Effects of angiotensin II on behavioral responses of defensive burying paradigm in rats

Behavioural assays to model cognitive and affective dimensions of depression and anxiety in rats

Animal models have been used extensively to investigate neuropsychiatric disorders, such as depression, and their treatment. However, the aetiology and pathophysiology of many such disorders are largely unknown, which makes validation of animal models particularly challenging. Furthermore, many diagnostic symptoms are difficult to define, operationalize and quantify, especially in experimental animals such as rats. Thus, rather than attempting to model complex human syndromes such as depression in their entirety, it can be more productive to define and model components of the illness that may account for clusters of co-varying symptoms, and that may share common underlying neurobiological mechanisms. In preclinical investigations of the neural regulatory mechanisms linking stress to depression and anxiety disorders, as well as the mechanisms by which chronic treatment with antidepressant drugs may exert their beneficial effects in these conditions, we have employed a number of behavioural tests in rats to model specific cognitive and anxiety-like components of depression and anxiety disorders. In the present study, we review the procedures for conducting four such behavioural assays: the attentional set-shifting test, the elevated-plus maze, the social interaction test and the shock-probe defensive burying test. The purpose is to serve as a guide to the utility and limitations of these tools, and as an aid in optimising their use and productivity.

Angiotensin as a target for the treatment of Alzheimer’s disease, anxiety and depression

The brain renin-angiotensin system (RAS), which is comprised of a variety of peptides including angiotensin II, angiotensin III and angiotensin IV acting on AT<inf>1</inf>, AT<inf>2</inf> and AT<inf>4</inf> receptors, is important in cognition and anxiety. Perturbation of the RAS improves basal cognition and reverses age-, scopolamine-, ethanol- and diabetes-induced deficits. In studies of dementias and Alzheimer's disease (AD), some studies have shown that antihypertensive drugs, including angiotensin-converting enzyme inhibitors, have some moderate effects on cognitive decline, but that the angiotensin receptor antagonist losartan has a significantly beneficial effect. These findings suggest that angiotensin receptor ligands may have potential in the prevention or even reversal of vascular dementias and AD. With respect to depression and anxiety, there is similar experimental evidence from animal models that drugs acting on the RAS may be antidepressant or anxiolytic, but insufficient clinical data exist. Such effects, if proven, could promote the use of such agents in the treatment of hypertension coexisting with depression or anxiety.

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.

Defensive burying in rodents: ethology, neurobiology and psychopharmacology

Defensive burying refers to the typical rodent behavior of displacing bedding material with vigorous treading-like movements of their forepaws and shoveling movements of their heads directed towards a variety of noxious stimuli that pose a near and immediate threat, such as a wall-mounted electrified shock-prod. Since its introduction 25 years ago by Pinel and Treit [J. Comp. Physiol. Psychol. 92 (1978) 708], defensive (shock-prod) burying has been the focus of a considerable amount of research effort delineating the methodology/ethology, psychopharmacology and neurobiology of this robust and species-specific active avoidance or coping response. The present review gives a summary of this research with special reference to the behavioral (face and construct) and pharmacological (predictive) validity of the shock-prod burying test as an animal model for human anxiety. Emphasis is also placed on some recent modifications of the paradigm that may increase its utility and reliability as to individual differences in expressed emotional coping responses and sensitivity to pharmacological treatments. Overall, the behavioral and physiological responses displayed in the shock-prod paradigm are expressions of normal and functionally adaptive coping patterns and the extremes of either active (i.e., burying) or passive (i.e., freezing) forms of responding in this test cannot simply be regarded as inappropriate, maladaptive or pathological. For this reason, the shock-prod paradigm is not an animal model for anxiety disorder or for any other psychiatric disease, but instead possesses a high degree of face and construct validity for normal and functionally adaptive human fear and anxious apprehension. However, the apparent good pharmacological validation (predictive validity) of this test reinforces the view that normal and pathological anxiety involves, at least partly, common neurobiological substrates. Therefore, this paradigm is not only suitable for screening potential anxiolytic properties of new drugs, but seems to be especially valuable for unraveling the neural circuitry and neurochemical mechanisms underlying the generation of active and passive coping responses as different expressions of anxiety.

The role of angiotensin II in cognition and behaviour

Polymorphisms of the renin-angiotensin system are associated with cardiovascular disorders, possibly as a consequence of increased brain angiotensin II activity. Within the brain, angiotensin controls blood pressure, fluid balance and hormone secretion; it also influences behaviour: reduction of central angiotensin function has both antidepressant-like and axiolytic-like actions. Evidence concerning the role of the renin-angiotensin system in learning and memory is contradictory, although more studies support the proposal that angiotensin reduces cognitive function. Studies of renin-angiotensin system genotype and psychological status have suggested an association between the angiotensin-converting enzyme deletion allele and age related cognitive decline, but a greater prevalence of the insertion allele in Alzheimer's disease. The deletion allele has also been associated with depressive illness, as has the M allele of the angiotensinogen gene although other studies have failed to replicate these findings. The role of the brain renin-angiotensin system in human psychopathology remains to be fully explored.

Anxiolytic-like actions of toluene in the burying behavior and plus-maze tests: differences in sensitivity between 5-HT(1B) knockout and wild-type mice

This paper compares the anxiolytic-like actions of toluene in two anxiety paradigms, the burying behavior and plus-maze tests, in 5-HT(1B) knockout (KO) and 129/Sv-ter wild-type (WT) mice. Static exposures were conducted in 29-l gas chromatographic jars. Animals were exposed to toluene (0, 1000, 2000 or 4000 ppm; n=8-12, each) for 30 min, and immediately after, tested in one of the anxiety paradigms. Motor coordination was evaluated in the rota-rod test in independent groups of mice. Toluene produced a dose-dependent decrease in anxiety-like levels in both anxiety paradigms and in both the strains. However, toluene exerted its effects at lower concentrations in KO mice than in the WT strain. These results cannot be attributed to a decrease in motor coordination since all the animals behaved similarly in the rota-rod test, regardless of the treatment. To discard any inherent difference in the nociception threshold between strains, mice were tested in the hot plate immediately after being exposed to either air or toluene. Toluene increased nociception in a similar fashion in both the strains. Our results suggest that 5-HT(1B) KO mice are more sensitive to those of toluene's actions related to anxiety, but not to those related with motor coordination or nociception. Data are discussed in terms of toluene's mechanisms of action and on differences between WT and KO animals.

Anxiolytic-Like effect of ejaculation under various sexual behavior conditions in the male rat

The purpose of the present study was to analyze the anxiety-like effect induced by ejaculation in male rats subjected to different sexual behavior conditions. The animal model of anxiety used was the conditioned defensive burying test. Results showed that experimental anxiety was reduced after one or six consecutive ejaculations. Six ejaculations did not induce a larger reduction in burying behavior than that produced by two, suggesting that this effect is not cumulative. This anxiolytic-like effect endured a short period (less than 24 h), and was not accompanied by a reduction in ambulatory behavior. The present results also showed a facilitating action of a previous ejaculation on the reduction in burying behavior induced by a second ejaculatory response. This potentiation occurred with an interval of 24 h between ejaculations. In sexually exhausted rats two populations are distinguished: one sexually unresponsive, and one achieving one ejaculation. Interestingly, in the ejaculatory population no reduction in burying behavior was observed, while in the unresponsive one a diminution in defensive burying was found. Data reveal differences in the anxiolytic-like properties of ejaculation between nonsatiated rats and the two populations of sexually exhausted animals.

The role of angiotensin II and of its receptor subtypes in the acetic acid-induced abdominal constriction test

The effects of angiotensin II (AngII), the AngII analogues saralasin--[Sar1, Ala8]AngII, sarmesin--[Sar1Tyr(Me)4]AngII, the nonpeptide AngII receptor antagonists DuP753 (losartan) (for AT1 receptor subtype) and PD123319 (for AT2 receptor subtype), as well as combinations of AngII and each of its analogues and receptor antagonists, administered intracerebroventricularly (ICV), were studied on mice using the acetic acid-induced abdominal constrictions test (acetic acid 1% intraperitoneally, IP). The abdominal constrictions were counted at 5-min intervals for 30 min. AngII at doses of 0.05, 0.1, and 1 microg exerted a dose-dependent antinociceptive effect. Saralasin, sarmesin, losartan, and PD123319 exhibited a dose-dependent effect on nociception: they either increased or decreased it. PD123319 antagonized the antinociceptive effect of AnglI while losartan was ineffective. The importance of AT2 receptor subtype for the nociception reducing effect of AngII is considered.

In vitro effect of angiotensin II on GABA release in rat hippocampus

In rat hippocampal slices [3H]GABA release evoked by 25 mM KCI consisted of Ca2+-dependent and Ca2+-independent fractions. Angiotensin II (AngII) at a concentration of 1 microM inhibited K+-stimulated [3H]GABA release. The effect of AngII (20% inhibition) on [3H]GABA release was decreased by the addition of 0.01 mM nipecotic acid to the superfusion medium. AngII also decreased the Ca2+-independent carrier-mediated [3H]GABA release (25% inhibition at a concentration of 1 microM). Different mechanisms of the neuromodulatory action of AngII on GABA release are discussed.

Effects of non-peptide angiotensin II-receptor antagonists on pentylenetetrazol kindling in mice

The effects of non-peptide AT1- and AT2-receptor antagonists DuP 753 (losartan) and PD 123319 on the intensity of pentylenetetrazol (PTZ)-kindled seizures in mice were studied. PTZ was injected intraperitoneally at a subconvulsive dose of 40 mg/kg at 48 h until the appearance of clonic seizures. DuP 753 administered intracerebroventricularly (i.c.v.) tended to decrease seizure intensity. Successive administration of ineffective doses of DuP 753 (losartan) and AT2 (angiotensin II) significantly decreased seizure intensity. PD 123319 (i.c.v.) decreased seizure intensity. Combination of ineffective doses of PD 123319 and AT2 also significantly decreased seizure intensity. The results suggest the role of AT2 receptor and its subtypes in PTZ-kindled seizures as well as an action of DuP 753 and PD 123319 similar to the action of AT2, an AT2-receptor agonist.

Ability of angiotensin II to modulate striatal dopamine release via the AT1 receptor in vitro and in vivo

1. The ability of angiotensin II to modulate dopamine release from rat striatal slices in vitro and in the intact rat striatum in vivo was assessed by the microdialysis technique. 2. In slices of rat striatum, angiotensin II (0.1-1.0 microM) induced a concentration-related increase in endogenous dopamine release which was maximal (approximately 250% above basal levels) within the first 2-4 min of agonist application and subsequently declined to near basal values. The angiotensin II-induced increase in dopamine release was Ca(2+)-dependent and was completely antagonized by the selective AT1 receptor antagonist, losartan (1.0 microM). In contrast, the AT2 receptor antagonist, PD123177 (1.0 microM) failed to modify the angiotensin II-induced response. Neither antagonist alone modified basal dopamine release from striatal slices. 3. In freely moving rats, angiotensin II (1.0-10 microM; administered via the microdialysis probe) induced a concentration-related increase in extracellular levels of dopamine which was maximal (approximately 150% above basal levels) within 20-40 min of agonist application and subsequently declined. The angiotensin II (10 microM)-induced increase in extracellular levels of dopamine was completely antagonized by the AT1 receptor antagonist, losartan (0.1-1.0 microM; administered via the microdialysis probe) but not by the AT2 receptor antagonist, PD123177 (1.0 microM; administered via the microdialysis probe). Neither antagonist alone modified basal extracellular levels of dopamine. 4. Homogenate radioligand binding studies with [125I]-angiotensin II (0.1 nm) identified relatively low levels of specific binding sites in rat striatal homogenates compared to homogenates of pyriform cortex (51.3 +/- 9.2 and 651.3 +/- 55.1 fmol g-1 wet weight, respectively, mean +/- s.e.mean, n = 3; non-specific binding defined by unlabelled angiotensin II). The majority of the specific [125I]-angiotensin II (0.1 nM) binding in the striatal and pyriform cortex homogenates was sensitive to the selective AT1 receptor antagonist, losartan (1.0 microM). 5. In conclusions the present study provides direct evidence that angiotensin II acting via the AT1 receptor subtype facilitates the release of dopamine in the rat striatum in vitro and in vivo. This receptor-mediated response may account for the modulation of dopamine-mediated behavioural responses by antagonists of the AT1 receptor and inhibitors of angiotensin converting enzyme.