An attenuated alpha-1 potentiation of beta adrenoceptor-stimulated cyclic AMP formation after repeated saline injections in Fischer 344 strain rats

Progesterone reduces the inhibitory effect of a serotonin 1B receptor agonist on lordosis behavior

Ovariectomized Fischer inbred rats were hormonally primed with 10μg estradiol benzoate and sesame seed oil (EO rats) or with estradiol benzoate and 500μg progesterone (EP rats). Four to six hours after progesterone or oil, rats were pretested for sexual behavior and then infused bilaterally into the ventromedial nucleus of the hypothalamus with 0, 50, 100 or 200ng of the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrol[3,2-bi]pyridin-5-one-dihydrochloride (CP 93129). Sexual receptivity was monitored by the lordosis to mount (L/M) ratio. EO rats showed a transient decline in lordosis behavior following infusion with the saline vehicle and this was amplified by CP 93129. There were no effects of any infusion in EP rats. These findings are discussed in terms of the possible stress effect of the intracranial infusion in EO rats and their implications for a role of 5-HT(1B) receptors in the response to a mild stress.

Different behavioral effects of maprotiline and fluxilan in rats

Serotonin and noradrenaline are involved in the mechanisms of action of most antidepressant drugs. We examined the effects of chronic treatment with maprotiline, a selective inhibitor of noradrenaline reuptake, and fluxilan, a selective inhibitor of serotonin reuptake, on the behavior of unstressed controls and chronic unpredictable mild stress (CUMS) model rats in the forced swim test (FST) and elevated plus maze test. Both selective reuptake inhibitors resulted in a significant reduction of time spent in immobility. Climbing was significantly increased in maprotiline- and swimming was exclusively elicited in the fluxilan-treated unstressed control and CUMS rats. Maprotiline-treated ani?mals displayed decreased anxiety and fluxilan-treated rats enhanced anxiety. The obtained results suggest that central noradrenergic and serotonergic systems might be affected differently during FST. The results also demonstrate that the anxiogenic effects of chronic fluxilan treatment are similar to those reported by many other studies. These differences observed for the effects of fluxilan in relation to those reported for maprotiline and probably due to the different phar?macological profiles of these drugs.

Alpha(1)-adrenergic and alpha(2)-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

RATIONALE

Central alpha(1)- and alpha(2)-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of alpha(1)-receptors may induce inactivity by leading to unopposed alpha(2) activity.

OBJECTIVE

The aim of this study was to test if central blockade of alpha(2)-receptor function restores behavioral activity in alpha(1)-receptor-blocked mice.

METHODS

Dose-response studies were undertaken on the effects of alpha(1)- and alpha(2)-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.

RESULTS

The behavioral inactivity resulting from blockade of alpha(1)-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an alpha(2)-antagonist, atipamezole, or a low dose of an alpha(2)-agonist, dexmedetomidine, but was exacerbated by a high dose of the alpha(2)-agonist.

CONCLUSION

The results support the hypothesis that blockade of alpha(1)-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of alpha(2)-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.

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.

Comparison of noradrenergic and serotonergic antidepressants in reducing immobility time in the tail suspension test

We examined the effects of two noradrenergic tricyclic antidepressants and two selective serotonin re-uptake inhibitors in the tail suspension test, with a suspension period of 30 min instead of the usual 10 min. Within the first 10 min, desipramine, nortriptyline and fluvoxamine significantly reduced the duration of immobility. Whereas desipramine and nortriptyline were also efficacious in the rest of the test period, fluvoxamine was not. Fluoxetine showed no significant effect throughout the study period. These results suggest that a prolonged tail suspension test results in functional changes in the noradrenergic and serotonergic systems and alters the sensitivity to antidepressants.

The suppressive effects of sucrose and cocaine, but not lithium chloride, are greater in Lewis than in Fischer rats: evidence for the reward comparison hypothesis

Rats suppress intake of a saccharin conditioned stimulus (CS) when it is paired with an aversive unconditioned stimulus (US), an appetitive US, or a drug of abuse such as morphine or cocaine. It is unclear, however, whether the reduction in intake induced by these drugs is mediated by their aversive or their rewarding properties. The present set of experiments addressed this question by comparing the suppressive effects of a known aversive US (LiCl), a known reinforcing US (sucrose), and a drug of abuse (cocaine) in two strains of rats (i.e., Lewis and Fischer 344 rats) that differ in their preference for rewarding stimuli. The results show that, although both strains readily acquired a LiCl-induced conditioned taste aversion (CTA), the suppressive effects of sucrose and cocaine were robust in the drug-preferring Lewis rats and absent in the Fischer rats. These data argue against a CTA account and in favor of the reward comparison hypothesis.

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.

Potentiation of cyclic AMP-mediated vasorelaxation by phenylephrine in pulmonary arteries of the rat

Alpha1-adrenoceptor agonists may potentiate relaxation to beta-adrenoceptor agonists, although the mechanisms are unclear. We compared relaxations induced by beta-adrenoceptor agonists and cyclic AMP-dependent vasodilators in rat pulmonary arteries constricted with prostaglandin F2alpha (PGF2alpha) or the alpha1-adrenoceptor agonist phenylephrine (PE). In addition, we examined whether differences were related to cyclic AMP- or nitric oxide (NO) and cyclic GMP-dependent pathways. Isoprenaline-induced relaxation was substantially potentiated in arteries constricted with PE compared with PGF2alpha. Methoxamine was similar to PE, whereas there was no difference between PGF2alpha and 30 mM KCl. The potentiation was primarily due to a marked increase in the NO-independent component of relaxation, from 9.1+/-1.7% for PGF2alpha to 55.1+/-4.4% for PE. NO-dependent relaxation was also enhanced, but to a lesser extent (50%). Relaxation to salbutamol was almost entirely NO-dependent in both groups, and was potentiated approximately 50% by PE. Relaxation to forskolin (activator of adenylate cyclase) was also enhanced in PE constricted arteries. Part of this relaxation was NO-dependent, but the major effect of PE was to increase the NO-independent component. Propranolol diminished but did not abolish the potentiation. There was no difference in response to CPT cyclic AMP (membrane permeant analogue) between PE and PGF2alpha, suggesting that mechanisms distal to the production of cyclic AMP were unchanged. Relaxation to sodium nitroprusside (SNP) was the same for PE and PGF2alpha, although relaxation to acetylcholine (ACh) was slightly depressed. This implies that potentiation by PE does not involve the cyclic GMP pathway directly. Mesenteric arteries constricted with PE did not show potentiation of isoprenaline-induced relaxation compared to those constricted with PGF2alpha, suggesting that this effect may be specific to the pulmonary circulation. These results clearly show that PE potentiates both the NO-independent and -dependent components of cyclic AMP-mediated relaxation in pulmonary arteries of the rat, although the effect on the former is more profound. We suggest that potentiation of both components is largely due to direct activation of adenylate cyclase via alpha1-adrenoceptors, within the smooth muscle and endothelial cells respectively.