Corticosterone responses in 5-HT1B receptor knockout mice to stress or 5-HT1A receptor activation are normal

RATIONALE: Previous research found no adaptations in presynaptic 5-HT1A receptors in mice lacking 5-HT1B receptors (5-HT1B KO). Stress and 5-HT1A receptor agonists induce corticosterone release in mice via hypothalamus-pituitary-adrenal (HPA) axis activation. 5-HT1B KO mice are hyperreactive to mild stressors and this might be reflected in altered postsynaptic 5-HT1A receptor sensitivity. OBJECTIVES: Our aim was to determine whether the activity of the HPA axis was increased in 5-HT1B KO mice in response to mild stress and pharmacological activation of 5-HT1A receptors as an indication of putative adaptive changes in postsynaptic 5-HT1A receptor function. METHODS: The effect of mild stress [i.e., the stress-induced hyperthermia (SIH) paradigm], induced by rectal temperature measurement, was determined on temperature and corticosterone over time (0, 5, 10, 20, 30, 60, and 90 min) in 5-HT1B KO and wildtype mice. In addition, corticosterone was measured 60 min after 5-HT1A receptor activation by flesinoxan (0, 0.03, 0.1, 0.3, 1, and 3 mg/kg s.c.). Blood was collected and plasma corticosterone levels were determined by radioimmunoassay. RESULTS: Both genotypes showed comparable time-dependent SIH responses, whereas basal temperature was higher in 5-HT1B KO mice. The effect of SIH on temperature was mirrored by mild increases in plasma corticosterone. Activation of 5-HT1A receptors caused a strong dose-dependent release of corticosterone in both genotypes. Neither response observed showed differences between both genotypes. CONCLUSIONS: Although 5-HT1B KO mice are hyperreactive to mild stress, this reactivity is not reflected by stronger corticosterone responses in the SIH paradigm. The lack of shift in dose-response curves for flesinoxan suggests that postsynaptic 5-HT1A receptor function is unaffected in 5-HT1B KO mice.     

Stress-induced hyperthermia is reduced by rapid-acting anxiolytic drugs independent of injection stress in rats

Background

Stress-induced hyperthermia (SIH) is the transient rise in body temperature after encountering a stressor. The SIH response can be blocked by administration of various anxiolytic drugs prior to inducing stress. However, a drug injection involves handling and injection stress and therefore induces a SIH response itself. In the standard SIH test, drugs are therefore injected 60 min before stress induction to allow injection-induced hyperthermia to decline. This makes it difficult to study putative anxiolytic compounds with a short half-life. The present study therefore aimed to compare the effects of standard (stressful) and stress-free anxiolytic drug administration on the subsequent SIH response with a 10-minute injection-stressor interval.

Methods

Anxiolytic drugs with short half-lives (midazolam, 8-OH-DPAT, nicotine) were injected subcutaneously in rats using either a stressful (manual injection) or stress-free injection (subcutaneous cannula) method 10 min before novel cage stress. Body temperature and locomotor activity were measured using telemetric transmitters.

Results

Stressful and stress-free drug administration resulted in comparable drug effects on the stress-induced hyperthermia and locomotor responses in rats.

Conclusion

The present study shows that both stressful and stress-free drug injection shortly before a stressor results in reproducible attenuation of the SIH response in rats. In rats, a short injection-stressor interval can therefore be applied using the SIH model, enabling the study of putative anxiolytic drugs with short half-lives.     

Studies on the effects of 5-HT1A drugs in the pigeon

A number of compounds with high receptor binding affinity for the 5-hydroxytryptamine (5-HT) receptor subtype designated as 5-HT_1A can produce anxiolytic and/or antidepressant effects in humans. In contrast to the traditional benzodiazepine anxiolytics, many of the clinically efficacious 5-HT_1A drugs are either ineffective or produce inconsistent results in traditional preclinical anxiolytic screens using behavioral procedures with rodents. In preclinical antidepressant models with these animals, however, effects of the 5-HT_1A drugs, as well as those of traditional antidepressant compounds, are predictive of their antidepressant activity in humans. In contrast, 5-HT_1A drugs are quite effective in pigeons studied under a rather conventional punishment or conflict-type procedure that is also sensitive to the benzodiazepine anxiolytics. However, traditional antidepressant compounds, such as the tricyclic drugs amitriptyline and imipramine, as well as the 5-HT reuptake blockers such as fluoxetine, do not show effects similar to the newer 5-HT_1A drugs in this procedure. Thus, in rodents, current antidepressant models are sensitive to drugs that appear to function through different mechanisms, whereas conflict-type procedures typically do not reveal anxiolytic-like effects with 5-HT_1A drugs. The pigeon conflict procedure, however, discriminates between the 5-HT_1A antidepressants and antidepressant drugs functioning through other systems, whereas the effects of known anxiolytics in this procedure are quite similar. Increases in punished responding with 5-HT_1A drugs correlates highly (r= +0.83) with affinity for the 5-HT_1A receptor in pigeons. This paper reviews behavioral studies conducted with the pigeon in which the focus has been on the analysis of 5-HT_1A compounds and addresses additional work that is required to answer many of the outstanding questions about this new class of anxiolytic and/or antidepressant drugs. © 1992 Wiley-Liss, Inc.     

Differential effects of methamphetamine and cocaine on behavior and extracellular levels of dopamine and 3,4-dihydroxyphenylalanine in the nucleus accumbens of conscious rats

The stress-induced hyperthermia procedure, in which effects of drugs on basal (T₁) and stress-induced body temperature (T₂) are measured, predicts anxiolytic drug effect. Serotonergic drugs alter these responses and here, we studied the role of 5-HT_1A receptors in stress-induced hyperthermia by using 5-HT_1A receptor knockout mice. Three strains (129/Sv, Swiss Webster and C57Bl6) were used because genetic background can significantly modulate the null phenotype. We found that GABA-ergic drugs with an anxiolytic profile and stimulate ₂ subunit containing GABA_A receptors, including diazepam and L838,417, result in reduced ΔT (ΔT = T₂ − T₁). The ₁ subunit containing GABA_A receptor was found to be primarily involved in regulation of basal body temperature T₁ and its stimulation can induce hypothermia. In addition, stimulation of 5-HT_1A receptors by buspirone results in a reduced ΔT, while stimulation of 5-HT₇ receptors primarily results in hypothermia. The null mutation of 5-HT_1A receptors resulted in differences in drug-sensitivity that was further modulated by the genetic background. In particular, the null mutation on the SW and C57Bl6 backgrounds resulted in differential diazepam/L838,417 and 5-CT responses respectively. This indicates an interaction between the 5-HT_1A receptor and genetic background and demonstrates the importance of selecting the background strain in a receptor knockout model.     

5-HT receptors as targets for the development of novel anxiolytic drugs: models,mechanisms and future directions

The introduction of buspirone for the treatment of anxiety, together with the eventual suggestion of a mode of action involving the serotonin (5-HT)_1A receptor subtype, has generated considerable research activity and renewed interest in the potential role of 5-HT in anxiety. The further identification of multiple 5-HT₁ receptors, coupled with the possibility that these subtypes potentially are involved in discrete biobehavioral regulation and pathophysiological conditions, has greatly expanded the search for tools capable of probing these receptors and has raised hopes for a new generation of more specific compounds to treat other disorders associated with the 5-HT system such as depression, aggression, and sleep and eating disturbances. The involvement of 5-HT in anxiety has prompted a careful reevaluation of several traditional areas of research. This has included those methods used in the in vivo evaluation of drugs in preclinical animal test procedures used to assess potential anxiolytic activity, as well as the mechanisms associated with adaptive changes occurring during long-term drug administration. The proliferation of various procedures for studying the anxiolytic effects of 5-HT drugs has not always been accompanied by systematic behavioral and pharmacological validation. At the present time, this area of research is characterized by numerous inconsistent findings. Procedures that are objective and impartial to the behavioral effects of drugs provide distinct advantages for addressing some of these issues, as will the results from carefully controlled clinical studies. The main objective of this article is to provide an overview of the recent developments in research involving the 5-HT system and anxiety. The emphasis will be on the 5-HT₁ receptor system and a review of the results in the predominant animal models used to evaluate these drugs, as well as an overview of the mechanisms currently believed to be responsible for the therapeutic activity of this class of compounds. Studies with the pigeon are reviewed, since this species appears distinctly sensitive to the anxiolytic-like effects of 5-HT_1A drugs in conflict procedures. Although chronic administration of 5-HT_1A drugs appears necessary for clinical anxiolytic and antidepressant activity, the most noteworthy neuropharmacological effects in animals seem to occur in 5-HT₂ and, perhaps, 5-HT₃ receptors which are downregulated. Studies summarizing the activity of drugs interacting with 5-HT_1C/2 and 5-HT₃ receptor sites are also discussed as they too may be involved in anxiety or the actions of anxiolytic drugs. The growing evidence suggesting an interaction between 5-HT receptor types, particularly between 5-HT_1A and 5-HT_1C/2 receptors, is reviewed, since drugs with these combined properties appear to be particularly efficacious in animal models of anxiety and warrant further detailed analyses. The development of drugs targeted specifically at multiple receptors may provide distinct therapeutic advantages for disorders such as anxiety and depression that appear to involve multiple neurotransmitter systems.     

S 14506: A novel,potent, high-efficacy 5-HT1A agonist and potential anxiolytic agent

The studies characterized the newly discovered 7-methoxy naphtylpiperazine S 14506 and examined the possible relationship between the apparent in vivo efficacy of 5-HT_1A ligands and the magnitude of their anxiolytic potential. In vitro radioligand binding studies indicated S 14506 to bind with high affinity and in an apparently competitive manner to 5-HT_1A sites, while having much less affinity for other 5-HT binding sites. S 14506 demonstrated exceptional anxiolytic potential in the pigeon conflict procedure; the magnitude of its effects exceeded that of the other 5-HT_1A ligands 8-hydroxy-2-(n-propylamino) tetralin (8-OH-DPAT), (+)-flesinoxan, buspirone, and BMY 7378. The anticonflict action of S 14506 was antagonized by the 5-HT_1A antagonist (–)-alprenolol. Like the prototypical 5-HT_1A agonist 8-OH-DPAT, S 14506 induced forepaw treading, hypothermia and spontaneous tail flicks, antagonized morphine-induced analgesia, and produced stimulus generalization in rats discriminating 8-OH-DPAT from saline. The apparent in vivo efficacy of the 5-HT_1A ligands in stimulating corticosterone release varied as follows: S 14506 > 8-OH-DPAT > (+)-flesinoxan > buspirone > BMY 7378. This rank order corresponded with the rank order of magnitude of the anxiolytic action which the same compounds produced in the pigeon conflict procedure. The rank order of in vivo potency of these compounds in producing 5-HT_1A agonist actions in rodents. (i.e., S 14506) > 8-OH-DPAT > (+)-flesinoxan > bus-pirone > BMY 7378) also corresponded with their rank order of potency in producing anticonflict effects. The data characterize S 14506 as an extremely potent, orally active, high-efficacy agonist at 5-HTl_1A receptors that demonstrates exceptional anxiolytic potential in the pigeon conflict procedure. The results provide strong evidence that the activation of central nervous system (CNS) 5-HT_1A receptors yields marked anticonflict effects and that the anxiolytic potential of 5-HT_1A ligands is proportional to their efficacy at 5-HT_1A re-ceptors. © 1992 Wiley-Liss, Inc.     

Pasteur Euroconference on Future Therapies for an Anxious World

The aim of this meeting was to explore the molecular targets for tomorrow’s drugs with a group of European and North American speakers from the pharmaceutical industry and academia presenting both data and ideas. Several speakers considered 5-hydroxytryptamine_1B/D (5-HT_1B/D) receptor antagonists to be a promising target for the treatment of obsessive compulsive disorders (OCD). 5-HT moduline, a peptide modulator of the 5-HT_1B/D receptor, presents a potentially subtle way of intervention. No clinical data are as yet available on 5-HT_1B/D receptor antagonists. The paradox of agonists, partial agonists and antagonists at the 5-HT_1A receptor (all having anxiolytic activity in animal models) was discussed. Although the 5-HT_1A agonists tested in clinical trials to date have been disappointing, the clinical potential of 5-HT_1A antagonists still has to be evaluated. Similarly, both agonists and antagonists at the 5-HT_2C receptor have been shown to be active in animal models of anxiety. It seems, however, that 5-HT_2C agonists may have a therapeutic potential in panic disorders and OCD, while the antagonists may be more appropriate in generalised anxiety. Cholecystokinin (CCK) has been shown to be implicated in panic attacks which can be induced by agonists at the CCK-B subtype. CCK-B antagonists are thus potential antipanic agents although this has not yet been confirmed by clinical trials. Finally, inhibition of glutamatergic neurotransmission produces anxiolytic effects in a wide-range of animal models and the new antagonists at the metabotropic glutamate receptors may have potential as anti-anxiety agents. This report focuses on the presentations that addressed novel drug candidates or potential new drug targets.     

Reduction of guinea pig pup isolation calls by anxiolytic and antidepressant drugs

 Guinea pigs possess central 5-HT_1D receptors similar to humans but different from rats and mice. In order to study the role of this receptor on animal behaviour, it may be of interest to develop a paradigm measuring affective states in the guinea pig. Therefore we assessed the effects of a variety of psychotropic drugs on guinea pig pup isolation calls. Anxiolytic compounds such as the benzodiazepine receptor agonists diazepam and alprazolam, the full 5-HT_1A receptor agonists 8-OH-DPAT and flesinoxan, and alcohol reduced isolation calling by the guinea pig pup. Moreover, mixed antidepressant/anxiolytic compounds like the 5-HT uptake inhibitors fluvoxamine and clomipramine or the MAO-inhibitor clorgyline as well as the antidepressant NA uptake inhibitors desipramine and maprotiline suppressed vocalizations. The 5-HT_1D/1A receptor agonist 5-CT was also very effective in reducing separation calls. Remarkably, the partial 5-HT_1A receptor agonists buspirone and BMY 7378 did not affect calling. The neuroleptic haloperidol, the psychostimulant d-amphetamine, the putative anxiogenics DMCM and m-CPP and the putative anxiolytics ondansetron and CI-988 had no effect on isolation calls of guinea pig pups. We propose this paradigm could be helpful to assess behavioural effects of anxiolytic and antidepressant drugs in a species different from rat or mouse, and in which the effects of 5-HT_1D receptor ligands may possibly be established. Received: 18 May 1995 / Final version: 20 June 1996     

Overview on 5-HT receptors and their role in physiology and pathology of the central nervous system

The present review gives an overview on the serotonin (5-hydroxytryptamine; 5-HT) system, its receptors and their relationship to central nervous system physiology and disorders. Additionally, we also introduce the recent knowledge about the 5-HT receptor ligands in preclinical research, clinical trials and as approved drugs.     

The selective serotonin (5-HT)1A receptor ligand, S15535, displays anxiolytic-like effects in the social interaction and Vogel models and suppresses dialysate levels of 5-HT in the dorsal hippocampus of freely-moving rats

RATIONALE: The benzodioxane, S15535, possesses low intrinsic activity and marked selectivity at 5-HT1A receptors, hippocampal populations of which are implicated in anxious states. OBJECTIVE: Herein, we examined its potential anxiolytic actions in relation to its influence upon extracellular levels of 5-HT in the dorsal hippocampus of freely-moving rats. Its effects were compared with those of other anxiolytic agents: the 5-HT1A agonists, buspirone and 8-hydroxy-2-(di-n-propylamino)-tetralin HBr (8-OH-DPAT), the 5-HT2C antagonist, SB206,553 and the benzodiazepine, diazepam. METHODS: Potential anxiolytic actions were evaluated in the Vogel conflict paradigm (increase in punished responses) and the social interaction (SI) test (increase in active SI) in rats. Extracellular levels of 5-HT were determined by microdialysis. RESULTS: In analogy to diazepam. S15535 increased punished responses in the Vogel test. This action was dose dependently expressed over a broad (16-fold) dose range. Buspirone and 8-OH-DPAT were likewise active, but yielded highly biphasic dose-response curves. SB206,553 was dose dependently active in this model. In the SI test, S15535 similarly mimicked the anxiolytic-like effect of diazepam and was active over a broad dose range. Buspirone and 8-OH-DPAT again showed biphasic dose-response curves, as did SB206,553. In both the Vogel and SI tests, the anxiolytic-like effects of S15535 were abolished by the selective 5-HT1A receptor antagonist, WAY100,635, which was inactive alone. S15535 exerted its anxiolytic-like effects with a more pronounced separation to motor-disruptive doses than the other drugs. Finally, S15535 suppressed dialysate levels of 5-HT in the dorsal hippocampus, an action abolished by WAY100,635. Buspirone, 8-OH-DPAT and diazepam, but not SB206,553, also reduced 5-HT levels. CONCLUSION: Likely reflecting its distinctive ability to selectively and preferentially activate pre- versus postsynaptic 5-HT1A receptors, S15535 suppresses hippocampal 5-HT release and displays marked anxiolytic-like effects over a broad dose range in the relative absence of motor perturbation.     

Pharmacogenetics and the serotonin system: initial studies and future directions

Serotonin (5-hydroxytryptamine, 5-HT) appears to play a role in the pathophysiology of a range of neuropsychiatric disorders, and serotonergic agents are of central importance in neuropharmacology. Genes encoding various components of the 5-HT system are being studied as risk factors in depression, schizophrenia, obsessive–compulsive disorder, aggression, alcoholism, and autism. Recently, pharmacogenetic research has begun to examine possible genetic influences on therapeutic response to drugs affecting the serotonin system. Genes regulating the synthesis (TPH), storage (VMAT2), membrane uptake (HTT), and metabolism (MAOA) of 5-HT, as well as a number of 5-HT receptors (HTR1A, HTR1B, HTR2A, HTR2C, and HTR5A), have been studied and this initial research is reviewed here. After a brief introduction to serotonin neurobiology and a general discussion of appropriate genetic methodology, each of the major 5-HT-related genes and their encoded proteins are reviewed in turn. For each gene, relevant polymorphisms and research on functional variants are discussed; following brief reviews of the disorder or trait association and linkage studies, pharmacogenetic studies performed to date are covered. The critical and manifold roles of the serotonin system, the great abundance of targets within the system, the wide range of serotonergic agents—available and in development—and the promising preliminary results suggest that the serotonin system offers a particularly rich area for pharmacogenetic research.     

The role of the amygdala in fear-potentiated startle: implications for animal models of anxiety.

Over the past several years, major advances have been made in understanding the pharmacology of anxiety, involving three broad classes of experimental approach. One approach studies the mechanism of action of drugs that are known to treat anxiety clinically, such as the benzodiazepines. A second approach uses various animal models of fear or anxiety that are sensitive to known anxiolytic drugs, to see if they will detect new compounds. A third approach involves describing the neural pathways and neurotransmitters that are active in a state of fear or anxiety; importantly, this approach is not derived from the mechanisms of known anxiolytics. In this review, Michael Davis describes such a 'neural systems' approach to the study of fear or anxiety that uses the paradigm of fear-potentiated startle.     

The dorsal raphé nucleus is a crucial structure mediating nicotine's anxiolytic effects and the development of tolerance and withdrawal responses

RATIONALE: Smokers frequently report that they obtain anxiety-reducing (anxiolytic) effects from smoking, and this may be one factor which contributes to nicotine dependence. OBJECTIVE: The aim of this study was to investigate the role of the dorsal raphé nucleus (DRN) in mediating the acute anxiolytic effect of nicotine, the development of tolerance to this effect and the anxiogenic response observed on withdrawal from chronic nicotine. METHODS: The social interaction test of anxiety was used to investigate the effects of a range of doses of (-)-nicotine (2.5-4000 ng) following DRN infusion, and whether co-administration of the specific 5-HT1A receptor antagonist WAY 100635 could antagonise the anxiolytic action of nicotine. We then examined the effects of intra-DRN nicotine (2.5-7 ng) following six daily injections of subcutaneous (s.c.) (-)-nicotine (0.1 mg/kg). Finally, we examined whether s.c. or intra-DRN (-)-nicotine could antagonise the anxiogenic response seen 72 h after the termination of 7 days of nicotine treatment. RESULTS: Acute nicotine administration into the DRN produced dose-related effects: low doses (2.5-10 ng) induced an anxiolytic effect, intermediate doses were behaviourally silent (100-1000 ng), and an anxiogenic effect was seen following administration of a high dose (4 micrograms). The anxiolytic effect of (-)-nicotine (5 ng) was reversed by co-administration of a behaviourally inactive dose of WAY 100635 (200 ng). Following 6 days of treatment with s.c. 0.1 mg/kg per day (-)-nicotine, tolerance developed to its anxiolytic action in the DRN. Rats withdrawn for 72 h following this chronic treatment showed an anxiogenic response which was reversed by (-)-nicotine injected s.c. (0.1 mg/kg) or into the DRN (5 ng). CONCLUSIONS: The present findings therefore suggest that the DRN plays an important role in mediating the acute effects of nicotine on anxiety, as measured in the social interaction test, and that the anxiolytic effect is mediated by activation of somatodendritic 5-HT1A autoreceptors. The DRN is also concerned with mediating the development of tolerance to nicotine's anxiolytic effects and because there is an anxiogenic response 72 h after withdrawal from chronic nicotine, this suggests that an oppositional, compensatory mechanism is mediating the tolerance.     

Neuroendocrine pharmacology of stress

Exposure to hostile conditions initiates responses organized to enhance the probability of survival. These coordinated responses, known as stress responses, are composed of alterations in behavior, autonomic function and the secretion of multiple hormones. The activation of the renin-angiotensin system and the hypothalamic-pituitary-adrenocortical axis plays a pivotal role in the stress response. Neuroendocrine components activated by stressors include the increased secretion of epinephrine and norepinephrine from the sympathetic nervous system and adrenal medulla, the release of corticotropin-releasing factor (CRF) and vasopressin from parvicellular neurons into the portal circulation, and seconds later, the secretion of pituitary adrenocorticotropin (ACTH), leading to secretion of glucocorticoids by the adrenal gland. Corticotropin-releasing factor coordinates the endocrine, autonomic, behavioral and immune responses to stress and also acts as a neurotransmitter or neuromodulator in the amygdala, dorsal raphe nucleus, hippocampus and locus coeruleus, to integrate brain multi-system responses to stress. This review discussed the role of classical mediators of the stress response, such as corticotropin-releasing factor, vasopressin, serotonin (5-hydroxytryptamine or 5-HT) and catecholamines. Also discussed are the roles of other neuropeptides/neuromodulators involved in the stress response that have previously received little attention, such as substance P, vasoactive intestinal polypeptide, neuropeptide Y and cholecystokinin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABA(A), histamine and serotonin receptors have been used to attenuate the neuroendocrine response to stressors. The neuroendocrine information for these drugs is still incomplete; however, they are a new class of potential antidepressant and anxiolytic drugs that offer new therapeutic approaches to treating anxiety disorders. The studies described in this review suggest that multiple brain mechanisms are responsible for the regulation of each hormone and that not all hormones are regulated by the same neural circuits. In particular, the renin-angiotensin system seems to be regulated by different brain mechanisms than the hypothalamic-pituitary-adrenal system. This could be an important survival mechanism to ensure that dysfunction of one neurotransmitter system will not endanger the appropriate secretion of hormones during exposure to adverse conditions. The measurement of several hormones to examine the mechanisms underlying the stress response and the effects of drugs and lesions on these responses can provide insight into the nature and location of brain circuits and neurotransmitter receptors involved in anxiety and stress.     

Selective activation of metabotropic G-protein-coupled glutamate 7 receptor elicits anxiolytic-like effects in mice by modulating GABAergic neurotransmission

We describe the anxiolytic-like effects of the first, selective metabotropic G-protein-coupled glutamate 7 (mGlu7) receptor agonist, N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082), as measured in the modified stress-induced hyperthermia (SIH) and the four-plate tests. Administration of AMN082 (3-6 mg/kg intraperitoneally) to Swiss mice produced anxiolytic-like effects in the modified SIH and four-plate tests. Moreover, it was ineffective as an anxiolytic in the SIH test in mGlu7 receptor knockout mice as compared with wild-type C57BL/6J littermate controls. In contrast, diazepam (1.25-5 mg/kg) significantly reduced SIH in both the wild-type and knockout animals. The anxiolytic-like effect of AMN082 in the SIH paradigm was abolished by pretreatment with flumazenil (10 mg/kg intraperitoneally). This indicates an involvement of gamma-aminobutyric acid-ergic neurotransmission in AMN's anxiolytic actions. The results indicate that activation of the mGlu7 receptor produces anxiolytic-like effects via the modulation of the gamma-aminobutyric acid system.     

Fear-potentiated startle response is remarkably similar in two laboratories

The fear-potentiated startle response paradigm is used to investigate anxiolytic properties of drugs. The first objective of the present study was to further investigate the predictive validity of this paradigm. The anxiolytics chlordiazepoxide (2.5–10 mg/kg IP) and oxazepam (1–10 mg/kg PO) and the putative anxiolytic flesinoxan (1–10 mg/kg PO) decreased startle potentiation dose-dependently, indicating an anxiolytic effect. The antidepressant fluvoxamine (5–20 mg/kg PO) did not affect startle potentiation. Ideally, anxiolytic drugs attenuate startle potentiation without affecting control startle levels, although some studies report altered control startle amplitudes. The second objective was to investigate whether different effects on control startle amplitudes are related to different startle devices. Therefore, the drugs were tested in two laboratories. Results showed no significant differences between laboratories, indicating that equipment is not a critical factor in the drug-induced alteration of control startle levels. In an additional experiment, it was shown that flesinoxan (10 mg/kg PO) did not affect strychnine-induced startle potentiation, supporting the idea that the attenuating effect of flesinoxan on the fear-potentiated startle response is due to its anxiolytic properties. Thus, the fear-potentiated startle response paradigm appears a valid and reliable model for anxiolytic properties of drugs.     

The comparison of benzodiazepine derivatives and serotonergic agonists and antagonists in two animal models of anxiety

The present paper compares the effects of different serotonergic agonists and antagonists with benzodiazepine derivatives in two animal models of anxiety; the Vogel's and the open-field tests. In the Vogel's conflict test, both diazepam and midazolam produced an anti-punishment action. The drugs 8-OH-DPAT (0.025 and 0.05 mg/kg), buspirone (0.62 mg/kg), gepirone and ipsapirone (0.3 and 0.62 mg/kg, respectively) increased punished intake of water. Ritanserin disinhibited the behaviour of rats at the doses of 2.5 and 5.0 mg/kg and ICS 205-930 (0.001 and 0.01mg/kg) exerted a marked increase in punished drinking, while ondansetron was active only after the largest dose (1.5 mg/kg). In the open-field test, all drugs increased the number of entries into the central area, as well as the time spent in the central sector of the open-field.

The present data indicate similar but not identical spectra of pharmacological sensitivity of both ethologically-oriented and conflict tests, for various classes of anxiolytic drugs. The 5-HT_1A receptor agonists and 5-HT₂ receptor antagonist have been shown to have similar anxiolytic-like profile to the benzodiazepines but in a narrower dose-range. The 5-HT₃ receptor antagonists appeared to be unique in respect to their very strong anti-emotional activity (ICS 205-930), devoid of any clear-cut general inhibitory properties upon locomotion.     

Comparison of acute and repeated treatment with the 5-HT1A receptor ligands 8-OH-DPAT and ipsapirone in animal models of anxiety and depression

The present study compared the 5-HT_1A receptor ligands 8-OH-DPAT and ipsapirone with diazpepam and imipramine in the shock induced ultrasonic vocalization anxiety test and the forced swimming depression test in the rat. Acutely, 8-OH-DPAT induced anxiolytic and antidepressive effects (ED₅₀: 0.12 and 1.4 mg/kg, i.p., respectively), whereas ipsapirone induced anxiolytic (ED₅₀: 0.6 mg/kg) and moderate antidepressive effects (33% at 3-10 mg/kg). Virtually no tolerance developed for the anxiolytic effects after 2 weeks of treatment with 0.03-1 mg/kg 8-OH-DPAT or 0.1-10 mg/kg ipsapirone (i.p., b.i.d.), with 10 mg/kg/day ipsapirone (s.c., mini-pumps), or with 1.5 μg/rat/hr 8-OH-DPAT (local infusion in the dorsal raphe nucleus, mini-pumps). However, some tolerance developed for the antidepressive effects of 8-OH-DPAT (ED₅₀: 0.6, 1.4, 2.5 and >3 mg/kg, after 2 weeks of pretreatment with vehicle, 0.3, 1, and 3 mg/kg 8-OH-DPAT, respectively, i.p., b.i.d.). In the case of ipsapirone, the dose-effect curve in the forced swimming test was shifted to the left after 2 weeks of pretreatment with ipsapirone (0.3-10 mg/kg, i.p., b.i.d.). Acutely, diazepam induced an anxiolytic effect (ED₅₀: 3.6 mg/kg, i.p.), but failed to induce an antidepressive effect; whereas imipramine induced an antidepressive effect (ED₅₀: 20.5 mg/kg) and a moderate anxiolytic effect (max. efficacy: 47% at 30 mg/kg). Upon repeated administration (2 weeks), diazepam (5 mg/kg) showed t0olerance for its anxiolytic effects and weak antidepressive effects emerged, whereas imipramine (20 mg/kg) showed weak sensitization for both effects. It is concluded that (a) with all compounds, tolerance, as well as sensitization can be observed, depending on the behavioral test, the dose and the type of compound; and (b) compared with the other compounds tested, relatively low doses of 5-HT_1A drugs offer the most attractive profile of mixed anxiolytic/antidepressive activity. © 1993 wiley-Liss, Inc.     

Pharmacological evaluation of a modified conflict procedure: punished drinking in non-water-deprived rats

Rationale: Conflict procedures used to detect anxiolytic-like activity of drugs often rely on maintaining strict schedules of water or food availability. It is ethically and practically desirable to reduce such states of deprivation in animal testing. Objective: The purpose of the present experiment was to develop and pharmacologically characterize a conflict drinking procedure that did not require the use of water-deprived animals. Methods: Rats were tested during daily sessions with alternating unpunished drinking (no tone: lick=sucrose solution) and signaled punished drinking (tone: lick=sucrose+shock) components, and developed individual steady baselines over a brief training period (approximately 3–4 weeks). The drugs tested i.p. were the positive allosteric modulators of γ-amino butyric acid_A (GABA)_A receptors, diazepam (0.03–30 mg/kg), chlordiazepoxide (0.03–30 mg/kg), lorazepam (0.03–10 mg/kg), zolpidem (0.3–10 mg/kg), pentobarbital (1–30 mg/kg), pregnanolone (1–30 mg/kg), and bretazenil (0.03– 10 mg/kg); the 5-hydroxy tryptamine_1A (HT)_1A-mediated anxiolytics, buspirone (1–10 mg/kg) and ipsapirone (1–17 mg/kg); and the negative controls d-amphetamine (0.3–3 mg/kg), haloperidol (0.01–0.3 mg/kg), morphine (0.3–17 mg/kg), and imipramine (0.3–30 mg/kg). Results: The experimental procedure was sensitive to increases in punished drinking by the GABA_A-positive modulators, consistent with their known or putative anxiolytic activity. Further, the 5-HT_1A-mediated anxiolytics increased punished drinking, although to a lesser extent and over a more narrow dose range than did the GABAergic drugs. In contrast, d-amphetamine, haloperidol, morphine, and imipramine failed to increase punished drinking up to doses that decreased unpunished drinking. Conclusions: The present results indicate that water deprivation is not a necessary condition to engender drinking conflict behavior or to obtain pharmacological effects similar to those obtained with other classical conflict procedures. Received: 23 November 1998 / Final version: 15 March 1999     

Metabotropic glutamate receptor 5 antagonist-induced stimulation of hypothalamic-pituitary-adrenal axis activity: interaction with serotonergic systems

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) produces anxiolytic or antidepressant effects in several rodent models through incompletely described mechanisms. Anxiolytics and antidepressants share several neuroendocrine features, including acute activation of the hypothalamic-pituitary-adrenal (HPA)-axis, desensitization of neuroendocrine responses with repeated dosing, and desensitization of the HPA axis to 5-HT1A agonist stimulation. We characterized these neuroendocrine parameters in rats treated systemically with MPEP and compared them to those induced by the anxiolytic buspirone. Acutely, MPEP dose-dependently (0.1-10 mg/kg i.p.) increased plasma corticosterone concentrations. These responses were blocked by 50% with the 5-HT1A antagonist WAY100635. The corticosterone responses to both 3 mg/kg MPEP and buspirone were decreased by 80% after 5 days of twice-daily injections. Repeated injection with MPEP decreased HPA-axis sensitivity to buspirone challenge by 75%. This desensitization was not associated with changes in mGluR5 or 5-HT1A receptor binding properties, expression of G-protein subunits coupled to these receptors, or in 5-HT-stimulated binding of [(3)H]-GTPgammaS to membranes.We conclude that MPEP acutely disinhibits the HPA axis, in part through uncharacterized changes in serotonergic signaling. Desensitization of 5-HT1A responses after repeated MPEP administration may indicate that, like other anxiolytics and antidepressants, plasticity in 5-HT signal transduction pathways has occurred.