma epinephrine, which increased after conditioning, systemic parameters were not affected. (3) Conditioned fear, elicited by transferring conditioned rats to the shock chamber, increased rCMRglu when compared to a control group that was conditioned to footshock using the same paradigm but not exposed to the shock chamber at the time rCMRglu was measured. The systemic parameters indicated a stress response in conditioned rats transferred to the shock chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain output dysregulation induced by olfactory bulbectomy: an approximation in the rat of major depressive disorder in humans?

Mounting evidence indicates that the emotional, cognitive, neurovegetative and behavioral symptoms of patients with major depressive disorder are due to abnormal neurochemical substrates in the brain. Although the specific neurochemical abnormalities responsible have not been identified, the presenting symptoms of major depression are consistent with a disruption of normal neural communications between the limbic system and hypothalamus. Following removal of the olfactory bulbs, rats display a syndrome of behavioral deficits that also reflect a disruption of the limbic-hypothalamic axis. Moreover, the bulbectomy induced deficits are selectively reduced by the chronic administration of the same drugs that alleviate the symptoms of depression when given chronically to the patients. In addition to this pharmacological similarity, there are also numerous behavioral parallels between bulbectomized rats and major depression patients. The bulbectomized rat provides a good model in which to study antidepressant drugs and also may provide neurochemical and neuroanatomical data that are relevant to understanding the biological substrates of emotion and the causes of depression in humans.

A further analysis of behavioral and endocrine effects of unpredictable chronic stress

Tolerance can develop when rats are repeatedly exposed to some predictable stressors. This does not occur, however, when rats are exposed to unpredictable chronic stress. In this study we have analyzed some behavioral and endocrine effects in male and female rats treated daily with unpredictable emotional stressor (ES-groups) or unpredictable physical stressors (PS-groups) over a 14-day period. Animals were then submitted to three behavioral tests at 24 hr intervals. Experiment 1 shows that when rats were tested in an enriched environment both total motor activity and exploration of the novel object were impaired by the PS treatment. This suggests the occurrence of motivational deficit. The fact that the PS-groups also showed increased intratest defecation suggests increased emotionality. When animals were submitted to an emergence test the PS-groups showed longer emergence latency, lower frequency of emergencies and lower time spent exploring the emergence compartment than the ES- and the C-group. This strongly supports that the PS treatment increased emotionality in rats. When ES- and PS-groups were exposed to a forced swim test they showed longer immobility duration (despair reaction) but only the PS-group displayed lower frequency of jumps (escape reaction). Results of all tests performed revealed that females were more resistant than males to the behavioral effects of the PS treatment. The day after the behavioral testing was completed, basal levels of corticosterone and prolactin were investigated in male subjects. The PS-group showed higher baseline levels of these "stress labile" hormones than the ES and the C-group.(ABSTRACT TRUNCATED AT 250 WORDS)

Is the forced swimming test a suitable model for revealing antidepressant activity?

The forced swimming test is reviewed. This test appears to be suitable for detecting antidepressant activity in rats but not in mice. Difference in experimental procedure may account for the different sensitivity to drugs of the two animal species.

Behavioural and biochemical studies on 6-methylamino-4,5,6,7-tetrahydrobenzothiazole (14.839JL), a new potent dopaminergic agonist

6-Methylamino-4,5,6,7-tetrahydrobenzothiazole monochlorhydrate (14.839JL) is a new, potent dopaminergic agonist. The stereotypy induced by this drug was greater than that induced by an equivalent dose of apomorphine, was antagonized by pretreatment with sulpiride and counteracted the hypomotility induced by reserpine. Striatal levels of the dopamine metabolites homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were significantly lowered for up to 4-6 h by doses from 0.05 to 1 mg/kg. The drug was also very effective in lowering prolactine secretion. 14.839JL displaced [3H]N-n-propylnorapomorphine [3H]NPA from striatal binding sites with an IC50 similar to dopamine (DA). Conversely, the ability of 14.839JL to displace 3H spiperone from its binding sites was 100 and 10 times lower than that of haloperidol and sulpiride, and similar to that of SCH 23390. Differently from the latter, however, 14.839JL did not modify adenylate cyclase activity. All these data suggest that 14.839JL is a new, potent, long-lasting direct DA agonist, probably acting on D2 receptors.

The comparative psychopathology of affective disorders in animals and humans

Reviews of animal models of affective disorders commonly concentrate on the behavioural features thereof, the supposed neurochemical substrates, the mode of production and the response to treatment of the state in question but ignore questions of psycho pathology. An attempt is made to deal critically with the psychopathology of human and animal affective disorders in the light of current operational criteria for the diagnosis of major depressive disorders. It is argued thatthe psychopathological tradition stemming from Jaspers may be more appropriate to a consideration of animal models of affective disorders than the psychopathological positions implicit in psychoanalysis, behaviourism or current cognitive psychologies and in addition more suited to meet these criteria. The adoption of such a perspective results in a shift of emphasis from abnormalities of psychological content to demonstrable neuropsychological deficits and a definition of affective disorders, whether in animals or humans, as psychosomatic illnesses, possibly involving a pathology of circadian rhythmicity. This perspective also suggests that animal models may be useful in the devel opment of more refined diagnostic criteria for affective disorders in humans.

The effects of carbamazepine on two animal models of depression

Some clinical reports on antimanic, antidepressant and prophylactic effects of carbamazepine (CBZ) in manic-depressive illness have appeared since its initial use as an anticonvulsant drug. The present report deals with the effects of CBZ on two animal models of depression, namely the potentiation of amphetamine-induced anorexia, and the behavioral despair model. Carbamazepine (10, 20 or 40 mg/kg) neither modified the methamphetamine anorectic effect, nor induced anorexia when administered alone. Subacute and chronic administration of imipramine (4 or 15 mg/kg) decreased immobility of rats in the behavioral despair model. Subacute and chronic administration of CBZ (40 mg/kg) also decreased immobility, whereas the dose of 10 mg/kg CBZ was effective only after chronic treatment. It was concluded that CBZ is similar to atypical antidepressants, since it did not potentiate the amphetamine-induced behavioral effect, but did have an effect on the behavioral despair model of depression.

Chronic stress depresses exploratory activity and behavioral performance in the forced swimming test without altering ACTH response to a novel acute stressor

The present study investigated the effects of chronic stress on some behavioral and endocrine variables in adult male rats. The variables were always measured the day that followed the stress session to chronically stressed rats. Two stress models were used: chronic shock (CS), and a combination of various stressors randomly chosen each day (CV). Chronic but not acute exposure to the stressors resulted in decreased exploratory activity in the holeboard. Likewise, only chronic exposure to shock or variable stressors resulted in increased immobility in the Porsolt's test. Taken together these results suggest that changes in exploratory and forced swimming activities might be affected by chronic stress. In contrast, neither CS nor CV rats showed altered basal or stress-induced levels of ACTH which suggests that the activity of the axis was unchanged. Although the present data indicate that chronic stress caused behavioral abnormalities closely related to that expected to take place in some type of depressive disorders, endocrine data are not in good agreement with those occurring in depression.

Neurochemical and behavioral consequences of mild, uncontrollable shock: effects of PCPA

The present experiments examined the role of the serotonergic system in the behavioral deficit produced by uncontrollable shock. In Experiment 1: Establishment of model, the behavioral potential of the Sprague-Dawley rat was defined. When exposed to mild uncontrollable stress such as a 0.8 mA electric footshock, a significant percentage of rats developed a shock escape deficit which was evident when subsequently placed in a shock escape paradigm. Serotonin depletion was produced by chronic treatment with p-chlorophenylalanine. Biogenic amine levels and 5-HT levels were monitored in various brain areas using HPLC. Following chronic treatment with PCPA, the shock escape capability of the Sprague-Dawley rat was assessed. The severe depletion of 5-HT in various brain regions was highly correlated with a dramatic improvement in the shock escape scores. Thus, the detrimental effects of exposure to a mild course of inescapable shock can be prevented by chronic treatment with PCPA. These experiments implicate the serotonergic system as a possible mediator of the "learned helplessness" phenomenon.

Behavioral consequences of antidepressant treatment in rodents

This review discusses the effects of antidepressant drugs on behaviors that are changed during the clinical treatment of depression. We first consider whether there is a similar subjective state produced by antidepressant drugs that might be akin to the mood changes caused clinically by these drugs. We thus review the evidence that antidepressant drugs can produce a distinctive enough subjective state to serve as a discriminative stimulus, and then discuss the nature of the cue produced. Secondly, we discuss whether there is any evidence that antidepressants enhance the rewarding aspects of stimuli since this aspect of behavior is reported to change during the successful treatment of depression. In this section we review the effects on electrical brain stimulation, water and food intake, exploratory and social behaviors. Finally, because of the proposed role of stress in the aetiology of depression, we review the effects of antidepressants on the responses to acute and chronic stress.

The effects of tricyclic and 'atypical' antidepressants on spontaneous locomotor activity in rodents

With the exception of amineptin, buproprion and nomifensine all tricyclic and 'atypical' antidepressants have been reported to reduce spontaneous motor activity in rodents, after both acute and chronic administration. However, with the diversity of chemical actions of these drugs it is unlikely that a single neurochemical mechanism is underlying this one behavioral effect. These widespread sedative effects have implications for interpreting behavioral changes in other test situations, since sedation generally occurs at doses that fall within the dose-range effective in other tests. We also review the effects on spontaneous motor activity of withdrawal from chronic antidepressant treatment.

Selective breeding for increased cholinergic function: development of a new animal model of depression

Two lines of rats that were selectively bred to vary in their sensitivity to the anticholinesterase DFP exhibited different degrees of behavioral depression after injection of the muscarinic agonist arecoline (2 mg/kg). The line of rats with increased behavioral depression after arecoline (the Flinders sensitive or S-line) also exhibited a greater reduction of activity in an open field chamber following exposure to foot shock and greater immobility in a forced swim test than the line of rats with reduced behavioral depression after arecoline (the Flinders resistant or R-line). In addition, the Flinders S-line exhibited a better memory on an inhibitory avoidance task. These differences were not related to differences in shock sensitivity between the lines. Thus, the Flinders S-line of rats reacts to both mild stressors and a cholinergic agonist with greater behavioral depression and may, therefore, be a useful new animal model of human depressive disorders, one that focuses on cholinergic supersensitivity.

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

Drugs with antidepressant properties in patients with severe depression also have various behavioral and neurochemical effects in animals. This has given rise to numerous animal models that have been suggested to be valid for research into the neurobiology of depression and the neurochemical mechanisms of the antidepressant drugs. However, considerable evidence from many avenues of research indicates that severe depression is a biochemical disorder that develops in those individuals with some predisposing neurochemical vulnerability. Although the predisposing biochemical abnormality has not been identified, it may be related to the neurochemical mechanisms that regulate impulse traffic in various neural systems and maintain the homeostatic balance of neural activity within the brain. Therefore, the appropriate animal model for severe depression should have some disruption of neural functioning that is returned to normal by the chronic administration of antidepressant drugs. Of the numerous animal models of depression that have been presented in the literature, only the rat with olfactory bulb lesions meets this requirement. The behavioral and endocrine abnormalities induced by the olfactory bulb lesions are reversed by chronic (but not acute) treatment with antidepressants of various classes. Of the existing animal models of severe depression, the olfactory bulbectomy model holds the most promise for elucidating the neurobiology of depression and the neurochemistry of antidepressant drugs.

Searching-waiting strategy: a candidate for an evolutionary model of depression?

The model proposed here assumes that depressive disorders could reflect an extreme state of a current behavioral strategy. According to this model, a subject facing a problem of survival without apparent solution may choose between two behavioral patterns: searching for a solution or waiting for that solution to occur. This choice can be made after one or several estimations of the cost and benefit attached to each of these alternatives. Two of the main behavioral models of depression are interpreted according to this model: namely, infant response to maternal separation in monkeys and "behavioral despair" in rodents. Practical and theoretical consequences of this model are discussed.

Drugs: guide and caveats to explanatory and descriptive approaches--II. Drugs in psychiatric research

Over the last two decades animal models, mediators and drugs have produced a mass of experimental data difficult to relate to the epidemiological and clinical side of psychiatric disorders. Antidepressant drugs are chosen as a model case to describe a more general scene: far from being a specific tool--a guide--in a complex situation, these drugs are acquiring a role which must be interpreted with caution (a caveat), in view of ample clinical evidence of responders to placebo, responders to any drugs, responders to many drugs, etc. Studies of biochemical descriptors of drug action as a marker of the disease and its outcome have drawn puzzling pictures, often contradictory and unstable in terms of the populations to whom they can be applied. Controlled clinical trials with antidepressant drugs over the last ten years have persisted in looking for short-term pharmacological effects rather than the medium- or long-term impact of medication in large populations. To establish a positive role for antidepressant drugs, they must be studied in a "natural" context where depressed patients are treated with all necessary follow-up.

Psychiatric progress. The learned helplessness model of depression

Attempts have been made to model certain human psychopathological states in the laboratory, with varying degrees of success. The animal model has emerged as an alternative to clinical studies in psychiatry because it is able to provide greater experimental control and allows the exercise of ethical discretion. Although numerous animal models of depression have been proposed in the literature, most, if not all, fail to mimic human depressive symptomatology; their main function is to act as selective screens for antidepressant drugs. The learned helplessness approach has been suggested as an animal analogue of depression because of its similarities to the human depressive state in terms of provocation, manifestation and treatment. Furthermore, the learned helplessness model, which was originally based on animal experimentation, has been shown to be reproducible in human subjects, a finding not observed with other animal models of depression. Although this model has been much criticized in the past, recent reformulation adds credence to it as a more valid analogue of human depression, given the additional cognitive constructs in depressed human subjects.

The validity of animal models of depression

Eighteen animal models of depression are reviewed in relation to three sets of validating criteria. Of the 18 models, five could only be assessed for predictive validity, seven could be assessed for predictive and face validity, and six could potentially have predictive, face and construct validity. Some traditional models (reserpine reversal, amphetamine potentiation) are rejected as invalid; the models with the highest overall validity are the intracranial self-stimulation, chronic stress and learned helplessness models in rats, and the primate separation model.

Dopamine and depression: a review of recent evidence. II. Theoretical approaches

This paper discusses 3 behavioural approaches to depression, 'learned helplessness', reward system dysfunction and reduced responsiveness to the environment, and the role of dopamine (DA) in their related animal models. The meso-limbic and nigrostriatal DA system appears to be related primarily to responsiveness to the environment. This conclusion is discussed in relation to the clinical symptomatology of depression.

Effects of acute and chronic administration of antidepressant drugs on the central cholinergic nervous system. Comparison with anticholinergic drugs

The purpose of this investigation was to study the effects of antidepressant drugs on the central cholinergic system of the rat after acute and chronic administration. Drugs (antidepressants and non-antidepressants) were first divided into highly potent, moderately potent or weak anticholinergic categories based upon the ability of each compound to displace [3H]-QNB [( 3H]quinuclidinyl benzilate from synaptosomal membranes. One antidepressant drug and one non-antidepressant drug, with similar anticholinergic properties, were chosen as representative agents of each category of anticholinergic potency. Acute administration of amitriptyline or atropine (highly potent anticholinergics) increased the level of high affinity uptake of choline in the hippocampus and striatum. Imipramine and thioridazine (moderately potent anticholinergics) increased the uptake of choline only in the striatum. After acute administration, the effects of nomifensine and d-amphetamine (weak anticholinergics) differed on striatal uptake of choline. Following 30 days pretreatment with any drug, an acute challenge dose of that drug no longer altered the uptake of choline in either region. After chronic administration, amitriptyline increased the density of muscarinic receptors in the cortex whereas atropine increased the density of receptors in the cortex, hippocampus and striatum. The other agents did not alter receptor parameters in the regions examined. Since the central cholinergic actions of the antidepressants were similar to the central actions of the non-antidepressants, it is concluded that the effects of the antidepressants on the central cholinergic nervous system are more closely related to the side effects of these agents than to their therapeutic mechanism of action.

Animal model of depression: tests of three structurally and pharmacologically novel antidepressant compounds

Previous studies have identified behavioral and neuroendocrine abnormalities in chronically stressed rats which resemble some of the more prominent features of clinical depression. These abnormalities have proved responsive to pharmacotherapy by standard antidepressant drugs and related somatic treatments. Several structurally and pharmacologically atypical compounds, resembling neither standard agents, nor each other, have recently been identified as clinically effective antidepressants. There drugs do not show typical preclinical response profiles in other drug screening tests and, therefore, represent critical instances for evaluating the selectivity of the chronic stress model. Three drugs were tested, these being iprindole, bupropion, and mianserine; a tricyclic indole, propriophenone, and tetracyclic compound respectively. Four circulating measures, which previously proved most useful in discriminating antidepressant potential, and a measure of circulating corticosterone were obtained for subjects examined factorially in a 2x2x2 experimental design (chronic stress vs none, acute stress vs none, and drugs vs control). All compounds proved capable of reversing chronic stress induced behavioral deficits, and all but one compound reversed the attendant basal hypersecretion of corticosterone. These findings argue that the chronic stress model provides an accurate and selective assessment of the therapeutic potential of both standard and structurally novel compounds.

Amitriptyline and scopolamine in an animal model of depression

Adult male Sprague-Dawley rats were subjected to acute (95 dB white noise) or chronic stress, or their combination. In comparison with unstressed controls, stressed rats were more active upon several measures of open field activity. A history of chronic stress eliminated the acute stress induced activation. Concurrent treatment of chronically stressed rats with amitriptyline or scopolamine, or with a combination of both drugs resulted in selective behavioral improvement (i.e., in motor activity, latency, defecation) for amitriptyline and combined treatment rats, with significant restoration of the normal behavioral response. Scopolamine however was only marginally effective. A higher dose of scopolamine proved effective, but only with a marked disruption of baseline activity. Examination of plasma corticosterone titers indicated that chronic stress induced an elevation of basal levels and that this was reversed by amitriptyline, scopolamine, and combined drug treatment. Thus while behavioral depression and elevated corticosteroids may covary they are not identically mediated.

Amphetamine and tranylcypromine in an animal model of depression: pharmacological specificity of the reversal effect

Amphetamine and tranylcypromine are structurally related chemical isomers with pharmacologically distinctive activity profiles. Since they are equimolar and structurally similar they may be used to assess the pharmacologically distinctive activity profiles. Since they are equimolar and structurally similar they may be used to assess the pharmacological specificity of a proposed animal model of depression. Adult male Sprague-Dawley rats were exposed to a chronic stress regimen or remained undisturbed. They were then acutely stressed with white noise. The monoamine oxidase inhibitor tranylcypromine was effective in restoring otherwise reduced stress elicited open field activity in chronically stressed rats. Amphetamine did not resemble tranylcypromine or other antidepressants, and produced a variety of effects at least some of which indicated a potential increase rather than reduction in depression consequent to chronic administration.