Protective effect of citalopram against the attenuation of the alpha 1-potentiation of cAMP formation in Fischer 344 strain rats

The tail suspension test as a model for assessing antidepressant activity: review of pharmacological and genetic studies in mice

Since its introduction almost 20 years ago, the tail suspension test has become one of the most widely used models for assessing antidepressant-like activity in mice. The test is based on the fact that animals subjected to the short-term, inescapable stress of being suspended by their tail, will develop an immobile posture. Various antidepressant medications reverse the immobility and promote the occurrence of escape-related behaviour. This review focuses on the utility this test as part of a research program aimed at understanding the mechanism of action of antidepressants. We discuss the inherent difficulties in modeling depression in rodents. We describe how the tail suspension differs from the closely related forced swim test. Further, we address some key issues associated with using the TST as a model of antidepressant action. We discuss issues regarding whether it satisfies criteria to be a valid model for assessing depression-related behavioural traits. We elaborate on the tests' ease of use, strain differences observed in the test and gender effects in the test. We focus on the utility of the test for genetic analysis. Furthermore, we discuss the concept of whether immobility maybe a behavioural trait relevant to depression. All of the available pharmacological data using the test in genetically modified mice is collated. Special attention is given to selective breeding programs such as the Rouen 'depressed' mice which have been bred for high and low immobility in the tail suspension test. We provide an extensive pooling of the pharmacological studies published to date using the test. Finally, we provide novel pharmacological validation of an automated system (Bioseb) for assessing immobility. Taken together, we conclude that the tail suspension test is a useful test for assessing the behavioural effects of antidepressant compounds and other pharmacological and genetic manipulations relevant to depression.

Emerging evidence for a central epinephrine-innervated alpha 1-adrenergic system that regulates behavioral activation and is impaired in depression

Currently, most basic and clinical research on depression is focused on either central serotonergic, noradrenergic, or dopaminergic neurotransmission as affected by various etiological and predisposing factors. Recent evidence suggests that there is another system that consists of a subset of brain alpha(1B)-adrenoceptors innervated primarily by brain epinephrine (EPI) that potentially modulates the above three monoamine systems in parallel and plays a critical role in depression. The present review covers the evidence for this system and includes findings that brain alpha(1)-adrenoceptors are instrumental in behavioral activation, are located near the major monoamine cell groups or target areas, receive EPI as their neurotransmitter, are impaired or inhibited in depressed patients or after stress in animal models, and are restored by a number of antidepressants. This "EPI-alpha(1) system" may therefore represent a new target system for this disorder.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Stress-induced subsensitivity to modafinil and its prevention by corticosteroids

Brain alpha(1)-adrenoceptors are known to be necessary for motor activity in rodents and have been shown to be altered by stress and corticosteroids but only in biochemical experiments. To determine if the behaviorally coupled receptors are also affected by stress, the present study examined the effect of stress and corticosteroids treatment on the motor activity response to modafinil, a putative alpha(1)-adrenoceptor agonist, which is unique in that it elicits extremely high levels of activity via these receptors. Mice were subjected to various schedules of restraint stress for 1-6 days and were subsequently tested for either modafinil-induced or dopaminergically induced behavioral activity in the home cage using videotape recording. In experiments on corticosteroid treatment, mice received exogenous corticosterone or dexamethasone in the drinking water before and during the stress and were tested for modafinil-induced activity as above. It was found that the stress significantly reduced the response to the drug by the third daily session. Motor responses to dopaminergic agents including apomorphine, amphetamine, dihydrexidine and quinpirole were either not altered or were increased at this time. Treatment of animals with corticosterone or dexamethasone prior to and during stress prevented the behavioral subsensitivity to modafinil. Corticosterone pretreatment markedly suppressed the plasma corticosterone response to the stress. The present results provide further support for the hypothesis that stress produces a selective desensitization or inhibition of motor-related brain alpha(1)-adrenoceptors and that this effect can be prevented by corticosteroid treatment.

Alpha-1-noradrenergic neurotransmission, corticosterone, and behavioral depression

BACKGROUND

Impaired brain alpha-1 noradrenergic neurotransmission has been implicated in some of the symptoms of depressive illness but has been difficult to investigate experimentally because of the insensitivity of current animal models of depression. The present experiment addressed this problem by examining the effects of pharmacologic blockade and corticosteroid-induced desensitization of alpha-1 receptors on two newer, more sensitive models in mice: the inhibition of nest-leaving and the tail suspension tests (TST).

METHODS

Male mice were administered either prazosin, betaxolol, atipamezole, corticosterone, or repeated restraint stress prior to measurement of either nest-leaving or TST. General behavioral function was assessed in horizontal wire, swim, and latency to escape footshock tests.

RESULTS

Prazosin increased depressive behavior in the nest-leaving and TSTs, whereas corticosterone and restraint stress did so only in the more sensitive nest-leaving test. Betaxolol also reduced nest-leaving, suggestive of an alpha-1 beta-1 receptor synergy. The effects of these agents could not be attributed to hypotension, sedation, or general behavioral impairment.

CONCLUSIONS

The fact that a reduction in alpha-1 noradrenergic neurotransmission increases depressive behavior, coupled with the fact that this change can result from elevated corticosteroid secretion, provides further support for a role of this factor in depressive illness. As not all alpha-1 functions are reduced in depression, it is likely that only a subgroup or specific locality of alpha-1 receptors are affected.