8-OH-DPAT acts on both 5-HT1A and 5-HT7 receptors to induce hypothermia in rodents

Synthesis, computational simulations and biological evaluation of new dual 5HT1A/5HT7 receptor ligands based on purine-2,6-dione scaffold

The new dual 5HT1A/5HT7 receptor ligands were designed based on the purine-2,6-dione scaffold with the fluorine atom. Twenty-one new derivatives were synthesized, and their structure-activity relationship was summarized. Compound 11 (7-(2-(3-fluorophenyl)-2-oxoethyl)-8-((4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)amino)-1,3-dimethyl-3,7-dihydro-1H-purine-2,6-dione) showed the highest affinity to 5HT1AR and 5HT7R, and was the most potent antagonist of 5-HT1AR (Kb = 0.26 ± 0.1 nM) which activity can be to reference compound NAN-190 (Kb = 0.26 ± 0.1 nM). The experimentally established physicochemical parameters of compound 11 showed that compound, as slightly ionized in the blood, could penetrate the blood-brain barrier. A molecular docking study showed that the fluorine substitution introduces additional stabilization effects on binding to 5HT1A/5HT7Rs. In animal assays of depression and anxiety, compound 11 revealed activity in terms of dosage compared to marketed psychotropics such as fluoxetine, citalopram, and sertraline.

Pharmacologic Activity of Substituted Tryptamines at 5-Hydroxytryptamine (5-HT)2A Receptor (5-HT2AR), 5-HT2CR, 5-HT1AR, and Serotonin Transporter

Novel psychoactive substances, including synthetic substituted tryptamines, represent a potential public health threat. Additionally, some substituted tryptamines are being studied under medical guidance as potential treatments of psychiatric disorders. Characterizing the basic pharmacology of substituted tryptamines will aid in understanding differences in potential for harm or therapeutic use. Using human embryonic kidney cells stably expressing 5-hydroxytryptamine (5-HT)_1A, 5-HT_2A, and 5-HT_2C receptors (5-HT_1AR, 5-HT_2AR, and 5HT_2CR, respectively) or the serotonin transporter (SERT), we measured affinities, potencies and efficacies of 21 substituted tryptamines. With the exception of two 4-acetoxy compounds, substituted tryptamines exhibited affinities and potencies less than one micromolar at the 5-HT_2AR, the primary target for psychedelic effects. In comparison, half or more exhibited low affinities/potencies at 5-HT_2CR, 5-HT_1AR, and SERT. Sorting by the ratio of 5-HT_2A to 5-HT_2C, 5-HT_1A, or SERT affinity revealed chemical determinants of selectivity. We found that although 4-substituted compounds exhibited affinities that ranged across a factor of 100, they largely exhibited high selectivity for 5-HT_2ARs versus 5-HT_1ARs and 5-HT_2CRs. 5-substituted compounds exhibited high affinities for 5-HT_1ARs, low affinities for 5-HT_2CRs, and a range of affinities for 5-HT_2ARs, resulting in selectivity for 5-HT_2ARs versus 5-HT_2CRs but not versus 5-HT_1ARs. Additionally, a number of psychedelics bound to SERT, with non-ring-substituted tryptamines most consistently exhibiting binding. Interestingly, substituted tryptamines and known psychedelic standards exhibited a broad range of efficacies, which were lower as a class at 5-HT_2ARs compared with 5-HT_2CRs and 5-HT_1ARs. Conversely, coupling efficiency/amplification ratio was highest at 5-HT_2ARs in comparison with 5-HT_2CRs and 5-HT_1ARs. SIGNIFICANCE STATEMENT: Synthetic substituted tryptamines represent both potential public health threats and potential treatments of psychiatric disorders. The substituted tryptamines tested differed in affinities, potencies, and efficacies at 5-hydroxytryptamine (5-HT)_2A, 5-HT_2C, and 5HT_1A receptors and the serotonin transporter (SERT). Several compounds were highly selective for and coupled very efficiently downstream of 5-HT_2A versus 5-HT_1A and 5-HT_2C receptors, and some bound SERT. This basic pharmacology of substituted tryptamines helps us understand the pharmacologic basis of their potential for harm and as therapeutic agents.

Understanding and countering opioid-induced respiratory depression

Respiratory depression is the proximal cause of death in opioid overdose, yet the mechanisms underlying this potentially fatal outcome are not well understood. The goal of this review is to provide a comprehensive understanding of the pharmacological mechanisms of opioid-induced respiratory depression, which could lead to improved therapeutic options to counter opioid overdose, as well as other detrimental effects of opioids on breathing. The development of tolerance in the respiratory system is also discussed, as are differences in the degree of respiratory depression caused by various opioid agonists. Finally, potential future therapeutic agents aimed at reversing or avoiding opioid-induced respiratory depression through non-opioid receptor targets are in development and could provide certain advantages over naloxone. By providing an overview of mechanisms and effects of opioids in the respiratory network, this review will benefit future research on countering opioid-induced respiratory depression. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Lopez S, Amodeo DA. Impact of specific serotonin receptor modulation on restricted repetitive behaviors

Restricted, repetitive behaviors (RRBs) are commonly divided into two behavioral categories, lower-order and higher-order RRBs. Individuals displaying lower-order motoric RRBs may express repetitive hand flapping behaviors, body rocking back and forth movements, and continuous body spinning. Higher-order RRBs most commonly cover the behavior inflexibility and cognitive rigidity commonly found in disorders such as autism spectrum disorder and obsessive-compulsive disorder. Various neuropsychiatric disorders are plagued by RRBs yet no FDA-approved treatments have been identified. In rodents, lower-order RRBs are commonly measured through various tasks, such as repetitive self-grooming, marble burying, and stereotypic motor behaviors. This review focuses on the effects that modulation of specific serotonin receptors have on lower-order RRBs. Although there is research examining how changes in 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT6, and 5-HT7 receptor modulation, more research has focused on the 5-HT1A, 5-HT2A, and 5-HT2C receptors. The accumulating data suggest that increasing 5-HT1A activation decreases RRBs while blocking 5-HT1A activation has no effect on RRBs. While there are mixed findings regarding the impact of 5-HT2A modulation on RRBs, the general trend shows mixed effects of 5-HT2A receptor activation RRB expression, whereas blockade generally decreases RRBs. 5-HT2C receptor activation can modulate RRBs in either direction depending on the 5-HT2C drug used, blocking 5-HT2C activation only seems to show therapeutic properties when 5-HT2C activation is already elevated. The other 5-HT receptors have been explored far less but show promise as potential targets for regulating RRBs. Although it is less clear due to the involvement of 5-HT1D, 5-HT1A activation increases RRBs, and blocking 5-HT1A tends to decrease RRBs. 5-HT2B activation could reduce RRBs, while inhibiting 5-HT2B does not impact RRBs. Increasing 5-HT3 has not been shown to affect RRBs. Yet, increases in RRBs have been observed in Htr3a KO mice. 5-HT6 receptor activation can increase RRBs, while blocking 5-HT6 activity tends to decrease RRBs. Lastly, neither increasing or blocking 5-HT7 activity can reduce RRBs. In sum, there is no uniform pattern in whether all specific 5-HT receptors affect RRBs in either direction, instead, there is evidence suggesting that different 5-HT receptors can modulate RRBs in different directions. Further researching the less explored receptors and aiming to understand why these receptors can differently modulate RRBs, may play a key role in developing therapeutics that treat RRBs.

Deletion of the Serotonin Receptor 7 Gene Changed the Development and Behavior of the Mosquito, Aedes aegypti

Serotonin (5-HT) plays a vital role in many physiological processes in insects, regulating physiological activities such as growth and movement through multiple 5-HT receptors (5-HTRs), which were potential targets for some new insecticides. However, the specific function of individual 5-HTRs in Ae. aegypti is still unclear. In this study, we investigated the function of the 5-HT7A receptor during Ae. aegypti development. 5-HTR7A transcripts were detected at all stages of development by real-time PCR. The results indicated that the gene expression was highest in the limbs (p < 0.01). We also generated 5-HTR7A mutant mosquitoes using CRISPR-mediated gene editing. The mutants had an abnormal phenotype at the larval stage, including an aberrant head-to-chest ratio and decreased motor activity. The mutant pupae developed abnormally, and most died (56.67%) (p < 0.0001). Using external stimuli to larvae and pupae with abnormal phenotypes, we found the mutant G1 and G2 generations responded to external stimuli in a longer time than the wild-type (WT) mosquitoes, and most of the mutants were 2 to 3 s slower than the WTs to respond to external stimuli (p < 0.01). Due to higher mortality, mutant larvae and pupae had fewer numbers than the WTs. The egg hatching rate of mutant G1 and G2 generations was lower than that of the WTs (p < 0.01). The expression level of 5-HTR7A in the mutants decreased by about 65% compared with the control group using real-time PCR (p < 0.05). In all, the 5-HT7A receptor plays an important role in the metamorphosis, development and motor function of Aedes aegypti.

Effects of the serotonin 5-HT1B receptor agonist CP 94253 on the locomotor activity and body temperature of preweanling and adult male and female rats

Serotonin 5-HT_1A receptor agonists increase locomotor activity of both preweanling and adult rodents. The part played by the 5-HT_1B receptor in locomotion is less certain, with preliminary evidence suggesting that the actions of 5-HT_1B receptor agonists are not uniform across ontogeny. To more fully examine the role of 5-HT_1B receptors, locomotor activity and axillary temperatures of preweanling and adult male and female rats was assessed. In the first experiment, adult (PD 70) and preweanling (PD 10 and PD 15) male and female rats were injected with the 5-HT_1B agonist CP 94253 (2.5-10 mg/kg) immediately before locomotor activity testing and 60 min before axillary temperatures were recorded. In the second experiment, specificity of drug action was determined in PD 10 rats by administering saline, WAY 100635 (a 5-HT_1A antagonist), or GR 127935 (a 5-HT_1B antagonist) 30 min before CP 94253 (10 mg/kg) treatment. CP 94253 significantly increased the locomotor activity of preweanling rats on PD 10, an effect that was fully attenuated by GR 127935. Conversely, CP 94253 significantly decreased the locomotor activity of male and female adult rats, while CP 94253 did not affect the locomotor activity of PD 15 rats. Regardless of age, CP 94253 (2.5-10 mg/kg) significantly reduced the axillary temperatures of preweanling and adult rats. When considered together, these results show that 5-HT_1B receptor stimulation activates motor circuits in PD 10 rats; whereas, 5-HT_1B receptor agonism reduces the overall locomotor activity of adult rats, perhaps by blunting exploratory tendencies.

Repeated lysergic acid diethylamide (LSD) reverses stress-induced anxiety-like behavior, cortical synaptogenesis deficits and serotonergic neurotransmission decline

Lysergic acid diethylamide (LSD) is a serotonergic psychedelic compound receiving increasing interest due to putative anxiolytic and antidepressant properties. However, the potential neurobiological mechanisms mediating these effects remain elusive. Employing in vivo electrophysiology, microionthophoresis, behavioral paradigms and morphology assays, we assessed the impact of acute and chronic LSD administration on anxiety-like behavior, on the cortical dendritic spines and on the activity of serotonin (5-HT) neurons originating in the dorsal raphe nucleus (DRN) in male mice exposed to chronic restraint stress. We found that while the acute intraperitoneal (i.p.) administration of LSD (5, 15 and 30 and 60 μg/kg) did not produce any anxiolytic or antidepressant effects in non-stressed mice, the dose of 30 µg/kg (daily for 7 days) prevented the stress-induced anxiety-like behavior and the stress-induced decrease of cortical spine densitiy. Interestingly, while LSD acutely decreased the firing activity of 5-HT neurons, repeated LSD increased their basal firing rate and restored the low 5-HT firing induced by stress. This effect was accompanied by a decreased inhibitory response of 5-HT neurons to microiontophoretic applications of the 5-HT_1A agonist 8-OH-DPAT (8-hydroxy-N,N-dipropyl-2-aminotetralin). In conclusion, repeated LSD prevents the exacerbation of anxiety-like behavior following chronic stress exposure, but has no behavioral effects in non-stressed mice. These effects are paralleled by increased cortical spinogenesis and an enhancement of 5-HT neurotransmission which might be due to 5-HT_1A receptors desensitization. Increased cortical spine density and enhancement of serotonergic neurotransmission may thus represent a candidate mechanism which mediate the therapeutic effects of serotonergic psychedelics on stress-induced anxiety.

Hyperthermia and Serotonin: The Quest for a “Better Cyproheptadine”

Fine temperature control is essential in homeothermic animals. Both hyper- and hypothermia can have deleterious effects. Multiple, efficient and partly redundant mechanisms of adjusting the body temperature to the value set by the internal thermostat exist. The neural circuitry of temperature control and the neurotransmitters involved are reviewed. The GABAergic inhibitory output from the brain thermostat in the preoptic area POA to subaltern neural circuitry of temperature control (Nucleus Raphe Dorsalis and Nucleus Raphe Pallidus) is a function of the balance between the (opposite) effects mediated by the transient receptor potential receptor TRPM2 and EP3 prostaglandin receptors. Activation of TRPM2-expressing neurons in POA favors hypothermia, while inhibition has the opposite effect. Conversely, EP3 receptors induce elevation in body temperature. Activation of EP3-expressing neurons in POA results in hyperthermia, while inhibition has the opposite effect. Agonists at TRPM2 and/or antagonists at EP3 could be beneficial in hyperthermia control. Activity of the neural circuitry of temperature control is modulated by a variety of 5-HT receptors. Based on the theoretical model presented the “ideal” antidote against serotonin syndrome hyperthermia appears to be an antagonist at the 5-HT receptor subtypes 2, 4 and 6 and an agonist at the receptor subtypes 1, 3 and 7. Very broadly speaking, such a profile translates in a sympatholytic effect. While a compound with such an ideal profile is presently not available, better matches than the conventional antidote cyproheptadine (used off-label in severe serotonin syndrome cases) appear to be possible and need to be identified.

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Systemic 8-OH-DPAT challenge causes hyperventilation largely via activating pre-botzinger complex 5-HT1A receptors

Systemic 8-OH-DPAT (a 5-HT_1A receptor agonist) challenge evokes hyperventilation independent of peripheral 5-HT_1A receptors. Though the pre-Botzinger Complex (PBC) is critical in generating respiratory rhythm and activation of local 5-HT_1A receptors induces tachypnea via disinhibition of local GABA_A neurons, its role in the respiratory response to systemic 8-OH-DPAT challenge is still unclear. In anesthetized rats, 8-OH-DPAT (100 μg/kg, iv) was injected twice to confirm the reproducibility of the evoked responses. The same challenges were performed after bilateral microinjections of (S)-WAY-100135 (a 5-HT_1A receptor antagonist) or gabazine (a GABA_A receptor antagonist) into the PBC. Our results showed that: 1) 8-OH-DPAT caused reproducible hyperventilation associated with hypotension and bradycardia; 2) microinjections of (S)-WAY-100135 into the PBC attenuated the hyperventilation by ˜60 % without effect on the evoked hypotension and bradycardia; and 3) the same hyperventilatory attenuation was also observed after microinjections of gabazine into the PBC. Our data suggest that PBC 5-HT_1A receptors play a key role in the respiratory response to systemic 8-OH-DPAT challenge likely via disinhibiting local GABAergic neurons.

Antidepressant and anxiolytic compounds isolated from Salvia elegans interact with serotonergic drugs

Salvia elegans belongs to a genus plants with biological activities in central nervous system. In this work, the purpose was to evaluate the anxiolytic and antidepressant effects of fractions and compounds isolated from S. elegans and its interaction with serotoninergic drugs by using behavioral tests in mice. Fractions from aerial parts of S. elegans were obtained by column chromatography, SeF1, SeF2, SeF3, and SeF4. Each of them was administered to 25 mg/k in ICR mice subject to forced swimming test (FST), or elevated plus maze test (EPM), or open field test (OFT). The most active fractions were chemically separated until compounds, which were analyzed as anxiolytic or antidepressant and the coadministration of these treatments with 5-HT_1A and 5-HT₂ drugs was measured in the different biological tests. All fractions were anxiolytic and antidepressant, oleanolic acid (OA) was found in SeF2, and from SeF3, a mixture of terpenes was found; a GC-MS analysis confirmed the presence of two main compounds: rosifoliol and agaraspirol (TM, mixture of terpenes). TM (doses-response curve, 0.01, 0.1, 0.5, 1.0, and 2.0 mg/kg) and OA (5 mg/kg) were also evaluated demonstrating an antidepressant and anxiolytic effect, respectively. The combination of TM (0.5 mg/kg) with 8-OH (selective 5-HT_1A receptor agonist) induced an increment of antidepressant activity, while with the antagonist WAY-100635, the effect diminished. But with DOI (5-HT₁c/5-HT₂ receptor agonist), there was no change, and with KET (5-HT₂ receptor antagonist), the activity was increased. When OA is co-administered with 8-OH or with DOI, the anxiolytic activity of this terpene, diminished; but with the combination with antagonists, the effect of OA shows no change. TM and OA were antidepressant and anxiolytic, respectively, on mice exposed to different tests, and these are able to interact with serotoninergic drugs.

Social interaction masking contributes to changes in the activity of the suprachiasmatic nucleus and impacts on circadian rhythms

Light is the most powerful temporal cue that entrains physiology and behavior through modulation of the suprachiasmatic nucleus (SCN) of the hypothalamus. However, on a daily basis, individuals face a combination of light and several non-photic cues, such as social interaction. In order to investigate whether SCN activity and SCN-driven rhythms are altered by social interaction, adult male C57BLJ/6 mice were maintained in groups of 3-4 animals per cage or 1 animal per cage (socially isolated) under 12:12 h / light:dark (LD) cycles or constant darkness (DD). Analysis of the two anatomical subdivisions (ventral, v and dorsal, d) of the medial SCN revealed an effect of housing conditions on the d-SCN but not on the v-SCN on the number of c-Fos immunoreactive (ir) neurons. As such, 2 h after the light-phase onset d-SCN c-Fos-ir number was lower in single-housed mice under LD. Importantly, under DD there were no effect of housing conditions in the number of c-Fos-ir SCN neurons. Social isolation increased the amplitude and strength of SCN-driven rhythm of body temperature (Tc) entrained to LD and it advanced its onset, uncoupling with spontaneous locomotor activity (SLA) rhythm, without altering endogenous Tc and SLA rhythms expressed under DD. Associated with reduced Tc in the light phase, single-housed mice showed reduced body weight. However, these phenotypes were not accompanied by changes in the number of c-Fos-ir neurons in the preoptic area (POA), which are known to regulate energy metabolism and Tc. Altogether, these results imply that the social interaction masking effect on the d-SCN is added to that of light stimulus, in order to achieve full c-Fos expression in the SCN, which, in turn seems to be required to maintain daily-phase coherence between the photo-entrained rhythms of Tc and SLA. There might be an inter-relationship between masking (social interaction) and entrainment stimulus (light) that impacts the circadian parameters of the photo-entrained Tc rhythm. As such, in the absence of social interactions a more robust Tc rhythm is shown. This inter-relationship seems to occur in the dorsal subdivision of the SCN but not in the POA.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

Eupnea and gasping in vivo are facilitated by the activation of 5-HT2A receptors

Eupnea and gasping in infancy depend on central nervous system (CNS) serotonin (5-hydroxytryptamine; 5-HT). Although previous in vitro preparations have provided some evidence that 5-HT acts through type 2 A receptors (5-HT_2A) to facilitate eupnea and gasping, here the hypothesis addressed is that 5-HT_2A receptor activation is necessary for eupnea and the proper generation of gasping in vivo. To test this, we administered 2,5-dimethoxy-4-iodoamphetamine (DOI; 0.25 mg/kg i.p.), a 5-HT_2A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.25 mg/kg i.p.), a 5-HT_1A agonist, or vehicle (saline) to 7-9-day-old tryptophan hydroxylase 2 knockout (TPH2^-/-) mice. A second experiment assessed the effect of MDL-11,939 (MDL; 10 mg/kg i.p.), the specific 5-HT_2A antagonist, or vehicle (DMSO) on the gasping of wild-type (TPH2^+/+) animals. Drugs were given 15 min prior to five episodes of severe hypoxia that elicited gasping. TPH2^-/- breathed more slowly but had the same V̇e and V̇e/V̇o₂ compared with TPH2^+/+. As previously reported, the gasping of TPH2^-/- was significantly delayed (P < 0.001) and occurred at a significantly lower frequency compared with TPH2^+/+ (P = 0.04). For both genotypes, DOI hastened eupneic frequency but had no effect on V̇e or V̇e/V̇o₂. The gasping of TPH2^-/-, although unaffected by 8-OH-DPAT, was indistinguishable from the gasping of TPH2^+/+ following DOI. In TPH2^+/+, application of MDL led to hypoventilation (P = 0.01), a delay in the appearance of gasping (P = 0.005), and reduced gasp frequency (P = 0.05). These data show that, in vivo, 5-HT_2A receptors facilitate both eupnea and gasping. As has been shown in vitro, 5-HT_2A probably promotes gasping by exciting hypoxia-resistant pacemaker neurons.NEW & NOTEWORTHY Previous in vitro studies suggest that 5-HT_2A receptors contribute to eupnea and are necessary for fictive gasping. The current study shows that the impaired gasping displayed by neonatal TPH2^-/- mice, deficient in CNS serotonin, is restored by 5-HT_2A receptor activation. Following 5-HT_2A blockade, wild-type mice hypoventilated and their gasping resembled that of TPH2^-/- mice. This study shows that both eupnea and gasping in vivo rely on the activation of 5-HT_2A receptors.

5-HT7 receptors as a new target for prostate cancer physiopathology and treatment: an experimental study on PC-3 cells and FFPE tissues

Prostate cancer (PCa) is one of the most common types of cancer seen among men worldwide. Previous studies have demonstrated that serotonin regulates cell proliferation, migration, and invasion in vitro; the presence of 5-HT receptors in cancer cells; and the role of serotonin in tumor development. The most recently discovered of these receptors is 5-HT7 but also least characterized receptors of serotonin. The aim of this study is to investigate the existence and possible role of 5-HT7 receptors in healthy and cancerous prostate tissues and also investigate effects of receptor agonists and antagonists on PC-3 cells to evaluate potential therapeutic effects. PC-3 cells were cultured and effects of 5-HT7 receptor agonist (LP-44) and antagonist (SB-269970) were evaluated on these cells. After proliferation analyses, relative expression of apoptotic markers and 5-HT7 receptor mRNA expression levels were determined through real-time PCR. Annexin V-FITC/PI double staining and Hoechst 33258 staining assay methods were applied to determine apoptosis. Additional PCR studies were performed on healthy and cancerous prostate tissue to see existence of receptors in human samples. The viability of PC-3 cells was decreased by SB-269970 after 48 and 72 h of incubation. However, LP-44 increased PC-3 cell proliferation at all time points. In 10^-6 M SB-269970 treated PC-3 cells, there was significant increase in the expression of CAS-3 (4-fold), CAS-9 (2.5-fold), BAX (1.9-fold), and Tp-53 (4.8-fold) gene mRNA levels when compared to non-treated control group. Conversely, there was a significant decrease in NF-κB (2.9-fold) and 5-HT7 receptor (3.6-fold) mRNA expression in cells treated with SB-269970 when compared to control. SB-269970 that antagonized 5-HT7 receptors also induced apoptosis in Annexin V-FITC/PI double staining assay and Hoechst 33258 staining assays when compared with other groups. In human samples, 5-HT7 receptor mRNA expression was approximately 200-fold higher than that of heathy ones. In this study, for the first time, the 5-HT7 receptor antagonist SB-269970 has been shown to inhibit proliferation in PC-3 cells and to be associated with an apoptosis-inducing effect. These results suggest blocking 5-HT7 receptors can be a novel therapeutic target for the treatment of prostate cancer.

Towards in vivo imaging of functionally active 5-HT1A receptors in schizophrenia: concepts and challenges

The serotonin 5-HT_1A receptor has attracted wide attention as a target for treatment of psychiatric disorders. Although this receptor is important in the pharmacological mechanisms of action of new-generation antipsychotics, its characterization remains incomplete. Studies based on in vitro molecular imaging on brain tissue by autoradiography, and more recently in vivo PET imaging, have not yielded clear results, in particular due to the limitations of current 5-HT_1A radiotracers, which lack specificity and/or bind to all 5-HT_1A receptors, regardless of their functional status. The new concept of PET neuroimaging of functionally active G-protein-coupled receptors makes it possible to revisit PET brain exploration by enabling new research paradigms. For the 5-HT_1A receptor it is now possible to use [¹⁸F]-F13640, a 5-HT_1A receptor radioligand with high efficacy agonist properties, to specifically visualize and quantify functionally active receptors, and to relate this information to subjects' pathophysiological or pharmacological state. We therefore propose imaging protocols to follow changes in the pattern of functional 5-HT_1A receptors in relation to mood deficits or cognitive processes. This could allow improved discrimination of different schizophrenia phenotypes and greater understanding of the basis of therapeutic responses to antipsychotic drugs. Finally, as well as targeting functionally active receptors to gain insights into the role of 5-HT_1A receptors, the concept can also be extended to the study of other receptors involved in the pathophysiology or therapy of psychiatric disorders.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Attenuation of stress-induced behavioral changes by activation of serotonin type 7 receptors in the median raphe nucleus of rats

BACKGROUND

Exposure to stressful aversive situations induces physiological and behavioral changes. Serotonin has been suggested to mediate such changes, as well as adaptation to stressful events. Serotoninergic projections arising from the median raphe nucleus to the dorsal hippocampus have been suggested to promote adaptation to chronic aversive stimuli. Such pathway may involve serotonin type 1a receptor-mediated neurotransmission. However, the serotonin 7 receptor can also be found in the median raphe nucleus and may be involved in mechanisms underlying response to stress.

AIMS

In this work we sought to investigate if activation of serotonin type 7 receptors would attenuate stress-induced deficits in different animal models of depression.

METHODS

Male Wistar rats with a guide-cannula aimed to the median raphe nucleus were submitted to restraint or forced swim stress and were tested in an elevated plus maze or forced swim test, respectively, 24 h later. SB 258741 (serotonin type 7 receptor antagonist) and/or LP 44 (serotonin type 7 receptor agonist) were administered intra-median raphe nucleus immediately before or after exposure to stress or before test. Control groups received intra-median raphe nucleus treatment 24 h or immediately before test in the elevated plus maze or forced swim test.

RESULTS

LP 44 attenuated restraint-induced exploratory deficits independently of the moment it was administered. Similar results were observed in the forced swim test, with the exception on post-stress condition. These effects on adaptation to stress induced by serotonin type 7 receptor activation were prevented by previous treatment with SB 258741.

CONCLUSIONS

Our data support the idea that activation of median raphe nucleus serotonin 7 receptor is important to the development of adaptation to stress.

Divergent pathways mediate 5-HT1A receptor agonist effects on close social interaction, grooming and aggressive behaviour in mice: Exploring the involvement of the oxytocin and vasopressin systems

BACKGROUND

5-HT_1A receptor (5-HT_1AR) abnormalities are implicated in aggression, and there has been considerable interest in developing 5-HT_1AR agonists for treating aggression. Endogenous oxytocin (OXT) released upon stimulation of 5-HT_1ARs in the hypothalamus mediates at least some of the effects of 5-HT_1AR agonists on social behaviour.

AIMS

Given 5-HT_1AR, OXT receptor (OXTR) and vasopressin V1a receptor (V1aR) agonists can all reduce aggression, the current study aimed to determine whether the anti-aggressive effects of 5-HT_1AR stimulation can also be explained by downstream actions at OXTRs and/or V1aRs in a mouse model of non-territorial, hyper-aggressive behaviour.

METHODS

Male Swiss mice (N=80) were socially isolated or group housed for six weeks prior to the start of testing. Testing involved placing two unfamiliar weight- and condition-matched mice together in a neutral context for 10 minutes.

RESULTS

Social isolation led to a pronounced increase in aggressive behaviour, which was dose-dependently inhibited by the 5-HT_1AR agonist 8-OH-DPAT (0.1, 0.3 and 1 mg/kg intraperitoneally (i.p.)), with accompanying increases in close social contact (huddling) and grooming. The effects of 8-OH-DPAT on aggression, huddling and grooming were blocked by pretreatment with a selective 5-HT_1AR antagonist (WAY-100635; 0.1 mg/kg i.p.). The anti-aggressive effects of 8-OH-DPAT were unaffected by an OXTR antagonist (L-368,899; 10 mg/kg i.p.), whereas the effects on huddling and grooming were inhibited. Pretreatment with a V1aR antagonist (SR49059; 20 mg/kg i.p.) had no effect.

CONCLUSIONS

Our study suggests that stimulation of endogenous oxytocin is involved in the effects of 5-HT_1AR activation on close social contact and grooming but not aggression.

5-HT7 receptorsare over-expressed in patients with nasal polyps

Nasal polyposis (NP) is an inflammatory disease of the paranasal sinuses and nasal cavity. The primary purpose of our study is to determine the expression of 5-HT7 receptors both in nasal polyps and in healthy tissue in the nasal cavity. The subsequent aim is to compare the expression of 5-HT7 receptors in patients with NP and in inferior turbinate tissue (control).The study included 60 participants (40 with NP and 20 controls) aged 35 to 62 years. Nasal polyp samples were collected from all patients and relative 5-HT7 receptor expression analyses were performed. RT-PCR analysis of nasal polyps and control tissue identified 5-HT7 receptor expression in the nasal cavities of controls. This expression was approximately 67 times higher in nasal polyp tissue than in healthy tissue. Our study identifies the expression of 5-HT7 receptors in the nasal cavity for the first time. It is also the first demonstration of increased 5-HT7 receptor expression in tissue from nasal polyps, which occur in the paranasal sinuses and nasal cavity.

Modulatory Effect of Serotonergic System in Pentylenetetrazole-Induced Seizures and Associated Memory Deficit: Role of 5-HT1A and 5-HT2A/2C

Background and Purpose

Recent studies have recognised the memory deficit as one of the most common psychiatric issues in the patients with epilepsy, which severely affects the quality of life. Our previous studies have demonstrated the possible involvement of serotonergic system in the pathogenesis of epilepsy and associated memory deficit. The possible involvement of 5-HT_1A and 5-HT_2A/2C receptor has not been explored yet. Therefore, this study has been envisaged to explore the effect of 5-HT_1A and 5-HT_2A/2C receptor modulation on epilepsy and memory deficit in pentylenetetrazole-kindled mice.

Methods

In the present experimental approach, we examined the efficacy of modulation of 5-HT_1A and 5-HT_2A/2C receptor in pentylenetetrazole-induced kindling in male Swiss mice (n=75). Mice were kindled by sub-convulsive dose of pentylenetetrazole (35 mg/kg, intraperitoneal injection), at the interval of 48±2 hours). Successfully kindled animals were treated with 5-HT_1A and 5-HT_2A/2C receptor modulators. The effect of different treatments on seizure severity score and memory impairment was analysed.

Results

5-HT_1A receptor agonist improved the memory functions while seizure severity was not improved, and the opposite effect was observed with 5-HT_1A receptor antagonist. On the other hand, 5-HT_2A/2C receptor agonist significantly improved memory deficit as well as seizure severity in the kindled animals.

Conclusions

The outcome of the study indicates the possible involvement of 5-HT_2A/2C receptor in the pathogenesis of epilepsy and associated memory deficit, which can be further explored for its management.

Alternative splicing in serotonergic system: Implications in neuropsychiatric disorders

BACKGROUND

The serotonergic system is a key component of physiological brain function and is essential for proper neurological activity. Numerous neuropsychiatric disorders are associated with deregulation of the serotonergic system. Accordingly, many pharmacological treatments are focused on modulation of this system. While providing a promising line of therapeutic moderation, these approaches may be complicated due to the presence of alternative splicing events for key genes in this pathway. Alternative splicing is a co-transcriptional process by which different mRNA transcripts can be produced from the same gene. These different isoforms may have diverse activities and functions, and their relative balance is often critical for the maintenance of homeostasis. Alternative splicing greatly increases the production of proteins, augmenting cell plasticity, and provides an important control point for regulation of gene expression.

AIM

The objective of this narrative review is to discuss the potential impact of alternative splicing of different components of the serotonergic system and speculate on their involvement in several neuropsychiatric disorders.

CONCLUSIONS

The specific role of each isoform in disease and their relative activities in the signalling pathways involved are yet to be determined. We need to gain a better understanding of the basis of alternative isoforms of the serotonergic system in order to fully understand their impact and be able to develop new effective pharmacological isoform-specific targets.

Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Since its discovery in 1993, the serotonin receptor subtype 7 (5-HT₇) has attracted significant attention as a potential drug target; due to its elucidated roles in conditions such as insomnia, schizophrenia, and more. Therefore, it is unsurprising that there has been relatively early efforts undertaken to develop a positron emission tomography (PET) imaging agent for said receptor system. PET can be clinically used to probe receptor systems in vivo, permitting information such as a drug's occupancy against this system to be investigated. This review focuses on the efforts towards the development of a 5-HT₇R selective PET CNS tracer over the last 20 years, critically reflecting on applied strategies and commonly employed chemical frameworks and suggests future considerations that are needed to successfully develop a PET tracer for this clinically relevant target. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Vapor inhalation of cannabidiol (CBD) in rats

RATIONALE

Cannabidiol (CBD), a compound found in many strains of the Cannabis genus, is increasingly available in e-cigarette liquids as well as other products. CBD use has been promoted for numerous purported benefits which have not been rigorously assessed in preclinical studies.

OBJECTIVE

To further validate an inhalation model to assess CBD effects in the rat. The primary goal was to determine plasma CBD levels after vapor inhalation and compare that with the levels observed after injection. Secondary goals were to determine if hypothermia is produced in male Sprague-Dawley rats and if CBD affects nociception measured by the warm water tail-withdrawal assay.

METHODS

Blood samples were collected from rats exposed for 30 min to vapor generated by an e-cigarette device using CBD (100, 400 mg/mL in the propylene glycol vehicle). Separate experiments assessed the body temperature response to CBD in combination with nicotine (30 mg/mL) and the anti-nociceptive response to CBD.

RESULTS

Vapor inhalation of CBD produced concentration-related plasma CBD levels in male and female Wistar rats that were within the range of levels produced by 10 or 30 mg/kg, CBD, i.p. Dose-related hypothermia was produced by CBD in male Sprague-Dawley rats, and nicotine (30 mg/mL) inhalation enhanced this effect. CBD inhalation had no effect on anti-nociception alone or in combination with Δ9-tetrahydrocannabinol inhalation.

CONCLUSIONS

The vapor-inhalation approach is a suitable pre-clinical model for the investigation of the effects of inhaled CBD. This route of administration produces hypothermia in rats, while i.p. injection does not, at comparable plasma CBD levels.

Serotonin receptor oligomerization regulates cAMP-based signaling

Protein-protein interaction is often investigated using quantitative molecular microscopy with Förster resonant energy transfer (FRET). Here, we combined 'linear unmixing FRET' (lux-FRET) with the simultaneous application of a FRET-based biosensor for cAMP to investigate the oligomerization between the 5-HT₇ receptor (5-HT₇R, also known as HTR7) and the 5-HT_1A receptor (5-HT_1AR, also known as HTR1A) and its importance for cAMP signaling. We found that the 5-HT₇R not only stimulates cAMP production, but also forms hetero-oligomers with 5-HT_1AR, which blocks the inhibitory effect of the latter. 5-HT₇R signaling, however, is not affected by this hetero-oligomerization. By modeling the kinetics of intracellular cAMP level changes in relation to the 5-HT₇R:5-HT_1AR stoichiometry, we were able to decipher the complex signaling characteristics of endogenous serotonin receptors in cultured hippocampal neurons. Our findings indicate that serotonergic signaling is not only modulated by the concentration of an individual receptor but also by its specific interaction with other receptors in endogenous systems. We conclude that the regulated ratio of serotonin receptors in immature and mature neurons may be critically involved in both the onset and response to treatments of psychiatric diseases, such as anxiety and depression.

8-OH-DPAT Induces Compulsive-like Deficit in Spontaneous Alternation Behavior: Reversal by MDMA but Not Citalopram

Rodents exhibit natural exploratory behaviors, which can be measured by the spontaneous alternation behavior (SAB) test. Perseverance in this test induced by the 5-hydroxytryptamine 1A receptor (5-HT_1AR) agonist, 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT), resembles compulsive behaviors observed in humans and manifests as reduced alternation ratio. This study characterized 8-OH-DPAT-induced perseverance in the SAB test in C57BL/6JOlaHsd male mice by coadministration of WAY100635, citalopram and the 5-HT releasing agent, 3,4-methylenedioxymethamphetamine (MDMA), to deepen the understanding of 5-HT-dependent mechanisms. The 5-HT_1AR mechanism of 8-OH-DPAT (1.0 mg/kg, p < 0.01) on perseverance was confirmed by coadministration of the 5-HT_1AR antagonist, WAY100635 (2.0 mg/kg, p < 0.05), which attenuated the effects of 8-OH-DPAT. Such effects could also be reversed by MDMA (1.0 mg/kg, p < 0.05; 10.0 mg/kg, p < 0.001) but not citalopram. These findings confirm the importance of 5-HT in regulating perseverative behavior. Future investigations are required to determine the predictive validity of the 8-OH-DPAT-disrupted SAB test as an inducible mouse model of compulsivity.

The investigation of possible roles of central 5-HT7 receptors in antipyretic effect mechanism of paracetamol in LPS-induced hyperthermia model of mice

AIM

This study aimed to investigate the role of the 5-HT₇ receptor in fever mechanisms and its possible effect on the antipyretic mechanism of paracetamol.

MATERIALS AND METHODS

The study consisted of eight experimental groups and one control group. Group I: healthy, II: LPS, III: LPS + PARA, IV: LPS + AGO, V: LPS + ANTA, VI: LPS + AGO + ANTA, VII: LPS + AGO + PARA, VIII: LPS + ANTA + PARA, and IX: LPS + AGO + ANTA + PARA. Rectal temperatures were measured with a rectal thermometer. At the end of the experiment, tissues were examined molecularly. Real-time PCR mRNA expression analyses were performed for the 5-HT7 receptor, IL-6, and TNF-α in hypothalamus tissue.

RESULTS

The mean differences in rectal temperature increased in the LPS, LPS + ANTA, and LPS + AGO + ANTA groups when compared to the healthy group and decreased in the LPS + PARA, LPS + AGO, LPS + AGO + PARA, and LPS + AGO + ANTA + PARA groups when compared to the healthy group. The IL-6 and TNF-α mRNA expression increased in the LPS, LPS + ANTA, and LPS + AGO + ANTA groups when compared to the healthy group in the 2nd and 4th hours. The IL-6 and TNF-α expression decreased in the LPS + PARA, LPS + AGO, LPS + AGO + PARA, and LPS + AGO + ANTA + PARA groups when compared to the LPS group in the 2nd and 4th hours. The 5-HT₇ receptor mRNA expression increased in the LPS group when compared to the healthy group in the 2nd hour. The 5-HT₇ receptor mRNA expression decreased in the LPS + AGO and LPS + AGO + PARA groups when compared to the LPS group in the 2nd hour. The 5-HT₇ receptor mRNA expression increased the in LPS + ANTA and LPS + ANTA + PARA groups when compared to the LPS group in the 2nd hour.

CONCLUSION

The 5-HT₇ receptor is a potential defense mechanism in stopping fever and the antipyretic property of paracetamol is not due to the 5-HT₇ receptor.

Potentiation of the glycine response by serotonin on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

The lamina II, also called the substantia gelatinosa (SG), of the trigeminal subnucleus caudalis (Vc), is thought to play an essential role in the control of orofacial nociception. Glycine and serotonin (5-hydroxytryptamine, 5-HT) are the important neurotransmitters that have the individual parts on the modulation of nociceptive transmission. However, the electrophysiological effects of 5-HT on the glycine receptors on SG neurons of the Vc have not been well studied yet. For this reason, we applied the whole-cell patch clamp technique to explore the interaction of intracellular signal transduction between 5-HT and the glycine receptors on SG neurons of the Vc in mice. In nine of 13 neurons tested (69.2%), pretreatment with 5-HT potentiated glycine-induced current (I_Gly). Firstly, we examined with a 5-HT₁ receptor agonist (8-OH-DPAT, 5-HT_1/7 agonist, co-applied with SB-269970, 5-HT₇ antagonist) and antagonist (WAY-100635), but 5-HT₁ receptor agonist did not increase I_Gly and in the presence of 5-HT₁ antagonist, the potentiation of 5-HT on I_Gly still happened. However, an agonist (α-methyl-5-HT) and antagonist (ketanserin) of the 5-HT₂ receptor mimicked and inhibited the enhancing effect of 5-HT on I_Gly in the SG neurons, respectively. We also verified the role of the 5-HT₇ receptor by using a 5-HT₇ antagonist (SB-269970) but it also did not block the enhancement of 5-HT on I_Gly. Our study demonstrated that 5-HT facilitated I_Gly in the SG neurons of the Vc through the 5-HT₂ receptor. The interaction between 5-HT and glycine appears to have a significant role in modulating the transmission of the nociceptive pathway.

CRISPR/Cas9-mediated in vivo gene editing reveals that neuronal 5-HT1A receptors in the dorsal raphe nucleus contribute to body temperature regulation in mice

Serotonin (5-HT) in the central nervous system regulates a variety of biological functions, from the basic homeostatic control to higher brain functions, by acting on fourteen known receptor subtypes. However, it is still usually unclear which receptor subtype is responsible for a specific function due to the lack of highly selective ligands for most of these receptors. Although 5-HT receptor knockout mice are useful, the brain-wide distribution of various receptors makes it difficult to dissect receptor functions in specific and brain regions and cell types. Recent advances in CRISPR/Cas9-mediated in vivo genome editing technology may overcome this problem. In this study, we constructed a viral vector expressing a single guide (sg)RNA targeting Htr1a (sgHtr1a) and Cre recombinase under the control of a neuron-specific promoter. Injection of the viral vector into the dorsal raphe nucleus (DRN) of Cre-dependent Cas9 knock-in mice induced Cre-dependent Cas9 expression mainly in DRN serotonin and GABA neurons. Mismatch cleavage assay and Sanger sequencing showed insertion or deletion formation at the target site. 5-HT_1A receptor agonist-induced hypothermia was attenuated and antidepressant effect of a selective serotonin reuptake inhibitor (SSRI) was enhanced by microinjection of the viral vector expressing sgHtr1a into the DRN of Cre-dependent Cas9 knock-in mice. These results suggest that this in vivo CRISPR/Cas9-mediated 5-HT receptor gene knockout strategy provides a reliable and low-cost method for elucidating 5-HT receptor functions in specific cell types and brain regions. Further, we demonstrate that the neuronal 5-HT_1A receptor in the DRN regulates body temperature and antidepressant effect of SSRI.

Fluorinated indole-imidazole conjugates: Selective orally bioavailable 5-HT7 receptor low-basicity agonists, potential neuropathic painkillers

The 5-HT₇ receptor has recently gained much attention due to its involvement in multiple physiological functions and diseases. The insufficient quality of the available molecular probes prompted design of fluorinated 3-(1-alkyl-1H-imidazol-5-yl)-1H-indoles as a new generation of selective 5-HT₇ receptor agonists. A potent and drug-like agonist, 3-(1-ethyl-1H-imidazol-5-yl)-5-iodo-4-fluoro-1H-indole (AGH-192, 35, K_{i 5-HT7R} = 4 nM), was identified by optimizing the halogen bond formation with Ser5.42 as the supposed partner. The compound was characterized by excellent water solubility, high selectivity over related CNS targets, high metabolic stability, oral bioavailability and low cytotoxicity. Rapid absorption into the blood, medium half-life and a high peak concentration in the brain C_max = 1069 ng/g were found after i.p. (2.5 mg/kg) administration in mice. AGH-192 may thus serve as the long-sought tool compound in the study of 5-HT₇ receptor function, as well as a potential analgesic, indicated by the antinociceptive effect observed in a mouse model of neuropathic pain.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

5-HT1A Receptor Function Makes Wound Healing a Happier Process

Skin wound healing is a multistage phenomenon that is regulated by cell–cell interplay and various factors. Endogenous serotonin is an important neurotransmitter and cytokine. Its interaction with the serotonin 1A receptor (5-HTR1A) delivers downstream cellular effects. The role of serotonin (5-hydroxytryptamine, 5-HT) and the 5-HT1A receptor has been established in the regeneration of tissues such as the liver and spinal motor neurons, prompting the investigation of the role of 5-HT1A receptor in skin healing. This study assessed the role of 5-HT1A receptor in excisional wound healing by employing an excisional punch biopsy model on 5-Ht1a receptor knockout mice. Post-harvest analysis revealed 5-Ht1a receptor knockout mice showed impaired skin healing, accompanied by a greater number of F4/80 macrophages, which prolongs the inflammatory phase of wound healing. To further unravel this phenomenon, we employed the 5-HT1A receptor agonist [(R)-(+)-8-Hydroxy-DPAT hydrobromide] as a topical cream treatment in an excisional punch biopsy model. The 5-HT1A receptor agonist treated group showed a smaller wound area, scar size, and improved neovascularization, which contributed to improve healing outcomes as compared to the control. Collectively, these findings revealed that serotonin and 5-HT1A receptor play an important role during the healing process. These findings may open new lines of investigation for the potential treatment alternatives to improve skin healing with minimal scarring.

Effects of 5-HT7 receptor antagonists on behaviors of mice that detect drugs used in the treatment of anxiety, depression, or schizophrenia

5-HT₇ receptors have been suggested to play a role in the regulation of psychiatric disorders. The experimental literature however is not fully consistent on this possibility. Two selective 5-HT₇ receptor antagonists, DR-4004 and SB-269970, were evaluated in mouse models used to detect drugs used to treat anxiety, depression, or schizophrenia. A 5-HT-induced hypothermia assay was used to define the doses of DR-4004 and SB-269970 predicted to impact 5-HT₇ receptors in the brain in vivo. 5-HT produced hypothermia in wildtype mice by either i.p. or i.c.v. routes but did not in 5-HT₇ receptor knockout mice. 5-HT-induced hypothermia was not attenuated by drugs selectively blocking alpha1 or 5-HT_1A receptors. Doses of DR-4004 and SB-269970 that blocked 5-HT-induced hypothermia, did not display significant anxiolytic-like (elevated plus maze; vogel conflict) or antidepressant-like efficacy (tail-suspension test) in mouse models. These compounds did demonstrate some antipsychotic-like properties in the PCP-induced hyperactivity assay and anxiolytic/anti-stress effects in the stress-induced cGMP assay. Negative findings were substantiated by positive control drugs that were active in each assay system. We conclude that 5-HT-induced hypothermia can be used to estimate blockade of central 5-HT₇ receptors. Effects of DR-4004 and SB-269970 in animal models are generally consistent with the experimental literature that the evidence is mixed or not robust regarding the potential efficacy of 5-HT₇ receptor antagonism in the treatment of anxiety, depression, or schizophrenia.

Inhibitory neurotransmitter serotonin and excitatory neurotransmitter dopamine both decrease food intake in Chinese perch (Siniperca chuatsi)

Aminergic neurotransmitters play important roles in the regulation of food intake. However, their effects on feeding in fish have been less explored and still unclear. In the present study, the effects of serotonin (5-HT) and dopamine (DA) on food intake were evaluated through intraventricular (ICV) administration in Chinese perch (Siniperca chuatsi) and the mRNA expression levels of neuropeptide Y (NPY), agouti gene-related protein (AgRP), and pro-opiomelanocortin (POMC) were detected. At 1 h post-injection, 5-HT significantly decreased food intake in a dose-dependent manner. The mRNA expression of NPY and AgRP were significantly decreased (p < 0.05), whereas the mRNA expression of POMC was significantly increased (p < 0.05), suggesting the involvement of NPY, AgRP, and POMC in inhibitory action of 5-HT on food intake in Chinese perch. DA significantly decreased (p < 0.05) food intake and AgRP mRNA expression at 1 h post-injection, indicating the inhibitory effect of DA on food intake might be mediated through AgRP. This might shed new light on the regulation of food intake in Chinese perch.

Testosterone promotes anxiolytic-like behavior in gonadectomized male rats via blockade of the 5-HT1A receptors

This study was designed to examine an anxiety-like behavior in the adult gonadectomized (GDX) male rats subjected to testosterone propionate (TP) treatment alone or in combination with 8-OH-DPAT, a 5-HT_1A receptor agonist, or with NAN-190, 5-HT_1A receptor antagonist. Two weeks after gonadectomy, GDX rats were subjected by treatments with the solvent, TP (0.5mg/kg, s.c.), 8-OH-DPAT (0.05mg/kg, s.c.), NAN-190 (0.1mg/kg, i.p.), TP in combination with 8-OH-DPAT or NAN-190 during 14days. Anxiety behavior was assessed in the elevated plus maze (EPM) and the open field test (OFT). 8-OH-DPAT treatment failed to modify the anxiety-like behavior of GDX rats in the EPM as compared to the GDX rats given with oil solvent. NAN-190 injected alone or in combination with TP to GDX rats resulted in a significant anxiolytic-like effect as compared to the GDX given with oil solvent or TP application. Our data indicate that the combination of NAN-190 and TP is more effective than TP alone in GDX rats inducing a more profound anxiolytic-like effect in the EPM. Thus, the results of this study suggest that effects of 5-HT_1A receptor agonist/antagonist can modify anxiety level in opposite direction in male rats after gonadectomy.

Development of a mechanism-based pharmacokinetic/pharmacodynamic model to characterize the thermoregulatory effects of serotonergic drugs in mice

We have shown recently that concurrent harmaline, a monoamine oxidase-A inhibitor (MAOI), potentiates serotonin (5-HT) receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT)-induced hyperthermia. The objective of this study was to develop an integrated pharmacokinetic/pharmacodynamic (PK/PD) model to characterize and predict the thermoregulatory effects of such serotonergic drugs in mice. Physiological thermoregulation was described by a mechanism-based indirect-response model with adaptive feedback control. Harmaline-induced hypothermia and 5-MeO-DMT–elicited hyperthermia were attributable to the loss of heat through the activation of 5-HT_1A receptor and thermogenesis via the stimulation of 5-HT_2A receptor, respectively. Thus serotonergic 5-MeO-DMT–induced hyperthermia was readily distinguished from handling/injection stress-provoked hyperthermic effects. This PK/PD model was able to simultaneously describe all experimental data including the impact of drug-metabolizing enzyme status on 5-MeO-DMT and harmaline PK properties, and drug- and stress-induced simple hypo/hyperthermic and complex biphasic effects. Furthermore, the modeling results revealed a 4-fold decrease of apparent SC₅₀ value (1.88–0.496 µmol/L) for 5-MeO-DMT when harmaline was co-administered, providing a quantitative assessment for the impact of concurrent MAOI harmaline on 5-MeO-DMT–induced hyperthermia. In addition, the hyperpyrexia caused by toxic dose combinations of harmaline and 5-MeO-DMT were linked to the increased systemic exposure to harmaline rather than 5-MeO-DMT, although the body temperature profiles were mispredicted by the model. The results indicate that current PK/PD model may be used as a new conceptual framework to define the impact of serotonergic agents and stress factors on thermoregulation.

Distinct effect of 5-HT1A and 5-HT2A receptors in the medial nucleus of the amygdala on tonic immobility behavior

The tonic immobility (TI) response is an innate fear behavior associated with intensely dangerous situations, exhibited by many species of invertebrate and vertebrate animals. In humans, it is possible that TI predicts the severity of posttraumatic stress disorder symptoms. This behavioral response is initiated and sustained by the stimulation of various groups of neurons distributed in the telencephalon, diencephalon and brainstem. Previous research has found the highest Fos-IR in the posteroventral part of the medial nucleus of the amygdala (MEA) during TI behavior; however, the neurotransmission of this amygdaloid region involved in the modulation of this innate fear behavior still needs to be clarified. Considering that a major drug class used for the treatment of psychopathology is based on serotonin (5-HT) neurotransmission, we investigated the effects of serotonergic receptor activation in the MEA on the duration of TI. The results indicate that the activation of the 5HT1A receptors or the blocking of the 5HT2 receptors of the MEA can promote a reduction in fear and/or anxiety, consequently decreasing TI duration in guinea pigs. In contrast, blocking the 5HT1A receptors or activating the 5HT2 receptors in this amygdalar region increased the TI duration, suggesting an increase in fear and/or anxiety. These alterations do not appear to be due to a modification of spontaneous motor activity, which might non-specifically affect TI duration. Thus, these results suggest a distinct role of the 5HT receptors in the MEA in innate fear modulation.

Synthesis and Structure-Activity Relationship Analysis of 5-HT₇ Receptor Antagonists: Piperazin-1-yl Substituted Unfused Heterobiaryls

A series of piperazin-1-yl substituted unfused heterobiaryls was synthesized as ligands of the 5-HT₇ receptors. The goal of this project was to elucidate the structural features that affect the 5-HT₇ binding affinity of this class of compounds represented by the model ligand 4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2). The SAR studies included systematical structural changes of the pyrimidine core moiety in 2 to quinazoline, pyridine and benzene, changes of the 3-furyl group to other heteroaryl substituents, the presence of various analogs of the 4-methylpiperazin-1-yl group, as well as additional substitutions at positions 5 and 6 of the pyrimidine. Substitution of position 6 of the pyrimidine in the model ligand with an alkyl group results in a substantial increase of the binding affinity (note a change in position numbers due to the nomenclature rules). It was also demonstrated that 4-(3-furyl) moiety is crucial for the 5-HT₇ binding affinity of the substituted pyrimidines, although, the pyrimidine core can be replaced with a pyridine ring without a dramatic loss of the binding affinity. The selected ethylpyrimidine (12) and butylpyrimidine (13) analogs of high 5-HT₇ binding affinity showed antagonistic properties in cAMP functional test and varied selectivity profile-compound 12 can be regarded as a dual 5-HT₇/5-HT2AR ligand, and 13 as a multi-receptor (5-HT₇, 5-HT2A, 5-HT₆ and D₂) agent.

Activation of 5-HT1A receptors in the preBötzinger region has little impact on the respiratory pattern

The preBötzinger (preBötC) complex has been suggested as the primary site where systemically administered selective serotonin agonists have been shown to reduce or prevent opioid-induced depression of breathing. However, this hypothesis has not been tested pharmacologically in vivo. This study sought to determine whether 5-HT1A receptors within the preBötC and ventral respiratory column (VRC) mediate the tachypneic response induced by intravenous (IV) (±)-8-Hydroxy-2-diproplyaminotetralin hydrobromide (8-OH-DPAT) in a decerebrated dog model. IV 8-OH-DPAT (19 ± 2 μg/kg) reduced both inspiratory (I) and expiratory (E) durations by ∼ 40%, but had no effect on peak phrenic activity (PPA). Picoejection of 1, 10, and 100 μM 8-OH-DPAT on I and E preBötC neurons produced dose-dependent decreases up to ∼ 40% in peak discharge. Surprisingly, microinjections of 8-OH-DPAT and 5-HT within the VRC from the obex to 9 mm rostral had no effect on timing and PPA. These results suggest that the tachypneic effects of IV 8-OH-DPAT are due to receptors located outside of the areas we studied.

Interplay between serotonin 5-HT1A and 5-HT7 receptors in depressive disorders

Serotonin (5-hydroxytryptamine or 5-HT) is an important neurotransmitter regulating a wide range of physiological and pathological functions via activation of heterogeneously expressed 5-HT receptors. Besides the important role of 5-HT receptors in the pathogenesis of depressive disorders and in their clinical medications, underlying mechanisms are far from being completely understood. This review focuses on possible cross talk between two serotonin receptors, 5-HT1A and the 5-HT7 . Although these receptors are highly co-expressed in brain regions implicated in depression, and most agonists developed for the 5-HT1A or 5-HT7 receptors have cross-reactivity, their functional interaction has not been yet established. It has been recently shown that 5-HT1A and 5-HT7 receptors form homo- and heterodimers both in vitro and in vivo. From the functional point of view, heterodimerization has been shown to play an important role in regulation of receptor-mediated signaling and internalization, suggesting the implication of heterodimerization in the development and maintenance of depression. Interaction between these receptors is also of clinical interest, because both receptors represent an important pharmacological target for the treatment of depression and anxiety.

Design, synthesis and biological evaluation of small molecule-based PET radioligands for the 5-hydroxytryptamine 7 receptor

A new prospective approach for PET imaging of 5-HT₇ by a small molecule ligand.

The lonely mouse - single housing affects serotonergic signaling integrity measured by 8-OH-DPAT-induced hypothermia in male mice

Male BALB/c mice single-housed for a period of three weeks were found to respond with a more marked hypothermia to a challenge with a selective serotonergic agonist (8-OH-DPAT) than their group-housed counterparts. This effect of single housing was verified by screening a genetically heterogeneous population of male mice on a C57BL/6 background from a breeding colony. Enhanced activity of the implicated receptor (5-HT1A) leading to an amplified hypothermic effect is strongly associated with depressive states. We therefore suggest that the 8-OH-DPAT challenge can be used to demonstrate a negative emotional state brought on by e.g. long-term single housing in male laboratory mice. The study emphasizes the importance of social housing, and demonstrates that male mice deprived of social contact respond with altered serotonergic signaling activity. Male mice not only choose social contact when given the option, as has previously been shown, but will also, when it is deprived, be negatively affected by its absence. We propose that the 8-OH-DPAT challenge constitutes a simple, but powerful, tool capable of manifesting the effect of social deprivation in laboratory mice. It potentially allows not only for an unbiased, biochemical evaluation of psychological stressors, but may also allow for determining whether the effect of these can be counteracted.

5-HT7 receptors as modulators of neuronal excitability, synaptic transmission and plasticity: physiological role and possible implications in autism spectrum disorders

Serotonin type 7 receptors (5-HT7) are expressed in several brain areas, regulate brain development, synaptic transmission and plasticity, and therefore are involved in various brain functions such as learning and memory. A number of studies suggest that 5-HT7 receptors could be potential pharmacotherapeutic target for cognitive disorders. Several abnormalities of serotonergic system have been described in patients with autism spectrum disorder (ASD), including abnormal activity of 5-HT transporter, altered blood and brain 5-HT levels, reduced 5-HT synthesis and altered expression of 5-HT receptors in the brain. A specific role for 5-HT7 receptors in ASD has not yet been demonstrated but some evidence implicates their possible involvement. We have recently shown that 5-HT7 receptor activation rescues hippocampal synaptic plasticity in a mouse model of Fragile X Syndrome, a monogenic cause of autism. Several other studies have shown that 5-HT7 receptors modulate behavioral flexibility, exploratory behavior, mood disorders and epilepsy, which include core and co-morbid symptoms of ASD. These findings further suggest an involvement of 5-HT7 receptors in ASD. Here, we review the physiological roles of 5-HT7 receptors and their implications in Fragile X Syndrome and other ASD.

Novel N-biphenyl-2-ylmethyl 2-methoxyphenylpiperazinylalkanamides as 5-HT7R antagonists for the treatment of depression

5-HT7 receptor (5-HT7R) is a promising target for the treatment of depression and neuropathic pain. 5-HT7R antagonists exhibited antidepressant effects, while the agonists produced strong anti-hyperalgesic effects. In our efforts to discover selective 5-HT7R antagonists or agonists, N-biphenylylmethyl 2-methoxyphenylpiperazinylalkanamides 1 were designed, synthesized, and biologically evaluated against 5-HT7R. Among the synthesized compounds, N-2'-chlorobiphenylylmethyl 2-methoxyphenylpiperazinylpentanamide 1-8 showed the best binding affinity with a Ki value of 8.69nM and it was verified as a novel antagonist according to functional assays. The compound 1-8 was very selective over 5-HT1DR, 5-HT2AR, 5-HT3R, 5-HT5AR and 5-HT6R and moderately selective over 5-HT1AR, 5-HT1BR and 5-HT2CR. The novel 5-HT7R antagonist 1-8 exhibited an antidepressant effect at a dose of 25mg/kg in the forced swimming test in mice and showed a U-shaped dose-response curve which typically appears in 5-HT7R antagonists such as SB-269970 and lurasidone.

The role of the sympathetic nervous system in the resuscitative effect of stimulating the central serotonin 1A receptors in haemorrhagic shock in rats

Haemorrhagic shock is a life threatening condition, and, as such, it is important to understand the mechanisms taking part in its reversal. In the 1990s, it was shown that activation of serotonin 1A receptors is responsible for the circulatory decompensation and development of the sympathoinhibitory phase. In previous reports, it was demonstrated that activation of serotonin 1A receptors induces resuscitative effects in haemorrhaged rats. However, the effectory mechanisms still require further investigation. The aim of the present study was to determine whether the sympathetic nervous system participates in the effects of serotonin through central serotonin 1A receptors in haemorrhagic shock in rats. In order to determine the role of the sympathetic nervous system alpha-1-, alpha-2-, and beta-adrenergic receptor agonists - prazosin, yohimbine and propranolol, respectively, were used. We found that stimulation of the central serotonin 1A receptors by the administration of a selective agonist - 8-hydroxy-2-(di-n-propylamino)tetralin, 1-(2,5-dimethoxy-4-iodophenyl)-aminopropane (8-OH-DPAT) into the lateral brain ventricle is connected with the activation of compensation mechanisms leading to the increase in the heart rate and blood pressure. The current results demonstrate that the stimulation of peripheral alpha-1-, alpha-2- and beta-adrenergic receptors plays an essential role in the resuscitative effect triggered by the stimulation of central serotonin 1A receptors.

5-HT1A receptor activation reduces fear-related behavior following social defeat in Syrian hamsters

Social defeat leads to selective avoidance of familiar opponents as well as general avoidance of novel, non-threatening intruders. Avoidance of familiar opponents represents a fear-related memory whereas generalized social avoidance indicates anxiety-like behavior. We have previously shown that serotonin signaling alters responses to social defeat in Syrian hamsters, although it is unclear whether serotonin modulates defeat-induced fear, anxiety, or both. In this study we focus on 5-HT1A receptors, in part, because their activation had been linked to the acquisition of conditioned fear. We hypothesized that pharmacological activation of 5-HT1A receptors prior to social defeat would reduce avoidance of familiar opponents and impair Arc expression in the basolateral amygdala (BLA), but not alter anxiety-like behavior. We administered 8-OH-DPAT, a 5-HT1A receptor agonist, prior to 3, 5-minute social defeats and 24h later exposed hamsters to a social interaction test to measure the conditioned defeat response immediately followed by either a Y-maze test or an open field test. In a separate experiment, we administered 8-OH-DPAT prior to 3, 5-minute social defeats and later removed the brains for Arc immunohistochemistry. Social defeat increased the number of Arc immunopositive cells in the central amygdala (CeA), prelimbic cortex (PL), and BLA, and 8-OH-DPAT treatment reduced Arc immunoreactivity in the PL. These results suggest that 5-HT1A receptor activation impairs the fear memory associated with social defeat, but does not alter defeat-induced anxiety. Overall, 5-HT1A receptor activation may impair Arc expression in select brain regions such as the PL and thereby disrupt the development of a fear memory essential for the conditioned defeat response.