Presynaptic 5-HT1A receptors mediate the effect of ipsapirone on punished responding in rats

Interaction of the natural anxiolytic Galphimine-B with serotonergic drugs on dorsal hippocampus in rats

AIM OF THE STUDY

Galphimine-B (G-B) is a nor-seco triterpene with an anxiolytic-like effect obtained from the plant species Galphimia glauca Cav. By means of a double blind clinical trial, it has been demonstrated that the extract from this plant, standardized in G-B content, possesses therapeutic effectiveness in patients with generalized anxiety. The mechanism of action of this compound remains unknown to date, but it has already demonstrated a non interaction with the γ-aminobutyric acid (GABA)ergic system. For this reason, the objective of this work was to evaluate the pharmacological interaction between G-B with the 5-hydroxytryptamine 1A (5HT(1A)) and 5-hydroxytryptamine 2A (5HT(2A)) serotonergic receptors on CA1 neurons of hippocampus.

MATERIALS AND METHODS

Electrophysiological records were performed as the frequency of discharge of in vivo CA1 cells from dorsal hippocampus in rats.

RESULTS

G-B was able to increase the frequency of discharge of neurons of the CA1 cells with some characteristics that support an interaction with the serotonergic system in this zone. It was demonstrated that this triterpene modulates the induced response of 5HT(1A) receptors, in an allosteric manner.

CONCLUSION

This effect demonstrated an interaction between G-B and the serotonergic system in dorsal hippocampus and evidenced that the mechanism of action of this compound could involve a complex series of actions on different neurotransmitter systems related with the anxiety disorder.

The neurobiology and control of anxious states

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

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Anxiogenic effect of median raphe nucleus lesion in stressed rats

Serotonin (5-HT) neurons located in the median raphe nucleus (MRN) may have a role in the development of behavioral changes to stress. The objective of the present work was to investigate the effects of a selective lesion of 5-HT neurons located in the MRN in previously stressed male Wistar rats submitted to the elevated plus maze (EPM). In an initial experiment, the animals (n=20-22) were submitted to one (acute) or seven (chronic) daily restraint stress periods (2 h) and tested in the EPM 24 h later. Results showed that acute restraint caused a significant decrease in the number of entries into the open arms, as compared to nonstressed controls. This effect disappeared when the animals were submitted to chronic restraint. In the next set of experiments, animals (n=6-8) received, 1 week before the behavioral studies, intra-MRN injection of 5,7-dihydroxytryptamine (5,7-DHT; 8 microg/1 microl). Neurochemical analysis showed that this treatment significantly decreases 5-HT and 5-hydroxy-indoleacetic acid (5-HIAA) levels in the hippocampus, but not in the striatum. No difference between lesioned and sham-operated animals in EPM performance was found in nonstressed animals or in those submitted to acute restraint. In chronically restrained animals, however, lesioned rats showed a significant decrease in the number of entries and time spent in the open arms. These results suggest that lesions of 5-HT neurons located in the MRN cause anxiogenic-like behavior in animals that have been chronically restrained.

Influence of anxiolytic drugs on the effects of specific serotonin reuptake inhibitors in the forced swimming test in mice

This study aimed at investigating the effect of several selective serotonin reuptake inhibitors (SSRIs), given alone or in combination with anxiolytic drugs, on the time spent immobile in the forced swimming test in mice. The time spent immobile was dose-dependently reduced by acute administration of fluoxetine (4-64 mg/ kg, i.p.), paroxetine (1-32 mg/kg, s.c.) or sertraline (4-32 mg/kg, s.c.), indalpine was active at only one dose (16 mg/kg, i.p.), fluvoxamine (up to 16 mg/kg, i.p.) and citalopram (up to 4 mg/kg, i.p.) were inactive. The anti-immobility effect of fluoxetine (32 mg/kg) was antagonized by an acute co-administration of all anxiolytics tested, the GABAA/BZD receptor agonists, diazepam (2 mg/kg, i.p.), chlordiazepoxide (8 mg/kg, i.p.), lorazepam (0.125 mg/kg, i.p.), triazolam (0.06 mg/kg, i.p.) and alpidem (8 mg/kg, i.p.) and the 5-HT1A receptor partial agonist, buspirone (0.5 mg/kg, s.c.). The sedative neuroleptic, thioridazine (4 mg/kg, i.p.), was also found to counteract the effect of fluoxetine. Lorazepam, triazolam and buspirone also reversed the anti-immobility effect of paroxetine and sertraline, while diazepam and chlordiazepoxide did not. Alpidem reduced the effect of sertraline but not paroxetine, whereas the reverse was found with thioridazine. These data indicate that the influence of anxiolytics on the action of SSRI antidepressants is variable, depending on both the SSRI and the anxiolytic considered. The co-administration of the GABAA/BZD receptor antagonist, flumazenil (16 mg/kg, i.p.), with behaviourally inactive doses of fluoxetine, fluvoxamine and citalopram, resulted in a reduction of immobility. The 5-HT1A receptor antagonist, (+)-WAY 100135 (8 mg/kg, s.c.), combined with a subactive dose of fluoxetine, but not with fluvoxamine, significantly reduced the time spent immobile. The 5-HT2A receptor antagonist, ketanserin (32 mg/kg, s.c.), which reduced immobility when given alone, did not interfere with fluoxetine given at a subactive dose. Although non-specific sedative and/or motor effects cannot be totally ruled out, these results suggest that pharmacodynamic interactions exist between various anxiolytics and SSRIs. These interactions probably involve both serotonergic and GABAergic processes.

Roles of 5-HT(1A) receptors in the dorsal raphe and dorsal hippocampus in anxiety assessed by the behavioral effects of 8-OH-DPAT and S 15535 in a modified Geller-Seifter conflict model

8-OH-DPAT [8-hydroxy-2-(di-N-propylamino)tetralin], a 5-HT(1A) receptor agonist, and S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine, a partial agonist at 5-HT(1A) receptors, were administered into the dorsal raphe nucleus and dorsal hippocampus and their behavioral effects were assessed in a modified Geller-Seifter conflict model. Injected into the dorsal raphe nucleus 8-OH-DPAT, 1 microg but not 0.04 or 0.2 microg 0.5 microl(-1), and S 15535, 2.5 microg but not 0.1 or 0.5 microg 0.5 microl(-1), significantly increased punished responding with no effect on rates of unpunished or time-out responding. WAY 100635, a selective 5-HT(1A) receptor antagonist, injected subcutaneously at 0. 3 mg kg(-1) 30 min before 1 microg 8-OH-DPAT or 2.5 microg S 15535 in the dorsal raphe, completely antagonized their effects on punished responding. At doses ranging from 1 to 10 microg microl(-1) injected into the CA1 region of the dorsal hippocampus neither 8-OH-DPAT nor S 15535 modified punished responding or the rates of time-out. At the highest doses, 8-OH-DPAT significantly reduced unpunished responding whereas S 15535 had the opposite effect. The results suggest that stimulation of 5-HT(1A) receptors in the dorsal raphe nucleus has anxiolytic-like effects whereas stimulation of postsynaptic receptors in the dorsal hippocampus has no anxiolytic or anxiogenic effects, at least judging from changes in rates of punished responding. These results are compatible with the hypothesis that 5-HT(1A) receptor agonists and partial agonists attenuate anxiety by reducing serotonergic transmission in brain areas innervated by the dorsal raphe nucleus.

Alnespirone and buspirone have anxiolytic-like effects in a conflict procedure in rats by stimulating 5-HT(1A) receptors

We studied the anxiolytic-like activity of alnespirone and buspirone, two 5-HT(1A) receptor agonists, in a modified Geller-Seifter conflict model, and examined the role of 5-HT(1A) receptors by studying whether WAY-100635, a selective antagonist at these receptors, blocked their effects. Administered s.c. 30 minutes before testing, 0.5 and 1mg/kg alnespirone significantly increased punished responding, whereas lower doses (0.125 and 0.25 mg/kg) had no effect. At 1mg/kg, alnespirone significantly reduced the rates of unpunished responding. One dose of buspirone (1mg/kg) significantly increased punished responding and reduced unpunished responding. Lower doses were ineffective. Administered s.c. 40 minutes before testing, WAY-100635 had no effect on any parameter but completely antagonized the effects of alnespirone (1mg/kg) and buspirone (1mg/kg) on punished responding. The ability of buspirone to reduce unpunished responding was not antagonized by WAY-100635, probably reflecting a sedative effect of buspirone due to dopamine D2 receptor blockade. The results suggest that alnespirone and buspirone have anxiolytic-like activity in a conflict procedure by stimulating 5-HT(1A) receptors, presumably at a presynaptic level. Like buspirone, alnespirone may have useful effects in the treatment of anxiety disorders.

Effects of 5-HT1A receptor ligands in a modified Geller-Seifter conflict model in the rat

In a modified Geller-Seifter conflict procedure, rats were trained to lever-press for food under a multiple variable interval-fixed ratio (VI30: food; FR10: food + shock) schedule of reinforcement. The ability to antagonize response suppression in the punished period is considered a good predictor for anxiolytic activity. Chlordiazepoxide and alprazolam increased punished responding. The 5-HT1A receptor agonists flesinoxan (R(+)-N-[2[4-(2,3-dihydro-2-2-hydroxymethyl-1,4-benzodioxin-5-yl)- 1-piperazinyl]ethyl]-4-fluorobenzoamide; 0.1-10.0 mg/kg) and 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.03-0.5 mg/kg) significantly increased punished responding, supporting a role of the 5-HT1A receptor in anxiety. 8-OH-DPAT and flesinoxan also reduced unpunished responding. The anxiolytic effects of 8-OH-DPAT and flesinoxan could only be antagonized with a high dose (1.0 and 3.0 mg/kg respectively) of the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). All doses of WAY-100635 antagonized the 5-HT1A-induced effects on unpunished responding. The dissimilarity in dose-response curve of WAY-100635 on punished and unpunished behaviour poses questions about the mediation of these effects.

Effect of repeated ipsapirone treatment on hippocampal excitatory synaptic transmission in the freely behaving rat: role of 5-HT1A receptors and relationship to anxiolytic effect

The effects of acute and repeated treatment with the 5-HT1A receptor ligand ipsapirone on hippocampal excitatory synaptic transmission and in an ultrasonic vocalization anxiety test were investigated in the rat. Synaptic responses in the CA1 region of the dorsal hippocampus of alert, freely behaving male Wistar rats were reduced after acute injection of ipsapirone (1 or 2 mg/kg, i.p.). This effect was prevented by pretreatment with the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide trihydrochloride, 0.25 or 0.5 mg/kg, i.p.) but not by the 5-HT-depleting agent para-chlorophenylalanine (300 mg/kg per day for 3 days, i.p.). WAY-100635 (0.1-0.3 mg/kg, i.p.) also blocked the acute anti-aversive effects of ipsapirone (3 mg/kg, i.p.) in the anxiety test. Repeated administration of ipsapirone (1 or 2 mg/kg per day for 7-8 days, i.p.) produced a gradual reduction in baseline synaptic transmission which was transiently reversed by WAY-100635 (0.25 mg/kg, i.p.). Ipsapirone (1 mg/kg per day for 7 days) produced a gradual and sustained reduction in the duration of vocalizations in the anxiety test which paralleled the reduction in baseline synaptic responses in the same animals. The data indicate that with repeated administration of ipsapirone, a prolongation and enhancement of the 5-HT1A receptor-mediated reduction in hippocampal excitatory synaptic transmission occurs. This delayed effect may contribute to the sustained anxiolytic and/or antidepressant effect of ipsapirone.

5-HT1A receptor full and partial agonists and 5-HT2C (but not 5-HT3) receptor antagonists increase rates of punished responding in rats

Drugs with different intrinsic activity at 5-HT1A receptors and antagonists at 5-HT2A/2C and 5-HT3 receptors were studied for their ability to increase the rates of punished operant responding in rats. Like chlordiazepoxide (5 and 10 mg/kg) and diazepam (1.25 and 2.5 mg/kg), 0.125 mg/kg 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and 5 and 10 mg/kg ipsapirone, a partial agonist at these receptors, increased the rates of punished responding, whereas (S)-WAY 100135, a 5-HT1A receptor antagonist, had no effect at doses from 1 to 10 mg/kg. 8-OH-DPAT and ipsapirone, like benzodiazepines, significantly reduced unpunished responding. The 5-HT2A/2C receptor antagonists ritanserin (2 mg/kg), mianserin (8 mg/kg), and mesulergine (0.1 mg/kg) significantly increased the rates of punished responding, whereas 0.5-2 mg/kg ketanserin, that has higher affinity for 5-HT2A than 5-HT2C receptors, had no effect. Antagonists, at 5-HT3 receptors such as ondansetron (0.001-0.1 mg/kg) and tropisetron (0.001-0.1 mg/kg), had no effect on punished or unpunished responding. The results show that agents acting as full or partial agonists at 5-HT1A receptors and blockers of postsynaptic 5-HT2C receptors have anxiolytic-like effects in a model of punished operant responding, whereas antagonists at 5-HT1A and 5-HT3 receptors have no such effect.