S 15535: a highly selective benzodioxopiperazine 5-HT1A receptor ligand which acts as an agonist and an antagonist at presynaptic and postsynaptic sites respectively

Advances in drug design and therapeutic potential of selective or multitarget 5-HT1A receptor ligands

5-HT1A receptor (5-HT1A-R) is a serotoninergic G-protein coupled receptor subtype which contributes to several physiological processes in both central nervous system and periphery. Despite being the first 5-HT-R identified, cloned and studied, it still represents a very attractive target in drug discovery and continues to be the focus of a myriad of drug discovery campaigns due to its involvement in numerous neuropsychiatric disorders. The structure-activity relationship studies (SAR) performed over the last years have been devoted to three main goals: (i) design and synthesis of 5-HT1A-R selective/preferential ligands; (ii) identification of 5-HT1A-R biased agonists, differentiating pre- versus post-synaptic agonism and signaling cellular mechanisms; (iii) development of multitarget compounds endowed with well-defined poly-pharmacological profiles targeting 5-HT1A-R along with other serotonin receptors, serotonin transporter (SERT), D2-like receptors and/or enzymes, such as acetylcholinesterase and phosphodiesterase, as a promising strategy for the management of complex psychiatric and neurodegenerative disorders. In this review, medicinal chemistry aspects of ligands acting as selective/preferential or multitarget 5-HT1A-R agonists and antagonists belonging to different chemotypes and developed in the last 7 years (2017-2023) have been discussed. The development of chemical and pharmacological 5-HT1A-R tools for molecular imaging have also been described. Finally, the pharmacological interest of 5-HT1A-R and the therapeutic potential of ligands targeting this receptor have been considered.

Nicotine and fluoxetine alter adolescent dopamine-mediated behaviors via 5-HT1A receptor activation

Introduction

Abuse or misuse of tobacco, e-cigarettes, or antidepressants may have serious clinical consequences during adolescence, a sensitive period during brain development when the distinct neurobiology of adolescent serotonin (5-HT) and dopamine (DA) systems create unique behavioral vulnerabilities to drugs of abuse.

Methods

Using a pharmacological approach, we modeled the behavioral and neurochemical effects of subchronic (4-day) nicotine (60µg/kg, i.v.) or fluoxetine (1mg/kg, i.v.) exposure in adolescent and adult male rats.

Results

Nicotine and fluoxetine significantly enhance quinpirole-induced locomotor activity and initial cocaine self-administration in adolescents, but not adults. These effects were blocked by serotonin 5-HT1A receptor antagonists, WAY-100,635 (100 µg/kg, i.v.) or S-15535 (300 µg/kg, i.v.). Neurochemical and anatomical autoradiographic analysis of 8-OH-DPAT-stimulated [35S]GTPγS reveal that prior exposure to nicotine and fluoxetine results in both overlapping and distinct effects on regional 5-HT1A receptor activity. Both fluoxetine and nicotine enhance adolescent 5-HT1A receptor activity in the primary motor cortex (M1), whereas fluoxetine alone targets prefrontal cortical neurocircuitry and nicotine alone targets the amygdala.

Discussion

Given their different pharmacological profiles, comparison between WAY-100,635 and S-15535 indicates that postsynaptic 5-HT1A receptors mediate the behavioral effects of prior nicotine and fluoxetine exposure. In addition, within the adolescent M1, maladaptive changes in 5-HT signaling and 5-HT1A activity after nicotine or fluoxetine exposure may potentiate hyper-responsiveness to dopaminergic drugs and prime adolescent vulnerability for future substance abuse.

Pharmacological Studies on the Role of 5-HT1 A Receptors in Male Sexual Behavior of Wildtype and Serotonin Transporter Knockout Rats

Brain serotonin (5-HT) neurotransmission plays an important role in male sexual behavior and it is well established that activating 5-HT_{1 A} receptors in rats facilitate ejaculatory behavior. However, the relative contribution of 5-HT_{1 A} somatodendritic autoreceptors and heteroreceptors in this pro-sexual behavior is unclear. Moreover, it is unclear whether the contribution of somatodendritic 5-HT_{1 A} autoreceptors and postsynaptic 5-HT_{1 A} heteroreceptors alter when extracellular 5-HT levels are chronically increased. Serotonin transporter knockout (SERT^-/-) rats exhibit enhanced extracellular 5-HT levels and desensitized 5-HT_{1 A} receptors. These rats model neurochemical changes underlying chronic SSRI-induced sexual dysfunction. We want to determine the role of presynaptic versus postsynaptic 5-HT_{1 A} receptors in the pro-sexual effects of 5-HT_{1 A} receptor agonists in SERT^+/+ and in SERT^-/- rats. Therefore, acute effects of the biased 5-HT_{1 A} receptor agonists F-13714, a preferential 5-HT_{1 A} autoreceptor agonist, or F-15599, a preferential 5-HT_{1 A} heteroreceptor agonist, and S15535 a mixed 5-HT_{1 A} autoreceptor agonist/heteroreceptor antagonist, on male sexual behavior were assessed. A clear and stable genotype effect was found after training where SERT^+/+ performed sexual behavior at a higher level than SERT^-/- rats. Both F-15599 and F-13714 induced pro-sexual activity in SERT^+/+ and SERT^-/- animals. Compared to SERT^+/+, the F13714-dose-response curve in SERT^-/- rats was shifted to the right. SERT^+/+ and SERT^-/- rats responded similar to F15599. Within both SERT^+/+ and SERT^-/- rats the potency of F-13714 was much stronger compared to F-15599. S15535 had no effect on sexual behavior in either genotype. In SERT^+/+ and SERT^-/- rats that were selected on comparable low sexual activity (SERT^+/+ 3 or less ejaculations and SERT^-/- 5 or less ejaculations in 10 weeks) S15535 also did not influence sexual behavior. The two biased compounds with differential effects on 5-HT_{1 A} auto- and hetero-receptors, exerted pro-sexual activity in both SERT^+/+ and SERT^-/- rats. Applying these specific pharmacological tools has not solved whether pre- or post-synaptic 5-HT_{1 A} receptors are involved in pro-sexual activity. Moreover, the inactivity of S15535 in male sexual behavior in either genotype was unexpected. The question is whether the in vivo pharmacological profile of the different 5-HT_{1 A} receptor ligands used, is sufficient to differentiate pre- and/or post-synaptic 5-HT_{1 A} receptor contributions in male rat sexual behavior.

Serotonin 5-HT1A Receptor Biased Agonists Display Differential Anxiolytic Activity in a Rat Social Interaction Model

When placed in an unfamiliar and brightly lit open-field, two adult male rats that have not previously interacted display a low level of social interaction (SI) attributed to an anxiety-like state. The SI test has therefore been used to explore anxiolytic/antistress activity. Here, we investigated the effects of serotonin 5-HT_1A receptor agonists displaying various activity profiles, i.e. partial vs full agonist efficacy and pre- versus postsynaptic 5-HT_1A receptor preferential activation by "biased agonists". Adult male Sprague-Dawley rats were housed singly before starting the social interaction session. At 30 min before being placed in an open-field, both rats of the dyad were injected (i.p or s.c.) with either vehicle, diazepam (as a reference compound), or one of six 5-HT_1A receptor agonists: NLX-101 (a.k.a. F15599), F13714, S15535, flesinoxan, 8-OH-DPAT, and buspirone. Time spent in SI (following, sniffing, playing) was recorded for 10 min. Time spent in SI was inversely correlated with light intensity, with values dropping nearly by half (212.6 ± 18.8 vs 113.7 ± 7.0 s) between 10 and 300 lx (measured at floor level). Under the high light intensity conditions (300 lx), diazepam showed a bell-shaped curve, significantly increasing SI (78% increase in interaction time above control) at 1 mg/kg i.p. only. In the case of 5-HT_1A receptor ligands, full agonists, whether nonpreferential (flesinoxan, (±)8-OH-DPAT) or preferential for presynaptic receptors (F13714), showed the strongest activity in this model. The preferential presynaptic receptor partial agonist, S15535, was also active over a wide dose-range, although with lower efficacy than F13714. In contrast, NLX-101, a high-efficacy biased agonist that preferentially activates postsynaptic 5-HT_1A receptors, exhibited little activity. The clinical anxiolytic, buspirone, showed a marked effect likely due to its partial agonist activity at 5-HT_1A presynaptic receptors. These data support the hypothesis that enhancement of SI in this model is mediated by preferential agonist activation of presynaptic 5-HT_1A receptors, and confirm previous studies using local microinjections of (±)8-OH-DPAT. They further support the utility of noninvasive administration of biased agonists for exploring the activity of 5-HT_1A receptor subpopulations.

The role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and its interaction in emotional learning and memory

Serotonin [5-hydroxytryptamine (5-HT)] is a multifunctional neurotransmitter innervating cortical and limbic areas involved in cognition and emotional regulation. Dysregulation of serotonergic transmission is associated with emotional and cognitive deficits in psychiatric patients and animal models. Drugs targeting the 5-HT system are widely used to treat mood disorders and anxiety-like behaviors. Among the fourteen 5-HT receptor (5-HTR) subtypes, the 5-HT_1AR and 5-HT₇R are associated with the development of anxiety, depression and cognitive function linked to mechanisms of emotional learning and memory. In rodents fear conditioning and passive avoidance (PA) are associative learning paradigms to study emotional memory. This review assesses the role of 5-HT_1AR and 5-HT₇R as well as their interplay at the molecular, neurochemical and behavioral level. Activation of postsynaptic 5-HT_1ARs impairs emotional memory through attenuation of neuronal activity, whereas presynaptic 5-HT_1AR activation reduces 5-HT release and exerts pro-cognitive effects on PA retention. Antagonism of the 5-HT_1AR facilitates memory retention possibly via 5-HT₇R activation and evidence is provided that 5HT₇R can facilitate emotional memory upon reduced 5-HT_1AR transmission. These findings highlight the differential role of these 5-HTRs in cognitive/emotional domains of behavior. Moreover, the results indicate that tonic and phasic 5-HT release can exert different and potentially opposing effects on emotional memory, depending on the states of 5-HT_1ARs and 5-HT₇Rs and their interaction. Consequently, individual differences due to genetic and/or epigenetic mechanisms play an essential role for the responsiveness to drug treatment, e.g., by SSRIs which increase intrasynaptic 5-HT levels thereby activating multiple pre- and postsynaptic 5-HTR subtypes.

Analysis of mechanisms for memory enhancement using novel and potent 5-HT1A receptor ligands

In light of the involvement of serotonergic 5-HT1A receptors in the mediation of the memory of aversive events, the potent and selective 5-HT1A receptor antagonists, MC18 fumarate and VP08/34 fumarate, were tested in the passive avoidance task (PA), a rodent model of instrumental conditioning. Either alone or in combination with the prototypical agonist 8-OH-DPAT, MC18 fumarate at doses (0.1, 0.3 and 1mg/kg given 15min prior to training) exerted a dose-dependent facilitation of PA memory retention. When administered 15min prior to 8-OH-DPAT (0.3 and 1mg/kg), MC18 fumarate at a dose of 0.3mg/kg, enhanced significantly the impairment of PA retention caused by 8-OH-DPAT (1mg/kg). However, VP08/34 fumarate given at the same doses exerted no statistically effect on PA retention memory. Furthermore, VP08/34 fumarate given 15min prior to 8-OH-DPAT (0.3 and 1mg/kg) only slightly enhanced the PA impairment induced by 8-OH-DPAT. In conclusion, the profile of MC18 fumarate is intriguing since it behaves in a manner which differs from both full receptor antagonists such as NAD-299 or partial receptor agonists. The results also illustrate the importance of subtle receptor interaction and probably ligand efficacy in determining the actions of two almost identical 5-HT1A receptor ligands in cognitive function such as instrumental learning.

Indanes--Properties, Preparation, and Presence in Ligands for G Protein Coupled Receptors

The indane (2,3-dihydro-1H-indene) ring system is an attractive scaffold for biologically active compounds due to the combination of aromatic and aliphatic properties fused together in one rigid system. This bicyclic structure provides a wide range of possibilities to incorporate specific substituents in different directionalities, thus being an attractive scaffold for medicinal chemists. Notably, many indane-based compounds are being used in the clinic to treat various diseases, such as indinavir, an HIV-1 protease inhibitor; indantadol, a potent Monoamine Oxidase (MAO)-inhibitor; the amine uptake inhibitor indatraline; and the ultra-long-acting β-adrenoceptor agonist indacaterol. Given the diversity of targets these drugs act on, one could argue that the indane ring system is a privileged substructure. In the present review, the synthetic and medicinal chemistry of the indane ring system is described. In more detail, it contains a comprehensive overview of compounds bearing the indane substructure with G protein coupled receptor (GPCR) activity, with particular emphasis on their structure-activity relationships (SAR).

Exploring the role of 5-HT1A receptors in the regulation of prepulse inhibition in mice: implications for cross-species comparisons

Prepulse inhibition (PPI) is a model of sensorimotor gating, a sensory filtering mechanism which is disrupted in schizophrenia. Here, investigation of the role of the serotonin-1A (5-HT(1A)) receptor in the regulation of PPI in two mouse strains, C57Bl/6 and Balb/c, was used to address findings in the PPI literature on species and mouse strain differences that question the usefulness of PPI as a cross-species preclinical test. Although the full 5-HT(1A) receptor agonist, 8-OH-DPAT, induced markedly different strain-specific responses in PPI, other selective 5-HT(1A) receptor ligands with partial agonist or antagonist activity elicited similar effects across strains. Pretreatment with the serotonin precursor, 5-HTP, to increase serotonergic activity in the brain, unmasked a decrease in PPI caused by 8-OH-DPAT in C57Bl/6 mice. Pretreatment with the serotonin synthesis inhibitor, PCPA, to decrease serotonergic activity in the brain, unmasked an 8-OH-DPAT-induced increase in PPI in this strain. These studies show that the strain-dependent involvement of 5-HT(1A) receptors in PPI can be modulated by the type of 5-HT(1A) ligand used, or increasing or decreasing serotonin levels in the brain. These results help to clarify some of the mouse strain and species differences in PPI regulation and strengthen its usefulness as a cross-species measure of sensorimotor gating.

Nicotine alters limbic function in adolescent rat by a 5-HT1A receptor mechanism

Epidemiological studies have shown that adolescent smoking is associated with health risk behaviors, including high-risk sexual activity and illicit drug use. Using rat as an animal model, we evaluated the behavioral and biochemical effects of a 4-day, low-dose nicotine pretreatment (60 μg/kg; intravenous) during adolescence and adulthood. Nicotine pretreatment significantly increased initial acquisition of cocaine self-administration, quinpirole-induced locomotor activity, and penile erection in adolescent rats, aged postnatal day (P)32. These effects were long lasting, remaining evident 10 days after the last nicotine treatment, and were observed when nicotine pretreatment was administered during early adolescence (P28-31), but not late adolescence (P38-41) or adulthood (P86-89). Neurochemical analyses of c-fos mRNA expression, and of monoamine transmitter and transporter levels, showed that forebrain limbic systems are continuing to develop during early adolescence, and that this maturation is critically altered by brief nicotine exposure. Nicotine selectively increased c-fos mRNA expression in the nucleus accumbens shell and basolateral amygdala in adolescent, but not adult animals, and altered serotonin markers in these regions as well as the prefrontal cortex. Nicotine enhancement of cocaine self-administration and quinpirole-induced locomotor activity was blocked by co-administration of WAY 100 635 (N-{2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide), a selective serotonin 1A (5-HT1A) receptor antagonist. Early adolescent pretreatment with the mixed autoreceptor/heteroceptor 5-HT1A receptor agonist, 8-OH-DPAT, but not the autoreceptor-selective agonist, S-15535, also enhanced quinpirole-induced locomotor activation. Nicotine enhancement of quinpirole-induced penile erection was not blocked by WAY 100 635 nor mimicked by 8-OH-DPAT. These findings indicate that early adolescent nicotine exposure uniquely alters limbic function by both 5-HT1A and non-5-HT1A receptor mechanisms.

Adaptations in pre- and postsynaptic 5-HT1A receptor function and cocaine supersensitivity in serotonin transporter knockout rats

RATIONALE

While individual differences in vulnerability to psychostimulants have been largely attributed to dopaminergic neurotransmission, the role of serotonin is not fully understood.

OBJECTIVES

To study the rewarding and motivational properties of cocaine in the serotonin transporter knockout (SERT-/-) rat and the involvement of compensatory changes in 5-HT1A receptor function are the objectives of the study.

MATERIALS AND METHODS

The SERT-/- rat was tested for cocaine-induced locomotor activity, cocaine-induced conditioned place preference, and intravenous cocaine self-administration. In addition, the function and expression of 5-HT1A receptors was assessed using telemetry and autoradiography, respectively, and the effect of 5-HT1A receptor ligands on cocaine's psychomotor effects were studied.

RESULTS

Cocaine-induced hyperactivity and conditioned place preference, as well as intravenous cocaine self-administration were enhanced in SERT-/- rats. Furthermore, SERT-/- rats displayed a reduced hypothermic response to the 5-HT1A receptor agonist 8-OHDPAT. S-15535, a selective somatodendritic 5-HT1A receptor agonist, reduced stress-induced hyperthermia (SIH) in wild-type controls (SERT+/+), while it increased SIH in SERT-/- rats. As 5-HT1A receptor binding was reduced in selective brain regions, these thermal responses may be indicative for desensitized 5-HT1A receptors. We further found that both 8-OHDPAT and S-15535 pretreatment increased low-dose cocaine-induced locomotor activity in SERT-/- rats, but not SERT+/+ rats. At a high cocaine dose, only SERT+/+ animals responded to 8-OHDPAT and S-15535.

CONCLUSION

These data indicate that SERT-/- -associated 5-HT1A receptor adaptations facilitate low-dose cocaine effects and attenuate high-dose cocaine effects in cocaine supersensitive animals. The role of postsynaptic and somatodendritic 5-HT1A receptors is discussed.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Differential role of the 5-HT1A receptor in aggressive and non-aggressive mice: An across-strain comparison

Differential role of the 5-HT(1A) receptor in aggressive and non-aggressive mice: an across-strain comparison. PHYSIOL BEHAV 00(0) 000-000, 2006. According to the serotonin (5-HT)-deficiency hypothesis of aggression, highly aggressive individuals are characterized by low brain 5-HT neurotransmission. Key regulatory mechanisms acting on the serotonergic neuron involve the activation of the somatodendritic inhibitory 5-HT(1A) autoreceptor (short feedback loop) and/or the activation of postsynaptic 5-HT(1A) receptors expressed on neurons in cortico-limbic areas (long feedback loop). In this study, we examined whether low serotonin neurotransmission is associated with enhanced 5-HT(1A) (auto)receptor activity in highly aggressive animals. Male mice (SAL-LAL, TA-TNA, NC900-NC100) obtained through different artificial-selection breeding programs for aggression were observed in a resident-intruder test. The prefrontal cortex level of 5-HT and its metabolite 5-HIAA were determined by means of HPLC. The activity of the 5-HT(1A) receptors was assessed by means of the hypothermic response to the selective 5-HT(1A) agonists S-15535 (preferential autoreceptor agonist) and 8-OHDPAT (full pre- and postsynaptic receptor agonist). Highly aggressive mice had lower serotonin levels in the prefrontal cortex and two out of three aggressive strains had higher 5-HT(1A) (auto)receptor sensitivity. The results strengthen the validity of the serotonin-deficiency hypothesis of aggression and suggest that chronic exaggerated activity of the 5-HT(1A) receptor may be a causative link in the neural cascade of events leading to 5-HT hypofunction in aggressive individuals.

Potentiation of the resetting effects of light on circadian rhythms of hamsters using serotonin and neuropeptide Y receptor antagonists

Circadian rhythms are entrained by light/dark cycles. In hamsters, the effects of light on circadian rhythms can be modulated by serotonergic input to the suprachiasmatic nucleus from the raphe nuclei and by neuropeptide Y containing afferents to the suprachiasmatic nucleus from the intergeniculate leaflet in the thalamus. In this study we measured effects of compounds acting on serotonergic 1A and neuropeptide Y Y5 receptors to determine if combined serotonergic-neuropeptide Y inhibition could synergistically potentiate effects of light on rhythms. We used mixed serotonergic agonist/antagonists BMY 7378 or NAN-190 as well as a neuropeptide Y Y5 antagonist CP-760,542. Both BMY 7378 and NAN-190 are thought to block serotonin release via acting as agonists at the 5-hydroxytryptamine 1A (5-HT1A) autoreceptors on cells in the raphe, and also block response of target cells by acting as antagonists at post-synaptic 5-HT1A receptors, for example, in the suprachiasmatic nuclei or the intergeniculate leaflet. Replicating prior work, we found that pretreatment with either drug alone increased the phase shift to light at circadian time 19. The combined effect of BMY 7378 and CP-760,542 given prior to light at circadian time 19 was to further potentiate the subsequent phase shift in wheel-running rhythms (the phase shift was 317% of controls; light alone: 1.35 h phase shift vs. BMY 7378, CP-760,542, and light: 4.27 h phase shift). Combined treatment with NAN-190 and CP-760,542 produced a light-induced phase shift 576% of controls (phase shift to light alone: 1.23 h vs. NAN-190, CP-760,542, and light: 7.1 h phase shift). These results suggest that the resetting effects of light on circadian rhythms can be greatly potentiated in hamsters by using pharmacological treatments that block both serotonergic and neuropeptide Y afferents to the suprachiasmatic nuclei.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.

Differences in the effects of 5-HT(1A) 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.

The influence of G protein subtype on agonist action at D2 dopamine receptors

In previous studies, we have shown that agonists influence the ability of D2 dopamine receptors to couple to G proteins and here we extend this work. The human D2Short dopamine receptor and a natural polymorphism of this D(2Short)(Ser311Cys), have been studied by co-expressing the receptors in insect cells with Gbeta1gamma2 and either Galpha(o), Galpha(i1), Galpha(i2) or Galpha(i3) G protein subunits. These preparations have been used to study the G protein coupling profiles of the two receptors and the influence of agonists. Receptor/G protein coupling was analysed in dopamine/[3H]spiperone competition binding experiments and through stimulation of [35S]GTPgammaS binding. Although the Ser311Cys polymorphism itself had no appreciable effect on the G protein coupling specificity of the D2 receptor, agonist stimulation of [35S]GTPgammaS binding, revealed that both dopamine and (+)-3PPP showed a clear preference for Galpha(o) compared to the Galpha(i) subtypes, but quinpirole did not. These results indicate that agonists are able to stabilise different receptor conformations with different abilities to couple to G proteins.

Basal and stress-induced differences in HPA axis, 5-HT responsiveness, and hippocampal cell proliferation in two mouse lines

To characterize individual differences in neuroendocrine and neurochemical correlates of stress coping, two lines of wild house mice were studied. These mice are genetically selected for high and low aggression and show distinctly different behavioral strategies toward environmental stimuli. Long attack latency (LAL), low aggressive mice display a passive coping style, whereas short attack latency (SAL), high aggressive mice show an active coping style. It was hypothesized that this difference in behavioral coping style is associated with differences in stress system reactivity. This was tested by investigating the regulation of the hypothalamus-pituitary-adrenal (HPA) axis and the serotonin (5-HT) system and hippocampal cell proliferation rate in these mice under baseline and stress conditions. Baseline corticosterone output in LAL mice was found to be more sensitive to adrenocorticotropic hormone, but showed less day/night variation than in SAL mice. Furthermore, LAL mice showed lower hippocampal 5-HT(1A) receptor gene expression and function. Basal hippocampal cell proliferation rate was slightly lower in LAL than in SAL mice. Exposure to acute stress (forced swimming for 5 min) resulted in a hyper-reactive HPA response, a reduced increase in brain 5-HT metabolism, and an almost 50% reduction in hippocampal cell proliferation rate in LAL compared with SAL mice. Chronic psychosocial stress (sensory contact stress) induced long-lasting changes in the HPA axis in LAL, but not in SAL mice. In conclusion, these studies show that a genetic trait in behavioral coping style in wild house mice is associated with differences in HPA regulation, 5-HT neurotransmission, and hippocampal cell proliferation rate. The results further indicate that LAL mice have a higher stress responsivity than SAL mice. These results may have implications for a differential susceptibility for stress-related mood disorders.

The control of feather pecking by serotonin

Feather-pecking behavior in laying hens (Callus gallus) may be considered a behavioral pathology, comparable to human psychopathological disorders. Scientific knowledge on the causation of such disorders strongly suggests involvement of the serotonergic (5-hydroxytryptamine; 5-HT) system in feather pecking. Previously, chicks from a high-feather-pecking (HFP) line were found to display lower 5-HT turnover levels than chicks from a low-feather-pecking (LFP) line (in response to acute stress; Y. M. van Hierden et al., 2002). The present study investigated whether low 5-HT neurotransmission modulates feather pecking. First. S-15535, a somatodendritic 5-HT-sub(1A) autoreceptor agonist, was demonstrated to be an excellent tool for reducing 5-HT turnover in the forebrain of LFP and HFP chicks. Second, the most effective dose of S-15535 (4.0 mg/kg body weight) significantly increased severe feather-pecking behavior. The results confirmed the postulation that the performance of feather pecking is triggered by low 5-HT neurotransmission.

Serotonergic serotonin (1A) mixed agonists/antagonists elicit large-magnitude phase shifts in hamster circadian wheel-running rhythms

The biological clock that generates circadian rhythms in mammals is located within the suprachiasmatic nuclei at the base of the hypothalamus. The circadian clock is entrained to the daily light/dark cycle by photic information from the retina. The retinal input to the clock is inhibited by exogenously applied serotonin agonists, perhaps mimicking an endogenous inhibitory serotonergic input to the clock arriving from the midbrain raphe. In the present study, a unique class of serotonergic compounds was tested for its ability to modulate retinal input to the circadian clock. The serotonergic ligands 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro(4.5)decane-7,9-dione dihydrochloride (BMY 7378), S 15535, and 8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride (MDL 73005 EF) can all be classified as mixed agonists/antagonists at type 1A serotonin receptors. Circadian wheel-running activity rhythms were monitored in Syrian hamsters maintained in constant darkness. Dim white-light pulses administered to the hamsters at circadian time 19 advanced the phase of their running rhythms by 1-2 h. Injection of BMY 7378, S 15535, and to a lesser degree MDL 73005 EF, prior to the light pulses resulted in phase advances from 5 to 6 h, and by as much as 8 h. Neither BMY 7378 nor S 15535 had any effect on light-induced phase delays in hamster activity rhythms at circadian time 14. Further, BMY 7378 is able to phase advance circadian rhythms by approximately 1 h at night even without light exposure. Finally, the effects of BMY 7378 on circadian rhythms is opposite to that observed with the prototypical serotonin 1A agonist (+/-)-8-hydroxy-2-(DI-n-propyl-amino)tetralin hydrobromide (8-OH-DPAT) (8-OH-DPAT elicits non-photic phase advances in the day and inhibits photic-induced phase advances at night). These results suggest that pharmacologically blocking raphe input to the suprachiasmatic circadian clock results in substantially larger photically induced phase advances in wheel-running rhythms. This is further evidence that raphe input to the circadian clock is probably acting to dampen the clock's response to light under certain conditions. The large-magnitude phase shifts, and temporal-activity profile seen with BMY 7378 and S 15535, suggest that compounds with this unique pharmacological profile may be beneficial in the treatment of circadian phase delays recently reported to be a complication resulting from Alzheimer's disease.

Improvement in the selectivity and metabolic stability of the serotonin 5-HT(1A) ligand, S 15535: a series of cis- and trans-2-(arylcycloalkylamine) 1-indanols

S 15535 (1) displays a distinctive profile of agonist and antagonist (weak partial agonist) activity at pre- and postsynaptic 5-HT(1A) receptors, respectively. It has proven to be active in several models predictive of anxiolytic, antidepressant, and procognitive properties. In an attempt to increase its selectivity and metabolic stability, and guided by the results of human metabolic studies, we prepared a series of cis- and trans-2-(arylcycloalkylamine) 1-indanols. Irrespective of the nature of the arylcycloalkylamine moiety or the presence of substituents on the indanol ring, trans isomers invariably showed the highest affinity for human, recombinant h5-HT(1A) receptors. Among them, compounds 39, 42, 45, 49, 52, 53, 54, 57, 61, 64, 67, and 70 displayed similar or higher affinity than the parent compound 1 (pK(i) > or = 9.1). Lack of selectivity toward alpha1-adrenoceptors has been frequently encountered with 5-HT(1A) ligands. While S 15535 itself presents reasonable selectivity (158-fold) in this respect, trans piperazine derivatives 4-trans,35, 39, 41, 47, 64, 68, 69, 70, 71 displayed even more pronounced selectivity vs alpha1-adrenoceptors, with the nitro derivative 70 being highly selective (1259-fold). However, among the set of trans piperidines prepared, only 64, which also bears a nitro on the indanol ring, displayed selectivity greater than the parent compound 1. All trans derivatives behaved as partial agonists at h5-HT(1A) receptors, as determined by their submaximal stimulation of [35S]GTPgammaS binding to a level comparable to that observed with S 15535. In metabolic stability studies in vitro using human microsomes and hepatocytes, only trans piperazines and, in particular, 35, 39, 41, 68, 69, and 70, showed an improvement relative to 1, whereas trans piperidines did not. Compounds 35, 39, 41, and 70, which combined both improved selectivity and metabolic stability, and which retained the distinctive pharmacological characteristics of S 15535, were evaluated in animal models of anxiety. Of these, 35, which showed the highest oral bioavailability in vivo in rats, was resolved into its two isomers 36 and 37. The eutomer 37 displayed 47% oral bioavailability in the rat and was potently active (0.1-0.5 mg/kg, s.c.) in the rat ultrasonic vocalization and social interaction models, predictive of anxiolytic activity. In conclusion, 2-(arylcycloalkylamine) 1-indanols represent a novel class of potent 5-HT(1A) ligands in which the presence of the hydroxyl group in the benzylic position enhances selectivity, while substituents on the phenyl ring of the indanol moiety improve both selectivity and metabolic stability.

Enhanced sensitivity of postsynaptic serotonin-1A receptors in rats and mice with high trait aggression

Individual differences in aggressive behaviour have been linked to variability in central serotonergic activity, both in humans and animals. A previous experiment in mice, selectively bred for high or low levels of aggression, showed an up-regulation of postsynaptic serotonin-1A (5-HT(1A)) receptors, both in receptor binding and in mRNA levels, in the aggressive line [Brain Res 736 (1996) 338]. The aim of this experiment was to study whether similar differences in 5-HT(1A) receptors exist in individuals from a random-bred rat strain, varying in aggressiveness. In addition, because little is known about the functional consequences of these receptor differences, a response mediated via postsynaptic 5-HT(1A) receptors (i.e., hypothermia) was studied both in the selection lines of mice and in the randomly bred rats. The difference in receptor binding, as demonstrated in mice previously, could not be shown in rats. However, both in rats and mice, the hypothermic response to the 5-HT(1A) agonist alnespirone was larger in aggressive individuals. So, in the rat strain as well as in the mouse lines, there is, to a greater or lesser extent, an enhanced sensitivity of postsynaptic 5-HT(1A) receptors in aggressive individuals. This could be a compensatory up-regulation induced by a lower basal 5-HT neurotransmission, which is in agreement with the serotonin deficiency hypothesis of aggression.

Drug action at the 5-HT(1A) receptor in vivo: autoreceptor and postsynaptic receptor occupancy examined with PET and [carbonyl-(11)C]WAY-100635

Serotonin(1A) (5-HT(1A)) receptors have been implicated in the pathophysiology and treatment of anxiety and depression and are a target for novel drug development. In this qualitative study, positron emission tomography (PET) and [carbonyl-(11)C]WAY-100635 were used to assess 5-HT(1A) autoreceptor and postsynaptic receptor occupancy in man in vivo by five different compounds with nanomolar affinity for this site. Occupancy by pindolol, penbutolol, buspirone, EMD 68843, and S 15535 was compared to test-retest data from 10 healthy volunteers. All drugs, apart from buspirone, displayed occupancy at the 5-HT(1A) receptor site. Pindolol demonstrated a preferential occupancy at the autoreceptor compared to the postsynaptic receptor over a plasma range of about 10-20 ng/mL. Differential occupancy may be an important component of novel drug action. The level of autoreceptor or postsynaptic occupancy needed to achieve significant physiological effects is not known, although it is of note that none of the drugs in this study achieved occupancies beyond 60%. Overall this study demonstrates the utility of PET in aiding novel drug development.

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.

S 15535, a benzodioxopiperazine acting as presynaptic agonist and postsynaptic 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

1 The effect of S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine), an agonist at presynaptic and antagonist at postsynaptic 5-HT1A receptors, on the impairment of spatial learning caused by intrahippocampal scopolamine in a two-platform spatial discrimination task was studied. 2 Scopolamine (4.0 microg microl-1), injected bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. 3 Administered subcutaneously 30 min before each training session, S 15535 1.0 (but not 0.3) mg kg-1 did not modify choice accuracy but prevented its impairment by intrahippocampal scopolamine. 4 WAY 100635, a 5-HT1A receptor antagonist, injected into the dorsal raphe at 1.0 microg 0.5 microl-1 5 min before scopolamine, had no effect on choice accuracy and latency or errors of omission and did not modify the effect of scopolamine but completely antagonized the effect of S 15535 (1.0 mg kg-1) on scopolamine-induced impairment of choice accuracy. 5 The results confirm a previous report (Carli et al., 1998) that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe counteracts the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. 6 Drugs that stimulate action on presynaptic 5-HT1A receptors, such as S 15535 and other partial 5-HT1A receptors agonists, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Agonist and antagonist actions of antipsychotic agents at 5-HT1A receptors: a [35S]GTPgammaS binding study

Recombinant human (h) 5-HT1A receptor-mediated G-protein activation was characterised in membranes of transfected Chinese hamster ovary (CHO) cells by use of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS binding). The potency and efficacy of 21 5-HT receptor agonists and antagonists was determined. The agonists, 5-CT (carboxamidotryptamine) and flesinoxan displayed high affinity (subnanomolar Ki values) and high efficacy (Emax > 90%, relative to 5-HT = 100%). In contrast, ipsapirone, zalospirone and buspirone displayed partial agonist activity. EC50s for agonist stimulation of [35S]GTPgammaS binding correlated well with Ki values from competition binding (r = +0.99). Among the compounds tested for antagonist activity, methiothepin and (+)butaclamol exhibited 'inverse agonist' behaviour, inhibiting basal [35S]GTPgammaS binding. The actions of 17 antipsychotic agents were investigated. Clozapine and several putatively 'atypical' antipsychotic agents, including ziprasidone, quetiapine and tiospirone, exhibited partial agonist activity and marked affinity at h5-HT1A receptors, similar to their affinity at hD2 dopamine receptors. In contrast, risperidone and sertindole displayed low affinity at h5-HT1A receptors and behaved as 'neutral' antagonists, inhibiting 5-HT-stimulated [35S]GTPgammaS binding. Likewise the 'typical' neuroleptics, haloperidol, pimozide, raclopride and chlorpromazine exhibited relatively low affinity and 'neutral' antagonist activity at h5-HT1A receptors with Ki values which correlated with their respective Kb values. The present data show that (i) [35S]GTPgammaS binding is an effective method to evaluate the efficacy and potency of agonists and antagonists at recombinant human 5-HT1A receptors. (ii) Like clozapine, several putatively 'atypical' antipsychotic drugs display balanced serotonin h5-HT1A/dopamine hD2 receptor affinity and partial agonist activity at h5-HT1A receptors. (iii) Several 'typical' and some putatively 'atypical' antipsychotic agents displayed antagonist properties at h5-HT1A sites with generally much lower affinity than at hD2 dopamine receptors. It is suggested that agonist activity at 5-HT1A receptors may be of utility for certain antipsychotic agents.

Interaction of the anxiogenic agent, RS-30199, with 5-HT1A receptors: modulation of sexual activity in the male rat

RS-30199 has been shown previously to have atypical interactions at 5-HT1A receptors. RS-30199 and RS-64459, an analogue of buspirone with a buspirone side chain, were compared with the classic, partial agonist at 5-HT1A receptors, 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) and buspirone. At human (h) 5-HT1A receptors in CHO cells, RS-30199-193 (racemate) and its enantiomers (-197, -198) inhibited [3H]-8-OH-DPAT binding (RS-30199-198, ki, 29.7 +/- 11.7 nM; RS-30199-197, ki, 74.1 +/- 11.7 nM) as did RS-64459 (ki, 24.9 +/- 6.0 nM), but RS-30199-197 and -198 were almost full agonists in a [35S]-GTPgammaS binding assay, whereas RS-64459 was a partial agonist, resembling buspirone and 8-OH-DPAT. RS-64459 and the enantiomers of RS-30199 had weaker affinity for 5-HT2C and 5-HT7 receptors. These compounds did not induce the 5-HT behavioural syndrome in male rats. However, in a model where naive male rats were introduced to estrogen-progesterone primed, sexually receptive female rats, RS-30199-197 (0.1, 1, 10 mg/kg, s.c.) had a profound inhibitory effect on sexual behaviour score. Neither buspirone nor 8-OH-DPAT reduced the sexual behaviour score. Unlike 8-OH-DPAT, which shortens intromission latency, RS-30199 prolonged intromission latency. RS-30199 (10 mg/kg s.c.) fully inhibited the facilitation of sexual behaviour caused by the alpha2-adrenoceptor antagonist, delequamine (0.1 mg/kg, p.o.). In contrast, RS-64459 (100, 250, 1000 and 4000 microg/kg, s.c.) failed to modify the sexual behaviour score and did not modify intromission latency. The differences between the effects of RS-30199 and RS-64459 in binding and functional experiments are supported by molecular models of the receptor-ligand interaction, where the compounds interact in different ways with the receptor; a model is proposed for the allosteric interaction of different agents with the receptor, resulting in different functional profiles. RS-30199 can be considered an atypical agonist at 5-HT1A receptors.

The putative 5-HT1A receptor antagonist DU125530 blocks the discriminative stimulus of the 5-HT1A receptor agonist flesinoxan in pigeons

Twelve homing pigeons were trained to discriminate the 5-HT1A receptor agonist flesinoxan (0.25 mg/kg p.o.) from its vehicle in a fixed ratio (FR) 30 two-key operant drug discrimination procedure. Tests for generalization and antagonism showed that compounds with agonistic action at the 5-HT1A receptor, such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), buspirone and ipsapirone all substituted for the flesinoxan cue. Compounds with mixed agonistic action at the 5-HT(1A/1B) receptor fully (eltoprazine) or partially (RU24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl-1H-indole)) substituted for flesinoxan. TFMPP (1-(3-trifluoromethylphenyl)piperazine) and mCPP (1-(3-chlorophenyl)piperazine), both acting at the 5-HT(1B/2C) receptor, did not substitute for flesinoxan, neither did the selective 5-HT re-uptake inhibitor fluvoxamine. The results of the antagonism tests showed that the 5-HT1A receptor antagonists NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine), WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-he xane-carboxamide) and the newly developed DU125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide) fully (more than 80%) blocked the flesinoxan cue without having substantial effects when given alone. WAY100135 (N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide ), (+/-)-pindolol and (S)-UH-301 ((S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin) all partially antagonized the flesinoxan cue. However, both WAY100135 as well as (+/-)-pindolol also partially substituted for flesinoxan in generalization tests. NAN190, (S)-UH-301, WAY100635 and DU125530 were without any activity in the generalization test at the doses tested. The putative 5-HT1A receptor antagonist S15535 (4-benzodioxan-5-yl) 1-(indan-2-yl)piperazine) was identified as a full agonist in the present procedure. Taken together these results suggest that the flesinoxan cue in pigeons is mediated by the 5-HT1A receptor and that DU125530 acts as a full antagonist on the 5-HT1A receptor.

Characterisation of a cloned human 5-HT1A receptor cell line using [35S]GTP gamma S binding

Compound potencies and efficacies depend upon receptor reserve and hence estimating this parameter in assay systems allows for a more meaningful interpretation of the data generated. This study describes a method whereby the degree of receptor reserve, with respect to 5-hydroxytryptamine (5-HT), was determined for a HeLa cell line expressing the human 5-HT1A receptor using the agonist-induced [35S]guanosine 5'[gamma-thio]triphosphate ([35S]GTP gamma S) binding assay, followed by a comparison of the potencies and relative efficacies of several compounds. Following irreversible antagonism with benextramine 5-HT yielded a pKA of 7.3, compared with a pKobs of 8.4 from saturation analysis, indicating the presence of high and low affinity state receptors. A 20% receptor occupancy elicited a half-maximal functional response consistent with the presence of receptor reserve. 5-HT, 5-carboxamidotryptamine (5-CT), 8-hydoxy-dipropylamino-tetralin (8-OH-DPAT), 5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)-1 H-indole (RU24969), buspirone, gepirone, mesulergine and sumatriptan were equally efficacious. 1-(2-Methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine (NAN 190) displayed reduced relative efficacy and methiothepin inverse agonism.

Binding of arylpiperazines, (aryloxy)propanolamines, and tetrahydropyridylindoles to the 5-HT1A receptor: contribution of the molecular lipophilicity potential to three-dimensional quantitative structure-affinity relationship models

A set of 280 5-HT1A receptor ligands were selected from available literature data according to predefined criteria and subjected to three-dimensional quantitative structure-affinity relationship analysis using comparative molecular field analysis. No model was obtained for serotonin analogues (19 compounds) and aminotetralins (60 compounds), despite a variety of alignment hypotheses being tried. In contrast, the steric, electrostatic, and lipophilicity fields alone and in combination yielded informative models for arylpiperazines (101 training compounds and 12 test compounds), (aryloxy)propanolamines (30 training compounds and four test compounds), and tetrahydropyridylindoles (54 training compounds) taken separately (models A, B, and C). Arylpiperazines and (aryloxy)propanolamines were then combined successfully to yield reasonably good models for 131 compounds (model D). In a last step, the three chemical classes (185 compounds) were combined, again successfully (model E). This stepwise procedure not only ascertains self-consistency in alignments but it also allows statistical signals (i.e., favorable or unfavorable regions around molecules) to emerge which cannot exist in a single chemical class. The models so obtained reveal a number of interaction sites between ligands and the 5-HT1A receptor, and extend the information gathered from a model based on homology modeling.

Pharmacological profile of (-)HT-90B, a novel 5-HT1A receptor agonist/5-HT2 receptor antagonist

1. HT-90B ((-)-N-([2-(8-methyl-l, 4-benzodioxane-2-ylmethyl)amino]ethyl) tricyclo[3,3,1,1(3.7)] decane-1-carboxamide) had high affinities for the 5-HT1A (Ki = 0.18 nM) and 5-HT2 (Ki = 9.2 nM) receptors. 2. HT-90B inhibited forskolin activated adenylate cyclase in rat hippocampal membranes as a 5-HT1A full agonist (IC50 = 2 nM), and the potency of the drug was higher than that of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a standard 5-HT1A agonist. 3. In the serotonin syndrome test, HT-90B behaved as a weak partial 5-HT1A agonist in reserpinized rats. 4. 5-HT2 receptor-mediated potentiation of rabbit platelet aggregation by serotonin (5-HT) was reduced by HT-90B (IC50 = 1.73 microM). 5. Head twitch response induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, was inhibited by HT-90B in mice. 6. It is concluded that HT-90B has potent 5-HT1A receptor agonist as well as 5-HT2 receptor antagonist properties in vitro and in vivo.

Neuroendocrine profile of the potential anxiolytic drug S-20499

The neuroendocrine profile of the serotonin 5-HT1A receptor agonist and potential anxiolytic drug (+)-4[N-(5-methoxy-chroman-3-yl)N-propylamino]butyl-8-azaspiro-(4, 5)-decane - 7,9-dione (S-20499) was examined in conscious male rats. S-20499 (0.01-20 mg/kg i.p.) dose-dependently elevated plasma adrenocorticotropin (ACTH) and corticosterone concentrations, with maximal effects observed at 15-30 and 30-60 min respectively. S-20499 also reduced plasma prolactin concentration, and did not alter plasma renin activity. S-20499 (1 mg/kg i.p.) also reduced blood pressure and heart rate within 10 min, suggesting reduced sympathetic output. Pretreatment with the 5-HT1A receptor antagonists (-)-pindolol (0.3 mg/kg i.p.) or spiperone (0.01 or 3 mg/kg s.c.) significantly attenuated the stimulatory effects of S-20499 on plasma ACTH and/or corticosterone concentrations. The data suggest that S-20499 stimulates the hypothalamic-pituitary adrenal axis by activating 5-HT1A receptors, although activation of dopamine D2 receptors may contribute to these responses. Like other 5-HT1A receptor agonists, S-20499 does not increase renin secretion. Additionally, it reduces prolactin secretion, presumably by acting as a weak dopamine D2 receptor agonist in the pituitary.

Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

The selective serotonin (5-HT) agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5-HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8-OH-DPAT (free ligand from 200 pM to 160 nM) revealed high-affinity 5-HT1A receptors (KH = 0.7-0.8 nM) and low-affinity (KL = 22-36 nM) binding sites. The kinetics of [3H]8-OH-DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high- and low-affinity binding sites. When the high-affinity sites were labeled with a 1 nM concentration of [3H]8-OH-DPAT, the competition curves of agonist and antagonist drugs were best fit to a two-site model, indicating the presence of two different 5-HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5-HT1AHIGH and 5-HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5-HT1A sites are modulated by 5'-guanylylimidodiphosphate, inhibition experiments with 5-HT were performed in the presence or in the absence of 100 microM 5'-guanylylimidodiphosphate. The binding of 1 nM [3H]8-OH-DPAT to the 5-HT1AHIGH site was dramatically (80%) reduced by 5'-guanylylimidodiphosphate; in contrast, the low-affinity site, or 5-HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high-affinity 5-HT1AHIGH site corresponds to the classic 5-HT1A receptor, whereas the novel 5-HT1ALOW binding site, labeled by 1 nM [3H]8-OH-DPAT and having a micromolar affinity for 5-HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5-HT1A sites and the 5-HT innervation, rats were treated with p-chlorophenylalanine or with the neurotoxin p-chloroamphetamine. The inhibition of 5-HT synthesis by p-chlorophenylalanine did not alter either of the two 5-HT1A sites, but deafferentation by p-chloroamphetamine caused a loss of the low-affinity [3H]8-OH-DPAT binding sites, indicating that these novel binding sites may be located presynaptically on 5-HT fibers and/or nerve terminals.

WAY 100,135 and (-)-tertatolol act as antagonists at both 5-HT1A autoreceptors and postsynaptic 5-HT1A receptors in vivo

In binding studies, WAY 100,135 (N-tertiobutyl-3-[4-(2-methoxyphenyl)-piperazinyl]-2-phenylpropana mide) and (-)-tertatolol showed affinities (Ki) of 29 nM and 10 nM, respectively, at 5-HT1A receptors. In vivo, they both dose dependently blocked the flat-body posture and corticosterone secretion provoked by an action of the 5-HT1A receptor agonist, S 14671 (1-[2-(2-thenoyl-amino)ethyl]-4-[1-(7- methoxynaphtyl)]piperazine), at postsynaptic 5-HT1A receptors. Alone, they exerted little effect. The firing rate of dorsal raphe neurones, which bear inhibitory 5-HT1A autoreceptors, was reduced by S 14671 whereas it was not affected by WAY 100,135 and was increased by (-)-tertatolol. Both WAY 100,135 and (-)-tertatolol blocked the ability of S 14671 to inhibit raphe firing. In conclusion, these data demonstrate that WAY 100,135 and (-)-tertatolol behave as antagonists at both 5-HT1A autoreceptors and postsynaptic 5-HT1A receptors in vivo.