Involvement of presynaptic 5-HT(1A) and benzodiazepine receptors in the anticonflict activity of 5-HT(1A) receptor antagonists

Interactions of a standardized flavonoid fraction from Tilia americana with Serotoninergic drugs in elevated plus maze

ETHNOPHARMACOLOGICAL RELEVANCE

Tilia americana var. mexicana (Schltdl) Hardin (Tiliaceae) aerial parts (bracts and flowers) are used in the traditional Mexican medicine to treat nervous disorders, as sedative and to treat insomnia. A fraction of this species called FC1 (organic fraction from this plant) was proposed, described as anxiolytic and characterized by the presence of flavonoids. In the present work, this fraction was standardized, and its interaction with different serotonergic drugs was tested. We used the elevated plus maze model as anxiety test and the open field test so as to observe a possible effect on mice׳s motor behavior.

MATERIAL AND METHODOLOGY

HPLC technique was used to quantify the flavonoids contained in a fraction called F1C. Different doses of F1C were administered to ICR mice (12.5, 25, 37.5 and 50mg/kg, oral pathway) then they were exposed to elevated plus maze or open field test. After, each dose of F1C fraction was co-administered with different drugs, in order to evaluate the animal׳s behavior: DOI agonist (2.0mg/kg) and KET antagonist (0.03mg/kg) of 5-HT2A receptors; 8-OH-DPAT (0.1mg/kg) selective agonist and WAY100635 (0.5mg/kg) antagonist of 5HT1 receptors.

RESULTS

The HPLC quantitative analysis revealed the F1C composition (mg/g of extract): tiliroside (28.56), glucoside of quercetin (16.25), quercitrin (7.96), rutin (3.93), Kaempferol (2.83). The Emax for F1C curve was 80.6% for time to open arms with an ED50 of 15.09 mg/kg. The combination of F1C with DOI gives a significant increase of the F1C anxiolytic effect (Emax=111% and ED50=13.51 mg/kg), while KET blocks it completely (Emax=12.25% and ED50=2.4 mg/kg). The administration of F1C with 8-OH-DPAT does not generate significant changes on the time to open arms, although it does induce a decrement in F1C potency (Emax=83.3% and ED50=33.3mg/kg). When F1C and WAY-100365 are combined, the anxiolytic activity of the fraction decreases (Emax=33.3% and ED50=102.10mg/kg).

CONCLUSIONS

The medicinal use attributed to Tilia americana for their effect on central nervous system, could be in part in the flavonoid fraction (F1C) with anxiolytic activity which is dose dependent, and has the ability to interact with the serotonergic system. It is necessary to advance in the study of the mechanism of action, using other techniques such in vitro analysis.

Distribution of serotonin 5-HT1A-binding sites in the brainstem and the hypothalamus, and their roles in 5-HT-induced sleep and ingestive behaviors in rock pigeons (Columba livia)

Serotonin 1A receptors (5-HT1ARs), which are widely distributed in the mammalian brain, participate in cognitive and emotional functions. In birds, 5-HT1ARs are expressed in prosencephalic areas involved in visual and cognitive functions. Diverse evidence supports 5-HT1AR-mediated 5-HT-induced ingestive and sleep behaviors in birds. Here, we describe the distribution of 5-HT1ARs in the hypothalamus and brainstem of birds, analyze their potential roles in sleep and ingestive behaviors, and attempt to determine the involvement of auto-/hetero-5-HT1ARs in these behaviors. In 6 pigeons, the anatomical distribution of [(3)H]8-OH-DPAT binding in the rostral brainstem and hypothalamus was examined. Ingestive/sleep behaviors were recorded (1h) in 16 pigeons pretreated with MM77 (a heterosynaptic 5-HT1AR antagonist; 23 or 69 nmol) for 20 min, followed by intracerebroventricular ICV injection of 5-HT (N:8; 150 nmol), 8-OH-DPAT (DPAT, a 5-HT1A,7R agonist, 30 nmol N:8) or vehicle. 5-HT- and DPAT-induced sleep and ingestive behaviors, brainstem 5-HT neuronal density and brain 5-HT content were examined in 12 pigeons, pretreated by ICV with the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (N:6/group). The distribution of brainstem and diencephalic c-Fos immunoreactivity after ICV injection of 5-HT, DPAT or vehicle (N:5/group) into birds provided with or denied access to water is also described. 5-HT1ARs are concentrated in the brainstem 5-HTergic areas and throughout the periventricular hypothalamus, preoptic nuclei and circumventricular organs. 5-HT and DPAT produced a complex c-Fos expression pattern in the 5-HT1AR-enriched preoptic hypothalamus and the circumventricular organs, which are related to drinking and sleep regulation, but modestly affected c-Fos expression in 5-HTergic neurons. The 5-HT-induced ingestivebehaviors and the 5-HT- and DPAT-induced sleep behaviors were reduced by MM77 pretreatment. 5,7-DHT increased sleep per se, decreased tryptophan hydroxylase expression in the raphe nuclei and decreased prosencephalic 5-HT release but failed to affect 5-HT- or DPAT-induced drinking or sleep behavior. 5-HT- and DPAT-induced ingestive and sleep behaviors in pigeons appear to be mediated by heterosynaptic and/or non-somatodendritic presynaptic 5-HT1ARs localized to periventricular diencephalic circuits.

Activation of postsynaptic 5-HT1A receptors improve stress adaptation

RATIONALE

Serotonin-1A (5-HT1A) receptors modulate the stress response and have been implicated in the etiology and treatment of depression and anxiety disorders. A reduction in postsynaptic 5-HT1A receptor function in limbic areas has consistently been observed following exposure to chronic stress.

OBJECTIVES

To investigate the hypothesis that increased activation of 5-HT1A receptors in rats having reduced 5-HT function may improve stress adaptation and the behavioral sequelae commonly associated with chronic stress.

METHODS

One hundred forty-four Sprague-Dawley rats received injections of para-chlorophenylalanine to partially deplete 5-HT then were given daily systemic pretreatment with the 5-HT1A receptor agonist, 8-hydroxy-2- (di-n-propylamino) tetralin (8-OH-DPAT), the antagonist, WAY 100635, or vehicle prior to either restraint stress (6 h/day for 10 daily sessions) or control conditions. Anxiety- and depressive-like behaviors were then assessed using the open field and sucrose preference tests. Protein level of hippocampal glucocorticoid receptors (GR) and mineralocorticoid receptors was detected by immunohistochemistry and brain-derived neurotrophic factor (BDNF) was determined by in situ hybridization.

RESULTS

8-OH-DPAT pretreatment prior to stress exposure attenuated later stress-induced anxiety- and depression-like behaviors and increased GR and BDNF mRNA expression in the hippocampus relative to vehicle- and WAY 100635-pretreated, stressed animals.

CONCLUSION

The stress-related impairments associated with 5-HT deficiency can be improved by 8-OH-DPAT pretreatment prior to stress exposure and are associated with an augmentation of GR-like immunoreactivity and BDNF mRNA expression in the hippocampus. It suggested that selective activation of 5-HT1A receptors may be a potential treatment strategy for stress-related disorders such as anxiety and depression.

5-HT1A receptor blockade reverses GABA(A) receptor alpha3 subunit-mediated anxiolytic effects on stress-induced hyperthermia

RATIONALE

Stress-related disorders are associated with dysfunction of both serotonergic and GABAergic pathways, and clinically effective anxiolytics act via both neurotransmitter systems. As there is evidence that the GABA(A) and the serotonin receptor system interact, a serotonergic component in the anxiolytic actions of benzodiazepines could be present.

OBJECTIVES

The main aim of the present study was to investigate whether the anxiolytic effects of (non-)selective alpha subunit GABA(A) receptor agonists could be reversed with 5-HT(1A) receptor blockade using the stress-induced hyperthermia (SIH) paradigm.

RESULTS

The 5-HT(1A) receptor antagonist WAY-100635 (0.1-1 mg/kg) reversed the SIH-reducing effects of the non-alpha-subunit selective GABA(A) receptor agonist diazepam (1-4 mg/kg) and the GABA(A) receptor alpha(3)-subunit selective agonist TP003 (1 mg/kg), whereas WAY-100635 alone was without effect on the SIH response or basal body temperature. At the same time, co-administration of WAY-100635 with diazepam or TP003 reduced basal body temperature. WAY-100635 did not affect the SIH response when combined with the preferential alpha(1)-subunit GABA(A) receptor agonist zolpidem (10 mg/kg), although zolpidem markedly reduced basal body temperature.

CONCLUSIONS

The present study suggests an interaction between GABA(A) receptor alpha-subunits and 5-HT(1A) receptor activation in the SIH response. Specifically, our data indicate that benzodiazepines affect serotonergic signaling via GABA(A) receptor alpha(3)-subunits. Further understanding of the interactions between the GABA(A) and serotonin system in reaction to stress may be valuable in the search for novel anxiolytic drugs.

The anxiolytic-like effect of 5-HT1B receptor ligands in rats: a possible mechanism of action

We have examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 times 10 mg kg−1), and the influence of flumazenil (Ro 15–1788, 10 mg kg−1), a benzodiazepine receptor antagonist, on the anxiolytic-like activity of CP 94253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1-H-pyrrolo[3,2-b]pyridine), a 5-HT1B receptor agonist, SB 216641 (N-[3-[3-(dimethylamino)ethoxy]-4-methoxyphenyl]-2′-methyl-4′-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1′-biphenyl]-4-carboxamide), a 5-HT1B receptor antagonist, and GR 127935 (N-[4-methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2′-methyl-4′-(5-methyl-1,2,4-oxadiazol-3-yl)-1, 1′-biphenyl-4-carboxamide), a 5-HT1B/1D receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. CP 94253 (2.5 mg kg−1), SB 216641 (2.5 mg kg−1), GR 127935 (10 mg kg−1) and diazepam (5 mg kg−1) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test in vehicle- and p-CA-pretreated rats. Flumazenil did not change the anxiolytic-like effect of CP 94253 (2.5 mg kg−1), but wholly blocked the anxiolytic-like effects of SB 216641 (2.5 mg kg−1), GR 127935 (10 mg kg−1) and diazepam (5 mg kg−1). p-CA and flumazenil alone were inactive in the conflict drinking test. The results suggested that the anxiolytic-like effect of the 5-HT1B receptor ligands CP 94253, SB 216641 and GR 127935 was possibly linked to the postsynaptic 5-HT1B receptors or/and 5-HT1B heteroreceptors. The results suggested also that benzodiazepine receptors were indirectly involved in the effects of SB 216641 and GR 127935 (but not of CP 94253), which might have been due to a possible interaction between the 5-HT and the GABA/benzodiazepine systems.

In-vivo effects of the 1,2,4-piperazine derivatives MM5 and MC1, putative 5-HT agonists, on dopamine and serotonin release in rat prefrontal cortex

Two 1,2,4-substituted derivatives of piperazine were tested for their effect on dopamine and serotonin (5-HT) release in rat prefrontal cortex. Both compounds, 1-[4-(4-chinolin-2-yl-piperazin-1-yl)-butyl]piperidin-2-on (MM5) and 1-[4-(2-methyl-4-chinolin-2-yl-piperazin-1-yl)-butyl]-8-azaspiro [4.5]decano-7,9-dion (MC1), produced hypothermia in mice and showed affinity for 5-HT1A receptors in-vitro. Like the selective 5-HT1A agonist 8-OH-DPAT (0.1 mg kg−1), MM5 given peripherally (30 mg kg−1) decreased the extracellular 5-HT level in rat prefrontal cortex, while MC1 suppressed 5-HT release at a higher dose (40 mg kg−1), but not at a lower one (30 mg kg−1). The effect of both compounds on 5-HT release was abolished by WAY 100635 (0.3 mg kg−1). MC1 (30 and 40 mg kg−1), but not MM5, raised cortical dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and extracellular homovanillic acid (HVA) levels. The effect of MC1 on dopamine release was reversed by neither WAY 100635 nor the non-selective 5-HT2 antagonist ritanserin (2 mg kg−1). However, ritanserin prevented the effect of the higher dose of MC1 on 5-HT release. The results of this study suggest that MM5 exhibits the profile of a 5-HT1A agonist devoid of dopaminergic activity. MC1 seems to possess moderate agonist activity at 5-HT1A and 5-HT2A receptors, while acting on 5-HT release in the rat prefrontal cortex. However, the facilitation of dopamine release by this compound does not seem to be related to its affinity for 5-HT1A and 5-HT2A receptors.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

The anxiolytic-like effect of the selective 5-HT6 receptor antagonist SB-399885: the impact of benzodiazepine receptors

The effect of lesion of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg/kg), and the influence of the benzodiazepine receptor antagonist flumazenil (10 mg/kg) on the anticonflict action of N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide (SB-399885), a selective 5-HT6 receptor antagonist, were investigated in the Vogel conflict drinking test in rats. In addition, the interaction between SB-399885 (0.3 mg/kg) and diazepam (2.5 mg/kg) was evaluated in that test. All the compounds tested were administered intraperitoneally. The anticonflict activity produced by SB-399885 (3 mg/kg) was not modified in p-CA-pretreated rats, but it was totally blocked by flumazenil. Combined administration of non-active doses of SB-399885 (0.3 mg/kg) and diazepam (2.5 mg/kg) produced a pronounced anticonflict effect in rats. The present results suggest that the anticonflict activity of SB-399885 is not conditioned by the integrity of 5-HT neurons, and that benzodiazepine receptors are indirectly involved in its effect, possibly due to a functional interaction between 5-HT6 receptors and the gamma-aminobutyric acid (GABA)/benzodiazepine system.

The influence of modifications in imide fragment structure on 5-HT1A and 5-HT7 receptor affinity and in vivo pharmacological properties of some new 1-(m-trifluoromethylphenyl)piperazines

New, flexible (7, 9, 11 and 13) and rigid (8, 10, 12 and 14) imides with a 1-(m-trifluorophenyl)piperazine fragment and a tetramethylene or a 1e,4e-cyclohexylene spacer, respectively, showed very high affinity (K(i)=0.3-34 nM) and agonistic in vivo activity for 5-HT(1A) receptors. Flexible new compounds and the previously described 5 also bound to 5-HT(7) receptors (K(i)=21-134 nM). Selected glutarimide derivatives, that is, the most potent postsynaptic 5-HT(1A) receptor agonist rigid compound 8 and its flexible analogue 7, as well as the previously described full agonist-rigid compound 6 and the partial agonist-its flexible counterpart 5 exhibited moderate affinity for alpha(1)-adrenoceptors (K(i)=85 - 268 nM), but were practically devoid of any affinity for dopamine D(2) sites. Those glutarimides demonstrated anxiolytic- (5 and 7) and antidepressant-like (5, 6 and 8) activity in the four-plate and the swim tests in mice, respectively; at the same time, however, they inhibited the locomotor activity of mice. The antidepressant-like effect of 8 was significantly stronger than that induced by imipramine used as a reference antidepressant.

Anxiolytic-like and antidepressant-like effects produced by the selective 5-HT6 receptor antagonist SB-258585 after intrahippocampal administration to rats

The purpose of this study was to investigate the effect of SB-258585, a selective 5-HT6 receptor antagonist, administered intrahippocampally to rats, in the conflict drinking and forced swim tests, that is models used for evaluating anxiolytic-like and antidepressant-like activity, respectively. Diazepam and imipramine were used as reference drugs. SB-258585 at a dose of 1 microg (but not 0.3 and 3 microg) showed an anticonflict effect that was weaker than that of diazepam (40 microg). SB-258585 at a dose of 3 microg (but not 1 and 10 microg) produced a marked anti-immobility effect comparable with that of imipramine (0.1 microg). The anxiolytic-like and antidepressant-like activity of SB-258585 seemed to be specific, as that compound--when given by the same route in doses effective in each model--did not affect the shock threshold, nonpunished water consumption, or exploratory activity of rats. The results obtained indicate that the hippocampus is one of the neuroanatomical sites involved in the anxiolytic-like and antidepressant-like activity of the selective 5-HT6 receptor antagonist SB-258585.

Indorenate modifies a1-adrenergic and benzodiazepine receptor binding in the rat brain: an autoradiography study

Indorenate (5-methoxytryptamine-beta-methylcarboxylate) is a 5-HT1A receptor agonist that produces antihypertensive, anxiolytic, antidepressant and anticonvulsant effects. However, there is evidence suggesting that these effects could involve the activation of benzodiazepine (BZD) receptors but not the activation of a1-adrenergic receptors. The goal of this study was to analyse the effect of indorenate on a1-adrenergic and BZD receptor binding in specific rat brain areas by using in-vitro autoradiography. Coronal brain sections from male Wistar rats were used for labelling 5-HT1A (3H-8-OH-DPAT, 2 nM), a1-adrenergic (3H-prazosin, 2 nM) and BZD (3H-flunitrazepam, 2 nM) receptor binding in the presence or absence of indorenate (1 microM). Indorenate totally displaced 3H-8-OH-DPAT binding in all the brain areas evaluated. It decreased 3H-prazosin binding just in the frontal (30%) and sensorimotor (32%) cortices and in the thalamus (21%). Additionally, indorenate diminished 3H-flunitrazepam binding only in the cingulate (16%) and piriform (18%) cortices as well as in the dorsal raphe nucleus (18%). These results confirm that indorenate is a 5-HT1A ligand and suggest the possible participation of a1-adrenergic and BZD receptors in its pharmacological properties.

Anticonvulsant effect of the selective 5-HT1B receptor agonist CP 94253 in mice

The effect of the selective 5-hydroxytryptamine1B (5-HT1B) receptor agonist 5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine (CP 94253) and the 5-HT1A/1B/1D receptor agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU 24969) in maximal electroshock- and pentylenetetrazol-induced seizures in mice was examined. CP 94253 (10-40 mg/kg) afforded no protection against maximal electroshock-evoked convulsions, but produced anticonvulsant action in the pentylenetetrazol-induced seizures (ED50 = 29 mg/kg). The anticonvulsant effect of CP 94253 was abolished by the selective 5-HT1B receptor antagonist N-[3-(2-dimethylamino)ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641; 20 mg/kg) but it was maintained following the p-chlorophenylalanine (p-CPA; 3 x 300 mg/kg)-induced 5-HT depletion. Interestingly, CP 94253 potentiated the anticonvulsant activity of diazepam in the pentylenetetrazol test; on the other hand, the benzodiazepine receptor antagonist, flumazenil (10 mg/kg), did not modify the anticonvulsant effect of CP 94253. RU 24969 (5 mg/kg) evoked no effect in the maximal electroshock model, but it produced anticonvulsant activity in the pentylenetetrazol assay, the latter effect being attenuated by the selective 5-HT1A receptor antagonist N-(2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl)-N-(2-pyridyl)-cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg) and SB 216641 (10-20 mg/kg). The obtained results suggest that CP 94253 exerts anticonvulsant activity on pentylenetetrazol-induced seizures in mice, as a consequence of stimulation of 5-HT1B receptors (probably located postsynaptically and/or as heteroreceptors); the antiseizure activity of RU 24969 seems to depend on the stimulation of both 5-HT1A and 5-HT1B receptors.

Structure–intrinsic activity relationship studies in the group of 1-imido/amido substituted 4-(4-arylpiperazin-1-yl)cyclohexane derivatives; new, potent 5-HT1A receptor agents with anxiolytic- like activity

Introduction of 1,4-disubstituted cyclohexane ring in the structure of flexible long chain arylpiperazines resulted in linearly constrained, potent serotonin (5-HT)(1A) ligands. In order to trace structure-intrinsic activity relationships in this group, a new series of 1-substituted 4-(4-arylpiperazin-1-yl)cyclohexane derivatives with different cyclic imide/amide termini, and their flexible, tetramethylene analogues were synthesized and pharmacologically evaluated for 5-HT(1A) receptors. In vitro binding experiments revealed that all the compounds were potent 5-HT(1A) receptor agents (K(i) = 1.9-74 nM). Some derivatives tested additionally showed also high affinity for alpha(1)-adrenergic receptors (K(i) = 2.9-101 nM) and for 5-HT(7) receptors. Functional in vivo examination revealed that rigid ligands with o-OCH(3) group in the aryl moiety and cyclic imide system in the opposite terminal behaved like postsynaptic 5-HT(1A) receptor antagonists. On the other hand, unsubstituted, m-Cl, or m-CF(3) substituted derivatives as well as those with cyclic amide group in the terminal fragment exhibited agonistic or partial agonistic activity. Three out of four derivatives tested, that is, postsynaptic 5-HT(1A) antagonists 9 and 10, and partial agonist 16, showed anxiolytic-like activity in the conflict drinking (Vogel) test in rats.

Effect of the postsynaptic 5-HT1A receptor antagonist MM-77 on stressed mice treated with 5-HT1A receptor agents

The pharmacological effect of the 5-HT1A receptor ligands, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), indorenate, and buspirone, alone or in combination with the antagonist MM-77, was studied in mice subjected to forced swimming. It was confirmed that this stressful factor produces an anxiolytic-like effect, which is reversed by the mentioned 5-HT1A receptor agonists. Only the 8-OH-DPAT-induced decrease of such an effect could be blocked by the postsynaptic antagonist of the 5-HT1A receptor 1-(2-methoxyphenyl)-4-[(4-succinimido)butyl]-piperazine (MM-77). Stressing by forced swimming seems to induce plastic changes in 5-HT1A receptors, which in turn modify the behavioural actions of 5-HT1A receptor agents.