Evidence for a functional interaction between 5-HT1A and 5-HT2 receptors in rats

 Evidence of a functional interaction between serotonin 5-HT_1A and 5-HT₂ receptor subtypes has been compromised by incomplete experimental designs and conflicting data. To test for such an interaction, combinations of the 5-HT_1A agonist 8-OH-DPAT and the 5-HT₂ agonist DOI were administered to rats prior to testing of locomotor activity in the Behavioral Pattern Monitor (BPM). The BPM is an activity and holeboard chamber that enables analyses of quantitative and qualitative changes in locomotor and investigatory activity. Dose-response studies of 8-OH-DPAT and DOI alone and in the presence of selected doses of the other drug were performed in order to allow isobolographic analysis, which characterizes the relationship of two drugs as either additive (no interaction), supra-additive, or infra-additive. Rats treated with saline, 8-OH-DPAT (6.25, 12.5, 25, or 50 μg/kg SC), DOI (0.15, 0.3, or 0.6 mg/kg SC), or selected combinations of both drugs were tested in the BPM for 1 h. Isobolographic analysis of the effects on locomotor activity revealed that 8-OH-DPAT and DOI interact in an infra-additive manner. Thus, at a functional level, 5-HT_1A and 5-HT₂ receptors interact antagonistically in the modulation of locomotor activity. Received: 15 December 1997 / Final version: 13 February 1998     

Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

Background and purpose:

Despite decreased presynaptic 5-HT_1A and altered 5-HT_2A receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT_1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT_2A receptor-mediated response.

Experimental approach:

Interactions between 5-HT_1A and 5-HT_2A receptors were assessed using the head-twitch response following 5-HT_1A and 5-HT_2A receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/−), and knockout (−/−) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

Key results:

WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT −/− versus SERT +/+ mice. In SERT −/− mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/− mice. Head twitches following the 5-HT_2A receptor agonist (+/−)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/− mice but much reduced in SERT −/− mice. DOI-induced head twitches were decreased by the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/− mice. All drug-induced head twitches were blocked by the 5-HT_2A receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

Conclusions and implications:

These data show that indirect activation of 5-HT_2A receptors via blockade of presynaptic 5-HT_1A receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.     

Cis-(+)-8-OH-1-CH3-DPAT, (+)ALK-3, a novel stereoselective pharmacological probe for characterizing 5-HT release-controlling 5-HT1A autoreceptors

Summary The somatodendritic 5-HT_1A autoreceptor regulating 5-HT neuronal activity is currently poorly defined pharmacologically because there are no specific antagonists, but also because potent and stereoselective agonists are scarce. Moreover, there have been few, if any, attempts to specifically investigate structure-activity relationships for agonists acting at this site. Employing brain microdialysis techniques, we have examined the effects of the enantiomers of cis-8-hydroxy-1-methyl-2-(di-n-propylamino)tetralin (ALK-3; 0.01-0.3 mg/kg s.c.), its trans-1-methyl analogue (ALK-4; 0.3 mg/kg s.c.) and the pure enantiomers of the parent compound - 8-OH-DPAT (0.3 mg/kg s.c.) — in an attempt to address stereochemical agonist structure-activity requirements of 5-HT release-controlling 5-HT_1A autoreceptors in brain. The cis-1-methylated 8-OH-DPAT analogue (+)ALK-3 was comparable to the parent compound in reducing the 5-HT output from rat ventral hippocampus. In comparison, both (–)ALK-3 and the racemic rans-diastereomer to ALK-3, ALK-4, were inactive, while the two stereoisomers of 8-OH-DPAT strongly reduced 5-HT release. Pretreatment with (–)pindolol (8 mg/kg s.c.), which has high affinity for 5-HT_1A radioligand binding sites, blocked the reduction of hippocampal 5-HT release induced by a submaximally effective dose of (+)ALK-3. The direct intrahippocampal administration of (+)ALK3 (10 M) via the perfusion medium did not affect 5-HT output.In summary, the data indicate that (+)ALK-3, like 8-OH-DPAT, is a very potent 5-HT receptor agonist which inhibits terminal 5-HT release in rat hippocampus, probably via activation of somatodendritic 5-HT_1A autoreceptors. However, unlike 8-OH-DPAT, (+)ALK-3 is highly stereoselective and may therefore represent a useful probe in the further characterization of 5-HT_1A receptor-mediated mechanisms and function. The present study defines some of the stereochemical requirements for 5-HT_1A receptor interaction, emphasizing the importance of the receptor region complementary to the C₁ and C₂ carbons of the 8-OH-DPAT molecule. These findings contribute to the establishment of structure-activity relationships for the cell body 5-HT_1A autoreceptors and might be of value in resolving structural features that determine agonist/antagonist activity at central 5-HT_1A receptors. Finally, in conjunction with our recent finding that (+)ALK-3 is a partial agonist at postsynaptic 5HT_1A receptors, the present study extends previous observations suggesting that pre- and postsynaptic 5-HT_1A receptor populations differ in their characteristics. Send offprint requests to S. Hjorth at the above address     

Effect of serotonergic agents on adenosine A2 receptor mediated catalepsy in mice

The effect of serotonergic agents was studied on the adenosine A₂ receptor agonist NECA-induced catalepsy in mice. The 5-HT releaser fenfluramine, the 5-HT_1A agonist 8-OH-DPAT, the 5-HT_2A/1C receptor agonist DOI and the 5-HT_2A/1C receptor antagonists ketanserin and mianserin reversed NECA-induced catalepsy. p-MPPI and ketanserin reversed the anticataleptic actions of 8-OH-DPAT and DOI, respectively. Further, the 5-HT reuptake inhibitor fluoxetine, the 5-HT_1B/1C receptor agonist TFMPP, the 5-HT synthesis inhibitor p-CPA, the selective 5-HT_1A receptor antagonist p-MPPI, the 5-HT_1A/1B receptor antagonist pindolol and the 5-HT₃ receptor antagonist LY 278, 584 had no effect on NECA-induced catalepsy. The anticataleptic action of fenfluramine was not affected by pretreatment with p-CPA. In p-CPA treated rats, ketanserin did not affect the anticataleptic effect of fenfluramine, whereas p-MPPI partially reversed this effect. It is concluded that modulation of serotonergic neurotransmission at 5-HT_1A and 5-HT_2A/1C receptors affects the cataleptic action of experimental antipsychotic agents with adenosine A₂ receptor agonistic activity. Received: 5 May 1997/Final version: 2 September 1997     

Age-related behavioural,neurochemical and radioligand binding changes in the central 5-HT system of Sprague-Dawley rats

Mature (3–4 months) and aged (18–19 months) Sprague-Dawley (SD) rats were treated with 5-HT receptor agonists and drug-induced behaviours monitored. The 5-HT_2/1C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), induced wet dog shakes and back muscle contractions which were significantly increased in aged, compared to mature, rats, suggesting an age-related enhancement of 5-HT₂ receptor function. In contrast, the selective 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) induced forepaw treading, flat body posture, hypothermia and hyperactivity which were not significantly different in aged compared to mature rats. Levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus and frontal cortex were measured using high performance liquid chromatography with electrochemical detection. There were no age-related changes in hippocampal 5-HT or 5-HIAA. However both 5-HT and 5-HIAA were increased in the frontal cortex of aged SD rats. 8-OH-DPAT reduced 5-HIAA in both regions examined in mature rats, an effect which was attenuated in the aged rats, suggesting an age-related reduction in presynaptic 5-HT_1A receptor function. DOI did not induce any changes in 5-HT or 5-HIAA in either of the regions examined. Radioligand binding studies with [³H] ketanserin showed there to be no significant age-related changes in cortical 5-HT₂ receptor density or affinity. In the samples taken from mature rats GTP shifted the competition curve to DOI and reduced the proportion of high affinity agonist binding sites; this effect was not observed in the aged samples, suggesting that there may be age-related changes in G-protein-mediated receptor-effector coupling mechanisms.     

Effects of serotonergic agents on apomorphine-induced locomotor activity

The interactions of serotonin 5-HT_1A, 5-HT_1C/2 and 5-HT₃ receptor subtypes with apomorphine-induced locomotor activity (AILA) were investigated in Sprague-Dawley rats. The 5-HT₃ antagonists ondansetron and ICS 205-930 had no significant effects on AILA. The 5-HT_1A agonist 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) produced an increase in locomotor activity that was independent of DA neurotransmission. The locomotor activity induced by co-administration of apomorphine (APO; 0.25 mg/kg) and 8-OH-DPAT (0.25–1.0 mg/kg) was not significantly higher than those induced by APO alone during the peak period of APO stimulation of locomotor activity, nor were they higher than activity induced by 8-OH-DPAT alone during the same time intervals. The 5-HT₁ antagonist (1)-propranolol had a depressant effect on AILA, but only at high doses. Co-administration of (1)-propranolol (5 mg/kg) and 8-OH-DPAT (1.0 mg/kg) elevated spontaneous locomotor activity for the first 10 min of the session when compared to 8-OH-DPAT (1.0 mg/kg) alone. The 5-HT₂ antagonist ketanserin along with moderate and high doses of mesulergine depressed AILA, effects which may be mediated by the 5-HT₂ antagonist properties of these drugs, by nonspecific sedation or by direct effects of these compounds on DA D₂ receptors. In contrast to the high-dose mesulergine depression of AILA, a low dose (0.1 mg/kg) of mesulergine elevated AILA, an effect which was blocked by the 5-HT_1C/2 agonist 1-(2,-5-dimethoxy-4-iodophenyl) -2-aminopropane (DOI: 1 mg/kg). Neither of these compounds at the doses tested had significant effects on spontaneous locomotor activity. In addition, stereotypy was increased in rats co-administered DOI and APO. These findings suggest that the 5-HT_1C and possibly the 5-HT₂ receptor subtypes are prime targets for drug interventions intended to modulate DA neurotransmission.     

Side effect profile of 5-HT treatments for Parkinson's disease and L-DOPA-induced dyskinesia in rats

BACKGROUND AND PURPOSE

Treatment of Parkinson''s disease (PD) with L-DOPA eventually causes abnormal involuntary movements known as dyskinesias in most patients. Dyskinesia can be reduced using compounds that act as direct or indirect agonists of the 5-HT_1A receptor, but these drugs have been reported to worsen PD features and are known to produce ‘5-HT syndrome’, symptoms of which include tremor, myoclonus, rigidity and hyper-reflexia.

EXPERIMENTAL APPROACH

Sprague-Dawley rats were given unilateral nigrostriatal dopamine lesions with 6-hydroxydopamine. Each of the following three purportedly anti-dyskinetic 5-HT compounds were administered 15 min before L-DOPA: the full 5-HT_1A agonist ±-8-hydroxy-2-dipropylaminotetralin (±8-OH-DPAT), the partial 5-HT_1A agonist buspirone or the 5-HT transporter inhibitor citalopram. After these injections, animals were monitored for dyskinesia, 5-HT syndrome, motor activity and PD akinesia.

KEY RESULTS

Each 5-HT drug dose-dependently reduced dyskinesia by relatively equal amounts (±8-OH-DPAT ≥ citalopram ≥ buspirone), but 5-HT syndrome was higher with ±8-OH-DPAT, lower with buspirone and not present with citalopram. Importantly, with or without L-DOPA, all three compounds provided an additional improvement of PD akinesia. All drugs tempered the locomotor response to L-DOPA suggesting dyskinesia reduction, but vertical rearing was reduced with 5-HT drugs, potentially reflecting features of 5-HT syndrome.

CONCLUSIONS AND IMPLICATIONS

The results suggest that compounds that indirectly facilitate 5-HT_1A receptor activation, such as citalopram, may be more effective therapeutics than direct 5-HT_1A receptor agonists because they exhibit similar anti-dyskinesia efficacy, while possessing a reduced side effect profile.     

Differences in the effects of 5-HT1A receptor agonists on forced swimming behavior and brain 5-HT metabolism between low and high aggressive mice

Rationale Male wild house-mice genetically selected for long attack latency (LAL) and short attack latency (SAL) differ in structural and functional properties of postsynaptic serotonergic-1A (5-HT_1A) receptors. These mouse lines also show divergent behavioral responses in the forced swimming test (FST, i.e., higher immobility by LAL versus SAL mice).Objectives We investigated whether the line difference in 5-HT_1A receptors is associated with a difference in brain 5-HT metabolism, and whether acute administration of a 5-HT_1A receptor agonist could differentially affect the behavioral responses of LAL and SAL mice.Methods 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were measured in homogenates of several brain regions using high-performance liquid chromatography. The behavioral effect of the full 5-HT_1A receptor agonist, 8-OH-DPAT, and of the somatodendritic 5-HT_1A autoreceptor agonist, S-15535, was examined in the FST. The effect of 8-OH-DPAT on forced swimming-induced 5-HT metabolism in brain homogenates was determined.Results In most brain regions, 5-HT and 5-HIAA levels and 5-HT turnover were not significantly different between LAL and SAL mice. 8-OH-DPAT abolished the behavioral line difference in the FST by reducing immobility in LAL mice and reducing climbing in SAL mice. S-15535 induced a similar behavioral effect to 8-OH-DPAT in SAL mice, but did not alter the behavior of LAL mice. Compared with LAL, forced swimming elicited in SAL mice a higher brain 5-HT turnover, which was potently attenuated by 8-OH-DPAT.Conclusions It is unlikely that the difference in 5-HT_1A properties between LAL and SAL mice is an adaptive compensatory reaction to changes in 5-HT metabolism. Although unspecific motor effects, at least in SAL mice, cannot be ruled out, it is suggested that the behavioral effects of 8-OH-DPAT and S-15535 may be mediated by predominant activation of postsynaptic 5-HT_1A receptors in LAL mice and by presynaptic 5-HT_1A receptors in SAL mice.     

Cross-tolerance studies of serotonin receptors involved in behavioral effects of LSD in rats

Like hallucinogenic 5-HT₂ agonists, LSD (d-lysergic acid diethylamide) produces characteristic decreases in locomotor activity and investigatory behaviors of rats tested in a novel environment. Because LSD is an agonist at both 5-HT_1A and 5-HT₂ receptors, however, the respective influences of these different receptors in the behavioral effects of LSD remain unclear. In particular, the paucity of selective 5-HT_1A antagonists has made it difficult to assess the specific contribution of 5-HT_1A receptors to the effects of LSD. An alternative approach to the delineation of receptor-specific effects is the use of cross-tolerance regimens. In the present studies, rats were pretreated with saline, 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) (0.5 mg/kg SC), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1.0 mg/kg SC), or LSD (60 µg/kg SC), every 12 h for 5 or 8 days. Thirty-six hours later, rats were tested in a behavioral pattern monitor 10 min after injection of saline, 0.5 mg/kg 8-OH-DPAT, 1.0 mg/kg DOI, or 60 µg/kg LSD. As expected, tolerance to the decreases in locomotor activity produced by acute administrations of 8-OH-DPAT, DOI, or LSD occurred when rats were pretreated chronically with 8-OH-DPAT, DOI, or LSD, respectively. Furthermore, pretreatment with either 8-OH-DPAT or DOI produced cross-tolerance to LSD. These results support the hypothesis that the effects of LSD in this model reflect a combination of 5-HT_1A and 5-HT₂ effects and support the view that there is an interaction between 5-HT_1A and 5-HT₂ receptors.     

5-HT(1A)-receptor over-expressing mice: genotype and sex dependent responses to antidepressants in the forced swim-test

Deficiencies in serotonergic neurotransmission are involved in the pathophysiology of depression. Due to its modulatory effect on serotonin (5-HT) release, the 5-HT_1A-receptor is thought to play a decisive role in the therapy of this mood disorder. However, it is not fully understood how antidepressant effects are mediated by pre- and postsynaptic receptor sites. In this study we examined the impact of postsynaptic 5-HT_1A-receptor over-expression in corticolimbic areas of male and female mice on the performance in the forced swim-test (FST). Furthermore, we investigated their response to the serotonin selective reuptake inhibitor (SSRI) citalopram in comparison to the selective noradrenaline reuptake inhibitor reboxetine, as well as the partial 5-HT_1A-receptor agonists, buspirone and S 15535. Additionally, these drugs were evaluated in the open field-test in order to observe effects on motor activity. The density of 5-HT_1A-receptors in discrete corticolimbic regions was determined in detail by quantitative autoradiography with [³H]8-OH-DPAT to investigate genotype as well as sex dependent differences in the expression pattern. [³H]8-OH-DPAT binding differed depending on sex with female mice of both genotypes displaying higher receptor binding in distinct brain areas. In the FST untreated male but not female over-expressing (OE) mice showed an antidepressant-like behaviour compared to wild-type (WT) mice. Citalopram yielded an antidepressant effect without influencing locomotor activity in OE mice but not in WT mice. Reboxetine had no antidepressant-like effect in OE mice, but sex-dependently in WT mice. The two partial agonists, buspirone and S 15535 produced no antidepressant-like activity in both genotypes and sexes, but aberrant motor effects. The antidepressant-like phenotype of male transgenic mice accounts for an involvement of postsynaptic 5-HT_1A-receptors in the FST behaviour. In addition, the selective over-expression of postsynaptic 5-HT_1A-receptors in mice contributes to the antidepressant response to citalopram in the FST. Although further pharmacological analysis is required, the data provide novel support for a role of postsynaptic 5-HT_1A-receptors in the effects of SSRIs.This article is part of a Special Issue entitled ‘Serotonin’.     

Mapping of CBV changes in 5-HT1A terminal fields by functional MRI in the mouse brain

Visualization of brain activity in humans and animals using functional magnetic resonance imaging (fMRI) is an established method for translational neuropsychopharmacology. It is useful to study the activity of defined brain structures, however it requires further refinement to allow more specific cellular analyses, like for instance, the activity of selected pools of brain cells. Here, we investigated brain activity in serotonergic pathways in the adult mouse brain by using acute pharmacological challenge of 5-hydroxytryptamine (5-HT) 1A receptors. We show that administration of the 5-HT_1A receptor agonist 8-OH-DPAT prompts a dose-dependent reduction in local cerebral blood volume (CBV) in brain areas rich in neurons expressing post-synaptic 5-HT_1A receptor, including the prefrontal cortex, hippocampus and amygdalar nuclei. Region-specific inhibition of the response by co-injection of 8-OH-DPAT with the selective 5-HT_1A receptor antagonist WAY-100635, or in 5-HT_1A knock-out mice, suggests that 5-HT_1A receptors are the primary targets of the agonist. Overall, the data demonstrate the feasibility of mapping region-specific serotonergic transmission in the adult mouse brain in vivo by non-invasive fMRI. The method opens novel perspectives for investigating 5-HT_1A receptor functions in mouse models of human pathologies resulting from a dysfunction of the 5-HT_1A receptor or the serotonergic system, including depression and anxiety.     

Antidepressant-like activity of 5-HT1A agonists measured with the forced swim test

This study examined the abilities of 5-hydroxytryptamine (5-HT) agonists with varying selectivity for different subtypes of 5-HT receptors to produce antidepressant-like behavioral effects in the forced swim test in rats. The 5-HT_1A agonists 8-OH-DPAT (0.125–1.0 mg/kg, SC) and tandospirone (SM-3997) (5–20 mg/kg, SC) both produced dose-related decreases in immobility time following subchronic treatment in rats. These effects were similar to those of the tricyclic antidepressants imipramine (5–15 mg/kg) and desipramine (5–15 mg/kg). In addition, the 5-HT_1A agonists, buspirone (20 mg/kg), gepirone (20 mg/kg) and ipsapirone (10 and 20 mg/kg) demonstrated antidepressant-like effects. Other groups of rats treated subchronically with each of the 5-HT_1A agonists or antidepressants showed no increase in locomotor activity, so that general changes in activity could not account for the reduction of immobility time in the forced swim test. 5-HT agonists selective for other receptor subtypes, such as the 5-HT_1B/1C agonistm-CPP (5 mg/kg) and the 5-HT_2/1C agonist DOB (1 mg/kg), were not effective in this behavioral test. The benzodiazepine diazepam (5 mg/kg) also failed to reduce immobility time, suggesting that anxiolytic properties of 5-HT_1A agonists did not mediate this behavioral effect. A common metabolite of some of the 5-HT_1A agonists, 1-PP, was ineffective in reducing immobility time. The stimulantd-amphetamine (2 mg/kg) significantly reduced immobility time but also significantly increased locomotor activity. Pretreatment with the 5-HT synthesis inhibitor PCPA alone did not alter immobility time, and did not alter the antidepressant-like effects of 8-OH-DPAT or tandospirone, suggesting that the 5-HT_1A agonists are producing their antidepressant-like effects through postsynaptic 5-HT_1A receptors. These results suggest that 5-HT_1A agonists may have antidepressant efficacy and act as a novel class of antidepressant drug.     

Direct effects of indorenate and 8-OH-DPAT on the blood pressure of pithed rats

Indorenate is a 5-HT_1A receptor agonist with antihypertensive properties. This study was aimed to determine if indorenate, like other 5-HT_1A receptor agonists, may also interact with α-adrenoceptors. Therefore, the effects of indorenate and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; which has affinity for 5-HT_1A receptors and, to a lesser extent, for α₁-adrenoceptors) on the blood pressure of pithed rats were compared. Both compounds produced dose-dependent increases in blood pressure; 8-OH-DPAT was the most potent whereas indorenate produced a higher maximum effect. Metitepine (a mixed 5-HT₁/5-HT₂ receptor antagonist), but not pindolol (a β-adrenoceptor and 5-HT₁ receptor blocker), antagonized the pressor responses produced by both agonists; only the pressor effects of 8-OH-DPAT, however, were antagonized by prazosin (an α₁-adrenoceptor antagonist). Interestingly, ketanserin (a 5-HT₂ and α₁-adrenoceptor blocker) strongly antagonized the pressor responses to indorenate whereas only a slight inhibition of 8-OH-DPAT responses was observed. Further, in pithed rats intravenously infused with norepinephrine (NE), 8-OH-DPAT, but not indorenate, produced dose-dependent hypotensive effects and both compounds were inactive in rats infused with quipazine. In conclusion, 8-OH-DPAT behaved as a partial agonist at α₁-adrenoceptors whereas indorenate produced pressor effects probably due to stimulation of 5-HT₂ receptors. Thus, 8-OH-DPAT, but not indorenate, showed activity at α₁-adrenoceptors in the pithed rat. © 1994 Wiley-Liss, Inc.     

Long lasting attenuation of 8-OH-DPAT induced corticosterone secretion after a single injection of a 5-HT1A receptor agonist

Summary The effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and some other 5-hydroxytryptamine_1A (5-HT_1A) receptor agonists (buspirone, ipsapirone and flesinoxan) on corticosterone secretion in rats were studied. The 5-HT_1A receptors mediating the corticosterone secretion appear to be postsynaptic to the 5-HT neurons, since the response to 8-OH-DPAT was not decreased but potentiated by depletion of 5-HT with p-chlorophenylalanine pretreatment of the animals. Rapid attenuation of the response was developed after a single dose of a 5-HT_1A receptor agonist. Thus, 1 mg/kg s.c. of 8-OH-DPAT attenuated the response of a challenge dose (0.1 mg/kg s.c.) of this compound within 4 h lasting between 7 and 14 d. The development of the subsensitivity was antagonized by pretreatment of the rats with the 5-HT_1A receptor antagonist S-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin ((–)-UH 301). This compound also antagonized the acute effect of 8-OH-DPAT in increasing serum corticosterone. The subsensitivity development was specific for the 5-HT_1A receptor-mediated corticosterone secretion, since the increase in serum corticosterone produced by stimulation of other receptor systems, e.g. ₂-adrenoreceptors (clonidine) or 5-HT₂ receptors [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, (DOI)] was not affected. Send offprint requests to S. B. Ross at the above address     

Labelling of recombinant human and native rat serotonin 5-HT1A receptors by a novel, selective radioligand, [3H]-S 15535: Definition of its binding profile using agonists, antagonists and inverse agonists

The novel benzodioxopiperazine, 5-HT_1A receptor weak partial agonist, S 15535 (4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine) bound with high affinity and selectivity to membranes of Chinese Hamster Ovary cells stably expressing the human (h) 5-HT_1A receptor (K_i = 0.6 nM versus [³H]-8-hydroxy-dipropylamino-tetralin, [³H]-8-OH-DPAT): its affinity at h5-HT_1A receptors was more than 70-fold higher than its affinity at > 50 other binding sites. S 15535 was tritiated to high specific activity (50 Ci/mmol) and its binding profile characterised. At 22° C, [³H]-S 15535 associated and dissociated from h5-HT_1A receptors with half-times of 2.9 and 5.0 min, respectively, yielding a K_d estimate of 3.6 nM. In saturation binding experiments, [³H]-S 15535 displayed a B_max value for h5-HT_1A receptors (1630 fmol/mg), higher than that obtained with the agonist [³H]-8-OH-DPAT (1023 pmol/mg). Guanylyl imidodiphosphate (GppNHp, 100 μM) reduced the binding of [³H]-S 15535 by only 25% compared with 79% for [³H]-8-OH-DPAT at h5-HT_1A receptors. [³H]-S 15535 also showed high affinity, saturable binding to rat hippocampal membranes (B_max = 820 fmol/mg versus 647 fmol/mg for [³H]-8-OH-DPAT). For both h5-HT_1A and rat 5-HT_1A receptors, the K_i values for competition binding of 15 serotonergic ligands with [³H]-S 15535 was highly correlated with that of [³H]-8-OH-DPAT. However, important differences were also observed. The agonist, 5-hydroxytryptamine (5-HT), displayed biphasic competition curves with [³H]-S 15535 but not with [³H]-8-OH-DPAT at h5-HT_1A receptors. Similarly, the ‘antagonists’, spiperone, methiothepin and (+)butaclamol, showed biphasic competition isotherms versus [³H]-S 15535 but not [³H]-8-OH-DPAT. When [³H]-S 15535 competition binding experiments were carried out in the presence of GppNHp (100 μM) the 5-HT and 8-OH-DPAT competition curves shifted to the right, whereas the spiperone and methiothepin competition curves shifted to the left. In contrast, in the presence of GppNHp, the competition isotherms for N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY 100,635) were not altered. Taken together, these data show that (i) [³H]-S 15535 is a highly selective 5-HT_1A receptor ligand which labels both G-protein-coupled and uncoupled 5-HT_1A receptors, (ii) antagonists, such as WAY 100,635, which yield monophasic isotherms in competition with both [³H]-agonists and [³H]-antagonists, are not sensitive to the G-protein coupling state of the receptor, but (iii) spiperone and methiothepin behaved as inverse agonists, their competition isotherms with [³H]-S 15535 being modulated in an opposite manner to those of agonists. Received: 12 September 1997 / Accepted: 1 December 1997     

Ethopharmacological studies differentiate the effects of various serotonergic compounds on aggression in rats

A series of experiments was performed to investigate the effects on aggression of various drugs affecting serotonergic transmission in rats. Using ethologically derived behavioural categories, the behaviour of treated animals was described. Drug effects were observed in two aggression models: resident–intruder aggression (RI) in male rats, and maternal aggression in lactating females during the postpartum period (MA). The 5-HT_1A agonists, buspirone, iosapirone, and 8-OH-DPAT, decreased aggression in RI and MA but simultaneously led to a marked decrease in social interest and activity, indicative of a nonspecific anti-aggressive profile. Nonselective 5-HT agonists, such as RU 24969, eltoprazine (DU 28853), and TEMPP, reduced aggression quite specifically and did not decrease social interest or exploration, but sometimes even increased these behaviours. In RI and MA, the behavioural effects of these drugs were roughly similar. By contrast, MA was more sensitive to the treatment with the 5-HT reuptake blocker fluvoxamine, which blocked RI aggression only nonspecifically at the highest dose. DOI, a 5-HT₂ and 5-HT_1C agonist, decreased aggressive behaviour and increased inactivity, without affecting social interest and exploration in RI as well as MA. This was, however, accompanied by “wet dog shaking,” characteristic of 5-HT₂-receptor stimulation. The nonspecific 5-HT agonist (and 5-HT₃ antagonist) quipazine also induced “wet dog shaking” at doses that suppressed aggression, social interest and exploration but increased inactive behaviours (sitting and lying). The discussion attempts to delineate a role for 5-HT receptor subtype involvement in the modulation of aggression, with the restrictions we clearly face with regard to the lack of specific serotonergic agonists and antagonists for certain receptor subtypes. By and large, male and female rats react similarly to treatment with serotonergic drugs stressing the consistent role of 5-HT in different forms of aggression. © 1992 wiley-Liss, Inc.     

MKC-242, a novel 5-HT1A receptor agonist, facilitates cortical acetylcholine release by a mechanism different from that of 8-OH-DPAT in awake rats

We have previously reported that 5-{3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy}-1,3-benzodioxole (MKC-242), a potent and selective serotonin (5-HT)_1A receptor agonist, exerts anxiolytic- and antidepressant-like effects in animal models and that the antidepressant-like effect may be mediated by postsynaptic 5-HT_1A receptors. The present study, using a microdialysis technique, was undertaken to characterize in vivo the effect of MKC-242 on cholinergic neurons. Subcutaneous injection of MKC-242 (0.5–1.0 mg/kg), like the typical 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), increased extracellular acetylcholine (ACh) levels in the rat cerebral cortex. The increase in ACh release by MKC-242 was also observed in the hippocampus. The effect of MKC-242 on cortical ACh release was attenuated by pretreatment with the 5-HT_1A receptor antagonists (10 mg/kg, s.c.) propranolol and N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropanamide. The increase in cortical ACh release by MKC-242 was blocked by lesion of serotonergic neurons with 5,7-dihydroxytryptamine, whereas that by 8-OH-DPAT was not. Lesion of noradrenergic neurons with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not affect the MKC-242-induced increase in ACh release. These results suggest that systemic injection of MKC-242 facilitates in vivo ACh release via an activation of somadendritic 5-HT_1A autoreceptors, and that MKC-242 and 8-OH-DPAT affect cholinergic neurons in the rat cerebral cortex via different mechanisms.     

Social instigation and aggressive behavior in mice: role of 5-HT1A and 5-HT1B receptors in the prefrontal cortex

Rationale  Social instigation is used in rodents to induce high levels of aggression, a pattern of behavior with certain parallels to that of violent individuals. This procedure consists of a brief exposure to a provocative stimulus male, before direct confrontation with an intruder. Studies using 5-HT_1A and 5-HT_1B receptor agonists show an effective reduction in aggressive behavior. An important site of action for these drugs is the ventral orbitofrontal cortex (VO PFC), an area of the brain which is particularly relevant in the inhibitory control of aggressive and impulsive behavior. Objectives  The objectives of the study are to assess the anti-aggressive effects of 5-HT_1A and 5-HT_1B agonist receptors [8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT) and CP-93,129] in the VO PFC of socially provoked male mice. To confirm the specificity of the receptor, 5-HT_1A and 5-HT_1B antagonist receptors (WAY-100,635 and SB-224,289) were microinjected into the same area, in order to reverse the agonist effects. Results  8-OH-DPAT (0.56 and 1.0 μg) reduced the frequency of attack bites. The lowest dose of CP-93,129 (0.1 μg) also decreased the number of attack bites and lateral threats. 5-HT1A and 5-HT1B receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming, and the latter, increasing these acts. Specific participation of the 1A and 1B receptors was verified by reversal of anti-aggressive effects using selective antagonists WAY-100,635 (10.0 μg) and SB-224,289 (1.0 μg). Conclusions  The decrease in aggressiveness observed with microinjections of 5-HT_1A and 5-HT_1B receptor agonists into the VO PFC of socially provoked mice, supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner.     

Effects of fluoxetine and buspirone on the panicolytic-like response induced by the activation of 5-HT1A and 5-HT2A receptors in the rat dorsal periaqueductal gray

Rationale Administration of 5-hydroxytryptamine (5-HT)_1A and 5-HT_2A receptor agonists into the dorsal periaqueductal gray (DPAG) inhibits escape, a defensive behavior associated with panic attacks. Long-term treatment with the antipanic compound imipramine enhances the DPAG 5-HT_1A- and 5-HT_2A-receptor-mediated inhibition of escape, implicating these receptors in the mode of action of panicolytic drugs. Objectives In the present study, we investigated whether the inhibitory effect on escape elicited by the intra-DPAG injection of 5-HT_1A and 5-HT_2A receptor agonists is also enhanced after treatment with fluoxetine, another widely used antipanic drug. The effects of fluoxetine were compared to those of buspirone, an anxiolytic drug without major effect on panic disorder. Methods Male Wistar rats, subchronically (3–6 days) or chronically (21–24 days) treated with fluoxetine (10 mg/kg i.p.) or chronically treated with buspirone (0.3 mg/kg i.p.), were intra-DPAG injected with 5-HT (20 nmol), the 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 8 nmol) or the preferential 5-HT_2A receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl) piperazine dihydrochloride (DOI; 16 nmol). The intensity of electrical current that applied to the DPAG-evoked escape behavior was measured before and after the microinjection of these agonists. Results The electrical current necessary to produce escape was increased after the microinjection of the three 5-HT receptor agonists in all groups of animals tested. However, this panicolytic-like effect was significantly higher in animals receiving long-term treatment with fluoxetine. Conclusions The results suggest that facilitation of the 5-HT_1A- and 5-HT_2A-receptor-mediated inhibition of DPAG neuronal activity is implicated in the beneficial effect of antidepressants in panic disorder.     

Behavioral and biochemical effects of the 5-HT1A receptor agonists flesinoxan and 8-OH-DPAT in the rat.

The 5-HT_1A receptor agonists flesinoxan (0.2–3.2 mg kg⁻¹ s.c.) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.025–0.4 mg kg⁻¹ s.c.) produced (1) a dose-dependent facilitation of male rat ejaculatory behavior and (2) characteristic, dose-dependent effects on spontaneous motor activity. Thus, total locomotor activity and rearing activity were decreased. However, forward locomotion and peripheral locomotion were increased relative to the total horizontal activity. Furthermore, (3) 5-HTP accumulation, after inhibition of cerebral decarboxylase, was dose dependency decreased by both compounds in the ventral striatum and in the prefrontal cortex. There was a statistically significant decrease in DOPA accumulation in the ventral striatum after administration of a high dose of flesinoxan (3.2 mg kg⁻¹), and a tendency for 8-OH-DPAT to produce the same effect. The efficacy of the compounds to affect male rat sexual behavior, spontaneous motor activity in the open-field and forebrain 5-HT synthesis was approximately the same, whereas flesinoxan was about an order of magnitude less potent than 8-OH-DPAT.