A water lick conflict paradigm using drug experienced rats

Behavioral effects of neuropeptides in rodent models of depression and anxiety

In recent years, studies have advocated neuropeptide systems as modulators for the behavioral states found in mood disorders such as depression and anxiety disorders. Neuropeptides have been tested in traditional animal models and screening procedures that have been validated by known antidepressants and anxiolytics. However, it has become clear that although these tests are very useful, neuropeptides have distinct behavioral effects and dose-dependent characteristics, and therefore, use of these tests with neuropeptides must be done with an understanding of their unique characteristics. This review will focus on the behavioral actions of neuropeptides and their synthetic analogs, particularly in studies utilizing various preclinical tests of depression and anxiety. Specifically, the following neuropeptide systems will be reviewed: corticotropin-releasing factor (CRF), urocortin (Ucn), teneurin C-terminal associated peptide (TCAP), neuropeptide Y (NPY), arginine vasopressin (AVP), oxytocin, the Tyr-MIF-1 family, cholecystokinin (CCK), galanin, and substance P. These neuropeptide systems each have a unique role in the regulation of stress-like behavior, and therefore provide intriguing therapeutic targets for mood disorder treatment.

Reconciling the role of central serotonin neurons in human and animal behavior

Animal research suggests that central serotonergic neurons are involved in behavioral suppression, particularly anxiety-related inhibition. The hypothesis linking decreased serotonin transmission to reduced anxiety as the mechanism in the anxiolytic activity of benzodiazepines conflicts with most clinical observations. Serotonin antagonists show no marked capacity to alleviate anxiety. On the other hand, clinical signs of reduced serotonergic transmission (low 5-HIAA levels in the cerebrospinal fluid) are frequently associated with aggressiveness, suicide attempts, and increased anxiety. The target article attempts to reconcile such human and animal findings by investigating whether anxiety reduction or increased impulsivity is more Likely to account for animal behavioral changes associated with decreased serotonergic transmission. The effects of manipulating central serotonin in experimental anxiety paradigms in animals (punishment, extinction, novelty) are reviewed and compared with the effects of antianxiety drugs. Anxiety seems neither necessary nor sufficient to induce control by serotonergic neurons on behavior. Further evidence suggests that behavioral effects of anxiolytics thought to be mediated by decreases in anxiety are not caused by the ability of these drugs to reduce serotonin transmission. Blockade of serotonin transmission, especially at the level of the substantia nigra, results in a shift of behavior toward facilitation of responding. This behavioral shift is particularly marked when there is competition between acting and restraining response tendencies and when obstacles prevent the immediate attainment of an anticipated reward. It is proposed that serotonergic neurons are involved not only in behavioral arousal but also in enabling the organism to arrange or tolerate delay before acting. Decreases in serotonin transmission seem to be associated with the increased performance of behaviors that are usually suppressed, though not necessarily because of the alleviation of anxiety, which might contribute to the suppression.

Anxiolytic-like profiles of histamine H3 receptor agonists in animal models of anxiety: a comparative study with antidepressants and benzodiazepine anxiolytic

RATIONALE

Histamine H3 receptor functions as a presynaptic auto- and hetero-receptor on histaminergic and non-histaminergic neurons in the brain regulating the synaptic release of numerous neurotransmitters. Therefore, the ligands for this receptor have been proposed to be of therapeutic interest for the treatment of various neuropsychiatric disorders. At present, however, the psychopharmacological profiles of H3 ligands, particularly H3 agonists, have not been extensively studied.

OBJECTIVE

The present study investigated the anxiolytic-like profiles of H3-selective agonists in a variety of classical (benzodiazepine-sensitive) and atypical (antidepressant-effective) animal models of anxiety. Comparator drugs used were diazepam and both fluvoxamine and desipramine in the former and latter models, respectively.

RESULTS

H3 agonist R-alpha-methylhistamine and immepip were inactive in rat elevated plus maze test and Vogel type conflict test where diazepam (5 mg/kg) produced significant anxiolytic-like effects. Meanwhile, these H3 agonists (10-30 mg/kg) significantly reduced isolation-induced vocalizations in guinea pig pups and isolation-induced aggressive behavior in mouse resident-intruder test. Moreover, in rat conditioned fear stress test, R-alpha-methylhistamine (30 mg/kg) and immepip (10 mg/kg) significantly decreased freezing time, which were completely reversed by concomitant treatment with H3 antagonist, thioperamide (10 mg/kg). In contrast to the limited efficacy obtained with desipramine (30 mg/kg), fluvoxamine (20-60 mg/kg) exhibited anxiolytic-like effects in all the latter three atypical models.

CONCLUSIONS

These data suggest that the H3 agonists may have anxiolytic-like effects similar to those of selective serotonin reuptake inhibitors but not benzodiazepine anxiolytics and represent a novel strategy for the treatment of some anxiety disorders in which selective serotonin reuptake inhibitors are prescribed.

Picrotoxin blocks the anxiolytic- and panicolytic-like effects of sodium valproate in the rat elevated T-maze

The effect of acute sodium valproate administration, an anxiolytic and putative panicolytic drug, was evaluated in rats tested in the elevated T-maze, an animal model that measures two defensive reactions: avoidance (inhibitory avoidance), related to generalized anxiety, and escape (escape from open arms), related to panic. Additionally, the involvement of gamma-aminobutyric acid (GABA) neurotransmission in sodium valproate effects was studied by picrotoxin co-administration. Sodium valproate (300 mg/kg, intraperitoneally, 30 min before the test) impaired both avoidance latency (time to leave the closed arm) and one-way escape (latency to enter the closed arm) indicating anxiolytic and panicolytic effects, respectively. Pre-treatment with picrotoxin (0.5 mg/kg, intraperitoneally, 5 min before sodium valproate administration) blocked the effects of sodium valproate on inhibitory avoidance and one-way escape. No locomotor effect was seen in the open-field. These data suggest that sodium valproate exerts anxiolytic-like and panicolytic-like effects in the elevated T-maze and that these effects were mediated by picrotoxin-sensitive GABA type A receptors.

Recent advances in animal models of chronic antidepressant effects: the novelty-induced hypophagia test

Animal models exhibiting sensitivity to chronic, but not acute, antidepressant treatment are greatly needed for studying the neural mechanisms of the antidepressant response. Although several models of acute antidepressant effects provide excellent tools for antidepressant discovery, they do not permit investigation into their therapeutic effects, which require several weeks of treatment to emerge. The inhibition of feeding produced by novelty, termed 'hyponeophagia', provides an anxiety-related measure that is sensitive to the effects of chronic, but not acute or subchronic, antidepressant treatment. This review evaluates the value of hyponeophagia-based tests as tools for investigating the neurobiology of the therapeutic response to antidepressant treatment. Criteria for the development and validation of animal models used to study neurobiological mechanisms of the antidepressant response are presented. Methodological considerations affecting the reliability, specificity, and ease of use of hyponeophagia-based models are also discussed. Lastly, we present a newly revised hyponeophagia paradigm, called the novelty-induced hypophagia (NIH) test, which attempts to maximize the predictive validity and practicality of the test. The NIH paradigm provides a promising new model for investigations into the neurobiology underlying the antidepressant response.

Retigabine: chemical synthesis to clinical application

Retigabine [D23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester] is an antiepileptic drug with a recently described novel mechanism of action that involves opening of neuronal K(V)7.2-7.5 (formerly KCNQ2-5) voltage-activated K(+) channels. These channels (primarily K(V)7.2/7.3) enable generation of the M-current, a subthreshold K(+) current that serves to stabilize the membrane potential and control neuronal excitability. In this regard, retigabine has been shown to have a broad-spectrum of activity in animal models of electrically-induced (amygdala-kindling, maximal electroshock) and chemically-induced (pentylenetetrazole, picrotoxin, NMDA) epileptic seizures. These encouraging results suggest that retigabine may also prove useful in the treatment of other diseases associated with neuronal hyperexcitability. Neuropathic pain conditions are characterized by pathological changes in sensory pathways, which favor action potential generation and enhanced pain transmission. Although sometimes difficult to treat with conventional analgesics, antiepileptics can relieve some symptoms of neuropathic pain. A number of recent studies have reported that retigabine can relieve pain-like behaviors (hyperalgesia and allodynia) in animal models of neuropathic pain. Neuronal activation within several key structures within the CNS can also be observed in various animal models of anxiety. Moreover, amygdala-kindled rats, which have a lowered threshold for neuronal activation, also display enhanced anxiety-like responses. Retigabine dose-dependently reduces unconditioned anxiety-like behaviors when assessed in the mouse marble burying test and zero maze. Early clinical studies have indicated that retigabine is rapidly absorbed and distributed, and is resistant to first pass metabolism. Tolerability is good in humans when titrated up to its therapeutic dose range (600-1200 mg/day). No tolerance, dependence or withdrawal potential has been reported, although adverse effects can include mild dizziness, headache, nausea and somnolence. Thus, retigabine may prove to be useful in the treatment of a diverse range of disease states in which neuronal hyperexcitability is a common underlying factor.

Implication of 5-HT2 receptor subtypes in the mechanism of action of antidepressants in the four plates test

RATIONALE

The selective serotonin reuptake inhibitors (SSRIs) and the serotonin and noradrenaline reuptake inhibitors (SNRIs) increase synaptic levels of serotonin, leading to an increased activation of a multitude of specific postsynaptic 5-HT receptors. However, it is not yet known which 5-HT receptor subtypes mediate the therapeutic effects of antidepressants.

METHODS

The effects of the SSRI, paroxetine and the SNRI, venlafaxine were evaluated in the mouse four plates test (FPT).

RESULTS

Paroxetine administered intraperitoneally (IP) (0.5, 2-8 mg/kg) potently augmented the number of punished passages accepted by mice in this paradigm. The effects of paroxetine (8 mg/kg) were not reversed by the selective 5-HT2C receptor antagonist, RS 10-2221 (0.1 mg/kg and 1 mg/kg) or the selective 5-HT2B/2C receptor antagonist SB 206553 (0.1 mg/kg and 1 mg/kg), at doses which lack an effect when administered alone. In contrast, the selective 5-HT2A receptor antagonist, SR 46349B (0.1 mg/kg and 1 mg/kg) completely abolished the paroxetine-induced increase in punished passages. The acute administration of venlafaxine induced an anxiolytic-like effect in the FPT at the doses of 2-16 mg/kg. This effect was reversed by the 5-HT2B/2C receptor antagonist as did SR 46349B, for both doses administered. Our results strongly suggest that activation of 5-HT2A receptors is critically involved in the anxiolytic activity of paroxetine, whereas the 5-HT2A and 5-HT2B receptors are involved in the anti-punishment action of venlafaxine in the FPT. The co-administration of selective 5-HT2A, 2B, 2C receptor agonists (DOI, 0.06 mg/kg and 0.25 mg/kg; BW 723C86, 0.5 mg/kg and 2 mg/kg and RO 60-0175, 0.06 mg/kg and 0.25 mg/kg), respectively, was subsequently investigated. The effects of sub-active doses of paroxetine (0.25 mg/kg and 1 mg/kg) were augmented by BW 723C86 and RO 60-0175 receptor agonist challenge. The anti-punishment effects of venlafaxine (0.25 mg/kg and 1 mg/kg) were potentialised only by DOI co-administration.

CONCLUSION

These results indicate that the co-administration of 5-HT2 receptor agonists with paroxetine and venlafaxine may provide a powerful tool for enhancing the clinical efficacy of these antidepressants.

A synthetic peptide ligand of NCAM affects exploratory behavior and memory in rodents

The neural cell adhesion molecule (NCAM) is an important modulator of neuronal development and plasticity associated with learning and memory. Previously, a synthetic peptide ligand of NCAM, called C3, has been identified and shown to modulate neuronal plasticity in vitro and memory in a step-through passive avoidance task in rats in vivo. In this study, we extended these findings and found that intraventricular injection of C3 prior to training impaired learning or memory processes in rats and mice in an approach avoidance task and decreased exploratory behavior in rats. The effect of C3 was additionally evaluated in the Morris water maze; memory impairment was observed in the second training trial 24 h after the injection of C3 only, indicating an effect on short-term memory. The C3-mediated memory impairment observed in the approach avoidance and water maze tests is suggested to be the result of C3-inhibiting NCAM functions in the brain. This study demonstrates that it is possible to modulate learning/memory processes in rodents in vivo with small synthetic NCAM-binding peptides that induce developmental plasticity in vitro.

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.

[Effects of fluvoxamine on both the desired anxiolytic effect and the adverse motor incoordination and amnesia induced by benzodiazepines]

Fluvoxamine, a selective serotonin reuptake inhibitor, is frequently used along with benzodiazepine anxiolytics in clinics. In this study, the effects of fluvoxamine on the anxiolytic effects as well as adverse effects of benzodiazepines were examined in the light/dark box, rota-rod and passive avoidance tests using mice. Diazepam, ethyl loflazepate and its active metabolite, CM7116, were used as benzodiazepine anxiolytics. The anxiolytic effects of diazepam, ethyl loflazepate and CM7116 were potentiated by intraperitoneal treatment with fluvoxamine at 10 mg/kg, whereas only those of ethyl loflazepate were potentiated by fluvoxamine at 45 mg/kg. The motor incoordination and amnesia induced by ethyl loflazepate and CM7116 were not affected by fluvoxamine, although these adverse effects of diazepam were potentiated by fluvoxamine at 45 mg/kg. Fluvoxamine itself showed no effects in any of the tests. These results suggest that low-dose fluvoxamine potentiates the anxiolytic effects of benzodiazepines, while high-dose fluvoxamine augments the adverse effects depending on the benzodiazepine used. Consequently, when fluvoxamine is administered along with benzodiazepines, the doses of both fluvoxamine and benzodiazepines should be carefully chosen to achieve anxiolytic effects without any adverse results.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Anxiolytic-like effect of paroxetine in a rat social interaction test

The effects of short- and long-term administration of the selective serotonin reuptake inhibitor paroxetine were investigated in a rat social interaction test. A single administration of paroxetine at oral doses of 0.3, 1, 3 and 10 mg/kg had no effect on social interaction between pairs of male rats under bright light (high anxiety) conditions. After 21 days of daily administration, paroxetine given orally at 3 mg/kg significantly (p < 0.01) increased the time spent in social interaction by pairs of rats tested under the same conditions, with no effect on locomotor activity, indicating an anxiolytic-like effect. The magnitude of increase (+97%) was comparable to that seen after a single dose of chlordiazepoxide (4 mg/kg orally). Although there was also an increase in time spent in social interaction after 21 days of repeated oral administration of paroxetine at 0.3, 1, and 10 mg/kg (+44, +56, and +54% increases, respectively), statistical significance was not achieved. These results indicate that in the long term paroxetine has an anxiolytic action, and thus support the clinical evidence for its therapeutic use in the treatment of anxiety disorders in addition to its established role as an antidepressant.

Anxiolytic effects of serotonergic interventions in the shock-probe burying test and the elevated plus-maze test

Although serotonergic neural systems have been implicated in the control of anxiety for a number of years, evidence in favour of this role is controversial. The present experiments were designed to further characterize the putative role of serotonin (5-HT) in anxiety, using two pharmacologically validated animal models: the elevated plus-maze and the shock-probe burying tests. If the integrity of 5-HT neural systems is necessary for the expression of 'anxious' behaviors, then disruption of 5-HT systems should produce effects in the plus-maze and shock-probe tests that are similar to those of anxiolytic drugs. In the present experiments, serotonergic function was disrupted in rats, either by chemical depletion using the synthesis inhibitor p-CPA, by inhibitory autoreceptor activation using the selective 5-HT1A receptor ligand 8-OH-DPAT, or by electrolytic lesions of the serotonin-containing, dorsal raphe nucleus. p-CPA and dorsal raphe lesions produced robust anxiolytic effects in the elevated plus-maze and the shock-probe burying tests, whereas 8-OH-DPAT produced anxiolytic effects only in the shock-probe burying test, and 'anxiogenic' effects in the elevated plus-maze test. Although these results generally support the view that serotonin plays a role in the expression of 'anxious' behavior, the opposite effects of 8-OH-DPAT in the two behavioral paradigms suggest that the 5-HT1A receptor subtype exerts differential control over different types of experimental anxiety.

Valproate potentiates and picrotoxin antagonizes the anxiolytic action of ethanol in a nonshock conflict task

The purpose of this study was to examine the effects of the indirect GABA agonist valproate and the indirect GABA antagonist picrotoxin on the anxiolytic (anti-conflict) activity of ethanol in a behavioral conflict task that does not employ electroshock. This task (negative contrast) quantifies how animals respond to an abrupt, unexpected reduction in reward. Treatment with valproate alone did not elevated depressed behavior engendered by abrupt reduction in reward. However, when administered together with a sub-effective dose of ethanol (0.5 g/kg), valproate (50-200 mg/kg) dose-dependently potentiated the anxiolytic action of ethanol. Picrotoxin (2 mg/kg) antagonized the anxiolytic effects of a larger dose of ethanol (1.0 g/kg) given alone, as well as the ability of valproate to enhance the anxiolytic effects of smaller dose of ethanol (0.5 g/kg). As such, these data support a role for GABA in mediating the anxiolytic activity of ethanol.

Effects of agonists and antagonists at the GABA/benzodiazepine receptor on conditioned suppression in rats

Certain drugs generally regarded as GABA agonists, such as valproate and combinations of muscimol and baclofen, have been reported to produce apparent anxiolytic effects in various animal behavioral tests. The present paper reports two experiments on the effects of these agents on conditioned suppression in rats. In the first study, muscimol (0, 1.25 micrograms/kg or 1 mg/kg), baclofen (0, 1 mg/kg) and combinations of these treatments failed to alleviate conditioned suppression. Experiment Two showed that valproate (200 mg/kg) did attenuate conditioned suppression, and that its effects were antagonised by picrotoxin (1.5 mg/kg), but not by bicuculline (1.5 mg/kg), Ro 15-1788 (10 mg/kg) or by delta-amino-n-valeric acid (10 mg/kg). The findings are discussed in the context of the proposed GABA/benzodiazepine receptor complex, with the conclusion that there is little evidence for a mediating role of GABAa or GABAb receptors in such drug actions, and that the site of valproate action is probably the chloride ion channel associated with the receptor complex.

Amperozide and emotional behaviour

The new putatively antipsychotic drug amperozide is characterized pharmacologically by a specific limbic mode of action. Thus amperozide is a potent antagonist of muricidal behaviour (ED50 = 0.16 mg/kg) as well as aggression between isolated male mice. Although amperozide displays anxiolytic properties in Vogel's conflict test as well as an antidepressive effect in the despair test, the drug does not interfere with motor coordination or cause sedation (ED50 greater than 50 mg/kg). These results could make amperozide very interesting as an antipsychotic drug in the clinic, with effect on both positive and negative symptoms.

Chlordiazepoxide and valproate enhancement of saline drinking by nondeprived rats: effects of bicuculline, picrotoxin and Ro15-1788

Drinking of 0.85% saline by nondeprived rats was significantly enhanced by chlordiazepoxide (5 or 10 mg/kg) and by valproate (100 or 300 mg/kg), drug effects being strongest in the earlier parts of a 30-minute test. When given alone, both bicuculline and picrotoxin significantly reduced saline drinking at 2.5 mg/kg, but not 1.5 mg/kg. Administration of valproate at either dose or of chlordiazepoxide (10 mg/kg) completely prevented bicuculline action and 5 mg/kg chlordiazepoxide reduced it. Picrotoxin, however, largely prevented the actions of both chlordiazepoxide and valproate. The increase in saline drinking induced by valproate (300 mg/kg) was also blocked by RO15-1788 (10 or 25 mg/kg). These findings are discussed in the context of the three-site model of the GABA/benzodiazepine receptor complex. It is concluded that drugs acting at the benzodiazepine site or the chloride ion channel affect saline drinking, but that there is little evidence of an important functional role for the GABAa site at present.

Potentiation of the anticonflict effects of diazepam, but not pentobarbital and phenobarbital, by aminooxyacetic acid (AOAA)

The Conditioned Suppression of Drinking (CSD) paradigm is an "animal model" for anxiety which has been used to study the anticonflict effects of the benzodiazepines. It has been postulated that benzodiazepines produce their effects through interactions with GABA. The present study examined this potential GABA-BZ interaction on CSD behavior. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.5 mA), electrification being signalled by a tone. Within 2-3 weeks control CSD responding had stabilized (16-24 shocks session and 10-14 ml water/session); drug tests were conducted at weekly intervals. As expected, diazepam (0.3-20.0 mg/kg), pentobarbital (0.6-10.0 mg/kg) and phenobarbital (10.0-40.0 mg/kg) alone markedly increased the number of shocks received at doses which did not depress background responding (i.e., water intake). Treatment with the GABA-transaminase inhibitor aminooxyacetic acid (AOAA: 2.5-10.0 mg/kg, 10- or 60-minute pretreatment) alone had no anticonflict effect on CSD behavior. However, pretreatment (60-minute) with 10.0 mg/kg AOAA significantly potentiated the effects of diazepam, as indicated by a significant shift to the left in the diazepam dose-response curve relative to diazepam alone. By contrast, the anticonflict effects of pentobarbital and phenobarbital were unaffected by this AOAA pretreatment. Thus, while increases in GABA transmission alone do not appear to affect CSD behavior, the anticonflict effect of benzodiazepines, but not barbiturates, appear to be potentiated by increases in GABA transmission.

Chlordiazepoxide and the drinking of water by rats: effects of shock and other suppressive measures

The effects of chlordiazepoxide (5 and 10 mg/kg) on fluid consumption in water deprived rats were assessed. Drinking was inhibited to approximately equal extents by a water preload, by d-amphetamine (1.5 mg/kg), by neophobia and by shock at mild (0.3 mA) or moderate (0.5 mA) intensities, the latter condition having an enhanced level of deprivation also. At both doses chlordiazepoxide significantly enhanced drinking in the neophobia, mild shock and, especially, the moderate shock condition but failed to increase drinking suppressed by preload or d-amphetamine. It is concluded that the increases in drinking suppressed by neophobia or shock which chlordiazepoxide induces may be due to anxiolytic actions of the drug or to enhanced palatability since they cannot be explained in terms of nonspecific enhancement of fluid consumption.

Sodium valproate decreases exploratory behaviour in mice: development of tolerance and cross-tolerance with chlordiazepoxide

Sodium valproate (400 mg/kg) significantly reduced exploratory head-dipping in mice, without significant reductions in locomotor activity or rearing. After 5 daily injections of sodium valproate (50 or 400 mg/kg) there was tolerance to the effects of a test dose of 400 mg/kg. Pretreatment for 5 days with chlordiazepoxide (5 mg/kg) did not change the effects of sodium valproate (400 mg/kg), but 5 days pretreatment with chlordiazepoxide (20 mg/kg) did produce cross-tolerance. Sodium valproate (50 mg/kg) was without significant effect acutely, or after 5 days of pretreatment with sodium valproate (50 or 400 mg/kg) or chlordiazepoxide (5 or 20 mg/kg). Daily injections of the benzodiazepine antagonist, flumazenil (10 mg/kg), immediately after the daily injection of sodium valproate (400 mg/kg) or chlordiazepoxide (20 mg/kg) did not prevent the development of tolerance or cross-tolerance. This suggests that benzodiazepine receptors may not mediate these phenomena; possible roles of GABA and 5-HT are also discussed.

Competition for sucrose-pellets in triads of male Wistar rats: effects of three serotonergic drugs

1. Within triads of rats, individuals can be discerned according to their competition for sucrose-pellets as a high-, a medium- and a poor-performing animal. 2. The competition-rates, expressed as scores, are affected by serotonergic drugs: upon inhibiting tryptophan-hydroxylase, the characteristic abstention of the poor-performing rats can temporarily be overcome; quipazine, on the other hand, leads to a dose-dependent decrease in the competition-rate of high-performing rats. 3. Such findings are indicative for a regulatory effect of some serotonergic mechanisms on a competition-behavior evoked within the familiar social context.

Valproate enhances fluid consumption suppressed by shock or neophobia, but not by partial satiation or d-amphetamine, in rats

The effects of sodium valproate (100 and 300 mg/kg) on fluid consumption in water deprived rats were assessed. Drinking was inhibited to approximately equal extents by a water pre-load, by d-amphetamine (1.5 mg/kg), by neophobia and by shock at mild (0.3mA) or moderate (0.5mA) intensities, the latter condition having an enhanced level of deprivation also. At both doses valproate significantly enhanced drinking in the neophobia, mild shock and moderate shock conditions but failed to increase drinking suppressed by pre-load or d-amphetamine. It is concluded that the increases in drinking suppressed by neophobia or shock which valproate induces are due to anxiolytic actions of the drug and not non-specific enhancement of fluid consumption. The present results also constitute a further parallel between the actions of valproate and those of benzodiazepines.

Future directions in anxiety research

More than 3.5 million patients suffer from anxiety in the United States of America alone. While our knowledge of the basic biological mechanisms of anxiety is very poor, we have a good understanding of the mechanism of action of various anxiolytic drugs at the molecular level. Their mechanism of action often relates to the inhibitory neurotransmitter GABA and it might be speculated that GABA has a role in anxiety. Future research will concentrate on designing better anxiolytic drugs, based for example on the discovery of partial benzodiazepine receptor agonists, and also on GABA uptake inhibitors as enhancers of GABAergic function. Furthermore, scientific studies will focus on a search for putative biological defects related to anxiety, such as defects in the GABA/benzodiazepine receptor chloride channel complex.

Selective agonists and antagonists for 5-hydroxytryptamine receptor subtypes, and interactions with yohimbine and FG 7142 using the elevated plus-maze test in the rat

The effects of some 5-HT receptor ligands were investigated on measures of anxiety in an elevated plus-maze test in the rat. Quipazine (2 and 4 mg kg-1), a non-specific 5-HT agonist and ritanserin (0.25-10 mg kg-1), a 5-HT2 receptor antagonist displayed anxiogenic profiles by reducing both of the measures of anxiety used in this test. Two 5-HT1A receptor ligands, buspirone (4 and 8 mg kg-1) and ipsapirone (2.5-10 mg kg-1) and the 5-HT1 agonist, RU 24969 (0.1875-1.5 mg kg-1) significantly reduced only the percentage of time spent on the open arms. (-)-Propranolol (5 and 10 mg kg-1), a 5-HT1 receptor antagonist significantly reduced only the percentage of entries made onto the open arms. Metergoline (4 mg kg-1), a non-specific 5-HT antagonist displayed anxiolytic effects in this test by increasing both measures of anxiety. The 5-HT1A receptor agonist, 8-OH-DPAT (0.0625-0.25 mg kg-1) had no effect on either of the measures of anxiety. The results from the non-specific ligands (quipazine and metergoline) are consistent with the theory that a reduction in 5-HT function reduces anxiety. However, in spite of their more selective effects on 5-HT receptors the results in this test from the more specific ligands are not consistent with a strong involvement of any single receptor subtype. The interaction studies with yohimbine and FG 7142 (beta-carboline-3-carboxylate methylamide) provided no clear evidence for a major role of 5-HT pathways in the mediation of their anxiogenic effects.

Modulation of anxiety by beta-carbolines and other benzodiazepine receptor ligands: relationship of pharmacological to biochemical measures of efficacy

Several beta-carbolines and other benzodiazepines (BZ) receptor ligands have been investigated for anxiolytic or anxiogenic action in 4 unrelated animal models of anxiety using rats. The substances could be grouped into essentially 2 groups. The first, anxiolytics, exhibited antipunishment activity in a lick-suppression test, antagonised the discriminative stimulus provided by pentylenetetrazol, resembled chlordiazepoxide (CDP) in a drug discrimination test, and reduced the rise in plasma corticosterone levels following swim stress. Such substances included several benzodiazepines, the beta-carboline ZK 93 423, and the triazolapyridazine CL 218 872. A subgroup of anxiolytics were active in only some of these tests. They included two beta-carbolines, ZK 91 296 and ZK 95 962, and the pyrazoloquinoline CGS 9896, and these 3 substances were also distinguishable in not producing rate-decreasing effects in any of the 3 operant tests. The second group were anxiogenic in that they produced a discriminative stimulus resembling that of PTZ, they antagonised the CDP cue, exhibited propunishment effects in the lick-suppression test, and themselves caused increases in plasma corticosterone in otherwise unstressed animals. Such substances included the beta-carbolines DMCM, FG 7142 and ZK 90 886, and the pyrazoloquinoline CGS 8216. Two substances, Ro 15-1788 and ZK 93 426 had little or only weak activity in any test. The classification of these substances into anxiolytics or anxiogenics could be predicted qualitatively both by their ability to enhance (anxiolytics) or decrease the binding of 35S-TBPS to rat brain membranes and by whether their own binding was increased (anxiolytics) by adding the GABA agonist muscimol to the in vitro incubation medium. For the limited number of substances for which full data was available, there was also a quantitative relationship between the degree of enhancement of 35S-TBPS binding by a substance and its potency in the CDP cue test when such potency was expressed as numbers of BZ receptors occupied at the ED50 value in the pharmacological test. Furthermore, for the anxiolytics, activity in the CDP cue correlated significantly with potency in 2 other tests. Otherwise, surprisingly weak correlations existed between potencies in the different tests. In particular, the beta-carboline ZK 95 962 was highly potent in antagonising the PTZ cue but inactive in both a conflict test and in protecting against stress. These results are discussed in terms of differences in the neuropharmacologies of the 4 tests and in selectivity of the BZ receptor ligands for subtypes of BZ receptor.

Behavioral effects of GABA agonists in relation to anxiety and benzodiazepine action

A considerable body of biochemical and neurophysiological evidence implicates GABA in anxiety and in benzodiazepine action. The present article surveys the behavioral effects of GABA agonists and their interactions with drugs acting at the benzodiazepine receptor in animal anxiety paradigms. Certain GABA agonists, notably valproate, simulate many behavioral actions of benzodiazepines. Moreover, several behavioral studies of the interaction of GABA agonists with benzodiazepines support the hypothesis of a benzodiazepine receptor complex with one or more GABA, benzodiazepine and probably other binding sites. However, there are also a number of anomalous findings of GABA agonist action alone and in combination with benzodiazepines. It is argued that these paradoxical results can better be accounted for in terms of the receptor complex and the distribution of the drugs, rather than by suggesting that the anxiolytic actions of benzodiazepines are not mediated by GABA systems. The potential clinical usefulness of GABA agonists in anxiety is commented upon.

A new method for screening anxiolytic drugs in rats

In order to evaluate the anxiolytic action of drugs, a simple experimental procedure using a corridor-type runway was designed. In this apparatus, five food pellets were set in a row on a plastic platform. Rats with one day food-deprivation take a food pellet and then usually return to the start box. The time required to take 5 pellets (total time) and the number of returns were recorded. Diazepam (DZP) at 1-3.2 mg/kg and zopiclone (ZOP) at 10 mg/kg caused decreases in both parameters. These effects were blocked by the benzodiazepine receptor blocker, Ro15-1788, at 10 mg/kg. However, tracazolate failed to produce any change in both parameters. Haloperidol and imipramine prolonged the total time while reducing the number of returns. In contrast to DZP and ZOP, pentetrazol, well known to possess an anxiogenic effect, prolonged the total time. These results suggest that decreases in both the total time and the number of returns produced by DZP and ZOP may be related to their anxiolytic action which is mediated by a benzodiazepine receptor. Therefore, this procedure would be a simple and selective method for detecting benzodiazepine-type anxiolytics.

Neurotransmitters, anxiety and benzodiazepines: a behavioral review

The possible involvement of serotonin, GABA and opioid peptides in anxiety and in the mechanism of action of benzodiazepine tranquilizers have recently been the subjects of intensive biochemical, neurophysiological and behavioral research. The present review examines the behavioral evidence, viewing anxiety and benzodiazepine action as far as possible separately. Four behavioral paradigms of experimental anxiety or "conflict behaviors" are described and assessed for soundness with some practical considerations. The functional significance and pharmacology of benzodiazepine receptors are discussed, and the cases for a number of putative endogenous ligands are examined. Conflict behavior is attenuated by drugs which reduce functional serotonin activity and enhanced by serotonin agonists, but there is little evidence to implicate serotonin in benzodiazepine action. GABA antagonists both intensify conflict and reduce benzodiazepine effects, but evidence of the reverse effects with GABA agonists is more equivocal. The interpretation of behavioral effects of opiate agonists and antagonists and their interactions with benzodiazepines is hindered by their actions on motivational systems other than anxiety, and evidence for an important role of opioid peptides is only suggestive. Some promising lines for future research are indicated.

Effects of chlordiazepoxide and sodium valproate in two tests of spatial behaviour

The effects of chlordiazepoxide (CDP) and sodium valproate (VPA) were studied in rats trained to asymptotic performance on two tests of spatial behaviour, the 8-arm radial maze and the 8-choice arena. The task in the 8-arm maze was to locate a single food pellet at the end of each arm. Both CDP and VPA caused an increase in errors, an increase in performance time, and the utilization of a non-spatial response strategy. The task in the 8-choice arena was to locate a single water bottle from an octagonal array of eight otherwise empty bottles. For one group the goal bottle remained in the same place from trial to trial; for a second group the position of the goal bottle was cued by a black card over the nozzle; for the third group the goal bottle was uncued and moved randomly from trial to trial. VPA had no effect on performance, but CDP impaired performance in all three groups. These patterns of effects suggest that VPA may specifically disrupt working memory, but that the impairment of spatial performance by CDP probably results from a non-specific perceptual or attentional deficit.

Differential effects of dorsal raphe lesions and intraraphe GABA and benzodiazepines on conflict behavior in rats

Both reductions in brain serotonin activity and injections of benzodiazepine drugs increase punished responding in rats, but the evidence is conflicting on the role of serotonin pathways in the benzodiazepine effect. Therefore a series of studies were carried out using a Geller-Seifter procedure with three components, to examine drug effects on rewarded, nonrewarded, and punished responding. Using male hooded Lister rats and chronic indwelling cannulae, it was found that neither chlordiazepoxide (1.5 and 5.0 micrograms in 0.5 microliter), midazolam (1.0 and 10.0 micrograms in 0.5 microliter), nor GABA (100, 500, 1000, and 5000 ng in 0.5 microliter), exerted significant anticonflict activity when injected into the dorsal raphe. Lesions of the dorsal raphe produced by injections of 5,6-dihydroxytryptamine significantly increased punished responding, and there were significant correlations between lesion size, extent of forebrain serotonin depletion, and increases in punished responding. Peripheral injections of chlordiazepoxide (5.0 and 10.0 mg/kg) and midazolam (1.25, 2.5, and 5.0 mg/kg) significantly increased punished responding both before and after raphe lesions. The increase in lesioned animals was significantly greater than after drug or lesion alone and represented a powerful additive effect which was specific to punished responding. As intraraphe benzodiazepines did not exert significant anticonflict activity, and raphe lesions did not attenuate the anticonflict activity of peripheral benzodiazepines, it is concluded that increases in punished responding seen after serotonin depletion and after benzodiazepine drugs may be dissociable.